Passivation vs. Electropolishing

A material is considered passivated when it can demonstrate a high resistance to corrosion in an environment where you would expect corrosion to take place. There are multiple means to accomplish the passivation of stainless-steel products and components. Passivation can be achieved through chemical processes like nitric acid passivation and citric acid passivation, or it can also be achieved through electropolishing.

The chromium in stainless steel forms a thin, oxygen enriched, passive layer on the surface of the metal. This layer is what makes stainless steel resistant to corrosion. Manufacturing process such as machining, cutting, stamping, laser slag and welding will disrupt the passive layer and embed free iron and impurities into the surface layer of the metal. These impurities lead to corrosion if left untreated.

Depending on the needs of the client, NEE typically will recommend using electropolishing to achieve the passivation. Passivation through electropolishing provides greater corrosion resistance than nitric or citric acid passivation while also enhancing the parts’ micro finish and producing an aesthetically pleasing finish.

How Electropolishing Improves the Surface of Stainless Steel

Ra and RMS are two representations used to measure the roughness of the surface of metal. Ra is calculated by measuring the average of a measured microscopic peaks and valleys. RMS is calculated as the root mean square of a metal surface’s measured microscopic peaks and valleys. Each value uses the same individual height measurements of the surface’s peaks and valleys but uses the measurements in a different formula. Ra is the most commonly used measurement today. Electropolishing can help reduce a part’s Ra by 10% to 30% depending on the starting finish.

An Electropolishing Case Study

A part is considered passivated when it is resistant to corrosion in an environment where you would expect corrosion to take place. One of those places one would expect corrosion to take place, would be in areas around salt water where exposure to sulfides is magnified. The following is a case study of historic Battery Wharf in Boston, Massachusetts. A series of buildings, museums, and condos had over 900 Stainless Steel panels installed over balconies. The panels had been cut and fabricated without any passivation treatment prior to installation.

In a short amount of time the surrounding environment caused many of the panels to exhibit corrosion and rust. Test panels were removed and shipped to New England Electropolishing to be electropolished, reinstalled and monitored over time. Electropolishing proved to be effective at removing corrosion and protecting the panels against further issues after being reinstalled.