Improving Corrosion Resistance for Metal Alloys
Electropolishing is widely specified to enhance corrosion resistance on a wide variety of metal alloys. Although most commonly used on stainless steels, electropolishing offers corrosion resistance on other alloys as well. Due to the versatility and superior effectiveness, electropolishing is fast becoming a replacement process for passivation.
Passivation is a chemical process that has been used for years to help restore contaminated stainless steel to original corrosion specifications. Passivation is routinely specified by engineers because of the many sources of contamination. The intent of passivation is to remove free iron or other foreign matter from the surface of the metal and to create a chromium rich surface that is resistant to corrosion. However, passivation is generally not effective in removing imbedded free iron and contaminants and will not remove heat tint or oxide scale on stainless steel.
Engineers looking to pass stringent salt spray and humidity tests have turned to electropolishing for a more aggressive replacement for passivation. The electropolishing process dissolves the outer skin of metal, removing deeply imbedded contamination. Heavily contaminated surfaces such as machined parts, welded or brazed assemblies or other components that typically respond poorly to passivation alone, are good candidates for electropolishing. Unlike passivation, all stainless alloys including the 400 series and precipitating grades can be processed without distortion, flash attack or hydrogen embrittlement.
Just as electropolishing is used to enhance corrosion resistance on stainless steel, it offers corrosion resistance on other alloys as well. Many companies use the process to delay or retard the corrosion properties of copper, brass, aluminum and carbon steels. On these and other alloys, the removal of surface skin and impurities enhances the corrosion resistant properties of the component.
The parts pictured on the right are made from type 303 stainless steel. After a series of machining operations, the parts needed to be decontaminated to remove imbedded steel and other impurities.
Under 40X magnification using the scanning electron microscope, you can see the passivated part is actually rougher, due to the chemical attack of the 303 stainless steel by normal passivation. In contrast, the electropolished part is smooth and clean. By carefully monitoring the amount and rate of metal removal, electropolishing dissolves the surface skin and its impurities, including impinged steel flecks and other contaminants.