Electrochemical polarisation

Fleischmann, M. and Thirsk, H.R. (1957) The electrochemical polarisation of silver single crystals in hydrochloric acid. Transactions of the Faraday Society, 53,91. 

Effect of noble metal addition on electrochemical polarisation behaviour of H2-oxidation and 02-reduction on 304 SS in high temperature water Conference Corrosion 98, San Diego, Ca, USA, 22-27 Mar. 1998 NACE, Houston, TX, USA, Paper 127,... 

Little et al. [12] reported that in one of their investigations, applying electrochemical polarisation could influence the number and types of bacteria associated with the surface. 

VA Marichev, SA Shipilov, Influence of electrochemical polarisation on crack-growth in corrosion cracking and corrosion fatigue of magnesium alloys, Soviet Materials Science, 1986, 33, 240-244. 

Lewis, E. C., Grain Boundary Corrosion of Sensitised Type 304 Stainless Steel by a Noble-to-active Polarisation Scan , J. Electrochem. Soc., 119, 219C (1972)... 

Langenegger, E. E. and Robinson, F. P. A., Effect of the Polarisation Technique on Dezincihcation Rates and Physical Structure of Dezincihed Zones , Corrosion, 24, 411 (1968) Brooks, W. B., Discussion of the De-alloying Phenomenon , Corrosion, 24, 171 (1968) Pickering, H. W., Volume Diffusion During Anodic Dissolution of a Binary Alloy , J. Electrochem. Soc., 115, 143 (1968)... 

Bond, A. P. and Lizlovs, E. A., Anodic Polarisation of Some Ferritic Stainless Steels in Chloride Media , J. Electrochem. Soc., 114, 199c (1%7)... 

Electrochemical corrosion of metals Since the aggressiveness of salt melts is governed by redox equilibria, and is often controlled by composition of the external atmosphere, effects analogous to electrochemical or oxygen-concentration corrosion in aqueous systems can occur in salt melts. Tomashov and Tugarinov determined cathodic polarisation curves in fused chlorides and concluded that the cathodic reactions of impurities could be represented as ... 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

Fig. 8.5 Potentiodynamic polarisation curves and the expected domains of electrochemical...

Electrochemical Techniques Although the linear polarisation resistance technique has moved beyond the infancy status attributed to it in the original material, its inherent limitations remain, i.e. it is a perturbation technique, sensitive to environmental conductivity and insensitive to localised corrosion. Two developments have occurred ... 

The electrochemical examination of fusion joints between nine pairs of dissimilar metal couples in seawater showed that in most cases the HAZ was anodic to the weld metals" . Prasad Rao and Prasanna Kumarundertook electrochemical studies of austenitic stainless steel claddings to find that heat input and 5Fe content significantly affected the anodic polarisation behaviour under active corrosion conditions whilst Herbsleb and Stoffelo found that two-phased weld claddings of the 24Cr-13Ni type were susceptible to inter-granular attack (IGA) as a result of sensitisation after heat treatment at 600°C /pa was unaffected by heat input. 

Although important contributions in the use of electrical measurements in testing have been made by numerous workers it is appropriate here to refer to the work of Stern and his co-workerswho have developed the important concept of linear polarisation, which led to a rapid electrochemical method for determining corrosion rates, both in the laboratory and in plant. Pourbaix and his co-workers on the basis of a purely thermodynamic approach to corrosion constructed potential-pH diagrams for the majority of metal-HjO systems, and by means of a combined thermodynamic and kinetic approach developed a method of predicting the conditions under which a metal will (a) corrode uniformly, (b) pit, (c) passivate or (d) remain immune. Laboratory tests for crevice corrosion and pitting, in which electrochemical measurements are used, are discussed later. 

A detailed and well-referenced account of electrochemical methods of testing has been written by Dean, France and Ketcham in a section of the book by Ailor. ASTM G5 1987 outlines standard methods for making potentiostatic and potentiodynamic anodic polarisation measurements and ASTM G3 1974 (R1981) gives conventions applicable to electrochemical measurements in corrosion testing. 

It is evident from previous considerations (see Section 1.4) that the corrosion potential provides no information on the corrosion rate, and it is also evident that in the case of a corroding metal in which the anodic and cathodic sites are inseparable (c.f. bimetallic corrosion) it is not possible to determine by means of an ammeter. The conventional method of determining corrosion rates by mass-loss determinations is tedious and over the years attention has been directed to the possibility of using instantaneous electrochemical methods. Thus based on the Pearson derivation Schwerdtfeger, era/. have examined the logarithmic polarisation curves for potential breaks that can be used to evaluate the corrosion rate however, the method has not found general acceptance. 

There is no difference between galvanostatic and potentiostatic polarisation experiments regarding the iR potential drop between the specimen and the tip of the probe used for measuring the electrochemical potential. In either case corrections should be made for accuracy. These could be quite large if the current density is high and/or the conductivity of the electrolyte is low. 

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