Pitting Resistance Of Stainless Steel Research Articles

(Invited) Recent Advances in Understanding the Critical Pitting Temperature

The critical pitting temperature (CPT) represents a critical condition for stable pit growth and is used as a principal parameter to evaluate the pitting resistance of stainless steels and nickel-based alloys. In the past several decades, many efforts have been made to clarify the underlying mechanism behind the CPT phenomenon. In this talk, we will summarize our existing understanding on this topic based on a recent framework for pit growth stability [1-8]. However, the previous theory mainly focused on the potentiostatic-CPT, which is associated with a sharp increase in current caused by pitting when the temperature increases above the CPT. Although many new insights were generated, the framework cannot fully rationalize the potentiodynamic-CPT, which is the sharp transition of breakdown potential from the transpassive region to pitting region when the CPT is exceeded. To fill the knowledge gap, we proposed that the CPT transition might be related to repassivation underneath the salt film of metastable pits. Our previous work has already confirmed the phenomenon of repassivation underneath the salt film in deep pits (~400 µm) [9]. We expected that this phenomenon also occurs in metastable pits (i.e. small pits) but it has not yet been reported. To verify this hypothesis, the critical conditions of potential (E) and temperature (T) for repassivation at relatively small depth (20 µm and 30 µm) were investigated using one-dimensional artificial pit electrodes. The repassivation underneath the salt film in small pits was indeed observed. Based on this finding, several possible models were proposed to interpret the potentiodynamic-CPT, and insights for a more comprehensive understanding on pit growth stability were also generated. Acknowledgments: Tianshu Li acknowledges the support of the National Natural Science Foundation of China (Grant No. 52201085). G. S. Frankel acknowledges the support of the US Office of Naval Research (Grant No. N00014-23-1-2202).Reference[1] G.S. Frankel, T. Li, J.R. Scully, Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability, Journal of the Electrochemical Society, 164 (2017) C180-C181.[2] T. Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability: Part II. A Model for Critical Pitting Temperature, Journal of The Electrochemical Society, 165 (2018) C484-C491.[3] T. Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation, Journal of The Electrochemical Society, 165 (2018) C762-C770.[4] T. Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part IV. The Role of Salt Film in Pit Growth: A Mathematical Framework, Journal of The Electrochemical Society, 166 (2019) C115-C124.[5] T. Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability: Part V. Validation of a New Framework for Pit Growth Stability Using One-Dimensional Artificial Pit Electrodes, Journal of The Electrochemical Society, 166 (2019) C3341-C3354.[6] T. Li, J. Wu, G.S. Frankel, Localized corrosion: Passive film breakdown vs. Pit growth stability, Part VI: Pit dissolution kinetics of different alloys and a model for pitting and repassivation potentials, Corrosion Science, 182 (2021) 109277.[7] T. Li, D.E. Perea, D.K. Schreiber, M.G. Wirth, G.J. Orren, G.S. Frankel, Cryo-based structural characterization and growth model of salt film on metal, Corrosion Science, 174 (2020) 108812.[8] T. Li, J. Wu, X. Guo, A.M. Panindre, G.S. Frankel, Activation energy of metal dissolution in local pit environments, Corrosion Science, 193 (2021) 109901.[9] T. Li, G.S. Frankel, Repassivation underneath salt film on stainless steel pits, Corrosion Science, 203 (2022) 110353.

Pitting behavior of austenitic stainless-steel welded joints with dense inclusions and methods to enhance pitting resistance

PurposeThis study aims to assess the pitting resistance of austenitic stainless steel welded joints fusion zone (FZ) with high density of inclusions before and after surface treatment, including potentiostatic pulse technique (PPT) and pickling.Design/methodology/approachThe potentiodynamic polarization tests and critical pitting temperature tests were carried out for estimating pitting resistance. The PPT and pickling were performed as surface treatment. Scanning electron microscope (SEM) and energy dispersive spectrometer were used for characterize the microstructure and elemental distribution. Electron back-scattered diffraction (EBSD) was used to assess the portion of phases and morphology of grains.FindingsThe weld metal exhibits a higher degree of alloying compared to the base metal, and it contains d-phase and sulfur-containing inclusions. Sulfur-containing inclusions serve as initiation sites for pitting, and they diminish the pitting resistance of weld metal. Both PPT and pickling can remove sulfur-containing inclusions, but PPT causes localized dissolution of the weld metal matrix around the inclusions, while pickling does not. Because of the high density of inclusions, certain pits initiated by PPT are significantly deeper, which makes the formation of stable pitting easier. Because of the high density of inclusions, certain pits initiated by the PPT are deeper. This characteristic facilitates the progression of these initial defects into fully developed, stable pits.Originality/valueAnalysis of pitting initiation in shielded metal arc welding FZ with PPT and ex situ SEM tracking observation. Explanation of why the PPT surface treatment is not able to enhance the pitting resistance of stainless steel with a high inclusion density.

