What is Passivation?
For all forms of stainless steel, corrosion begins on or near the surface. Unfortunately, all fabricating and handling practices invariably degrade surface conditions. Surface contaminants, including grease, dirt, iron, and other metallic particles are inherent to the metal fabrication process. Additional mechanical cutting, handling and polishing will leave iron particles and abrasives embedded in the surface. The presence of surface contaminates disrupt the formation of stainless steel's naturally corrosion resistant oxide layer and are the origin of future corrosion.
Traditional Nitric Acid Passivation
The traditional Nitric Acid passivation process removes surface contaminants and facilitates the formation of a very thin, transparent oxide film, which protects the stainless steel from oxidation.
Prior to a traditional passivation process at NEE, parts are subjected to an extensive cleaning cycle to remove oils, grease, and metallic residue left behind during forming. General degreasing and cleaning can be accomplished by a variety of commonly accepted methods, including Acid Pickle and an Alkaline Soak. After removal of the organic and metallic residues, the parts are placed into one of NEE’s Nitric Acid baths, specifically calibrated for traditional passivation
There are a number of acceptable parameters and ASTM Specifications for Chemical Passivation. The primary three variables of Nitric passivation are (1) time, (2) temperature and (3) concentration of Nitric Acid. Typical immersion times are between 20 minutes and 2 hours. Typical bath temperatures range between room temperature and 160°F. Nitric acid concentrations in the 20% to 50% by volume range are generally specified.
Unlike the electropolishing process, traditional Nitric Acid passivation does not apply electrical current or require specialized racking. It will not alter the visual appearance or surface finish (RA, RMS) of stainless steel.
Passivation through Electropolishing
The team at NEE believes that a mild electropolishing cycle is the most effective means of passivation. Electropolishing will meet/exceed all traditional passivation requirements for corrosion resistance. The electropolishing process removes all free iron from the surface of the work piece, which has the obvious effect of eliminating free iron corrosion up front. Unlike Nitric passivation, a mild electropolishing cycle removes a small amount of surface material including additional free iron beneath the surface of a part. Electropolishing increases the chromium oxide layer to a greater degree than traditional passivation methods, thus enhancing the part’s corrosion resistance.
Due to its increased corrosion resistance benefits, NEE generally recommends passivation through mild electropolishing. However there are select situations that don’t lend themselves to this surface treatment method. Economic considerations, extremely tight tolerances, and engineering guidelines will prompt customers to seek a traditional Nitric Acid passivation over mild electropolishing.