Question: This question relates to stress corrosion cracking and pitting in ISO tanks. “Please see attached pictures of the inside of my tanks. What is causing the pitting in this area?”
Tank Guru: I suspect that the reason for this damage may be chloride stress corrosion. Chlorides are a problem for 300 series grades of stainless steel, such as 304 or 316 (most common in tank containers), and stress corrosion cracking can occur when you have the simultaneous presence of tensile stresses along with specific environmental factors such as having a steam-heated/insulated tank. Most insulation contains chlorides, especially polyurethane, and so it is usually advisable to not have this type of insulation in direct contact with the barrel when the tanks are going to be steam-heated. Chloride stress corrosion is an increased risk with temperatures above 125° F. The tensile stresses could be the result of certain operations to the tank, or due to residual stress from the fabrication process such as bending and/or welding. I think this is why you are seeing the corrosion problem between the head seam and knuckle radius.