materials
Article Passivation Characteristics of Alloy Corrosion-Resistant Steel Cr10Mo1 in Simulating Concrete Pore Solutions: Combination Effects of pH and Chloride
Zhiyong Ai 1,2, Wei Sun 1,2, Jinyang Jiang 1,2,*, Dan Song 1,2,3, Han Ma 4, Jianchun Zhang 4 and Danqian Wang 1,2
1 School of Materials Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, China; [email protected] (Z.A.); [email protected] (W.S.); [email protected] (D.S.); [email protected] (D.W.) 2 Jiangsu Key Laboratory of Construction Materials, Nanjing 211189, Jiangsu, China 3 College of Mechanics and Materials, Hohai University, Nanjing 210098, Jiangsu, China 4 Research Institute of Jiangsu Shasteel Iron and Steel, Zhangjiagang 215625, Jiangsu, China; [email protected] (H.M.); [email protected] (J.Z.) * Correspondence: [email protected]; Tel.: +86-25-5209-1084
Academic Editors: Peter J. Uggowitzer and Daniel J. Blackwood Received: 2 July 2016; Accepted: 29 August 2016; Published: 1 September 2016
Abstract: The electrochemical behaviour for passivation of new alloy corrosion-resistant steel Cr10Mo1 immersed in alkaline solutions with different pH values (13.3, 12.0, 10.5, and 9.0) and chloride contents (0.2 M and 1.0 M), was investigated by various electrochemical techniques: linear polarization resistance, electrochemical impedance spectroscopy and capacitance measurements. The chemical composition and structure of passive films were determined by XPS. The morphological features and surface composition of the immersed steel were evaluated by SEM together with EDS chemical analysis. The results evidence that pH plays an important role in the passivation of the corrosion-resistant steel and the effect is highly dependent upon the chloride contents. In solutions with low chloride (0.2 M), the corrosion-resistant steel has notably enhanced passivity with pH falling from 13.3 to 9.0, but does conversely when in presence of high chloride (1.0 M). The passive film on the corrosion-resistant steel presents a bilayer structure: an outer layer enriched in Fe oxides and hydroxides, and an inner layer, rich in Cr species. The film composition varies with pH values and chloride contents. As the pH drops, more Cr oxides are enriched in the film while Fe oxides gradually decompose. Increasing chloride promotes Cr oxides and Fe oxides to transform into their hydroxides with little protection, and this is more significant at lower pH (10.5 and 9.0). These changes annotate passivation characteristics of the corrosion-resistant steel in the solutions of different electrolyte.
Keywords: passivation; pH; chloride; corrosion-resistant steel; film composition
1. Introduction Most reinforced concrete structures are expected to service for at least 75 years without major repairs [1]. However, this is difficult to achieve, not because of a structural problem but a durability issue. Corrosion of reinforcing steel inside concrete is one of the most important factors that reduce concrete structures durability. In order to minimize or prevent steel corrosion, various methods and techniques [2,3] have been developed and applied, being the most important: concrete cover optimization, electrochemical protection (including cathodic protection, electrochemical realkalization and electrochemical chloride extraction), chemical inhibitors incorporation, epoxy coating on rebar,
Materials 2016, 9, 749; doi:10.3390/ma9090749 www.mdpi.com/journal/materials Materials 2016, 9, 749 2 of 17 reinforcement galvanization, etc. However, these mentioned techniques have drawbacks or limitations, failing to prevent steel corrosion of concrete for enough long time. As one of the most reliable solutions, using “non-corroding” stainless steels is recommended. Some long-term field practice has proved that stainless steel, which provides surpassing chemical resistance, can avoid corrosion problems for a very long time, even in highly aggressive environments [4,5]. However, for their quite high initial costs [6], stainless steels are limited for decades and presently only used in the most critical areas of new structures (such as tidal zones of bridges in maritime regions). Alloy corrosion-resistant steels, which are more resistant to chlorides or carbonation than plain carbon steel, although less corrosion-resistant than stainless steels but much less costly, have aroused great concern. In recent years, China accelerates the development of independent innovational corrosion-resistant steels. Research Institute of Jiangsu Shasteel Iron and Steel designed and prepared a new alloy corrosion-resistant steel, Cr10Mo1, which has been successfully declared for Chinese invention patents [7]. This steel is alloyed with about 10 wt % Cr and 1 wt % Mo, consisting of granular bainite with ferrite between the grains. Laboratory tests showed that Cr10Mo1 steel has a critical chloride threshold level more than 10 times that of carbon steel, and could be well matched with MMFX steel [8,9], which is one of the most significant and typical corrosion-resistant steels [6], developed by MMFX Steel Corporation, Irvine, CA, USA, in 1998. Basically, steel reinforcement embedded in concrete forms a thin oxides layer (5~10 nm) in the strong alkaline conditions of concrete pore solution (pH 13~14), referred to as passive film [10], which acts a protective coat and forbids the metal from corroding. For carbon steel, the growth and formation of this passive film is highly affected by both alkalinity and chloride-contamination of the media, as mentioned in many works [11,12]. However, there are limited systematic studies on the combination effects of pH and chloride on the passivation of alloy corrosion-resistant steel and the film surface chemistry. This work aimed at studying the passive behaviour of Cr10Mo1 alloy corrosion-resistant steel in simulating fresh and carbonated concrete pore solutions with different chloride contents, presenting the combination effects of pH and chloride on the composition and electrochemical characteristics of surface film formed on the steel metal, for a comprehensive understanding of passivation performances of alloy corrosion-resistant steel in different aggressive environments.
2. Experimental Procedures
2.1. Materials
2.1.1. Test Solutions
According to the reports [13], an alkaline solution with 0.03 M Ca(OH)2 (saturated) + 0.2 M KOH + 0.1 M NaOH (pH 13.3), prepared with analytical grade chemicals and Millipore water (18.2 MΩ·cm), was used to simulate the electrolyte in fresh concrete pores. Solutions with pH 12.0, 10.5, and 9.0 were prepared by addition of NaHCO3 powder into the 0.03 M Ca(OH)2 (saturated) solution (pH about 12.5), to simulate the carbonated or less alkaline concrete pore solutions [14], which could have low pH values even close to 9.0 [15]. The pH of the solutions was carefully checked and monitored throughout by a pH meter (Thermo Scientific Orion pH 2100, Thermo Fisher Scientific, Waltham, MA, USA). The aggressive chloride ions supplied by sodium chloride (also analytical grade) were added into the solutions with the molar concentrations of 0.2 and 1.0 M, which are very often used in corrosion research.
2.1.2. Steel Samples Experimental materials were alloy corrosion-resistant HRB400 steel Cr10Mo1, designed by Research Institute of Jiangsu Shasteel Iron and Steel. The chemical composition (in wt %) was 0.01% C, Materials 2016, 9, 749 3 of 17
Materials 2016, 9, 749 3 of 17 0.49% Si, 1.49% Mn, 0.01% P, 0.01% S, 0.06% V, 10.36% Cr, 1.16% Mo, and the residual Fe. Results of OpticalResults Microscopyof Optical Microscopy (OM) observation (OM) observation of the steel of are the shown steel are in shown Figure1 in, which Figure indicates 1, which granularindicates bainitegranular with bainite ferrite with between ferrite thebetween grains the for grains the microstructure for the microstructure of the steel. of the steel. Steel samples of of 10 10 mm mm length length were were cut cut from from ribbed ribbed rebars rebars with with a diameter a diameter of 25 of mm. 25 mm.The Thecross-sections cross-sections of steel of steel samples samples were were mechanically mechanically ground ground with with grades grades 200, 200, 600, 600, 1000 1000 and and 2000 SiC emery papers successively, and polished with alumina paste up to 2.5 µμm gritgrit toto eliminateeliminate thethe heterogeneities ofof thethe steelsteel surface.surface. After polishing,polishing, the samples were degreased with alcohol, rinsed withwith distilleddistilled waterwater andand dried with a stream ofof air just before immersion,immersion, toto ensure theirtheir samesame initial surface states.states. The samples were kept in test solutions for up to 10 days, days, during during which which electrochemical electrochemical responsesresponses werewere recordedrecorded forfor allall thethe electrodes afterafter 6 h, 1 day, 3 days, 7 days and 10 days immersions, toto tracktrack thethe formationformation processprocess ofof passivepassive filmsfilms onon thethe exposedexposed surfacessurfaces [[16,17].16,17].
Figure 1. Microstructure of Cr10Mo1 steelsteel obtainedobtained byby OM.OM.
