Passivation Effect Utilizing Electropolishing Methods
Passivation refers to the chemical neutralization of the surface - increasing corrosion resistance.
Almost any stainless steel that you can buy has been rolled, machined and/or manipulated with carbon or tempered steel implements. This means that, in general, all of the stainless steel that you will ever buy off the shelf or re-manufacture will have an appreciable amount of steel worked into its surface. Without Passivation iron or iron oxide exists on the surface of stainless steel. When any machining or fabrication operations are performed, the condition is worsened. Aside from iron being deposited by tooling, it is also being entrapped underneath the “skin” created by the mechanical operation. In the metal industry this surface impregnated steel is referred to as "free iron".
This free iron corrodes easily. The corrosion process of iron is a very aggressive reaction. This reaction will in almost all cases start corrosion in the stainless steel. Once started, the corrosion of the stainless will continue to take place without the presence of free iron.
Electropolishing of stainless steel has two significant benefits besides the leveling of the work piece surface. First, the Electropolishing process will remove all free iron from the surface of the work piece. This has the obvious effect of eliminating the free iron corrosion up front. Secondly, Electropolishing removes material from the surface of the metal selectively. For example, Electropolishing does not readily remove the carbon from the metal because carbon is very electrochemically neutral. Further, the process does not readily remove chromium or nickel. The chromium, nickel and carbon, for all practical purposes, becomes uncovered and remains sitting on the surface of the metal as the Electropolishing process takes place.
As you Electropolish a part you start enriching the surface with chromium and nickel. At some point, a chemical reaction takes place during processing of the part. The chromium reacts and forms chromium oxide. Further, if the surface is very rich in chromium, the chromium oxide will form what you can think of as a layer over the metal surface. This is referred to as a chromium enriched surface oxidizing to form a chromium oxide Passivation layer. This mechanism is referred to in the industry as "Passivation."
The term "Passivation" is used widely in the stainless steel processing industry. Many people refer to this oxide surface as a chrome-nickel oxide.
As Electropolishing solution removes the iron ions from the surface of the part, it leaves much of the chromium, nickel and carbon behind on the surface of the metal.
In 1962, General Dynamics/Astronautics Division in San Diego recommended the use of Electropolishing to increase the corrosion resistance of stainless steel in propellant systems to NASA.Their conclusions were based on actual accelerated seacoast exposure tests.
Tests conducted to determine the Passivation effects of the Electropolishing solution on stainless steel showed that this treatment greatly increased the passivity over that of conventional methods of Passivation. Specimens of 300 series stainless steel, contaminated and then treated by various methods, were exposed for approximately 560 hours to a salt spray environment. All of the specimens passivated conventionally (nitric acid at varying temperatures and concentration) showed a considerable amount of rust after 8 hours and were removed from the test while the Electropolished specimens did not show any change until after 40 hours. At this point light stain was only observed in the center of specimens. After 146 hours, light rust, lighter than that formed on conventionally passivated samples, was observed on the Electropolished parts.