Corrosion and Corrosion Fatigue of Steels in Downhole CCS Environment—A Summary

processes

Review Corrosion and Corrosion Fatigue of Steels in Downhole CCS Environment—A Summary

Anja Pfennig 1 , Marcus Wolf 2 and Axel Kranzmann 2,*

1 HTW Berlin, Wilhelminenhofstraße 75A, 12459 Berlin, Germany; [email protected] 2 BAM Federal Institute of Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-501-94231

Abstract: Static immersion tests of potential injection pipe steels 42CrMo4, X20Cr13, X46Cr13, X35CrMo4, and X5CrNiCuNb16-4 at T = 60 ◦C and ambient pressure, as well as p = 100 bar were

performed for 700–8000 h in a CO2-saturated synthetic aquifer environment similar to CCS sites in the Northern German Basin (NGB). Corrosion rates at 100 bar are generally lower than at ambient

pressure. The main corrosion products are FeCO3 and FeOOH with surface and local corrosion phenomena directly related to the alloy composition and microstructure. The appropriate heat treatment enhances corrosion resistance. The lifetime reduction of X46Cr13, X5CrNiCuNb16-4, and duplex stainless steel X2CrNiMoN22-5-3 in a CCS environment is demonstrated in the in situ corrosion fatigue CF experiments (axial push-pull and rotation bending load, 60 ◦C, brine: Stuttgart

Aquifer and NGB, flowing CO2: 30 L/h, +/− applied potential). Insulating the test setup is necessary to gain reliable data. S-N plots, micrographic-, phase-, fractographic-, and surface analysis prove that the life expectancy of X2CrNiMoN22-5-3 in the axial cyclic load to failure is clearly related to the surface finish, applied stress amplitude, and stress mode. The horizontal grain attack within corrosion pit cavities, multiple fatigue cracks, and preferable deterioration of austenitic phase mainly Citation: Pfennig, A.; Wolf, M.; cause fatigue failure. The CF life range increases significantly when a protective potential is applied. Kranzmann, A. Corrosion and Corrosion Fatigue of Steels in Keywords: steel; high alloyed steel; corrosion; corrosion fatigue; CCS; carbon capture and storage Downhole CCS Environment—A Summary. Processes 2021, 9, 594. https://doi.org/10.3390/pr9040594 1. Introduction Academic Editor: Aneta Magdziarz The carbon capture and storage process (CCS [1,2]) is a well acknowledged technique Received: 3 March 2021 to mitigate climate change. Emission gases—mostly from combustion processes of power Accepted: 24 March 2021 or cement production plants—are compressed into safe deep onshore or offshore geological Published: 29 March 2021 layers. However, steels used as pipes for transport or injecting into, e.g., a saline aquifer (onshore CCS site) are susceptible to CO2-corrosion [3–9] influenced by: Publisher’s Note: MDPI stays neutral • Temperature and CO2 partial pressure; with regard to jurisdictional claims in • alloy composition and compositions of the corrosive media; published maps and institutional affil- • contamination of alloy and media; iations. • flow conditions and injection pressure and; • protective corrosion scales [5,6,10–21]. The corrosion resistance of various steels is mostly dependent on the composition of the alloys [22] and their heat treatment [23–25]: Ni- and Cr reduce surface corrosion phe- Copyright: © 2021 by the authors. nomena [26,27] and retained austenite reduces local corrosion [26]. The higher temperature Licensee MDPI, Basel, Switzerland. during austenitizing of martensitic steels [28–30] and annealing of lean duplex stainless This article is an open access article steels [22,23,28] decreases the potential for local phenomena. If C-Mn (carbon) steels are distributed under the terms and heat treated to the martensitic microstructure, grain boundaries react in a H S-containing conditions of the Creative Commons 2 NaCl resulting in lower corrosion resistance compared to the ferritic or ferritic-bainitic Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ microstructure [31]. 4.0/).

Processes 2021, 9, 594. https://doi.org/10.3390/pr9040594 https://www.mdpi.com/journal/processes Processes 2021, 9, 594 2 of 33

Corrosion also leads to early failure of stainless steels that are mechanically loaded [32–34] since chemical reactions as well as local changes of lattice energy at the surface and mechanical load enhance pitting [35,36]. Pit-, selective-, and intergranular corrosion are generally correlated to the local lattice mismatch leading to crack formation [33] at dual or triple points of grain/phase boundaries with higher grain/phase boundary accelerating crack formation and propagation [32,34]. Pfennig et al. [35,36] modified a possible crack initiation first presented by Han et al. [37]. When exposing steels to the CO2-environment, the corrosion products found on the surface and within pits are similar [15,17], mostly comprising of siderite FeCO3 [3,38]. ◦ The low solubility of FeCO3 in water is low (pKsp = 10.54 at 25 C[29,37]) causing anodic iron dissolution initialized by forming of transient iron hydroxide Fe(OH)2 [6,30]. The pH increases locally causing reactions (Equations (1) to (6) [15,29]) that lead to the precipitation of an internal and external ferrous carbonate film:

+ − CO2 (g) + H2O (L) → H + HCO3 (aq) (1)

− − 2− Cathodic: 2 HCO3 + 2 e → 2 CO3 + H2 (2) Anodic: Fe → Fe2+ + 2e− (3) 2+ 2− Fe + CO3 → FeCO3 (4) 2+ − Fe + 2 HCO3 → Fe(HCO3)2 (5) + Fe(HCO3)2 → FeCO3 CO2 + H2O (6) The literature widely describes the influence of frequency, temperature and chloride concentration [35,36,39–43], topography [44], geometric [45], and compression pre- cracking [46,47], as well as foreign impact [48–51] on the corrosion fatigue behavior and especially on the crack initiation and crack growth [45–51]. In general, corrosion processes with or without the applied mechanical stress are enhanced [49,52], especially in steels with low chromium content [6,10,11], with the presence of chloride [48,50,53,54], hydrogen [55], hydrogen sulfide (H2S) [56,57], and CO2 [8,58,59]. The stress corrosion resistance under the unidirectional load in CO2-saturated brines and the endurance limit decreases with the increasing temperature, increasing mechanical load, and decreasing pH for high alloyed steels [60–62]. On the contrary, it increases with the increasing chromium content in ferritic, austenitic, and duplex steels [63], as well as the internal compressive stress in surface regions [49,64]. The martensitic microstructure shows a brittle fracture under the cyclic load [65], but improves the corrosion fatigue [66], while precipitations of copper or oxide inclusions will cause early failure already at a small number of cycles [36,67]. Standard duplex stainless steel DSS X2CrNiMoN22-5-3 (AISI 2205), usually used in operational units of desalinations plants, heat exchangers, and chemical and petroleum industries [68], is a forward-looking candidate regarding corrosion fatigue in both CCS and geothermal applications since it is highly resistant to stress corrosion cracking [69] as well as corrosive environments [70,71]. The surface quality of steels directly influences the corrosion fatigue behavior [72–80], which is improved by smooth surfaces [81], deeper plastic deformations and compressive surface stress [82–84], shot peeing [85], and in ferritic stainless steel when Ra exceeded 0.5 µm [86]. The corrosion fatigue resistance of X2CrNiMoN22-5-3 under different environments [87–90] has first been tested under in situ geothermal conditions [91–93] and the results are now combined. This paper comprises and compares summarized data of previously published work by the authors.

2. Materials and Methods To examine the inﬂuence of the corrosive media on the corrosion fatigue behavior of high alloyed steels, the specimens were tested by choosing conditions similar to those Processes 2021, 9, 594 3 of 33

occurring during carbon capture and storage CCS (static corrosion) and conditions in geothermal energy production (corrosion fatigue).

2.1. Steels Static corrosion tests at ambient pressure as well as at high pressure (100 bar) were car- ried out using samples of mild steel AISI 4140, 42CrMo4 1.7225 [9,15] (Table1), martensitic and duplex stainless steels (Tables2–6): 1. AISI 420 (X20Cr13, 1.4021) (Table2); 2. AISI 420C (X46Cr13, 1.4043) (Table3); 3. No AISI (X35CrMo17, 1.4122) (Table4); 4. AISI 630 (X5CrNiCuNb 16-4, 1.4542) (Table5); 5. AISI A182 F51 (329LN)) SAF 2205 (X2CrNiMoN 22-5-3 (UNS S31803) 1.4462) (Table6) . In order to conﬁrm the material’s chemical composition, samples were analyzed via spark emission spectrometry SPEKTROLAB M and by the electron probe microanalyzer JXA8900-RLn (Tables1–6).

Table 1. Chemical composition of 1.7225 (42CrMo4, AISI 4042), in mass percent.

Elements C Si Mn P S Cr Mo Ni Co Fe acc standard a 0.38–0.45 <0.40 0.6–0.9 ≤0.035 ≤0.035 0.90–1.20 0.15–0.30 rest analysed b 0.43 0.32 0.70 0.014 0.025 1.05 0.22 0.04 <0.01 97.1 a Elements as speciﬁed according to DIN EN 10088-3 in %; b spark emission spectrometry.

Table 2. Chemical composition of 1.4021 (X20Cr13, AISI 420A), in mass percent.

Elements C Si Mn P S Cr Mo Ni Co Fe acc standard a 0.17–0.25 <1.00 ≤1.00 ≤0.045 ≤0.03 12.0–14.0 0.20–0.45 analysed b 0.22 0.39 0.32 0.007 0.006 13.3 - 0.123 - rest a Elements as speciﬁed according to DIN EN 10088-3 in %; b spark emission spectrometry.

Table 3. Chemical composition of 1.4043 (X46Cr13, AISI 420C), in mass percent.

Elements C Si Mn P S Cr Mo Ni Co Fe acc standard a 0.42–0.5 <1.00 ≤1.00 ≤0.045 ≤0.03 12.5–14.5 0.20–0.45 analysed b 0.46 0.25 0.45 0.018 0.003 13.39 0.03 0.13 0.03 85.4 a Elements as speciﬁed according to DIN EN 10088-3 in %; b spark emission spectrometry.

Table 4. Chemical composition of 1.4122 (X35CrMo17), in mass percent.

Elements C Si Mn P S Cr Mo Ni Co Fe acc standard a 0.33–0.45 <1.00 ≤1.00 ≤0.045 ≤0.03 15.5–17.5 0.8–1.3 ≤1.00 0.20–0.45 a Elements as speciﬁed according to DIN EN 10088-3 in %.

Table 5. Chemical composition of 1.4542 (X5CrNiCuNb16-4, AISI 630), in mass percent.

Elements C Si Mn P S Cr Mo Ni Cu Nb acc standard a ≤0.07 ≤0.70 ≤1.50 ≤0.04 ≤0.015 15.0–17.0 ≤0.60 3.00–5.00 3.00–5.00 0.20–0.45 analysed b 0.03 0.42 0.68 0.018 0.002 15.75 0.11 4.54 3.00 0.242 a Elements as speciﬁed according to DIN EN 10088-3 in %; b spark emission spectrometry. Processes 2021, 9, 594 4 of 33

Table 6. Chemical composition of 1.4462 X2CrNiMoN 22 5 3 (UNS S31803), in mass percent.

Phases C Si Mn Cr Mo Ni N α & γ ** 0.023 0.48 1.83 22.53 2.92 5.64 0.15 α * 0.02 0.55 1.59 24.31 3.62 3.81 0.07 γ * 0.03 0.47 1.99 20.69 2.17 6.54 0.28 * PREN α = 37.4; γ = 32.4; ** p = 0.024; S = 0.008. Stainless steels 1.4043 and 1.4021 contain 13% chromium and serve as piping, shafts or axles in pumps in the geothermal energy production [38,54,94,95]. The higher carbon content of 1.4034 (0.46 mass% C) compared to 1.4020 (0.2 mass% C) most likely increases the corrosion rates. The precipitation hardening martensitic stainless steel 1.4542 (AISI 630, X5CrNiCuNb16- 4) contains 3% small grained copper [96]. Niobium and copper carbides are distributed in the layered body cubic centered (BCC) martensitic matrix [97]. The precipitation hardened microstructure shows good mechanical properties and corrosion resistance but is susceptible to stress corrosion cracking (SCC). (Note that although the strength is lower in the solution treated state, its corrosion resistance is higher [98,99]). Stainless steel 1.4462 (X2 CrNiMoN 22-5-3 (standard duplex stainless steel, Table6)) was continuously casted, tempered appropriately, and quenched in water resulting in a phase equilibrium of ferrite and austenite (Table7). In general, the corrosion resistance is directly related to the percentage of austenite demonstrated by the PREN number (35.1), which is twice as high for 1.4462 than for 1.4542 [90,100].

Aquifer Water To simulate the in situ geothermal condition, the geothermal aquifer water (known to be similar to the Stuttgart Aquifer [56–58,101] and Northern German Basin (NGB) [88,102]) was synthesized in a strictly orderly way to avoid the precipitation of salts and carbonates (Table7).

Table 7. Chemical composition of the Northern German Basin (NGB) and Stuttgart Formation electrolyte or according to the Stuttgart Formation.

According to the Northern German Basin or according to Stuttgart Formation

NaCl KCl CaCl2 × 2H2O MgCl2 × 6H2O NH4Cl ZnCl2 SrCl2 × 6H2O PbCl2 Na2SO4 Ph Value g/L 98.22 5.93 207.24 4.18 0.59 0.33 4.72 0.30 0.07 5.4–6

NaCl KCl CaCl2 × 2H2O MgCl2 × 6H2O Na2SO4 × 10H2O KOH NaHCO3 g/L 224.6 0.39 6.45 10.62 12.07 0.321 0.048 + 2+ 2+ 2+ − 2− − Ca K Mg Na Cl SO4 HCO3 pH value g/L 1.76 0.43 1.27 90.1 14.33 3.6 0.04 8.2–9

2.2. Static Corrosion Experiments Coupons of the as-received and thermally treated steel qualities of 8 mm thickness, 20 mm width, and 50 mm length were exposed to: 1. CO2-saturated aquifer brine and 2. water saturated CO2.

Heat Treatment One set of coupons was austenitized at 950, 1000, and 1050 ◦C for 30, 60, and 90 min, quenched in water and annealed at 650 ◦C for 30 min according to the usual heat treatment protocols. Another set of coupons was heat treated according to Table8[ 9,15–17,38,52,98, 99,103–106]. ProcessesProcesses 20212021,, 99,, x 594 FOR PEER REVIEW 55 of of 33 33

TableTable 8. 8. HeatHeat treatment treatment protocol protocol for for X20Cr X20Cr13,13, X46Cr13, X46Cr13, and X5CrNiCuNb16-4.