2.2. Electrochemical Measurements 2.2. Electrochemical Measurements The electrochemical responses of surface films formed on the steel in solutions with different pH The electrochemical responses of surface films formed on the steel in solutions with different pH and chloride contents were monitored by electrochemical tests. The electrochemical tests, including and chloride contents were monitored by electrochemical tests. The electrochemical tests, including linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and capacitance linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and capacitance measurements (Mott–Shottky approach), were performed at room temperature (25 °C) and under measurements (Mott–Shottky approach), were performed at room temperature (25 ◦C) and under natural aeration, in a classical electrochemical cell with three electrodes, where steel sample as the natural aeration, in a classical electrochemical cell with three electrodes, where steel sample as the working electrode was installed with an exposed working area of 1 cm2, the reference electrode was working electrode was installed with an exposed working area of 1 cm2, the reference electrode was a saturated calomel electrode (SCE, all electrode potentials reported in this study were referred to a saturated calomel electrode (SCE, all electrode potentials reported in this study were referred to SCE) SCE) and a platinum counter electrode was also used. and a platinum counter electrode was also used. The LPR measurements were carried out with polarization within ±20 mV from the open-circuit The LPR measurements were carried out with polarization within ±20 mV from the open-circuit potential (OCP) in the anodic direction with a scan rate of 0.1667 mV/s. The EIS response was potential (OCP) in the anodic direction with a scan rate of 0.1667 mV/s. The EIS response was recorded recorded following LPR, in a frequency range from 104 Hz down to 10−2 Hz with the applied AC following LPR, in a frequency range from 104 Hz down to 10−2 Hz with the applied AC amplitude amplitude of 10 mV at OCP. Capacitance measurements [18] were performed at a fixed frequency of of 10 mV at OCP. Capacitance measurements [18] were performed at a fixed frequency of 1000 Hz 1000 Hz (The parameters obtained from Mott–Schottky plots are almost independent when the (The parameters obtained from Mott–Schottky plots are almost independent when the frequency on the frequency on the order of kHz is used, according to our previous trials and some references [19]) and order of kHz is used, according to our previous trials and some references [19]) and a sinusoidal signal a sinusoidal signal of 10 mV, with the polarization applied in successive steps of 50 mV in the cathodic of 10 mV, with the polarization applied in successive steps of 50 mV in the cathodic direction from the direction from the potential +0.25 V to −1.5 V vs. SCE. There were 3 replicates for each specimen. The potential +0.25 V to −1.5 V vs. SCE. There were 3 replicates for each specimen. The equipment used equipment used was a PARSTAT 4000 electrochemical system (Princeton Applied Research Inc., was a PARSTAT 4000 electrochemical system (Princeton Applied Research Inc., Oakridge, TN, USA). Oakridge, TN, USA). 2.3. Surface Analysis 2.3. Surface Analysis Steel samples immersed in the test solutions for 7 d at OCP were withdrawn, rinsed with distilled waterSteel and driedsamples with immersed ethanol, andin the then test kept solutions in a vacuum for 7 dryer.d at OCP were withdrawn, rinsed with distilledThe water chemical and composition dried with andethanol, thickness and then of the kept surface in a vacuum films on thedryer. steel samples were determined by X-rayThe photoelectronchemical composition spectroscopy and (XPS). thickness A PHI of Quantera the surface SXM films X-ray on photoelectron the steel samples spectrometer were (ULVAC-PHIdetermined by Inc., X-ray Chigasaki, photoelectron Japan) spectroscopy equipped with(XPS). a A monochromatic PHI Quantera SXM Al K X-rayα radiation photoelectron source spectrometer (ULVAC-PHI Inc., Chigasaki, Japan) equipped with a monochromatic Al Kα radiation source (1486.6 eV), a hemispherical electron analyser operating at a pass energy of 55 eV and an analytical chamber with a base pressure of 10−7 Pa, was used to collect XPS spectra. The depth profile information was obtained by sputtering the specimens with a scanning argon-ion gun operating at
Materials 2016, 9, 749 4 of 17
(1486.6 eV), a hemispherical electron analyser operating at a pass energy of 55 eV and an analytical chamber with a base pressure of 10−7 Pa, was used to collect XPS spectra. The depth profile information was obtained by sputtering the specimens with a scanning argon-ion gun operating at ion energy of 2 keV. The sputtering rate was estimated to be about 0.055 nm·s−1. The spectra were calibrated by setting the main line for the C 1s signal of adventitious carbon to 284.6 eV. All XPS spectral analysis was performed by the commercial software XPSpeak version 4.1, which contains the Shirley background subtraction and Gaussian–Lorentzian tail function for better spectra fitting. The surface morphologies of the samples were examined by scanning electron microscopy (SEM), using a FEI 3D microscope (FEI, Hillsboro, OA, USA) equipped with energy dispersive spectrometer (EDS) microanalysis hardware, which aims to examine the chemical composition of the surfaces.
3. Results and Discussion
3.1. Surface Film Composition and Structure (XPS Analysis) The survey spectra (not shown) from samples exposed to all media only exhibit signals from the alloy constituents, carbon and oxygen. No traces of components from solution (Na+,K+, Ca2+, and Cl−) were detected. The high resolution XPS spectra of Fe-2p, Cr-2p and O-1s signals were deconvoluted into some chemical states which are most probable components needed for corresponding chemical assignments using a deconvolution software XPSpeak version 4.1, based on the average of the binding energies reported in the Handbook of X-ray photoelectron spectroscopy [20] and previous works [21]. According to the deconvoluting results (Figure2), the Fe 2p 3/2 signal (Figure2a) consists 2+ of four components, including metallic state (Femet, 706.5 eV), Fe in oxide form (FeO, 709.5 eV) and 3+ Fe in oxide (Fe2O3, 710.6 eV) and hydroxide forms (FeOOH/Fe(OH)3, 712.0 eV). For the Cr 2p3/2 spectrum (Figure2b), there presents three constituent peaks which are assigned to Cr met (573.6 eV), Cr2O3 (576.3 eV) and CrOOH/Cr(OH)3 (577.1 eV), respectively. The peak intensity of the Cr2O3 is apparently higher than that of CrOOH/Cr(OH)3, indicating Cr2O3 is the dominant Cr species in the passive film for the steel. The O 1s signal (Figure2c) was fitted using two contributions: one peak at 530.2 eV corresponding to O2− in Fe and Cr oxides, and another one at 531.8 eV corresponding to OH− in hydroxides. The XPS measurements did not detect the presence of Mo in the passive film on Cr10Mo1 steel; therefore, its performance will not be discussed in this paper, although Mo may produce some considerable effects on electronic properties and electrochemical processes of the passive film, as presented in some works [22]. Figures3 and4 display the XPS depth profiles of the surface films formed on the corrosion-resistant steel in all test solutions. As can be observed, Fe-metal and Cr-metal concentrations increase progressively with depth and do dramatically where the oxidation state components almost disappear, as expected. This indicates that the predominant metallic state component comes from the substrate when the surface film is sputtered out. The concentration of oxygen, on the other hand, follows an increase initially (about 1~2 nm), and then decreases significantly approaching the metal surface. The high carbon concentration at the surface of each specimen is mainly due to organic carbon contamination (the absorbed alcohol), as referred in other studies [12]. The percentage composition of Fe-oxidation has a sharp rise at first, then reduces gradually and vanishes. Similarly, the Cr-oxidation component also shows an evolution of increasing initially and decreasing afterwards. However, the cut-off points are not identical: for the Fe species the cut-off point is at about 1 nm, while for the Cr species that is at a greater depth of about 2~3 nm. This important difference reveals that the constituents within passive film of the corrosion-resistant steel vary with depth into the layer: the inner region that is adjacent to the metal substrate is a Cr species concentrated layer, while the outer layer is mostly composed of Fe oxides and hydroxides. If Fe-oxidation or Cr-oxidation content tends to zero, it was considered as an interface where the layer is sputtered out [23]. In this term, the surface films on the steel in solutions with 0.2 M chloride are approximately 5 nm, 5 nm, 6 nm and 6 nm thick Materials 2016, 9, 749 5 of 17 for pH 13.3, 12.0, 10.5, and 9.0, respectively (Figure3), while in solutions with 1.0 M chloride are 5 nm, Materials 2016, 9, 749 5 of 17 5 nm, 4Materials nm and 2016 4, nm9, 749 thick for pH 13.3, 12.0, 10.5, and 9.0, respectively (Figure4). 5 of 17
