Measurement 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)... 

Gray, H. R., Ion and Laser Microprobes Applied to the Measurement of Corrosion-Produced Hydrogen on a Microscopic Scale , Corrosion, 28, 47 (1972)... 

More recently, attempts have been made to correlate mathematically the chemical composition of natural waters and their aggressivity to iron by direct measurements on corrosion coupons or pipe samples removed from distribution systemsThis work has been of limited success, either producing a mathematical best fit only for the particular data set examined or very general trends. The particular interest to the water supply industry of the corrosivity of natural waters to cast iron has led to the development of a simple corrosion rig for the direct measurement of corrosion ratesThe results obtained using this rig has suggested an aggressivity classification of waters by source type i.e. 

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. 

In order to evaluate the tests determinations were carried out on the steels that had been exposed to the atmosphere for 1,2, 3, 4 and 6-month periods. It was established that the initial open-circuit potential and the decrease in potential (more negative) with time varied with the nature of the steel and the time of exposure to the atmosphere, and the maximum negative potential was taken as a measure of corrosion resistance the more negative the... 

The most precise measurements of corrosion resistance require the use of specimens that can be weighed accurately after careful removal of corrosion products by the techniques described earlier. 

Practice for measurement of corrosion potentials of aluminium alloys Practice for conducting and evaluating galvanic corrosion tests in electrolytes Practice for liquid impingement erosion 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... 

Polarization resistance (Rp) techniques can be used to continuously monitor the instantaneous corrosion rate of a metal. Mansfeld provided a thorough review of the use of the polarization resistance technique for the measurement of corrosion currents. R is defined as ... 

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. 

Finally, in Chapter 5, the applied side of electrochemistry is served by Bech-Neilsen et al. in the review of recent techniques for automated measurement of corrosion. 

Similarly, cooling water that is fully softened tends to lack sufficient buffering capacity and creates the potential for significant corrosion within the cooling system, so care and attention to design detail are needed if future corrosion problems are to be avoided. Also, most chemical treatments incorporate polymers or phosphonates of some sort to provide a measure of corrosion inhibition. These products usually require the presence of at least 25 to 35 ppm of calcium hardness to provide satisfactory inhibition. 

Data Analysis. Data analysis is, of course, directly related to data acquisition. However, not all good data is or can be completely analyzed. For example, McIntyre (J5) has observed that "a broad base in chemical shift data has been slow in developing" for XPS data. Until such a data base existed, it was difficult for both expert and non-expert to interpret spectra from corrosion products, particularly on complex alloys. The Handbook of X-Ray Photoelectron Spectroscopy (27) and collections of Auger parameter data (32 ) are examples of data compilations very useful to a researcher trying to interpret measurements of corrosion products. 

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. 

Herb Uhlig was the advisor and intellectual father of many U.S. scientists who are at work in numerous companies in the United States. Among the best known of his students were Milton Stern, originator of the Stern-Geary method for the measurement of corrosion, and Winston Revie, who made the first Auger measurements on passive films. 

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) ... 

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. 

Annual Book ofASTM Standards, Standard Practice for Measurement of Corrosion Potentials of Aluminum Alloys, ASTM, Philadelphia, PA, 1994, G 69-81. 

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

Atmospheric Corrosion. The aluminum-based alloys in general are corrosion resistant to outdoor exposure with the exception of copper-bearing alloys. The Alclad alloys gave the best performance. The loss in tensile strength has been used as a measure of corrosivity and the loss of 1-2% of tensile strength over a period of 1 yr and in particular a loss of 17% was observed with 2017T alloy in 1 yr of outdoor exposure. 

Testing. The measurement of corrosion, wear, and corrosion-wear interactions as well as erosion-corrosion interactions is a multistep process. Each component of the interaction must be measured separately. The results may then be combined to identify the synergistic effects and create a complete picture of the damage process. Measurement of the interaction between corrosion and wear modes or damage is more difficult. The standard (ASTM, G119)4 applies to systems in liquid solutions or slurries and some aspects of it can be adapted to dry corrosion and wear interactions as well. (Tylczak and Adler)5... 

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