◦ ◦ ◦ ◦ HeatHeat Treatment Treatment T TAustenitizing/Austenitizing/°C CTT AustenitizingAustenitizing/°C / CTT AnnealingAnnealing/°C/ CTTAnnealing Annealing//°CC TimeTime Cooling X20Cr13/X46 X5CrNi-X5CrNi- X20Cr13 and X5CrNi- X20Cr13/X46Cr13 X20Cr13 and X5CrNiCuNb16-4 MinMin MediumMedium Cr13 CuNb16-4CuNb16-4 X46Cr13X46Cr13 CuNb16-4 HT1HT1 normalizing normalizing HT1HT1 785785 850 850 30 30 oil HT2HT2 hardening hardening 10001000 1040 1040 30 30 oil HT3HT3 hardening hardening + tempering 1 1 10401040 100 100 550 550 655 30 30 oil HT4HT4 hardening hardening + tempering 2 2 10401040 1000 1000 650 650 670 30 30 oil HT5HT5 hardening hardening + tempering 3 3 10401040 1000 1000 700 700 755 30 30 oil

SpecimensSpecimens were were positioned positioned by by a a hole hole of of 3.9 3.9 mm mm and and tested tested in in the the vapor vapor phase phase and and liquidliquid phase. phase. A Acapillary capillary meter meter GDX600_ma GDX600_mann by QCAL by QCAL Messtechnik Messtechnik GmbH, GmbH, Munic Munic con- trolledcontrolled the CO the2 COflow2 ﬂow (purity (purity 99,995 99,995 vol%) vol%) into intothe aquifer the aquifer water water in ambient in ambient pressure pressure experimentsexperiments at at3 NL/h. 3 NL/h. Specimens Specimens were were exposed exposed for for 700 700 to to 8000 8000 h husing using separate separate reaction reaction vesselsvessels at at 60 60 °C◦C and and 100 100 bar, bar, as as well well as as ambient ambient pressure pressure [9,15,16] [9,15,16] and and at at 100 100 bar bar [9,15– [9,15– 17,38,52,98,99,103–107].17,38,52,98,99,103–107]. Beforehand,Beforehand, steel surfacessurfaces werewere grinded grinded under under water water with with SiC-paper SiC-paper down down to 120to 120µm. µm.After After corrosion corrosion testing, testing, samples samples were were dissected dissected with with corrosion corrosion scales scales (for (for scale scale analysis) analysis)and descaledand descaled with with 37% 37% HCl HCl (for (for kinetic kinetic analysis). analysis). Sample Sample parts parts were were embedded embedded using us- ingEpoxicure, Epoxicure, Buehler Buehler cold cold resin, resin, then then cut andcut and polished polished from from 180 to180 1200 to 1200µm with µm SiC-paperwith SiC- paperunder under water water and ﬁnished and finished with 6 with and 6 1 andµm using1 µm using diamond diamond paste. paste.

2.3.2.3. Corrosion Corrosion Fatigue Fatigue Experiments Experiments TestTest Setup Setup CorrosionCorrosion fatigue fatigue was was tested tested using using a a Schenck-Erlinger Schenck-Erlinger Puls Puls PPV PPV test test machine machine at at a a frequencyfrequency of of 33 33 Hz Hz with with geothermal geothermal brine brine constantly constantly flowing ﬂowing around around the the specimen. To To excludeexclude the the specimen-machine specimen-machine interaction, interaction, the the corrosion corrosion chamber chamber is is directly directly fixed ﬁxed on on the the testtest specimen specimen [33–35,108] [33–35,108] (Figure (Figure 11).).

FigureFigure 1. 1. SchematicSchematic setup setup of of in in situ situ corrosion corrosion fatigue fatigue testing. testing.

Processes 2021, 9, 594 6 of 33 Processes 2021, 9, x FOR PEER REVIEW 6 of 33

The temperature temperature of of the the corrosion corrosion medium medium is is 369 369 K Kcontrolled controlled via via thermal thermal sensors sensors in thein the reservoir reservoir and and corrosion corrosion chamber. chamber. The The specially specially designed designed electromagnetically electromagnetically powered pow- gearered type gear pumps type pumps the corrosion the corrosion medium medium from the from reservoir the reservoir to the pump, to the into pump, the corrosion into the chamber,corrosion and chamber, back into and the back reservoir into the (Figures reservoir 1 and (Figures 2) at a1 realand 2flow) at arate real of flowV* = rate2.5× 10 of −6 −6 3 −3 mV*3/s = and 2.5 × the10 theoreticalm /s and flow the rate theoretical of ω0 = 1.7 flow × 10 rate−3 m/s of ωat0 the=1.7 critical× 10 specimenm/s at section. the critical For CCSspecimen simulation, section. the For technical CCS simulation, CO2 flows the into technical the closed CO2 systemflows into at approximately the closed system 9 L/h at [35,36,38,52,79,89,91,93,100,106].approximately 9 L/h [35,36,38,52 ,79,89,91,93,100,106].

Figure 2. Test setup for horizontal corrosion fatigue tests (top, right). Horizontal resonant testing machine (bottom, Figure 2. Test setup for horizontal corrosion fatigue tests (top, right). Horizontal resonant testing machine (bottom, right), peripheryright), periphery with gear with pump, gear pump,reservoir reservoir and measuring and measuring units (top, units right (top,), righttest specimen), test specimen (bottom (bottom left), (1: left Reservoir,), (1: Reservoir, 2: Tem- 2: Temperatureperature control control unit, unit, 3: Magnetically 3: Magnetically driven driven gear gear pump, pump, 4: Heating 4: Heating element, element, 5: Control 5: Control unit). unit).

A titanium/titanium-mixed oxide oxide electrode electrode wi withth no no electrical electrical contact contact with with the the speci- speci- men or chamber provides a constant potential [[100,106].100,106]. AnAn Ag/AgClAg/AgCl electrode electrode fixed fixed in a TeflonTeflon channel serves as a reference andand measuresmeasures thethe freefree corrosioncorrosion potentialpotential [[100,106].100,106]. Thirty specimens were tested in each test series between 150 and 500 MPa. Due to the Thirty specimens were tested in each test series between 150 and 500 MPa. Due to the rather heterogeneous fine machined surfaces (surface roughness Rz = 4), the specimens are rather heterogeneous fine machined surfaces (surface roughness Rz = 4), the specimens comparable with prefabricated parts. The fatigue strength in air (theoretically an infinite are comparable with prefabricated parts. The fatigue strength in air (theoretically an infi- number of load cycles without failure) has a relatively smooth slope. nite number of load cycles without failure) has a relatively smooth slope. 2.4. Analysis 2.4. Analysis The morphology and layer structure of corrosion scales were analyzed using light opticalThe and morphology electron microscopy. and layer structure The phase of analysis corrosion was scales done were via X-ray analyzed diffraction using usinglight opticalCoK α-radiation and electron with microscopy. an automatic The slit phase adjustment, analysis step was 0.03 done◦, andvia countX-ray ofdiffraction 5 s in a URD-6 using CoK(Seifert-FPM). α-radiation Peak with positions an automatic were slit identified adjustme automaticallynt, step 0.03°, withand count PDF-2 of (2005) 5 s in powdera URD- 6patterns, (Seifert-FPM). most likely Peak structurespositions pickedwere identified from the automatically ICSD and refined with to PDF-2 fit the (2005) raw-data-files powder patterns,using POWDERCELL most likely structures 2.4 [98] and picked AUTOQUAN from the ICSD® by Seifertand refined FPM. to Three-dimensional fit the raw-data-files im- usingages were POWDERCELL produced via 2.4 the [98] double and AUTOQUAN optical system ® Microprof by Seifert TTVFPM. by Three-dimensional the FRT characterized im- ageslocal were corrosion. produced Surface via corrosion the double rates optical were system derived Microprof from the massTTV changeby the FRT of the character- coupons izedbefore local and corrosion. after exposure Surface to corrosion the corrosive rates environment were derived following from theDIN mass 50 change 905 part of 1–4the coupons before and after exposure to the corrosive environment following DIN 50 905

Processes 2021, 9, 594 7 of 33

(Equation (7)). Moreover, the semi-automatic analyzing program Analysis Docu ax-4 by Aquinto allowed for analyzing corrosion kinetics. h i hours · mm · [ ] mm 8760 year 10 cm weight loss g corrosion rate = h i (7) year area[ 2] · density g · time[ ] cm cm3 hour

3. Results

In general, the CO2 is injected in its supercritical phase [9,15,17] and reacts with salts of the aquifer to mineralize in a rather short time [88,101,102]. However, in the case of injection intermissions and technical revisions, the aquifer water may rise backwards into the borehole creating a three phase boundary consisting of CO2, aquifer water, and steel from the injection pipe. Here, the steels are most susceptible to a corrosive attack as summarized by Pfennig and Kranzmann [108]. Stable corrosion rates are reliably determined after 1 year of exposure [9,15].

3.1. Surface Corrosion Data regarding surface corrosion was compiled from references: [9,15,36,98,99,105,108]. The pitting resistance equivalent (PRE = %Cr + 3.3% Mo + 16% N) is a measure to describe the pitting resistance of high alloyed stainless steel in corrosive media containing halogen-ions (e.g., Cl−,F−, etc.). The PRE is identified by the chromium, nitrogen, and molybdenum content of a steel, the latter particularly increasing the resistance to local and crevice corrosion. A high PRE guarantees a greater reliability and broader field of application since the steel is more resistant against a corrosive attack. Both the ferrite and austenite phase of duplex stainless steel X2 CrNiMoN 22-5-3 with a high PRE number (53.1) do not deteriorate or show corrosion phenomena, neither after exposure up to 1 year to CO2-saturated Stuttgart Aquifer water [101] nor to the Northern German Basin [88,102]. The microstructural change and crack propagation under the dynamic load was analyzed in detail by Wolf et al. [52,100,109]. The influence of chromium as a corrosion resistant element is clearly shown for 42CrMo4. The stainless steel 42CrMo4 has a relatively high Mo-content of 0.22 wt% exceeding that of X46Cr13 by a factor of 7 (0.03 wt%). However, corrosion rates are high due to the low chromium content (1.05 wt%) compared to the much lower corrosion rates resulting in a smaller corrosion layer of X46Cr13 (PRE = 12.5–14.5, Cr = 13.39 wt%). Under pressure at 100 bar corrosion rates are lower than at ambient pressure, as shown for the mild steel 42CrMo4 in Figure3[ 9,15,105,108] and martensitic stainless steels X20Cr13, X46Cr13, X35CrMo17, and X5CrNiCuNb 16-4 in Figure4[ 9,15,105,108]. Higher corrosion rates at ambient pressure may be attributed to the corrosion layer with an open capillary system (not present at 100 bar) enabling a fast mutual diffusion of ionic species as a requirement for scale growth [105]. Corrosion scale: The complicated multi-layered corrosion scale analyzed after exposure at ambient pressure [15,105,108] reveals a carbonate/oxide structure that mainly comprises of siderite FeCO3, goethite a-FeOOH, mackinawite FeS, and akaganeite Fe8O8(OH)8Cl1.34. Additionally, various chemically different spinel-phases and carbides (Fe3C or Cr-rich iron carbides, first described by Hünert et al. [110]) are present. Note that the corrosion products in pits are the same as in the surface corrosion layers [16,105,108]. Processes 2021, 9, 594 8 of 33 Processes 2021, 9, xProcesses FOR PEER 2021 REVIEW, 9, x FOR PEER REVIEW 8 of 33 8 of 33

1.0 1.0 1 bar 1 bar 0.8 42CrMo4_vapour0.8 42CrMo4_vapour 42CrMo4_liquid 42CrMo4_liquid 0.6 0.6

0.4 0.4

0.2 0.2 corrosionin ratemm/y 0.0 corrosionin ratemm/y 0.0 0 2000 0 4000 20006000 40008000 6000 8000 1.0 1.0 100 bar 100 bar 0.8 42CrMo4_supercritical0.8 42CrMo4_supercritical 42CrMo4_liquid 42CrMo4_liquid 0.6 0.6

0.4 0.4

0.2 0.2 corrosion rate in mm/y 0.0 corrosion rate in mm/y 0.0 0 2000 0 4000 20006000 40008000 6000 8000 exposure time in hexposure time in h Figure 3. ComparisonFigure of 3. corrosion Comparison rates ofof of corrosioncorrosion 42CrMo4 ratesrates and of ofX46Cr1342CrMo4 42CrMo4 after and and 8000 X46Cr13 X46Cr13 h of exposure after after 8000 8000 to h haqui- of of exposure exposure to to aquifer aquifer brine water atbrinefer 60 brine °C water and water ambient at 60 at◦ 60C and°Cpressure, and ambient ambient as well pressure, pressure, as at 100 aswell asbar. wellas at as 100 at 100 bar. bar.

1.0 X20Cr13 vapour X20Cr13 vapour 1.0 0.5 1 bar 0.5 X20Cr13 intermediate X20Cr13 intermediate X46Cr13_vapour 1 bar 0.8 0.8 X46Cr13_vapour 1 bar 1 barX20Cr13 liquid 0.4 0.4 X20Cr13 liquid X46Cr13_liquid X46Cr13_liquid 0.6 0.6 0.3 0.3 0.4 0.4 0.2 0.2 0.2 0.2 0.1 0.1

mm/y in rate corrosion 0.0 mm/y in rate corrosion 0.0 0.0 0.0 0 2000 4000 6000 8000 corrosion rate in mm/y in rate corrosion 0 2000 4000 6000 8000 corrosion rate in mm/y in rate corrosion 0 2000 0 4000 2000 6000 4000 60001.0 1.0 X46Cr13_supercritical 100 bar 100 bar y X20Cr13 supercritical CO X46Cr13_supercritical y X20Cr132 supercritical CO 0.020 0.8 2 0.8 0.020 X20Cr13 liquid - aquifer X46Cr13_liquid 100 bar 100 bar X20Cr13 liquid - aquifer X46Cr13_liquid 0.6 0.6 0.015 0.015 0.4 0.4 0.010 0.010 0.2 0.2 0.005 0.005 corrosion rate in mm/y in rate corrosion 0.0 mm/y in rate corrosion 0.0 0 2000 04000 20006000 40008000 6000 8000 0.000 0.000

corrosion rate in mm/ exposure time in h 0 2000corrosion rate in mm/ 40000 20006000 40008000 6000 8000 exposure time in h

exposure time in hexposure time in h y 0.030 y 0.030 0.30 0.30 1 bar 1 bar 1 barX35CrMo17 vapour 0.025 0.025 1X5CrNiCuNb16-4_vapour bar X5CrNiCuNb16-4_vapour 0.25 0.25 X35CrMo17 vapour X35CrMo17 intermediateX35CrMo17 intermediate0.020 0.020 X5CrNiCuNb16-4_liquidX5CrNiCuNb16-4_liquid 0.20 0.20 X35CrMo17 liquid X35CrMo17 liquid0.015 0.015 0.15 0.15 0.010 0.010 0.10 0.10 0.005 0.005

0.05 0.05 0.000 0.000 0.00 -0.005

0.00 in mm/ rate corrosion -0.005 corrosion rate in mm/ rate corrosion corrosion rate in mm/y 0 2000corrosion rate in mm/y 40000 20006000 40008000 6000 80000 2000 04000 20006000 40008000 6000 8000 y y y -2 y 0.030 0.030 1·10 1·10-2 X35CrMo17_supercriticalX35CrMo17_supercritical 100 bar 100 bar 0.025 100 bar X5CrNiCuNb16-4_supercritical -2 X35CrMo17_liquid 0.025 100 bar X5CrNiCuNb16-4_supercritical 0.8·10 0.8·10-2 X35CrMo17_liquid 0.020 0.020 X5CrNiCuNb16-4_liquidX5CrNiCuNb16-4_liquid -2 0.6·10 0.6·10-2

0.015 0.015 0.4·10-2 -2 0.4·10 0.010 0.010 -2 0.2·10 0.2·10-2 0.005 0.005 0·10-2 -2 0.000

0·10 in mm/ rate corrosion 0.000 corrosion rate in mm/ rate corrosion corrosionrate in mm/ 0 2000corrosionrate in mm/ 4000 6000 8000 0 2000 04000 20006000 40008000 6000 8000 0 2000 4000 6000 8000 exposure time in hexposure time in h exposure time in hexposure time in h Figure 4. Comparison of corrosion rates in the liquid and vapor/supercritical phase after 8000 h of Figure 4. Comparison of corrosionFigure 4. rates Comparison in the liquid of corrosion and vapor/supercritical rates in the liquid phase and vapor/supercritical after 8000 h of exposure phase toafter aquifer 8000 h of exposure to aquifer brine water at 60 °C and ambient pressure, as well as at 100 bar. (Up left): brine water at 60 ◦C and ambientexposure pressure, to aquifer as well brine as atwater 100 bar.at 60 ( Up°C and left): ambient X20Cr13, pressure, (up right as): well X46Cr13 as at 100 ambient bar. ( pressureUp left): X20Cr13, (up rightX20Cr13,): X46Cr13 (up ambientright): X46Cr13 pressure ambient 100 bar, pressure (down left 100): bar, X35CrMo17-1, (down left): ( downX35CrMo17-1, right): (down right): 100 bar, (down left): X35CrMo17-1, (down right): X5CrNiCuNb16-4. X5CrNiCuNb16-4.X5CrNiCuNb16-4.

