Galvanic corrosion tests in electrolytes

Guidance on conducting and evaluating galvanic corrosion tests in electrolytes is given in ASTM G71 1981 (R1986). 

ASTM G71-81, Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes, Philadelphia, PA, 1981. 

Unlike galvanic corrosion testing in electrolytes, galvanic corrosion testing in the atmosphere can be standardized because special circumstances exist that limit the number of test variables. 

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... 

Three factors that are different in atmospheric testing from most other galvanic corrosion testing are the presence of a limited amount of electrolyte, the need for a very long duration of exposure, and the importaince of specimen... 

Figure 4.17 illustrates the corrosion occurring on high-purity AZ31 and ZW3 in contact with steel bolts. Tested alone in sea-water, the corrosion rate of the former is much the lower. It is evident from the illustration, however, that the governing factor in galvanic corrosion is the type of electrolyte present rather than the composition of the alloy. 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

In several installations, Ti-Au thin fihn metaUization was patterned with an iodine etch. One installation had problems with corrosion of the metallization after patterning but the others did not have the same problem. Extensive tests showed that the corrosion was due to iodine in the fihn which, when combined with water, formed an electrolyte that allowed galvanic corrosion to occur. The main difference in the installations was the ventilation in the etching areas. Improved ventilation and the use of a Ti-Pd-Au metallization cured the problem. 

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