Pit Growth Kinetics Research Articles

Assessing Concurrent Uniform Corrosion of Aluminum Alloys As a Function of Temperature, pH, Time, and Chloride Ion Concentration

One of the greatest challenges for aircraft maintenance is localized corrosion induced fatigue and cracking, which is induced by aluminum alloys susceptibility to aggressive environment. Although pit growth kinetics was thoroughly investigated and nicely modeled by power law, the uniform corrosion impact on pit growth kinetics was ignored in the past, and therefore resulted in pit growth deviation from the ideal power law as indicated in a preliminary result. Uniform corrosion dissolved the surface material and thus induced reference plane recession and pit depth reduction. This non-trivial difference was proved to have a significant influence on pit growth modelling especially in high pH conditions. In this work, temperature and pH influence on uniform corrosion of 2024-T3, 6061-T6, and 7075-T6 aluminum alloys were reported through the free immersion test method. Extensive results performed in various pH (3 to 10), chloride concentration (0.01M to 1M), and temperatures (20⁰C to 80⁰C) from up to 1 month is reported to mimic real applications. A comparison with the weight loss experiment was demonstrated to provide a quantitative significance of pitting and uniform corrosion, respectively. Severity of uniform corrosion were measured and versus various pH, temperature, time, and chloride concentration. The key finding in this work was that uniform corrosion rate is oxygen-dependent with a dynamic influence of passivation. Therefore, to identify the contribution of oxygen and passivation (as affected by pH and temperature), experimental approaches including electrochemical impedance spectroscopy (EIS) and cathodic polarization were utilized. Furthermore, TEM images and electron diffraction were performed to assess the structure and morphology of the protective corrosion product driven by temperature. In addition, the Artificial Neural Network (ANN) modelling was employed to analyze the data obtained. A statistic-based model is able to recognize corrosion behavior pattern versus various conditions, therefore a prediction model can be generated according to the environmental inputs. In summary, the ultimate goal of this study is to suggest an effective maintenance for high value DoD and industrial assets according to an experiment-based prediction model and to understand the impact of different environmental parameters. In this study, a successful uniform corrosion predictive model was developed within specified conditions, which has a significant modification over pit depth growth. Therefore, a predictive model based on lab experimental data is applicable and an efficient low cost maintenance is doable.

Morphology Characterization and Kinetics Evaluation of Pitting Corrosion of Commercially Pure Aluminium by Digital Image Analysis

The pit morphology and growth kinetics of commercially pure aluminium in naturally aerated NaCl solutions were studied using an image processing method based on reflected light microscopy. In order to distinguish between pits and pre-existing cavities, metallographic examination and statistical analysis were carried out before and after corrosion testing. The results show that the pit shapes and sizes are more dependent on the immersion time than the chloride concentration. Pits are predominantly hemispherical, but they undergo reasonable geometric transitions associated with increased immersion time and occur without significant depth variation. The role of chloride ions is more closely associated with the pit nucleation phenomenon.

Characterization of the Effect of Corrosion Inhibitors on Pit Growth Kinetics and Damage Accumulation Rates in Aluminum Alloy 7075 T-6

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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.

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.

The Effect of Nickel on the Local Corrosion Behavior of Amorphous Al-Cu-Mg Based Alloys

The effect of a minor amount of Ni on local corrosion behavior was investigated using Al-Cu-Mg amorphous alloys. Cyclic polarization studies revealed that the addition of a small amount of Ni can enhance both pitting and repassivation potentials significantly, in both the amorphous and devitrified crystalline state. The pit growth kinetics were also ennobled and slowed by the minor solute Ni. Electron microscopy analysis revealed a porous layer depleted with Al and Mg in pits, indicating preferential dissolution. The porous layer was finer in the presence of Ni. A new mechanism of corrosion is discussed. The possible roles of Ni are also discussed.

