Pitting Of Stainless Steel Research Articles

Selective Initiation of Corrosion Pits in Stainless Steel Using Scanning Electrochemical Cell Microscopy.

Scanning electrochemical cell microscopy is a useful technique for determining variations in corrosion behavior across a surface. However, the numerous options for experimental parameters and little understanding of their effect on the corroding system render comparisons of results between studies difficult. Herein, we explore changes in corrosion behavior of two martensitic stainless steels, a cast CA6NM and a wrought S41500, as a result of the chosen experimental parameters, including scan rate, approach potential, surface oil immersion, and tip aperture diameter. The study demonstrates that these experimental parameters can be controlled to probe oxide passivation kinetics and single pitting events by changing the surface state and cathodic currents. We measured the pitting and repassivation kinetics of a single pit and determined the compositional change of the Al2O3 inclusion site initiation point. Hundreds of data points were measured within 17 h of experimental time on the stainless steel samples, allowing statistical averages of corrosion and pitting values. This work will open new avenues for fine-tuning various corrosion aspects at the microscale, thereby contributing to a deeper understanding of the corrosion processes and mechanisms of diverse materials.

Repassivation Temperature of One-Dimensional Artificial Pits in Stainless Steel

A new framework has been developed in recent works to describe pit growth stability 1-8, based on which many critical scientific issues in localized corrosion were addressed, such as the rate-controlling step in pit growth, the salt film growth and self-regulating mechanism, and the kinetics and the activation energy of metal dissolution in the local pit environment. Although many new insights were generated, there are still some phenomena that are not well understood, such as the pit repassivation temperature (T rp). According to the new framework, T rp is predicted to decrease with the increase of applied potential (E app). However, T rp sometimes exhibits an unexpected dependence on E app that contradicts with the prediction, and the underlying mechanism is not clear.In this study, T rp of one-dimensional artificial pits made from 316L stainless steel was investigated in 3.5 wt.% NaCl by downward temperature scanning. For a given pit depth, T rp exhibits an unexpected “S” dependence on applied potential, which has never been reported previously. Further analysis shows that this relationship is derived from repassivation underneath the salt film, which is different from repassivation that occurs when the pit surface concentration decreases below a critical value. This indicates that there is a reversal of the effect of the salt film on pit growth stability. The systematic study shows that such a reversal depends on temperature, E app, and pit depth. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.References G. S. Frankel, T. Li, and J. R. Scully, Journal of the Electrochemical Society, 164 (4), C180-C181 (2017).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (9), C484-C491 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (11), C762-C770 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (6), C115-C124 (2019).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (11), C3341-C3354 (2019).T. Li, J. Wu, and G. S. Frankel, Corrosion Science, 182 109277 (2021).T. Li, J. Wu, X. Guo, A. M. Panindre, and G. S. Frankel, Corrosion Science, 193 109901 (2021).T. Li, D. E. Perea, D. K. Schreiber, M. G. Wirth, G. J. Orren, and G. S. Frankel, Corrosion Science, 174 108812 (2020).

2-Dimensional Simulations of Pit Propagation and Multi-Pit Interactions

This work presents a reaction-transport model for pit propagation coupled with a phase field method to model the moving boundary at the corroding surface. This enables numerical simulations of the simultaneous propagation of multiple pits in close proximity to each other to study the interactions between pits under galvanostatic conditions, with limited applied currents. Results show the formation of lacy covers over pits in stainless steel, which is due to undercutting of the surrounding surface, and reveal the development of other complex morphologies arising from the interaction between neighboring pits; e.g., the growth of “pits within pits” and the evolution of “champion pits.” Such observations are due to the shrinkage of the total active surface to sustain the limited current as pits grow larger. The proposed model can be a valuable tool for studying the evolution of pit morphology in more realistic scenarios when multiple pit initiation sites are present.

Investigation of the Microstructure Dependence of Critical Pitting Temperature and Pitting Potential in a 2205 Duplex Stainless Steel

Dependence of 2205 duplex stainless steel pitting resistance on its microstructure was investigated using microstructural examinations and potentiodynamic and potentiostatic polarization techniques. An as-received hot-rolled alloy was subjected to different heat treatments to understand how microstructural characteristics individually affect the corrosion resistance in terms of critical pitting temperature (CPT), pitting potential (E pit), and metastable pitting characteristics. Results revealed that there is a transition temperature interval for the 2205 DSS. By promoting pit initiation and pit propagation, the Widmanstätten structure significantly deteriorated both CPT, by 10 °C, and E pit, compared to the as-received hot-rolled specimen. The σ phase appearance was also associated with a minor drop in CPT, but a considerable reduction in E pit as it increased the occurrence frequency of metastable pits. The specimen having equiaxed grains had the highest CPT, underwent stable pitting at 47 °C, but still it had a low E pit compared to the hot-rolled specimen due to its higher occurrence frequency of metastable pits. The formation of the Widmanstätten austenite needles weakened the ferrite phase in terms of the pitting resistance by reducing the contents of Cr and Mo in it and made it more susceptible to pit propagation.

