A stochastic analysis of the effect of magnetic field on the pitting corrosion susceptibility of pure magnesium

The effect of magnetic field on the pitting corrosion susceptibility of pure magnesium was investigated by stochastic approaches. Stochastic models had been applied to simulate pitting corrosion as the combination of two physical processes: pit initiation and pit growth. The results revealed that magnetic field increased the pitting corrosion susceptibility of pure magnesium. For the pit initiation process, the presence of magnetic field changed the mechanism of pit initiation from the parallel birth and death stochastic model (B1) to the parallel birth stochastic model (A3). Magnetic field increased the pit generation rate λ while decreased the repassivation rate µ, which indicated that magnetic field accelerated the pit initiation process of pure magnesium. For the pit growth process, magnetic field didn't change the pit growth mechanism, but it could decrease the ability of repassivation of pitting corrosion resulting in a great probability for the stable pitting corrosion to grow up with a higher growth rate and finally develop into larger pit cavity.     

用随机分析方法研究磁场对纯镁点蚀过程的影响机理

通过恒电位的方法测量了在有无磁场条件下纯镁的电流-时间响应曲线,并用随机的方法对结果进行分析,研究了磁场对纯镁点蚀产生过程和点蚀成长概率的影响。磁场通过洛仑兹力产生磁流体动力学效应（Magnetohydrodynamic）,显著地增加了纯Mg的点蚀敏感性：对于点蚀的产生过程,磁场的存在改变了纯镁的点蚀产生机制。在无磁场作用的条件下,纯镁的点蚀产生过程都遵循“生死异地”的B1机制;在有磁场作用的条件下,纯镁的点蚀产生过程都遵循“相同点蚀并联”的A3机制。磁场不仅增大了点蚀产生速度,而且降低了点蚀的再钝化速度。对于点蚀的成长过程,磁场提高了稳态点蚀的成长概率,更容易成长为较大的点蚀坑。     

Extreme value analysis applied to pitting corrosion experiments in low carbon steel: Comparison of block maxima and peak over threshold approaches

The block maxima and peak over threshold approaches to extremes have been applied to pitting corrosion data from laboratory-simulated buried line pipe steel. Both approaches have been useful for gaining a better understanding of pitting in low carbon steel. We show that the parent distribution of the extreme depths is the right tail of the observed pit depth distribution. The threshold approach proved more robust to maximum depth reduction, which results from the dependency between pit depths. A simplified sampling method is proposed for applying the threshold approach without the need for measuring the entire pit depth distribution.     

Corrosion behavior of Al-3.0 wt%Mg alloy in NaCl solution under magnetic field

The influences of applied magnetic field on the corrosion behavior of Al-3.0 wt%Mg alloy in 3.5 wt% NaCl solution were investigated by electrochemical measurements,scanning electron microscopy(SEM)and energy-dispersive spectroscopy(EDS).Stochastic analysis was applied to investigate the influences of applied magnetic field.The results indicate that the application of horizontal magnetic field of 0.4 T would increase the pitting corrosion potential(E_(pit)),decrease the corrosion current density(i_(corr)),prolong the pit initiation time,slow down the pit generation rate and inhibit the growth of pitting of the tested alloys in 3.5 wt% NaCl solution.The applied magnetic field would also change the mechanism of pit initiation of Al-3.0 wt%Mg alloy from A_3 model(without magnetic field)to A_3+A_4 model(with magnetic field).The intermediate product Al_((ad))~+ is the paramagnetic ion that would be influenced by magnetic field sensitively.     

Probability distribution of pitting corrosion depth and rate in underground pipelines: A Monte Carlo study

The probability distributions of external-corrosion pit depth and pit growth rate were investigated in underground pipelines using Monte Carlo simulations. The study combines a predictive pit growth model developed by the authors with the observed distributions of the model variables in a range of soils. Depending on the pipeline age, any of the three maximal extreme value distributions, i.e. Weibull, Fréchet or Gumbel, can arise as the best fit to the pitting depth and rate data. The Fréchet distribution best fits the corrosion data for long exposure periods. This can be explained by considering the long-term stabilization of the diffusion-controlled pit growth. The findings of the study provide reliability analysts with accurate information regarding the stochastic characteristics of the pitting damage in underground pipelines.     

Stochastic modeling of pitting corrosion: A new model for initiation and growth of multiple corrosion pits

In this work, a new stochastic model capable of simulating pitting corrosion is developed and validated. Pitting corrosion is modeled as the combination of two stochastic processes: pit initiation and pit growth. Pit generation is modeled as a nonhomogeneous Poisson process, in which induction time for pit initiation is simulated as the realization of a Weibull process. In this way, the exponential and Weibull distributions can be considered as the possible distributions for pit initiation time. Pit growth is simulated using a nonhomogeneous Markov process. Extreme value statistics is used to find the distribution of maximum pit depths resulting from the combination of the initiation and growth processes for multiple pits. The proposed model is validated using several published experiments on pitting corrosion. It is capable of reproducing the experimental observations with higher quality than the stochastic models available in the literature for pitting corrosion.     

