A Comparative Study of Localized Corrosion and Stress Corrosion Cracking of 13Cr Martensitic Stainless Steel Using Acoustic Emission and X-Ray Computed Tomography

materials

Article A Comparative Study of Localized Corrosion and Stress Corrosion Cracking of 13Cr Martensitic Stainless Steel Using Acoustic Emission and X-ray Computed Tomography

Kaige Wu 1,* , Kaita Ito 2 , Ippei Shinozaki 3, Pornthep Chivavibul 1 and Manabu Enoki 1

1 Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2 Research and Services Division of Materials Data and Integrated System, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 3 Materials Department, Research Laboratory, IHI Corporation, 1 Shin-Nakahara-Cho, Isogo-ku, Yokohama 235-8501, Japan * Correspondence: [email protected]; Tel.: +81-3-5841-7129

Received: 16 July 2019; Accepted: 9 August 2019; Published: 12 August 2019

Abstract: An accurate evaluation of stress corrosion cracking (SCC) in 13Cr martensitic stainless steel (MSS) is still missing due to the lack of an in-situ insight into the process evolution and full characterization of the corrosion morphology. In this work, two main regimes involved in the SCC progression, including localized corrosion and cracking, were comparatively studied using in-situ acoustic emission (AE) monitoring and three-dimensional (3D) X-ray computed tomography (XCT) scanning. The stress corrosion tests were conducted with u-bent smooth specimens subjected to a single droplet of 1 µL 1% neutral NaCl solution. Localized corrosion and cracking evolution were controlled in tempered and quenched steel specimens, respectively. From XCT scanning, localized corrosion was featured by an irregular corrosion pit with deposited corrosion products containing cracks. The single dominant SCC crack was observed to initiate from corrosion pit and propagate with a 3D tortuous and discontinuous morphology. AE signals were detected in both cases. Correlated with in-situ observations and clustering analysis, source identiﬁcation of AE signals was proposed. AE signals during localized corrosion were assessed to be mainly from cracking within the deposited corrosion products. Comparatively, hydrogen-bubble evolution, plastic deformation, and crack-branches coalescence were proposed as the AE sources of cracking evolution.

Keywords: martensitic stainless steel; pitting; stress corrosion cracking; plastic deformation; acoustic emission; X-ray CT

1. Introduction Due to its good mechanical properties and reasonable corrosion resistance, 13Cr martensitic stainless steel (MSS) has been widely used as a structural material in many ﬁelds like civil engineering, nuclear power plants, and the oil & gas industry. However, it can undergo stress corrosion cracking (SCC) when exposed to the combination of stress and corrosive environment, and especially the chloride-containing environment [1]. In many applications, localized corrosion serves as the precursor to SCC [2]. Compared with the rapid cracking regime, the long-term localized corrosion can largely determine the overall life of an exposed component. Therefore, surveillance of the early localized corrosion and subsequent cracking evolution are of equal importance to SCC monitoring. In practical

Materials 2019, 12, 2569; doi:10.3390/ma12162569 www.mdpi.com/journal/materials Materials 2019, 12, 2569 2 of 19 application, to reduce the risk of premature failures of high-strength structural components in hostile fields, establishment of in-situ monitoring methodologies for the SCC is always required. Acoustic Emission (AE) method is a promising nondestructive testing (NDT) method with high sensitivity to various corrosion processes [3]. The AE method has been used to study steel corrosion like localized corrosion [4–13] and SCC [14–26]. Numerous experimental works have deepened our understanding of the damages during corrosion processes based on the recorded AE signals. First, AE activities during localized corrosion have been assigned with different damage processes, including instantaneous stress changes on the metal surface [4], rupture of pit cover [5,6], breakdown of the passivating film [7], corrosion potential fluctuations [8], and hydrogen bubble evolution [5,6,9–13]. Comparatively, the different damages process during SCC evolution, including hydrogen bubble evolution [14–16,25], plastic deformation of the advancing crack tip [17,18], cracking within the corrosion products [14,19–21], falling off of the surface grain [14,19,21], and propagation of the primary crack [15–19,22–26] have been proposed in literature as the possible mechanisms for generation of AE signals. On the other hand, some AE signal-derived secondary parameters have been introduced as a discriminant factor to estimate the morphological evolution of pitting corrosion [6], or to monitor the SCC mode [18]. Furthermore, the detected AE signal has been combined with electrochemical noise to distinguish the different damage stages with a predominant electrochemical or mechanical contribution to crack propagation depending on the SCC evolution [15]. Despite extensive work and the advances that have been made, many challenges still exist in evaluating and understanding AE behavior during SCC. First of all, most source mechanisms during SCC proposed in literature are not undisputed since the interpretation of AE data is mainly based on post-mortem microstructural analysis. The primary reason for this challenge lies in the fact that most accelerating corrosion tests, mostly derived from ASTM G129 [27], ASTM G36 [28], or NACE TM0177 [29] are basically conducted in bulk solutions relevant to complete immersion conditions. This greatly increases the difficulty in observing the surface of immersed specimen in-situ. Although some AE signals during SCC have been interestingly correlated with the concurrent electrochemical noise [14,15,26], limited in-situ observational data seems to make their conclusions not so visualized. Meanwhile, some SCC testing in hot magnesium chloride solution [15] can introduce additional uncertainty in extracting the AE events from the artificial noises. Moreover, the localized corrosion as the precursor to cracking has not gained enough attentions compared with the crack propagation. In many reports [24,26,30,31], early localized corrosion during SCC was often determined to be undetected or detected with rare AE events, which obviously conflict the AE detectability of localized corrosion verified by many other works [4–13]. This may be attributed to the principle of accelerated corrosion testing in most laboratory study for shortening experimental time periods. By this consideration, the time to crack initiation is always intended to be reduced either by tailoring the specimen’s microstructure to be more susceptible or by applying more aggressive environment. This can prematurely trigger the condition for pit-to-crack transition when the pit is still in its infancy, corresponding to the phenomenon of “delay time” (i.e., a time delay for pit growth to reach a critical size required for AE detection) [5,6,9,11–13] and therefore unmeasured by AE method [30,31]. This can lead to lost sight of some secondary AE source mechanisms like the cracking of deposited corrosion products and hydrogen bubble evolution in developed pits that should have gained attention in practical applications. On the other hand, any corrosion evolution is a process not just evolving on the specimen surface, but developing spatially in three dimensions (3D). Therefore, two-dimensional (2D) techniques cannot characterize all the local features with the inner corrosion morphology. The recently-developed nondestructive 3D technique of X-ray computed tomography (XCT), whether synchrotron-based or laboratory setups, has been applied to study the localized corrosion, intergranular corrosion, and SCC of stainless steels and aluminium alloys [32–34]. The application of tomographic 2D images and rendered 3D views has enabled remarkable progress in characterizing and understanding the evolution of corrosion damages. A combination of in-situ and ex-situ experiments has produced a quantitative evaluation of the morphology of localized corrosion, as well as the pit-to-crack transition in steels [32]. Materials 2019, 12, 2569 3 of 19

A counterintuitive fact was uncovered from 3D tomographic imaging that the crack is not necessarily initiated from the pit base but tends to primarily develop from the pit mouth [33]. Moreover, some transgranular cracks that seem to be discontinuous from the surface observation were confirmed by XCT to be actually continuous inside the specimen by demonstrating the angular extending of some secondary micro-cracks emanated from the main propagation direction of a main crack [34]. XCT has gained wide popularity with many interesting results in corrosion studies. However, study on the localized corrosion and SCC in MSSs was rarely reported. In this work, motivated by the steel corrosion under a thin aqueous layer relevant to atmospheric corrosion [19,22,30,31,35], a stress corrosion test under a single droplet of neutral NaCl solution was improved in the u-bent SUS420J2 MSS specimen to facilitate AE monitoring with in-situ optical microscopy observations. A direct support to AE data analysis was expected to be provided based on in-situ observational data of the corrosion process. On the other hand, the evolution of localized corrosion and cracking development was controlled by the specimen preparation with different heat treatment procedures (i.e., tempered vs. as-quenched). XCT was used to demonstrate the corrosion morphologies of both cases. Meanwhile, the AE behaviors of both cases were comparatively studied. From an AE clustering analysis, in-situ observations, correlative post-observations, and source identification of AE signals was discussed.

2. Materials and Methods

2.1. Materials and Specimen Preparations A commercial 13Cr MSS of SUS420J2 (provided by Kusayama Unique Special Steel, Kawasaki, Japan.) was used in this study. The chemical composition of the steel is given in Table1. Two rolled ﬂat bars of 22 mm in thickness were austenitized at 950 ◦C for 2 h, followed by quenching in argon gas. Then, one quenched steel bar was additionally tempered at 500 ◦C for 20 h, followed by air cooling. The other bar was used as as-quenched specimen. After heat treatment, cylindrical tensile specimens with a gauge length of 30 mm and a diameter of 6 mm were produced from the two steel bars for characterization of the mechanical properties of both steels in accordance with Japan Industrial Standard (JIS) G0567 [36] at room temperature. The tensile tests were conducted using a universal testing machine (Instron 5982, Norwood, MA, USA) with a loading capacity of 100 kN. The mechanical properties of both steels are summarized in Table2, where E is the modulus of elasticity, σ0.2 is the 0.2% proof stress, UTS is the ultimate tensile stress, EL, RA, and HV represent the elongation, reduction in area, and Vickers hardness. Two steels with distinct properties were intended to have diﬀering corrosion behavior. Two kinds of corrosion specimens with identical dimensions of 170.0 mm (L) × 6.3 mm (W) 1.5 mm (T) were produced for corrosion testing from the tempered bar (marked as × specimen A) and quenched bar (marked as specimen B). All corrosion specimens were polished down to 1 µm diamond suspension before corrosion testing.

Table 1. Chemical compositions of as-received SUS420J2 steel in this study, mass%.

