Cellular automaton model for simulation of metastable pitting

Abstract

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.     

PO43-对304不锈钢在氯离子水溶液中小孔腐蚀形核过程的影响

利用动电位循环扫描的方法研究了PO₄^3-对304不锈钢亚稳态孔蚀及稳定孔蚀形核与生长阶段的影响。随着PO₄^3-浓度的增大,亚稳态孔蚀电位E_m和稳定孔蚀电位E_b值均增大,即PO₄^3-浓度的增大,抑制了亚稳态孔蚀和稳定孔蚀的形核。PO₄^3-增加导致亚稳态孔蚀的平均生长速度和电流峰值降低,从而增大了亚稳态孔蚀转化为稳定孔蚀的难度,抑制了稳定孔蚀形核。但PO₄^3-增加导致孔蚀再钝化电位E_p降低,使充分发展的稳定蚀孔更难于再钝化,其原因可能是磷酸盐膜在小孔孔口的沉淀会促进蚀孔生长的稳定性。     

Real time pit initiation studies on stainless steels: The effect of sulphide inclusions

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.     

The inhibition of pitting corrosion of stainless steels by chromate and molybdate ions

The inhibitive effects of chromate and molybdate on pitting corrosion in stainless steel AISI 304 and AISI 316 were studied in acidified chloride solution. The results presented show that these known inhibitors affect both the nucleation of pitting and metastable pitting by reducing the numbers and sizes of these events. This makes attainment of stable pit growth more difficult.     

In Situ Characterization of Pitting Corrosion of Stainless Steel by a Scanning Electrochemical Microscopy

In this work, a scanning electrochemical microscopy (SECM) was used to characterize in situ the metastable and stable pitting processes occurring on a stainless steel in the chloride solution. It was found that metastable pitting would occur on the steel that was at corrosion potential and passive potential. The positive shift of potential would enhance the metastable pitting current. On application of a potential exceeding pitting potential, the pit became stabilized and maintained a continuous growth. The SECM is capable of detecting the microdissolution event and provides a “visual” observation of the pitting processes.     

混凝土孔隙液中Cl-浓度对304不锈钢亚稳态点蚀的影响

通过动电位极化和恒电位极化研究了Cl-浓度对304不锈钢在模拟混凝土孔隙液(pH 12.6)中亚稳态点蚀的影响,并利用极值统计分布研究了亚稳态点蚀的峰值电流密度(Ipeak),亚稳态点蚀半径(rpit),亚稳态点蚀稳定性乘积(Ipeak·rpit)以及单个亚稳态点蚀的生长时间(tgrow),再钝化时间(trep),生长率(Kgrow)和再钝化率(Krep)。结果表明,在碱性介质中,Cl-浓度增加会促使亚稳态点蚀更易转变为稳定点蚀。     

A micro-electrode study of the nitrate effect on pitting of stainless steels

Stainless steel micro-electrodes have been used to measure the effect of nitrate on pitting dissolution in sodium chloride solutions. No inhibiting effect of nitrate on active (film-free) dissolution is observed, even when the metal salt solution in the pit is supersaturated. However, nitrate causes abrupt passivation during diffusion-controlled dissolution across a salt film formed at higher potentials. The passivation potential of the salt-covered surface is highly reproducible and decreases with increasing ((NO₃⁻/(Cl⁻) ratio. This behaviour is probably related to redox reactions or electrochemical reduction of nitrate within the salt film, coupled with an increase in the pH of the salt environment with potential; a related observation in pure NaCl solutions is that local passivation-reactivation events occur under the salt film above a critical potential.     

Effect of Sulphur and Manganese on Nucleation of Corrosion Pits in Iron

Abstract

A study has been made of the effect of sulphur and manganese concentrations in iron on its tendency to pitting in a buffered potassium chloride solution. As revealed by electron microprobe and microscopic examinations, (Mn, Fe)S_x inclusions are the main sources of pit nucleation. Corrosion most frequently starts within the boundary region between the inclusion and the passive metal. Electrochemical investigations have shown that the critical pitting potential of the alloys under investigation, irrespective of S content, is lower than the corresponding value for ultra-pure iron. Mn has a dual effect on fhe resistance of Fe to pitting: it slightly increases the critical pitting potential, but it forms the sulphide inclusions at which the pits nucleate.     

