Why Stainless Steel Parts Need Passivation

Date:15-11-2016

Passivation remains a critical step in maximizing the essential corrosion resistance of stainless steel parts and components. It can make the difference between satisfactory performance and premature failure. Incorrectly performed passivation can actually induce corrosion.

Passivation is a post-fabrication method of maximizing the inherent corrosion resistance of the stainless alloy from which the workpiece was produced. It is not a scale removal treatment, nor is it like a coat of paint.

There is no universal agreement on the precise mechanics of how passivation works. But it is certain that a protective oxide film is present on the surface of passive stainless steel. This invisible film is considered to be extremely thin, less than 0.0000001 inch thick, which is about 1/100,000 the thickness of a human hair.

A clean, freshly machined, polished or pickled stainless steel part automatically acquires this oxide film from exposure to oxygen in the atmosphere. Under ideal conditions, this protective oxide film completely covers all surfaces of the part.

In actual practice, however, contaminants such as shop dirt or particles of iron from cutting tools may be transferred to the surface of the stainless steel parts during machining. If not removed, these foreign particles can reduce effectiveness of the original protective film.

During the machining process, a microscopic amount of free iron may be worn off the cutting tool and transferred to the surface of the stainless steel workpiece. Under certain conditions, a thin coating of rust may appear on the part. This is actually corrosion of the steel from the tool and not the parent metal. Sometimes the crevice at the embedded particle of steel from the cutting tool or its corrosion products may cause an attack of the part itself.

Similarly, small particles of iron-containing shop dirt may adhere to the part surface. Although the metal may appear shiny in the as-machined condition, the invisible particles of free iron can lead to rusting on the surface after exposure to air.

Exposed sulfides also can be a problem. They come from the addition of sulfur to stainless steels to improve machinability. Sulfides improve the alloy’s ability to form chips that break away cleanly from the cutting tool during the machining process. Unless the part is properly passivated, sulfides can act as initiation sites for corrosion on the surface of the fabricated product.

In both cases, passivation is needed to maximize the natural corrosion resistance of the stainless steel parts. It can remove surface contamination, such as particles of iron-containing shop dirt and iron particles from cutting tools that can form rust or act as initiation sites for corrosion. Passivation also can remove sulfides exposed on the surface of free-machining stainless alloys.