C Si Mn P S Ni Cr Fe 0.32 0.17 0.38 0.031 0.002 0.34 12.85 Bal

Table 2. Heat treatment procedures and mechanical properties of the two kinds of steel specimens used in this study.

Heat Treatment Mechanical Properties Specimen # σ UTS EL RA Annealing Quenching Tempering E (GPa) 0.2 HV (MPa) (MPa) (%) (%)

A 950 ◦C/2 h Argon gas 500 ◦C/20 h 218 1027 1169 12.5 45.6 367 B 950 ◦C/2 hcooling - 211 1119 1699 9.6 28.4 511 Materials 2019, 12, 2569 4 of 19

2.2. Microstructure Characterization and Precipitation Identification Specimens were observed using optical microscopy (OM) and field-emission scanning electron microscopy (FE-SEM, JSM7000F, JEOL Ltd., Tokyo, Japan) equipped with energy-dispersive spectroscopy (EDX) after being chemically etched in Villella’s reagent (5cc hydrochloric acid + 2g Picric acid +100cc Ethyl alcohol) for 5 s. Moreover, X-ray diffraction (XRD) measurements were conducted using a 9-kW diffractometer (Cu Kα radiation source; Rigaku SmartLab, Rigaku Corp., Tokyo, Japan) after the specimens were wet-polished down to 0.05 µm colloidal silica suspension. The 2θ-scan range was set as 40◦ to 90◦ with a step size of 0.01◦ and a scan speed of 2◦/min to analyze the main structure of each specimen. To analyze the detailed information of the precipitates [37], a scan ranging from 47◦ to 53◦ of 2θ was further performed with a 0.01◦ step size and a 0.1◦/min scan speed.

2.3. SCC Testing and AE Measurements Figure1 shows the experimental setup. The specimen was bent and fixed in an experimental jig with a curvature radius of 125 mm leading to a strain of 0.6% at the apex of the specimen surface, which is approximated by the equation [38]: ε = t/2R, where t is the specimen thickness and R is the radius of the bend. A single droplet of 1 µL 1.0 mass% neutral NaCl solution was dropped in the center area of the specimen surface using a microliter syringe. The experimental apparatus was placed in a thermostatic bath (298 K, 99%–100% humidity) with pure water. The bath was covered with a transparent glass cap to allow in-situ observation using a digital microscope (VHX-5000, Keyence Corp., Osaka, Japan). Considering the constant deformation of the u-bending specimen and localized corrosion region under the droplet, the overall stress and strain of specimen during the testing was believed to be uniform and constant compared to the initial bending state. But as the corrosion proceeded, the local stress and strain near the corrosion region were estimated to increase and drive the crack nucleation. Simultaneously, continuous AE measurement was performed using a custom AE acquisition system developed in our group, continuous wave memory (CWM) [39]. Two high-sensitivity R-CAST sensor systems with a resonant frequency of 200 kHz (M204A, Fuji Ceramics Corp., Fujinomiya, Japan, 55 dB gain) were positioned 10 mm and 30 mm away from the droplet site to record the AE signals. A threshold of 29.7 dB and a 100 kHz high-pass filter (HPF) were used as thresholds to isolate AE events from the continuously recorded waveform. Then the target signals were extracted by filtering out the noise outside the corrosion region using the source location method. K-means clustering algorithm was used to classify the extracted AE data of both cases [31]. The optimum number k of clustering analysis was determined by using the average silhouette coefficient [30,31]. After different AE clusters were recognized, the distributions of AE data were compared with the in-situ observational data of the corrosion process over the time evolution. The source mechanisms of detected AE signals were proposed by both considerations of the AE features and corresponding observations of the corrosion process. Materials 2019, 12, 2569 5 of 19 Materials 2019, 12, x FOR PEER REVIEW 5 of 19

FigureFigure 1. 1. SchematicSchematic of of the the modified modiﬁed stress stress corrosion corrosion cracking cracking ( (SCC)SCC) test test system system incorporated incorporated with with an an inin-situ-situ digital digital microscope microscope and and acoustic acoustic emission emission ( (AE)AE) measurement measurement systems. Online Online version version in in color. color.

2.4.2.4. X X-ray-ray CT CT and and SEM SEM-EBSD-EBSD Measurements Measurements AfterAfter corrosion testing,testing, the the specimens specimens were were cut cut to an to appropriate an appropriate length length of 20 mm of 20 to fitmm the to Scanning fit the Scanningelectron microscopyelectron microscopy (SEM) chamber (SEM and) chamber ultrasonically and ultrasonically cleaned with cleaned ethanol at with room ethanol temperature at room to temperatureremove the surface to remove corrosion the products. surface corrosion The surfaces products. of specimens The were surfaces first ofinspected specimens with SEM.were Next, first inspectedthe spatial with features SEM. of Next, the corrosionthe spatial morphologies features of the were corrosion obtained morphologies by a 3D scanning were obtained using lab-based by a 3D scanningX-ray computed using lab tomography-based X-ray (XCT) computed system tomography (ZEISS Xradia (XCT) 520 Versa,system Carl (ZEISS Zeiss Xradia XRM, 520 Pleasanton, Versa, Carl CA, ZeisUSA).s XRM, A full Pleasanton, scan of 360 CA,◦ rotation USA). of A thefull specimen scan of 360° was rotation conducted of the at aspecimen tube voltage was ofconducted 140 kV and at a a tubepower voltage of 10 Wof with140 kV a scanningand a power period of of10 8.5W h.with A resolutiona scanning of period 1.0 µm of/voxel 8.5 h. was A resolution achieved withof 1.0 a μm/voxelfield of view was (FoV) achieved width with of 2 a mm. field After of view scanning, (FoV) thewidth acquired of 2 mm. projection After scanning, images were the imported acquired projectioninto the software images ORS were Dragonfly imported Pro into (Object the software Research ORS System, Dragonfly Montreal, Pro (Object Canada) Research to produce System, a 3D Montreal,reconstruction Canada) of the to specimen’s produce volume. a 3D reconstruction Moreover, to investigate of the specimen the local feature’s volume around. Moreover, the crack tip, to investigateelectron backscatter the local di ff featureraction (EBSD) around measurement the crack was tip, performedelectron backscatter after a fine polishing diffraction of the (EBSD crack) measurementarea with a 0.05 wasµm performed colloidal silica after suspension a fine polishing for 5 h. of EBSD the crack data werearea with collected a 0.05 using μm acolloidal field-emission silica suspensionSEM (JSM70001, for 5 h. JEOL, EBSD Tokyo, data were Japan) collected equipped using with a afield EBSD-emission detector. SEM A 15-kV (JSM70001, acceleration JEOL, voltageTokyo, Japanwas selected) equipped with with a step a EBSD size ofdetector. 0.25 µm A for15-kV EBSD acceleration scanning. voltage EBSD datawas selected were analyzed with a step using size OIM of 0.25(EDAX-TSL) μm for EBSD v7 software scanning. (EDAX EBSD Inc., data Mahwah, were analyzed NJ, USA). using The OIM Kernel (EDAX Average-TSL) Misorientation v7 software (EDAX (KAM) Inc.,analysis Mahwah, was used NJ, toUSA assess). The the Kernel local plasticAverage deformation Misorientation around (KAM) the crack analysis tip [ 30was].KAM used duringto assess EBSD the localanalysis plastic is a deformation qualitative quantification around the crack of the tip average [30]. KAM local duringmisorientation EBSD analysis around isa measurement a qualitative quantificationpoint with respect of the to average all of its local nearest misorientation neighbor pointsaround by a measurement definition in thepoint perimeter with respect of the to kernel. all of itsDeformed nearest neighbor locals due points to high by dislocation definition densityin the perimeter correspond of tothe high kernel. KAM Deformed intensity. locals KAM due can to be high used dislocationto evaluate density the localized correspond deformation to high on KAM the well-prepared intensity. KAM specimen can be surfaceused to containing evaluate the high-density localized deformationdislocation. For on the the as-quenched well-prepared specimen specimen B containing surface containing high-density high dislocation-density with dislocation. itself, KAM For map the aswas-quench calculateded specimen using third B containing nearest-neighbor high-density EBSD dislocation points and awith maximum itself, KAM of 6◦ tomap track was the calculated localized usingplasticity third along nearest the- crackneighbor tip. Meanwhile,EBSD points the and cross-sectional a maximum morphology of 6° to track of localizedthe localized corrosion plasticity was alongobserved the crack using tip. SEM Meanwhile, with a serial the sectioning cross-sectional of the morphology specimen. of localized corrosion was observed using SEM with a serial sectioning of the specimen.