Processes 2021, 9, 594 9 of 33

Exposure time: For 42CrMo4, X20Cr13, and X46Cr13, the corrosion rates increase with time at ambient pressure and decrease with time at 100 bar. In the vapor phase at ambient pressure, the corrosion rate for 42CrMo4 doubles from 4000 h (0.45 mm/year) of exposure to 8000 h (0.8 mm/year). In the liquid phase, the exposure time has little impact on the corrosion rate. The same applies for both atmospheres at 100 bar [105,108]. This corresponds with the thickness of the precipitation layer that is greater in the vapor/supercritical than in the liquid phase. Possibly, the experimental low pressure system is not completely gas tight and excess oxygen accelerates the corrosive degradation of the steel rates as a function of time. At 100 bar, however, fast forming hydroxides [3,12,37,105,108] passivate the steel surface initially then react to form a siderite-layer that increases with time and acts as a diffusion barrier. For X5CrNiCuNb16-4 and X35CrMo17, corrosion rates generally increase slightly (at ambient pressure as well as at 100 bar) [105] attributed to the passivating layer breaking down. Although the alloy composition accounts for a stable passivating layer, additional local corrosion phenomena increase the overall corrosion rates. Pressure: The corrosion rates at ambient pressure are much higher for 42CrMo4, X20Cr13, and X35CrMo4 than at 100 bar. Stainless steels X20Cr13 and X5CrNiCuNb16-4 show less dependence on pressure. Atmosphere with regards to pressure: At ambient pressure, corrosion rates in the vapor phase (water-saturated CO2) are higher by a factor of 3–8 compared to the liquid phase (CO2-saturated water). At 100 bar, corrosion rates in the liquid phase can be both higher or lower than in the vapor/supercritical phase. The atmosphere (liquid, vapor or supercritical atmosphere) does not significantly influence the general corrosive behavior. That is: It cannot be stated that the vapor/supercritical atmosphere (water saturated CO2) leads to higher or lower corrosion rates than the liquid atmosphere (CO2-saturated water). In the vapor phase, the supercritical CO2 is saturated with H2O in contrast to the liquid phase where liquid H2O saturated with CO2. Although it is known that the corrosion rate increases with the increasing CO2-partial pressure [3,105,108], the explanation that higher corrosion rates in the vapor (supercritical) phase correspond with the high CO2 partial pressure compared to the liquid phase with a lower CO2 partial pressure does not count here. Higher corrosion rates in the liquid phase at ambient pressure for X5CrNiCuNb16-4, X20Cr13, and X35CrMo4 and less significant for X20Cr13 and 42CrMo4 may generally be explained by slower diffusion kinetics. Moreover, when comparing phase precipitations in the supercritical phase and in water at high pressure, the diffusional flow is much slower in the water phase than in the supercritical phase. This has been explained in detail by Pfennig et al. [105,108]: Slower diffusion leads to the stable growths of the {10-10}- planes leading to the typical tabular crystal habit [105,108]. Due to the low density of the supercritical phase compared to the liquid greater diffusion rates in the supercritical phase, this enhances the nucleation of siderite crystals and results in small crystals forming a dense layer on the metal surface (Figure5). The faster kinetics are supported by the higher CO2-partial pressure in the supercritical phase, also enhancing nucleation and therefore smaller siderite crystals [105,108]. FeCO3-grains decrease in size with the increasing pressure. However, with similar corrosion rates measured in the liquid phase and in the vapor/supercritical phase (<0.1 mm/year in the liquid and 0.03 mm/year in the vapor phase) it may be assumed that the grain size is not decisive, but the thickness of the corrosion scale counts for diffusional pathways. The corrosion rates decrease with the longer diffusion pathways. After 8000 h of exposure, the scale has grown to a diffusion barrier preventing the mutual diffusion of elements from the base material and from the CO2 aquifer mixture (O2 and CO2 diffuse into the steel’s surface and Fe2+ diffuses from the metal bulk to the surface). The dissolution of the iron base material leads to an increase in the local pH at the steel surface due to the accumulation of ferrous ions that then reacts to ferrous carbonate after super-saturation at the steels surface [12]. The reaction kinetics are not significantly related to the precipitation morphologies and pressure [12,105,108], but to the oxygen partial pressure [111]. Processes 2021, 9, x FOR PEER REVIEW 10 of 33

Processes 2021, 9, x FOR PEER REVIEW 10 of 33 precipitation morphologies and pressure [12,105,108], but to the oxygen partial pressure [111]. Processes 2021, 9, 594 10 of 33 precipitation morphologies and pressure [12,105,108], but to the oxygen partial pressure [111].

Figure 5. Surface precipitation on 42CrMo4 after exposure to CO2 saturated saline aquifer water for 8000 h at ambient pressure and 100 bar. (Top left and right): Cross section (vapor), (bottom

left): Vapor, (right): Liquid. Figure 5. SurfaceFigure precipitation 5. Surface precipitation on 42CrMo4 on after 42CrMo4 exposure after to exposure CO2 saturated to CO2 salinesaturated aquifer saline water aquifer for water 8000 h at ambient pressure andfor 100 8000 bar. h (Top at ambient left3.2. and Local pressure right Corrosion): Cross and 100 section bar. (vapor),(Top left ( bottomand right left): ):Cross Vapor, section (right (vapor),): Liquid. (bottom left): Vapor, (right): Liquid. Data regarding local corrosion was compiled from references: 3.2. Local Corrosion 3.2. Local Corrosion[9,15,16,36,98,99,105,108]. DataStainless regarding steel 42CrMo4 local corrosion is highly was compiledsusceptible from towards references: surface [9,15 corrosion,16,36,98,99 so,105 that,108 no]. Data regarding local corrosion was compiled from references: pits wereStainless distinctively steel 42CrMo4 measured. is highly Initial susceptible pits grow towards very quickly surface to corrosiona discontinuous so that surface no pits [9,15,16,36,98,99,105,108]. werelayer distinctivelythat can be divided measured. into Initial an inner pits (dissolu grow verytionquickly of base tometal) a discontinuous and outer (oxide surface growth layer Stainless steel 42CrMo4 is highly susceptible towards surface corrosion so that no thaton metal can besurface) divided corrosion into an innerlayer covering (dissolution the ofentire base surface metal) of and the outer sample (oxide (Figures growth 5 and on pits were distinctively measured. Initial pits grow very quickly to a discontinuous surface metal6). surface) corrosion layer covering the entire surface of the sample (Figures5 and6). layer that can be divided into an inner (dissolution of base metal) and outer (oxide growth on metal surface) corrosion layer covering the entire surface of the sample (Figures 5 and 6).

◦ FigureFigure 6.6. SEM micrographs ofof X46Cr13X46Cr13 afterafter 17,20017,200 hh ofof exposureexposure atat 6060 °CC and 100 bar to water saturated supercriticalsupercritical COCO22 clearly showing the inner and outer corrosion layer. clearly showing the inner and outer corrosion layer. Figure 6. SEM micrographs of X46Cr13 after 17,200The h internalof exposure and at external 60 °C and corrosion 100 bar to layer water grow saturated dependingdepending supercritical onon thethe CO variousvarious2 carboncarbon andand clearly showing the inner and outer corrosion layer. oxygen partial pressures [[17,36,99].17,36,99]. Due to mismatchesmismatches ofof thermalthermal expansionexpansion coefﬁcientscoefficients and large differences in surface morphologies, the corrosion layer detaches in lateral The internaldirection and external once a criticalcorrosion thickness layer grow of the depending surface corrosion on the various layer is carbon exceeded. and oxygen partial pressures [17,36,99]. Due to mismatches of thermal expansion coefficients

Processes 2021, 9, x FOR PEER REVIEW 11 of 33

Processes 2021, 9, 594 and large differences in surface morphologies, the corrosion layer detaches in lateral11 of di- 33 rection once a critical thickness of the surface corrosion layer is exceeded. This local surface degradation is enhanced since oxygen vacancies, as a result of EquationsThis local(1)–(6), surface consolidate degradation and condense is enhanced at the hydroxide/brine since oxygen vacancies, interface. as The a resultdepend- of enceEquations on the (1)–(6), anionic consolidate concentration and of condense the cons atecutive the hydroxide/brine reactions has been interface. discussed The by depen- Wei etdence al. [112]. on the As anionic a consequence, concentration the siderite of the detaches consecutive from reactions a transient has hydroxide been discussed film de- by Weiscribed et al. in [ 112detail]. As by a consequence,various authors the [6,17,26 siderite,36] detaches with fromvacancies a transient generated hydroxide by carbonate film de- ionsscribed being in detailthe main by various cause for authors the precipitation [6,17,26,36] of with oxygen vacancies vacancies generated (Equations by carbonate 8 and 9a,b): ions being the main cause for the precipitation of oxygen vacancies (Equations (8) and (9)a,b): Fe + 2H2O → [Fe(OH)2]ads +2H+ + 2e− (8) + − Fe + 2H2O → [Fe(OH)2]ads +2H + 2e (8) [Fe(OH)2]ads + H2O → alpha-FeOOH + 3H+ + 3e− (9a) + − [Fe(OH)2]ads + H2O → alpha-FeOOH + 3H + 3e (9a) [Fe(OH)2]ads + [H2CO3]ads → FeCO3 + 2H2O (9b) [Fe(OH)2]ads + [H2CO3]ads → FeCO3 + 2H2O (9b) The flowing corrosive media removes the remaining film causing the pit to grow widerThe and flowing eventually corrosive covers media the entire removes surface. the remaining film causing the pit to grow widerIn and contrast, eventually the other covers steel the qualities entire surface. investigated showed distinct local corrosion phe- nomenaIn contrast,(Figure 7) the [9,15,16,36,98,99,105,108] other steel qualities investigated with generally showed higher distinct number local of pits corrosion under aquiferphenomena water (Figure conditions.7)[ 9,15 Moreover,,16,36,98,99 a,105 higher,108] chromium with generally content higher of numberthe steels of pitsleads under to a aquifer water conditions. Moreover,2 a higher chromium content of the steels leads to a higher number of pits per m2 . (Note that these steels only show very little surface corro- sionhigher but number therefore of pitsare perhighly m .susceptible (Note that theseto local steels corrosion). only show The very number little surfaceof pits formed corrosion at but therefore are highly susceptible to local corrosion). The number of pits formed at 100 bar, either, in CO2-saturated aquifer water (liquid phase) or in water saturated CO2 100 bar, either, in CO -saturated aquifer water (liquid phase) or in water saturated CO (supercritical/vapor phase)2 exceeds those formed at ambient pressure by a factor of 10.2 (supercritical/vapor phase) exceeds those formed at ambient pressure by a factor of 10. This is due to the fact that after similar exposure times, the corrosion scale is much thicker This is due to the fact that after similar exposure times, the corrosion scale is much thicker when precipitated at ambient pressure and therefore complicates the analysis. Moreover, when precipitated at ambient pressure and therefore complicates the analysis. Moreover, kinetics at 100 bar are faster, pressing CO2 and water onto the metal’s surface resulting in kinetics at 100 bar are faster, pressing CO and water onto the metal’s surface resulting in a a lower pH and faster local degradation 2of the steel [104]. lower pH and faster local degradation of the steel [104].

30,000

2 30,000 25,000 1 bar 25,000 1 bar X46Cr13_liquid X20Cr13_liquid 20,000 20,000 15,000 15,000 10,000 10,000 5,000 5,000

number ofpits per m 0 0 number pitsof perm² 0 2000 4000 6000 8000 0 2000 4000 6000 8000

300,000 ² 300,000 100 bar 250,000 X46Cr13_liquid 250,000 X20Cr13_liquid 100 bar 200,000 200,000 150,000 150,000 100,000 100,000 50,000 50,000 number ofpits per m² 0 number pitsof perm 0 0 2000 4000 6000 8000 0 2000 4000 6000 8000 exposure time in h exposure time in h Figure 7. Cont.

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Processes 2021, 9, 594 12 of 33

40,000 2 1 bar X5CrNiCuNb16-4_liquid 30,000

20,000 6·106 10,000 5·106 100 bar

number of pits in m pits of number 0 4·106 X35CrMo17-1_liquid 0 2000 4000 6000 8000 3·106 X35CrMo17-1_supercritical 400,000 100 bar 2·106 300,000 1·106

6 200,000 m² per pits of number 0·10 0 2000 4000 6000 8000 100,000 X5CrNiCuNb16-4_supercritical exposure time in h X5CrNiCuNb16-4_liquid

number of pits per m² per pits of number 0 0 2000 4000 6000 8000 exposure time in h

Figure 7. Number of pits of X20Cr13 in the liquid phase after 6000/8000 h of exposure to aquifer brine water at 60 ◦C and Figure 7. Number of pits of X20Cr13 in the liquid phase after 6000/8000 h of exposure to aquifer brine water at 60 °C and ambient pressure, as well as at 100 bar. (Up left): X20Cr13, (up right): X46Cr13, (down left): X35CrMo17-1, (down right): ambient pressure, as well as at 100 bar. (Up left): X20Cr13, (up right): X46Cr13, (down left): X35CrMo17-1, (down right): X5CrNiCuNb16-4.