Reexamining the Role of Mns Inclusions in Pitting of Stainless Steel

Most models of pit initiation in austenitic stainless steels involve the anodic dissolution of manganese sulfide (MnS) inclusions. This work reexamined the role of MnS inclusions and looked at whether they are anodic or cathodic to the matrix metal surrounding them. Specifically, pit formation and the initial stages of growth were investigated using potentiodynamic polarization paired with microscopy.Stainless steel (SS) 304 samples were exposed to 0.5 M NaCl and slowly polarized from 0.2 VSCE to 0.4 VSCE. Polarization scans showed evidence of metastable pits and, in some cases, pit growth. Microscopy of sample surfaces revealed many small open pits with an occasional larger covered pit. The small open pits were associated with inclusions of various types that had fallen to the bottom (Figure 1a). The larger covered pits had MnS inclusions that remained attached to the sample surface (Figure 1b). The attached inclusions acted as a pit cover during initial stages of growth. Growth directions indicated by the perforated pit covers usually showed asymmetric enlargement that is likely controlled by local metallurgical conditions such as crystal orientation and grain boundaries.Examination of MnS inclusions in both open and covered pits showed no evidence of significant corrosion of the inclusion. The electrochemical behavior of MnS samples was analyzed subsequently using potentiodynamic polarization scans. MnS was passive in 0.5 M NaCl up to the maximum potential applied, 0.85 VSCE. The open circuit potential of MnS was approximately -0.26 VSCE, which is slightly more active than that for bulk SS 304, and the passive current density ranged from slightly less than to slightly greater than that of SS 304 up to the pitting potential of SS 304.Based on the results from this study, the following model for pit initiation and initial growth is proposed for SS 304 in NaCl solutions: Pits initiate at cathodic inclusions (MnS, oxides, etc.) with corrosion occurring in the matrix metal at the inclusion / metal interface.When corrosion occurs around the entire interfacial area of the inclusion, the inclusion falls into the pit and repassivation can occur.When corrosion does not occur evenly around the inclusion due to local metallurgical conditions (e.g., composition, crystal orientation, grain boundaries, etc.), the inclusion acts as a cover and pit growth can occur. Figure 1

Long-Term Effects of Humidity on Stainless Steel Pitting in Sea Salt Exposures

Ground 304 stainless steel (SS) samples were exposed to sea salt particles at 35 °C and two relative humidity (RH) levels for durations ranging from 1 week to 2 years. For all exposure times, pit number density and total pit volume at 40% RH were observed to be considerably greater than those at 76% RH. Statistical analysis of distributions of pit populations for both RH conditions showed that pit number density and total pit volume increased rapidly at first but slowed as exposure time increased. Cross-hatched features were observed in the 40% RH pits while ellipsoidal, faceted pits were observed at 76% RH. Optical profilometry indicated that most pits were not hemispherical. X-ray tomography provided evidence of undercutting and fissures. Piecewise curve fitting modeled the 40% RH data closely, predicting that corrosion damage would eventually plateau. However, a similar treatment of the 76% RH data suggested that corrosion damage would continuously increase, which implied that the piecewise power-law fit was limited in its ability to model atmospheric corrosion generally. Based on these observations, the operative mechanisms determining long-term corrosion behavior were hypothesized to be different depending on the RH of exposure.

