Intergranular Attack of Alloy 600: High-Temperature Electrochemical Tests. Final Report

摘要

In some steam generators, intergranular attack (IGA) has occurred on Alloy 600 tubes in the tube-tube sheet crevice region. In order to evaluate the physico-chemical parameters which influence this type of corrosion, potentiostatic tests were implemented on Alloy 600 C-rings at 320 exp 0 C in a 10% caustic media. From metallographic examinations performed after 120 hour duration tests it was observed that IGA is a stress assisted phenomenon and occurs on mill-annealed Alloy 600 near the corrosion potential or in the cathodic range. At higher applied potentials only stress-corrosion cracking was observed in the most-highly-stressed regions. Both carbonate and sulfate have a pronounced deleterious effect on IGA. Silicate has an inhibiting effect while phosphate appears to be moderately deleterious. Thermal treatments at 700 exp 0 C, with or without chromium depletion, strongly improve the resistance to IGA. (ERA citation 08:044137)