Metastable Pitting Research Articles

Effects of Corrosion and Cracking of Sulfide Inclusions on Pit Initiation in Stainless Steel

Microelectrochemical measurements were carried out to ascertain the effects of applied stress on pit initiation behavior at (Mn,Cr)S inclusions, which contained around 15 at% chromium. During anodic polarization in 1.5 mol kg−1 MgCl2, the MnS selectively dissolved and the composition changed from (Mn,Cr)S to CrS covered with a surface oxide film. Stable pitting did not always occur when no stress was applied, even when the MnS dissolved and meta-stable pits formed. Under applied stress, stable pitting was consistently initiated in the potential range of the MnS dissolution, and micro-cracks were generated perpendicular to the tensile direction. The cracks were thought to be stress corrosion cracking induced by the formation and rupture of the oxide film on the inclusion under applied stress. The inclusion was perforated by the crack, and the steel matrix under the inclusion was then exposed to the solutions, and stable pitting was initiated.

Atmospheric pitting corrosion of AA7075-T6 under evaporating droplets with and without inhibitors

Pitting corrosion of polished and etched AA7075-T6 in the presence of 3.5 wt% NaCl electrolyte was investigated both with standard full immersion testing, and under evaporating electrolyte droplets in a fixed humidity environment. The effects of chromate-, vanadate-, and cerous-based corrosion inhibiting salts were also examined. A scanning Kelvin probe (SKP) was used to investigate the corrosion potential behavior in the electrolyte droplets. During immersion testing, widespread pitting at intermetallic particles was observed in uninhibited solution. Similar but less severe attack was observed with additions of 0.03 mM vanadate and cerous ions. At higher concentration of vanadate, small numbers of very large pits were found, while higher concentrations of cerous ions provided very good inhibition. Chromate provided nearly complete inhibition of corrosion in all concentrations of immersion testing. In droplet testing, corrosion potentials measured by SKP were associated with metastable pitting, inhibition/passivation, and a form of corrosion attack in which pitting at the edge of the test droplet resulted in the formation of an adjacent, secondary droplet. A model is described for the formation of the secondary droplet and associated attack, by deliquescence of water vapor due to an excess of ions from the anode diffusing to the edge of the droplet.

Pitting corrosion of cold rolled solution treated 17-4 PH stainless steel

In the present paper the effects the of cold rolling on pitting corrosion of 17-4 precipitation hardening stainless steel in 3.5wt% NaCl solution was investigated. In order to clarify the effect of cold rolling the metastable pitting has been examined in more details. The results presented show that cold rolling increases the dissolution rate of metastable pitting in a manner which facilitates the transition from metastable to stable pitting. On the other hand, the frequency of occurrence of metastable pits decreases with cold working. In overall, cold rolling has no significant effect on pitting potential.

The metastable pitting potential and its relation to the pitting potential for four materials in chloride solutions

The metastable pitting potential for four materials, Q235 steel, X70 steel, pure iron and 316L stainless steel in chloride solutions were measured by slow scan potentiodynamic polarization. For the four materials, good linear relationship exists between the metastable pitting potential Em and pitting potential Eb. The close relationship between the two parameters is attributed to the passivity of the metals in environments, which plays the key role in both the nucleation processes of metastable pitting and stable pitting. Higher passivity results in higher average pitting potential and larger difference between average Eb and Em values.

The reliability of electrochemical noise and current transients characterizing metastable pitting of Al–Mg–Si microelectrodes

Current transient measurements and electrochemical noise (EN) method have been used to characterize the metastable pitting events for Al–Mg–Si microelectrodes (diameter 50μm) in deaerated neutral 0.5M NaCl solution. The results are compared with those obtained by atom force microscopy and scanning electron microscopy. The pit numbers determined by shot noise analysis at open circuit potential and by current transient analysis at −1.2V (SCE) are both significantly less than those determined by corrosion morphology image analysis. These results indicate that larger discrepancies can be introduced if the anodic and cathodic reactions localize severely on the same electrode.

