Stainless steel passivation is the result of a process that helps to protect the metals surfaces from corrosion. Stainless steel is considered “passivated” when it demonstrates a high resistance to corrosion. Electropolishing is an electro-chemical process that removes the outer layer on the surface of a part and is a process that results in the passivation of stainless steel. However, there are other chemical methods that can also be used to passivate stainless steel. This article will explore the difference between electropolishing and passivation as well as the different methods used to passivate stainless steel and the benefits of each.
What Is Passivation and How Does it Work?
Stainless steel is passive by nature, but after the manufacturing process, is often left with embedded free iron on the surface. This can result in a corrosive reaction between the two metals, which is why these free irons on the surface of the stainless steel need to be removed. Once the free irons have been removed from the surface of the metal and the metal can demonstrate a high tolerance to corrosive elements found in the environment, it can then be considered “passivated“. While passivation is one of the benefits of electropolishing, passivation can be accomplished without electropolishing through other chemical means.
Passivating Stainless Steel Through Electropolishing
Scientific studies have shown that electropolishing is the most effective process for achieving passivation in stainless steel alloys. Electropolishing is an electrochemical process that performs a surface treatment, reducing surface roughness as it dissolves material from the surface of a metal part. Electropolishing smooths the surface of metal by lifting high points, removing micro-burrs, and surface impurities. The electropolishing process can improve surface roughness or Ra up to 30-40%. Electropolished surfaces possess increased corrosion resistance, and as a result, are more resistant to corrosion in harsh environments. This finishing process is applied on a variety of metals, with the most common being 304 stainless steel, 316 and 316 L stainless steel, and 400 series stainless steel.
Prior to the electropolishing process, parts are subjected to an extensive pre-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 the removal of the organic and metallic residues, the parts are placed into one of NEE’s passivation baths, specifically calibrated to meet ASTM finishing standards.
The metal to be electropolished is immersed in a temperature-controlled electrolyte and is subjected to direct electrical current. Anodic dissolution is what removes the surface material while in the electropolishing “bath.” This electropolishing bath is a blend of phosphoric and sulfuric acids and is often agitated with air. Temperature is controlled and specific gravity typically ranges from 1.70-1.75.
This combination of chemistry and current creates a film over the metal. The electrical resistance is most effective where the film is the thinnest, which is over any projections on the surface of the metal. This results in the greatest rate of material removal. This is a highly controlled, precise, and repeatable process. Often, during a standard run, total material removal is limited between 0.0002” to 0.0003” per surface. The removal rate can be adjusted depending on a part’s tolerances.
Chemical Passivation Methods
Chemical passivation doesn’t alter the visual appearance or surface finish (Ra, RMS) of stainless steel. Instead, the passivation process removes surface contaminants from the base metal and facilitates the formation of a very thin, transparent oxide film, which protects the stainless steel from corrosion.
DIFFERENCES AND BENEFITS OF BOTH
Advantages and uses of electropolished metals include:
- Polished, lustrous finish
- 8X more effective than passivation for enhancing corrosion resistance
- Non-abrasive/non-mechanical finishing process; smooths out threads of materials to prevent galling and seizing
- Ease of cleaning
- Removal of heat tint or oxide scale due to heating or welding
- Complex shapes can be processed with speed and cost efficiency
- Economic method to deburr large volume of metal parts
- Used for applications in food, pharmaceutical, semi-conductor, architectural markets
Advantages and uses of passivated metals include:
- Removal of contaminates on surface
- Does not change appearance of material
- Improved corrosion resistance; can positively alter the composition of stainless steel’s protective oxide layer
- Used for medical device, aerospace, and maritime industry applications—may include combination of both processes