Intergranular attack electrochemical tests

Attempts to assess sensitization in Type 304 alloy by means of 5-15 min potentiostatic electrochemical tests in solutions that simulated exactly the plant conditions have been shown to be invalid [42]. This testing period is much too short for testing in simulated service conditions, i.e., the electrode potential applied to the specimen in the laboratory test, the solution compositions, and temperature were the same as those to be used in plant service. Thus, there was no accelerating factor in the laboratory test and it would have had to be just as long as a plant test, perhaps 3-12 months, to determine whether or not the material would be subject to intergranular attack in plant service. 

There are a number of tests available for the evaluation of the sensitivity of a given stainless steel to intergranular corrosion (Table 7.41). They differ in severity of corrosion conditions and therefore do not reveal the same phenomena. The oxalic acid test consists of an electrochemical attack at constant current. The morphology of the corroded surface is then compared with reference samples. The main advantage of this technique is its rapidity (the attack takes about one minute). The applied potential lies in the transpassive region, where the chromium easily dissolves. The oxalic acid test therefore does not reveal the zones depleted in chromium, but rather exposes the presence of carbides and intermetallic phases. 

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