A hybrid machine learning strategy for pitting probability prediction of stainless steels

Pitting corrosion represents a substantial threat to passive metals, potentially leading to stress-corrosion cracking and the abrupt failure of materials. The complex relationship between the internal alloy compositions and external environmental factors complicates the precise prediction of pitting probability. In this study, a hybrid machine learning strategy is developed to predict the probability and boundary of pitting corrosion in stainless steel, considering the collaborative effects of different factor pairs rather than a single factor. The strategy integrates both black-box (adaptive boosting) and white-box (symbolic classification) algorithms, where the former is constructed to determine the pitting probability with an F1 score of 0.85, and the latter is designed to achieve recognition of critical factor pairs. This work aims to offer a systematic diagram and new insights in understanding and optimizing the pitting resistance of stainless steel.

Pitting Initiated by MnO·Cr2O3–MnS and MnS Inclusions in a Si‐/Mn‐Killed Stainless Steel

Two types of inclusions in the stainless steels are detected: MnO·Cr2O3–MnS dual‐phase ones and pure MnS ones. The increase of sulfur content can inhibit the formation of MnO–Cr2O3–MnS inclusions and reduce the harm of MnO·Cr2O3–MnS inclusions on the pitting corrosion of stainless steels. When the sulfur content increases from 26 to 70 ppm, the average pitting potential increases from 0.34 to 0.4 VSCE due to the decrease in the number density of MnO–Cr2O3–MnS inclusions. A Cr‐depletion zone exists in the steel matrix around MnO–Cr2O3–MnS inclusions. The size of pits induced by MnO·Cr2O3–MnS inclusions is larger, which poses a more serious threat to the pitting resistance of stainless steels.

Regularities of the Influence of Substitutional and Interstitial Alloying Elements on the Corrosion Properties of Austenitic Stainless Steels

Pitting corrosion studies were carried out on nitrogen containing austenitic stainless steels of different compositions and concentrations of alloying elements. As was shown there is a certain predicted influence of the concentration of each alloying elements as chromium, manganese, nickel, carbon and nitrogen on the pitting potential (Eb) of investigated steels with the nitrogen content less than 0.169 wt. %. However with an increase of the nitrogen content to a certain value (in our study up to 0.82 wt. %) the predicting of alloying elements influence on pitting potential of the steels requires a new approach. Based on the analysis of the experimental results and to take into account the influence of all alloying elements in steel on the pitting potential, a regression equation is proposed. In the presence of nitrogen, the positive role of carbon on the pitting resistance of stainless steel was shown, and the critical values of the total content (C + N) and the C / N ratio were determined, allowing prediction of the best composition of stainless steel.

Prediction of Critical Pitting Temperature of 316L Stainless Steel in Gas Field Environments by Artificial Neutral Network

Abstracts: 316L stainless steel is widely used for enhancing the pitting resistance of pipelines in gas field. The corrosion environment is complex and diversified in different working districts of gas field. Therefore, it is necessary to develop a model for predicting the pitting resistance of pipelines serving in different corrosive environments. Critical pitting temperature (CPT) is considered as a criterion for evaluating the pitting resistance of stainless steel. Based on a survey on the operation situations of gas field, the relevant data of CPT for 316L stainless steel is acquired by potentiodynamic polarization method in solutions with various Cl concentrations and pH values, which are selected to correspond with the real environments in operation. Then, an artificial neutral network (ANN) model is developed to predict the CPT, and therewith to compare with the measured data.