3.3.3.3. Influence Influence of of Heat Heat Treatment Treatment DataData regardingregarding thethe influence influence of of heat heat treatment treatment was was compiled compiled from references:from references: [17,38, [17,38,98,99,103,104,108,113,114].98,99,103,104,108,113,114]. 3.3.1.3.3.1. Surface Surface Corrosion Corrosion AllAll steels steels meet meet the the requirements for pressurepressure vesselsvessels (DIN(DIN 6601<6601< 0.1 mm/year),mm/year), since since thethe corrosion corrosion rate rate generally does not exceed 0.04 mm/yearmm/year for for differently differently heat heat treated X20Cr13,X20Cr13, X46Cr13,X46Cr13, andand X5CrNiCuNb16-4—independent X5CrNiCuNb16-4—independent of of the the heat heat treatment, treatment, atmosphere atmosphere (liquid, supercritical/vapor) or pressure (1 and 100 bar). At 100 bar and high CO2 (liquid, supercritical/vapor) or pressure (1 and 100 bar). At 100 bar and high CO2 partial, partial,the lower the corrosionlower corrosion rates pressure rates pressure could co beuld a consequencebe a consequence of closed of closed capillary capillary systems sys- temswithin within the corrosion the corrosion scale. scale. The denseThe dense layer layer prevents prevents fastdiffusion fast diffusion processes processes after after the long the longexposure exposure and sufficientand sufficient thickness thickness of the of corrosion the corrosion layer. layer. TheThe influence influence of of the the corrosion corrosion behavior behavior of st ofeels steels on their on heat their treatment heat treatment is well known is well [11,17,22,28,29,98,104].known [11,17,22,28,29 ,The98,104 lowest]. The corrosion lowest corrosion rates and ratestherefore and good therefore corrosion good resistance corrosion regardingresistance surface regarding corrosion surface in corrosion water saturated in water supercritical saturated supercriticalCO2 and CO2 CO—saturated2 and CO sa-2— linesaturated water salineare accomplished water are accomplished by hardening by and hardening tempering and the tempering steel at low the temperature steel at low (600–670temperature °C) to (600–670 obtain ◦aC) martensitic to obtain amicrostructure. martensitic microstructure. Generally,Generally, surface corrosion rates increase as a function of exposure time. At At ambient ambient pressure,pressure, surfacesurface corrosioncorrosion rates rates of of heat heat treated treated X20Cr13 X20Cr13 and and X46Cr13 X46Cr13 increase increase significantly significantly(factor (factor 3). At 3). 100 At bar, 100 the bar, corrosion the corrosion rates dorates not do increase not increase when when exposed exposed for 4000 for 4000 to 8000 to 8000h assuming h assuming a sufficient a sufficient thick thick carbonate carbonate layer layer reduces reduces the diffusionthe diffusion of ionic of ionic species species into 2− − intothe basethe base materials materials (CO 3(CO-3 and2− -and O2 O-species)2− -species) and and towards towards the the outer outer surface surface (Fe-ions) (Fe-ions) and andprevents prevents further further degradation degradation [17,104 [17,104].]. ExceptExcept for for steel steel coupons coupons hardened hardened and tempered at 700 °C,◦C, corrosion corrosion rates at 100 100 bar bar werewere compared toto thosethose obtained obtained for for X5CrNiCuNb16-4 X5CrNiCuNb16-4 (below (below 0.005 0.005 mm/year). mm/year). Generally, Gener- ally,corrosion corrosion rates rates do not do differnot differ significantly significan aftertly after a long a long exposure exposu tore the to CCSthe CCS environment. environ- ◦ ment.However, However, hardening hardening and tempering and tempering at a high at a temperature high temperature of 700/755 of 700/755C may °C lead may to lead the toprecipitation the precipitation of Cr-carbides of Cr-carbides in X20Cr13 in X20Cr13 and X46Cr13and X46Cr13 after after a long a long exposure exposure depleting depleting the themetal metal matrix matrix of free of free chromium chromium because because passivation passivation of the of surfacethe surface then then hinders hinders the base the basematerial material degrades. degrades.

Processes 2021, 9, 594 13 of 33 Processes 2021, 9, x FOR PEER REVIEW 13 of 33

Generally, low corrosion rates in the liquid and even lower in the supercritical phase for X5CrNiCuNb16-4Generally, low are corrosion attributed rates to in passivation the liquid andand possiblyeven lower insufﬁcient in the supercritical electrolytes phase[38, 89]. for X5CrNiCuNb16-4 are attributed to passivation and possibly insufficient electrolytes Moreover, the cathodic reactions (Equations (1) and (2)) result in a higher H2CO3 concen- [38,89]. Moreover, the cathodic reactions (Equations (1) and (2)) result in a higher H2CO3 tration and therefore more acidic and reactive environment as in the CO2 saturated liquid phaseconcentration [7,26]. Corrosion and therefore rates more increase acidic at and 100 reactive bar in the environment supercritical as phasein the andCO2 remainsaturated at the sameliquid level phase kept [7,26]. in the Corrosion liquid phase rates increase (0.003 mm/y at 100after bar in 4000 the h).supercritical Depassivation phase after and 1000re- h ofmain exposure at the same in the level supercritical kept in the phase liquid isphase the result(0.003 ofmm/y fast after reaction 4000 kineticsh). Depassivation and carbide after 1000 h of exposure in the supercritical phase is the result of fast reaction kinetics and precipitation. The accompanied chromium depletion of the matrix (Figure8) prohibits new carbide precipitation. The accompanied chromium depletion of the matrix (Figure 8) pro- passivation and degrades the material [98,113]. hibits new passivation and degrades the material [98,113].

FigureFigure 8. 8.Left: Left: SEM SEM micrographs micrographs andand elementelement dist distributionribution of of the the ellipsoidal ellipsoidal corrosion corrosion layer layer formed formed on onX5CrNiCuNb16-4 X5CrNiCuNb16-4 hardened and tempered at 670 °C prior to exposure after 8000 h of exposure at 60 °C and 100 bar to water saturated hardened and tempered at 670 ◦C prior to exposure after 8000 h of exposure at 60 ◦C and 100 bar to water saturated supercritical CO2. supercritical CO2. The influence of the heat treatment is more significant at 100 bar than at ambient The influence of the heat treatment is more significant at 100 bar than at ambient pressure. However, since the data is not distinct, the heat treatment for X5CrNiCuNb16-4 pressure.seems not However, decisive—rather since the the data chromium is not distinct, content theand heat atmosphere. treatment Good for X5CrNiCuNb16-4 surface corro- seemssion resistance not decisive—rather at ambient pressure the chromium can be a contentttributed and foratmosphere. steels hardened Good or hardened surface corrosion and resistance at ambient pressure can be attributed for steels hardened or hardened and tempered. Good surface corrosion resistance at 100 bar under supercritical CO2 conditions tempered.hardened and Good tempered surface at corrosion 670 °C (<0.001 resistance mm/year, at 100 martensitic bar under microstructure) supercritical CO and2 conditions in the ◦ hardenedliquid phase and normalized tempered at(ca. 670 0.004C mm/year, (<0.001 mm/year, ferritic-perlitic martensitic microstructure) microstructure) [98,113]. and in the liquidThe phase time normalized of austenitizing (ca. 0.004plays mm/year, a significant ferritic-perlitic role regarding microstructure) the surface corrosion [98,113 of]. X46Cr13The timeand ofX20Cr13 austenitizing (Figure plays 9) abut significant is neglectable role regarding regarding the surfacelocal corrosion corrosion of X46Cr13[58,103,104,114]. and X20Cr13 Surface (Figure corrosion9) but decreases is neglectable as a function regarding oflocal increasing corrosion the austenitizing[58,103,104,114 ]. Surfacetime and corrosion decreasing decreases the austenitizing as a function temperat of increasingure [114]. the The austenitizing lowest corrosion time and rates decreasing were thefound austenitizing for specimens temperature heated to [950114 ].°C The and lowest annealed corrosion for 30 min rates compared were found to the for highest specimens ◦ heatedcorrosion to 950rates Caustenitized and annealed at 1050 for 30°C minfor 60 compared min after to700 the h [114]. highest The corrosion significance rates of austeni- the tizedinfluence at 1050 decreases◦C for with 60 min the increasing after 700h exposu [114].re The time significance to a geothermal of the environment. influence decreases After with4000 theh of increasing exposure, austenitizing exposure time becomes to a geothermal insignificant environment. (Figure 9) [103,104,114]. After 4000 h of exposure, austenitizing becomes insignificant (Figure9)[103,104,114]. 0.07 r 0.06 austenitizing time austenitizing temperature

0.05 30 min 950 min 60 min 1000 min 0.04 90 min 1050 min 0.03

0.02

0.01

corrosion rate in mm/yea 0.00 700 2000 4000 700 2000 4000 exposure time in h exposure time in h

Processes 2021, 9, x FOR PEER REVIEW 13 of 33

Generally, low corrosion rates in the liquid and even lower in the supercritical phase for X5CrNiCuNb16-4 are attributed to passivation and possibly insufficient electrolytes [38,89]. Moreover, the cathodic reactions (Equations (1) and (2)) result in a higher H2CO3 concentration and therefore more acidic and reactive environment as in the CO2 saturated liquid phase [7,26]. Corrosion rates increase at 100 bar in the supercritical phase and remain at the same level kept in the liquid phase (0.003 mm/y after 4000 h). Depassivation after 1000 h of exposure in the supercritical phase is the result of fast reaction kinetics and carbide precipitation. The accompanied chromium depletion of the matrix (Figure 8) prohibits new passivation and degrades the material [98,113].

Figure 8. Left: SEM micrographs and element distribution of the ellipsoidal corrosion layer formed on X5CrNiCuNb16-4 hardened and tempered at 670 °C prior to exposure after 8000 h of exposure at 60 °C and 100 bar to water saturated supercritical CO2.

The influence of the heat treatment is more significant at 100 bar than at ambient pressure. However, since the data is not distinct, the heat treatment for X5CrNiCuNb16-4 seems not decisive—rather the chromium content and atmosphere. Good surface corrosion resistance at ambient pressure can be attributed for steels hardened or hardened and tempered. Good surface corrosion resistance at 100 bar under supercritical CO2 conditions hardened and tempered at 670 °C (<0.001 mm/year, martensitic microstructure) and in the liquid phase normalized (ca. 0.004 mm/year, ferritic-perlitic microstructure) [98,113]. The time of austenitizing plays a significant role regarding the surface corrosion of X46Cr13 and X20Cr13 (Figure 9) but is neglectable regarding local corrosion [58,103,104,114]. Surface corrosion decreases as a function of increasing the austenitizing time and decreasing the austenitizing temperature [114]. The lowest corrosion rates were found for specimens heated to 950 °C and annealed for 30 min compared to the highest Processes 2021, 9, 594 corrosion rates austenitized at 1050 °C for 60 min after 700 h [114]. The significance 14of ofthe 33 influence decreases with the increasing exposure time to a geothermal environment. After 4000 h of exposure, austenitizing becomes insignificant (Figure 9) [103,104,114].

0.07 r 0.06 austenitizing time austenitizing temperature

0.05 30 min 950 min 60 min 1000 min 0.04 90 min 1050 min 0.03

0.02

0.01

corrosion rate in mm/yea 0.00 700 2000 4000 700 2000 4000 exposure time in h exposure time in h Figure 9. Inﬂuence of austenitizing: Surface corrosion rate (combined: X20Cr13 and X46Cr13 annealed at 650 ◦C for 30 min)

as a function of austenitizing time and temperature prior to exposure to CO2-saturated saline aquifer water.

3.3.2. Pit Corrosion Pit formation is most likely also described by Han et al. as the initial formation of Fe(OH)2 (Equations (1)–(6)) [6,37] locally increasing the pH near the hydroxide film followed by the internal and external formation of a ferrous carbonate film [37]. Chlorides enhance local corrosion [46]. Additionally, dislocations, grain boundaries, and precipitation phase boundaries, such as carbides result in a local lattice mismatch thus the local boundary energy increases [36]. Therefore, the distinct microstructure itself is susceptible to a corrosive attack. When the hydroxide film is then locally damaged, e.g., at grain boundaries, pitting is initialized by dissolving the film, depassivating the base material, and finally detaching the carbonate film (Figure 10)[17,38,98,99,103,104,108,113,114].

Figure 10. Schematic drawing of pit precipitation on injection steels initiated at grain boundaries.

Local corrosion as a function of the austenitizing time and temperature promises a better pit corrosion resistance for X20Cr13 than for X46Cr13 (X20Cr13: ca. 3508814 pits per m2, X46Cr13: ca. 9622 pits per m2). The same result applies for pit diameters revealing an average pit diameter for X46Cr13 of 249 µm, which is five times larger than the average pit on the X20Cr13 (49 µm) [54,103]. The majority of pits after 700 h of exposure are 51 to 100 µm in diameter, but the number of pits for X46Cr13 is three times higher than for X20Cr13. Even if the maximum pit diameter of pits that precipitated on X46Cr13 succeed those precipitated on X20Cr13 by a factor of 3.5 for steel coupons austenitized at 950 ◦C for 90 min, the average pit diameter of both steel qualities does not differ significantly. Furthermore, there is no significant influence on the austenitizing routines prior to exposure to the CCS environment. The average pit diameter does not succeed at 100 µm after 700 h of exposure. (Note that the critical parameter to assess the influence of pit precipitation is preferably not the diameter, but the depth of pits). However, after 700 to 4000 h, the depth of pits was shown to be quite comparable and these will be evaluated more closely in the future.

Processes 2021, 9, 594 15 of 33

At ambient pressure and at 100 bar, as well as the number of pits per unit area are not significantly influenced by the particular heat treatment (no significant lowest number of pits) [16,17,103,108,114]. Hardening and tempering between 600 and 670 ◦C realize the lowest number of pits after 6000 h of exposure. (Note that the pits may consolidate to a shallow pit corrosion phenomena and therefore lead to unusual low numbers of pits). Generally, pitting is independent of the heat treatment. Steels with martensitic microstructure and higher carbon-content were developed in fewer pits with smaller maximum intrusion depths. At ambient pressure, the number of pits levels off for X20Cr13, Processes 2021, 9, x FOR PEER REVIEWX46Cr13, and X5CrNiCuNb16-4 (<40,000 per m2). At 100 bar, the number of pits15 is of in-33

dependent of the heat treatment and atmosphere (water saturated supercritical CO2 and ◦ CO2 saturated aquifer) except for coupons hardened and tempered at 670 C. Although known as corrosion resistant, X5CrNiCuNb16-4 shows a high number of pits per m2 in as corrosion resistant, X5CrNiCuNb16-4 shows a high number of pits per m2 in both at- both atmospheres at ambient pressure and 100 bar [98]. mospheres at ambient pressure and 100 bar [98]. The maximum pit depth is not inﬂuenced signiﬁcantly by the heat treatment. For the The maximum pit depth is not influenced significantly by the heat treatment. For the heat treated specimen (Table8), pit intrusion depths for all steel qualities were obtained heat treated specimen (Table 8), pit intrusion depths for all steel qualities were obtained metallographically and via the optical volume measurement (Figure 11). The pit depth metallographically and via the optical volume measurement (Figure 11). The pit depth increases with the exposure time (max. 300 µm for X20Cr13, hardened after 6000 h of increases with the exposure time (max. 300 µm for X20Cr13, hardened after 6000 h of ex- exposure at ambient pressure) [38,104,105]. The average pit depths for X5CrNiCuNb16-4 is posure at ambient pressure) [38,104,105]. The average pit depths for X5CrNiCuNb16-4 is 10–250 µm after exposure at 100 bar and 60 ◦C (Figure 11). Normalizing and hardening 10–250 µm after exposure at 100 bar and 60 °C (Figure 11). Normalizing and hardening + + tempering at 600 ◦C seems to be favorable for X20Cr13 and X46Cr13 (intrusion depth: tempering at 600 °C seems to be favorable for X20Cr13 and X46Cr13 (intrusion depth: 8– 8–25 µm), whereas hardening + tempering between 670 and 755 ◦C is recommended for 25X5CrNiCuNb16-4 µm), whereas hardening (intrusion depth:+ tempering 10 µm) between [104]. 670 and 755 °C is recommended for X5CrNiCuNb16-4 (intrusion depth: 10 µm) [104].

