Passivation vs. Electropolishing
Whats the difference between electropolishing vs. passivation? Are both necessary for processing a part? Which one should be done first?
These common questions are often mistaken in industry because of specification callouts vs. true understanding of each process.
Passivation Definition is…
a non-electrolytic process typically using nitric or citric acid which removes free iron from the surface and forms an inert, protective oxide layer that in turn renders the stainless steel more rust-resistance due to lack of iron to react with atmosphere. During machining manufacturing proceses, the stainless steel parts may have imperfections from iron being embedded or smeared onto the surface from the machining tool steels. These free irons on the surface of the stainless steel need to be removed to prevent a corrosive reaction can occur between the two different metals. The passivation of stainless steel chemically removes these free irons and forms a passive oxide “film” layer which further improves corrosion resistance. When exposed to air, the stainless steel undergoing passivation will form a chemically inactive or inert surface. This is one advantage of using citric acid vs. nitric acid for passivation.
Electropolishing Definition is…
an electrolytic/ electrochemical process which dissolves the electrically active asperities to leave an ultra smooth surface for such reasons as cleanliness, deburring, polishing and to prevent materials from sticking. Through the electropolishing process, surface material is removed by anodic dissolution using electrical current passing through electrolytic solution.
So Whats the Difference between Electropolishing and Passivation?
As a non-electrolytic process, passivation uses solutions like citric and nitric acids instead of an electrical current used in electropolishing to create a inert oxide layer / film and remove free iron and foreign matter from metal surfaces.
Electropolishing, a non-mechanical interactive process, can be used on objects with complex geometries. The electropolishing process uses a combination of electrolytic chemicals and an electrical current to carefully eliminate imperfections and contaminants of metal part surfaces. This particular process is an alternative to abrasive fine polishing. Electropolishing can be used to polish, deburr, and smooth metal components whereby doing so also causes material removal to occur including the free irons on the surface of the part.
Electropolishing would be the more appropriate approach when aesthetics are necessary. A smoother, polished surface can result from electropolishing. Passivation, on the other hand, does not change or brighten the surface appearance and is not an effective method for removing surfaces that has been welded with oxide heat effect scale or heat treated.
Why do some specifications require electropolishing AND passivation?
There is some debate that electropolishing since it uniformly removes ALL material from the part surface could remove enough chromium from the part surface and expose iron that the corrosion resistance properties of stainless steel could be compromised. The other side of the debate says that electropolishing forms a deeper, thicker oxide layer than any type of passivation because electropolishing actually effects more than the part surface whereas passivation only reacts with surface level elements.
Most common in the medical device and aerospace industries, both electropolishing and passivation are called out together on specifications. The proper process then is to electropolish to utilize the surface removal, polishing advantages of electropolishing and then follow it up with passivation to remove possibly any free irons on the surface which were exposed during electropolishing. Share your knowledge about electropolishing vs. passivation of stainless steel with your coworkers or employees!