Stable Pitting Research Articles

The effect of chromate on the pitting susceptibility of AA7075-T651 studied using potentiostatic transients

The effect of chromate on metastable pitting of AA7075-T651 as determined via potentiostatic polarisation is reported. A systematic study of metastable pitting and its correlation with stable pits was conducted in various concentrations of sodium chromate (Na2CrO4), revealing the metastable pitting rate was able to provide a quantitative metric for pitting corrosion. The size and number of metastable pits decreased significantly in the presence of chromate. The present study is intended as a general baseline for the assessment of future chromate replacement technologies, as elaborated herein.

Influence of surface preparation and microstructure on the passivity and corrosion behaviour of duplex stainless steels

The influence of various surface preparation methods (mechanical polishing, electropolishing and etching) on the passivity and the corrosion behaviour of a duplex stainless steel (UNS S32202) was studied using Auger spectroscopy and the electrochemical microcell technique. From surface analyses, the thickness and chemical compositions (ratios Cr/Fe and O2−/OH−, distribution of chloride and nitrogen) were determined. The corrosion behaviour of samples was investigated from local polarization curves. The presence of oxidation peaks and stable pitting was discussed considering the specimen microstructure and the properties of the passive film.

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.

Computer Simulation of Pitting Corrosion of Stainless Steels

However, these selections are typically very conservative because the limits tend to be driven by particular incidents or test results, rather than by fundamental understanding. Decision-making can be very challenging, especially in today’s mega-facilities, where the cost of production downtime is often staggeringly large. Thus significant practical benefits could be gained from reliable quantitative models for pitting corrosion of stainless steels. There have been several attempts to develop purely stochastic models of pitting corrosion. 3-6 On the other hand, purely deterministic models 7,8 have also been proposed. The critical processes involved in pitting corrosion can be clearly separated into those involved in initiation, nucleation of a ‘metastable’ pit, and propagation or maintenance of the stability of growth of the initially formed defect. 9 Fluctuations in current signal the nucleation, temporary growth, and cessation of growth (‘death’) of metastable pits. A steadily increasing current signals the formation of a pit whose propagation continues. Analysis of these current fluctuations leads to a simple idea connecting the frequency of initiation of metastable pits, λ, with the frequency of formation of stable pits, Λ Λ = λ exp(−µtc)

Passive Behavior and Passivity Breakdown of AISI 304 in LiBr Solutions through Scanning Electrochemical Microscopy

The passive behavior and passivity breakdown of AISI 304 stainless steel in LiBr solutions has been investigated by means of scanning electrochemical microscopy (SECM). The sample generation – tip collection (SG-TC) mode was used to operate the SECM and the tip potential was biased to detect the electroactive species. The evolution of the current at the ultramicroelectrode tip with the applied potential within the passive range was followed at different LiBr concentrations. Results show that the absolute value of the current at the tip increases with the applied potential. Additionally, SECM was also used to detect stable pits formed on the stainless steel surface in a 0.2 M LiBr solution. The results show clear evidence of the presence of high amounts of other reducible species (metal cations) apart from oxygen. Also, the dish-shape morphology of the pits observed using Confocal Laser Scanning Microscopy will be discussed in relation to the kinetics of the reactions observed using SECM.

Direct Observation of Pit Initiation Process on Type 304 Stainless Steel

In situ and real-time optical microscopic observations of pit initiation process on a commercial Type 304 stainless steel with low-sulfur content (0.004mass%) were performed in 3M NaCl solution at 298K. MnS inclusions with diameters of ca. 1μm were found to act as the initiation sites of pitting, as was the case in a re-sulfurized Type 304 stainless steel (0.027mass%). The pit was initiated at the boundary between the MnS inclusion and the steel matrix, and grew with time. After a few seconds, no visible change was observed for 1 s, even though the anodic current was measured, suggesting that the steel dissolution proceeded in the depth direction. After that, a small hole suddenly appeared on the steel surface. The hole widened with time, steadily becoming a large stable pit. This pit initiation process in the low-sulfur stainless steel is much like that found in the re-sulfurized stainless steel. [doi:10.2320/matertrans.M2014028]

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.

Scanning electrochemical microscopy study on the electrochemical behavior of CrN film formed on 304 stainless steel by magnetron sputtering

Abstract The electrochemical behavior of CrN film formed on 304 stainless steel (304 SS) by magnetron sputtering was investigated by scanning electrochemical microscopy (SECM) in redox competition mode and X-ray photoelectron spectroscopy (XPS) in 0.2 M H 3 BO 3 + 0.05 M Na 2 B 4 O 7 (pH = 8.4) buffer solution containing 3.5 wt% NaCl. The results demonstrated that passive film could form on the surface of CrN film with the chemical compositions of Cr, Cr 2 O 3 , CrN, Cr(OH) 3 , and CrO 3 . When applied with 0.9 vs SCE/V potential, the pitting propagation sites corresponded to the high oxygen consumption region. While above stable pit, the Pt microelectrode showed an enhanced faradic current. Separated anodes and cathodes were observed in the scratch, which suggested that the dominant corrosion mechanism is pitting corrosion induced and promoted by galvanic corrosion. These results provide a more explicit description of the corrosion mechanism and processes of CrN film.