Figure 11. MicrographMicrograph and optical measurement of pit and pit intrusion depth of X5CrNiCuNb16-4.

SinceSince local local corrosion is a highly statistical phenomenon it is not predictable. Hence, itit is is not not possible possible to to give give reliable reliable corrosion corrosion rates rates and and lifetime lifetime predictions predictions regarding regarding pit cor- pit rosioncorrosion in CCS in CCS technology technology [104]. [104 ].

3.4. Statistical Approach in Corrosion Fatigue Specimens were tested at different stress levels each (the string of pearls method). Logarithms of stress amplitude σa and the number of cycles to failure Nf were used in the following method of linear regression analysis assuming a logarithmic normal distribution of the number of cycles to failure Nf [36,52]. As a result, the scatter bands for the survival probabilities PS are parallel to the PS = 50% straight (regression) line according to:

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3.4. Statistical Approach in Corrosion Fatigue Specimens were tested at different stress levels each (the string of pearls method). Logarithms of stress amplitude σa and the number of cycles to failure Nf were used in the following method of linear regression analysis assuming a logarithmic normal distribution of the number of cycles to failure Nf [36,52]. As a result, the scatter bands for the survival probabilities PS are parallel to the PS = 50% straight (regression) line according to:

• log(σa): Logarithm of stress amplitude σa, independent (error-free) variable x; • log(Nf): Logarithm of cycles to failure Nf, dependent variable y (inaccurate); • m: Estimate for the slope of the regression line; • b: Estimate for the intercept of the regression line; • tX: One-sided barrier of standard normal distribution for PS = x %; • sN: Standard error of the estimate of N; • PS: Probability of survival. Therefore, Equation (10) of the S-N curve follows: log Nf = m log(σa) + b + txsN

respectively (10)

log(Nf )−b−txsN log(σa) = m

The important standard error sN of the regression calculation is determined from the mean square deviation of each value of the regression line (Equation (11)). The residual 2 variance sN reveals the scatter which cannot be explained by regression: v u n u 2 u ∑ {log(Ni) − b − m log(σa,i)} = s = t i 1 (11) N n − 2

The linear regression was calculated according to Equation (12) as follows:

−k Sa N50 = NA· f or Sa ≥ SA (12) SA given:

• N50 50% probability cycle value; • NA reference cycle value; • SA reference stress value; • Sa selected stress value.

The scatter range TN is the quotient between the probability cycle value of 10% and 90% (Equation (13)): log N90 TN = 1 : (13) log N10 given:

• N90 90% probability cycle value; • N10 10% probability cycle value; • TN scatter range. The R2 value varies from 0 to 1. The higher number indicates higher coherence and statistical certainty. The R2 value is unusually high (0, 99 therefore, very close to 1) possibly due to the scarce dataset or the same fatigue mechanism.

3.5. Corrosion Fatigue of X46Cr13 (Air: σts = 680 MPa, σy = 345 MPa, Fatigue Limit: 260 MPa) Data regarding corrosion fatigue of X46Cr13 was mainly compiled from references: [35,36]. Processes 2021, 9, 594 17 of 33

Unlike fatigue tests in air results obtained in geothermal environment does not show Processes 2021, 9, x FOR PEER REVIEW 17 of 33 in a distinguished fatigue limit. The S-N curve from tests conducted in CO2-saturated saline aquifer at 60 ◦C decreases continuously for X46Cr13 (soft annealed microstructure ∼ ∼ = ferritic matrix with coagulated cementite, σtsair = 680 MPa, σyair = 345 MPa) [35,36] • σee: Estimated(Figure endurance 12). The endurance-limitlimit in air from the in air yield (σee and= 262 tensile MPa, strength Equation in MPa; (14)) is represented by • σy: Yieldthree strength run-outs in MPa; at stress amplitudes: 260, 280, and 288 MPa [35]: • σts: Tensile strength in MPa. The influence of corrosion significantly decrσee =eases0.2 theσy +fatigueσts + life57 represented by the (14) comparatively steep slope of the S-N curve that is described by k = 6.321. After 210 h of • σee: Estimated endurance limit in air from the yield and tensile strength in MPa; exposure to CO2-saturated geothermal brine (21 × 106 cycles) at 60 °C, the endurable stress • σ : Yield strength in MPa; amplitude is ca. 160y MPa and thus only 60% of the fatigue limit in air (ca. 260 MPa, • σ : Tensile strength in MPa. Figure 12) [35,36,106,113].ts

Figure 12. S-NFigure curve 12.of X46Cr13S-N curve at ofR = X46Cr13 −1, frequency at R = 30−1, Hz: frequency Corrosion 30 Hz:fatigue Corrosion compared fatigue to the compared endurance to thelimit endurance in air, limit in air, fatigue crack, pits,fatigue and crack, striations pits, on and the striations fatigue onfracture the fatigue surface. fracture surface.

Mostly cracksThe are influenceassociated of with corrosion rather significantlysmall pits (diameter decreases << the 0.1 fatiguemm) in life the represented sam- by the ple middle sectioncomparatively with the highest steep slope effective of the stress. S-N curveLarger that corroded is described areas (>0.5 by k mm)= 6.321. lo- After 210 h 6 ◦ cated in the peripheralof exposure section to CO of2-saturated the specimen geothermals reveal only brine a (21few× cracks10 cycles) (Figure at 12). 60 OneC, the endurable or more surfacestress cracks amplitude developed is ca. inde 160pendently MPa and thusof the only stress 60% amplitude of the fatigue Sa between limit in S aira = (ca. 260 MPa, 187 and 255 MPaFigure [36]. 12 )[Cracks35,36 ,initiated106,113]. from the shallow pit corrosion are predominantly dendritic and singleMostly when cracks initiated are associatedfrom small with pits. rather [35] small pits (diameter << 0.1 mm) in the sample Crack propagationmiddle section and withfinally the the highest corrosion effective fatigue stress. failure Larger of corrodedX46Cr13 areasis related (>0.5 to mm) located in the peripheral section of the specimens reveal only a few cracks (Figure 12). One or more more than one initial micro crack (100 µm width and 50 µm depth) at stress amplitude σa surface cracks developed independently of the stress amplitude Sa between Sa = 187 and = 220 MPa and 4 to 5 cracks of various sizes at Sa = 173 and 230 MPa. At stress amplitudes 255 MPa [36]. Cracks initiated from the shallow pit corrosion are predominantly dendritic Sa < 230 MPa, initial crack regions are associated with pits. Exceeding σa > 230 MPa, only and single when initiated from small pits [35]. mechanical stress causes failure. Although striations are selectively present in all samples Crack propagation and finally the corrosion fatigue failure of X46Cr13 is related to (Figure 12) fatigue cracking cannot clearly be distinguished from corrosion fatigue in the more than one initial micro crack (100 µm width and 50 µm depth) at stress amplitude passive state (passivated crack flanks reducing corrosion fatigue). σa = 220 MPa and 4 to 5 cracks of various sizes at Sa = 173 and 230 MPa. At stress amplitudes S < 230 MPa, initial crack regions are associated with pits. Exceeding σ > 230 MPa, 3.6. Corrosion Fatiguea of X5CrNiCuNb16-4 (Air: Fatigue Limit: 620 MPa) a only mechanical stress causes failure. Although striations are selectively present in all Data regardingsamples corrosion (Figure 12 fatigue) fatigue of X5Cr crackingNiCuNb16-4 cannot clearly was mainly be distinguished compiled from corrosionref- fatigue erences: [79,90,91,99,106,113,115].in the passive state (passivated crack flanks reducing corrosion fatigue). The endurance limit in a corrosive environment of X5CrNiCuNb16-4 is lower than in air (620 MPa) by 60% [116] revealing a much steeper fatigue limit line [98,99,108]. First, results showed that the S-N curve did not reveal the expected fatigue strength and non- linear very steep slopes of the possible fatigue strength for finite life (k = 3.59, TN = 1:34,

Processes 2021, 9, 594 18 of 33

3.6. Corrosion Fatigue of X5CrNiCuNb16-4 (Air: Fatigue Limit: 620 MPa) Data regarding corrosion fatigue of X5CrNiCuNb16-4 was mainly compiled from references: [79,90,91,99,106,113,115]. The endurance limit in a corrosive environment of X5CrNiCuNb16-4 is lower than Processes 2021, 9, x FOR PEER REVIEW 18 of 33 in air (620 MPa) by 60% [116] revealing a much steeper fatigue limit line [98,99,108]. First, results showed that the S-N curve did not reveal the expected fatigue strength and non- linear very steep slopes of the possible fatigue strength for ﬁnite life (k = 3.59, TN = 1:34, 2 × 7 r2r = =0.33, 0.33, max. max. N Nf f= =10 10 × 10107) at) atSa S =a 150= 150 MPa) MPa) [79,99,100]. [79,99,100 ].New New results results showed showed a asmaller smaller scatterscatter but but also also a a steep slope ofof thethe fatiguefatigue limit limit line line pointing pointing to to a fasta fast degradation degradation and and low × 7 lowmaximum maximum Nf (10Nf (1010 × 10at7 at Sa S=a = 130 130 MPa) MPa) (Figure (Figure 13 13))[103 [103,106,108].,106,108].

350

300

250 X5CrNiCuNb 16-4 X2 CrNiMoN 22 5 3

in MPa N

a 10 N 200 50 N 90

150 X2 CrNiMoN 22-5-3 R = −1, f = 33 Hz, k = 1 t Stress amplitude S North German Basin = T 96 °C, Dcrit = 12.5 mm push/pull isolated 100 104 105 106 107 Cycles N

0,1 1 10 100 Test Duration in h

Figure 13. S-N curve and crack formation on the 1.4542 first and second set of experiments exposed to saline aquifer water Figure 13. S-N curve and crack formation on the 1.4542 first and second set of experiments exposed to saline aquifer water and technical CO2. Comparison results for 1.4462 are given. and technical CO2. Comparison results for 1.4462 are given. Pits are a source of multiple cracks [113]. Local corrosion is enhanced by the me- chanicalPits are load a source increasing of multiple the local cracks microstructural [113]. Local boundarycorrosion is energy enhanced of X5CrNiCuNb16-4 by the mechan- icaldue load to theincreasing local lattice the local mismatch microstructural [36], increased boundary dislocation energy number, of X5CrNiCuNb16-4 grain boundaries, due toprecipitation the local lattice phase mismatch boundaries, [36], andincreased carbides dislocation [36], as well number, as the grain presence boundaries, of chlorides precip- [46]. itationThe phase specimen boundaries, with and a presumably carbides [36], short as well number as the of presence cycles to of failure chlorides typically [46]. con- sistedThe of specimen non-metallic with inclusionsa presumably communized short number with ofδ cycles-ferrite to [ failure99,100, 108typically,113]. consisted Moreover, ofaluminum non-metallic was inclusions in specific communized samples but with could δ not-ferrite be identified[99,100,108,113]. as a significant Moreover, reason alumi- for numthe lowwas numberin specific of samples cycles and but earlycould failure not be [identified90]. Early as failure a significant could notreason significantly for the low be numberassociated of cycles with and heterogeneously early failure [90]. distributed Early failure aluminum could [not90] significantly and other microstructural be associated withimpurities, heterogeneously artificial aquifer distributed water aluminum [106,107,113 [90]], stress and amplitude,other microstructural number of striationsimpurities, or artificialthickness aquifer of the water corrosion [106,107,113], layer. stress amplitude, number of striations or thickness of the Acorrosion cathodic layer. potential clearly enhances the corrosion fatigue life expectancy by a factor of 20A cathodic to 70 (U SHEpotential= −400 clearly to − e150nhances mV) the [116 corrosion]. The specimen fatigue life degrades expectancy slower by a factor at low ofpotentials 20 to 70 (U andSHE faster= −400 atto high−150 potentials.mV) [116]. The specimen fatigue life degrades expectancy slower (number at low ofpotentials cycles to andfailure) faster is at increased high potentials. with the decreasingThe fatigue potential life expectancy considering (number the drawback of cycles that to failure) a cathodic is increasedpotential with discharges the decreasing H2 from thepotential specimen’s considering surface the and drawback accelerates that embrittlement a cathodic poten- of the tialalloy, discharges which causes H2 from earlier the specimen’s and unpredicted surface failure and acce [116lerates]. embrittlement of the alloy, which causes earlier and unpredicted failure [116]. 3.7. Corrosion Fatigue of X2CrNiMo22-5-3 (Air: Fatigue Limit: 485 MPa) 3.7. CorrosionData regarding Fatigue of corrosion X2CrNiMo22- fatigue5-3 (Air: of X2CrNiMo22-5-3 Fatigue Limit: 485 was MPa) mainly compiled from references:Data regarding [52,79,90 corrosion,92,100,106 fatigue,109]. of X2CrNiMo22-5-3 was mainly compiled from references:In air,[52,79,90,92,100,106,109]. the fatigue limit of duplex stainless steel X2CrNiMoN22-5-3 is 485 MPa (Pf = 50%, push/pull, N = 107 cycles) [33–35]. The steep slope of the S-N curve (k = 8.78) typical In air, the ffatigue limit of duplex stainless steel X2CrNiMoN22-5-3 is 485 MPa (Pf = in a corrosive environment reveals a low scatter range TN (1:1.35), therefore generally 50%, push/pull, Nf = 107 cycles) [33–35]. The steep slope of the S-N curve (k = 8.78) typical in a corrosive environment reveals a low scatter range 𝑇 (1:1.35), therefore generally indicating the same failure mechanism. The maximum number of cycles of (9.2 × 106) in a corrosive environment at a free corrosion potential (resting potential) was determined at a stress amplitude of 240 MPa. Failure is always characterized by an abrupt drop of potential, but here, at approximately 10 min prior to the potential, failure began to decrease steadily indicating an upcoming failure [82].

Processes 2021, 9, 594 19 of 33

indicating the same failure mechanism. The maximum number of cycles of (9.2 × 106) in a corrosive environment at a free corrosion potential (resting potential) was determined at a stress amplitude of 240 MPa.

Processes 2021, 9, x FOR PEER REVIEW Failure is always characterized by an abrupt drop of potential, but here, at approx-19 of 33 imately 10 min prior to the potential, failure began to decrease steadily indicating an upcoming failure [82].

3.7.1.3.7.1. Influence Influence of of Machine Machine Insulation Insulation SinceSince electric electric insulation insulation was was found found to to affect affect the the corrosion corrosion fatigue fatigue behavior behavior of of the the steel, steel, thethe subsystem subsystem specimen/corrosion specimen/corrosion chamber chamber were were electrically electrically grounded grounded and and shielded shielded [81]. [81]. (Note(Note that that grounding grounding only only does does not not supply supply reliable reliable corrosion corrosion fatigue fatigue data data due due to the to in- the sufficientinsufficient electric electric shielding shielding of ofthe the specimen). specimen). AnAn insulated insulated setup setup is characterized byby aa higherhigher S-N S-N coefficient coefficient of ofk =k = 4.7 4.7 in in comparison compari- sonto anto an uninsulated uninsulated setup setup (k (=k 8.78)= 8.78) (Figure (Figure 14 14))[ 52[52],], which which dependsdepends onon the more more cathodic cathodic electrochemicalelectrochemical potential. potential. Starting Starting from from the the same same value value at at 400 400 MPa, MPa, the the two two S-N S-N curves curves divergediverge as as the the stress stress amplitude amplitude decreases. decreases. Insu Insulationlation shifts shifts the the potential potential to to a a more more noble noble regime.regime. At At the the same same time, time, the range reduces toto ca.ca. UUSHESHE = −−55 to to −−6565 mV. mV. Failure Failure is is also also presagedpresaged by by a a distinct distinct potential potential drop. drop.

