Stable Pitting Research Articles

Inducing pitting in low carbon steel via chloride contamination: Electrochemical noise analysis in concrete gel pore solutions

This study explores an alternative approach to examining the corrosion of reinforced concrete, a persistent and challenging issue. The research utilizes an agar-based gel as a synthetic pore solution to simulate the environment of reinforcing steel, with the goal of confining pitting corrosion induced by chlorides to specific areas of the steel. By employing this innovative approach, metastable and stable pits were successfully induced according to the Cl−OH− ratio at pre-selected locations on the steel surface. The presence of metastable pits was determined in the range of 31.6 >Cl−OH− > 3.2, as indicated by electrochemical noise analysis. The findings support the efficacy of this experimental procedure for enabling precise manipulation and contamination of localized regions with chloride ions.

Water Separation and Formation of Cells with Differential Aeration as Factors Controlling Corrosion of Steel Pipelines in a Crude Oil Storage Facility

The mechanism causing the dramatic intensification of the corrosion deterioration of carbon steel pipes in a crude oil storage facility has been investigated. This study considers a number of factors affecting corrosion in crude oil, such as the water content, the corrosivity of the aqueous phase, the kinetics of water–oil separation, the effect of dissolved oxygen, the effect of the crude oil quality, the degree of stagnancy inside of the pipes, the possible contribution of microbially induced corrosion (MIC) and the presence of deposits. The key root of the corrosion intensification was the separation of the water phase, supported by stagnancy, which eventually led to the formation of stable shallow pits surrounded by cathodic areas.

New insights into the influence mechanism of carbides on the localized corrosion of N80 carbon steel: Experiments and first-principles study

Carbides significantly influenced the corrosion behavior of N80 carbon steel. The specific types of carbides and their adsorption behavior at the interface with the steel matrix were particularly complex. This study comprehensively evaluated the impact of carbides on the localized corrosion of N80 steel using both computational and experimental techniques. The findings revealed that the predominant carbides in the steel were Fe3C, with a potential difference of approximately 65.3 mV greater than that of the steel matrix, leading to preferential dissolution of the steel matrix. The corrosion film that formed on the steel surface was loose and porous, enriched with Cl-, and insufficient to effectively prevent the penetration of corrosive media. The work function relationship between Fe3C and the steel matrix was Fe < Fe3C, and O exhibited a higher adsorption energy on the Fe surface than on the Fe3C surface. This resulted in a strong galvanic coupling corrosion effect between Fe3C and the steel matrix. Fe3C, acting as the cathodic phase in the galvanic corrosion process, accelerated the adsorption of O2 on the steel surface, which in turn led to significant dissolution of the steel matrix. This process facilitated the nucleation of pits, ultimately resulting in the formation of stable pits on the surface.

Energy control and block performance optimization of bench blasting

In this study, rock-like model blast tests with varying toe burdens are conducted and the evolution of blast-induced fractures is analyzed using the digital image correlation (DIC) method. In addition, post-blast fragmentations are image-processed to quantitatively investigate the blasting performance. A three-dimensional numerical model is developed and validated against the experimental results. The effects of blasthole inclination (70°, 80°, and 90°) and short delays on fragment size distribution (FSD) are numerically investigated. Field trials are conducted to verify the numerical results. The experimental results reveal that the overall fragment size increases significantly in proportion to the toe burden. The explosion energy utilization rates range from 3.56 % to 13.16 %, with a tendency to initially increase and subsequently decrease. The numerical results demonstrate that fragments are more evenly distributed with a blasthole inclination of 70°, resulting in a remarkable improvement in rock fragmentation compared to a vertical blasthole. Compared to the toe burden, the influence of short delays on concrete fragmentation is insignificant. The field test results indicate that a narrower FSD range and stable pit walls are achieved by utilizing inclined blastholes in bench blasting. This study provides theoretical guidance for optimizing bench blasting parameters, which has practical significance for improving explosive energy utilization and production efficiency in open-pit mines.

Effect of Nitrogen on the Precipitation and Pitting Corrosion Susceptibility of High Nitrogen 316SS Weld

The present study aimed to understand the effect of nitrogen on the carbide precipitation and its influence on pitting corrosion of shielded metal arc welded high nitrogen 316SS weld. Nitrogen addition to weld reduces Cr availability sites in ferrite and enhances Cr in nearby austenite. Although higher carbon and continuous ferrite with ferritic austenitic mode exist, the changes in the degree of sensitisation were avoided at all the stages, and marginal changes in pitting potential (Epit) at 898 K/100 h and 998 K/24 h were observed. Nitrogen addition to weld further reduced the number of stable pits in ferrite and austenite, and the protectiveness of passive film was affected by nitrogen in ferrite and austenite, which affected pit propagation. This results in stable pits in ferrite and austenite, which affects pit propagation during thermal ageing.