Eine charakterisierung der lochfraßkorrosion des nickels—I. Die bestimmung der lochfraßpotentiale in neutralen und alkalischen lösungen

Quasistationary values for the characteristic pitting potentials for nickel were determined by means of potentiokinetic polarization measurements and their dependence on chloride and hydroxide ion concentration was investigated. The pit nucleation potential U_np is a linear function of the logarithm of the anion concentration. The value being determined by the adsorption equilibrium of the anions on the passivated metal surface.The critical pitting potential U_cp depends on the pH of the solution only. Above pH 6-5 U_cp decreases with increasing pH. On the basis of the mechanism for crevice corrosion this behaviour may be explained by the influence of OH^?-ions being known to take part in the ionization of the metal atoms.     

Determining the probability of stable pit initiation on aluminium alloys using potentiostatic electrochemical measurements

A method is described that determines the probability of stable corrosion pits being formed on aluminium alloys. The method employs potentiostatic measurements of metastable pit equivalent volumes, in coulombs, at the open circuit potential. The metastable pit volumes are statistically analysed using an extreme value statistics procedure that returns an expected distribution of metastable pit volumes. This enables the probabilities of the occurrence of metastable pits of given volumes with time to be calculated. The size requirement for a metastable pit to transition into a stable pit was then determined using both optical microscopy and electrochemical tests. This size requirement when combined with the metastable pit size distributions was used to determine the probability of the formation of a stable pit. The method is demonstrated for aluminium alloy AA2024-T3 in chloride media with and without chromate additions and validated against optically measured pitting rates.     

Environmental factors affecting the corrosion behavior of reinforcing steel III. Measurement of pitting corrosion currents of steel in Ca(OH)2 solutions under natural corrosion conditions

Using a simple electrolytic cell, the pitting corrosion current of reinforcing steel is measured in Ca(OH)₂ solutions in presence of chloride and sulfate as aggressive ions. Pitting corrosion current starts to flow after an induction period which depends on the concentration of both the aggressive and the passivating anions. The pitting corrosion current densities reach steady-state values which depend also on the type and concentration of the corrosive and passivating anions. The corrosive action of the aggressive species decreased in the order: SO₄²⁻ > Cl⁻. Corrosion of the steel is found to be governed by a single electron transfer reaction. Raising the temperature decreases the induction period associated with pit initiation and increases the corrosion current associated with pit propagation. From Arrhenius plots, the activation energies for both pit initiation and pit propagation in presence of chloride and sulfate ions are calculated.     

Electrochemical ac and dc response of copper in sea water at different anodic potentials

Abstract

The electrochemical behaviour of copper in sea water includes the formation of passive products on the surface and their breakdown by pit nucleation and growth. The aim of the present paper is to report this behaviour, paying particular attention to the electrochemical current noise response. On the basis of this information a possible pitting mechanism is proposed.     

Evaluation of metastable pitting on titanium by charge integration of current transients

The metastable pitting of titanium has been studied under potentiostatic control in solutions containing chloride ions. An approach based on the charge integration of current transients was proposed for a quantitative determination of metastable pitting. A pit density (d_mpit) was defined as the number of metastable pits per unit area per unit time (cm⁻² h⁻¹) with a typical size, instead of a size distribution. The calculated d_mpit of titanium at 0.5 V_SCE in 0.6 M NaCl was about 1.0 × 10³ cm⁻² h⁻¹ with a typical radius of 0.12 μm. An exponential potential dependence of d_mpit was obtained through the integration approach.     

Die Bedeutung der metastabilen Lochkorrosion bei hochlegierten Stählen

The importance of metastable pitting corrosion in the case of high alloy steels A suitable computer-aided experimental method allows during potentiostatic tests to recognize and quantitatively treat current transients. The current transients result from metastable pitting phenomena below the pit propagation potential. It is possible under certain conditions to detect metastable pitting which in the SEM appears in the form of small (approximately 1 μm), in most cases hemispherical pits. A detailed study of metastable pitting has brought about fundamental knowledge about the mechanism of pit initiation, stable pit growth and repassivation in pitting and crevice corrosion processes.     