3. Results

Materials 2019, 12, 2569 6 of 19

Materials 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 66 of 1919 3. Results 3.1. Microstructure and Precipitates 3.1. Microstructure and Precipitates Figure 2 shows a general view of the microstructure of tempered and quenched specimens. FigureFigure 3 shows2 shows the localized a general microstructure view of the microstructure with EDX mapping, of tempered indicating and that quenched carbides specimens. rich in Cr andFigure C 3decorate shows the the localized hierarchic microstructure lath structured with martensitic EDX mapping, matrix indicating which thatagrees carbides well with rich inthe Cr other and workC decorate [40].]. P theriorrior hierarchic austeniteaustenite lath graingrain structured boundariesboundaries martensitic appear matrixmore visibly which in agrees quenched well with microstructure the other work B with [40]. smallePriorsmalle austeniterr precipitatesprecipitates grain upup boundaries toto 0.50.5 μm;μm; whilewhile appear inin moretemperedtempered visibly specimenspecimen in quenched A,A, somesome microstructure nanonano--sizedsized precipitatesprecipitates B with smaller (<(< 0.1 precipitatesμm) are also up observed to 0.5 µm; in while addition in tempered to the submicro specimen--sized A,sized some carbides carbides nano-sized with with precipitates sizes sizes up up to to ( < 0.9 0.90.1 μm. μm.µm) Allare All precipitatesalso observed exhibit in addition similar to circular the submicro-sized or elongated carbides shape. with X--rayray sizes difdiffractionfraction up to 0.9 ((XRDµm.) All) spectraspectra precipitates ofof specimensspecimens exhibit Asimilar and B circular are shown or elongated in Figure shape. 4, indicating X-ray di theﬀraction (110), (XRD) (200), spectraand (211) of specimensplanes of martensite A and B are (α shown′′)) phasephase in only.Figure Some4, indicating weak peaks the (110), within (200), 47° and to 53° (211) of planes both steel of martensite are determined ( α0) phase to be only. 48.60°, Some 50.90°, weak 51.80°, peaks within 47 to 53 of both steel are determined to be 48.60 , 50.90 , 51.80 , which could indicate the (440), which could◦ indicat◦ e the (440), (531), and (600) planes of◦ Cr--richrich◦ MM2323C◦ 66 carbides. It is consistent with (531),thethe resultsresults and (600) ofof TEMTEM planes observationobservation of Cr-rich ofof M thethe23C samesame6 carbides. steelsteel temperedtempered It is consistent atat 500°500° with [[37 the]] andand results 550°550° of [[41 TEM].]. Therefore,Therefore, observation thethe of the same steel tempered at 500 [37] and 550 [41]. Therefore, the submicro-sized precipitates in submicrosubmicro--sizedsized precipitatesprecipitates inin bothboth◦ steelssteels seemseem ◦toto bebe undissolvedundissolved CrCr--richrich MM2323C66 carbides, and some both steels seem to be undissolved Cr-rich M C carbides, and some additional nano-sized Cr-rich additional nano--sizedsized Cr Cr--richrich M M2323C66 carbides23 6 were formed during the tempering process for specimenMspecimen23C6 carbides A.A. were formed during the tempering process for specimen A.

Figure 2.2. MicrostructuresMicrostructures ofof chemically chemically etched etched specimens specimens (Viella’s (Viella’s reagent reagent for for 5 s): 5 ss ():a):) (( specimena)) specimenspecimen A; (A;A;b) (specimen(b)) specimenspecimen B. B.B.

Figure 3.3. Scanning electroneelectronlectron microscopy-energy-dispersivemicroscopyicroscopy--energyenergy--dispersive spectroscopy (SEM-EDX)((SEM--EDX)) map showingshowing carboncarbon andand chromiumchromiumchromium enrichment enrichmentenrichment and andand iron ironiron depletion depletiondepletion on on precipitates. precipitates. ( a()a specimen)) specimenspecimen A; A;A; (b ()(b specimen)) specimenspecimen B. B.Online Online version version in in color. color.

Materials 2019, 12, x FOR PEER REVIEW 7 of 19

Materials 2019, 12, 2569 7 of 19 Materials 2019, 12, x FOR PEER REVIEW 7 of 19 (a) Specimen A (b) Specimen A Specimen B Specimen B (110) (531) M C 23 6 Specimen A (440) M C Specimen A (a) (b) 23 6 (600) M C Specimen B Specimen 23B 6 (110) (531) M C 23 6

(440) M C 23 6 (600) M C 23 6 (211) (200)

Intensity, a.u.

950Ch

Intensity, a.u.

(211) (200)

950Ch Intensity, a.u. Intensity, a.u. 950Ch – 500Ch

40 50 950Ch – 60500Ch 70 80 90 47 48 49 50 51 52 53 2, deg. 2, deg. 40 50 60 70 80 90 47 48 49 50 51 52 53 2, deg. Figure 4. X-ray diffraction (XRD) patterns of the two steel specimens: (a) 2diffraction, deg. angles from 40° to 90°Figure and 4. (b )X - detailray diffractioned information (XRD) patterns between of the diffraction two steel specimens: angles from (a) 47°diffraction to 53°. angles Online from version 40° in Figure 4. X-ray diffraction (XRD) patterns of the two steel specimens: (a) diffraction angles from 40◦ to color.to 90° and (b) detailed information between diffraction angles from 47° to 53°. Online version in 90◦ and (b) detailed information between diffraction angles from 47◦ to 53◦. Online version in color. color. 3.2.3.2. Corrosion Corrosion Processe Processess 3.2. Corrosion Processes ThreeThree parallel parallel experimentations experimentations of of both both steels steels were were conducted conducted in the in same the conditionsame condition with similar with Three parallel experimentations of both steels were conducted in the same condition with similarresults. results. Only one Only typical one typical data for data each for case each was case reported was rep inorted this paper.in this paper. similar results. Only one typical data for each case was reported in this paper. Figure5 shows the in-situ observations of the corrosion process in specimen A. Early in the FigureFigure 5 shows 5 shows the the in -insitu-situ observations observations ofof the corrosioncorrosion process process in inspecimen specimen A. EarlyA. Early in the in the beginningbeginningbeginning of of the theof theexperiment, experiment, experiment, localized localized localized corrosion corrosioncorrosion began began asas as the the the droplet droplet shrank. shrank. A typicalA A typical typical feature feature feature of of of corrosioncorrosioncorrosion “ “atoll”atoll “atoll” was was” was gradually gradually gradually visualized visualized visualized as as thethe formation formation of of ofan an an annular, annular, annular, ring ring ring-like-like-like surface surface surface stains stains stains underunderunder the the droplet dropletthe droplet (Figure (Figure (Figure 55aa–c). –5ac).–c). This This corrosion corrosion atollatoll is is attributed attributed to to the the atmospheric atmospheric corrosion corrosion under under locallocal localNaCl NaCl NaCl attack attack attack developed developed developed with with with spatial spatial spatial separation separationseparation ofof of thethe the anodic anodic anodic and and and cathodic cathodic cathodic areas areas areas [30,31, [30,31, [3042,31]. The,42]. The The depositeddepositeddeposited corrosion corrosion corrosion products products products were were were yellowish yellowish yellowish initially initially and and and then then then became became became dark, dark, dark, spreading spreading spreading over over overthe the the entireentireentire corrosion corrosion atoll atoll (Figure(Figure (Figure5 5dd–g). 5d–g).–g). During During During this thisthis process, process, many many many secondarysecondary secondary droplets droplets droplets formed formed formed adjacent adjacent adjacent to to the original droplet. As corrosion proceeded, some secondary droplets coalesced to larger tothe the original original droplet. droplet. As corrosion As corrosion proceeded, proceeded, some secondarysome secondary droplets droplets coalesced coalesced to larger to droplets larger droplets (Figure 5c–e). In the late stage (Figure 5f–h), these secondary droplets gradually droplets(Figure5 c–e). (Figure In the 5c late–e). stage In the (Figure late 5f–h), stage these (Figure secondary 5f–h), droplets these secondary gradually droplets disappeared gradually away disappeared away from the localized corrosion site. This phenomenon was also captured by another from the localized corrosion site. This phenomenon was also captured by another researcher [42] who disappearedresearcher away [42] fromwho thought the localized that these corrosion secondary site. droplets This phenomenon originated from was both also the captured initial droplet by another reseathoughtandrcher moisture that [42 these] who in secondary thethought atmosphere that droplets theseand may originatedsecondary serve as fromdropletsthe cathodic both originated the area. initial Anodic from droplet location both and the in moisture theinitial central droplet in the andatmosphere moisturecorrosion and sitein maythe with atmosphere serve high acidity, as the and cathodic low may potential, area.serve and Anodicas thehigh cathodic location Cl enhanced inarea. the the Anodic central metal corrosion locationdissolution in site withinthe with central high  corrosionacidity,the lowhydrogen site potential, with reduction high and acidity, high[30]. Moreover, Cl low− enhanced potential, a filiform the and metal leakage high dissolution ofCl corrosion enhanced within solution the the metal (Figure hydrogen dissolution 5f–h), reduction similar within [30]. theMoreover, hydrogento the observation a filiform reduction leakage in [[30].42], offromMoreover, corrosion the localized a solution filiform corrosion (Figureleakage was5 f–h),of captured, corrosion similar but tosolution no the AE observation event (Figure was recorded5f in–h), [ 42 similar], from tothe the localizedalong observation with corrosion this inphenomenon. [42 was], from captured, theFinally, localized butno crack no corrosion AE initiation event was was was captured, triggered, recorded andbut along onlyno AE with localized event this wascorrosion phenomenon. recorded alongFinally,deposited with no this crack with phenomenon. initiationcorrosion products was Finally, triggered, was no observed. crack and initiation only localized was triggered, corrosion and deposited only localized with corrosion corrosion depositedproducts waswith observed. corrosion products was observed.

Figure 5. Localized corrosion in the u-bent specimen A with 311 h’ exposure to a single droplet of 1 μL neutral 1% NaCl solution in 99% relative humidity at room temperature: (a–g) in-situ observation; (h) post-observation. Figure Figure 5. 5. LLocalizedocalized corrosion corrosion in in the the u u-bent-bent specimen specimen A A with with 311 311 h h’’ exposure exposure to to a a single single droplet droplet of of 1 1 μLµL neutral neutral 1% 1% NaCl NaCl solution solution in in 99% 99% relative relative humidity humidity at at room room temperature temperature:: (a (a––g)g) in in-situ-situ observation; observation; (h)(h) post post-observation.-observation.

