Galvanic corrosion electrochemical tests

Other questions can be answered by electrochemical testing. By measuring the behavior of the different materials in a complex structure, it is possible to determine if the corrosion rate of certain components of a structure will be enhanced by galvanic interactions. The basis for making this assessment was given in Sect. 1.3 of this volume. 

In order to be able to properly design and interpret the results from galvanic corrosion tests, it is necessary to have some appreciation of the electrochemical theory behind galvanic corrosion. Metal corrosion consists of at least two reactions. The first is the metal going into solution in the electrolyte... 

Accelerated testing to get a result in a shorter time period than would be possible naturally should be avoided whenever possible, because the mechanism of g2dvtinic corrosion ctin change if the rate is altered significMtly. For this reason, electrochemical tests are frequently chosen to evaluate galvanic corrosion performance. The sensitivity of these tests allows for obtaining results fairly quickly without accelerating the corrosion process. 

Using electrochemical tests to predict galvanic corrosion performance has the same caveats as in any other type of predictive testing. Conditions must be as close as possible to those being modeled. This includes not only solution chemistry and flow, but also the surface condition of the samples. Polished samples can behave quite differently from samples with more representative surface conditions emd corrosion products developed by pre-exposure. 

More detail on using electrochemical tests for galvanic corrosion can be found in Refs 6 and 7. 

Ishikawa, Y., Hosaka, N., and Hioki, S., Galvanic Corrosion of Copper Alloys, Electrochemical Corrosion Testing, ASTM STP 727,1981, pp. 327-338. 

Tran et al. studied the kinetics of carbonation of molten alkali hydroxides [228], the air flow pattern [229], and the corrosion of Type 304L stainless steel by molten hydroxides at air port openings [230], The presence of oxygen was found to make the corrosion inevitable in the presence of molten hydroxides. Electrochemical testing showed that a galvanic cell is set up between carbon steel and stainless steel with the latter protecting the former. 

Oldfield JW. Electrochemical theory of galvanic corrosion. In Hack HP, ed. Galvanic Corrosion. Philadelphia, Pa. American Society for Testing of Materials, 1988 5-22. 

Electrochemical techniques measure electrochemical potentials and currents that are fundamentally related to the thermodynamics and kinetics of corrosion reactions. They are often used to measure the rates of uniform corrosion, to determine the tendency of localized corrosion, to study a wide range of corrosion-related phenomena such as passivation, galvanic corrosion and sensitization effects. Electrochemical corrosion testing and monitoring can be performed in a diverse range of environments in the laboratory or in the field, in a pipeline or in an autoclave. For instance, they have been successfully employed to monitor corrosion in multiphase oil/water conditions with as little as 1-2% water " ... 

Most laboratory tests, including electrochemical tests, say nothing about the intensity of possible galvanic corrosion of aluminium in contact with other metals. 

Laboratory tests for evaluating/predicting galvanic corrosion f l into two categories electrochemical tests, in which the data are analyzed and reported in a way that assists galvanic corrosion predictions (e.g., potential measurements and polarization measurements) and specimen exposures, which can or cannot be electrochemically monitored (e.g., immersion tests... 

Figure 16.4 Electrochemical corrosion cell used for testing the ennobling effect of a conducting polymer. The mild steel sample (1) is coated by a thin epoxy layer and is in contact with a 0.1 M UCIO4 solution (aerated or deaerated) through a small hole. Polymethylthiophene (PMT) is deposited on a Pt electrode (2) a saturated calomel electrode (3) and a Pt counter-electrode (4) are used with either (1) or (2) as the working electrode. Electrodes (1) and (2) are galvanically connected and can be disconnected from the electrical circuit for specific measurements. (Reprinted with permission from Electrochimica Acta, Corrosion protection by ultrathin films of conducting polymers by U. Rammelt, P.T. Nguyen and W. Plieth, 48, 9, 1257-1262. Copyright (2003) Elsevier Ltd)...

---