Metastable Pit Growth Research Articles

Corrosion behavior and passivation property of CoCrFeNi HEA in a simulated seawater environment containing CO32−/HCO3−

AbstractThis work systematically studied the corrosion behavior and passivation property of CoCrFeNi high‐entropy alloys (HEAs) in a simulated seawater environment. The results reveal that the addition of CO32−/HCO3− results in the secondary passivation of the HEA in NaCl solution, and secondary passivation film possesses higher dissolution rate in comparison with primary passive film. H+ ions ionized by HCO3− facilitate the charge transfer process, thin the thickness of passive film, and increase the disordered degree of the film. Moreover, the presence of HCO3− promotes the nucleation and growth of metastable pits, and enhances the pitting sensitivity. Furthermore, an increase in CO32− concentration accelerates the dissolution of passive film, weakens the compactness and protective performance of the film, and increases the probability that metastable pitting evolves into stable pitting, as well aggravates the corrosion of the HEA. In addition, the corrosion resistance of the HEA is diminished due to the formation of microgalvanic corrosion cells between matrix and inclusion. The selective dissolution of elements occurred in the HEA.

Effect of pH on corrosion behaviour of CoCrNi MEA imposed by alternating current in Na2CO3/NaHCO3 solution

ABSTRACT The corrosion behaviour and mechanism of CoCrNi medium-entropy alloy (MEA) imposed by alternating current (AC) in HCO3 −/CO3 2− solution with various pH were systematically investigated via a series of corrosion tests. The results showed that with the augment of pH value, the polarisation resistance of the MEA decreased significantly, Ep shifted negatively, and ipI value increased, as well as corrosion rate was enhanced, revealing that the corrosion resistance was weakened. As solution pH increased, the formation of secondary passivation zone became easier, which may be attributed to the reaction conversion process of Cr(III) to Cr(VI) species. Moreover, the increase in pH led to an increase in defect density and a thinning of passivation film, facilitating the corrosion of the MEA. AC interference accelerated the ion exchange process and promoted the formation possibility of stable pitting. Higher iAC and pH inhibited the repassivation of metastable pitting, and increased the pitting susceptibility. Highlights The effect of pH on the corrosion behaviour of CoCrNi MEA with imposed AC was clarified. High pH and iAC accelerated the nucleation and growth of metastable pits. With increased pH and iAC, the stability and protective ability of passive film were weakened. The secondary passivation of the MEA was mainly due to the conversion of Cr(III) to Cr(VI) species.

Beneficial effect of copper on pitting resistance of Ni-Cr-Fe alloys

This study examines the effect of copper alloying on pitting resistance in a model solid solution FCC Ni-13%Cr-10%Fe alloy through potentiodynamic and potentiostatic polarization in 0.1 M NaCl in conjunction with an analysis using first-principles competitive electro-chemisorption modeling. The pitting potential increased with increasing Cu content in the alloy. Furthermore, the extent of metastable pit growth was suppressed and the incubation time for metastable to stable pit transition increased with Cu content. The first-principles competitive adsorption calculations suggested that Cu alloying suppresses chloride ion adsorption on the alloy surface in a simulated pit environment, which inhibits active dissolution at the pit bottom, enhances proton adsorption, and thereby increases the local pH at the pit bottom. We propose that these two effects of Cu in solid solution combine to reduce pit stability and may act in addition to the enrichment of Cu on the corroding pit surface.

Development of an In Situ Micro-Corrosion Cell for the Investigation of Pitting Corrosion on Austenitic and Ferritic Stainless Steels

In order to investigate the electrochemical pitting corrosion in more detail, a micro-corrosion cell was developed, allowing real-time in situ optical observations of steel surfaces in direct correlation with electrochemical measurement results. In this study, the austenitic 1.4301–X5CrNi18-10 and the ferritic 1.4016–X6Cr17 stainless steel grades were examined in electrolytes containing chloride ions. The micro-corrosion cell revealed a stable pitting corrosion of the ferritic 1.4016 and metastable pitting corrosion of the austenitic 1.4301. The pits were characterized by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM) in detail. A clear correlation between current peaks in the current density-potential curve and the growth of many small pits on the test surface was established and was identified as metastable pit growth. In general, the pitting corrosion potential increased as the diameter of the test surface decreased for both stainless steels. In contrast to the complex precipitates of 1.4301, chromium precipitates with a significantly higher amount was detected on the entire surface of the 1.4016. The corrosion initiation was identified at the interface between the precipitates and the base material for both stainless steels. By comparing both materials, the ferritic 1.4016 had a lower pitting corrosion potential than the austenitic 1.4301 under all test conditions.