The Role of Pitting at Mns Inclusions in Intergranular Corrosion of Sensitized Type 304 Stainless Steel in NaCl Solution

The degree of sensitization and susceptibility of intergranular corrosion of austenitic stainless steels are properly evaluated from immersion testing in acidic solutions as rapid screening methods. However, most of stainless steels are used in near-neutral pH environments, and the typical corrosion morphology in these environments is pitting. Acidification is known to occur inside pits due to the hydrolysis reaction of dissolving metal ions. Under practical service conditions around neutral pH such as seawater and river water, pitting is assumed to be closely related to the initiation of intergranular corrosion. It is well-known that non-metallic inclusions, such as MnS, act as the initiation sites for pitting of stainless steels. Chiba et al. demonstrated that a small inclusion-free area on Type 304 indicated superior pitting corrosion resistance even in 3 M NaCl. 1) It would appear that MnS inclusions strongly affect pitting and intergranular corrosion of sensitized stainless steels. To clarify the role of pitting at MnS inclusions in intergranular corrosion of sensitized Type 304 stainless steel (sensitized at 923 K), in situ observations were carried out during micro-scale anodic polarization in 1 M NaCl (pH 5.4). Micro-scale potentiodynamic anodic polarization was performed for a small area with a MnS inclusion at a sensitized grain boundary. Intergranular corrosion was initiated from the pit that was generated at the MnS inclusion located at the sensitized grain boundary. Acidification inside the pit probably caused intergranular corrosion due to the transition from passive to active state. In 1 M NaCl, the depassivation pH values at the sensitized grain boundary with and without MnS inclusions were 1.0 and 0.4, respectively. It is likely that the dissolution of MnS accelerates the depassivation and promote the initiation pf intergranular corrosion. The dissolution products from MnS are supposed to accelerate depassivation of Cr-depleted regions. 1) A. Chiba, I. Muto, Y. Sugawara, N. Hara, J. Electrochem. Soc., 160, C511-C520 (2013).

Stainless steel corrosion scale formed in reclaimed water: Characteristics, model for scale growth and metal element release

Stainless steels generally have extremely good corrosion resistance, but are still susceptible to pitting corrosion. As a result, corrosion scales can form on the surface of stainless steel after extended exposure to aggressive aqueous environments. Corrosion scales play an important role in affecting water quality. These research results showed that interior regions of stainless steel corrosion scales have a high percentage of chromium phases. We reveal the morphology, micro-structure and physicochemical characteristics of stainless steel corrosion scales. Stainless steel corrosion scale is identified as a podiform chromite deposit according to these characteristics, which is unlike deposit formed during iron corrosion. A conceptual model to explain the formation and growth of stainless steel corrosion scale is proposed based on its composition and structure. The scale growth process involves pitting corrosion on the stainless steel surface and the consecutive generation and homogeneous deposition of corrosion products, which is governed by a series of chemical and electrochemical reactions. This model shows the role of corrosion scales in the mechanism of iron and chromium release from pitting corroded stainless steel materials. The formation of corrosion scale is strongly related to water quality parameters. The presence of HClO results in higher ferric content inside the scales. Cl− and SO42− ions in reclaimed water play an important role in corrosion pitting of stainless steel and promote the formation of scales.

Can the Point Defect Model Explain the Influence of Temperature and Anion Size on Pitting of Stainless Steels

(Received July 22, 2015; Revised November 03, 2015; Accepted November 03, 2015)The pitting behaviours of 304L and 316L stainless steels were investigated at 3 °C to 90 °C in 1 M solutions of NaCl, NaBr and NaI by potentiodynamic polarization. The temperature dependences of the pitting potential varied according to the anion, being near linear in bromide but exponential in chloride. As a result, at low temperatures grades 304L and 316L steel are most susceptible to pitting by bromide ions, while at high temperatures both stainless steels were more susceptible to pitting by small chloride anions than the larger bromide and iodide. Thus, increasing temperature appears to favour attack by smaller anions. This paper will attempt to rationalise both of the above findings in terms of the point defect model. Initial findings are that qualitatively this approach can be reasonably successful, but not at the quantitative level, possibly due to insufficient data on the mechanical properties of thin passive films.Keywords: stainless steel, pitting corrosion, anion size

Experimental and Modeling Studies on Mass Transport and Electrochemical Factors Influencing Stainless Steel Pitting and Repassivation

Studies on the critical conditions describing pitting stability and repassivation have focused on either determining the dissolution flux and solution chemistry or the potential. Experimental and modeling techniques toward building a quantitative relationship among the critical conditions were performed in this study. The artificial pit (lead-in-pencil) electrode constructed from stainless steel wires of small diameter was used as the tool to obtain measurable estimates of the critical dissolution flux and potential in aqueous sodium chloride (NaCl) solution. Both estimates were obtained using a single experimental technique. Experimental results so obtained were utilized to develop a one-dimensional diffusional model with representative boundary conditions. Results from these analyses indicated that the critical surface concentration required for stable pitting (or to prevent repassivation) may be more dilute than previously accepted in the literature.

