Stainless steel is famous for its superior resistance to corrosion. It gets this resistance from a chromium oxide layer, forming when chromium present in the alloy reacts with oxygen in the environment.
In order to be effective at protecting a metal part from corrosion, this protective layer needs to have formed properly. The presence of free iron, dirt or other contaminants on the surface can impair the formation of the chromium oxide layer and thus reduce corrosion resistance.
This can be avoided by applying treatments like passivation or electropolishing to the alloy, thus improving the part’s surface and ensuring that the protective chromium oxide layer is formed correctly.
Although passivation and electropolishing share many similarities, there are also some important differences between the two processes that should be considered.
The Passivation Process
When passivation is used, the part’s surface becomes less chemically reactive and is thus called “passive.” Even though stainless steel will passivate on its own to some degree, using the process will help get rid of the vast majority of impurities present on the surface, which enhances its ability to form a protective barrier.
Pre-cleaning is done before passivation, removing oils, scale and debris to increase the effectiveness of the treatment.
Passivation is done by submerging the stainless steel part in either nitric acid or citric acid. These acids are used due to some of their unique properties, most notably the way they react with surface contaminants like iron.
Contaminants that can reduce corrosion resistance are dissolved during the process. The end result is a part that has a clean and uniform surface, rich in chromium and possessing the ability to naturally form a stronger chromium oxide protective layer. While passivation is very beneficial, it isn’t an optimal solution in some cases.
The process shouldn’t be used on brazed or welded parts, as well as parts made from stainless steel grades which lack enough nickel and chromium.
The Electropolishing Process
While electropolishing is a type of passivation process, it is done in a different manner and is able to produce superior results, leading to increased corrosion resistance. The part is submerged in a solution of electrolytes, usually a mixture of sulfuric acid and phosphoric acid.
An electric circuit is constructed in a way that uses the part as the anode, with another metal conductor submerged in the solution serving as the cathode. Electricity moving through the circuit electropolishes the part by releasing metal ion by ion.
The process results in a very small layer of metal, ranging from 20 to 40 μm in thickness being removed from the part’s surface.
This electrochemical process passivates the part and cleans it, getting rid of debris and contaminants found on its surface. The part will end up having a much cleaner and smoother surface, with its corrosion resistance enhanced around 30 times more than with passivation alone.
For this reason, electropolishing is commonly used on parts that require the highest possible resistance to corrosion, such as items that will be constantly exposed to the elements.
The smoother surface reduces bacterial adhesion and makes the part easier to clean, which are highly desirable properties in medical and food-related applications. The electropolishing process can be effectively used on welded or brazed parts and all grades of stainless steel.