Temperature-dependent corrosion behaviour of the amorphous steel in simulated wet storage environment of spent nuclear fuels

Temperature dependence of corrosion behaviour in simulated wet storage conditions for spent nuclear fuels were investigated systematically by comparing SAM2X5 amorphous steel and 316 L SS. Results indicated that with elevating temperature, the passive current for the amorphous steel increased distinctly but without pitting corrosion. The sensitivity of the passive current with temperature for the amorphous steel was related to the reduction of Cr2O3 content and altered semiconductor characteristics for its passive film, and the prominent pitting resistance at higher temperatures was associated with the remarkably inhibited pit initiation by the uniform passive film and the suppressed stable growth of metastable pits.

Pitting resistance of 316 stainless steel after laser shock peening: Determinants of microstructural and mechanical modifications

• Grain refinement was avoided through modified LPwC. • Compressive residual stress after LSP reduces the metastable pitting rate. • Microdefects induced by LSP dominate the growth of metastable pits. • Adjacent metastable pits combine easily to transform into a stable pit after LSP. Laser shock peening (LSP) was performed on a 316 stainless steel for microstructural and mechanical modifications. To identify the dominant factor controlling pitting corrosion resistance, a single LSP impact was performed and various characterization techniques, such as scanning electron microscopy, X-ray diffraction, transmission electron microscopy, electron backscattered diffraction, microhardness and residual stress analysis were conducted. The results show that a considerable increase in the microdefects, microhardness, and compressive residual stress occurred at a depth of ∼200 μm from the surface, while no evident grain refinement was observed. The compressive residual stress reduced the metastable pitting nucleation rate. However, microdefects after LSP treatment resulted in larger metastable pits, which are more likely to combine to transform into a stable one. It is summarised that the substantial microdefects can deteriorate the superiority of compressive residual stress induced by LSP in the pitting resistance of 316 stainless steel.

Effects of U‐Bending on Pitting Corrosion of 304L Stainless Steel

The influence of cold U‐bending on pitting corrosion of 304L stainless steel in an acidic NaCl solution is investigated using surface analyses and electrochemical measurements. The pitting behavior of the exterior and interior surfaces of U‐bended specimen exhibiting tensile and compressive residual stress respectively are both considered. The results show that microstructural modification by U‐bending promotes the pit nucleation. Considering the passive film properties after U‐bending, no significant change in both the thickness and Cr/Fe ratios of the passive films formed on the two deformed surfaces can be found. The accompanying residual stresses after U‐bending, especially the tensile residual stress, exhibits a remarkable acceleration of the metastable pit growth rate. Therefore, the pitting potential declines significantly for the exterior surface of U‐bended specimen. However, the compressive residual stress in the interior surface facilitates the repassivation of the metastable pit, can nullify the negative effect of microdefects for the pitting corrosion resistance, so that the pitting potential of the interior surface of U‐bended specimen does not decrease.

Effect of Austenite Reformation on Localized Corrosion Resistance of Hyper-Duplex Stainless Steel in Hot Chloride Solution

The materiality of duplex stainless steels for deep-sea applications, where high corrosive conditions prevail, was studied in the present work. The effect of annealing temperature on the phase balance of the new hyper-duplex grade and subsequent variation in their corrosion resistance at hot chloride environments were estimated. ASTM A890 grade 7A samples were soaked at 1130 °C, 1160 °C, and 1190 °C and furnace-cooled till 1060 °C followed by water quenching. Electron microscope and optical micrograph images confirmed the decrease in austenite with the rise in solutionizing temperature; but on slow cooling, major austenite reformation takes place. Energy-dispersive spectroscopy (EDS) and line-scan methods were used to find out the distribution of constituent alloying elements. The migration of Cr and Mo to the austenite phase occurred as a result of the heat-treatment processes. Potentiodynamic polarization technique was conducted at different temperatures of 30 °C, 40 °C, 50 °C, and 60 °C on all heat-treated samples. At higher temperatures, corrosion resistance improved with an increase in austenite percentage. Optical images of the corroded samples showed the metastable pit growth with an increase in chloride medium temperature.

