Electrochemical Techniques for Determination of Corrosion Rate

This section will discuss a number of different electrochemical techniques used for determination of corrosion resistance or corrosion rate. It will be assumed that a modem, computer ontroUed, potenhostat is available for use. Such instruments are now relatively inexpensive. The menu-driven software for experiment control and data collection, as well as powerful 

A number of good sources of information exist on the topic of electrochemical and nonelectrochemical test methods for corrosion. This chapter refers to many ASTM standards, most of which are in Volume 3.02 of the ASTM Annual Book of Standards entitled Wear and Erosion Metal Corrosion . Books [7-9] and review articles [10-12] on corrosion testing provide more detail than can be given here. 

---

The information about the tendenqf for corrosion to occur that can be obtained from thermodynamic calculations is important and useful. However, most of the science and engineering aspects in the field of corrosion focus on knowing and reducing the rate of corrosion. The rate of corrosion is not addressed by thermodynamics it falls instead within the purview of kinetics. So the kinetics of electrochemical reactions in general, and corrosion reactions specifically, are at the heart of the subject of corrosion. This chapter will introduce electrochemical kinetics at a simple level, with sufficient detail to develop the concept of mixed potential theory. The interested reader is referred to other volumes of this encyclopedia and to textbooks in corrosion [1-9] for a more detailed description. The kinetic underpinnings of some of the electrochemical techniques for determination of corrosion rate will also be presented. The influence of transport on the rates of electrochemical reactions will be discussed in the next chapter (see Chapter 1.4). 

Electrochemical techniques for determination of corrosion rate generate a value in units of A cm , which is current density. 

V. Kucera and E. Mattsson, Electrochemical technique for determination of the instantaneous rate of atmospheric corrosion. Corrosion in Natural Environments, ASTM STP 558, American Society for Testing and Materials, Philadelphia, 1974, p. 239. 

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. 

S. Feliu, J. A. Gonzales, C. Andrade, Electrochemical methods for onsite determinations of corrosion rates of rebars , in Techniques to Assess the Corrosion Activity of Steel Reinforced Concrete Structures,... 

The electrochemical techniques described in Sects. 7.3.1 and 7.3.2 measure the response of the system to an externally applied perturbation. It is possible to use the inherent noise in the system as a stimulus and measme the system response [27-36]. The fundamental basis to use noise analysis as a determination of corrosion rate is not as strong as that for the other techniques. However, the advantage of noise analysis is that there is no need to apply any external signal, and the system is not polarized at all away from its natural... 

Feliu, S., Gonzalez, J.A. and Andrade, C. (1995). Electrochemical Methods for onsite Determination of Corrosion Rates of Rehars, in Symposium on Techniques to Assess the Corrosion Activity of Steel in Reinforced Concrete Structures, December 1994, American Society for Testing and Materials. 

Potentiodynamic polarization measurements are quite appropriate for determination of the pitting susceptibility of aluminum coatings, and/or the corrosion current density/ corrosion rate of coated steel products in general. ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, describes the calculation of corrosion rates and other information from electrochemical measurements. Another example of the use of DC electrochemical methods to examine the corrosion performance of coated sheet materials is a study by D. A. Jones et al. [48]. The study used polarization resistance measurements to examine the mechanism of steel and coated sheet degradation under conditions of alternate immersion. Jones compared the polarization resistance of samples of low-carbon steel, unpainted galvanized, aluminum-coated, and Zn-Ni alloy coated steel during continuous immersion and alternate immersion. Alternate immersion cyclic exposure produced a thick oxide that led to significant underfilm attack. Jones found that phosphate pretreatment tends to increase the resistance of these materials to underfilm attack. This study is an excellent example of the way electrochemical measurements can be used as a complement to other techniques to elucidate mechanistic information. 

Specifics on the types and rates of microbiological attack. These must be determined by using other methods such as chemical and microbiological analysis of the solution and materials from the corrosion sites. Consideration must be given to limitations of electrochemical techniques for MIC studies, noted previously under Corrosion Testing Laboratory Tests and subsequent subsections. 

A general scheme for the development of corrosion models based on electrochemical principles has been described, and a number of examples for active, passive, and localized corrosion has been given. This chapter is by no means comprehensive, and a search of the scientific and technical literature will unearth many additional examples. The value in using electrochemical methods both to develop understanding of the corrosion process and to measure the values of specific modeling parameters is obvious. However, their application alone would not provide all the elements and parameter values required for the development of corrosion models, so the use of supplementary techniques is necessary. It is necessary also to keep in mind that electrochemical techniques inevitably accelerate the corrosion process one is interested in. Consequently, the scaling of electrochemi-cally determined parameter values to the rates and time periods of interest in the corrosion process to be modeled should be undertaken carefully and with a full knowledge of the limitations involved. 

Several electrochemical techniques for the assessment of corrosion rate have been presented in this chapter. It is of use to summarize and compare the different techniques. Table 1 provides a summary of the data generated by the different techniques for Fe in 0.5 M H2SO4, and the corrosion current densities determined from those data. 

Corrosion occurs at a rate determined by equilibrium between opposing electrochemical reactions. The rate of any given electrochemical process depends on the rates of two conjugate reactions proceeding at the surface of the metal. Transfer of metal atoms from the lattice to the solution (anodic reaction) with the liberation of electrons and consumption of these electrons by some depolarisers (cathodic reaction). When these two reactions are in equilibrium, the flow of electrons from each reaction of balanced and no net electron flow (current) occurs. Various methods are available for the determination of dissolution rate of metals in corrosive environments but electrochemical methods employing polarisation techniques are by far most widely used. The corrosion rate (CR) is evaluated by mass loss method considering uniform corrosion. The Corrosion rate is determined by the following formula as per standard [102]. 

There are many other techniques for measuring corrosion. Some of these may rely on chemical solution analysis or physical measurement of metal loss. One widely used additional technique is provided by ASTM G 96, Practice for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods). This guide covers two basic procedures. In one, a sample of the material, usually in the form of a continuous wire, is immersed into the environment of interest (liquid, air, solid, or multiphase). The electrical resistance through the wire is determined. As corrosion consumes the cross section of the wire, the resistance increases proportionately. The second procedure uses the polarization resistance technique described in ASTM G 59 to determine the corrosion rate in the environment. 

---