(a) 4500 (b) 3000 (a) 4500 (b) 3000 4000 4000 Fe O Fe2 O3 2 3 Cr2O3 Cr2O3 3500 3500 FeOOH FeOFeO FeOOH CrOOH CrOOH (Fe(OH) (Fe(OH) )) 3000 33 2500 3000 2500 (Cr(OH) ) (Cr(OH)3) 3
Intensity (cps) Intensity Intensity (cps)
Intensity (cps) Intensity Cr 2500 Fe Intensity (cps) Cr 2500 metFemet metmet
20002000
15001500 20002000 704704 706 706 708 708 710 710 712 712 714 714 716 716 718 570570 572 572 574 574 576 576 578 578 580 580 582 582 584 584 Bingding energy (eV) Binding energy (eV) Bingding energy (eV) Binding energy (eV) 8000 (c)(c)8000 2 O 2 7000 O 7000
6000 6000 OH 5000 OH 5000
4000 4000 Intentsity (cps)
Intentsity (cps) 3000 3000 2000 2000 1000 526 528 530 532 534 536 538 1000 526 528Binding 530 energy 532 (eV) 534 536 538 Binding energy (eV) Figure 2. Deconvolution of the Fe 2p3/2, Cr 2p3/2 and O 1s XPS spectra detected for the passive film Figure 2. Deconvolution of the Fe 2p3/2, Cr 2p3/2 and O 1s XPS spectra detected for the passive film Figureon the 2. steelDeconvolution after 7 d immersion of the Fe in 2p solution3/2, Cr 2p of3/2 pH and 13.3 O with1s XPS 0.2 spectraM Cl−: (adetected) Fe 2p3/2 for; (b )the Cr passive2p3/2; and film on the steel after 7 d immersion in solution of pH 13.3 with 0.2 M Cl−:(a) Fe 2p ;(b) Cr 2p ; on (thec) O steel 1s. after 7 d immersion in solution of pH 13.3 with 0.2 M Cl−: (a) Fe 2p3/2; (b)3/2 Cr 2p3/2; and 3/2 and (c(c)) OO 1s.1s. (a) 80 (b) 80 Fe-metal Fe-oxidation Cr-metal Fe-metal Fe-oxidation Cr-metal 70 (a) 80 70 Cr-oxidation O C (b) 80 Cr-oxidation O C Fe-metal Fe-oxidation Cr-metal Fe-metal Fe-oxidation Cr-metal 70 60 Cr-oxidation O C 6070 Cr-oxidation O C 60 50 5060
50 40 4050
30 30 40 40
20 20 30 30 Atomic concerntration (%) 10 (%) Atomic concerntration 10 20 20 0 0 Atomic concerntration (%) 10 (%) Atomic concerntration 10 012345678910 012345678910 0 Supttered depth (nm) 0 Supttered depth (nm) (c) 80 (d) 80 012345678910 Fe-metal Fe-oxidation Cr-metal 012345678910 Fe-metal Fe-oxidation Cr-metal 70 Cr-oxidationSupttered O depth (nm) C 70 Cr-oxidation Supttered O depth C (nm) (c) 80 (d) 80 60 Fe-metal Fe-oxidation Cr-metal 60 Fe-metal Fe-oxidation Cr-metal 70 70 50 Cr-oxidation O C 50 Cr-oxidation O C 60 60 40 40
50 30 3050
40 20 2040
Atomic concerntration (%) concerntration Atomic 30 10 (%) concerntration Atomic 1030
20 0 200 012345678910 012345678910 Atomic concerntration (%) concerntration Atomic 10 (%) concerntration Atomic 10 Supttered depth (nm) Supttered depth (nm) 0 0 Figure012345678910 3. Composition depth profiles obtained from XPS analysis012345678910 for the surface films on the steel in − solutions of differentSupttered pH with depth 0.2 (nm) M Cl : (a) pH 13.3; (b) pH 12.0; (c) pH 10.5;Supttered and (d depth) pH 9.0.(nm)
Figure 3. Composition depth profiles obtained from XPS analysis for the surface films on the steel in Figure 3. Composition depth profiles obtained from XPS analysis for the surface films on the steel in solutions of different pH with 0.2 M Cl−: (a) pH 13.3; (b) pH 12.0; (c) pH 10.5; and (d) pH 9.0. solutions of different pH with 0.2 M Cl−:(a) pH 13.3; (b) pH 12.0; (c) pH 10.5; and (d) pH 9.0.
Materials 2016, 9, 749 6 of 17 Materials 2016, 9, 749 6 of 17
(a) 80 (b) 80 Fe-metal Fe-oxidation Cr-metal Fe-metal Fe-oxidation Cr-metal 70 Cr-oxidation O C 70 Cr-oxidation O C 60 60
50 50
40 40
30 30
20 20 Atomic concerntration (%) Atomic concerntration(%) 10 10
0 0 012345678910 012345678910 Supttered depth (nm) Supttered depth (nm) (c) 80 (d) 80 Fe-metal Fe-oxidation Cr-metal Fe-metal Fe-oxidation Cr-metal 70 Cr-oxidation O C 70 Cr-oxidation O C 60 60
50 50
40 40
30 30
20 20 Atomic concerntration (%) Atomic concerntration (%) concerntration Atomic 10 10
0 0 012345678910 012345678910 Supttered depth (nm) Supttered depth (nm)
FigureFigure 4. Composition 4. Composition depth depth profiles profiles obtained obtained from XPS analysis analysis for for the the surface surface films films on the on thesteel steel in in solutions of different pH with 1.0 M Cl−: (a) pH 13.3; (b) pH 12.0; (c) pH 10.5; and (d) pH 9.0. solutions of different pH with 1.0 M Cl−:(a) pH 13.3; (b) pH 12.0; (c) pH 10.5; and (d) pH 9.0.
It is noteworthy that, in solutions with 0.2 M Cl−, the Cr-oxidation content as a function of pH − showsIt is noteworthy an increasing that, trend in with solutions pH falling, with indicating 0.2 M Cl the, the Cr species Cr-oxidation have a gradual content enrichment as a function in the of pH showssurface an increasing film. Literatures trend with[24,25] pH attributed falling, indicating this to preferential the Cr species solubility have of aCr/Fe gradual oxides enrichment at different in the surfacepH: film.at high Literatures pH, Cr species [24,25 become] attributed more thissoluble to preferential as chromite solubilityions (CrO2− of and Cr/Fe CrO3 oxides3−), whereas at different Fe 3+ 2− 3− pH:oxides at high are pH, more Cr stable; species at becomelow pH, morethe Cr soluble concentration as chromite within ions the surface (CrO filmand increases CrO3 ),due whereas to the Fe oxidestendency are more for higher stable; stability at low pH,of Cr the oxides Cr3+ togetherconcentration with the withinhigh dissolution the surface of Fe film oxides increases into aqueous due to the tendencysolution for when higher the stability pH drops. of However, Cr oxides the together Cr species with enrichment the high dissolution at low pH cannot of Fe oxidesbe explained into aqueous just in terms of Fe oxides relative decreasing resulted by selective dissolution. It is most probably the solution when the pH drops. However, the Cr species enrichment at low pH cannot be explained consequence of the further formation and growth of Cr oxides, because the Cr-concentrated inner just in terms of Fe oxides relative decreasing resulted by selective dissolution. It is most probably layer in the surface film becomes more thicken when pH drops from 13.3 to 9.0 (Figure 3), which the consequence of the further formation and growth of Cr oxides, because the Cr-concentrated might result from excessive dissolution of Fe and Cr from the substrate. When exposed to less alkaline innermedia, layer Fe in oxides the surface in the outer film layer becomes decompose more grad thickenually when and release pH drops more soluble from 13.3 ions tointo 9.0 solution, (Figure 3), whichpromoting might resultmetallic from Fe beneath excessive the dissolution film to dissolve of Fe into and the Cr film from layer the as substrate. Fe(OH)aq When[26]. Although exposed to lesslower alkaline pH media,facilitates Fe dissolution oxides in theof the outer excessive layer Fe, decompose little new graduallyFe species form and releasein the film more layer soluble for the ions intohigh solution, dissolution promoting rate of metallic Fe oxides Fe at beneath the film/sol the filmution to interface, dissolve so into the theFe-based film layer outer as layer Fe(OH) has aqno [26]. Althoughgrowth, lower as illustrated pH facilitates in Figure dissolution 3. Following of metallic the excessive Fe dissolution, Fe, little Cr new from Fe the species metal also form dissolves in the film layerexcessively for the high and dissolutionproduces new rate Cr of oxides Fe oxides due to at the the tendency film/solution for its very interface, high stability. so the Fe-based The newly outer layerformed has no Cr growth, species asprecipitate illustrated in the in Figureinner layer3. Following and therefore metallic the Fethickness dissolution, of the CrCr-oxides from the layer metal alsoexperiences dissolves excessively an increase. andFreire produces et al. [18] new inve Crstigated oxides the due effect tothe of pH tendency on passive for itsbehaviour very high of AISI stability. The316 newly stainless formed steel Cr in species alkaline precipitate media in the in theabsence inner of layer chloride, and thereforeand recognized the thickness that Cr oxides of the has Cr-oxides an increasing enrichment accompanied by the passive film thickening at lower pH. It should be stated layer experiences an increase. Freire et al. [18] investigated the effect of pH on passive behaviour of that this is also the same case for Cr10Mo1 corrosion-resistant steel, at least if the environment is AISI 316 stainless steel in alkaline media in the absence of chloride, and recognized that Cr oxides moderately contaminated by chloride. When the chloride reaches 1.0 M (Figure 4), there is still a has an increasing enrichment accompanied by the passive film thickening at lower pH. It should be gradual enrichment of the Cr species in the surface film following pH decreasing, however, the film statedexhibits that thisimportant is also changes, the same i.e., case as for pH Cr10Mo1 falls below corrosion-resistant 10.5, the thickness steel, of the at film least suffers if the a environment decline, is moderatelycoming to 4 contaminatednm, in contrast with by chloride. 