Corrosion testing electrochemical methods

Key words corrosion inhibition, rare earth metal inhibitors, corrosion testing, electrochemical methods, surface analytical technique. 

ASTM G 96, Practice for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)—This standard outlines procedures for online corrosion monitoring in operating systems. The test methods described in this standard are used to determine the cumulative metal loss (electrical resistance method) or instantaneous corrosion rates (electrochemical method). Reference 15 provides a summary of electrical resistance and polarization resistance theory. 

Specifics on the type of biological attack. This must be done by some other method such as chemical analysis of the solution (plus consideration given to limitations to the use of these several electrochemical techniques for MIC studies, noted previously under Corrosion Testing Laboratoiy Tests and subsequent subsections). 

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. 

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... 

Corrosion Monitoring in Industrial Plants Using Nondestructive Testing and Electrochemical Methods , Proc. Symposium Sponsored by ASTM Committee E-7 and G-1, Montreal, Canada, May 1984, ASTM Special Technical Publication 908 Internal Corrosion Control and Monitoring in the Oil, Gas and Chemical Industries Proc. 

In 1980 Bemhardsson et introduced an automated electrochemical method for CPT determination. The specimen is mounted as described in Section IV.2 (ii) using a stream of argon to avoid crevice corrosion and 0.02-5% sodium chloride as electrolyte. The CPT is determined by a potentiostatic test method using an instrument called the Santron CDT 400 for potential control, temperature control, and current measurements. 

Chapter 4 describes how the electrical nature of corrosion reactions allows the interface to be modeled as an electrical circuit, as well as how this electrical circuit can be used to obtain information on corrosion rates. Chapter 5 focuses on how to characterize flow and how to include its effects in the test procedure. Chapter 6 describes the origins of the observed distributions in space and time of the reaction rate. Chapter 7 describes the applications of electrochemical measurements to predictive corrosion models, emphasizing their use in the long-term prediction of corrosion behavior of metallic packages for high-level nuclear waste. Chapter 8 outlines the electrochemical methods that have been applied to develop and test the effectiveness of surface treatments for metals and alloys. The final chapter gives experimental procedures that can be used to illustrate the principles described. 

It is preferable to carry out laboratory corrosion tests and to validate the data with service tests for the selection of materials. It is needless to note that the chosen test method be reliable and cost effective. Some of the test methods in use in industry are service tests, field tests, laboratory tests, and rapid electrochemical methods such as potentiodynamic polarization, linear polarization, electrochemical impedance and electrochemical noise. 

Thompson, N.G., Payer, J.H., DC Electrochemical Test Methods, in Corrosion Testing Made Easy, B.C. Syrett (ed.), NACE International, Houston, TX, USA, pp. 72-77, 1998. 

S. Hamyama and T. Tsum, A Corrosion Monitor Based on Impedance Method, Electrochemical Corrosion Testing, STP 727, F. Mansfeld and U. Bertocci, Ed., ASTM, 1981, p 167-186... 

Electrochemical and Electrical Methods. Electrochemical and electrical methods for studying film properties and corrosion phenomena have been extensively reviewed (29-31). Comparisons of corrosion test results with direct current measurements of conductivity suggest that visible corrosion is associated with film resistance less than about 1 Mohm/cm, but this condition may well correspond with the occurrence of virtual pores in the film allowing development of local conductive pathways. In studies of the equivalent alternating current resistance as a function of frequency, Kendig and Leidheiser (44) found that the development of a region of slope -1 on a log permittivity versus log frequency plot... 

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