An Example of Blistering Due to Hydrogen Occlusion

The interior of a cylindrical tank used for the road transport of concentrated sulfuric acid revealed severe blistering of the plates, mainly over the crown and more particularly on the first ring. The tank, made in 1958, was of welded construction, the material being mild steel plate. Some of the blisters were pierced by drilling a hole in the center and at the same time applying a small flame. In several cases combustion of the escaping gas caused minor explosions, a result characteristic of hydrogen. Etching showed the material to be a low-carbon steel in the partly spheroidized condition. There was no evidence of cracking of the material in the region of the blisters and bend tests demonstrated it possessed satisfactory ductility. The primary cause of the blistering was ascribed to the presence of discontinuities within the plate. This provided cavities in which the hydrogen was able to accumulate and build up pressure. Had the material been free from discontinuities of appreciable size, the blistering would not have occurred.