6 nm thickn Wheness of thethe layer chloride in the reaches presence 1.0 of M0.2 (FigureM Cl−. This4), thereindicates is still a gradual enrichment of the Cr species in the surface film following pH decreasing, however, the film exhibits important changes, i.e., as pH falls below 10.5, the thickness of the film suffers a decline, coming to 4 nm, in contrast with 6 nm thickness of the layer in the presence of 0.2 M Cl−. This indicates Materials 2016, 9, 749 7 of 17 that increasing chloride has some negativity on the passivation of the steel and this effect is more obviousMaterials in low 2016 pH, 9, 749 (10.5 and 9.0) media. 7 of 17 Materials 2016, 9, 749 7 of 17 Figures5 and6 show the composition profiles (for atomic ratios of Fe hy/Feox and Crhy/Crox at that increasing chloride has some negativity on the passivation of the steel and this effect is more various sputtered depths) of the surface films on the corrosion-resistant steel in different test solutions, thatobvious increasing in low pHchloride (10.5 andhas some9.0) media. negativity on the passivation of the steel and this effect is more obtainedobvious from in quantitative low pH (10.5 XPS and analysis9.0) media. according to the peak intensity of components. Generally, with Figures 5 and 6 show the composition profiles (for atomic ratios of Fehy/Feox and Crhy/Crox at increasingvariousFigures sputtered sputtered 5 and depth, depths)6 show the theof contents thecomposition surface of Fefilms profiles and on Cr (forthe hydroxides atomiccorrosion-resistant ratios decrease of Fe hysteel gradually/Feox inand different Cr andhy/Cr disappear oxtest at finally,varioussolutions, indicating sputtered obtained the hydroxides depths)from quantitative of the are surface the XPS predominant analysisfilms on according the components corrosion-resistant to the peak near intensity the steel free in ofsurface differentcomponents. of test the film. solutions, obtained from quantitative XPS analysis according to the peak intensity of components. It canGenerally, be observed with that increasing with the sputtere decreasingd depth, of pH,the contents the Fehy of/Fe Fe oxandratio Cr hydroxides has a continuous decrease rise gradually at the same depthGenerally,and into disappear the film with (Figures finally,increasing 5indicatinga andsputtere6a). thed In depth, fact,hydroxides lessthe contents alkaline are the of media Fepredominant and favours Cr hydroxides Fecomponents hydroxides decrease near formation gradually the free [ 26]. and disappear finally, indicating the hydroxides are the predominant components near the free surface of the film. It can be observed that with the decreasing of pH, the Fehy/Feox ratio has a When the surface film is exposed to lower pH, FeO (Fe3O4) and Fe2O3 decompose gradually, converting surfacecontinuous of the rise film. at the It same can depthbe observed into the that film with (Figure the 5a decreasing and 6a). In of fact, pH, less the alkaline Fehy/Fe mediaox ratio favours has a into porouscontinuous and rise loose at the FeOOH/Fe(OH) same depth into 3the. This film induces(Figure 5a the and Fe-oxides 6a). In fact, layer less alkaline becomes media more favours and more Fe hydroxides formation [26]. When the surface film is exposed to lower pH, FeO (Fe3O4) and Fe2O3 defective.Fe hydroxides From comparison formation [26]. of When the Fe thehy/Fe surfaceox in film Figures is exposed5a and to6 lowera for pH, an identicalFeO (Fe3O pH,4) and increasing Fe2O3 decompose gradually, converting into porous and loose FeOOH/Fe(OH)3. This induces the Fe-oxides chloridedecompose also brings gradually, about converting some rise in ofto the porous Fehy and/Fe looseox ratio FeOOH/Fe(OH) at the same3 depth. This induces into the the film, Fe-oxides indicating layer becomes more and more defective. From comparison of the Fehy/Fe− ox in Figures 5a and 6a for an a similarlayer effect becomes to that more of and decreasing more defective. pH. This From is expected,comparison because of the Fe Clhy/Fefavoursox in Figures adsorbing 5a and 6a on for a an passive identical pH, increasing chloride also brings about some rise of the Fehy/Feox ratio at the same depth − film andidenticalinto thenthe film, pH, occupying indicatingincreasing oxygen a chloride similar vacancies effectalso brings to that by about of taking decreasing some place rise pH. of of Thisthe OH Fe ishy expected,in/Fe theox ratio oxides. because at the Thesame Cl− favours continuous depth − increaseintoadsorbing in the the film, vacancy on indicating a passive production a film similar and induces effect then tooccupying thethat formationof decreasing oxygen of vacancies cavitiespH. This inis by expected, the taking oxides, place because eventually of OHCl favours− in leading the to − the oxidesadsorbingoxides. destruction The on continuous a passive [27]. increase Thus,film and both in then the carbonation occupyingvacancy production andoxygen chloride vacancies induces could theby causeformationtaking damageplace of ofcavities ofOH the in Fe-oxides the oxides.oxides, eventuallyThe continuous leading increase to the inoxides the vacancy destructio productionn [27]. Thus, induces both carbonationthe formation and of chloridecavities incould the layer and therefore reduce its protection, as mentioned in references [26,28]. Similar to the Fehy content oxides,cause damage eventually of the leading Fe-oxides to the layer oxides and destructio thereforen reduce [27]. Thus, its protection, both carbonation as mentioned and chloride in references could evolution,cause Crdamagehy concentration of the Fe-oxides in Crlayer species and therefore also has reduce an increasing its protection, trend as withmentioned pH falling in references (Figures 5b [26,28]. Similar to the Fehy content evolution, Crhy concentration in Cr species also has an− increasing and6b), but the change is dependent on chloride contents. In solutions with 0.2 M Cl , the Cr /Crox [26,28].trend with Similar pH falling to the (Figure Fehy content 5b and evolution, 6b), but the Cr chanhy concentrationge is dependent in Cr on species chloride also contents. has an In increasing solutionshy valuetrend remains with small,pH falling at levels(Figure below5b and 6b), 0.25 but (Figure the chan5b),ge indicatingis dependent most on chlori of Crde contents. oxides maintainIn solutions intact. with 0.2 M Cl−, the Crhy/Crox value remains small, at levels below 0.25 (Figure 5b), indicating most of −− However,withCr oxides with0.2 M maintain ClCl , contentthe Crintact.hy/Cr increasing However,ox value remains towith 1.0 Cl M, small,− content this at ratio levels increasing at below the film to 0.25 1.0 surface (FigureM, this enhances ratio5b), indicatingat the sharply film mostsurface from of 0.24 − to 0.58Crenhances when oxides pH sharplymaintain drops from intact. from 0.24 However, 13.3 to 0.58 to 9.0when with (Figure pH Cl drops content6b), from meaning increasing 13.3 to high 9.0 to (Figure 1.0 chloride M, this 6b), ratio promotesmeaning at the high film the chloride surface substantial formationenhancespromotes of Cr sharplythe hydroxides substantial from 0.24 atformation to low 0.58 pH, when of which Cr pH hydroxides drops are also from at low 13.3low protective topH, 9.0 which (Figure [are29 6b),]. also meaning low protective high chloride [29]. promotes the substantial formation of Cr hydroxides at low pH, which are also low protective [29].
(a) 0.7 (b) 0.3 0.7 0.3 (a) pH 13.3 (b) 0.6 pH 13.3 pH 13.312.0 0.6 pH 13.312.0 pH 12.010.5 0.5 0.2 pH 12.010.5 pH 10.59.0 0.5 pH 10.59.0 pH 9.0 0.2 0.4 pH 9.0 0.4 0.3 0.1 0.3 Atomic ratio Atomic ratio Atomic 0.2 0.1 Atomic ratio Atomic ratio Atomic 0.2 0.1 0.1 0.0 0.0 0.0 0.0 012345 0123456 012345Supttered depth (nm) 0123456Supttered depth (nm) Supttered depth (nm) Supttered depth (nm) Figure 5. Composition depth profiles obtained from XPS analysis for the surface films formed on the Figure 5. Composition depth profiles obtained from XPS analysis for the surface films formed on the Figure 5. Composition depth profiles obtained −from XPS analysis for the surface films formed on the steel in solutions of different pH with 0.2 M Cl−: (a) Fehy/Feox; and (b) Crhy/Crox. steel insteel solutions in solutions of different of different pH pH with with 0.2 0.2 M M Cl Cl−::( (aa)) Fe Fehyhy/Fe/Feox; andox; and(b) Cr (bhy)/Cr Croxhy. /Crox.