Pitting corrosion behaviour of 2101 duplex stainless steel in chloride solutions

ABSTRACT

The corrosion behaviour of 2101 duplex stainless steel (DSS) in NaCl solution was studied and compared with that of 2205 DSS. The effects of chloride concentration and solution temperature on pitting corrosion behaviour were focused. The relative sensitivity to pitting corrosion of the constituent phases in both 2101 and 2205 DSSs was also explored. Pitting corrosion susceptibility was evaluated by conducting cyclic polarisation curve measurement. The corrosion morphology was examined by a scanning electron microscope equipped with energy dispersive spectrometer. The experimental results showed that the pitting corrosion resistance of 2101 DSS was inferior to that of 2205 DSS by exhibiting a lower threshold chloride concentration and a lower critical pitting temperature. For both 2101 and 2205 DSSs, the ferrite phase was more susceptible than austenite phase to pitting corrosion.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Impedance studies of the passive film on aluminium

Impedance spectroscopy has been used to study the characteristics of aluminium in 0.10 M NaCl solution at potentials below the pitting potential (−1.50V_SCE to −0.70V_SCE). Multiple capacitive (or pseudocapacitive) impedance features have been identified and analyzed over this range of potentials. Higher frequency resistances have been compared with surface analytical data, establishing a correlation between higher frequency impedances and the concentration of chloride in the passive film. The relationship shows that both charging resistance and chloride concentration reach a maximum immediately prior to passive film breakdown. At the onset of metastable pitting, both the impedance and chloride concentration decrease from the measured maxima. In contrast to X-ray photoelectron spectroscopy studies, the impedance results do not reveal oxide thinning prior to the onset of metastable pitting. Modelling from impedance data indicates that the thickness of the oxide layer sampled by impedance is much thinner than the oxide measured with surface analytical techniques, suggesting that the impedance method senses only the space charge layer in the oxide and not the total film thickness.     

Corrosion behaviour of aluminium in fresh waters with special reference to pipelines

Abstract

This review is largely concerned with work carried out on the long-term behaviour of aluminium in fresh waters and the methods developed for predicting service behaviour. The corrosion behaviour of aluminium in waters is discussed and it is shown that aluminium corrodes by pitting. The effect of pitting on tensile properties, on the bursting strength of pipes, and on the hydraulic efficiency of pipes is described. It is shown that the most important consideration is that of preventing of perforation by pitting. The mechanism of pit initiation and pit growth and its practical Implications are described. The methods used by these Laboratories for determining the corrosivities of waters are given. The alloys in general use.in fresh waters are given together with their chemical compositions. The types of all aplicationS are briefly mentioned, and a specific application, that of water pipelines, is then dealt With in detail. Finally, the methods usedfor preventing perforation are summarised.     

Investigation of pitting resistance of titanium based on a modified point defect model

The pitting resistance of titanium was studied under potential control in solutions containing chloride ions. The results evidenced that Cl⁻ concentration had an effect on the metastable pitting intensity, but no significant influence on the uniform corrosion. XPS characterization revealed that some Cl⁻ ions were present in the outer hydroxide layer, but few in the inner oxide layer, indicating that the inner oxide layer was impervious to Cl⁻ ions. A cation–anion-vacancy condensation mechanism was considered for pit initiation based on the point defect model (PDM). The experimental results analyzed by the charge integration technique were in agreement with the derived relations.     

Depassivation current transients measured between identical twin microelectrodes in open circuit

Current transients between identical twin titanium microelectrodes have been measured in open circuit. The electrodes showed transient breakdown of passivity in 1.5 M HCl although the potential is far below the pitting potential. Measurement of the current between the twin microelectrodes in sodium sulphate solution containing no chloride showed no detectable breakdown transients above the background noise. The current transients represent the breakdown of passivity and associated nucleation of corrosion pits. Some of the open circuit breakdown events show evidence of brief metastable pit propagation in the titanium surface.     

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 3.5 wt% 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.