Electrochemical Noise Analysis of Pitting Corrosion of Type 304L Stainless Steel

Electrochemical current and potential noise were simultaneously acquired from Type 304L stainless steel (UNS S30403) in 0.05 M ferric chloride (FeCl3) using a three-electrode configuration. Power spectral, statistical, and wavelet analyses have been used to know the uniqueness of the parameters proposed for the identification of various types of corrosion processes. Roll-off slopes derived from power spectral analysis and statistical parameters such as standard deviation, localization index, and kurtosis corroborated with pitting as the corrosion mechanism. Energy distribution plots (EDP) obtained from wavelet analysis of current noise was found to be useful to derive mechanistic information on the progress of corrosion. Discrete wavelet transform was used to decompose the signals into a D1, D2, D3…D8, S8 set of coefficients. The EDP showed that the contribution from the medium time scale crystal, D5, prevailed over the smaller time scale crystals and larger time scale crystals during the initial stages of immersion. With an increase in the time of immersion, the energy deposition on the larger time scale crystals increased and the maximum energy was concentrated on the D8 crystals, indicating that the dominant process occurring on the specimen surface was stable pitting. The results of the investigation are detailed in the paper.

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.

Corrosion Mechanism of 316L Stainless Steel in a Expansion Joint of Blast Furnace Gas Pipeline in a Power Plant

Severe corrosion phenomenon appeared in an expansion joint of blast furnace gas (BFG) pipeline after booster fan in a power plant. The sediment adhered to the inner wall of the expansion joint was analyzed by ion chromatography. The corrosion site of expansion joint was examined by scanning electron microscope (SEM), and the behavior of electrochemical corrosion of stainless steel electrode was discussed through electrochemical impedance spectroscopy (EIS) and polarization curve. The results reveal that the leaching solution of the sediment is a kind of strong electrolyte solution. Along with the increase of immersion period, the impedance of stainless steel electrode decreases dramatically, the impedance spectroscopy experiences such a procedure that a single capacitive reactance arc gradually develops into the contraction of inductive reactance and then tends to be two capacitive reactance arc, which indicates that pitting corrosion transfers from induction period to evolution period until metastable pitting translates into stable pitting. There exists no passive area in polarization curves of stainless steel and shows the characteristics of active dissolution. In conclusion, the failure of the expansion joint stainless steel shows typical pitting characteristic.

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.

Effect of microstructure on the corrosion behaviour of extruded heat exchanger aluminium alloys

Aluminium extrusions used for heat transfer applications are exposed to corrosive environments, which can eventually lead to perforation of the tube by pitting. In this study, the effect of microstructure on the corrosion behaviour of extruded AA3XXX series alloys has been investigated. It was revealed that pits developed were purely crystallographic and a drop testing procedure was developed to simulate the seawater acetic acid test in a laboratory environment. The drop testing experiment has been successfully employed to study the early stages of pitting for different surface finishes. Selected pits were studied with in‐SEM nanotomography. Stable pits initiated on areas free of second‐phase particles, in the vicinity of grain boundaries. When the corrosion front reached a grain boundary plane, pit propagation was interrupted in the direction perpendicular to the plane and the corrosion front followed grain boundaries, corroding one grain body more than its neighbour. A mechanistic understanding of crystallographic pitting corrosion has been developed. Copyright © 2013 John Wiley & Sons, Ltd.

Microstructure-Based Numerical Modeling of Pitting Corrosion in 316 Stainless Steel

Pitting involves localized corrosion of metals that can result in catastrophic events through crack initiation and failure. Beside the magnitude and mode of loading, the transition from pit to crack is also influenced by the pit shape, which in turn is affected by the microstructure of the corroding material. Thus, predicting the influence of microstructure on pit shape can assist in developing microstructures more resistant to stress corrosion cracking. This work presents numerical modeling of the stable pit growth based on direct incorporation of the actual microstructure in simulations. While many microstructural attributes can contribute towards corrosion, the current work focuses on the effects of the crystallographic orientation of the grains on pit growth through variation in corrosion potential with orientation. Two-dimensional simulation results show the strong influence of the crystallographic orientation on pit shapes and growth under the assumed 5% and 10% variation in corrosion potential, delineating the critical place of crystallographic orientation in pit growth to failure.

Potentiodynamic polarization behaviour of AISI type 316 stainless steel in NaCl solution

Potentiodynamic polarization behaviour of AISI type 316 SS in NaCl solution was investigated in terms of the potential scan rate effect. The critical pitting potential, Epit, of the stainless steel appeared to be strongly dependent on the potential scan rate. A cumulative anodic electric charge density of the steel was defined as the total charge density from the open circuit potential, Eocp, and calculated using the potentiodynamic polarization curves. It was found that, plotted as a function of the polarization time, the values of the cumulative charge density consisted of two lines with different slopes. It was confirmed that the deflection of the cumulative charge density vs. time plots corresponded to Epit and the values of the cumulative charge density at the deflection were little dependent on the applied scan rate. The cumulative charge density at the deflection was defined as a critical electric charge density for the stable pitting. Also, it was suggested that this electric charge density should be associated with the critical condition for the stable pitting and the critical electric charge for stable pitting should be a representative parameter for the pitting resistance of a material.

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.

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.

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.