R.A. Cottis

Interpretation of Electrochemical Noise Data

Abstract The measurement of the electrochemical noise associated with corroding metals places a number of requirements on the measurement system if good quality data are to be obtained. In particular, aliasing and quantization noise should be avoided. Analysis methods may ignore the ordering of the measured potential or current values by using sequence-independent parameters such as the mean, standard deviation, skew, and kurtosis, or they may take the sequencing into account by computing the autocorrelation function, power spectra, or higher-order spectra. When potential and current noise are measured simultaneously, additional methods are available, including the calculation of electrochemical noise resistance, electrochemical noise impedance, characteristic charge, characteristic frequency, and various cross-correlation methods. Newer, somewhat more speculative methods include wavelet and chaos analysis. One of the most attractive prospects of electrochemical noise measurement methods is the ability to...

THE DETERMINATION OF pH, POTENTIAL AND CHLORIDE CONCENTRATION IN CORRODING CREVICES ON 304 STAINLESS STEEL AND 7475 ALUMINIUM ALLOY

Abstract The conditions in a simulated crevice have been studied for type 304 stainless steel and for 7475 T651 AlZnMg alloy in 0.6 M NaCl bulk solutions. The pH has been measured with a palladium/hydrogen diffusion electrode, and the chloride concentration with a silver/silver chloride electrode. For stainless steel the results conform with the expectations of the classical crevice corrosion model, with the crevice becoming a net anode, and the pH falling as a result of chromium ion hydrolysis. For the aluminium alloy more complex results are obtained, with parts of the crevice becoming mildly acidic (pH 3–4), while the deeper parts of the crevice become slightly alkaline (pH 8). The latter observation is not readily explained in terms of the chemistry and electrochemistry of the crevice.

Extrapolation of Extreme Pit Depths in Space and Time

A four parameter model is proposed for data collected on maximum pit depths enabling simultaneous extrapolation into the future and over large areas of exposed metal. This model is based on the generalized extreme value distribution whose use in this context is here justified mainly on statistical, rather than metallurgical, reasoning. Those aspects of the model which allow for extrapolation in time rely on reported power law dependencies for mean pit depths. Use of the model for predicting means, standard deviations, percentiles, bounds, order statistics, hole counts, and size of perforated areas, is demonstrated, both in general and for a particular data set. Comparisons are made with other reported techniques

Validation of a Percolation Model for Passivation of Fe‐Cr Alloys: Two‐Dimensional Computer Simulations

Two-dimensional computer simulations have been used to demonstrate the main atomistic features of a percolation model for alloy passivity. The simulations reproduce all the qualitative features of passivation in Fe-Cr alloys, including a critical alloy composition and a passivation potential which decreases with incresing Cr content. New features which would appear in three dimensions are discussed

Phenomenological modelling of atmospheric corrosion using an artificial neural network

World-wide data for the atmospheric corrosion of steel and zinc were used to train and test neural networks. The cross validity technique was employed as the criterion to stop training. The statistical performance of the neural network was expressed as an average of five training and testing results. Multiple correlation coefficients showed that the neural network could account for about 70% of the variance in the corrosion data for steel and zinc. The testing results showed that predictions of corrosion data not included in the original training were close to practical data. Sensitivity analysis also demonstrated the effects of sulphur dioxide, chloride and exposure time on atmospheric corrosion in specific environments.

Electrochemical Noise Generation during Stress Corrosion Cracking of Alpha-Brass

Abstract An investigation of the electrochemical noise generation during the stress corrosion cracking (SCC) of alpha-brass was conducted in Mattsson solution (ammoniacal copper sulfate solution) o...

Computer simulation of alloy passivation and activation

Abstract Two important problems in passivity have been studied by atomistic computer simulation: the primary passivation of FeCr alloys, and the activation of AlX alloys, where X may be Hg, Sn, In, Ga, Zn etc. The basis of the simulation is a Monte Carlo procedure with site dependent probabilities for dissolution and surface diffusion on 2-D square and 3-D simple cubic lattices, the latter incorporating screw dislocations. The rule for passivation (zero or low probability of dissolution) requires the presence of two or more surface nearest-neighbours or second-nearest neighbours around a passivating (Cr or Al) atom which are also Cr or Al atoms. This permits the formation of a …X(OH)-O-X(OH)-O-X(OH)… chain (X = Cr or Al). Several aspects of alloy passivity have been rationalized with the model, including the critical Cr content for passivation (equivalent to a site percolation threshold), active-passive transitions with variation of dissolution probabilities, and activation by noble additives of high surface diffusivity.

Extrapolation of Extreme Pit Depths in Space and Time Using the r Deepest Pit Depths

A four‐parameter model for extreme pit depths is considered which allows the simultaneous extrapolation into the future and over large areas of exposed metal. Estimation of the model is proposed using the deepest pit depths from individual metal coupons or distinct areas, rather than using just maximum pit depths alone. The increased precision in estimates and also in predictions obtained through using the deepest pit depths is illustrated. This estimation is based on the asymptotic joint distribution of the largest order statistics, which is itself a generalization of the generalized extreme value distribution. The use of this joint distribution for predicting means, standard deviations, and bounds, and also for finding the distributions of the number of holes, the total area of holes, and perforation times is demonstrated both in general and for a particular set of data.

Electrochemical noise measurements for corrosion studies

This paper examines the various techniques that may be used to analyse electrochemical noise data, and explores the theoretical basis of the more important methods. Methods considered include: ○ Time domain analysis methods - including the direct measurement of the properties of transients from the time record. ○ Statistical methods - including the measurement of the mean, standard deviation/variance, noise resistance, coefficient of variation, localisation parameter and other statistical parameters. ○ Frequency domain analysis methods - including spectral estimation using various methods, including MEM and FFT, the determination of noise impedance, the determination of bispectra and wavelet analysis. ○ Discriminant analysis - this is a general method for determining the ability of a particular measurement to discriminate between alternative behaviours, and its application to electrochemical noise analysis is indicated.

Crack initiation in the corrosion fatigue of structural steels in salt solutions

Abstract The growth of short cracks in a structural steel 3.5% NaCl has been studied using smooth bending specimens. The influence of sodium nitrite additions has also been investigated. Cracks initiated at non-metallic inclusions, primarily oxide inclusions in air, and sulphides in the corrosive environments. At lower stresses pitting at inclusions preceded the formation of identifiable cracks. Different behaviours of ferrite and pearlite were detected, with the ferrite appearing more susceptible to cracking in the absence of corrosion, while cracking in the pearlite became significant at lower stresses in the corrosive environment. The observed behaviour has been related to corrosion processes in the region of the crack tip, with some evidence of corrosion-product induced crack closure in the presence of the sodium nitrite.