Cleaning Q&A: Necessity for Passivation
What is the importance of passivation?
Q. I work for an organization that makes equipment for the beer, wine and liquor industries. All equipment is made from stainless steel. We passivate all equipment with citric acid. What is the importance of this? Is it even necessary?
A. The passivation of stainless steel is a common industrial practice intended to mitigate the effects of processing and restore the stainless steel’s corrosion resistant properties to optimal condition. All stainless steels have one element in common: chromium, which is responsible for corrosion resistance associated with stainless steels and works by forming a very thin chromium oxide layer on the surface. Since this corrosion resistant layer is very thin, it can be subject to disruption and degradation by many industrial methods such as welding, grinding and other handling and fabrication processes.
Generally, when a stainless steel assembly is completed (after fabricating, welding, grinding and so on), the entire assembly is citric acid passivated, particularly the parts expected to need and retain the most corrosion resistance. The two typical methods of doing this are with nitric or citric acid. Since both are oxidizing acids, they restore the chromium oxide layer by accelerating oxide growth in the depleted regions. Nitric acid has been the historical choice for passivating stainless steel, but citric acid has seen significant growth due to similar or better effectiveness, less hazard to workers and fewer disposal issues when spent.
It is very common to passivate stainless equipment used in any of the food and beverage industries since the need for cleanliness and product safety for consumption is extremely high. Without it, it is likely the heat affected zone of welds, weld seams and ground areas would start to rust and contaminate the end product. Eventually it would lead to localized corrosion that could perforate the assembly; however, in the case of the food and beverage industry, it would never last long. Such equipment would need to be taken out of service prior to further use.
Originally published in the November 2015 issue.
A more realistic way to perform salt spray tests.
Specific questions about zinc phosphate and pretreatment are answered in one article...
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.