The Artificial Pit Electrode as a Technique to Evaluate Critical Mass Transport and Electrochemical Parameters in Stainless Steel Pitting and Repassivation

The critical conditions of mass transport and localized chemistry required for a 1-D pit to grow stably were first examined by Galvele (1), resulting in the pit stability product (i*x) as the key parameter of interest. Experimentally determining this parameter requires a quasi-steady-state configuration (2) such as that under a salt film (3, 4). The lowest potential at which this film is stable has been termed the transition potential Et (5). However, pitting is known to proceed stably at potentials lower than Et (6, 7), and hence Et cannot be used as a conservative lower bound. This means that a salt film is not critical to stability and the critical pit stability product (i*x)crit = f*(i*x)salt film, where f is some fraction of saturation. The repassivation potential Erp, the potential below which all existing pits passivate, has been proposed as an alternative critical potential (8). In this regard, Erp has to be examined in terms of diffusive transport in order to relate it to pitting stability. Moreover, studying the effects of experimental variables on (i*x) and Erp enables understanding their influence on pitting stability, providing information for corrosion mitigation strategies. A cyclic artificial pit technique was developed to enable high-efficiency, automated data collection and analysis. This approach utilized an artificial pit constructed from stainless steel wire (diameter 50-100 μm), grown to different depths applying a high potential in neutral chloride solution for different time periods, after which a cathodic potentiodynamic sequence was employed, a variation of the method of Sridhar and Cragnolino (8). The entire sequence was looped multiple times. The pit stability product under a salt film, (i*x)salt film was obtained from the diffusion-limited dissolution current density at different depths (5), and Erp was noted as the potential at which a significant number of current density data points were lower than 30 μA/cm2 (8, 9). This approach enabled the measurement of both critical parameters from the same experiment and the direct correlation of Erp to pit depth and diffusive transport. The high level of automation permitted sampling a wide region of parameter space. The large volume of data from this technique facilitated replication and a statistical treatment of (i*x) and Erp. Data obtained were also utilized to develop a mass-transport-based model relating Erp and the surface concentration at the edge of pit stability, based on which an estimate of the critical degree of saturation, f was obtained. Simultaneous examination of the effects of different variables on Erp, salt film solubility, and kinetics was possible. As shown in Figure 1, (i*x)salt film values of three stainless steels tested were the same within the limits of experimental scatter. Erp for 17-4 PH was seen to be slightly higher than for 316L. Erp for 17-7 PH was much lower, leading to the hypothesis that electroactive copper in 17-4 PH may contribute to interfering redox reactions (10), yielding an artificially high Erp. Current-peak analysis (11) using data from the same experiment allowed this hypothesis to be tested. Studies have focused on the inhibitory effects of oxyanions on the pitting potential and therefore, pit initiation (12). However, examining the effects of these anions on pit repassivation and stability is important to understand their influence on pit growth. As shown in Figure 2, salt film solubility and repassivation potential are affected differently by the oxyanions. Preliminary analysis shows that the oxyanions may enhance the cathodic reaction in the pit solution, resulting in a higher measured Erp. Real-time ohmic compensation could also be included in the polarization sequence inside the loop to examine the approach to Erp, which may help understand the inhibitory mechanisms better.

Correlative tomography.

Increasingly researchers are looking to bring together perspectives across multiple scales, or to combine insights from different techniques, for the same region of interest. To this end, correlative microscopy has already yielded substantial new insights in two dimensions (2D). Here we develop correlative tomography where the correlative task is somewhat more challenging because the volume of interest is typically hidden beneath the sample surface. We have threaded together x-ray computed tomography, serial section FIB-SEM tomography, electron backscatter diffraction and finally TEM elemental analysis all for the same 3D region. This has allowed observation of the competition between pitting corrosion and intergranular corrosion at multiple scales revealing the structural hierarchy, crystallography and chemistry of veiled corrosion pits in stainless steel. With automated correlative workflows and co-visualization of the multi-scale or multi-modal datasets the technique promises to provide insights across biological, geological and materials science that are impossible using either individual or multiple uncorrelated techniques.

Mass Transport and Electrochemical Factors Influencing Stainless Steel Pitting and Repassivation in Neutral Chloride Media

not Available.

Stainless steel pitting and early-stage stress corrosion cracking under ultra-low elastic load

Pitting and crack nucleation at the early stages of stress corrosion cracking (SCC) under ultra-low elastic load were studied. Pits initiated on the sample surface and grew faster in the thickness direction rather than other directions. This corresponds with the stress and strain concentrations calculated using the finite element analysis. At the ultra-low elastic stress levels it is essential for the stress corrosion cracks to emanate from the pits/defects on the sample surface. Furthermore, the preferential SCC initiation sites are at the shoulders, rather than at the bottoms of the surface pits/defects.