(a) 0.7 (b) 0.7 0.7 0.7 (a) pH 13.3 (b) 0.6 0.6 pH 13.3 pH 13.312.0 0.6 0.6 pH 13.312.0 pH 12.010.5 0.5 0.5 pH 12.010.5 pH 10.59.0 0.5 0.5 pH 10.59.0 pH 9.0 0.4 0.4 pH 9.0 0.4 0.4 0.3 0.3 0.3 0.3 Atomic ratio 0.2 ratio Atomic 0.2 Atomic ratio 0.2 ratio Atomic 0.2 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 01234 012345 01234Supttered depth (nm) 012345Supttered depth (nm) Supttered depth (nm) Supttered depth (nm) Figure 6. Composition depth profiles obtained from XPS analysis for the surface films formed on the Figure 6. Composition depth profiles obtained from XPS analysis for the surface films formed on the steel in solutions of different pH with 1.0 M Cl−: (a) Fehy/Feox; and (b) Crhy/Crox. Figure 6. Composition depth profiles obtained− from XPS analysis for the surface films formed on the steel in solutions of different pH with 1.0 M Cl−: (a) Fehy/Feox; and (b) Crhy/Crox. steel in solutions of different pH with 1.0 M Cl :(a) Fehy/Feox; and (b) Crhy/Crox.
Materials 2016, 9, 749 8 of 17
Figure7 presents the growth processes of passive films on the steel in solutions of different pH with chloride contents. The thermodynamic stability of oxides or hydroxides, which is affected by the concentrations of hydroxyl ions and chloride ions [30], determines passive film formation and growth, as confirmed by the results obtained by XPS analysis in the present study. The following mechanism for passive film formation and growth on Cr-series stainless steels in alkaline solutions has been proposed in published works [31,32]. When the alloy is exposed to an alkaline electrolyte, Fe would prefer to dissolve and faster diffuse into the solution and therefore be enriched at the film/solution interface while dilatory dissolving and slower diffusing component like Cr will remains in the region nearer the metal, with little movement. Contacting with OH− predominantly, dissolved Fe/Cr initially forms hydroxides. With increasing passivation time, the hydroxides will dehydrate into oxides and then the oxides layer grows gradually. 2+ − Fe + 2OH → Fe(OH)2↓ (1)
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3 (2)
2Fe(OH)3 → Fe2O3 + 3H2O (3) − − 2Fe(OH)3 + Fe + 2OH → Fe3O4 + 4H2O + 2e (4) 3+ − Cr + 3OH → Cr(OH)3↓ (5) − − Cr(OH)3 + Cr + 3OH → Cr2O3 + 3H2O + 3e (6) The outer Fe-oxides layer grows by the diffusion of metallic ions through micropores in the film and the inner Cr-oxides layer grows by access of OH− to the film/alloy interface through the micropores in the film. The continuously growing oxides layer will block the transport of metallic ions and hydroxyl ions and further slow down the growth of itself. As a result, the passive film is constructed by a Cr oxides and hydroxides concentrated inner layer (grown from alloy matrix) and an outer layer enriched in Fe oxides and hydroxides (formed by a diffusion and precipitation process). It should be noted that in solutions of high pH (13.3), high chloride not more than 1.0 M depresses little the growth and formation of the passive film on the corrosion-resistant steel (Figure7a,b). When in solutions of low pH (9.0), metallic Fe/Cr dissolves and diffuses faster, but the precipitation of the hydroxides and oxides, especially Fe hydroxides and oxides, become more difficult for the diluted OH− concentration. This would allow the excessive dissolution of the Fe and Cr, in another vein, more metallic Fe and Cr dissolve from the metal. Thus, the inner Cr-oxides layer has further growth and becomes more thicken, but the outer Fe-species layer is not in the same case, due to its very high solubility at low pH (Figure7c). However, in the presence of 1.0 M chloride, the inner Cr-oxides layer has no growth as in the presence of 0.2 M chloride, but decreases dramatically in the layer thickness, together with further reduction of the Fe-species outer layer (Figure7d), due to the standard free energies of Fe and Cr oxides in more severe environments are substantially enhanced [30].
Figure 7. Cont. Materials 2016, 9, 749 9 of 17
Figure 7. Schematic illustration of the growth processes of passive films on alloy corrosion-resistant steel Cr10Mo1 in solutions with different pH and Cl− contents: (a) with pH 13.3 and 0.2 M Cl−;(b) with pH 13.3 and 1.0 M Cl−;(c) with pH 9.0 and 0.2 M Cl−; and (d) with pH 9.0 and 1.0 M Cl−.
3.2. Electrochemical Results
3.2.1. Capacitance Measurements (Mott–Schottky Plots) The semi-conductive behaviour and electronic properties of the passive oxides film can be assessed by measuring the capacitance of the electrode/electrolyte interface. When the oxides film is in contact with an electrolyte, the capacitance of the electrode/electrolyte interface (C) can be described by the capacitance of space charge layer (Csc) and the capacitance of the Helmholtz layer (CH) as two condensators in series, neglecting the Guoy–Chapman diffuse layer and surface states contribution for the total capacitance [33]. When the used frequency is high enough (on the order of kHz), the measured C is considered to be approximately equal to the Csc, for the CH contribution could be negligible [34]. Assuming that space charge layer of the semiconductor is under depletion conditions, the determined capacitance of electrode/electrolyte interface, C, and the applied potential, E, can be described by Mott–Schottky relationship: for n-type semiconductor −2 2 KT C = E − EFB − (7) εε0eNd e for p-type semiconductor −2 2 KT C = − E − EFB + (8) εε0eNa e where ε is the dielectric constant of the semiconductor (usually taken as 15.6 [33,35] for the oxides −14 −1 films formed on alloys), ε0 is the vacuum permittivity (ε0 = 8.85 × 10 F·cm ), Nd and Na are the donor density and acceptor density, respectively, e is the elementary charge (e = 1.602 × 10−19 C), k is −23 −1 the Boltzmann constant (k = 1.38 × 10 J·K ), T is the absolute temperature, and EFB the flat band potential. The KT/e term can be neglected as it is only about 25 mV at room temperature. The carrier density can be calculated from the slope of the experimental C−2 versus E plot (a negative slope is for a p-type semiconductor response inversely proportional to the acceptor density Na, while a positive slope for a n-type semiconductor also inversely proportional to the donor density Nd). Figure8 displays the Mott–Schottky plots recorded for the passive films formed on the steel after 7 d immersion in all test solutions. It can be observed that in the Mott–Schottky plots two linear regions are presented, indicating the passive film formed on the steel exhibits both n-type (positive slope) and p-type (negative slope) semiconducting characteristics irrespective of the exposure conditions. The duplex semiconductor character of the passive film is related to its chemical composition. Its n-type semiconductor behaviour can be attributed to Fe oxides and hydroxides, and p-type semiconductor behaviour to Cr species, according to previous investigations [33,35]. Materials 2016, 9, 749 10 of 17 Materials 2016, 9, 749 10 of 17
2 (a) 4 (b) pH 13.3 pH 13.3 pH 12.0 pH 12.0 pH 10.5 3 pH 10.5
pH 9.0 ) ) 4
4 pH 9.0 cm cm -2 -2 F F 2 1
9 9 (10 (10 -2 -2 C C 1
0 0 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 E (V vs. SCE) E (V vs. SCE)
FigureFigure 8.8. Mott-ShottkyMott-Shottky plotsplots forfor passivepassive filmsfilms formedformed onon thethe steelsteel inin solutionssolutions withwith differentdifferent pHpH andand − ClCl− contentscontents at at 7 7 d d immersion: immersion: ( (aa)) 0.2 0.2 M; M; and and ( (bb)) 1.0 1.0 M. M.
