Measurement of Corrosion Rate

The Stem-Geary equation provides a direct relationship between the steady state corrosion current and the dc resistance across the interface (Stem and Geary [1957]) 

Measurement of the series resistance at the high-frequency limit normally presents few problems, because Z(J(o) becomes nonreactive at frequencies as low as lOkHz, in most cases. On the other hand, in the low-frequency region, reactance is commonly observed at frequencies in the vicinity of 10 Hz, so that special precautions 

Because most impedance measurements are made sequentially at discrete frequencies, the total data acquisition time can be expressed as 

The structured noise method stems from the elegant work of Smith and coworkers (Smith [1966]), who developed a multifrequency technique for ac polarog-raphy. Subsequently, structured noise techniques have been used in corrosion studies by Smyrl and coworkers (Smyrl [ 9%5a,b] Smyrl and Stephenson [1985]) and by Pound and Macdonald [1985], In all cases, the perturbation applied to the system is of the form 

Flow Rate = 1.62 m/s Temperature = 26°C Specimen Area = 11.05 cm Oxygen Content = 0.85 mg/dm  

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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. 

It is significant that most of the data from which a remarkable uniformity of attack is deduced are derived from small isolated panels. This is the most convenient form of specimen for measurements of corrosion rates by loss of weight but it eliminates the important effect of galvanic currents passing between remote parts of a large structure. It is believed that the experience of civil engineers and other users would not support the conclusion suggested by panel tests that corrosion is no faster in tropical than in temperate waters. 

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. 

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. 

Thus we note that current density gives a measure of corrosion rate. Corrosion rates are generally expressed in practice as mpy (mils per year), ipy (inches per year), ipm (inches per month) and mdd (loss of weight in milligrams per square decimetre per day), and a nomograph for interconversion of one unit into another unit is depicted in Figure 1.22. 

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

Such measurements, however, constitute only indirect information about material corrosion, while the direct measurements of corrosion rates give straight information about anomalous behaviours of the structural materials. 

C. Andrade, ). A. Gonzales, Quantitative measurements of corrosion rate of reinforcing steels embedded in concrete using polarization resistance measurement . Materials and Corrosion, 1978, 29, 515-519. 

The understanding of the electrochemical phenomena underlying corrosion processes provides a basis for experimental techniques that allow simple and accurate measures of corrosion rates. The electrochemical fundamentals are discussed in Sects. 1.2-1.4 of this volume and in other volumes of this series. This chapter wiU discuss a wide range of experimental techniques commonly used in the field of corrosion and issues associated with their use. Electrochemical techniques will be the focus, but some nonelectrochemical techniques will also be discussed. Electrochemical techniques take advantage of our ability, with modern instrumentation, to utilize feedback control and measure very small currents. These techniques allow highly sensitive measurements that far exceed the capabilities of most nonelectrochemical techniques based on, for instance, weight loss or appearance. On the other hand, some nonelectrochemical techniques are also extremely sensitive to small amounts of material loss. An example is the quartz crystal microbalance (QCM), which provides submonolayer sensitivity as will be described in the following sections. 

EIS is a method that is not very well suited for the study of pitting corrosion because, as described earlier, it should be applied to electrodes at steady state, and pitting is a non-steady state condition. Nonetheless, EIS can be used to assess the low-frequency impedance at open circuit, which provides a measure of corrosion rate without the need to polarize away from open circuit. It is therefore possible to use EIS to determine if pitting is occurring at open circuit. 

K.B. Oldham, F. Mansfeld, On the so-called hnear polarization method for measurement of corrosion rates, Corrosion 27 (1971) 434-436. 

Corrosion current density is a particularly suitable measure of corrosion rate when treating corrosion theory and in connection with electrochemical corrosion testing. Current density is also directly applicable for cathodic and anodic protection (Sections 10.4 and 10.5). In corrosion testing the unit pA/cm is most often used. When dealing with cathodic protection the units mA/m and A/m are used for the cathode (sfructure to be protected) and the anode, respectively. 

Comparisons of corrosion rates before and after treatment have consistently showed significant reductions. In the Tees Viaduct field measurements of corrosion rates were in the range 0.33-1.66 with a mean of... 

A different situation exists when high levels of water saturation and low levels of oxygen lead to the black rust that is deposited in the concrete without exerting stresses (see Section 2.2). In this case the corrosion rate measurement can be taken as showing a section loss that will eventually lead to failure. However, it is very difficult to extrapolate an instantaneous measurement of corrosion rate to a total section loss measurement. If we can make a series of corrosion rate measurements at different locations and we can come up with a compensation for variations in relative humidity and temperature we can estimate the average corrosion rate. We also have to estimate the original time to corrosion, and assume that the corrosion rate has either been constant or increased in some sensible manner (say linear or logarithmic) to the present condition. 

Electrode kinetics is the study of reaction rates at the interface between an electrode and a liquid. The science of electrode kinetics has made possible many advances in the understanding of corrosion and the practical measurement of corrosion rates. The interpretation of corrosion processes by superimposing electrochemical partial processes was developed by Wagner and Traud [1]. Important concepts of electrode kinetics that wifi be introduced in this chapter are the corrosion potential (also called the mixed potential and the rest potential), corrosion current density, exchange current density, and Tafel slope. The treatment of electrode kinetics in this book is, of necessity, elementary and directed toward application of corrosion science. For more detailed discussion of electrode kinetics, the reader should refer to specialized texts Usted at the end of the chapter. 

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

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