Pit Slope Configuration for Open Pit Mining – A Case Study

To achieve stable pit wall slopes, it is imperative to obtain a fair knowledge of the rock mass characterisation before designing the pit. Insufficient knowledge of the competency of the country rock could lead to using unsupported slope configuration in the design process which can consequently lead to slope failure. In this study, the geomechnical properties of the Bremen-Nkosuo concession are analysed using Bieniawski’s classification scheme to determine the Rock Mass Rating (RMR) for defining safe pit slope configuration of the Nkosuo pit. The findings show that the rockmass are best described as ‘fair’ for the two main lithologies existing at the concession. Subsequently, localised adjustment factors are applied to the calculated RMR to arrive at Mining Rock Mass Ratings (MRMR). These MRMR values are correlated with 50 m fixed stack height and 1.2 safety factor to determine optimum Bench Slack Angle (BSA) of 54° and 57° for host sedimentary and granitic rocks respectively. For individual benches, optimum slope design configurations were 10 m, 800, and 6.6 m respectively for bench height, bench face angle and catch berm for metasedimentary rocks. Likewise, that for granitic formation were 10 m bench height, 800 face angle and 6.0 m catch berm width. These configurations are in conformance with mineral and mining regulations of Ghana. Slope stability assessment was performed which included Slope Mass Rating (SMR), Kinematic and Limit equilibrium analysis. From the analysis, multi-bench scale slope instability occurrence was found to be rare but single-double scale could be possible at the western wall of the planned pit with probability of failure of about 0.4. Presplit and trim shots perimeter blasting techniques are recommended to maintain the integrity of the final pit walls at certain areas.

PREDICTION OF ELECTROCHEMICAL PROCESSES OF LOCAL DISSOLUTION OF STAINLESS STEELS. PART 2. ANALYTICAL AND SIMULATION MODELING OF PROCESS DYNAMICS

To analyze the process of pitting corrosion, a state graph was used, which displays the possible states of local dissolution of chromium-nickel steels, taking into account the additional state "unstable passivation of macropitting". The developed analytical model for calculating the process before the formation of stable pitting is based on homogeneous Markov chains. The calculation of conditional probabilities of system changes from one possible state to another determines the time in the VisualStudio 2010 environment on the platform of NetFramework using the C# programming language. A comparison of the calculation of states with “unstable passivation of macropitting” (UPM) and without it allowed us to establish for stainless steels (12X18N10T, 10X17N13M2T, 08X22N6T) that the formation time of stable pitting taking into account UPM is less than the formation time without it, but more for 08X22N6T steel, which is due to the increased chromium content in its composition. A simulation model (SM) has been created, which is based on the Monte Carlo method, in which a random number generator is used to implement random processes of local dissolution due to SM, a sequence of states in which the system is located before its transition to a stable state is obtained. Also, due to this model, it became possible to consider the change in the potential of the system in the process of local dissolution of the studied steels. The use of two modeling approaches taking into account the state of the UPM allowed us to establish that IT is better consistent with experimental results both on a quantitative and qualitative level. An algorithm for selecting the values of the mode parameters is proposed, which became the basis for predicting the formation of macropitting processes. For citation: Vinogradova S.S. Prediction of electrochemical processes of local dissolution of stainless steels Part 2. Analytical and simulation modeling of process dynamics. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 6. P. 119-126. DOI: 10.6060/ivkkt.20246706.6972.

Statistical analysis of metastable pitting behavior of 2024 aluminum alloy based on deep learning

In this study, a processing flow model for recognizing, statistically analyzing, and assessing pitting images in 2024 aluminum alloy was established based on deep learning object detection algorithms. By analyzing the statistical outcomes, the process of pitting development can be intuitively distinguished. Furthermore, the critical size for the transition from metastable pitting to stable pitting in aluminum alloy was accurately determined. In addition, by combining X-ray computed tomography testing, the evolution of pitting in aluminum alloys and its correlation with microstructure were delineated. Integrating deep learning into analysis provides valuable insights into the behavior and mechanisms of metastable pitting.

Effect of structural heterogeneity on stable pit growth of Co-based metallic glasses

The artificial pit technique was employed to investigate the stable pit growth of Co68.15Fe4.35Si12.5B12Cr3 metallic glasses (MGs) exposed to 0.6 M NaCl solution. Critical parameters of pit growth stability were assessed to clarify the correlation between structural heterogeneity and stable pitting kinetics. Thermally induced degeneration of structural heterogeneity increases the repassivation potential and critical pit solution concentration required for pit survival, thus strongly inhibits the activation-controlled growth of salt-film-free pits. Nevertheless, the inhibitory effect on the diffusion-controlled growth of salt-film-covered pits is relatively weak, as revealed by the fact that the saturation concentration required for salt precipitation remains unaffected.