Challenges in modelling the evolution of stress corrosion cracks from pits

The evolution of stress corrosion cracks from pits is important in many industrial applications but continues to be a challenge in both measurement and prediction. Life prediction in these circumstances has to account for pit growth kinetics, the conditions for the transition from pits to cracks, and the growth rate of cracks in the short and long crack domain. An example of importance is the performance of steam turbine rotors in power generation. Although stress corrosion failures are comparatively rare, the consequences can be severe and occasionally catastrophic. Consequently, considerable effort is being focused on evaluating the effect of operational variables on pitting and crack growth and in developing an improved basis for structural integrity assessment. A preliminary mathematical model based on deterministic equations with statistically variable input parameters was developed for simulating the evolution of the pit depth distribution at different exposure times, and the transformation to stress corrosion cracks was based on the Kondo criteria. The model predicted some features of the damage well but recent novel measurements of the evolution of stress corrosion cracks from pits combined with finite element analysis of the strain distribution suggest that the transition from a pit to a crack is more complex than can be accounted for by the Kondo approach.

Localized Corrosion of Al-Based Amorphous-Nanocrystalline Alloys with Solute-Lean Nanocrystals: Pit Stabilization

The localized corrosion resistance of several partially nanocrystalline Al-based metallic glasses was examined in fully amorphous, partially nanocrystalline, and fully crystalline microstructures. In all of the alloys examined, , , , and , the pitting potential remained elevated relative to pure Al in all partially nanocrystalline states. The pit growth kinetics of the alloys, which were examined using an artificial pit technique, showed that the anodic kinetics were ennobled and the cathodic kinetics were faster than those of pure Al. The pit stability product was also found to be lower for Al than for the fully amorphous alloys. Artificial pit growth experiments also showed that the difference in the repassivation potential between pure Al and the amorphous alloys is not due to the differences in the potential dependence of the transition to tunneling. There was no significant difference between the pit growth kinetics of the fully amorphous and partially nanocrystalline states of the alloys. Because the nanocrystals do not affect the growth kinetics of large ( radius) artificial pits, it is concluded that any effects nanocrystals have on the pitting resistance of alloys must be related to pit initiation or early growth.

Computer simulation of the corrosion pit growth

Numerical simulations of the corrosion pit growth are performed using both Monte Carlo (MC) and Cellular Automata (CA) techniques. The theoretical framework of these two methods is presented. In the MC technique, a dissolution algorithm is proposed for elementary dissolution events. Varying the repassivation probability at the atomic scale on the pit walls is shown to control the pit growth kinetics ( t n law, with n varying from 1/2 to 2). In the CA technique, the Metal/Film/Electrolyte system is considered as an automaton with some specific transition rules. The dissolution probability is shown to strongly influence both pit morphologies and the t n law. Last, predominant effect of the IR drop inside the pit is evidenced. These preliminary investigations motivate future works to simulate in more details the pit growth process and the repassivation of metastable pits.

Stochastic approach to the pit growth kinetics of Inconel alloy 600 in Cl − ion-containing thiosulphate solution at temperatures 25–150 °C by analysis of the potentiostatic current transients

The pit growth kinetics of Inconel alloy 600 in aqueous 0.1 M Na 2S 2O 3 + 0.1 M NaCl solutions was investigated as a function of solution temperature by analysis of the potentiostatic current transient, based upon a stochastic theory. Potentiostatic current transients revealed that time necessary for pit embryo formation, t pit,form decreased with increasing solution temperature. Analysis of the statistical distribution of t pit,form demonstrated that the pit embryo formation rate function at higher solution temperature exhibits a weaker dependence on exposure time compared with that at lower solution temperature. This is attributable to the increase in the active sites necessary for pit embryo formation due to the increase of the defects in oxide film grown at higher solution temperature. Moreover, it was found that the rate of pit growth decreased with increasing solution temperature over the whole applied potential range. Based upon the experimental findings, the kinetics of pit growth with respect to solution temperature is discussed in terms of the changes of the shape parameter derived from the stochastic theory, the pit growth rate parameter determined from the logarithmic potentiostatic current transient and fractal dimension of the pits formed on the electrode surface.