Materials 2019, 12, 2569 8 of 19 Materials 2019, 12, x FOR PEER REVIEW 8 of 19

FigureFigure 66 showsshows thethe in-situin-situ observationsobservations of the corrosion process in specimen B. As expected, initiationinitiation and and pr propagationopagation of of SCC SCC was was observed observed with with relatively relatively shorter shorter exposure exposure than than specimen specimen A. A. Generally,Generally, the crack initiatedinitiated fromfrom thethe earlyearly localized localized corrosion. corrosion. In In similarity similarity to to the the case case of of specimen specimen A, the early stage of corrosion evolution in specimen B could be mentioned as localized corrosion which A, the earlyMaterials stage 2019, 12of, x corrosionFOR PEER REVIEW evolution in specimen B could be mentioned as localized8 of corrosion 19 whichwas featured was featured by a corrosion by a corrosion atoll with atoll localized with localized attack (Figure attack6 (Figurea–d). Thereafter, 6a–d). Thereafter, the crack the was crack observed was observedto develop to fromFiguredevelop both 6 shows from sides theboth of in the -sidessitu localized observations of the corrosionlocalized of the corrosioncorrosion normal process to normal the u-bendin specimento the load u -B.bend direction. As expected,load direction. The crack initiation and propagation of SCC was observed with relatively shorter exposure than specimen A. Thegrew crack with grew a low with rate a inlow the rate beginning in the beginning (Figure6e), (Figure then transitioned6e), then transitioned to a high to growth a high rate growth with rate the increasingGenerally, crack lengththe crack (Figure initiated6f–g). from Manythe early secondary localized corrosion. droplets In spreading similarity to along the case the of perimeter specimen of the with theA, increasingthe early stage crack of corrosion length evolution (Figure in 6f specimen–g). Many B could secondary be mentio dropletsned as localized spreading corrosion along the localized corrosion area, which is similar to the case of specimen A, were also observed. A phenomenon perimeterwhich of thewas featured localized by corrosiona corrosion atoll area, with which localized is similar attack (Figure to the 6a – cased). Thereafter, of specimen the crack A, was we re also observed.of crack-branchobserved A phenomenon to coalescence develop from of during crack both -sidesbranch crack of propagationthe coalescence localized corrosion was during captured normal crack andtopropagation the would u-bend be load was discussed direction. captured in detail and wouldwith AE beThe data.discussed crack The grew test in with detail was a low ﬁnally with rate AE stoppedin thedata. beginning withThe test the (Figure was occurrence finally6e), then stopped of transitioned crack arrest,with to thea whichhigh occurrence growth was believedrate of crack to arrest,be controlled whichwith the by was increasing the believed electrochemical, crack to length be controlled mechanical, (Figure 6f by–g). and the Many geometric ele secondaryctrochemical, factors droplets [ mechanical,43 ], spreading but was alongand not included geometric the in perimeter of the localized corrosion area, which is similar to the case of specimen A, were also factorsthe primary [43], but concern was not of thisincluded work. in the primary concern of this work. observed. A phenomenon of crack-branch coalescence during crack propagation was captured and would be discussed in detail with AE data. The test was finally stopped with the occurrence of crack arrest, which was believed to be controlled by the electrochemical, mechanical, and geometric factors [43], but was not included in the primary concern of this work.

FigureFigure 6. 6. AA single single crack crack evolution evolution in in the the u u-bent-bent specimen specimen B B exposed exposed to to a a single single droplet droplet of of 1 1 μLµL Figure 6. A single crack evolution in the u-bent specimen B exposed to a single droplet of 1 μL neutralneutral 1% 1% NaCl NaCl solution in 99% relative humidityhumidity atat roomroom temperaturetemperature until until the the crack crack arrest arrest in in 48.5 48.5 h: neutral 1% NaCl solution in 99% relative humidity at room temperature until the crack arrest in 48.5 h(a: –(ag–)g) in-situ in-situ observation; observation; (h (h)) post-observation. post-observation. h: (a–g) in-situ observation; (h) post-observation. 3.3. Corrosion Morphologies 3.3. Corrosion3.3. Corrosion Morphologies Morphologies TheThe localThe morphology morphology local morphology of of corrosion corrosion of corrosion areas areas areas of two of of two two cases cases cases after after after testing testing testing was was compared was compared compared from from the the from surface the surfaceobservations. observations.surface Inobservations. specimen In specimen In A, specimen some A, cracks someA, some can cracks cracks be recognized can can be recognizedrecognized within within some within some corrosion some corrosion corrosion products products depositedproducts deposited inside an irregular pit, as shown in Figure 7a. Figure 7b shows the local features related to depositedinside an irregular inside an pit, irregular as shown pit, in Figureas shown7a. Figure in Figure7b shows 7a. Figure the local 7b featuresshows the related local to features the crack related initiation to in specimenthe crack B. A initiation single dominantin specimen crack B. A single is observed dominant to becrack from is observed a corrosion to be pit. from Some a corrosion small pit. secondary the crackSome initiation small secondaryin specimen cracks B. wereA single also observeddominant to havecrack emanate is observedd from to the be main from propagation a corrosion pit. cracks were also observed to have emanated from the main propagation direction of the main crack. Some smalldirection secondary of the main cracks crack. were also observed to have emanated from the main propagation direction of the main crack.

Figure 7. Scanning electron microscopy (SEM) images showing the surface morphologies of corroded specimens A (a) and B (b).

Materials 2019, 12, x FOR PEER REVIEW 9 of 19

MaterialsFigure2019 , 7.12 , 2569Scanning electron microscopy (SEM) images showing the surface morphologies of9 of 19 corroded specimens A (a) and B (b).

Figure8 shows the reconstructed slices and volume rendering images from the XCT data of Figure 8 shows the reconstructed slices and volume rendering images from the XCT data of localized corrosion in specimen A. From the unique observation of inner morphology in different localized corrosion in specimen A. From the unique observation of inner morphology in different sections, the the localized corrosion corrosion featured featured by an irregular pit with depositeddeposited corrosion products containing cracks as small as the micron scale is clearly confirmed. No SCC was observed from the containing cracks as small as the micron scale is clearly confirmed. No SCC was observed from the bottom or the shoulder of pit thanks to the full scan of the corrosion region, indicating that damages damages during the corrosion process of specimen A were mainly the pitting corrosion and precipitation of during the corrosion process of specimen A were mainly the pitting corrosion and precipitation of corrosion productsproducts withwith cracking.cracking. A A 3D 3D view view and and a segmentationa segmentation of theof the localized localized corrosion corrosion were w alsoere alsotried. tried. Despite Despite some some uncertainty uncertainty existing existing in segmenting in segmenting the small the crack small inside crack the inside corrosion the corro products,sion products,the spatial the geometry spatial geometry of the corrosion of the pitcorrosion inside apit MSS inside was a firstMSS provided. was first provided. The main meritThe main of this merit 3D ofvisualization this 3D visualization of a real pit of isa probablyreal pit is theprobably potential the inpotential triggering in triggering a data-driven a data modeling-driven modeling of the crack of theinitiation crack initiation in relation in to relation the consideration to the consid oferation real pit of geometry, real pit geometry, rather than rather the introduction than the introduction of a fictive ofsemi-sphere a fictive semi or ellipse-sphere as or the ell initialipse as defect the initial [33,34 defect]. [33,34].

Figure 8. XCTXCT data data of of localized localized corrosion corrosion of of specimen specimen A: A: ( (aa)) Virtual Virtual slices slices in in different diﬀerent sections; ( b) 3D tomogram; ( (cc)) Segmented pit and cracks inside the corrosion products with transparent volume of material. Here, Here, the color is is based on the volume. Online Online version version in in color. color.

Figure9 9 shows shows the the XCT XCT data data of SCC of SCC in specimen in specimen B. First, B. First, it allows it allows a direct a illustration direct illustration the relative the relatispatialve positionspatial position of the corrosion of the corrosion pit and pit crack. and Combinedcrack. Combined with surface with surface observation observation in Figure in Figure7b, it 7clearlyb, it clearly supports supports that the that crack the initiatedcrack initiated from the from corrosion the corrosion pit was pit as similarwas as assimilar the mostly as the reported mostly

Materials 2019, 12, 2569x FOR PEER REVIEW 10 of 19 reported cases [2,15,16,19,29,30–32,40]. A zig-zag morphology could be related to the crack casespropagation [2,15,16 , from19,29 – the32,40 X].-Z A zig-zagand Y-Z morphology sections. Some could be voids related up to to the dozens crack propagation of micrometers from arethe X-Zdispersedly and Y-Z visible sections. in Somematerial. voids The up void to dozens formation of micrometers is probably are associated dispersedly with visible the inlocalized material. shear The voiddeformation formation and is distortion probably associated during the with quenching the localized process. shear The deformation caused residual and distortion stress around during these the quenchingvoids may process. accelerate The thecaused crack residual initiation stress and around propagation. these voids Additionally, may accelerate the the maincrack initiation crack is andseemingly propagation. continuous Additionally, at large as thea single main crack, crack which is seemingly is similar continuous to the claim at in large the SCC as a singleof austenitic crack, whichstainless is similar steel [34]. to the However, claim in the SCC crack of evolution austenitic inside stainless the steel present [34]. material However, is the more crack complex evolution as insidetortuous the morphology present material from isa more3D view complex than the as tortuouszig-zag morphology morphology from from 2D a 3D tomographic view than the images zig-zag or morphologySEM surface fromobservation. 2D tomographic First, some images voids or appearing SEM surface at the observation. crack plane First, locally some separated voids appearing the crack at theto be crack discontinuous. plane locally Secondly, separated compared the crack towith be discontinuous.the reported 3D Secondly, crack in compared Al-7Si-1Mg with alloy the reported[44], the 3Dreal crack morphology in Al-7Si-1Mg of SCC alloy in [MSS44], the is real believed morphology to be largely of SCC affected in MSS is by believed an uneven to be growth largely a fromffected a bylocalized an uneven competition growth from among a localized the crack competition fronts among because the of crack a stress fronts shielding because of effect a stress inside shielding the eheterogeneousffect inside the material. heterogeneous A detailed material. interpretation A detailed of interpretation the fracture ofmechanisms the fracture around mechanisms the crack around-tip thefields crack-tip requires fields time requires-lapse imaging time-lapse and imaging in-situ and AE in-situ monitoring AE monitoring of the crack of the growth crack, growth,which will which be willreported be reported in another in another paper. paper.

Figure 9. X-ray computed tomography (XCT) data of SCC of specimen B: (a) Virtual slices in Figure 9. X-ray computed tomography (XCT) data of SCC of specimen B: (a) Virtual slices in diﬀerent differentsections; (b sections;) 3D tomogram; (b) 3D tomogram; (c) Segmented (c) crack Segmented and voids crack with and transparent voids with volume transparent of materials. volume Here, of materials.the color is Here, based the on color the volume. is based Onlineon the versionvolume. in Online color. version in color.