Effect of repeated TIG welding cycles on microstructure evolution and pitting corrosion behavior of UNS 32760 super duplex stainless steel

Microstructure evolution and pitting corrosion behavior in repeated TIG welding cycles of UNS32760 super duplex stainless steel were studied. The microstructural evolution was observed through optical electron microscope. Meanwhile, pitting corrosion was investigated by means of potentiodynamic polarization and gravimetric test. The specimens were evaluated to simulate repeated welding cycles of original weld, Repair-1 (R1), Repair-2 (R2) and Repair-3 (R3) respectively. The result show that the chromium nitride precipitates starts appear in heat-affected zone R-2 welding cycles which followed by the growth of metastable pits.

Cellular Automata Study of the Combined Effects of Passive Film Breakdown and Repassivation on Metastable Pits on Sputtered Nanocrystalline Stainless Steel

The growth of metastable pits on sputtered nanocrystalline stainless steel (NCss) was simulated by a cellular automata technology. Pit morphologies, passive film breakdown, and the metal cation concentration inside the metastable pit during its growth were analyzed. The combined effects of passive film breakdown and repassivation hindered metastable pit growth on the NCss, even though their isolated effects may vary significantly. The calculated values of the critical pitting temperature are shown to be statistically distributed because of the difficulty of passive film breakdown, repassivation, and the metastable/stable pitting transition.

Computational Evaluation of the Influence of Various Uniaxial Load Levels on Pit Growth of Stainless Steel under Mechanoelectrochemical Interactions

Numerical modelling of the effects of different applied load levels on the growth of metastable single pit in stainless steel is the subject of the present work. A cellular automata model integrated with finite element analysis is used, taking into account both the electrochemical reactions during metastable pitting growth process and the real-time influence of local stress/strain distribution at pit surface on anodic dissolution. Specifically, in the current work the impacts of uniaxial load levels on pitting corrosion for various conditions of passive layer degradation extents and hydrogen ions diffusion coefficients are quantitatively assessed in terms of current transient. For a general condition with uniaxial applied load level increases from 0 MPa to 250 MPa, the ascending trend of current transient and current density transient is observed due to the increase of plastic strain accumulation at pit bottom, revealing the enhancement of metastable pit growth rate. Meanwhile, the critical magnitude of load level enabling metastable pitting to alter into the development of stable pit for each condition is discussed and predicted.

Electrolyte Composition for Distinguishing Corrosion Mechanisms in Steel Alloy Screening

The formation and breakdown of passive layers due to pitting corrosion are a major cause of failure of metal structures. The investigation of passivation and pitting corrosion requires two different electrochemical measurements and is therefore a time consuming process. To reduce time in material characterization and to study the interactions of both mechanisms, here, a combined experiment addressing both phenomena is introduced. In the presented electrolyte the different corrosion mechanisms are distinguished and investigated by cyclic voltammograms and polarization scans. The measurements show a passive area, metastable pit growth, and pitting corrosion as well as repassivation. The pitting corrosion is separated from additional dissolution processes and the standard deviation of the corrosion potential is smaller than in other electrolytes. Both passivation and pitting corrosion can be observed in one measurement without additional corrosion attacks. The deviation between different measurements of the same steel is small; this is helpful for the screening of similar materials.

Effect of Thiosulfate on Metastable Pitting of 304L and S32101 in Chloride- and Thiosulfate-Containing Environment

The effect of thiosulfate addition on the metastable pitting behavior of stainless steels 304L (UNS S30403) and lean duplex stainless steel (LDX2101, UNS S32101) in 0.6 M NaCl solution was studied. It was found that metastable pits initiate preferentially on the ferritic phase of S32101. Current transients during potentiostatic polarization were used to study the pitting behavior of the tested steels. Statistical analysis of peak current, growth time, and occurrence time of the metastable pitting events was used to evaluate pitting behavior. From the distribution curves of these parameters, it was concluded that the addition of thiosulfate stabilizes metastable pit growth and promotes pitting kinetics in chloride solution. This effect is more pronounced for 304L compared to S32101.

