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Cleaning Stainless Steel

Jun 27th 2013

What is Passivation? What is Electropolishing?

Cleanliness and stainless steel go hand-in-hand in a cleanroom environment and sterile lab areas. In the handling of food, chemicals, and pharmaceuticals, stainless steel provides corrosion resistance that is necessary to prevent product contamination or surface rusting. Stainless steel performs best when clean―cleanliness is essential for maximum resistance to corrosion.

There are various methods for cleaning stainless steel during manufacturing and when being used in the cleanroom. This includes removing free-iron contamination on stainless steel surfaces that may have been picked up from metal working tools, and for removing general accumulation of dirt, grime and surface stains that occur during normal handling.

Cleaning of Stainless Steel

Stainless steel is protected from corrosion by a thin layer of chromium oxide. A clean, freshly made piece of stainless steel equipment automatically acquires the oxide film from exposure to oxygen in the atmosphere. Oxygen from the atmosphere combines with the chromium in the stainless steel to form this passive chromium oxide film that protects from further corrosion. Any contamination of the surface by dirt, or other materials, hinders this passivation process and traps corrosive agents, reducing corrosion protection. Some form of routine cleaning is necessary to preserve the appearance and integrity of the surface. Stainless steels are easily cleaned by many different methods. They actually thrive with frequent cleaning, and, unlike some other materials, it is impossible to “wear out” stainless steel by excessive cleaning.

Clean Water and Wipe ― The simplest, safest, and least-costly method that will adequately do the job is always the best method. Stainless steel surfaces thrive with frequent cleaning because there is no surface coating to wear off. A soft cloth and clean warm water should always be the first choice for mild stains, and loose dirt and soil. A final rinse with clean water and a dry wipe will complete the process and eliminate the possibility of water stains.

Solvent Cleaning ― Organic solvents can be used to remove fresh fingerprints, oils and greases that have not had time to oxidize or decompose. The preferred solvent is one that does not contain chlorine, such as acetone, isopropyl alcohol and mineral spirits. Cleaning can be accomplished by immersing smaller articles directly into the solvent, wiping with solvent-impregnated cloths, or by sophisticated vapor or spray methods. The wiping technique sometimes leaves a streaked surface.

How does stainless steel rust? How to fix it?

Stainless steel will rust if the chromium oxide layer on the surface is removed from a scratch or chlorine solution. When the oxide layer is removed it exposes the iron and causes corrosion. Stainless steel can also be contaminated by carbon steel if scratched with tools or equipment made from carbon steel. The effect is iron-to-iron affinity and readily rusts. Fixing the stainless steel would need to be done with passivation.

What is passivation?

According to ASTM A380, passivation is “the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself.” In addition, it also describes passivation as “the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.”

Stainless steel passivation is a preventative measure. The passivation process of stainless steel is a chemical treatment of the surface with a mild nitric or citric acid solution. The process removes iron compound contamination left behind on the surface of the stainless steel from machining and fabricating. Passivation is done to make the stainless steel more passive and corrosion resistant. This facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation (corrosion). Passivation can also be done with reverse osmosis, deionized water, which assures that the stainless steel is ultra clean.

What Is Electropolishing?

Electropolishing is an electrochemical process similar to, but the reverse of, electroplating. The electropolishing process smoothes the microscopic surface of a metal object. As a result, the surface of the metal is void of the smallest micro-depression or micro-projection. In electropolishing, the metal is removed ion by ion from the surface of the stainless steel object being polished. The object to be electropolished is immersed in an electrolyte which would be a concentrated acid solution such as sulfuric acid and phosphoric acid. Than it is subjected to a direct electrical current. The stainless steel object or the anode along with a cathode are placed in the solution. The cathode is the negatively charged electrode and the anode is the positively charge electrode. The electrical charge will dissolve the microscopic metal ions from the stainless steel part.

Smoothness of the metal surface is one of the primary and most advantageous effects of electropolishing. During the process, a film of varying thickness covers the surfaces of the metal. This film is thickest over microdepressions and thinnest over microprojections. Electrical resistance is at a minimum wherever the film is thinnest, resulting in the greatest rate of metallic dissolution. Electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro-depressions or “valleys.”

Electropolishing removes metal. It does not move it or wipe it. The basic metal surface is left bright, clean, microscopically smooth, and has a “mirror like” finish. Electropolished stainless steel is an excellent choice for a cleanroom environment. Electropolished stainless steel tables, gowning benches and garment racks are often a good choice for use in the pharmaceutical, laboratory, aerospace and semiconductor industries.

Cindy Weist
Western States Sales Inc

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Cleanroom World is a cleanroom specialist in Centennial Colorado