Table3 3summarized summarized the the experimental experimental results results of bothof both steels. steels. In summary, In summary, localized localized corrosion corrosion with nowith SCC no initiation SCC initiation was observed was observed in tempered in tempered specimen specimen A until 311 A until h’ exposure. 311 h’ exposure. Localized Localizedcorrosion corrosion is featured by an irregular corrosion pit where corrosion products deposited with cracks; while in quenched specimen B, crack initiation and propagation were observed and finally stopped

Materials 2019, 12, 2569 11 of 19 isMaterials featured 2019 by, 12 an, x FOR irregular PEER REVIEW corrosion pit where corrosion products deposited with cracks; while11 of in 19 quenched specimen B, crack initiation and propagation were observed and ﬁnally stopped with crack arrestwith in crack 48.5 h. arrest The crack in 48.5 was h. observed The crack to bewas initiated observed from to a corrosion be initiated pit and from propagated a corrosio withn pit a 3D and tortuouspropagated and discontinuouswith a 3D tortuous path, ratherand discontinuous than the pseudo-continuous path, rather thanfrom the 2D pseudo characterization.-continuous from 2D characterization. Table 3. Summary of the corrosion behaviors and AE results of two cases. Table 3. Summary of the corrosion behaviors and AE results of two cases. Specimen # Corrosion Test Exposure Corrosion Behavior AE Events Specimen # Corrosion Test Exposure Corrosion Behavior AE Events A U-bend, 1% NaCl-droplet 311 h Localized corrosion 106 BA U99%-bend, RH, 1% RT. NaCl-droplet 48.5311 h hrs Localized SCC corrosion 106 610 B 99% RH, RT. 48.5 hrs SCC 610 3.4. AE Results 3.4. AE Results As summarized in Table3, 106 and 610 AE signals were extracted from the localized corrosion As summarized in Table 3, 106 and 610 AE signals were extracted from the localized corrosion and crack evolution, respectively. Figure 10 shows a comparison of extracted AE signals over the and crack evolution, respectively. Figure 10 shows a comparison of extracted AE signals over the time evolution and in cross-plot. Overall, all AE signals distribute in a resemble frequency range time evolution and in cross-plot. Overall, all AE signals distribute in a resemble frequency range between 200–350 kHz. However, crack evolution in specimen B produced more AE activity with higher between 200–350 kHz. However, crack evolution in specimen B produced more AE activity with amplitude-level than the localized corrosion of specimen A. K-means clustering analysis was applied higher amplitude-level than the localized corrosion of specimen A. K-means clustering analysis was to the AE data and the results of AE clustering analysis, as well as the source identiﬁcation of two applied to the AE data and the results of AE clustering analysis, as well as the source identification cases will be discussed separately. of two cases will be discussed separately. (b) (a) 4 AE signals from localized corrosion of specimen A 10 70 AE signals from SCC of specimen B AE signals during localized corrosion of specimen A

N AE signals during SCC of specimen B

103 60

50 102

Amplitude, dB 40 101

Cumulative AE events, 30 100 200 225 250 275 300 325 350 0 50 100 150 200 250 300 350 Average frequency, kHz Time, t / hr

Figure 10. Comparison of the AE signals extracted from the corrosion processes of specimens A and B: (aFigure) Cumulative 10. Comparison AE events of over the time; AE signals (b) Cross-plot extracted of from amplitude the corrosion to average processes frequency. of specimens Online version A and inB: color. (a) Cumulative AE events over time; (b) Cross-plot of amplitude to average frequency. Online version in color. 3.4.1. AE Signals during Localized Corrosion in Specimen A 3.4.1.As AE presented Signals during in Figure Localized 11a, two Corrosion clusters in with Specimen diﬀerent A frequency level were obtained, i.e., low-frequencyAs presented cluster in 1 andFigure high-frequency 11a, two clusters cluster with 2. Figure different 11b shows frequency the time level evolution were obtained, of twoAE i.e., clusters.low-frequency A weak cluster tendency 1 and could high be-frequency recognized, cluster i.e., low-frequency 2. Figure 11b shows cluster the 1 was time ﬁrst evolution detected of and two high-frequencyAE clusters. A clusterweak tendency 2 beganto could appear be inrecognized, the middle i.e., and low later-frequency periods cluster of the corrosion1 was first duration. detected Toand assist high in-frequency interpreting cluster AE signals, 2 began a further to appear cross-sectional in the middle observation and later of periods the localized of the corrosion corrosion wasduration. implemented. To assist Figurein interpreting 12 shows AE three signals, sectional a further morphologies cross-sectio ofnal the observation selected sections. of the localized More clearly,corrosion corrosion was implemented. products containing Figure 12 cracks shows are three observed sectional inside morphologies the irregular of pit. the Theselected cracks sections. seem toMore cross clearly, through corrosion the corrosion products products containing in a disorderly cracks are fashion observed (Figure inside8). the Some irregular micro-pits pit. The are cracks also shownseem atto thecross base through of the th macro-pit.e corrosion They products were marked in a disorderly as pseudo-crack fashionsince (Figure no high-amplitude 8). Some micro signals-pits are werealso recordedshown at during the base the of localized the macro corrosion-pit. They compared were marked to the as SCC pseudo in- specimencrack since B. no The high mechanism-amplitude accountingsignals were for localizedrecorded corrosion during the has localized been deeply corrosion studied compared in relation to to the the SCC chloride in speci ionmen attack. B. By The consideringmechanism theaccounting transport for of localized ionic species, corrosion the metal has been dissolution deeply studied followed in by relation hydrolysis to the of chloride metal ionsion could attack. maintain By considering an active dissolution the transport interface of ionic which species, drives the the pitmetal growth dissolution [35,42,45 , followed46]. Along by hydrolysis of metal ions could maintain an active dissolution interface which drives the pit growth [35,42,45,46]. Along with this process, the involved hydrogen-bubble evolution was mostly proposed in literature as the source for AE signals with higher-frequency or lower-strength than cracking-signals recorded during localized corrosion [5–7] and SCC [24,25]. Therefore, the

Materials 2019, 12, 2569 12 of 19 withMaterialsMaterials this 2019 2019 process,, 12, 12, x, xFOR FOR the PEER PEER involved REVIEW REVIEW hydrogen-bubble evolution was mostly proposed in literature12 as12 of the of 19 19 source for AE signals with higher-frequency or lower-strength than cracking-signals recorded during localizedhighhigh-frequency-frequency corrosion cluster cluster [5–7 ] 2 and 2 is is SCCattributed attributed [24,25 ]. to to Therefore, the the hydrogen hydrogen the high-frequency-bubble-bubble evolution evolution cluster despite despite 2 is attributed being being without without to the hydrogen-bubbledirectdirect-observations.-observations. evolution despite being without direct-observations.

(a)(a)5050 (b)(b)5050 Cluster Cluster 1 1 Cluster Cluster 1 1 Cluster Cluster 2 2 Cluster Cluster 2 2 4545 4545

4040 4040

3535 3535

Amplitude, dB

3030 3030

200200 225225 250250 275275 300300 325325 350350 0 0 5050 100100 150150 200200 250250 300300 350350 AverageAverage frequency, frequency, kHz kHz Time,Time, t / t hr/ hr

FigureFigure 11. 11. ((aa ())a TheThe) The resultresult result ofof of AEAE AE clusteringclustering clustering analysisanalysis analysis withwith with twotwo two clusters;clusters clusters; ((;b ())b EvolutionEvolution) Evolution of of cluster cluster 1 1 and and 2 2 duringduring localized localized corrosion corrosion in in specimenspecimen specimen A.A. A. OnlineOnline Online versionversion version inin in color.color. color.

FigureFigure 12. 12. SEM SEM images images via via serial serial sectioning sectioning of of the the localized localized corrosion corrosion area area in in specimen specimen A: A: (a ())a surfacesurface) surface morphologymorphology and and (b (–bd–))d ObservationsObservations) Observations ofof of cross-sectionscross cross-sections-sections 1–3.1 –13.–3. Online Online version version in in color.color. color.

CracksCracks within within the the corrosion corrosion products products are are believedbel believedieved to to be be generated generated with with the the precipitation precipitation and and accumulationaccumulation of of corrosion corrosion products products that that were were frequently frequently quoted quoted as as a a secondary secondary AE AE source source during during localizedlocalized corrosion corrosion andand and SCCSCC SCC [[8,14,8 [8,14,,14,191919––2121].–21].]. The The low-frequencylow low-frequency-frequency cluster cluster 1 1 waswas was accordinglyaccordingly accordingly associatedassociated associated withwith the the crackingcrack crackinging within within the the corrosioncorrosion corrosion products.products. products. Moreover, Moreover, hydrogen-bubblehydrogen hydrogen-bubble-bubble evolution evolution is is generally generally thoughtthought to to occur occur after after after the the the dissolution dissolution dissolution of of metal metal metal and and and formation formation formation of of occluded occluded occluded localsl ocals locals with with low-pH low low-pH-pH conditioncondition [5 [5,6,9,13,30,, [5,6,9,13,30,6,9,13,30,4545,4546,46,].46]. The ]. The The distribution distribution distribution of cluster-2 of of cluster cluster in- middle2- 2 in in middle middle and later and and periods late later r of periods periods the corrosion of of the the process,corrosioncorrosion as process, well process, as lagging as as well well behind as as lagging lagging cluster behind1, behind may logically cluster cluster 1support, 1 , maymay the logically logically source support identiﬁcation support the the source of source AE signalsidentificationidentification during of localizedof AE AE signals signals corrosion. during during localized localized corrosion. corrosion. Moreover,Moreover, an an obvious obvious increase increase in in the the AE AE population population was was observed observed around around 140 140–150–150 h, h, as as shownshown in in Figure Figures s10 10a aand and 11b 11b. This. This phenomenon phenomenon was was also also confirmed confirmed in in repeated repeated trials. trials. However, However, fromfrom the the in in-situ-situ surface surface observations observations (Figure (Figure 5), 5), no no exact exact change change could could be be determined determined around around this this pointpoint because because of of the the covered covered corrosion corrosion products. products. Considering Considering the the most most si signalsgnals from from cracking cracking within within