450

400 Unisolated unisolated isolated

N10 350 N50 N Isolated 90

in MPa 300 a

250

X2 CrNiMoN 22-5-3 R = −1, f = 33 Hz, k = 1 t 200 Stress amplitude S North German Basin = T 96 °C, Dcrit = 12.5 mm Push/pull isolated, Technological surface 150 3x104 105 106 107 3x107 Cycles N

0.25 1 10 100 Test Duration in h FigureFigure 14. 14. CorrosionCorrosion fatigue fatigue X2CrNiMoN22-5-3. S-N curves,curves, uninsulateduninsulated and and insulated insulated experimental experi- mentalsetup. setup.

3.7.2.3.7.2. Influence Inﬂuence of of Electrochemical Electrochemical Potential Potential WhenWhen a a critical critical potential potential is is applied applied the the corrosion corrosion effects can be repressed [[52,90.52,90]. The The applicationapplication of of the the critical critical cathodic cathodic potential potential at at a a stress stress amplitude amplitude of of 275 275 MPa MPa increases increases the the numbernumber of of cycles cycles compared compared to to the the free free corrosion corrosion potential. potential. The The initiation initiation of of local local corrosion corrosion isis suppressed suppressed that that otherwise otherwise initiates initiates crack crack formation formation and and therefore therefore increases increases fatigue fatigue life. life. 5 TheThe cathodic cathodic potential potential at at 275 275 MPa MPa increases increases the the number number of ofcycles cycles from from 4.7 4.7× 10×5 (free10 (free po- 6 6 7 tentialpotential at: at:ca. ca.−3 mV)−3 mV) to 2.6 to 2.6× 10×6 (10−150(− mV)150 mV)up to up 5.4 to × 5.4 10×6 at10 −300at −mV300 and mV 10 and7 cycles 10 cycles at a potentialat a potential of −450 of −to450 −900 to mV.−900 As mV. a back As a draw, back draw,hydrogen hydrogen evolves evolves from fromthe specimen’s the specimen’s surfacesurface at very atvery low potentials low potentials [90] (an [90 anodic] (an anodic potential potential causes causes severe severe pit formation pit formation and there- and foretherefore a more a morerapid rapidfailure failure of the ofspecimen). the specimen).

3.7.3. Influence of Load Type The superior corrosion fatigue resistance in a geothermal environment under the rotation bending load is significant by the factor 1.11–1.25 in the cycle range 105 to 106 [90]. The initially better corrosion fatigue properties under the rotation bending load (higher stress amplitudes at a certain number of failure) become less significant with the increasing number of cycles. The S-N 50% probability graphs converge at approximately 3.5 × 106 cycles (30 h of cyclic loading), eliminating the advantage of the rotation bending load

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3.7.3. Inﬂuence of Load Type The superior corrosion fatigue resistance in a geothermal environment under the rotation bending load is signiﬁcant by the factor 1.11–1.25 in the cycle range 105 to 106 [90]. The

Processes 2021, 9, x FOR PEER REVIEWinitially better corrosion fatigue properties under the rotation bending load (higher20 stress of 33 amplitudes at a certain number of failure) become less signiﬁcant with the increasing number of cycles. The S-N 50% probability graphs converge at approximately 3.5 × 106 cycles (30 h of cyclic loading), eliminating the advantage of the rotation bending load (slope (slopefactors factors of rotating of rotating bending bendingk = 8.2 k = and 8.2 and push/pull push/pullk = k 19).= 19). The The greater greater negativenegative slope coefficientcoefﬁcient achieved for rotation bending experimentsexperiments describes the higher rate of alloy degradation and therefore greater susceptibility to the corrosive attack, especially after high cycle numbers and therefore long exposure to a corrosivecorrosive environment.environment.

3.7.4. Influence Influence of Surface Conditions Data regardingregarding the the influence influence of surfaceof surface conditions conditions on the on corrosion the corrosion fatigue offatigue X2CrNi- of X2CrNiMo22-5-3Mo22-5-3 was mainly was compiledmainly compiled from references: from references: [52,91,92 [52,91,92,109].,109]. The surface roughness influences influences the corrosioncorrosion fatigue behavior of the duplex steel significantlysignificantly (note that there is no influenceinfluence on the corrosion fatigue behavior within one experimental series). The The line line of of regression for technical surfaces has a greater negativenegative slope (steeper) ( k = 8.78) than for polished surfaces (k == 19.006). (Note that the lines of regression with a small number of k declinedecline greater than those withwith large numbers of k (Figure 1515)).)). Rather,Rather, the the low low scatter scatter ranges ranges (technical (technical surface: surface: TN TN = 1:1.35, = 1:1.35, polished polished surface: surface:TN = TN 1:1.95) = 1:1.95) indicate indicate no change no change in failure in failure mechanism. mechanism.

Figure 15. InfluenceInﬂuence of of surface surface finish ﬁnish quality quality and and local local corrosion corrosion init initiationiation on on CF, CF, S-N S-N curves, curves, and and insulated test setup. ( Top right): Turned, ((bottom rightright):): Polished.Polished.

As discussed discussed in in detail detail [92,109], [92,109], a apolished polished surface surface finish ﬁnish (approximate (approximate RZ R= Z1.4–1.59= 1.4– 1.59µm) µinhibitsm) inhibits degradation degradation compared compared to the to turned the turned surface surface finishes ﬁnishes (approximate (approximate RZ = R3.2Z µm).= 3.2 Theµm). specimen The specimen with technical with technical surfaces surfaces(Rz = 2.6–4.6) (Rz =tested 2.6–4.6) at stress tested amplitudes at stress ampli-above 275tudes MPa above (9 × 275105 cycles) MPa (9 (P50%× 105 atcycles) Sa 300 (P50% Mpa = at 5Sa × 10 3005) reveal Mpa = a5 higher× 105 )life reveal expectancy a higher than life 5 theexpectancy specimen than with the polish specimened surfaces with polished(P50% at surfacesSa 300 Mpa (P50% = 1.5 at × S10a 5300) (Figure Mpa =15). 1.5 The× 10 life) expectancy(Figure 15). for The polished life expectancy surfaces, forhowever, polished increases surfaces, with however, the test increasesduration withand decreas- the test ingduration stress andamplitude. decreasing Smoother stress polished amplitude. surfaces Smoother provide polished little to surfaces no additional provide stress little con- to centrationno additional associated stress concentration with the base associated region of withmicro-notches the base region and inhibit of micro-notches crack initiation and andinhibit early crack failure initiation [52]. and early failure [52].

4. Discussion In general, a higher higher Ni and and Cr Cr content content in in the the heat heat treated treated steels steels improve improve the corrosion resistance [[22,27],22,27], alsoalso provenproven byby thethe steels steels investigated investigated in in this this study. study. Moreover, Moreover, the the inter- in- termediatemediate heat heat treatment treatment protocols protocols for steels for steel withs continuouswith continuous martensitic martensitic microstructures microstruc- are tures are favorable in terms of good corrosion resistance and are widely discussed before [9,17]. The corrosion rate is more susceptible to the heat treatment at 100 bar than at ambient pressure. Therefore, the heat treatment is less significant compared to the chromium content and atmosphere. Hardening or hardening and tempering coupons are promising at ambient pressure with regards to low surface corrosion rates. Considering surface corrosion only, at 100 bar the martensitic microstructure (hardened and tempered at 650 °C for the supercritical phase: < 0.001 mm/year) is superior in water saturated supercritical

Processes 2021, 9, 594 21 of 33

favorable in terms of good corrosion resistance and are widely discussed before [9,17]. The corrosion rate is more susceptible to the heat treatment at 100 bar than at ambient pressure. Therefore, the heat treatment is less significant compared to the chromium content and atmosphere. Hardening or hardening and tempering coupons are promising at ambi- Processes 2021, 9, x FOR PEER REVIEWent pressure with regards to low surface corrosion rates. Considering surface corrosion21 of 33 only, at 100 bar the martensitic microstructure (hardened and tempered at 650 ◦C for the supercritical phase: <0.001 mm/year) is superior in water saturated supercritical CO2. However, in the CO saturated aquifer, the water normalized ferritic-perlitic microstructure CO2. However, in the2 CO2 saturated aquifer, the water normalized ferritic-perlitic micro- isstructure sufficiently is sufficiently resistant againstresistant corrosion against corrosion (ca. 0.004 (ca. mm/year) 0.004 mm/year) [98,104 [98,104].]. Here,Here, wewe will briefly briefly discuss discuss specifically specifically pick pickeded topics topics that that we we find find most most interesting. interesting.

4.1.4.1. PrecipitationPrecipitation of Corrosion Corrosion Scales Scales in in a aGeothermal Geothermal Environment Environment High alloyed steels exposed to water saturated CO show little dependence on the High alloyed steels exposed to water saturated CO2 show2 little dependence on the exposureexposure timetime or heat treatment. treatment. Corrosion Corrosion rates rates remain remain nearly nearly constant constant at approximately at approximately 0.0040.004 (X5CrNiCuNb16-4)-0.4(X5CrNiCuNb16-4)-0.4 (X46Cr13) (X46Cr13) mm/year. mm/year. The The sufficiently sufﬁciently thick thick carbonate carbonate layer layer actsacts asas aa diffusiondiffusion barrier passivating passivating the the steel steel [36,89,90,99,104,113,115,116]. [36,89,90,99,104,113,115,116 ]. HeterogeneousHeterogeneous carbonate carbonate layers layers are are formed formed in in a aCO CO2-saturated2-saturated saline saline aquifer aquifer and and covercover the specimens’ surface surface and and pits pits because because carbonic carbonic acid acidforms, forms, which which reduces reduces the si- the sideritederite FeCO FeCO3-solubility3-solubility in inCO CO2-containing2-containing water water with with a rather a rather low low pH pH [24]. [24 Main]. Main phases phases atat ambientambient pressure are: Siderite Siderite FeCO FeCO3 3andand cementite cementite Fe Fe3C.3 C.Moreover, Moreover, goethite goethite α-FeOOHα-FeOOH atat 100100 barbar andand mackinawitemackinawite FeS, akaganeite Fe Fe88OO8(OH)8(OH)8Cl8Cl1.341.34, and, and spinel-phases spinel-phases of various of various compositionscompositions [[98,99,103,113].98,99,103,113]. TheThe corrosion rates rates first ﬁrst increase increase in ina CO a CO2-saturated2-saturated saline saline aquifer, aquifer, then passivate then passivate re- resultingsulting in indecreasing decreasing corrosion corrosion rates rates but late butr laterthe rates the increase rates increase again (ca. again 0.004 (ca. to 0.014 0.004 to 0.014mm/year mm/year for X5CrNiCuNb16-4). for X5CrNiCuNb16-4). The passivating The passivating layer breaks layer down breaks and down gaseous and CO gaseous32-- 2− COand3 O2- -species and O2 -speciesdiffuse into diffuse the metal into the and metal iron di andffuses iron towards diffuses the towards surface. the Here, surface. siderite Here, sideriteFeCO3 layers FeCO form3 layers (Figure form 16) (Figure [9,15,17,36,99,113]. 16)[9,15,17,36 Due,99 ,to113 the]. Duefact that to the the fact larger that areas the largerof areasthe corrosion of the corrosion layer locally layer detach, locally a fresh detach, surface a fresh is exposed surface to is the exposed brine and to the corrosion brine and corrosionreactions accelerated reactions accelerated [113]. [113].

FigureFigure 16. 16.Schematic Schematic drawing drawingof of thethe precipitationprecipitation of the corrosio corrosionn layer layer on on X5CrNiCuNb16-4 X5CrNiCuNb16-4 (1.4542) (1.4542) and and an anexample example of of sample surfaces after 8000 h of exposure to water saturated supercritical CO2. sample surfaces after 8000 h of exposure to water saturated supercritical CO2.

However,However, X5CrNiCuNb16-4 shows shows an an unusual unusual corrosion corrosion behavior behavior forming forming homoge- homoge- neousneous ellipsoidsellipsoids on the surfaces surfaces (Figures (Figures 12 12 and and 16 16 (leopard (leopard shaped). shaped). The The corrosion corrosion layer layer revealsreveals inhomogeneouslyinhomogeneously thick carbonate corrosi corrosionon products products on on the the surface surface such such as as sider- siderite ite FeCO3, goethite FeOOH, and carbides, mainly Fe3C. The center of the typical ellipsoids FeCO3, goethite FeOOH, and carbides, mainly Fe3C. The center of the typical ellipsoids at at exposure greater than 4000 h reveals mainly hematite Fe2O3 but also iron sulfate FeSO4, exposure greater than 4000 h reveals mainly hematite Fe2O3 but also iron sulfate FeSO4, siderite FeCO3, and goethite FeOOH—the latter also after corrosion fatigue tests. siderite FeCO3, and goethite FeOOH—the latter also after corrosion fatigue tests. Corrosion ellipsoids may form since carbides are distributed heterogeneously within Corrosion ellipsoids may form since carbides are distributed heterogeneously within the microstructure and these are more susceptible to initial corrosion [34] steels, which are the microstructure and these are more susceptible to initial corrosion [34] steels, which are locally depassivated [98]. locally depassivated [98]. Another likely explanation depends on water solubility, pressure, and temperature Another likely explanation depends on water solubility, pressure, and tempera- [98]. Generally, the solubility of water increases with temperature. However, under pres- ture [98]. Generally, the solubility of water increases with temperature. However, un- sure, the solubility of water decreases from 0 to 50 bar and then increases again slightly der pressure, the solubility of water decreases from 0 to 50 bar and then increases again [31]. In water saturated supercritical CO2 at 100 bar and 60 °C, the metal◦ surface is wetted slightly [31]. In water saturated supercritical CO2 at 100 bar and 60 C, the metal surface is due to the decreasing water solubility in the supercritical CO2. Consequently, small and very thin “leopard”-shaped droplets form and thin corrosion layers grow. Due to the oxygen content (Figure 12), these are most likely iron carbonate or iron hydroxide. Consolidating droplets form small pits around the former droplet and corrosion processes are enhanced at this distinct multiphase boundary (water, metal, supercritical CO2). Since small droplets are formed much faster, the consolidated water diffuses back into the

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wetted due to the decreasing water solubility in the supercritical CO2. Consequently, small and very thin “leopard”-shaped droplets form and thin corrosion layers grow. Due to the oxygen content (Figure 12), these are most likely iron carbonate or iron hydroxide. Consolidating droplets form small pits around the former droplet and corrosion Processes 2021, 9, x FOR PEER REVIEWprocesses are enhanced at this distinct multiphase boundary (water, metal,22 supercritical of 33

CO2). Since small droplets are formed much faster, the consolidated water diffuses back into the supercritical CO2. As a consequence, the area of consolidated droplets reduces inwardssupercritical with CO the2. sulphatesAs a consequence, (FeSO4) the found area in of the consolidated outer and hematitedroplets reduces (Fe2O3) inwards in the centered areaswith the (Figure sulphates 12)[ 98(FeSO]. 4) found in the outer and hematite (Fe2O3) in the centered areas (Figure 12) [98]. 4.2. Inﬂuence of Austenitizing 4.2. InfluenceResults of from Austenitizing the DoE analysis after Klein [117] prove that the surface corrosion resis- tanceResults is dependent from the only DoE on analysis the austenitizing after Klein time,[117] butprove neither that the the surface austenitizing corrosion temperature re- norsistance the carbonis dependent content only of on the the baseaustenitizing material time, [17] but (Figure neither 17 the). Theaustenitizing carbon contenttemper- shows theature largest nor the impact carbon on content the local of the corrosion base mate butrial still [17] no (Figure signiﬁcant 17). The dependence carbon content on the local corrosionshows the behaviorlargest impact [103 ].on the local corrosion but still no significant dependence on the local corrosion behavior [103].

