Electrochemical measurements of corrosion

Electrochemical Measurement of Corrosion Rate There is a link between elec trochemical parameters and actual corrosion rates. Probes have been specifically designed to yield signals that will provide this information. LPR, ER, and EIS probes can give corrosion rates direc tly from electrochemical measurements. ASTM G102, Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, tells how to obtain corrosion rates directly. Background on the approximations made in making use of the electrochemical measurements has been outlined by several authors. 

Cowan, R. L, and Kaznoff, A. I., Electrochemical Measurements of Corrosion Processes in a Boiling Water Nuclear Reactor , Corrosion, 29, 123 (1973)... 

In most electrochemical measurements of corrosion kinetics a potentiostat is used. This description will cover the rudimentary operation of a potentiostat using the concept of an ideal operational amplifier (op amp) as a basis. An op amp is a three-terminal device as shown in Fig. 16 with two input terminals and one output terminal. A perfect op amp follows five basic rules (19) ... 

Electrochemical Measurements of Corrosion of Iron Alloys in Supercritical Water... 

R.L. Cowan, A.I. Kaznoff. Electrochemical measurements of corrosion processes in a Boiling Water Nuclear Reactor. Corrosion Vol. 29 (4), 123, (1973). 

The Electrochemical Measurement of Corrosion Rates in CathodicaUy Protected Systems, EPRI Report CS-2858 on Project 1689-7, SRI International, Menlo Park, Calif. 

In the past, electrochemical measurements of corrosion as well as processes in batteries or other electrochemical processes were averaged over many atoms and molecules. Atomic details of electron- and ion-transfer reactions associated with dissolution and passivation are now ripe for further progress given nanoscale experimental and computational advances. Recent advances such as scanning electro-... 

Kutznelnigg, Pitting, the Typical Corrosion of Nickel Coatings , Korrosion, 13, 64 (1960) Schmeken, H., Electrochemical Measurements on Corrosion of Zinc with Regard to Pitting Possibilities , Korrosion, 13, 65 (I960)... 

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. 

In view of the electrochemical nature of corrosion, it has seemed reasonable to many investigators to assume that suitable accelerated corrosion tests could be made by observing the response to electrolytic stimulation of the corrosion processes, or by attaching particular significance to the results of quickly made electrode potential and current measurements. 

Test method for sandwich corrosion test Recommended practice for preparing, cleaning, and evaluating corrosion test specimens Practice for aqueous corrosion testing of samples of zirconium and zirconium alloys Test method for corrosion testing of products of zirconium, hafnium and their alloys in water at 633 K or in steam at 673 K [metric] Recommended practice for conventions applicable to electrochemical measurements in corrosion testing... 

Developments in electrochemical methods since 1976 for measurement of corrosion have been rapid. Research and development has produced several new techniques, e.g. a.c. impedance and electrochemical noise. These methods require corrosion expertise for both operation and interpretation. Industry generally prefers instrumentation that can be operated by process... 

The measurement of corrosion current has provided, as is well known, a quite useful electrochemical technique for determining corrosion rates. However, contrary to homogeneous corrosion, pitting corrosion is a typical heterogeneous reaction on a metal surface, so that it is difficult to estimate the actual corrosion state from the usual corrosion current data. 

Nagy, Z. DC Electrochemical Techniques for the Measurement of Corrosion Rates 25... 

Measurements of corrosion rates and other parameters connected with corrosion processes are important, first as indicators of the corrosion resistance of metallic materials and second because such measurements are based on general and fundamental physical, chemical, and electrochemical relations. Hence improvements and innovations in methods applied in corrosion research are likely to benefit basic disciplines as well. A method for corrosion measurements can only provide reliable data if the background of the method is fully understood. Failure of a method to give correct data indicates a need to revise assumptions regarding the basis of the method, which sometimes leads to the discovery of as-yet unnoticed phenomena. 

There are basically three kinds of approaches to the measurement of corrosion. Although the mechanism of corrosion is certainly electrochemical, only one of the methods available uses an ostensibly electrochemical technique. 

The impossibility of a direct measurement of corrosion rate using electrochemical testing would seem to be discouraging. Application of mixed potential theory allows determination of the corrosion rate using a method known as Tafel extrapolation. 

However, since this corrosion reaction is short-circuited on the corroding surface, no current will flow in any external measuring circuit. Consequently, a direct electrochemical measurement of the corrosion current (convertible to corrosion rate by the application of Faraday s law) cannot be made. Despite this limitation, electrochemical techniques can be used to decouple the two half-reactions, thereby enabling each to be separately and quantitatively studied. This involves the determination of the current-potential relationships for each half-reaction. Subsequently, the behavior under electrochemically unperturbed (open-circuit or natural corrosion) conditions can be reconstructed by extrapolation of these relationships to Ecorr-... 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

The electrochemical methods of measurement of corrosion rates have been described in Chapter 1. Some features of these methods are noted below ... 

Apparatus for electrochemical measurements during corrosion fatigue. CF tests can be done using an apparatus designed by the Continental Oil Company, as shown in Figure 6.52.110,111 The polarization potential and current can be controlled for the four samples tests at the same time. The apparatus consists of a Monel tank in which four specimens are subjected to cyclic bending. The preliminary step in the experiment is to determine the displacement caused by the desired applied load. The exact stresses are then determined with the use of strain gages. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

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