Why did my stainless steel leak?
What you should know to avoid failures.
18 April 2017
Banner image description: A wine tank perforated in 14 days after being filled with bore water to stabilise it against strong wind conditions when delivered to a winery. The tanks had poor weld fabrication and when filled with high salinity water containing bacteria, the 316 grade of stainless steel did not stand a chance.
Stainless steel is an excellent material, but it requires a great deal of attention, something that is often missing when engineers simply specify stainless steel for corrosion resistance, thinking problem solved.
Stainless steels are just that, they tend to corrode less than most steels but liquid containing equipment can perforate very quickly. Only one attack site is needed to cause a failure. These groups of materials rely on a complex passive layer that requires a suitable oxidising environment; too little and the protective passive layer does not adequately form or reform, too much and pitting will accelerate as the passivity is locally destroyed.
What can cause these issues?
- Aggressive environments, and in near neutral pH conditions it is often the salt content, most notably chloride concentrations. There is a limit to chlorides and dissolved chlorine that the various grades of stainless steel can tolerate.
- Grades of stainless steel with increasing chromium, molybdenum and nitrogen provide increased resistance albeit at a cost penalty. This can be quite significant, but so can repairs and complete replacement plus lost production.
- Design detail to avoid crevices, dead spots with poor or no fluid flow and the collection of debris.
- Fabrication of components entails many processes, some dictated in part by design detail. Welds are commonly used in fabrication and they can be a concern for several reasons. Their contours are excellent points for collection of debris and they can be associated with weld imperfections such as lack of full weld penetration that act as crevices.
*Image at right: Image of a radiograph showing pits, associated with a lack of full weld penetration, which act as crevices.
- The presence of weld scale and heat tinting has a detrimental effect on the ability of the stainless steel to form and maintain the protective passive layer.
*Image at right: Pitting associated with scale on a 304 stainless steel weld.
- Contamination of stainless steel surface by foreign material such as carbon steel working debris from nearby structures or from the use of contaminated tooling during dressing of welds.
- Inadequate maintenance to remove build-up of deposits. Stainless steels require an oxidising environment at the steel surface to maintain the protective layer and this generally requires a clean surface.
They are based at Intertek-AIM, Adelaide, South Australia.
Dr. Peter Kentish is a Metallurgist with a PhD on the subject of stress corrosion cracking in high pressure gas pipelines. He has extensive experience in failure analyses, corrosion investigations and materials testing for mining, manufacturing, construction, fabrication, energy generation, insurance loss adjusting and as an expert witness in litigation.