Corrosion rate evaluation

This element has proved to be most effective for reducing the corrosion rate of lead—antimony alloys and has therefore been intensively studied [17,30—32]. Alloy corrosion rate (evaluated by weight loss) and electrode potential have been measured vs Ag content for Pb—Ag and Pb—Sb—Ag alloys [33]. The obtained curves are presented in Fig. 4.14. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

Combination electrical methods Tomashov and Mikhailovsky describe a method developed in the Soviet Union. This test is essentially a combination of resistivity measurement and polarisation rates on iron electrodes in soil in situ. The usefulness and value of this procedure has not as yet been determined by practical application by corrosion engineers. The development of this combination test does, however, represent an attempt to integrate some of the complex factors controlling corrosion rates in soil. Much more research on these factors and methods of measurement should in the future enable the corrosion engineer to evaluate soil properties with respect to application of corrosion-alleviating operations. 

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. 

Bureauand others have tried to apply the technique for evaluating the corrosion rate of painted metals, and although the results are controversial the method has also been used successfully in the study of canning materials and lacquered surfaces . 

The Schwerdtfeger polarisation break and the polarisation resistance methods have been studied by Jones and Lowe " in relation to their effectiveness in evaluating corrosion rates of buried metals. A Holler bridge circuit was used to remove IR contributions during the measurement of the polarised potential. Jones and Lowe, on the basis of their studies of buried steel and aluminium specimens, concluded that the polarisation resistance was the most useful, and that the polarisation break had the serious limitation that it was difficult to identify the breaks in the curve. 

A relationship was also established between the oxide-reduction time and time of exposure, and the results for a mild steel and a lCu-3Ni weathering steel were similar to those obtained by mass loss. The authors give various expressions that relate oxide-reduction time (min) with corrosion rate (mm/y), and claim that a short exposure to a laboratory SO2 atmosphere followed by determining the E vs. time and oxide-reduction time provides a rapid method of evaluating weathering steels. 

It should be noted that although ASTM A262 1986 provides details of test procedures no information is given on typical corrosion rates or acceptable limits for various heat-treated alloys, which are regarded as outside the province of a specification that describes test procedures. Table 19.4, taken from a paper by Brown, shows the maximum acceptable evaluation test rates specified by the Du Pont Company for various alloys tested by the acid... 

Corrosion of metals and alloys. Methods for the determination of corrosion rate of standard specimens for the evaluation of corrosivity... 

Adsorption is, of course, of major importance in the inhibition of corrosion by organic compounds (adsorption inhibitors) that have the ability to adsorb strongly on the metal surface, thus impeding the dissolution reaction and reducing the corrosion rate. It follows that the coverage of a metal surface by adsorbed inhibitor can be evaluated from the relationship... 

The corrosion rate of a metal in terms of weight loss per unit area (g m" d ) or rate of penetration (mm y" ) can be calculated from Faraday s law if the current density is known. Conversely, the corrosion current density can be evaluated from the weight loss per unit area or from the rate of penetration. The following symbols and units have been adopted in deriving these relationships in which it is assumed that corrosion is uniform and the rate is linear ... 

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths," comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits," corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe... 

The kinetic model described by Eq. (8) can be used to evaluate the material impact on the carbon corrosion rate. As shown for 50% Pt/Vulcan in Fig. 4 and Table 3 (first row), its carbon corrosion rate is essentially equal to the 02 crossover current. Since the... 

Figure 16 Illustration of the procedure used to evaluate fuel corrosion performance in a nuclear waste vault (A) fuel corrosion rate as a function of radiation dose rate [from (B) in Figure 15] (B) calculated radiation dose rate decay curve (C) fuel corrosion rates as a function of time in a waste vault. The dashed line indicates that there is a limit to the acceptable extrapolation of rates determined electrochemically.

ISO 9226, Corrosion of Metals and Alloys, Corrosivity of Atmospheres, Determination of Corrosion Rate of Standard Specimens for the Evaluation of Corrosivity, Geneva, Switzerland, 1992. 

Thermal stability evaluated as per ASTM G-28A after sensitization and ASTM G-28B showed Alloy 59 to be superior to other alloys with corrosion rates 40-51 mpy and 4 mpy, respectively. The comparative data are given in Tables 4.45 and 4.46, respectively. 

In order for this instrument to be used successfully, the effect of system temperature fluctuations on resistance of the wire must be nullified. This may be accomplished by having an identical loop of wire in the probe but protected from the corrosive fluid. The difference in resistance of the two loops gives one of the values for evaluation of corrosion rate. This instrument is only useful if corrosion occurs evenly over the wire loop. Pitting will cause excessively high readings and the error becomes increasingly great towards the end of the probe s life. 

The temperature dependence of corrosion rate is given by the temperature dependence of all the parameters mentioned above and participating in the corrosion process. The main roles are played by the temperature dependence of the diffusion coefficient and that of viscosity which determines the convection rate. Solubility and the other characteristics are of lesser significance. As the parameters involved do not have the same temperature coefficienis, the activation energy evaluated directly from the corrosion kinetics is not reliable for interpretation of the corrosion mechanism. 

---