Anodic Polarization Behavior of Cold Worked Austenitic Stainless Steel

The anodic polarization behavior of Sanicro 28 austenitic stainless steels has been established in the cold worked (10 to 80% reduction in thickness) samples in 1N HCl, 1M H2SO4+ 1.5N HCl and 3N HCl solutions at room temperature. The current oscillations during the potentiodynamic scans appeared in 3N HCl implying formation of meta stable pits and this solution more severe than 1N HCl and 1M H2SO4+ 1.5N HCl. Two anodic peaks were observed in as-received and the cold worked samples in 3N HCl. In as-received state, the first anodic peak appeared at-3.95 mVSCEand the second anodic peak appeared at 116 mVSCE. It was established that selective dissolution started from 18mVSCE. The grains and grain boundaries were not attacked at the beginning of the first peak-168 mVSCEand started revealing at 18 and 216 mVSCEin the potentiodynamic polarization test. The fore-scatter detector (FSD) attached to FEI Quanta EBSD revealed the pitting morphology of the specimens. It is shown that pit dimensions, types, distribution, the shape of pits in all deformation conditions is hemispherical and number of pits did not depend on the microstructural features, irrespective of the plastic deformation and it may not be related to severity of plastic deformation.

Metastable and stable pitting events at zinc passive layer in alkaline solutions

The electrochemical behavior of Zn in 0.5 M NaOH solutions containing various concentrations (0.01–0.1 M) of ClO3 − or ClO4 − anions was studied with potentiodynamic anodic polarization and chronoamperometry techniques. Microstructural and topographical characterization of the pitted surfaces was carried out by ex situ scanning electron microscopy and atomic force microscopy examinations. Addition of either ClO3 − or ClO4 − stimulated general corrosion and ruptured the passive layer (stable pitting), with ClO3 − being more aggressive than ClO4 −. Metastable pitting events appear as current oscillations (spikes) at potentials close to the pitting potential when Cl− ions are produced by cathodic reduction of ClO3 − and ClO4 − before passive layer growth. Current–time measurements are performed at fixed potential after production of Cl− ions and show that the rate of metastable pitting and the intensity of current spikes increase with the potential and the concentration of aggressive anions. Concepts of thin film growth are applied to the passive layer formation in order to explain those results. Metastable events are related to the presence of defects in the passive layer because their frequency and intensity are enhanced in conditions that favor defect formation and roughening in growing films, while stable pitting typically occurs at regions of high metal disorder.

Potential change and corrosion behavior of two AlMgSi alloys with different Si content under MgCl2 drops in 33% relative humidity

The Volta potential change and corrosion behaviors of alloys Al‐0.63Mg‐0.28Si and Al‐0.52Mg‐0.63Si under MgCl2 drops with the initial concentration of 0.1–1.0 M in environments with relative humidity (RH) 33% were studied using a Kelvin probe (KP). For the alloy Al‐0.63Mg‐0.28Si, the Volta potential curve was mainly featured with many potential transients in the form of a sudden drop followed by an immediate recovery along the potential baseline at a higher potential value of −0.35 to −0.5 VSHE, which were associated with initiation and repassivation of metastable pits with diameter less than 3 μm. For the alloy Al‐0.52Mg‐0.63Si containing excess Si particles, the potential curve was featured with potential change in the form of slow increase followed by a sudden drop along the potential baseline at a lower potential value of about −0.7 VSHE. The sudden potential drop and stabilizing at the lower potential level were associated with corrosion initiation and growth, respectively. In the case of lower initial concentration and higher Si content leading to excess Si particles, the corrosion rating was higher. However, the excess Si particles showed a greater impact than the initial concentration.

Study of the correlation between pitting corrosion and the component ratio of the dual phase in duplex stainless steel welds

Three duplex stainless steel weldments were produced by changing the chromium element to study the correlation between the pitting corrosion characteristics and the component ratio of the dual phase. The pit morphologies showed that metastable pits were generated at a lower pitting resistance equivalent number (PREN) phase. The secondary austenite phases seemed to serve as a path for the corrosive environment regardless of the ferrite number (FN). There is some discrepancy between the measured values (pitting potential (Ep) through polarization test) and expected values (sequence ranked by PREN of weaker phase) in 1moll−1 NaCl solution at 60°C.

Pitting corrosion of 304ss nanocrystalline thin film

Pitting corrosion behavior of coarse crystalline (CC) 304ss and its nanocrystalline (NC) thin film have been investigated by electrochemical measurement and in situ AFM observation in 3.5% NaCl solution. Results show two effects of nanocrystallization on pitting corrosion behavior: (1) more frequent occurrence of metastable pits, but with lower probability of transition to stable pits, which is attributable to differences in morphologies of sulfur and manganese as well as outstanding repassivation ability of NC thin film; (2) nanocrystallization decreases stable pit generation rate and its propensity to form larger pit cavities, and modifies the morphology of stable pit cavity.