Study on initiation of localised corrosion on copper thin film electrode by combinational use of an EQCM with a liquid-phase ion gun

A new method allowing simultaneous measurements of anodic currents and small mass changes during initiation and early growth of a single localised corrosion site was discussed. An electrochemical quartz crystal microbalance (EQCM) was combined with a liquid-phase ion gun consisting of an Ag/AgCl microelectrode which produces chloride ions, causing local breakdown of passive film and pit growth. The method was applied to copper thin films polarised anodically at 0.6 or 0.8 V (SHE) in pH 8.4 borate buffer solution. It was found from comparison between coulometry and gravimetry that copper dissolves as Cu 2+ during the pit growth. The shape of the pit was almost circular and the average pit current density, i p=25 A cm −2 was evaluated from the kinetics of 2D pit growth.

Pit growth studies in stainless steel foils. I. Introduction and pit growth kinetics

Pits were grown from the edges of thin 304 SS (stainless steel, SS) foils, and the resulting cavities were viewed by video microscopy, allowing the study of real-time pit growth under different applied conditions, e.g. potential and temperature. This method also allowed direct comparison of the current recorded with the development of the pit. All pits developed a lacy metal cover, which was shown to provide a diffusion barrier necessary to sustain stable pit growth, especially in the early stage of a pit's life. Pit growth, morphology and cover formation are explained with the help of a semi-quantitative model based upon concentration gradients and active/passive transitions. Pits grown in plate material and high alloy stainless steel have the same basic morphology.

Metastable Pitting of Carbon Steel under Potentiostatic Control

The metastable pitting of A516‐70 carbon steel was studied under potentiostatic control in solutions containing chloride ions. It was shown that there were different current fluctuation patterns and spectral slopes, that is, roll‐off slopes, in passivity, general corrosion, and metastable pitting. Pits were often covered by a deposit which played an important role in the current fluctuation, with a quick current rise followed by a slow drop. There was a transitional potential (about 0 mV vs. Ag/AgCl electrode) below which the metastable pitting initiation rate increased with the potential, because more sites would be activated. Above the transitional potential, the decay of the pitting occurrence rate with increased potential was due to the elimination of available pit sites. When the applied potential was between −50 and 100 mV, pit growth kinetics was controlled by the potential drop through the deposit over the pit mouth. The potential dependence of repassivation time was mainly due to the effect of applied potential on the deposit over the pit mouth. There seemed to be good agreement between the calculated pit size and the measured values by optical microphotography. The assumption of hemispherical pit geometry was reasonable in calculating the pit radii. © 1999 The Electrochemical Society. All rights reserved.

Localised corrosion behaviour of Fe-N model alloys

The present investigations were carried out to understand the role played by nitrogen in affecting the passivation and pitting kinetics of Fe-N model alloys. Synthetic alloys of Fe-N, Fe-10%Ni-N and Fe-10%Ni-3%Mo-N were prepared by surface nitriding process with 0.9 wt% nitrogen on the surface. Pit initiation studies were conducted on pure Fe and Fe-N alloy in an acidic sulphate solution (0.01 M H 2SO 4 and 0.99 M Na 2SO 4) containing 0.0007, 0.001, 0.007 and 0.07 M NaCl at three different electrode potentials, namely, +800, +1000 and +1200 mV (NHE). Pit growth kinetics was assessed by studying the dissolution behaviour of these alloys during potentiostatic transient in an acidic sulfate solution containing 0.5, 1, 2 and 4.5 M NaCl. The steady state dissolution current was considered as the parameter reflecting the pit dissolution behaviour of the alloys. Potentiodynamic anodic polarisation of the alloys was also carried out in the acidic sulphate solution at room temperature. The results of the investigation indicated that as the chloride content increased nitrogen addition delayed the pit initiation as well as reduced the dissolution kinetics of the pits. The polarisation study of the model alloys indicated that nitrogen played a greater role in reducing the corrosion behaviour of the alloys by reducing the active peak current density and widening the passive range in which pitting is less probable.