Materials 2019, 12, 2569 13 of 19

Moreover, an obvious increase in the AE population was observed around 140–150 h, as shown in Figures 10a and 11b. This phenomenon was also confirmed in repeated trials. However, from the in-situMaterials surface 2019, observations12, x FOR PEER REVIEW (Figure 5), no exact change could be determined around this point because13 of 19 of the covered corrosion products. Considering the most signals from cracking within the corrosion the corrosion products, this phenomenon may reflect a transition associated with the accumulation products, this phenomenon may reflect a transition associated with the accumulation of corrosion of corrosion products. A reliable interpretation requires an interrupted experiment in future works. products. A reliable interpretation requires an interrupted experiment in future works. 3.4.2. AE Signals during SCC in Specimen B 3.4.2. AE Signals during SCC in Specimen B Figure 13a shows the AE results with the profile of visible cracks from the in-situ observation Figure 13a shows the AE results with the profile of visible cracks from the in-situ observation of of specimen B. By the appearance of corrosion evolution and AE data, four stages were identified. specimen B. By the appearance of corrosion evolution and AE data, four stages were identified. Stage I Stage I and II were observed to be localized corrosion and slow crack growth with relatively and II were observed to be localized corrosion and slow crack growth with relatively moderate AE moderate AE events. However, the detailed separation between the localized corrosion and crack events.initiation However, was difficult the detailed to obtain separation because between the whole the cracklocalized-profile corrosion was masked and crack by the initiation corrosion was diffiproducts.cult to obtain In stage because III, the the crack whole shifted crack-profile to a high growth was masked rate with by the profuse corrosion AE signals. products. Finally, In stage crack III, thearrest crack occurred shifted to in a stage high growthIV. According rate with to a profuse detailed AE discussion signals. Finally,of the initiation crack arrest and occurredpropagation in stage of IV. AccordingSCC in another to a detailedwork [30], discussion the slow ofand the fast initiation crack growth and propagation were supposed of SCC to inbe anothercorresponding work [30 with], the slowthe and initiation fast crack and growth propagation were supposed of SCC to which be corresponding were predominantly with the controlled initiation and with propagation active path of SCCcorrosion which were and predominantlyhydrogen-assisted controlled cracking with mechanisms, active path respectively. corrosion and The hydrogen-assisted distinction in number cracking of mechanisms,AE signals respectively. of stage II and The III distinction may reflect in number the initiation of AE-to signals-propagation of stage transition II and III dur maying reflect SCC theprogression. initiation-to-propagation Figure 13b shows transition the results during of clustering SCC progression. analysis and Figurethree clusters 13b shows with different the results AE of clusteringfeaturesanalysis were recognized, and three i.e., clusters cluster with 1 with different low- AEamplitude features and were high recognized,-frequency,i.e., cluster cluster 2 and 1 with 3 low-amplitudewith resemble andd frequency high-frequency, components cluster 2but and with 3 with a distinct resembled strength frequency in amplitude. components By but a direct with a distinctcorrelation strength with in amplitude. the hydrogen By-bubble a direct evolution correlation captured with the in hydrogen-bubble [31], high-frequency evolution clustercaptured 1 was in [attributed31], high-frequency to be the hydrogen cluster 1-bubble was attributed evolution. to be the hydrogen-bubble evolution.

III (a) 70 3 25002500 (b) 70 AE amplitude IV 10 Cumulative AE events N Cluster 1 The profile of visible crack Cluster 2 60 20002000 Cluster 3 dB 60 II m

Stage I + II: Localized corrosion, slow crack growth  102 Stage III: Rapid crack propagation 15001500 50 Stage IV: Crack arrest 50

Stage I 10001000 length, 101 40 40

Amplitude, dB

AE Amplitude,

500500 Crack

30 Cumulative AE events, 100 0 0 30 0 5 10 15 20 25 30 35 40 45 50 200 225 250 275 300 Time, t / hr Average frequency, kHz

FigureFigure 13. 13.(a) ( Thea) The AE AE activity activity of of amplitude amplitude and and cumulative cumulative AE events and and the the profile proﬁle of of visible visible crack crack overover time time evolution evolution and and (b ()b the) the result result of of clustering clustering analysisanalysis showingshowing three three AE AE clusters clusters with with different diﬀerent AEAE features features during during SCC SCC evolution evolution in in specimen specimen B. B. Online Online versionversion inin color.color.

To interpretTo interpret clusters clusters 2 and 2 and3, the 3, distribution the distribution of AE of data AE and data in-situ and in observations-situ observations during during a selected a periodselected were period compared were as compared shown in as Figure shown 14 .in Figure Figure 14 14.a shows Figure the 14a evolution shows the of AEevolution data and of FigureAE data 14 b showsand the Figure successive 14b capture shows ofthe crack successive propagation. capture A phenomenon of crack propagation. of crack-branches A phenomenon coalescence wasof monitored,crack-branches as previously coalescence shown was inmonitored, another workas previously [31], i.e., shown the crack in another propagated work [31], to both i.e., the directions crack propagated to both directions with the extending and coalescence of two crack branches at each with the extending and coalescence of two crack branches at each crack tip. Images 6–8 highlight one crack tip. Images 6–8 highlight one coalescence event with the formation of a “metal island”. coalescence event with the formation of a “metal island”. Interestingly, the high-amplitude cluster 3 Interestingly, the high-amplitude cluster 3 was always concurrent with the occurrence of was always concurrent with the occurrence of crack-branch coalescence. Therefore, the crack-branch crack-branch coalescence. Therefore, the crack-branch coalescence may account for the AE cluster 3. coalescence may account for the AE cluster 3. Cluster 2 during the crack propagation may be from the Cluster 2 during the crack propagation may be from the plastic deformation process along with plasticcracking deformation growth. processTo support along it, EBSD with crackinganalysis of growth. an advancing To support crack it, tip EBSD is shown analysis in Figure of an advancing15. First, crackthe tip coalescence is shown in site Figure of the 15 . crack First,-branch the coalescence and the site metal of the island crack-branch were well and identified the metal for island the werephenomenon well identiﬁed of crackfor the propagation. phenomenon A of plasticity crack propagation. zone along A the plasticity crack zone path along and tip the was crack well path andconfirmed tip was well with conﬁrmed the high-intensity with the from high-intensity the KAM map, from indicating the KAM the map, local indicating plastic deformation. the local plastic As an important source for AE signals, plastic deformation during crack propagation is proposed to be the source mechanism of cluster 2 that is with resembling frequency components, but lower amplitude compared with the cluster 3 of cracking evolution. In a summary, the parameter of

Materials 2019, 12, 2569 14 of 19

Materials 2019, 12, x FOR PEER REVIEW 14 of 19 deformation. As an important source for AE signals, plastic deformation during crack propagation is proposedaverage- tofrequency be the source separated mechanism the signal of of cluster hydrogen 2 that-bubble is with evolution resembling (cluster frequency 1); Cluster components,2 and 3 but lowerhaving amplitude resembling compared frequency with components the cluster but 3 a of different cracking amplitude evolution. level In a were summary, attributed the to parameter the of average-frequencyplastic deformation separated and cracking the propagation, signal of hydrogen-bubble respectively. evolution (cluster 1); Cluster 2 and 3 Materials 2019, 12, x FOR PEER REVIEW 14 of 19 having resembling frequency components but a diﬀerent amplitude level were attributed to the plastic deformationaverage and-frequency cracking separated propagation, the signal respectively. of hydrogen-bubble evolution (cluster 1); Cluster 2 and 3 having resembling frequency components but a different amplitude level were attributed to the plastic deformation and cracking propagation, respectively. (a) 900 AE cluster 1 70 AE cluster 2 AE cluster 3 Profile of the visible crack

60  (a) 900800 AE cluster 1 70 AE cluster 2 50 AE cluster 3 Profile of the visible crack

length,

60  800700

Amplitude, dB 40

50 Crack

30 length, #1 #2 #3 #4 #5 #6 #7 #8 #9 700 Amplitude, dB 40 600

163500 163530 163560 163590 163620 Crack 30 Time, t / s #1 #2 #3 #4 #5 #6 #7 #8 #9 600 163500 163530 163560 163590 163620 Time, t / s (b)

(b)

FigureFigure 14. ( 14a).The (a) The evolution evolution of of three three AE AE clusters clusters duringduring the the selected selected duration duration;; (b) (Ab )phenomenon A phenomenon of of Figure 14. (a) The evolution of three AE clusters during the selected duration; (b) A phenomenon of crack-branch coalescence during crack propagation in specimen B. Marked numbers are corresponding crack-crackbranch-branch coalescence coalescence during during crack crack propagation propagation in in specimen specimen B. B. Marked Marked numbers numbers are are to Figurecorresponding 14correspondinga. Online to Figure version to Figure 14a in14a. Online color.. Online version version in in color. color.