In Table 1, the carrier concentrations (donor and acceptor species, Nd and Na, respectively) for In Table1, the carrier concentrations (donor and acceptor species, N d and Na, respectively) for the semiconductor passive films are listed. In solutions of 0.2 M Cl−, the steel has declined carrier the semiconductor passive films are listed. In solutions of 0.2 M Cl−, the steel has declined carrier concentrations as pH varies from 13.3 to 9.0, meaning the film behaves as a semiconductor with concentrations as pH varies from 13.3 to 9.0, meaning the film behaves as a semiconductor with poorer poorer and poorer electrical conductivity. This evolution indicates a more effective corrosion and poorer electrical conductivity. This evolution indicates a more effective corrosion protection of protection of the surface film at lower pH. According to the point defect model (PDM) proposed by the surface film at lower pH. According to the point defect model (PDM) proposed by Macdonald Macdonald and co-workers [36], the passive film contains a number of point defects, such as oxygen and co-workers [36], the passive film contains a number of point defects, such as oxygen and/or and/or cation vacancies, which act as donors and acceptors, respectively. Carbonation promotes the cation vacancies, which act as donors and acceptors, respectively. Carbonation promotes the excessive excessive dissolution of Fe and Cr from the metal and results in more Fe and Cr species formed in dissolution of Fe and Cr from the metal and results in more Fe and Cr species formed in the film, the film, which have decreased oxygen and cation vacancy concentrations. However, in test solutions which have decreased oxygen and cation vacancy concentrations. However, in test solutions with − with 1.0− M Cl , this trend is reversed. Na and Nd of the steel have far higher values at pH 9.0 than that 1.0 M Cl , this trend is reversed. Na and N of the steel have far higher values at pH 9.0 than that at at pH 13.3. This substantial change in carrierd concentrations corresponds to non-stoichiometry defects pH 13.3. This substantial change in carrier concentrations corresponds to non-stoichiometry defects in in the space charge region or disordered character of the surface film [37]. Thus, the surface film the space charge region or disordered character of the surface film [37]. Thus, the surface film becomes becomes more and more defective with pH decreasing, and this is associated with the gradual more and more defective with pH decreasing, and this is associated with the gradual destruction of destruction of the protective oxides, as shown by XPS measurements. the protective oxides, as shown by XPS measurements.
Table 1. Effect of pH values and chloride contents on semiconducting properties of passive films Table 1. Effect of pH values and chloride contents on semiconducting properties of passive films formed on the steel. formed on the steel. 0.2 M 1.0 M pH 20 −3 0.2 M20 −3 20 −3 1.0 M20 −3 pH Nd (10 cm ) Na (10 cm )Nd (10 cm )Na (10 cm ) 20 −3 20 −3 20 −3 20 −3 13.3 N29.65d (10 cm )N36.53a (10 cm )N49.11d (10 cm )N41.26a (10 cm ) 13.312.0 18.21 29.65 19.72 36.53 45.90 49.1143.47 41.26 12.010.5 16.41 18.21 14.61 19.72 59.51 45.9068.28 43.47 10.59.0 15.98 16.41 12.54 14.61 63.26 59.5178.02 68.28 9.0 15.98 12.54 63.26 78.02
3.2.2. Linear Polarization Resistance 3.2.2. Linear Polarization Resistance Linear polarization resistance (LPR) monitoring is a non-destructive technique for measuring Linear polarization resistance (LPR) monitoring is a non-destructive technique for measuring the the corrosion rate of reinforcing steel and accurately evaluating its condition, which has been corrosion rate of reinforcing steel and accurately evaluating its condition, which has been discussed in discussed in detail in many works [38]. From LPR tests, the corrosion potential, Ecorr, which directly detail in many works [38]. From LPR tests, the corrosion potential, Ecorr, which directly indicates the indicates the corrosion state, and polarization resistance, Rp, which is related to the corrosion rate of corrosion state, and polarization resistance, R , which is related to the corrosion rate of a corrosion a corrosion process, can be obtained directly byp the built-in fitting software. process, can be obtained directly by the built-in fitting software. The Ecorr and Rp values of the steel in all test solutions against time (6 h, 1 day, 3 days, 7 days, The Ecorr and Rp values of the steel in all test solutions against time (6 h, 1 day, 3 days, 7 days, and and 10 days) are presented in Figures 9 and 10. The results show that both Ecorr and Rp values are 10 days) are presented in Figures9 and 10. The results show that both E corr and Rp values are affected affected by the exposition conditions. In solutions with 0.2 M Cl−, Ecorr values of the steel get by the exposition conditions. In solutions with 0.2 M Cl−,E values of the steel get continuous continuous increasing and reach above −200 mV after 3 d immersion,corr with variations from −200 to increasing and reach above −200 mV after 3 d immersion, with variations from −200 to −150 mV −150 mV vs. SCE, indicating complete passivity of the steel at all the pH values [39]. However, when vs. SCE, indicating complete passivity of the steel at all the pH values [39]. However, when the Cl− the Cl− content rises to 1.0 M, high Ecorr values above −200 mV could stay up only when at pH 13.3 and 12.0. At pH below 10.5, the Ecorr values have no significant change from the beginning of
Materials 2016, 9, 749 11 of 17 content rises to 1.0 M, high E values above −200 mV could stay up only when at pH 13.3 and 12.0. Materials 2016, 9, 749 corr 11 of 17 At pH below 10.5, the Ecorr values have no significant change from the beginning of immersion, and showimmersion, slight riseand andshow fall slight over theriseimmersion and fall over time, the but immersion remains below time, −but350 remains mV, suggesting below −350 difficult mV, passivatingsuggesting difficult of the steel. passivating of the steel. p AtAt earlyearly immersion,immersion, thethe RRpp is relatively small,small, andand thenthen increasesincreases graduallygradually andand tendstends toto remainremain stablestable after 7 d d immersion. immersion. This This behaviour behaviour should should be be attributed attributed to tothe the fact fact that that at the at the beginning beginning of the of p theimmersion, immersion, the surface the surface of the of steel the samp steelle sample is active is activeand consequently and consequently the Rp is the low, R pbutis low,as the but exposure as the exposuretime increasing, time increasing, the surface the of surface the steel of the sample steel sampleis covered is covered by a corrosion by a corrosion products products film, film,and p andconsequently consequently the R thep enhances Rp enhances and gets and to gets stable to stablefeature feature until basically until basically maturemature passive passive film is formed film is formed(after processing (after processing for 7 days). for 7Certainly, days). Certainly, this trend this is trendin cases is inexcept cases for except low pH for (10.5 low pH and (10.5 9.0) andwith 9.0) 1.0 − − − p − withM Cl 1.0−. Generally, M Cl . Generally, at the same at pH, the sameRp values pH, obtained Rp values in obtained 0.2 M Cl in− condition 0.2 M Cl arecondition higher than are higherthat in − − p thansolutions that incontaining solutions 1.0 containing M Cl−. The 1.0 Mhigher Cl . the The R higherp value, the the R pstrongervalue, the the stronger corrosion the prevention corrosion preventioncapability of capability the surface of the film surface [40]. filmSo chloride [40]. So chloridereduces reducesthe passivity the passivity for the forsteel the and steel this and is thismore is p moresignificant significant at low at pH. low pH.It is Itnoteworthy is noteworthy that that the the RpR hasp has different different evolution evolution with with the the pH pH varying, − − p dependingdepending onon thethe chloridechloride contents. contents. In In solutions solutions with with 0.2 0.2 M M Cl Cl−,R, Rp valuesvalues have marked raise at lower pH,pH, andand almostalmost showshow anan incrementincrement ofof 1/41/4 when pH drops from 13.3 to 9.0.9.0. However, withwith exposureexposure p p toto highhigh chloridechloride (1.0 M), M), R Rpp decreasesdecreases prominently prominently with with pH pH.. When When pH pH isis below below 10.5, 10.5, Rp R fluctuatesp fluctuates in 2 2 inthe the range range of of 5~20 5~20 kΩ kΩ·cm·cm2, which, which are are very very low low values, values, with with little little enhancement enhancement even ifif somewhatsomewhat declinedecline afterafter 77 dd immersion, revealingrevealing thethe steelsteel hardlyhardly passivates forfor the surface filmsfilms are very electric conductive,conductive, inin consistentconsistent withwith Mott–SchottkyMott–Schottky plotsplots analysis.analysis.
-100 (a) -100 450k (a) (b) 450k
400k -150 350k )
2 350k ) 2 cm cm
-200 300k (
-200 pH 13.3 300k ( p mV vs. SCE) pH 13.3 p
mV vs. SCE) pH 13.3 ( pH 12.0 R pH 13.3 ( pH 12.0 R pH 12.0 corr pH 10.5 250k pH 12.0 corr
E pH 10.5 250k
E pH 9.0 pH 10.5 -250 pH 9.0 pH 10.5 pH 9.0 200k
-300 150k -300 0 50 100 150 200 250 150k 0 50 100 150 200 250 0 50 100 150 200 250 Immersion time (h) Immersion time (h) Immersion time (h)
Figure 9. The corrosion potential (Ecorr) and polarization resistance (Rp) of the steel as a function of FigureFigure 9.9. TheThe corrosioncorrosion potentialpotential (E(Ecorrcorr)) and polarization resistance (Rp)) of of the steel as aa functionfunction ofof − time in solutions of different pH with 0.2 M Cl−−: (a) Ecorr; and (b) Rp. timetime inin solutionssolutions ofof differentdifferent pHpH withwith 0.20.2 MM ClCl ::( (aa))E Ecorrcorr; and; and (b (b) )RRp.p .
