凍結融解腐食環境下にあるSUS304溶接材の孔食促進

概要

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Welded structures in coast regions in Hokkaido prefecture are exposed to the severe environment such as a freeze-thawing of a liquid included with a chlorine ion Cl- that is brought by a sea breeze and a snow melting agent during a winter season. On the other hand, it is well-known that Cl- solution produces a pitting corrosion in an austenitic stainless steel. However, the relationship between the pitting corrosion occurred in the welded austenitic stainless steel and the freeze-thawing environment have not been clarified yet. In this work, the influence of freeze-thawing environment on a pitting corrosion occurred in the welded austenitic stainless steel in Cl- solutions was investigated using SUS304 specimen with bead-on-plate welding. The specimen was fully immersed in FeCl3 solution. The freeze-thawing environment was recreated by a temperature cycling from 20°C to -20°C. As a result, large-scale pitting corrosions was induced mostly in the area of a high tensile residual stress and a sensitive metallographic structure near weld line under the freeze-thawing environment. It is confirmed that the accelerated pitting corrosion phenomenon was introduced by a concentration cell corrosion due to a local concentration of chlorine ion Cl- around weld line during freezing and thawing of the solution.