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.

The formation of pitting under dynamic DC interference: Corrosion product film defects or passive film induction

The cause of pitting under dynamic DC interference is unclear. This paper studies the mechanisms of pitting in simulated solutions under neutral and alkaline conditions using weight loss, electrochemical testing, and microscopic characterization. The results indicate that the metal mainly undergoes active corrosion in the neutral solution, and the formation of millimeter-scale pits is due to local defects in corrosion products. Metals in the alkaline solution undergo passive corrosion, resulting in micro-scale pits. Initially, stable pits form at damaged areas of the passivation film. Then, pits can grow rapidly under self-catalysis, with the growth rate surpassing other surface locations.

Effect of hydrogen on the pitting corrosion of 2205 duplex stainless steel welded joints

The effect of hydrogen on the pitting susceptibility of 2205 duplex stainless steel welded joints was investigated using metallographic characterisation, electrochemical techniques and immersion experiments. The results show that the diffusible hydrogen content in the welded joints is approximately twice that of base material (BM). Hydrogen significantly decreases the pitting potential (Epit) of the welded joints, while increasing the carrier density in the passive film, suggesting that hydrogen weakens the protective properties of the passive film and increases the pitting susceptibility of the welded joints. According to immersion experiments, the pitting susceptibility of the welded joint from high to low is heat-affected zone (HAZ), BM and weld metal (WM), the HAZ tends to be more prone to the formation of larger and more stable pits, and the number of inclusions at the HAZ is significantly greater than that at the BM and WM. For BM under hydrogen charging, austenite phase is usually corroded first, because hydrogen tends to be enriched in the austenitic phase of BM. However, in some regions of WM, ferrite phase corroded first, which is related to the enrichment of hydrogen in the ferrite in the weld area.

Correlative evaluation of the corrosion resilience and passivation properties of zinc and aluminum alloys in neutral chloride and acid-chloride solutions

Abstract Comparative analysis of the corrosion resilience and passivation characteristics of pure zinc (Zn) and aluminum (Al) alloys in neutral chloride and acid-chloride solutions at 0.5–3% NaCl concentrations was done with potentiodynamic polarization, potentiostatic evaluation, optical characterization, and open circuit potential measurement. Results show Al alloy was more resistant to general corrosion in both solutions with values ranging from 0.031 to 0.082 mm/year, and 0.037 to 0.389 mm/year compared to Zn alloys with values of 0.432–0.691 mm/year and 0.465–5.016 mm/year. Corrosion potential values of Zn alloy were significantly more electronegative than the values for Al alloy. The passivated region of the polarization plots for Al was thermodynamically unstable with visible current transients compared to that of Zn. Passivation of Al occurred at the early onset of anodic polarization in the neutral chloride solution. Zn alloy passivated at specific potentials, coupled with stable passivation behavior. The passivation range values of Al were generally greater than the values for Zn due to delayed stable pitting activity. Optical images for Al showed extensive localized degradation along specific regions and grain boundaries, whereas Zn morphology indicates general surface degradation. Open circuit potential plots indicate significant growth of Al2O3 oxide on Al coupled with active–passive transition behavior of the oxide. This contrasts the observation for Zn where the plot configuration indicates limited oxide formation and growth but significant thermodynamic stability.

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.

An investigation on the effect of molybdenum alloy element and molybdate inhibitor on the stable and metastable pits and its correlation with the pit morphology

BackgroundMolybdenum (Mo) has been introduced as a useful alloying element by increasing the resistance to pitting corrosion of steels. To prove this opinion, the effect of this element on pitting corrosion parameters in corrosive environments can be investigated. In addition, adding a molybdate (MoO42−) inhibitor to the media can answer the question of whether there is a synergistic effect between molybdenum in the alloy and molybdate in the environment to increase corrosion resistance or not. The study holds relevance by suitable select materials based on the presence of corrosion-resistant alloying elements and inhibitors based on the reduction of environmental acidity or the occurrence of polymerization reactions for industries frequently affected by pitting corrosion. MethodsIn this paper, the influence of the molybdenum alloying element and molybdate inhibitor on the metastable and stable pits of 316 SS and 304 SS in chloride and molybdate-bearing media is studied. The evaluation is conducted using potentiodynamic and potentiostatic techniques on a flat sample. The morphology of the pits is observed through scanning electron microscopy in solutions of NaCl and Na2MoO4. FindingsThe results suggest that the presence of molybdate ions in chloride solution leads to an increase in the pitting potential and repassivation potential of both steels. Furthermore, molybdate is suggested to decrease the peak current, radius, and stability product of metastable pits. A comparative analysis between the corrosion behavior of 316 SS and 304 SS suggests that the addition of molybdenum enhances the resistance to pitting corrosion and facilitates the repassivation process. Additionally, adding molybdate in chloride solutions results in smaller stable cavity sizes without altering their appearance.