Pitting corrosion of steel 12Kh18N10T under heat transfer conditions

The effect of laminar movement and heat transfer on the occurrence and development of stainless steel pitting is studied, using a device based on a system of two disks, one of which (working heat transfer specimen) is stationary, and rotation of a second disk providing solution movement. It is shown that the effect of heat transfer between metal and solution on pitting formation depends on metal temperature and heat flux direction. With a metal temperature of 50 °C under heat transfer conditions from metal to solution with an increase in temperature drop pitting corrosion potential increases, and under conditions of heat transfer from solution to metal it decreases. Presence of a heat flux in any direction through a metal–solution phase boundary facilitates the development of pits, but makes their repassivation difficult, and thereby deepens corrosion.

(Invited)Localized Dissolution Kinetics From Fast In Situ Radiography of Propagating Pits in Stainless Steel and Implications for Modeling Pitting Corrosion over Long Time-Scales

In-situ synchrotron radiography has been used to observe propagating 2D pits in a thin foil of 304 stainless steel sandwiched between glass slides. Key kinetic parameters have been extracted from these radiographs such that they can be used as input parameters for numerical simulations of pitting corrosion. Simulation results are presented for pits growing under potentiostatic control, with a focus on propagation stability in the concentrated (> 1M) chloride solutions that cause atmospheric pitting corrosion of 304 stainless steel.

Improvement of Pitting Corrosion Resistance of Type 316L Stainless Steel by Potentiostatic Removal of Surface MnS Inclusions

The beneficial effect of the removal of MnS inclusions on the pitting of stainless steels has been demonstrated in two ways. (1) High-purity Type 316L stainless steel with no inclusions was used as a specimen in the measurement of anodic polarization curves in 0.5 M NaCl and (2) commercial Type 316L stainless steel with MnS and slag-related inclusions was first polarized at different potentials for 30 min in 1 M Na2SO4of pH 3 and then anodic polarization measurements were taken in 0.5 M NaCl. Pitting did not occur in the passive or transpassive region of the high-purity steel. The polarization treatment dissolved MnS and some oxide inclusions (CaO and SiO2) on the surface of the commercial steel. An increase in pitting potential of the commercial steel was noted after treatment at potentials above 0.2 V. At the same time, the number of current spikes due to metastable pits decreased significantly. These results are more likely due to the beneficial effect of removing MnS inclusions from the steel surface rather than the modification effect of the chemical composition of passive films on the surface.

Localised Dissolution Kinetics From Fast In Situ Radiography of Propagating Pits in Stainless Steel and Implications for Modeling Pitting Corrosion over Long Time-Scales

not Available.

Computational simulation of metastable pitting of stainless steel

Abstract Metastable pit growth, a critical stage of pitting corrosion, has an important role in understanding pitting corrosion which involves a series of complicated interface electrochemical reactions. In this paper, a computational model which simulates metastable pit growth and its transition to stable growth is developed. The model based on the cellular automata approach contains several evolution rules that govern electrochemical reactions, solution reactions and diffusion. The results of simulated metastable pit growth kinetics are compared with the experiment data quoted from the literature. The influence of different factors including the diffusion of cations, the extent of the rupture of protective film, the pit radius and the existence of salt film on growth transition is analyzed. In order to improve the simulation efficiency, the Block algorithm is introduced which can accelerate the simulation of reaction–diffusion system. The mechanism of transition to passivity is discussed through comparing the simulation results with experimental data.

Pitting resistance criteria for corrosion-resistant steels

Comparative analysis is provided for currently used and proposed criteria for stainless steel pitting resistance, i.e., the potential for forming a salt film. It is shown that the potential for forming a salt film is specified by current oscillations, that is a typical sign of pitting corrosion, and it makes it possible to predict stainless steel pitting corrosion limits more precisely. Comprehensive use of the applied and proposed pitting resistance criteria make it possible to separate clearly the possible conditions of a passive film in three areas: an area of a passive condition or stable passivity, where the probability of formation and development of previously occurring pitting is zero; an area of a metastable condition or unstable passivity within which there is a non-zero probability fore existence of previously occurring pitting and the generation of new repassivating pitting; an area of pitting formation where formation and a stable increase in pitting is possible.