Steel-toe boot heat treating tank stress corrosion cracking / hydrogen embrittlement

Heat-treating shop near Boston airport built a large welded steel tank for sodium nitrate quench bath of steel-toe boot toes. Tank cracked at the welds. Literature said sodium nitrate causes stress corrosion cracking; Tom's U-bend test with anodic polarization wouldn't crack the specimen, but reversing the polarity (cathodic) cracked it within 24 hours. The real mechanism was hydrogen embrittlement, not SCC.

There's a little heat-treating shop over here near the airport. A number of years ago, they put in a big steel tank where they were going to heat-treat steel toes — I'm surprised they're doing this in the United States, but the steel toes for steel-toe boots. Very high-strength steel, very ductile steel, because you want lightweight steel toes, and you don't want it to crack if you drop a brick on them. They were putting in a big heat-treating tank, all welded construction, and they were filling it up like a swimming pool — about 50 yards long, 5 yards wide, 3 yards deep — and all of a sudden it started leaking. There were cracks, and I looked at the cracks around the welds. Weld residual stresses, clearly looked like stress corrosion cracking. I go to look in the literature, and they were using sodium nitrate salts for the heat-treating bath, and I found, oh yeah, this is sodium steel and sodium nitrate is susceptible to stress corrosion cracking.