(a) -150 350k (a) -150 (b) 350k 300k -200 250k
) 200k pH 13.3
-250 2 pH 13.3 ) 200k pH 13.3 -250 pH 13.3 2 pH 12.0
cm pH 12.0
pH 12.0 cm 150k pH 10.5 pH 12.0 150k pH 10.5 (
pH 10.5 -300 ( V vs. SCE) pH 9.0 pH 10.5 p -300
V vs. SCE) pH 9.0 p
pH 9.0 R
pH 9.0 R 25k (m 25k (m
corr -350 20k
corr -350 E 20k E 15k -400 10k 5k -450 0 -450 00 50 100 150 200 250 0 50 100 150 200 250 0 50 100 150 200 250 Immersion time (h) Immersion time (h) Immersion time (h)
Figure 10. The corrosion potential (Ecorr) and polarization resistance (Rp) of the steel as a function of Figure 10. The corrosion potential (Ecorr) and polarization resistance (Rp) of the steel as a function of Figure 10. The corrosion potential (Ecorr) and polarization− resistance (Rp) of the steel as a function of time in solutions of different pH with 1.0 M Cl−: (a) Ecorr; and (b) Rp. time in solutions of different pH with 1.0 M Cl−: (a) Ecorr; and (b) Rp. time in solutions of different pH with 1.0 M Cl :(a)Ecorr; and (b)Rp.
3.2.3. Electrochemical Impedance Spectroscopy Figures 11 and 12 show the EIS spectra in Nyquist and Bode forms obtained for the − corrosion-resistant steel after 7 d immersion in solutions of pH from 13.3 to 9.0 with different Cl−
Materials 2016, 9, 749 12 of 17
3.2.3. Electrochemical Impedance Spectroscopy
MaterialsFigures 2016 , 119, 749 and 12 show the EIS spectra in Nyquist and Bode forms obtained12 for of 17 the corrosion-resistant steel after 7 d immersion in solutions of pH from 13.3 to 9.0 with different Cl− − contents.contents. It It is is evident evident that that the the EIS EIS spectra spectra profilesprofiles ofof thethe steel with the the pH pH varying varying for for the the two two Cl Cl− − − contentscontents are are materially materially different. different. In In solutions solutions with with 0.20.2 MM Cl−, ,the the steel steel shows shows capacitive-like capacitive-like behaviour behaviour 2 withwith the the maximum maximum phase phase angles angles close close to to− −9090°◦ andand high values of of |Z| |Z| above above 300 300 kΩ kΩ·cm·cm2 in2 inthe the region region of low frequencies, with regard to the Bode plots, suggesting that the passive films formed on the of lowof low frequencies, frequencies, with with regard regard to theto the Bode Bode plots, plot suggestings, suggesting that that the the passive passive films films formed formed on on the the steel steel at all pH offer high corrosion resistance. It is noted that the impedance response is increasing at allsteel pH at offer all pH high offer corrosion high corrosion resistance. resistance. It is noted It is that noted the that impedance the impedance response response is increasing is increasing following following the pH dropping, and the capacitive arc radius and overall impedance are even larger at the pH dropping, and the capacitive arc radius and overall impedance are even larger at pH 9.0, pH 9.0, indicating the film formed at lower pH exhibits more protective behaviour. In contrast, when indicating the film formed at lower pH exhibits more protective behaviour. In contrast, when chloride chloride rises to 1.0 M, the capacitive arc radius markedly decreases as the pH does. The overall rises to 1.0 M, the capacitive arc radius markedly decreases as the pH does. The overall impedance impedance has very low values in the order of 10~20 kΩ·cm2 when the pH falls below 10.5, signifying has very low values in the order of 10~20 kΩ·cm2 when the pH falls below 10.5, signifying that the that the steel hardly passivates in low pH media. These results are in good agreement with the steelMott–Schottky hardly passivates approach in low and pH LPR media. observation. These results Indeed are, inthis good change agreement is ascribed with theto the Mott–Schottky chemical approachcomposition and LPR evolution observation. of the surface Indeed, film this formed change on is the ascribed steel with to the pH chemical and Cl− compositioncontents varying, evolution as − ofmentioned the surface above. film formed on the steel with pH and Cl contents varying, as mentioned above.
400k (a) 6 106 -90 pH 13.3 (b) 10 -90 350k pH 12.0 pH 12.0 -80 pH 10.5 5 300k 105 pH 9.0 10 -70 ) ) 2
2 250k 250k Fitting -60 ) ) cm 4 2
cm 4 2 10
-50 200k cm -50 ( 200k cm
( im im ( Z pH 13.3 ( Z
-40 - 3 pH 13.3 -40 - 150k 3 Z
Z 10 10 pH 12.0 pH 12.0 pH 10.5 -30 100k pH 10.5 pH 9.0 2 -20 10 (degrees) angle Phase 50k 10 (degrees) angle Phase 50k Fitting -10
0 1 0 50k 100k 150k 200k 101 0 -2-2 -1-1 0 1 2 3 4 2 10 10 10 10 10 10 10 Zre ( cm ) re Frequency (Hz) Frequency (Hz) Figure 11. Measured EIS (in Nyquist and Bode forms) of the steel in solutions of different pH with Figure 11. Measured EIS (in Nyquist and Bode forms) of the steel in solutions of different pH with − 0.20.2 M M Cl Clafter− after 7 7 d d immersion: immersion: (a) Nyquist plots; plots; and and (b (b) )Bode Bode plots. plots.
300k 106 -90 (a) (b) 10 -90 pH 13.3 pH 12.0 -80 250k pH 12.0 pH 10.5 105 pH 10.5 10 -70 pH 9.0
) 200k -60 ) 200k 2 -60 2 ) 4 ) Fitting 2 4 Fitting 2 10 cm 10 cm -50 15k cm -50 cm 15k
( 150k
( 150k
( im (
-40 im ) 2 3 -40 ) Z
2 3 10k Z Z - Z
10k 10 - cm
10 cm
100k
pH 13.3 ( 100k -30
pH 13.3 ( -30
im im 5k pH 12.0 -Z 5k pH 12.0 -Z 2 pH 10.5 -20 10 pH 10.5 (degrees) angle Phase 50k 10 (degrees) angle Phase 0 pH 9.0 00 2k 4k 6k 8k 10k 12k 14k 16k 18k 20k pH 9.0 -10 0 2k 4k 6k 8k 10k 12k2 14k 16k 18k 20k -10 Z cm2 re ( ) Fitting Zre (cm ) Fitting 0 1 0 10 0 0 50k 100k 150k 200k -2-2 -1-1 0 1 2 3 4 2 10 10 10 10 10 10 10 Z ( cm2) Zre ( cm ) Frequency (Hz) Figure 12. Measured EIS (in Nyquist and Bode forms) of the steel in solutions of different pH with FigureFigure 12. 12.Measured Measured EIS EIS (in (in Nyquist Nyquist andand BodeBode forms) of the the steel steel in in solutions solutions of of different different pH pH with with 1.0 M Cl− after 7 d immersion: (a) Nyquist plots; and (b) Bode plots. 1.01.0 M M Cl −Clafter after 7 7 d d immersion: immersion: ((aa)) NyquistNyquist plots; plots; and and (b (b) )Bode Bode plots. plots.
