Stainless steel’s corrosion resistance is due to its protective chromium oxide layer, which forms when the chromium in the alloy reacts with oxygen in its surrounding environment. The metal’s surface can be affected by dirt, free iron, and other residues, impairing formation of this protective layer and increasing the vulnerability to corrosion. Treatments such as passivation or electropolishing are applied to stainless steel to improve the surface of the part. As a result the chromium oxide coating can form more comprehensively and provide enhanced corrosion resistance.
What is Passivation?
Passivation is a process to make a metal’s surface “passive” or less chemically reactive. Stainless steel naturally passivates, but the passivation process on stainless steel improves and purifies the steel’s surface to strengthen this protective barrier. A pre-cleaning step is required to make the passivation treatment effective, removing scale, debris and oils from the surface of the part.
Passivation begins when the part is submerged into nitric acid or citric acid. The acid reacts with iron and other contaminants on the surface of the part, dissolving away these agents that can form initiation sites for corrosion. The result is a uniform, chromium-enriched surface, able to form a reinforced chromium oxide protective coating. Passivation is not recommended for certain stainless steel grades lacking sufficient levels of chromium and nickel, and for parts that have been welded or brazed.
What is Electropolishing?
Electropolishing occurs when an electric current is used to remove a small layer of metal (20‑40 μm) from the surface of the steel. The part is immersed in a solution able to carry an electric current. The circuit is constructed so the part is the anode, being electropolished as the metal is released ion by ion from the surface as electricity moves through the circuit. The cathode is another metal conductor immersed in the same solution to complete the circuit.
The part is simultaneously cleaned and passivated as the electrochemical process removes contamination and debris, as well as the outer metal layer from the surface. In addition, the preferential action on the metal peaks of the impeller’s surface results in a net smoothing, eliminating burrs and micro-crevices and enhancing corrosion resistance 30 times more than passivation alone. Electropolishing is also effective for all grades of stainless steel, and for welded and brazed parts in contrast to passivation. The microscopically smooth, uniform finish resists product and bacterial adhesion to the stainless steel surface. Benefits include reduced cleaning time and lower bacterial growth on the impeller, especially desirable in food, beverage and pharmaceutical applications.
Both passivated and electropolished stainless steels deliver long serviceable life for an impeller. The protective oxide layer that results from these processes is integrated into the metal, so unlike paint, plating, or coatings it cannot chip or peel. All Caframo shafts are passivated stainless steel to offer enhanced corrosion resistance, and we have a comprehensive selection of impellers and blades made of electropolished stainless steel. Contact us today to find the impeller that serves your needs.
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