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Passivation

Overview

Passivation is a cleaning and corrosion-protection treatment appropriate for a variety of machined metals. Although it is useful for many types of metals, passivation is particularly popular as a method of cleaning the impurities left on the surface of stainless steel after the manufacturing process. During passivation, the steel or other metal is treated with an acid solution which removes contaminants from the metal surface and which coats the surface in a protective film. Passivation lengthens the life of the metal without affecting its physical appearance.

Semi-finished or finished parts are usually treated for passivation. It is a usual practice to passivate parts after machining, grinding or lapping. This is because during forming machining particles of iron may be imbedded in or smeared on the surfaces of stainless steel components. If allowed to remain the iron corrodes and often gives the appearance of large or small rust parts on the stainless steel. If the parts have flux or slag from welding or high-temperature brazing they should be removed by chipping, brushing with stainless-steel wire brush, grinding, or polishing with an iron-free abrasive. When machining or grinding oils are to be removed. Cleaning should begin with solvent cleaning which should be followed by an alkaline soap cleaning and thorough water rinsing. Best results are obtained in passivation when the parts are cleaned as thoroughly as possible.

Technical Information

How Passivation Works
When fresh metal is exposed from a machining process it will immediately begin to react with the atmosphere. Depending on the atoms present on the surface of the metal it can react in a variety of ways. For example, a free iron present on the surface of a sheet of metal will react with the oxygen in the air to form rust. On the other hand, chromium on the surface of stainless steel will react with oxygen to form chromium oxide which can help to protect the metal.

Passivation works in two ways. First, passivation rids the surface of the metal of free iron and other impurities that would be likely sources of corrosion. Secondly, it deactivates the active surface of the freshly exposed metal, forming a protective film and making it less likely to react further with the atmosphere.

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