Effect of simulated outage conditions in acid environment containing copper on corrosion of steam generator tubes

In some cases, the stress corrosion cracking (SCC) of steam generator (SG) tubes seems to depend on the number! of cycles and shutdowns, more than on time of exposure itself.(1) On the other hand, corrosion tests performed at Laborelec have shown the deleterious influence of copper oxide in acid environment, especially for alloys 600 and 690 with iron and sodium sulfate solutions. (2) If copper is present in the SG, it may become oxidized during the shutdown in presence of air and stagnant water :in particular at-the top of tubesheet. Therefore, Laborelec completed a series of capsule tests to verify the influence of-cold shutdowns on corrosion in the case of acid sulfate environment containing metallic copper: the tests were interrupted every 24 to 120 h and the capsules were aerated either in "dry" or in "wet" conditions. To check the influence of mere temperature variation at shutdown, without possibility of copper oxidation, tests were performed without aerating the capsules during the stoppages. Reference tests were also performed without interruption, with metallic copper or with copper oxide, to evaluate the influence of the stoppages. The overall effect of outages was difficult to evaluate because the test periods used in this program (between two outages) appeared too short to allow initiation of cracks, at least for outage simulations in reducing conditions. On the other hand, significant results were obtained in oxidizing conditions. The main result-from this test program is that the "natural" oxidation of metallic copper during outage at roots temperature (particularly in dry conditions); increases the risk of SCC for alloys 600 and 690 when subsequently exposed to acid sulfate environments. For alloy 690, the presence of metallic copper in reduding conditions can also lead to SCC in acid sulfate environments whereas this alloy::is totally immune in absence of copper.