Figure 15. (a) In-situ observation of one coalescence event at the advancing crack tip; (b) KAM map of the corresponding area calculated using third nearest-neighbor EBSD points and a maximum of 6◦ showing the localized plastic deformation zone. Online version in color. Materials 2019, 12, 2569 15 of 19

4. Discussion

4.1. Effect of Heat Treatment on the Corrosion Behavior of 13Cr MSSs 13Cr MSSs, based on a Fe-Cr-C ternary system, are usually thermally-treated to austenitic condition at elevated temperatures and then quenched to martensite state with body centered tetragonal crystal structure. Subsequently, as-quenched steels are often tailored to be with specific microstructure by tempering for a required combination of strength and ductility. For a 13Cr MSS with given chemical composition, the susceptibility to localized corrosion and SCC is largely dependent on its microstructure in relation to the residual stress due to austenite-to-martensite phase transformation during quenching treatment and Cr-rich secondary precipitation (mainly carbides) during tempering [37,40,41]. The matrix sites next to Cr-rich carbides have been confirmed to be a preferential site to localized corrosive attack [37]. Despite the disadvantage in precipitating Cr-rich secondary carbides (Figures3 and4), tempering treatment at 500 ◦C is usually preferred as a standard procedure for 13Cr MSSs because of effective stress relaxation. In this work, localized corrosion as the early precursor stage to SCC was intentionally controlled in a 500 ◦C-tempered steel specimen. Initiation and propagation of SCC were intended to be accelerated in the specimen of as-quenched condition with high-residual stress and low toughness. From the results, the heat treatment-based corrosion behaviors were confirmed to be reliable to simulate two main regimes of SCC evolution, i.e., early precursor of localized corrosion and the later stage of crack evolution. Moreover, the improved stress corrosion testing relevant to droplet corrosion was proven to be a simple preparation, with high repeatability and effectiveness in accommodating the in-situ optical observation and AE monitoring experiments.

4.2. Relationship between Corrosion Process and AE Behavior AE method as an effective NDT method has long been used to study the SCC of stainless steel. Extensive works in literature can be found on the AE behavior during SCC with face-centered cubic structured austenitic-grade stainless steel. However, less is known to the case with MSSs. In this work, the AE behaviors with two typical corrosion processes were comparatively studied with improved droplet SCC test. First, the AE activity both in population and feature is largely determined by the corrosion processes. In details, localized corrosion produced relatively weaker signals over longer-term exposure than the accelerated initiation and propagation of SCC. Second, with 3D views of the internal information of corrosion morphologies by XCT, the corrosion products containing cracks were confirmed in the localized corrosion of tempered specimen-A only, indicating that the condition for pit-to-crack transition in quenched specimen-B had been met early when pit was at its smaller size (Figures7–9, Figure 12) before corrosion products deposited inside it. Considering the standard heat treatment of specimen-A, AE signals during localized corrosion are worth being paid more attentions despite being weak. Since long-term localized corrosion in early stage of SCC evolution largely determines the lifetime of an exposed structure and component. The distinction of AE features between localized corrosion in specimen A and cracking propagation in specimen B could be used for in-situ corrosion monitoring against SCC-induced premature failures. Moreover, it is of great significance to separate the localized plastic deformation (Figure 14, Figure 15) from the cracking process using the amplitude-parameter. It was proposed that AE amplitude is fundamentally related to the length scale and velocity of the AE source [47]. As further demonstrated in [48], various AE sources could be related to approximate ranges of amplitude, indicating the potential of AE amplitude in discriminating the processes of dislocation motion during plastic deformation (approx. 30–40 dB at sensor) and cracking (approx. 40–80 dB at sensor). This rough criterion may support the classification of present AE data during cracking propagation (Figure 13, Figure 14). It is conventionally recognized that the hydrogen-assisted cracking mechanism in relation to the hydrogen embrittlement (HE) plays a dominant but not necessarily exclusive role in the SCC evolution of high-strength martensitic steels [1,30,40]. However, there is still a strong disagreement among researchers on the inherent nature of HE mechanism. The localized plasticity around the crack Materials 2019, 12, 2569 16 of 19 tip and the concurrent AE signals (cluster 2) seem to be inclined to support a role of hydrogen-enhanced localized plasticity (HELP) model in cracking process based on hydrogen-enhanced dislocation mobility for HE mechanisms [49,50]. Considering the amplitude-parameter is relatively independent on the selected threshold, the use of AE amplitude parameter would demonstrate another potential application in in-situ revealing the HE mechanism, which has some advantages over the traditional methods of microscopy.

4.3. Uncertainty on AE Classification and Source Identification SCC evolution is a complex process involving various events depending on both spatial and temporal evolution. The small dimensions, short timescale, and dynamic interplay between microstructure and changing electrochemical variables complicate the development of a complete understanding of the process evolution and whole morphology. From this work, 3D XCT technique and in-situ AE method were employed to comparatively study the localized corrosion and cracking evolution during SCC of a 13Cr MSS. Some spatial and temporal features related to SCC evolution were uncovered. With AE clustering analysis, in-situ observation, and correlative analysis, source mechanisms for recorded AE data were proposed. The identification of AE sources seems to be plausible. However, it must be emphasized that some uncertainty in AE classification and source identification still exist for several reasons. First, the damage events during corrosion process are always featured by mixture of long-termed incubation and instantaneous occurrence under corrosive electrolyte. The microscopic characteristics collected by conventional optical observation, either post-mortem or in-situ, cannot be corresponded one-on-one with their AE responses that are on the order of microseconds. Although in this work an attempt of in-situ observation was performed to reduce the time gap between recorded AE signals and observed damage events, the conclusions were indeed drawn based on some assumptions and speculations. Second, there is no universal clustering strategy in data science which can guarantee a perfect recognition between different AE populations. The unsupervised K-means algorithm used in this work presented reasonability in AE classification and interpretation. However, further proof based on improved clustering algorithm like supervised-machine learning and deep learning should be explored in future works. Concluding this section, it should hold an open view with respect to the statistical analysis of AE data and source identification of the SCC process because uncertainty can be caused by a number of factors. More experimental works including finer testing design and advanced analysis are necessary to get closer to the nature of corrosion evolution and the concomitant AE behavior.

5. Conclusions To better understand the initiation and propagation of SCC in 13Cr MSSs, localized corrosion as precursor evolution to SCC was simulated in a tempered specimen and cracking evolution was accelerated in an as-quenched steel specimen of SUS420J2. A comparative study of localized corrosion and cracking evolution were conducted using AE and XCT methods. Some conclusions were drawn based on the present experimental work.

(1) The corrosion susceptibility of SUS420J2 seemed to largely depend on the microstructure in relation to the Cr-rich M23C6 carbides and residual stress due to phase transformation during quenching treatment. With an improved droplet SCC testing, localized corrosion as the early precursor stage to SCC was intentionally controlled in a 500 ◦C-tempered steel specimen-A. Initiation and propagation of SCC were intended to be accelerated in the specimen-B of as-quenched condition with high-residual stress and low toughness. (2) Localized corrosion was featured by an irregular pit with deposited corrosion products containing cracks; while the SCC was observed to be initiated from a corrosion pit and propagated with a 3D tortuous and discontinuous morphology that might be segmented by some voids, rather than the pseudo-continuous from 2D characterization. Materials 2019, 12, 2569 17 of 19

(3) AE signals were detected in both cases. In case of localized corrosion, two clusters with diﬀerent frequency level were recognized. Low-frequency AE cluster was believed to be from the cracking within the deposited corrosion products; high-frequency cluster was proposed to be associated with hydrogen-bubble evolution. Contrastingly, three AE clusters were divided in the case of crack propagation. Except for the high-frequency hydrogen-bubble evolution, plastic deformation and cracking process seemingly could be separated by the use of AE amplitude-parameter. The distinction of AE features between localized corrosion and cracking propagation could be used for in-situ corrosion monitoring against SCC-induced premature failures.

Author Contributions: Conceptualization, K.W. and M.E.; Investigation, K.W.; Methodology, K.W., K.I., I.S. and P.C.; Writing—original draft, K.W.; Writing—review & editing; K.W. and M.E. Funding: This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Materials Integration for revolutionary design system of structural materials” (Funding agency: JST). Acknowledgments: Chihiro Nagasaki (The University of Tokyo) is acknowledged for his assistance with the XRD measurements. Masayoshi Yamamoto (ZEISS, Tokyo, Japan) is acknowledged for his assistance with XCT measurements. The authors also thank Fabien Briffod (The University of Tokyo), Takayuki Shiraiwa (The University of Tokyo) and Chao Yang (The University of Tokyo) for their kind helps in the experiments and data analysis. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Truman, J.E. Stress-corrosion cracking of martensitic and ferritic stainless steels. Int. Mater. Rev. 1981, 26, 301–349. [CrossRef] 2. Turnbull, A. Corrosion pitting and environmentally assisted small crack growth. Proc. R. Soc. A 2014, 470, 20140254. [CrossRef][PubMed] 3. Ono, K. Acoustic emission in materials research–a review. J. Acoust. Emission 2011, 29, 284–308. 4. Magaino, S.; Kawaguchi, A.; Hirata, A.; Osaka, T. Spectrum analysis of corrosion potential ﬂuctuations for localized corrosion of type 304 stainless steel. J. Electrochem. Soc. 1987, 134, 2993–2997. [CrossRef] 5. Wu, K.; Jung, W.S.; Byeon, J.W. In-situ monitoring of pitting corrosion on vertically positioned 304 stainless steel by analyzing acoustic-emission energy parameter. Corros. Sci. 2016, 105, 8–16. [CrossRef] 6. Wu, K.; Byeon, J.W. Morphological estimation of pitting corrosion on vertically positioned 304 stainless steel using acoustic-emission duration parameter. Corros. Sci. 2019, 148, 331–337. [CrossRef] 7. Jirarungsatian, C.; Prateepasen, A. Pitting and uniform corrosion source recognition using acoustic emission parameters. Corros. Sci. 2010, 52, 187–197. [CrossRef] 8. Shiwa, M.; Masuda, H.; Yamawaki, H.; Ito, K.; Enoki, M. In-situ observation and acoustic emission analysis

for corrosion pitting of MgCl2 droplet in SUS304 stainless steel. Mater. Trans. 2012, 53, 1069–1074. [CrossRef] 9. Fregonese, M.; Idrissi, H.; Mazille, H.; Renaud, L.; Cetre, Y. Initiation and propagation steps in pitting corrosion of austenitic stainless steels: monitoring by acoustic emission. Corros. Sci. 2001, 43, 627–641. [CrossRef] 10. Darowicki, K.; Mirakowski, A.; Krakowiak, S. Investigation of pitting corrosion of stainless steel by means of acoustic emission and potentiodynamic methods. Corros. Sci. 2003, 45, 1747–1756. [CrossRef] 11. Xu, J.; Wu, X.; Han, E.H. Acoustic emission during pitting corrosion of 304 stainless steel. Corros. Sci. 2011, 53, 1537–1546. [CrossRef] 12. Wu, K.; Jung, W.S.; Byeon, J.W. Acoustic emission of hydrogen bubbles on the counter electrode during pitting corrosion of 304 stainless steel. Mater. Trans., 2015, 56, 587–592. [CrossRef] 13. Wu, K.; Jung, W.S.; Byeon, J.W. Evaluation of corrosion critical variables of 304 stainless steel by delay time of acoustic emission. Mater. Trans., 2015, 56, 398–403. [CrossRef] 14. Yonezu, A.; Cho, H.; Takemoto, M. Monitoring of stress corrosion cracking in stainless steel weldments by acoustic and electrochemical measurements. Meas. Sci. Technol. 2006, 17, 2447–2454. [CrossRef] 15. Calabrese, L.; Bonaccorsi, L.; Galeano, M.; Proverbio, E.; Pietro, D.D.; Cappuccini, F. Identiﬁcation of damage evolution during SCC on 17–4 pH stainless steel by combining electrochemical noise and acoustic emission techniques. Corros. Sci. 2015, 98, 573–584. [CrossRef] Materials 2019, 12, 2569 18 of 19

