Posted on March 14, 2019
In factories and factories, stainless steel pipes and pipes are something we often see in removable insulated installation phones. In these industrial environments, stainless steel pipes are often exposed to extreme temperatures and conditions; therefore, insulating them is important to save energy. Perhaps just as important is the inspection of stainless steel pipes.
Corrosion Under Insulation (CUI) is an important reason why stainless steel pipes need to be inspected. Stainless steel contains more chromium than carbon steel, and chromium forms an invisible, passivating film of chromium oxide that provides corrosion resistance. Although stainless steel is less corrosive than regular carbon steel, it must be remembered that stainless steel is corrosion resistant, not corrosion resistant. A common store statement is "stainless steel is not stain resistant - it just stains less."
Austenitic stainless steels (AKA 300 series stainless steels) have received the most attention in terms of CUI - due to their popularity (about 70% of stainless steel produced is austenitic) and the CUI issues associated with austenitic stainless steel piping systems. While CUI generally causes uniform wall damage in carbon steel pipes, CUI is generally very localized in stainless steel pipes. Maintaining in-situ insulation already makes corrosion difficult to detect and often stimulates corrosion; hyperlocalized CUIs are difficult to detect and compromise structural integrity. CUI often manifests as pitting or stress corrosion cracking in stainless steel pipes.
pitting corrosion
Pitting corrosion occurs when corrosion attacks small holes or cavities in the metal. Alloys that are resistant to corrosion due to the passivation layer, such as stainless steel, are actually the most prone to pitting. If the passive film is damaged and then attacked by corrosion, the corrosion will not spread to the metal surface, but will penetrate inward. Pitting can lead to stress cracking, and - if pitting occurs as a critical point - it can cause massive damage.
stress corrosion cracking
Stress corrosion cracking is as the name suggests. Stress corrosion cracking is difficult to detect with the naked eye, and fine (sometimes microscopic) cracks can penetrate into the steel and can rapidly propagate or grow, potentially leading to catastrophic failure. Austenitic stainless steels are particularly susceptible to corrosion by chlorides, including chlorine, seawater (the salt is sodium chloride), and many cleaning solutions.
Stress cracking from chloride corrosion is more likely to occur at temperatures that promote evaporation (100 to 200°F) due to higher chloride deposits and moisture content. In addition, many paints and insulating coatings contain chlorides, so be sure to choose insulation for stainless steel pipes carefully! When susceptible alloys such as 300 series stainless steel are exposed to corrosive chemicals such as chlorides, cracking is much more likely if the pipe is subjected to significant stress.