Effect of pretreatment on electrochemical etching behavior of Al foil in HCl–H2SO4

The aluminum foil for high voltage aluminum electrolytic capacitor was immersed in 0.5 mol/L H3PO4 or 0.125 mol/L NaOH solution at 40 °C for different time and then DC electro-etched in 1 mol/L HCl+2.5 mol/L H2SO4 electrolyte at 80 °C. The pitting potential and self corrosion potential of Al foil were measured with polarization curves (PC). The potentiostatic current—time curve was recorded and the surface and cross section images of etched Al foil were observed with SEM. The electrochemical impedance spectroscopy (EIS) of etched Al foil and potential transient curves (PTC) during initial etching stage were measured. The results show the chemical pretreatments can activate Al foil surface, facilitate the absorption, diffusion and migration of Cl− onto the Al foil during etching, and improve the initiation rate of meta-stable pits and density of stable pits and tunnels, leading to much increase in the real surface area and special capacitance of etched Al foil.

Analysis of Electrochemical Current Noise From Metastable Pitting of SS304L in NaCl Solutions

Electrochemical noise emanating from a corrosion situation gives indication about the nature and form of corrosion. An attempt has been made to analyse electrochemical current noise signal generated under potentiostatic condition (for metastable pitting) for SS304L–NaCl system. To begin with polarisation plots of SS304L were obtained in various test solutions to precisely know pitting potential of SS304L. It is found that Epit and ipass increase on increasing chloride content. The electrochemical current noise was measured at potentials 20–30 mV below the pitting potentials. The current time record shows two types of current transients; (i) slow rise and rapid decay and (ii) rapid rise and slow decay. Power spectral density analysis of current noise shows that the power (A2/Hz) of the signal measured at metastable pitting range increases with increasing chloride. Sampling frequency has to be properly selected otherwise some of the spikes are not recorded and as a result, size and shape of few current transients is altered.

Pit Initiation Mechanism at MnS Inclusions in Stainless Steel: Synergistic Effect of Elemental Sulfur and Chloride Ions

Microscopic polarization, scanning transmission electron microscopy, and Raman spectroscopy were performed to ascertain the pit initiation mechanism at MnS inclusions in stainless steels. While the inclusion surfaces dissolved under anodic polarization in 0.1 M Na2SO4 as well as 0.1 and 3 M NaCl solutions, the boundaries between the inclusions and the steel matrix dissolved selectively only in the NaCl solutions. This selective dissolution resulted in the formation of trenches, in which metastable and stable pits were initiated. The trenches were shown to be formed by the active dissolution of the steel sides of the boundaries, where no anomalous phase and no compositionally altered zone was observed. It was found that elemental sulfur was deposited on the inclusions and at the boundaries after anodic polarization in 3 M NaCl. The active dissolution of the steel matrix occurred in solutions in which chloride ions and elemental sulfur coexist. The synergistic effect of the elemental sulfur produced by MnS inclusion and chloride ions is likely to cause the trenches, and the decrease in both pH and potential inside the trenches results in pit initiation.

Simulation of metastable corrosion pit development under mechanical stress

A cellular automata model coupled with finite element method is used to describe metastable pitting corrosion growth and its transition to stability of stainless steel under mechanical stress. The model includes anodic dissolution, passivation, diffusion and hydrolysis of salt film. The local stress and strain distributions on the pit surface are analyzed by the finite element method in real time during the pit growth. The effects of local stress and strain on anodic current at every micro-region of the pit surface obtained by Gutman model are used as the boundary conditions for the cellular automata model. The result shows that the growth rate of metastable pitting corrosion under stress is far higher than that under no stress. The effects of rupture time and rupture extent of pit cover and diffusion of hydrogen ions on current transients under stress and no stress are analyzed.

Stochastic modeling of pitting corrosion in aluminum alloys

The stochastic theory of pitting corrosion has been successfully used to analyze the statistical nature of pitting. In this paper a Monte Carlo model is used to predict the extent of damage accumulation in aluminum alloys. This model uses experimental parameters obtained by electrochemical noise measurements on electrodes arrays. The algorithm is based on the random occurrence of the metastable pit birth/death or the stable pit growth. Simulated pit depth distributions are compared to experimental data obtained by Optical Profilometry (OP), leading to an improvement of the model and challenging the existence of a metastable/stable transition in free corrosion conditions.