16. Calabrese, L.; Galeano, M.; Proverbio, E.; Pietro, D.D.; Cappuccini, F.; Donato, A. Monitoring of 13% Cr martensitic stainless steel corrosion in chloride solution in presence of thiosulphate by acoustic emission technique. Corros. Sci. 2016, 111, 151–161. [CrossRef] 17. Xu, J.; Wu, X.; Han, E.-H. Acoustic emission response of sensitized 304 stainless steel during intergranular corrosion and stress corrosion cracking. Corros. Sci. 2013, 73, 262–273. [CrossRef] 18. Zhang, Z.; Wu, X.; Tan, J. In-situ monitoring of stress corrosion cracking of 304 stainless steel in high-temperature water by analyzing acoustic emission waveform. Corros. Sci. 2019, 146, 90–98. [CrossRef] 19. Shiwa, M.; Masuda, H.; Yamawaki, H.; Ito, K.; Enoki, M. In-situ observation and acoustic emission analysis

for SCC of MgCl2 droplet in SUS304 stainless steel. Mater. Trans. 2014, 55, 285–289. [CrossRef] 20. Cho, H.; Takemoto, M. Acoustic Emission from Rust in Stress Corrosion Cracking. In Proceedings of the 26th European Conference on Acoustic Emission Testing, Berlin, Germany, 15–17 September 2004. 21. Yonezu, A.; Taniyama, Y.; Cho, H.; Ogawa, T.; Takemoto, M. Acoustic emission and potential ﬂuctuation during chloride SCC with oxide in its surface. J. Soc. Mater. Sci. Jpn. 2006, 55, 211–217. [CrossRef] 22. Ito, K.; Yamawaki, H.; Masuda, H.; Shiwa, M.; Enoki, M. SCC monitoring of chloride droplets on thin SUS304 plate specimens by analysis of continuous recorded AE waveform. Mater. Trans. 2010, 51, 1409–1413. [CrossRef] 23. Takemoto, M.; Ueno, S.; Nakamura, M. Source of AEs from IG-SCC of face centered cubic metals under static or dynamic straining. J. Acoust. Emission. 2017, S32–S37. 24. Smanio, V.; Kittel, J.; Fregonese, M.; Cassagne, T.; Normand, B.; Ropital, F. Acoustic emission monitoring

of wet H2S cracking of linepipe steels: Application to hydrogen-induced cracking and stress-oriented hydrogen-induced cracking. Corrosion 2011, 67, 065002-1–065002-12. [CrossRef] 25. Du, G.; Li, J.; Wang, W.K.; Jiang, C.; Song, S.Z. Detection and characterization of stress-corrosion cracking on 304 stainless steel by electrochemical noise and acoustic emission techniques. Corros. Sci. 2011, 53, 2918–2926. [CrossRef] 26. Kovac, J.; Alaux, C.; Marrow, T.J.; Govekar, E.; Legat, A. Correlations of electrochemical noise, acoustic emission and complementary monitoring techniques during intergranular stress-corrosion cracking of austenitic stainless steel. Corros. Sci. 2010, 52, 2015–2025. [CrossRef] 27. ASTM G129-00. Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking; ASTM International: West Conshohocken, PA, USA, 2013. 28. ASTM G36-94. Standard Practice for Evaluating Stress-Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution; ASTM International: West Conshohocken, PA, USA, 2013. 29. NACE TM0177. Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments; NACE International (NACE): Houston, TX, USA, 2016. 30. Wu, K.; Briffod, F.; Ito, K.; Shinozaki, I.; Chivavibul, P.; Enoki, M. In-situ observation and acoustic emission monitoring of the initiation-to-propagation transition of stress corrosion cracking in SUS420J2 stainless steel. Mater. Trans. 2019.[CrossRef] 31. Wu, K.; Ito, K.; Shinozaki, I.; Enoki, M. Stress corrosion cracking monitoring of 13Cr martensitic stainless steel under chloride solution droplet by in-situ observation and acoustic emission clustering analysis. Corros. Sci.. (under review). 32. Connolly, B.J.; Horner, D.A.; Fox, S.J.; Davenport, A.J.; Padovani, C.; Zhou, S.; Turnbull, A.; Preuss, M.; Stevens, N.P.; Marrow, T.J.; et al. X-ray microtomography studies of localised corrosion and transitions to stress corrosion cracking. Mater. Sci. Technol. 2006, 22, 1076–1085. [CrossRef] 33. Horner, D.A.; Connolly, B.J.; Zhou, S.; Crocker, L.; Turnbull, A. Novel images of the evolution of stress corrosion cracks from corrosion pits. Corros. Sci. 2011, 53, 3466–3485. [CrossRef] 34. Zhu, L.K.; Yan, Y.; Li, J.X.; Qiao, L.J.; Volinsky, A.A. Stress corrosion cracking under low stress: Continuous or discontinuous cracks? Corros. Sci. 2014, 80, 350–358. [CrossRef] 35. Frankel, G.S.; Sridhar, N. Understanding localized corrosion. Mater. Today 2008, 11, 38–44. [CrossRef] 36. JIS G0567. Method of Elevated Temperature Tensile Test for Steels and Heat-Resisting Alloys; Japanese Industrial Standards Committee: Tokyo, Japan, 2019. 37. Lu, S.Y.; Yao, K.F.; Chen, Y.B.; Wang, M.H.; Liu, X.; Ge, X. The effect of tempering temperature on the microstructure and electrochemical properties of a 13 wt.% Cr-type martensitic stainless steel. Electrochim. Acta 2015, 165, 45–55. [CrossRef] Materials 2019, 12, 2569 19 of 19

38. Davis, J.R. Corrosion Testing: Tests for Stress-Corrosion Cracking. In Metals Handbook Desk Edition, 2nd ed.; Davis, J.R. ASM International: Ohio, OH, USA, 2001; pp. 1348–1351. 39. Ito, K.; Enoki, M. Acquisition and analysis of continuous acoustic emission waveform for classification of damage sources in ceramic fiber mat. Mater. Trans. 2007, 48, 1221–1226. [CrossRef] 40. Anantha, K.H.; Örnek, C.; Ejnermark, S.; Thuvander, A.; Medvedeva, A.; Sjöström, J.; Pan, J. Experimental and modelling study of the effect of tempering on the susceptibility to environment-assisted cracking of AISI 420 martensitic stainless steel. Corros. Sci. 2019, 148, 83–93. [CrossRef] 41. Bonagani, S.K.; Bathula, V.; Kain, V. Influence of tempering treatment on microstructure and pitting corrosion of 13 wt.% Cr martensitic stainless steel. Corros. Sci. 2018, 131, 340–354. [CrossRef] 42. Li, S.X.; Hihara, L.H. Atmospheric corrosion initiation on steel from predeposited NaCl salt particles in high humidity atmospheres. Corros. Eng. Sci. Techn. 2010, 45, 49–56. [CrossRef] 43. Sanchez, J.; Andrade, C.; Fullea, J. Reasons for crack arrest in stress corrosion cracking tests-crack propagation rate in high-strength steels. Corrosion 2009, 65, 368–375. [CrossRef] 44. Qian, L.; Toda, H.; Uesugi, K.; Kobayashi, M.; Kobayashi, T. Direct observation and image-based simulation of three-dimensional tortuous crack evolution inside opaque materials. Phys. Rev. Lett. 2008, 100, 115505. [CrossRef] 45. Galvele, J.R. Transport processes and the mechanism of pitting of metals. J. Electrochem. Soc. 1976, 123, 464–474. [CrossRef] 46. Pistorius, P.C.; Burstein, G.T. Metastable pitting corrosion of stainless steel and the transition to stability. Philos. T. R. Soc. A 1992, 341, 531–559. 47. Robertson, I.M. The effect of hydrogen on dislocation dynamics. Eng. Fract. Mech. 1999, 64, 649–673. [CrossRef] 48. Martin, M.L.; Dadfarnia, M.; Nagao, A.; Wang, S.; Sofronis, P. Enumeration of the hydrogen-enhanced localized plasticity mechanism for hydrogen embrittlement in structural materials. Acta Mater. 2019, 165, 734–750. [CrossRef] 49. Scruby, C.B.; Wadley, H.N.G.; Hill, J.J. Dynamic elastic displacements at the surface of an elastic half-space due to defect sources. J. Phys. D Appl. Phys. 1983, 16, 1069–1083. [CrossRef] 50. Barsoum, F.F.; Suleman, J.; Korcak, A.; Hill, E.V. Acoustic emission monitoring and fatigue life prediction in axially loaded notched steel specimens. J. Acoust. Emission 2009, 27, 40–63.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).