Localized Electrochemical Impedance Spectroscopy for Studying Pitting Corrosion on Stainless Steels

Pitting corrosion of stainless steel has been extensively studied by electrochemical impedance spectroscopy (EIS). However, the frequency dependence of the current distribution is much different in the vicinity of a pit than on the passivated part of the surface. Therefore a localized electrochemical impedance technique has been developed and used in order to study the kinetics of pit initiation and growth on stainless steels. AC current distribution maps have been recorded during the initiation and propagation of pits. It is shown that the technique can detect localized attacks at an early stage. Traditional EIS and locally measured impedance spectra have been recorded at the same time on active and passive sites. It has been found that traditional EIS spectra contain contributions from both the pit and the passive area. On the other hand, the locally measured impedance displays a feature which agrees with the behavior observed for the dissolution paths of iron‐chromium alloys at the active current of the anodic polarization curve in chloride‐containing acidic media. This approach can provide useful information on the kinetics of initiation and propagation of pitting corrosion.

Surface morphologies for Br-etched Si(100)-2×1: Kinetics of pit growth and step retreat

Temperature-dependent surface morphologies of Si(100) resulting from Br etching at 700 and 800 K have been studied using scanning tunneling microscopy. Quantitative analysis of linear pit distributions allows us to interpret the mechanisms involved in surface pattern formation. We show that linear chains growing at 800 K on 450-Å-wide terraces are in quasi-steady-state conditions in that their lengths follow the most probable Flory–Schulz distribution. Results obtained under equivalent conditions at 700 K, however, demonstrate that this distribution is not reached. The difference reflects the rate of step retreat relative to the rate of pit growth. At 700 K, pits that develop on terraces are erased before their length distributions reach quasi-steady-state values even though the overall morphology does not change. The surface morphologies that are reached at long times can then be understood by considering the temporal evolution of linear chains on terraces where quasi-steady-state conditions are not fulfilled.

塩化物イオンを含む液滴下でのステンレス鋼の孔食成長機構

塩化物イオンを含む液滴下でのステンレス鋼の孔食成長機構

A pitting mechanism for passive 304 stainless steel in sulphuric acid media containing chloride ions

The pitting behavior of passive 304 stainless steel in 0.25 M Na 2SO 4 media of pH 1 containing chloride ions of different concentrations has been investigated using EIS technique. The characteristics of measured impedance spectra have been analysed. It is proposed that an intermediate complex (MOHCl)com will form in both the pit initiation and propagation stages. The reaction mechanism models for pit initiation and propagation, and an equivalent circuit model to simulate the electrode impedance in pit propagation stage have been suggested. The factors which control the pit growth kinetics have been discussed.

The effect of surface degradation on fatigue and fracture behaviour

This project has attempted to quantitatively describe and model the effect of corrosion pitting on the high-cycle fatigue behaviour of engineering components, using a novel fracture mechanics approach. The work has incorporated into the model not only crack initiation and growth parameters but also the interactive effects of corrosion pit nucleation and growth kinetics. The results of a 13% Cr turbine blade steel are reported. With respect to the scatter of the measured values, the modelling results are in good agreement with the experiments.

Review of modelling of pit propagation kinetics

AbstractA critical evaluation of empirical and mechanistically based modelling of pit propagation kinetics has been made. Empirical models of pit growth kinetics based on the application of Faraday s law to measurement of the total current flowing to potentiostatically controlled specimens have the limitation that only average properties are measured and have limited practical relevance. Extreme value statistics applied to materials exposed for varying periods of time provide a more effective method of prediction of maximum pit depths at a given time. However, the approach assumes that the material and environmental conditions are nominally uniform in the regions of interest, and variations in conditions with time are difficult to account for. Mechanistically based models are more flexibile and more informative from the viewpoint of understanding. However, absolute prediction of pit growth kinetics in service conditions is uncertain because of specific limitations in input data. An implicit fitting to exp...