Carbon content Temperature 95% Time

95% Influence in % in Influence

parameters pitting FigureFigure 17. TheThe impact impact of of the the experimental experimental parameters parameters on the on corrosion the corrosion rate after rate 4000 after h 4000of expo- h of exposure sure time combined with confidence intervals. time combined with confidence intervals. Generally, martensitic microstructures are highly corrosive (up to two orders of mag- Generally, martensitic microstructures are highly corrosive (up to two orders of mag- nitude higher than ferritic or ferritic-bainitic microstructures) since grain boundaries are nitudemore reactive higher [25,118]. than ferritic However, or ferritic-bainitic increasing the austenitizing microstructures) temperature since grainlowers boundaries the pit- are moreting potential reactive of [ 25lean,118 high]. However,alloyed steels increasing [24,28,82] thewith austenitizing a maximum improvement temperature by lowersan- the pittingnealing potentialat 1200 °C of[24]. lean Since high carbides alloyed dissolve steels at [24 high,28, 82annealing] with a temperatures maximum improvementthe corro- by ◦ annealingsion resistance at1200 of martensiticC[24]. stainless Since carbides steels with dissolve 13% Cr increases at high annealingat higher austenitizing temperatures the corrosiontemperatures resistance (980–1050 of °C) martensitic [25,30,91]. stainless (Note that steels Cr-rich with M 13%23C6 and Cr increasesM7C3 carbides at higher gener- austeni- ◦ tizingally increase temperatures the mechanical (980–1050 propertiesC) [25 due,30, 91to]. secondary (Note that hardening Cr-rich M[30],23C but6 and reduce M7C the3 carbides generallyresistance increaseof a passivating the mechanical film and enhance properties pitting due [82]). to secondary However, hardeningat the beginning, [30], butthe reduce thecorrosion resistance rates ofof a X46Cr13 passivating and filmX20Cr13 and are enhance significantly pitting higher [82]). However,after austenitizing at the beginning, at thehigher corrosion temperatures rates ofand X46Cr13 longer holding and X20Cr13 times, arebut significantlyafter 4000 h of higher exposure after to austenitizingthe CCS at higherenvironment temperatures the corrosion and rates longer are holdingof not much times, difference. but after Most 4000 likely h ofthe exposure advantageous to the CCS environmentcarbide dissolution the corrosion at high austenitizing rates are of temp not mucheratures difference. is not the Mostdriving likely corrosion the advantageous force carbideand the dissolutiongrain boundaries at high are austenitizing corrosively attacked. temperatures Diffusion is not processes the driving are slowed corrosion down force and with the increasing thickness of the growing corrosion layer. Additionally, shorter aus- the grain boundaries are corrosively attacked. Diffusion processes are slowed down with tenitizing at a low temperature limits the grain growth by not allowing for equilibration. the increasing thickness of the growing corrosion layer. Additionally, shorter austenitizing The slow grain growth leads to coarse grain sizes resulting in lower corrosion rates. More- atover, a low the temperatureretained austenite limits at thea low grain austenit growthizing by temperature not allowing may for be equilibration.beneficial against The slow grainpitting growth of 13%-chromium leads to coarse steels grain(13CrNiMo) sizes resulting[26]. However, in lower in a geothermal corrosion rates.environment, Moreover, the retainedaustenitizing austenite does not at ainfluence low austenitizing the corrosion temperature behavior after may long be exposure beneficial times against (Figure pitting of 13%-chromium13). steels (13CrNiMo) [26]. However, in a geothermal environment, austenitizing does not influence the corrosion behavior after long exposure times (Figure 13). 4.3. Crack Initiation and Corrosion Fatigue Failure of High Alloyed Steels X46Cr13 and X5CrNiCuNb16-4 The deterioration of grain boundaries and pit formation initiate crack formation thus failure of high alloyed steels under cyclic load in a (CO2-saturated) corrosive geothermal environment [32–34,36,99,113].

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4.3. Crack Initiation and Corrosion Fatigue Failure of High Alloyed Steels X46Cr13 and X5CrNiCuNb16-4 Processes 2021, 9, x FOR PEER REVIEW 23 of 33 The deterioration of grain boundaries and pit formation initiate crack formation thus failure of high alloyed steels under cyclic load in a (CO2-saturated) corrosive geothermal environment [32–34,36,99,113]. PitsPits onon stainlessstainless steels form form due due to to chemical chemical reactions, reactions, local local changes changes of lattice of lattice energy energy withinwithin thethe steel’s surface, surface, and and mechanical mechanical load load [9,15]. [9,15 During]. During cyclic cyclic stress-strain stress-strain loading, loading,plastic plastic deformation deformation is a isreason a reason for forstrain strain and and work work hardening hardening effects effects increasing increasing the the grain/phasegrain/phase boundary energy energy at at dual dual or or triple triple points points of of grain/phase grain/phase boundaries, boundaries, residual residual perliteperlite grains,grains, and more more likely likely carbides carbides [34]. [34 This]. This local local lattice lattice mismatch mismatch initiates initiates pit -, se- pit -, selective-,lective-, and and intergranular intergranular corrosion, corrosion, as as well well as ascrack crack formation formation [33] [33 and] and accelerates accelerates crack crack propagationpropagation [[32,34].32,34]. AtAt thethe beginning, the the pits pits form form grain grain and and phase phase boundaries boundaries catalyzing catalyzing corrosion corrosion re- reactions.actions. In In geothermal geothermal water water with with high high chloride chloride concentrations, concentrations, the the phases phases arrange arrange at at equilibriumequilibrium andand dissolutedissolute an outer metastable metastable oxide/hydroxide oxide/hydroxide film film before before siderite siderite then then precipitateprecipitate [[9,15]9,15] despitedespite a passivating inner inner chromium chromium oxide oxide film film [31,53]. [31,53 Mu]. Mu et al. et al.[6] [6] illustratedillustrated thatthat sideritesiderite forms when when carbonate carbonate io ionsns absorb absorb into into the the base base material material at the at the hydroxide/brinehydroxide/brine interface interface and and combine combine with with oxygen oxygen vacancies vacancies (cation/oxygen (cation/oxygen vacancy vacancy pairspairs of of Mott-Schottky-type). Mott-Schottky-type). AtAt thethe hydroxide/brinehydroxide/brine interface, interface, oxygen oxygen vacancies vacancies and and ad- addi- tionalditional carbonate carbonate ions ions form form excess excess cation cation vacancies. vacancies. These These traveltravel toto the hydroxide/sider- hydroxide/siderite interface,ite interface, consolidate, consolidate, and and condense condense to to form form larger larger vacancy vacancy areas. areas. As As a a consequence, consequence, the sideritethe siderite and hydroxideand hydroxide film film detach, detach, enhancing enhancing pitting pitting and and lateral lateral surface surface degradation degradation [6,36 ]. [6,36]. A possible crack initiation model was presented by Han et al. [37] and modified A possible crack initiation model was presented by Han et al. [37] and modified by by Pfennig et al. [17,35,36,99,113] (Figure 18): Initially, pits catalyze corrosion on steels Pfennig et al. [17,35,36,99,113] (Figure 18): Initially, pits catalyze corrosion on steels ex- exposed to CO2-environment [15,17,29] and siderite FeCO3 [3,38] precipitates (a). Anodic posed to CO2-environment [15,17,29] and siderite FeCO3 [3,38] precipitates (a). Anodic iron dissolution forms transient Fe(OH)2 [6,30] (grey area in: a) The pH rises locally at the iron dissolution forms transient Fe(OH)2 [6,30] (grey area in: a) The pH rises locally at the hydroxide film and a thin ferrous carbonate layer precipitates internally and externally hydroxide film and a thin ferrous carbonate layer precipitates internally and externally (a (a and b) (Equations (1)–(6)) [15,29,37]. Both corrosion layers grow with regards to carbon and b) (Equations (1)–(6)) [15,29,37]. Both corrosion layers grow with regards to carbon andand oxygen oxygen partialpartial pressures (b). (b). When When the the ferrous ferrous hydroxide hydroxide film film (c) (c) is locally is locally damaged damaged (either(either mechanicallymechanically or chemically) the the non-protective non-protective porous porous ferrous ferrous carbonate carbonate film film is is exposedexposed to to thethe brinebrine (local low low pH). pH). It It dissol dissolvesves and and depassivates depassivates the the steel steel (d) (d) intensifying intensifying locallocal surfacesurface degradation.degradation. Oxygen Oxygen vacancies vacancies consolidate consolidate and and condense condense at the at thehydrox- hydrox- ide/brineide/brine interface resulting resulting in in the the lateral lateral spallation spallation of ofthe the siderite siderite from from the the hydroxide hydroxide alignedaligned to to the the mechanical mechanical stressstress appliedapplied (e).(e). The remaining film film is dissipated dissipated by by the the flow- flowing brineing brine inducing inducing further further pitenlargement, pit enlargement, since since the the newly newly exposed exposed surface surface reactions reactions will will start repeatedlystart repeatedly from thefrom top the (f). top Concurrently, (f). Concurrently, cracks cracks initiate initiate and and propagate propagate due due to theto the cyclic loadcyclic and load additional and additional forces forces at the at pit the bottom, pit bottom, where where the stress the stress concentration concentration and and plastic deformationplastic deformation produce produce slip bands slip bands [26] at[26] the at crackthe crack flanks. flanks. Newly Newly formed formed slip slip bands bands are susceptibleare susceptible to corrosive to corrosive reactions reactions as explained as explained and and once once wetted wetted the corrosionthe corrosion steps steps repeat withinrepeat thewithin crack the itself. crack itself.

Figure 18. Cont.

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FigureFigure 18. Schematic18. Schematic crack crack initiation initiation model model modiﬁed modified by by Han Han et et al. al. [ 37[37].]. Figure 18. Schematic crack initiation model modified by Han et al. [37]. 4.4. Influence of Surface Quality on the Corrosion Fatigue of Duplex Stainless Steel 4.4.X2CrNiMoN22-5-3 Inﬂuence of Surface Quality on the Corrosion Fatigue of Duplex Stainless Steel X2CrNiMoN22-5-34.4. Influence of Surface Quality on the Corrosion Fatigue of Duplex Stainless Steel X2CrNiMoN22-5-3Generally, technical surfaces of duplex stainless steel X2CrNiMoN22-5-3 (1.4462, equilibriumGenerally, at 98 technical °C in a corrosive surfaces environm of duplexent stainless of the Northern steel X2CrNiMoN22-5-3 German Basin (NGB) (1.4462, at Generally, technical◦ surfaces of duplex stainless steel X2CrNiMoN22-5-3 (1.4462, equilibrium275 MPa) reveal at 98 goodC in corrosion a corrosive fatigue environment resistance ofin a the CCS Northern or geothermal German environment Basin (NGB) at at equilibrium at 98 °C in a corrosive environment of the Northern German Basin (NGB) at 275low MPa) stress reveal amplitudes. good corrosionNote: at Safatigue > 270 MPa, resistance technical in surfaces a CCS or (average geothermal surface environment roughness at 275 MPa) reveal good corrosion fatigue resistance in a CCS or geothermal environment at lowof 3.2 stress µm) amplitudes. the fatigue Note:life succeeds at Sa > 270that MPa,of polished technical surfaces surfaces (average (average surface surface roughness roughness low stress amplitudes. Note: at Sa > 270 MPa, technical surfaces (average surface roughness ofof 3.2 1.59µm) µm). the At fatigue Sa < 270 life MPa, succeeds polished that surfaces of polished succeed surfaces technical (average surfaces surface (Figures roughness 15 and of of 3.2 µm) the fatigue life succeeds that of polished surfaces (average surface roughness 1.5919) µ[81–84]m). At. Sa < 270 MPa, polished surfaces succeed technical surfaces (Figures 15 and of 1.59 µm). At Sa < 270 MPa, polished surfaces succeed technical surfaces (Figures 15 and 19)[81–84]. 19) [81–84].

Figure 19. Schematic S-N curves of 1.4462 X2CrNiMoN22-5-3 in air and the corrosive media Northern German Basin at 98 °C after machine turning (technical surface) and polishing (polished surface). Figure 19. Schematic S-N curves of 1.4462 X2CrNiMoN22-5-3 in air and the corrosive media Northern German Basin at 98 Figure 19. Schematic S-N curves of 1.4462 X2CrNiMoN22-5-3 in air and the corrosive media Northern German Basin at °C after machine turning (technical surface) and polishing (polished surface). 98 ◦C after machine turning (technical surface) and polishing (polished surface).

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Cracks usually propagate perpendicular to the laminar duplex duplex structure structure (Figure (Figure 20).20). Sanjuro etet al.al. [[82]82] statedstated that that residual residual stress stress only only contributes contributes 10% 10% to theto the enhanced enhanced corrosion corro- sionfatigue fatigue behavior. behavior. Mostly Mostly surface surface ﬁnishing finish accompanieding accompanied with thewith removal the removal of surface of surface stress stressraisers raisers contribute. contribute. However, However, the specimen the specim withen technical with technical surfaces surfaces plastically plastically deform andde- formharden and during harden machining. during machining. The inﬂuence The of influence this residual of this compressive residual stresscompressive might weighstress mighthigher weigh than thehigher increased than the surface increased roughness surface and roughness is responsible and is responsible for the high for fatigue the high life fatigueexpectancy life expectancy at amplitudes at amplitudes above 275 MPa above [109 275]. MPa [109].

Figure 20.20. MicrosectionMicrosection ofof macro macro crack crack with with signiﬁcant significant residual residual surface surface opening. opening. Etchant: Etchant: Beraha Beraha II (top II ()[top93)]. [93]. Microsections Microsec- tions of major crack initiation (left) and major crack termination zone (right) (380 MPa; 0.36 × 106 cycles). of major crack initiation (left) and major crack termination zone (right) (380 MPa; 0.36 × 106 cycles).