Effect of 1, 2, 3-benzotriazole on the corrosion properties of 316L stainless steel in synthetic tap water

This study examines the effect of the benzotriazole on the corrosion properties of 316L stainless steel in synthetic tap water. Electrochemical tests, surface analyses and quantum study were conducted for evaluating corrosion behavior and adsorption mechanism. In case of stainless steel in the synthetic tap water, the adsorption layer of benzotriazole is not uniform due to the low adsorption. The benzotriazole is preferred to ionic state in alkaline condition like synthetic tap water. Thus, the formation of metal-benzotriazole complex was difficult, and which confirmed in FT-IR analysis. The ionic state of benzotrizole in alkaline environment caused the electrostatic repulsion force and the strong tendency of donating electron obstructed the adsorption of ionic benzotirzole. In addition, the lack of d-orbital of Cr made hard to adsorption of the benzotrizle on the stainless steel. This non-uniform adsorption layer of benzotriazole on the stainless steel surface induced the localized defect sites which decrease the pitting resistance of stainless steel.

On the problem of setting a criterion for pitting resistance of stainless steels

A comparative analysis of the currently existing criteria for the pitting resistance of stainless steels and the proposed criterion based on the formation potential of a salt film is carried out. The formation potential of the salt film is shown to provide an explanation of the current oscillations, which are a typical feature of pitting corrosion, as well as to fairly accurately predict the pitting-resistance ranges of stainless steels. The combination of the existing and proposed criteria for pitting resistance enabled us to more accurately distinguish the three possible states of a passive film, namely, the range of a passive state or stable passivity where the probability of the formation and development of the earlier appeared pits equals zero, the range of a meta-stable state or instable passivity where the existence probability of the earlier formed pits and the nucleation probability of repassivating pits are nonzero, and the range of pit formation where the formation and stable growth of pits are possible.

Effect of austenite stability on the pitting corrosion resistance of cold worked stainless steels

The influence of cold rolling on the pitting resistance of stainless steels is investigated with respect to the austenite stability. A maximum in pitting initiation frequency for 20% reduction is confirmed whatever the austenite stability, but the value of this maximum fairly correlates to the amount of deformation-induced martensite. A direct influence of dislocations piling-up and an indirect role of martensite as a pile-up stabilizer is postulated. In the potentiostatic regime, cold work is shown to lower the repassivation ability and to increase the number of stable pits. Then, addition of alloying elements (such as copper) affects the pitting resistance not only for chemical but also for mechanical reasons (such as their effect on the staking fault energy and austenite stability), confirming the decisive role of microstructure in pitting corrosion of industrial alloys.

Microstructure, corrosion and mechanical properties of 304 stainless steel containing copper, silicon and nitrogen

The effects of 0.086–0.336% nitrogen additions on the microstructure, corrosion and mechanical properties of type 304 austenitic stainless steel (SS) containing 2% copper and 2–3% silicon were studied. This study was carried out using scanning electron microscope (SEM), potentiodynamic, weight loss, hardness number and ball-punch bulge measurements. Mutual effects between Si and N were observed in the matrix of SS. Nitrogen offset the ferrite-forming tendencies of Si and was more efficient than Ni as austenitizer, but Si decreased the solubility of N in solid solution. N additions improved the pitting resistance of SS in acidic and neutral chloride solutions. This was more evident in more aggressive solutions than in solutions with low chloride concentrations. Segregation of second N-rich phases, like Cr2N, in SS containing 2% Cu, 3% Si and 0.237% N was occurred. This steel exhibited less pitting corrosion resistance than the plain 304 SS in most chloride solutions under study. Addition of 2% Si to 304 SS containing Cu has negative effect on the mechanical behavior. But presence of N improved the mechanical strength of steel irrespective of the drop solubility of N affected by Si.

A statistical study of the role of molybdenum in the pitting resistance of stainless steels

The local breakdown of passive films grown on stainless steels in the presence of chloride ions generally leads to pitting. To study this phenomenon, the pitting potential of several stainless steels in neutral chloride media has been determined, by a statistical technique. This parameter is obtained when the statistical pit generation rate corresponds to a preset critical value. Two cases were considered: (a) stainless steels without alloyed Mo (AISI 430 or 304) in NaCl or NaCl + Na 2MoO 4, aerated or de-aerated, solutions, and (b) Mo alloyed stainless steels (AISI 434 or 316) in NaCl aerated or de-aerated solutions. The experimental results are compared, and discussed in terms of the influence of alloyed or dissolved molybdenum. It appears that the role of this element is different in each case, and also that dissolved oxygen in the solution plays a significant role in the improvement of the passivity due to molybdate ions action.