Insight into thermodynamics and corrosion behavior of Al–Ni–Gd glassy alloys from atomic structure

To clarify the relation between the thermodynamics and metastable pitting behavior of Al–Ni–Gd glassy alloys, the as-spun ribbons were investigated using various experimental methods and ab initio molecular dynamic simulations. The results show that the relative area of main peak shoulder in XRD patterns, onset crystallization temperature, contraction degree of samples increase with increasing cNi. The peak for the metastable pitting events in the polarization curves with Cl− containing solutions is accelerated by increasing cNi. The variation of thermal and electrochemical properties of samples is attributed to the strong interaction of Al–Ni bond based on the atomic structure analysis.

Study of pitting corrosion on mild steel during wet–dry cycles by electrochemical noise analysis based on chaos theory

Electrochemical noise (EN) was used to investigate the corrosion behaviour of mild steel (Q235) in 0.1M Na2SO4 and 0.1M NaCl aqueous solutions during wet–dry cycles. The positive fraction and value of the Largest Lyapunov Exponent (LLE) of electrochemical current noise (ECN) were found out to represent the number and isolation degree of the pits formed in two electrolyte conditions. The calculated results indicate that metastable pits are more plentiful and uniformly distributed in wet cycles and in Na2SO4 solution than those in dry cycles and in NaCl solution respectively.

Corrosion behavior of micro-arc oxidation coating on AZ91D magnesium alloy in NaCl solutions with different concentrations

Ceramic oxide coatings were prepared on AZ91D magnesium alloys in alkaline silicate solution using micro-arc oxidation (MAO) technique. The corrosion behavior of MAO coating on AZ91D magnesium alloys in NaCl solutions with different concentrations (0.1%, 0.5%, 1.0%, 3.5% and 5.0% in mass fraction) was evaluated by electrochemical measurements and immersion tests. The results showed that the corrosion rate of the MAO coated AZ91D increased with increasing chloride ion concentration. The main form of corrosion failure was localized corrosion for the MAO coated AZ91D immersed in higher concentration NaCl solutions (1.0%, 3.5% and 5.0%), while it was general corrosion in dilute NaCl solutions (0.1% and 0.5%). Two different stages of the failure process of the MAO coated AZ91D could be identified: 1) occurrence of the metastable pits and 2) growth of the pits. Different equivalent circuits were also proposed based on the results of electrochemical impedance spectroscopy (EIS) for the MAO coated AZ91D immersed in different concentrations of NaCl solutions for 120 h.

Corrosion dependence of an AlMgSi alloy under MgCl2 solution drop on initial concentration and humidity

The corrosion behavior of an Al–0.63Mg–0.28Si alloy under MgCl2 solution droplets with different initial concentration in environments of 33, 75, and 88% relative humidity (RH) was studied using a Kelvin probe. The available cathodic limiting current associated with oxygen reduction was highly dependent on the environment RH and initial concentration. In the 33% RH environment, metastable pitting was the main form of corrosion. In some cases at 75 and 88% RH, the potential baseline decreased slowly by hundreds of mV and remained at the lower value, indicating stable filiform-like corrosion grew. The initiation time of the stable corrosion was determined by the environment RH and initial concentration. With increasing initial concentration or decreasing environment RH, the available cathodic limiting current decreased, stable corrosion was difficult to occur. The stable corrosion could occur outside the drop and/or inside the drop, which was dependent on not only the cathodic limiting current but also the environment RH.

Pitting susceptibility of induction-quenched pipeline with microstructural heterogeneity

Pitting corrosion was found to be the immature failure mechanism for a slurry pipe that has an internal surface hardened with induction-quenching technique. The metallographic examination shows that the microstructure of surface layer is a banded mixture of martensite, ferrite, and pearlite. The pits initiate preferably in the ferrite bands. Electrochemical measurements are conducted to evaluate the pitting susceptibility of each constituent. When the small amount of chloride is present, the breakdown potential of ferrite is lower than those of martensite and pearlite. The high likelihood of metastable pit initiation in ferrite is also demonstrated by the electrochemical noise records. The development of pits in ferrite in the mixed microstructure is further promoted by the galvanic effect when ferrite is coupled with pearlite or martensite. The passive film of steel is an n-type semiconductor. The Mott–Schottky measurements indicate that the passive film formed on the ferrite specimen contains more oxygen vacancies, suggesting that the lower breakdown resistance is likely due to a highly defective film structure. The high-to-low pitting susceptibility is ferrite > martensite > pearlite. The experimental observations of this study imply that a practical approach to improve the performance of tailing pipes is to eliminate the microstructural heterogeneity in the surface layer by modifying the induction-heating process.