Another explanation is related to a changing mechanism of crack growth depending on the stress amplitude (note: At 275 MPa, th thee overlapping regression lines prove that the surface condition does not inﬂuenceinfluence the corrosion fatigue behavior). At S a >> 275 275 MPa, MPa, the the overall overall tensile stress in the specimen during push-pull loading enhances the micro crack formation (Figure 2020).). During cyclic loading the crack constantly opens and closes exposing the (newly formed) crack ﬂanksflanks to the geothermal media.media. As a consequence of the corrosive reactions described earlier [[26,81,105],26,81,105], sideritesiderite precipitatesprecipitates and segregates. Since the strain of the corros corrosionion layer and base metal do not match further the mechanical stress causes crack propagationpropagation until failure. High endurance limits are generally found found for for high high strength strength material material and and because because technical technical surfaces surfaces may may reveal reveal in- creasedincreased strength strength from from compressive compressive stress, stress, the the number number of of cycles cycles to to fatigue fatigue is higher for technical surfaces at Sa >> 275 275 MPa MPa [109]. [109]. At S a << 275 275 MPa, MPa, pit pit corrosion corrosion is is responsible responsible for crack formation. During exposure to the mechanical load and corrosive environment pits grow to a critical size and due to a notch effect, the fatigue cracks form (Figure 1919))[ [68].68]. Local corrosion is a reason for micro crack initiation at Sa << 270 270 MPa MPa and and therefore, therefore, polished polished surfaces surfaces suppressing pit formation are a reason for higher numbers of cycles to failure [[93,109].93,109]. Generally, the surface roughness may be neglectedneglected for small stress amplitudes where pitting is the driving forceforce forfor crackcrack initiation.initiation. At high stressstress amplitudes,amplitudes, mechanicallymechanically induced micro cracks are responsible forfor crackcrack initiationinitiation andand propagation.propagation.

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4.5.4.5. Failure Failure of of Duplex Duplex Stainless Stainless Steel Steel X2CrN X2CrNiMoN22-5-3iMoN22-5-3 in a Geothermal Environment WhenWhen X2CrNiMoN22-5-3 isis testedtested under under axial axial push-pull push-pull load, load, the the crack crack path path is normalis nor- malto the to loadthe load direction direction [93]. [93]. Cracks Cracks propagate propagate predominantly predominantly perpendicular perpendicular to the to lamellar the la- austenitic as well as ferritic phases [91,93] (Figure 20, top). mellar austenitic as well as ferritic phases [91,93] (Figure 20, top). Linear crack propagation leaves a 30 µm residual crack width. Multiple small sec- Linear crack propagation leaves a 30 µm residual crack width. Multiple small secondary micro cracks ca. 3 mm below the sample surface (Figures 20 and 21) are associated ondary micro cracks ca. 3 mm below the sample surface (Figures 20 and 21) are associated with pits and disappear soon after initiation. The remaining pits form cavities. with pits and disappear soon after initiation. The remaining pits form cavities.

Figure 21. Left: CrossCross sectionsection through through the the corrosion corrosion pit. pit. Etchant: Etchant: Beraha Beraha II. Right:II. Right: Schematic Schematic drawing drawing showing showing the preferred the pre- ferreddeterioration deterioration of the austeniticof the austenitic phase (phaseγ). (γ).

FiguresFigures 20 and 21 proof that mainly the austenitic phase corrodes at crack flanks flanks and within cavities, whereas the ferritic phase remains uncorroded since the electric potential dominatesdominates [91,93]. [91,93]. Due Due to tohigher higher contents contents of molybdenum, of molybdenum, chromium, chromium, and andnitrogen, nitrogen, ele- mentselements known known to toimprove improve corrosion corrosion resistance, resistance, the the pitting pitting resistance resistance equivalent equivalent number (PREN)(PREN) is is lower lower for for the austenitic 32.37 than the ferritic phase [91,93] [91,93] resulting in a lower corrosioncorrosion resistance of the austenitic phase (total: 37.39). (Note that a high PREN = high corrosioncorrosion resistant for high alloyed alloyed steels). steels). Mo Moreover,reover, due to to its its face face cubic cubic centered centered (FCC) (FCC) structure,structure, the the large large octahedral octahedral interstitial interstitial si sitestes of of the the austenitic austenitic ph phasease (mean (mean diameter diameter ca. ca. 0.410.41 R R (R (R = = average average atomic atomic radius)) radius)) can can incorporate incorporate more more corrosive corrosive elements elements than the body cubiccubic centered (BCC)(BCC) ferritic ferritic phase phase with with only only small small octahedral octahedral interstitial interstitial sites sites (ca. 0.20(ca. µ0.20m). µm).Since Since diffusion diffusion is enhanced is enhanced in structures in structures with a largewith voidsa large intrusion voids intrusion of, e.g., chlorides of, e.g., chlo- from ridesthe brine from and the therefore,brine and therefore, pitting is preferentialpitting is preferential in the austenitic in the austen phaseitic or phase austenite-ferrite or austenite-ferritephase boundary. phase boundary. TheThe initiation of of corrosion fatigue failure in the passiv passivee state may be firstfirst derived fromfrom local local corrosion corrosion effects effects and and secondly secondly from from the the decay decay of of passivation passivation [91,93] [91,93] (Figure(Figure 22):22): 1. Pitting causes the formation of of initial micro micro cracks. The The area area of of micro micro cracks cracks is is depas- depas- sivatedsivated followed followed by by crack propagation when the stress rises above a critical level. 2. The passivation layerlayer isis locally locally destroyed destroyed and and since since the the mechanical mechanical axial axial push-pull push-pull load load develop de- a slip band that grows at the metal surface the enlarged material surface is exposed to the velop a slip band that grows at the metal surface the enlarged material surface is exposed corrosive environment. Pits now originate easily and lead to micro crack formation. Both to the corrosive environment. Pits now originate easily and lead to micro crack formation. mechanisms result in crack formation that degrades the crack flanks and consequently lead Both mechanisms result in crack formation that degrades the crack flanks and conse- to failure (Figure 21, right and Figure 22, right). Environmentally induced cracks in railway quently lead to failure (Figure 21, right and Figure 22, right). Environmentally induced axels exposed to artificial rainwater tend to initiate from localized corrosion pits for smooth cracks in railway axels exposed to artificial rainwater tend to initiate from localized cor- surface specimens [50], but Hu et al. [50] did not find that corrosion pits are observed at rosion pits for smooth surface specimens [50], but Hu et al. [50] did not find that corrosion the crack nucleation sites for scratched specimens. Therefore, the surface condition also pits are observed at the crack nucleation sites for scratched specimens. Therefore, the surface condition also influences crack nucleation with or without pitting. Intergranular

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crackinginﬂuences was crackstated nucleation for both conditions with or without [50], while pitting. the Intergranularduplex steel also cracking shows was transgran- stated for ularboth cracking. conditions [50], while the duplex steel also shows transgranular cracking.

FigureFigure 22. 22.SchematicSchematic failure failure routes routes during during corrosion corrosion fatigue. fatigue. Left: Left: Pitting Pitting results results in depassivation in depassivation andand crack crack formation. formation. Middle: Middle: Depassivation Depassivation leads leads to pitting to pitting and crack and crackformation. formation. Right: Crack Right: Crack formationformation and and degradation degradation of the of the microstr microstructureucture within within the the crack crack flank ﬂank region. region.

5. Conclusions5. Conclusions ThisThis paper paper comprises andand compares compares data da ofta previously of previously published published work: [ 9,15work:,17,35 , [9,15,17,35,36,38,52,53,79,89–9336,38,52,53,79,89–93,97–100,103,97–100,103–106,108,109,113,115–117.–106,108,109,113,115–117]. TheThe corrosion corrosion and and corrosion corrosion fatigue fatigue behavior ofof AISIAISI 420 420 (X20Cr13, (X20Cr13, 1.4021), 1.4021), AISI AISI 420C 420C(X46Cr13, (X46Cr13, 1.4043), 1.4043), No AISI No (X35CrMo17,AISI (X35CrMo17, 1.4122), 1.4122), AISI 630 AISI (X5CrNiCuNb 630 (X5CrNiCuNb 16-4, 1.4542), 16-4, and 1.4542),AISI A182and AISI F51 (329LN))A182 F51 SAF(329LN)) 2205 (X2SAF CrNiMoN 2205 (X2 CrNiMoN 22-5-3 (UNS 22-5-3 S31803) (UNS 1.4462) S31803) were 1.4462) tested in the laboratory CO2-saturated CCS environment of the onshore saline aquifer (Stuttgart were tested in the laboratory CO2-saturated CCS environment of the onshore saline aqui- Aquifer and Northern German Basin). fer (Stuttgart Aquifer and Northern German Basin). 5.1. Static Corrosion 5.1. Static Corrosion 1. Corrosion products: The continuous corrosion scale and pits of all steels mainly 1. Corrosion products: The continuous corrosion scale and pits of all steels mainly com- comprises of FeCO3 and FeOOH at ambient pressure as well as 100 bar. X5CrNiCuNb prises16-4 of shows FeCO an3 and unusual FeOOH non-uniform at ambient ellipsoidal pressure as corrosion well as layer100 bar. additionally X5CrNiCuNb containing 16- 4 shows an unusual non-uniform ellipsoidal corrosion layer additionally containing Fe3C and in the center Fe2O3 and FeSO4. 2. Fe3InfluenceC and in the of pressure:center Fe2 AfterO3 and 8000 FeSO h4 of. exposure, corrosion rates are generally much 2. Influencelower atof 100pressure: bar than After at ambient8000 h of pressure exposure, (depending corrosion onrates the are atmosphere: generally much (Vapor lowerorliquid) at 100 bar by athan factor at ambient of ca. 10–80 pressure for 42CrMo4, (depending a factor on the of atmosphere: ca. 1.5–30 for (Vapor X46Cr13, or a liquid)factor by of a factor ca. 10–100 of ca. for 10–80 X20Cr13, for 42CrMo4, and a factor a factor of ca.of ca. 10 1.5–30 for X35CrMo17-1. for X46Cr13,(Note a factor that of ca.X5CrNiCuNb16-4 10–100 for X20Cr13, shows and similar a factor surface of ca. corrosion10 for X35CrMo17-1. rates in the liquid(Note that phase). X5CrNi- 3. CuNb16-4Influence shows of atmosphere: similar surface Generally, corrosion the corrosion rates in the rates liquid at ambient phase). pressure are higher 3. Influencein the vaporof atmosphere: phase, at 100 Generally, bar higher the corrosion corrosion rates rates are at found ambient in the pressure liquid phase.are higherIndependent in the vapor of pressure phase, at the 100 higher bar high numberer corrosion of pits was rates found are found for the in liquid the liquid phase. 4. phase.Influence Independent of alloy composition:of pressure the Higher higher chromium number of content pits was increases found surface for the corrosion liquid phase.resistance, while carbon has no significant influence (ambient pressure, vapor phase: 4. Influence0.8 mm/year of alloy for composition: 42CrMo4 (1% Higher Cr), 0.3chromium mm/year content for X46Cr13 increases (13% surface Cr), 0.3 corrosion mm/year resistance,for X20Cr13 while (13% carbon Cr), has 0.1 mm/yearno significant for X35CrMo17-1 influence (ambient (17% Cr), pressure, and 0.01 vapor mm/year phase: for 0.8X5CrNiCuNb16-4 mm/year for 42CrMo4 (16% Cr)).(1% Cr), However, 0.3 mm/year the local for corrosion X46Cr13 behavior(13% Cr), of 0.3 X35CrMo17-1 mm/year forand X20Cr13 X5CrNiCuNb16-4 (13% Cr), 0.1 mm/year compares for to theX35CrMo17-1 less costly X20Cr13(17% Cr), and and X46Cr13. 0.01 mm/year At 100 for bar X5CrNiCuNb16-4and 8000 h of exposure (16% Cr)). with However, no significant the local regard corrosion of atmosphere, behavior the of highest X35CrMo17-1 corrosion andrates X5CrNiCuNb16-4 are 0.01 mm/year compares for 42CrMo4, to the less X20Cr13 costly (liquid X20Cr13 phase) and X46Cr13,X46Cr13. andAt 100 less bar than 0.01 mm/year for X35CrMo4 and X5CrNiCuNb16-4.

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5. Inﬂuence of heat treatment: In general, the shorter austenitizing time and lower austenitizing temperature result in better corrosion resistance regarding surface corrosion, but has no signiﬁcant impact on the number of pit and pit sizes. At 100 bar as well as at ambient pressure and 60 ◦C, hardening and tempering at low temperatures (600 to 670 ◦C) for X20Cr13, X46Cr13, and X5CrNiCuNb16-4 result in the lowest corrosion rates and reveal good resistance against local corrosion. Therefore, the best corrosion resistance is achieved by a continuous martensitic microstructure.

5.2. Corrosion Fatigue In general, the fatigue strength of steels in a CCS environment is reduced and no typical endurance limit of S-N curves exists compared to non-corrosive conditions. Primarily, pitting (pit/local corrosion) driven by the formation of carbonic acid, not the mechanical loading, induces crack formation and propagation and therefore, is most likely the cause for failure. X46Cr13 (air: Endurance limit: 260 MPa). Max. number of cycles to failure is 1.10 × 107 at stress amplitude 173 MPa under CCS conditions. Corrosion fatigue in the passive state is assumingly the main cause for failure. Above Sa = 170 MPa, 90% of the specimen has typical multiple cracks that originate from a central 0.2 mm pit. The initial local corrosion initiates cracks that then condition the inter crystalline corrosion. X5CrNiCuNb16-4 (air: Endurance limit: 620 MPa). Corrosion phenomena are unusual, such as: Elliptic surface pattern, no typical corrosion fatigue limits of S-N plot, high Wöhler- exponent (k = 3.5 and scatter range (TN = 1:34), and very small coefficient of correlation 2 6 (r = 0.33). The max. number of cycles to failure of 10 × 10 at Sa = 150 MPa is even lower 7 in a second experimental set (10 × 10 at Sa = 130 MPa). Local corrosion and inclusions at the fracture surface as well as the inter crystalline corrosion of crack flanks during crack propagation lower the number of cycles to failure independent of: Hardness (335 HV10), aquifer water composition, and microstructure. X2CrNiMoN22-5-3 (air: Endurance limit: 485 MPa): Max. number of cycles for standard duplex stainless steel X2CrNiMoN22-5-3 without insulation of the testing machine 6 6 is 4.2 × 10 at Sa = 290 MPa and 9.2 × 10 at Sa = 240 MPa with insulation. Initially, the crack follows a linear path perpendicular to the axial load followed by a delta-like micro crack structure with an overall curved path with considerable horizontal degradation of the austenitic phase only. Crack initiation may be allocated to both: First, early pit formation results in depassivation and then second, local depassivation results in pit formation. At the critical electrical potential at USHE = −450 to −900 mV at Sa = 275 MPa, the specimen did not fail (107 cycles to failure). However, at low potentials hydrogen evolves to degrading the steel. Polished surface finishes result in a higher corrosion fatigue life expectancy at medium to low stress amplitudes (long exposure time). At low stress amplitudes, local corrosion is likely to initialize the crack growth. Technical surfaces perform better at high stress amplitudes (and shorter exposure). At high stress amplitudes, micro cracks are the reason for crack propagation and failure.

Author Contributions: Conceptualization, A.P. and M.W.; methodology, A.P. and M.W.; software, M.W.; validation, A.P., M.W. and A.K.; formal analysis, A.P. and M.W.; investigation, A.P. and M.W.; resources, A.P., A.K. and M.W.; data curation, A.P. and M.W.; writing—original draft preparation, A.P.; writing—review and editing, A.P.; visualization, A.P.; supervision, A.P. and A.K.; project administration, A.P.; funding acquisition, A.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Processes 2021, 9, 594 29 of 33

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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