The Zsimpwin program was used to fit the EIS data. Based on some trials and references [18,23], theThe equivalent Zsimpwin circuit program depicted was usedin Figure to fit the11a EISwas data. adopted Based to on fit some the experimental trials and references data, which [18,23 ], theprovided equivalent a right circuit fitting depicted with errors in Figure within 11a 10%. was The adopted constant to fit phase the experimental element (CPE) data, is used which to descript provided a rightthe frequency fitting with dispersion errors withinbehaviour 10%. of Thenon-ideal constant capacitors phase with element its impedance (CPE) is used (ZCPE to) defined descript by the frequencyEquation dispersion (9) behaviour of non-ideal capacitors with its impedance (ZCPE) defined by Equation (9) 1 Z = (9) Z = (9) Y jw
Materials 2016, 9, 749 13 of 17
1 ZCPE = n (9) Y0(jw) where Y0 is the CPE electrical constant admittance, ω is the angular frequency (in rad/s), j is the imaginary number (j2 = −1) and n is the CPE exponent, as an adjustable parameter affected by non-homogeneities and roughness of the surfaces, that always lies from 0 to 1. For the meaning of the circuit elements in this circuit model, the following physical interpretation is adopted [18,23]: the resistance connected in series with the two time constants corresponds to the ohmic resistance of the solution (Rsol), which changes with ion concentrations of the test solution. The high frequency time constant (R1, CPE1) can be attributed to the charge transfer processes in the active surface areas (film defects/pores) and it is represented by the charge transfer resistance (R1) coupled with the double layer capacitance (simulated by CPE1). The low frequency time constant (R2, CPE2) was assigned to the redox processes taking place in the areas covered with the passive film (protective oxides) and it is composed of the passive layer resistance (R2) and the passive film capacitance (CPE2). Table2 presents the fitting parameters obtained from the experimental EIS spectra of the steel in − all test solutions at 7 d immersion time. It can be observed that in presence of 0.2 M Cl , both R1 and R2 have the highest values at pH 9.0. This is obviously ascribed to the further growth and formation of protective Cr oxides on the metal when pH decreasing, which provides higher resistance to the corrosion processes as proposed in literatures [18]. The admittance evolution of CPE2 agrees well −5 −5 −1 −2 n with that of R2. It varies in the range 2.1 × 10 ~1.7 × 10 Ω ·cm ·s as the pH drops from 13.3 to 9.0, reflecting the passive film capacitance behaviour are somewhat more significant at lower pH. However, the CPE1 values show an upward tendency as the media becomes less alkaline, suggesting that the dispersion effect of the double layer capacitance becomes more highlighted. This reveals the overall film surface becomes rougher and more heterogeneous, for more Fe hydroxides with porous and loose structure form in the outer layer at lower pH. Even so, the film formed on the steel at lower pH has greater protection, as evidenced by the higher R1 and R2 values, in agree well with the LPR − and Mott–Schottky analysis results. On the contrary, in the presence of 1.0 M Cl ,R1 and R2 suffer an important drop as pH, and exhibit extremely low values less than 10 kΩ·cm2 at pH 9.0, suggesting the very fast electrochemical corrosion process. Certainly, this is related with the formation of a surface film with more defects. CPE1 increases about one order of magnitude and CPE2 exhibits a similar trend when pH below 10.5, indicating the steel more approachable to accomplish depolarization [41]. All these signify that the film becomes less and less protective owing to its deterioration.
Table 2. Best fitting parameters for the experimental EIS of the steel in test solutions with different pH and Cl− contents after 7 d immersion.
− 2 2 CPE1 2 CPE2 Cl Contents (M) pH Rsol (Ω·cm ) R1 (Ω·cm ) R2 (Ω·cm ) −1 −2 n −1 −2 n Y0 (Ω ·cm ·s ) n Y0 (Ω ·cm ·s ) n 13.3 18.2 3.54 × 105 2.47 × 10−5 0.92 11.37 × 105 2.07 × 10−5 0.82 12.0 121.2 4.93 × 105 2.54 × 10−5 0.91 13.23 × 105 1.94 × 10−5 0.82 0.2 10.5 106.3 6.84 × 105 2.75 × 10−5 0.90 15.62 × 105 1.85 × 10−5 0.80 9.0 96.8 7.31 × 105 2.87 × 10−5 0.89 17.36 × 105 1.73 × 10−5 0.80 13.3 11.8 2.96 × 105 2.58 × 10−5 0.91 8.85 × 105 2.23 × 10−5 0.81 12.0 35.7 2.31 × 105 2.91 × 10−5 0.90 6.13 × 105 2.07 × 10−5 0.81 1.0 10.5 30.4 1.06 × 104 2.12 × 10−4 0.81 1.89 × 104 1.63 × 10−4 0.59 9.0 23.2 3.21 × 103 2.93 × 10−4 0.79 9.76 × 103 2.41 × 10−4 0.53
3.3. Surface Morphology Figure 13 presents several representative images of the steel samples in different exposure conditions. It is observable that, in presence of 0.2 M Cl−, the SEM images show a clean and bright steel surface for all the pH values, with almost no spots. When exposed to 1.0 M Cl−, the steel has the same case as above for pH 13.3, but for low pH values (pH 9.0), some pits rendering black Materials 2016, 9, 749 14 of 17 dish-like holes were observed on the metal surface, and different attack morphologies could also be found. Selecting typical black dish-like holes as areas (e.g., area A) for EDS chemical analysis. TheMaterials results 2016 reveal, 9, 749that the composition of chemical species inside the area A includes about14 of 17 iron (36.0%), calcium (19.4%), chromium (1.1%), oxygen (18.6%), and also small amounts of other inclusion (19.4%), chromium (1.1%), oxygen (18.6%), and also small amounts of other inclusion elements from elements from the metal (The considerable amounts of Al (6.61%) may originate from alumina paste the metal (The considerable amounts of Al (6.61%) may originate from alumina paste adhering to the adheringsurface to of the steel surface when of steelthe steel when is thepolished). steel is polished).This indicates This some indicates corrosion some products corrosion (as products Fe/Cr (as Fe/Crhydroxides) hydroxides) and andcalcium calcium hydroxide hydroxide crystals crystals (from (from the thesolution) solution) attached attached into intothe pitting the pitting hole, hole, suggestingsuggesting the the area area has has suffered suffered some some corrosion corrosion degree.degree.
− FigureFigure 13. 13.SEM SEM images images of of the the steel steel surfaces surfacesimmersed immersed for 7 7 d d in in solutions solutions with with different different pH pH and and Cl Cl− contents, and EDS spectra registered on the characteristic areas of SEM images. contents, and EDS spectra registered on the characteristic areas of SEM images. 4. Conclusions 4. Conclusions The passive behaviour of alloy corrosion-resistant steel Cr10Mo1 in simulating concrete pore solutionsThe passive with behaviourdifferent pH of values alloy (from corrosion-resistant 13.3 to 9.0) and steel chloride Cr10Mo1 contents in simulating(0.2 M and 1.0 concrete M) was pore solutionsinvestigated. with different Analytical pH and values electrochemical (from 13.3 results to 9.0) proved and chloridethat the exposure contents conditions (0.2 M and modify 1.0M) the was investigated.chemical composition Analytical and electrochemi electrochemicalcal responses results proved of the surface that the film. exposure conditions modify the chemicalSurface composition composition and electrochemical analysis performed responses by XPS of re thevealed surface that film.the passive film formed on the corrosion-resistant steel consists of Fe and Cr oxides/hydroxides, which presents in two layers with
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Surface composition analysis performed by XPS revealed that the passive film formed on the corrosion-resistant steel consists of Fe and Cr oxides/hydroxides, which presents in two layers with the outer layer mainly composed of Fe oxides and hydroxides, and the inner one enriched with Cr species. In presence of 0.2 M chloride, as the pH drops, Fe oxides in the outer layer become more soluble but Cr oxides in the inner layer maintain good stability and have further growth, on account of excessive dissolution of metallic Fe and Cr from the substrate promoted by lower pH. However, in presence of 1.0 M chloride, both the Fe-oxides outer layer and Cr-oxides inner layer suffer a significant decrease in thickness, and substantial Fe and Cr hydroxides form in the surface film when pH drops below 10.5, indicating high chloride produce much negative effect on the passive film formation of the steel in low pH (10.5 and 9.0) media. Mott–Schottky analysis suggests the passive film performs n-type and p-type semiconductor behaviour related to its bilayer structure composed of Fe and Cr species. In presence of 0.2 M chloride, the surface film has poorer electrical conductivity with pH dropping mainly due to Cr oxides with excellent protection get enriched in the film. However, high chloride (1.0 M) facilitates Cr oxides to convert into their hydroxides which are defective, and the case is reversed: the lower the pH value, the more electric conductive the film. LRP and EIS tests evidence that, in solutions with relatively moderate chloride (0.2 M), Cr10Mo1 steel gets good and stable passivity after 7 d immersion no matter the pH drops, and in view of the electrochemical responses, lower pH provides better conditions for the steel passivation, due to more protective Cr oxides forming and precipitating in the inner layer. However, in presence of high chloride (1.0 M), there is an inversion of this trend. The passivity is dramatically weakened with increasing carbonation, and even pits occur when pH is below 10.5 as shown by SEM/EDS analysis. This can be related to the major decomposition of protective Fe and Cr oxides, which induces the formation of a defective and porous surface layer rich in hydroxides.
Acknowledgments: The authors gratefully acknowledge the financial support from National Basic Research Program of China (No. 2015CB655100), National Natural Science Foundation of China (Nos. 51278098 and 51308111), Industry-University-Research Cooperative innovation fund of Jiangsu Province (No. BY2013091), and Transformation Projects of Major Scientific and Technological Achievements of Jiangsu Province (No. 85120000220). Author Contributions: Jinyang Jiang, Wei Sun and Zhiyong Ai conceived and designed the experiments; Han Ma and Jianchun Zhang were responsible for preparation of the steel samples; Zhiyong Ai performed the experiment works, analysed the data and wrote the manuscript; and Dan Song and Danqian Wang contributed to the result analysis and the revision of the paper. Conflicts of Interest: The authors declare no conflict of interest.
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