MISIF, Nickel Institute and International Molybdenum Association jointly organized a webinar titled “ Pickling, Passivation and Cleaning of Stainless Steels – What Specifiers Really Need to Know” on 6 October 2022.
The webinar session was conducted by Mr. Geir Moe, Technical Inquiry Co-Ordinator for the Nickel Institute. Mr. Geir has assisted end-users for 30 years in the selection, application and fabrication of stainless steels and nickel alloys for corrosive and high temperature environments. He has worked in stainless steel distribution, as senior metallurgist for Hatch Engineering. Geir is a Professional Engineer registered in Ontario, Canada and a member of AMPP since 1991.
Stainless steels obtain their corrosion resistance by the formation of a corrosion resistant passive layer. This layer is extremely durable in service and is self-healing if disturbed, but its corrosion resistance can be compromised by poor fabrication practices. The presentation identified those practices and how proper pickling, passivation and cleaning can ensure optimum corrosion resistance in completed fabrications. The differences between pickling, passivation and cleaning were explained during the webinar session.
Pickling – a chemical cleaning using acid solutions, such as nitric/hydrofluoric, which are corrosive to the stainless steel. Typically used to remove:
- Scale resulting from heat treatment at the steel mill,
- Deeply embedded iron particles in the surface from fabrication,
- Heat tint and underlying chromium depleted layer from the HAZ after welding.
Passivation – a chemical cleaning using solutions, such as nitric, citric, phosphoric, and hydrogen peroxide which are not corrosive to the stainless steel but remove surface iron contamination. In essence it is simply a surface cleaning process because it does not remove any stainless steel.
Cleaning – a chemical treatment, typically using solvents or caustic, to remove contaminants, such as grease, oils, ink, paint, and labels, that are resistant to removal by acids. Pre-cleaning prior to pickling or passivation is necessary to ensure none of the surface area is masked by these contaminants which would prevent effective pickling or passivation.
A total of 56 participants joined the webinar session from various steel related companies.