Pitting corrosion behaviour of austenitic stainless steels with Cu and Sn additions

The influence of Cu and Sn on the pitting corrosion resistance of AISI 304 and 316 stainless steels in chloride-containing media has been investigated. The corrosion behaviour was evaluated by cyclic polarization, potentiostatic CPT measurements and electrochemical impedance spectroscopy in 3.5 wt% NaCl. The corrosion resistance was also studied in FeCl₃ under Standard ASTM G-48. According to the results, Cu addition favours pit nucleation but inhibits its growth, whereas Sn exerts the opposite effect, favouring pit growth and inhibiting its nucleation. Studies by SEM, X-ray mapping and EDS analysis showed Cu-, Cl- and O-rich corrosion products that reduce the extent of corrosion damage.     

Markov chain modelling of pitting corrosion in underground pipelines

A continuous-time, non-homogenous linear growth (pure birth) Markov process has been used to model external pitting corrosion in underground pipelines. The closed form solution of Kolmogorov’s forward equations for this type of Markov process is used to describe the transition probability function in a discrete pit depth space. The identification of the transition probability function can be achieved by correlating the stochastic pit depth mean with the deterministic mean obtained experimentally. Monte-Carlo simulations previously reported have been used to predict the time evolution of the mean value of the pit depth distribution for different soil textural classes. The simulated distributions have been used to create an empirical Markov chain-based stochastic model for predicting the evolution of pitting corrosion depth and rate distributions from the observed properties of the soil. The proposed model has also been applied to pitting corrosion data from pipeline repeated in-line inspections and laboratory immersion experiments.     

Effect of microcrystallization on pitting corrosion of pure aluminium

A microcrystalline aluminium film with grain size of about 400 nm was prepared by magnetron sputtering technique. Its corrosion behaviour was investigated in NaCl containing acidic solution by means of potentiodynamic polarization curves and electrochemical noise (EN). The polarization results indicated that the corrosion potential of the sample shifted towards more positive direction, while its corrosion current density decreased compared with that of pure coarse-grain Al. The EN analysis based on stochastic model demonstrated that there existed two kinds of effect of microcrystallization on the pitting behaviour of pure aluminium: (1) the rate of pit initiation is accelerated, (2) the pit growth process was impeded. This leads to the enhancement of pitting resistance for the microcrystallized aluminium.     

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.     

Stable pitting corrosion of stainless steel as diffusion-controlled dissolution process with a sharp moving electrode boundary

This paper is devoted to explanation and prediction of pit propagation or stable pit growth, governed by ion transport properties in electrolytic solutions. Therefore, we vigorously derive the (3D) mass conservation law for a body hosting a sharp metal/solution interface separating the solid electrode from liquid electrolyte. The model for stable pitting corrosion is completed by Fick’s law of diffusion, governing the behavior of the dissolved metal ions. There are only three model input values, which are directly accessible from experiments, namely the ion concentration in the solid metal, as well as the diffusion coefficient and the saturation concentration of the dissolved metal ions in the electrolyte. The partial differential equations describing stable pitting corrosion as diffusion-controlled dissolution process are solved for boundary conditions related to 1D pencil electrode tests as well as to 2D foil electrode tests. The mathematical solution functions (model predictions) are related to pit growth in terms of depth and width, and to electric current evolution. Such model predictions reasonably agree with corresponding experiments, which shows the relevance of the proposed approach. In the case of pits with lacy covers, the boundary conditions for the ionic flux probably depend on complex depassivation-repassivation phenomena. However, once the extent of perforation of the lacy cover is known, the entire pit propagation characteristics, in particular the pit shape, can be predicted by the proposed model, relating to a classical Stefan problem.     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

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.     

A novel accelerated corrosion test for exhaust systems by means of power ultrasound

Cycling corrosion tests have been performed to simulate corrosion conditions in the cold end of an automotive exhaust system. A middle range 1.4512 (AISI 409) stainless steel is submitted to a conventional dip dry test (DDT) parallel to a similar test but including an additional external stress thanks to an ultrasonic transducer. This new ultrasonic test (so called UST) is expected to reduce the diagnostic time of the corrosion test by the combined action of the chemical corrosion process and of mechanical degradations. Both corrosion tests are performed in two different media in order to simulate internal corrosion due to exhaust gas condensate and external cosmetic corrosion, greatly enhanced by road salt during winter. It respectively concerns a synthetic gas condensate, the composition of which is derived from what is obtained from motor gasoline combustion, and NaCl solution. In both electrolytes the stainless steel suffers from pitting corrosion. Samples are examined by optical micrography and surface profilometry. The degradation state is quantified according to three parameters: maximum pit depth, average pit diameter and porosity rate. The efficiency of each test is then related to the time of immersion. As expected, use of ultrasound allows pits growth to be achieved from the beginning of the exposure time, so that the maximum pit depth recorded after 180 h of immersion is twice than with the classical dip dry test. It seems that it does not modify the pit initiation mechanism but only increases growth kinetics.     