Unveiling the pitting corrosion mechanism of borated stainless steel in the wet storage environment of spent nuclear fuels

Borated stainless steels (BSSs) with high neutron absorbability are promising alloys for the wet storage of spent nuclear fuel. However, consensus on the underlying mechanisms for the effects of B content in BSS and H3BO3 in wet storage environment on the pitting corrosion resistance of BSSs is still lacking. To determine the mechanisms, we designed three hypoeutectic BSSs containing 0.6, 1.2 and 1.8 wt.% B and systematically investigated their electrochemical behavior in solutions with various Cl−/H3BO3 ratios through experimental and computational studies. Results revealed that the pitting corrosion resistance of BSS was dependent on the synergistic effect of the pit initiation induced by localized Cr depletion and the pit stable growth caused by microgalvanic effect, causing the pitting corrosion preferentially occurred at the (Cr, Fe)2B/matrix interface. A higher B content increased the density and width while reduced the Cr contents of the Cr-depleted regions, enhancing the susceptibility to pitting initiation. Meanwhile, the strengthened micro-galvanic effect between the (Cr, Fe)2B phase and γ matrix further facilitated the stable pit growth. For the H3BO3 effect, the first-principles calculations indicated that the competitive adsorption of B(OH)4− to Cl− inhibited pitting initiation in solutions with lower Cl−/H3BO3 ratios. However, as the Cl−/H3BO3 increased, the inhibition effect of the H3BO3 was weakened, and the acidizing effect became the dominant factor deteriorating the corrosion property. Ultimately, based on the Cr depletion-microgalvanic synergetic mechanism, the corrosion resistance of the BSS was significantly improved by tailoring the size and distribution of the (Cr, Fe)2B phase.

Effect of tellurium treatment on the pitting behavior of 38MnVS6 Non-quenched and tempered steel in alkaline environment

This paper investigates the impact of tellurium (Te) treatment on the pitting behavior of 38MnVS6 Non-quenched and Tempered steel (NQTS) in an alkaline environment. The electrochemical tests demonstrate an increase in pitting potential within the Te content from 0 to 500 ppm and a decrease from 500 to 1600 ppm. Pitting can be ascribed to the preferential dissolution at the inclusion/matrix interface, which is caused by the stress concentration. At the presence of micro-crevices, it is easier for inclusions to induce stable pitting.

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.

Pitting corrosion behavior and corrosion protection performance of cold sprayed double layered noble barrier coating on magnesium-based alloy in chloride containing solutions

Nitrogen processed, cold sprayed commercially pure (CP)-Al coatings on Mg-based alloys mostly lack acceptable hardness, wear resistance and most importantly are highly susceptible to localized corrosion in chloride containing solutions. In this research, commercially pure α-Ti top coating having good pitting potential (∼ 1293 mVSCE), high microhardness (HV0.025: 263.03) and low wear rate was applied on a CP-Al coated Mg-based alloy using high pressure cold spray technology. Potentiodynamic polarization (PDP) curves indicated that the probability of transition from metastable pits to the stable pits for cold spayed (CS) Al coating is considerably higher compared to that with the CS Ti top coating (for Ti/Al/Mg system). In addition, CS Ti top coating was in the passivation region in most pH ranges even after 48 h immersion in 3.5 wt% NaCl solution. The stored energy in the CS Ti top coating (as a passive metal) was presumed to be responsible for the easy passivation. Immersion tests indicated no obvious pits formation on the intact CS Ti top coating surface and revealed effective corrosion protection performance of the CS double layered noble barrier coatings on Mg alloys in 3.5 wt% NaCl solution even after 264 h.

Quantitative analysis of initiation site of pitting corrosion on type 304 austenitic stainless steel

The initiation site of pitting corrosion on type 304 stainless steel was quantitatively analysed using an automatic inclusion analyser and electron backscatter diffraction. In a sample anodically polarised at 0.43 VSSE, the fraction of stable pits initiated from inclusions was only 23 %. Pits initiated from sites other than inclusions probably initiated from grain boundaries, strain-induced martensite and scratches. In mixed inclusions of MnS and oxides, the probability of pit initiation increased with the S concentration. The Ti concentration had no effect on the probability of pit initiation, suggesting that whether Ti exists as TiO2 or TiN is important.