Influences of niobium and solution treatment temperature on pitting corrosion behaviour of stabilised austenitic stainless steels

Effects of niobium and solution treatment temperature on pitting corrosion of Super304H austenitic stainless steels were investigated by potentiodynamic and potentiostatic polarization tests and surface analysis. Results indicated that niobium in solid solution stabilised passive films against pit initiation and formed insoluble niobium-rich corrosion products partially covering pit walls inhibiting pit growth, whereas niobium as carbonitride accelerated metastable pit growth by reducing nitrogen in austenite matrix. Niobium additions more than a critical amount ennobled the pitting potential of Super304H. Moreover, increasing solution treatment temperature increased pitting potential for niobium-bearing Super304H samples but had little influence on that for niobium-free ones.

Critical conditions for pit initiation and growth of austenitic stainless steels

The pitting behaviour of 316 stainless steel in NaCl solution was investigated using potentiodynamic (PD) and potentiostatic (PS) polarization. Tests with the mode change from PD to PS at potentials close to the pitting potential showed that the potential could play a role in the stable pitting by maintaining the pit stability product. The metastable pit growth was studied by changing the mode at potentials far below the pitting potential and by characterizing the first current density spikes. The results suggested that the cumulative electric charge density governs the metastable pit initiation as a critical condition.

Cellular automaton model for simulation of metastable pitting

As a new simulation method, cellular automata (CA) approach had been used in a number of areas, including electrochemistry and corrosion science research. In this work, a probability CA model was developed to simulate the growth of metastable pits. The simulation imaged the morphology of the pit and calculated the time dependences of pitting current and pitting current density. Results demonstrated that the CA approach is capable of simulating corrosion pit in terms of the pit morphology and pit growth kinetics. During growth of a hemispherical metastable pit controlled by diffusion, the time dependence of pitting current follows I = f(t2) law. Furthermore, it is determined that, for the simulation results to be consistent with the experimental results, an optimal range for variable d should be 0·1–0·25.

Cellular automata modeling on pitting current transients

In order to simulate pitting current transient, a cellular automata (CA) model is reported in this paper, which involves a series of evolution rules. Block algorithm is used in this model to speed up simulation process. Different current transients for pitting metastable growth and stable growth are obtained by setting different simulation factors. The variation of simulated current transient or iR with time is found to be related to the statement of the pitting growth for stainless steel. The simulation demonstrates that metastable pit growth is likely controlled by anodic reaction.

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.

Interactions among metastable pits on heterogeneous electrodes

A previously developed spatiotemporal model of the interactions among metastable pits on homogeneous electrodes is extended to heterogeneous surfaces with randomly distributed inclusion sites in an otherwise passive electrode. The inclusions are treated as favored sites for pit initiation events. The effect of the spatial density of inclusions on susceptibility to explosive growth in pitting corrosion is investigated. It is shown that there exist critical values of the pit density for the explosive growth of metastable pits that leads to a transition to pitting corrosion. Convective mass transport is shown to alter the density required for explosive growth. The effect of a limitation on the number of times that an inclusion can initiate a pit on the transition to the pitting corrosion is also considered.

Investigation of Hydrogen-Promoted Pitting by the Electrochemical Noise Method and the Scanning Reference Electrode Technique

Hydrogen-promoted metastable pitting of iron and its development into stable pitting were first investigated by the combination of the electrochemical noise method and the scanning reference electrode technique (SRET). Time records of current noise show that hydrogen significantly increases the number of current fluctuations, inferring that hydrogen might increase the initiation of pitting, and hydrogen also increases the magnitude of current fluctuations, suggesting that hydrogen might promote the growth of metastable pits. In addition, the duration of the time period for initiation of metastable pitting on the charged specimen is longer than that on the uncharged specimen, supporting that the active sites which induce pitting on the charged specimen might be much more numerous than on the uncharged specimen. An analysis of the power spectrum density shows that the level of the frequency-independent plateau of the charged specimen is higher than that of the uncharged specimen and the roll-off frequency t...