Influence of cold working on the pitting corrosion resistance of stainless steels

This paper addresses the influence of cold rolling and tensile deformation on the pitting corrosion resistance of AISI 304 and AISI 430 stainless steels, investigated using some electrochemical techniques specifically designed for the different pitting stages to be analyzed separately. Cold work is shown to act differently depending on the pitting stage under consideration. (i) The pit initiation frequency shows a maximum after 20% cold-rolling reduction or 10% tensile deformation. This maximum is also observed on the ferritic grade, contradicting the hypothesis of a direct effect of strain-induced martensite, and is more likely related to the dislocations pile-ups. (ii) The pit propagation rate increases monotonously with cold rolling reduction, and pit repassivation ability decreases (leading to a larger number of stable pits), suggesting that the overall dislocation density is the controlling factor in these stages. Last, the significance of pitting potential measurements is discussed in the light of the effect of the cold-rolling reduction on the measured values.     

Modelling the effects of production rates and physico-chemical parameters on pitting rate and pit depth growth of onshore oil and gas pipelines

To estimate the pitting rate of internally corroded oil and gas gathering pipelines, a multivariate regression modelling was carried out, using pitting rates and operating parameters. These operating parameters, temperature, pH, CO₂ partial pressure, water cut, wall shear stress, chloride ion concentration, sulphate ion concentration, operating pressure, oil production rate, gas production rate and water production rate, were obtained from routine monitoring of the pipelines, whereas, the pitting rates (mean pit depths over time) were determined by the ultrasonic thickness measurement technique. The operating parameters and pitting rates were also used to estimate the pit depth growth of the pipelines using Monte Carlo simulation, and field data were used to test the developed models. The results obtained indicated that the pipelines under severe pitting corrosion rate were, more conservatively predicted than those under low, moderate and high pitting corrosion rates.     

Initial stages of corrosion pits on AISI 1040 steel in sulfide solution analyzed by temporal series micrographs coupled with electrochemical techniques

This work presents a study of the initial instants in the pitting corrosion of AISI 1040 steel, analyzed by temporal series micrographs coupled to an open circuit potential (E_oc) and polarization curves. During the E_oc measurement, the pit induction time and the initial pit growth in MnS inclusions was detected in alkaline sulfide solution. The pit area behavior has two distinct rate of area changes in specific regions directly associated to current slope changes. Finally, it was possible to create a three-dimensional model of the pit depth evolution on the metal, using Faraday’s law and the bullet-shaped geometry.     

The effect of impurity inclusions on the pitting corrosion behaviour of beryllium

An electrochemical polarisation study has been conducted to determine the effect of impurity inclusions on the pitting corrosion behaviour of beryllium. Three grades of commercial beryllium differing in impurity levels were used. This work showed a strong relationship between the pitting potential of the beryllium and the level of impurity inclusions, the grade with the least number of overall inclusions being the most resistant to pitting corrosion. The work also revealed the preferred sites for pit initiation, with most corrosion pits initiating at intermetallic Fe/Al/Be inclusions, and to a smaller extent at inclusion sites containing either elemental silicon or carbides.     

Pitting corrosion of intermetallic compound Ni3(Si,Ti) in sodium chloride solutions

The pitting corrosion of intermetallic compound Ni₃(Si,Ti) was investigated as functions of test temperature and chloride concentration in sodium chloride solutions by using a potential step method. In addition, the pitting corrosion of solution-annealed austenitic stainless steel type 304 and pure nickel was also studied under the same experimental condition for comparison. The pitting potential obtained for the intermetallic compound decreased with increasing chloride concentration and test temperature. A critical chloride concentration below which no pitting corrosion took place was found to exist and to decrease with increasing test temperature. The specific pitting potential at the critical chloride concentration also decreased with increasing test temperature. In addition, the pitting potential at various constant chloride concentrations above the critical chloride concentration decreased with increasing test temperature. The pitting potential of Ni₃(Si,Ti) was higher than pure nickel, but lower than that of type 304.     

A numerical study of damage caused by combined pitting corrosion and axial stress in steel pipes

Localized pipe wall damage accounts for many failures. Numerical modelling of pipes under increasing axial load and constant internal pressure when there is corrosion pits on the exterior surface of the pipe is reported herein. It is shown that for the assumed ideal elastic–plastic material the shape and volume of the plastic field depend on pit depth and its geometry. Pipe wall fracture around a pit can be associated with a critical plastic section. The results reported herein should be relevant for estimating of the risk of perforation and of loss of contents for steel pipes under different loading.