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The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a “passive” film which resists further “oxidation” or rusting. The formation of this film is...

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Does it can rust?

STAINLESS RUST
The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a “passive” film which resists further “oxidation” or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become “passivated” and the oxidation or “rusting” rate will slow down to less than 0.002″ per year (0,05 mm. per year).

Stainless gets its non corrosive properties from the chromium in the alloy. It’s created when oxygen combines with the chrome in the stainless to form chrome oxide which is more commonly called “ceramic”. This protective oxide or ceramic coating is common to most corrosion resistant materials. The chromium atoms combines with oxygen and forms a passive surface film over the base steel very much like the paint protects your car. Once this layer is removed the base metal is exposed to the moisture in the atmosphere and rust forms. Chlorine in any form combines with the chromium and removes this protective layer and exposes the base metal and rust will occur.

NEVER use any chemicals that contain chlorine near any stainless. This includes any cleaners, acids to clean quarry tile or brick, and some detergents. Even the vapors can attack stainless steel. The only inorganic acid that is friendly to stainless is nitric and it is used to remove any iron particles left on the surface from manufacturing or machining. Never use steel wool to clean stainless. Particles of the steel wool will get trapped in the grain of the stainless and these steel particles will rust. Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters “ine”. Listed in order of their activity they are:

fluorine

chlorine

bromine

iodine

astatine (very unstable.)

This type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steel. It’s the easiest to recognize as the entire surface of the metal shows a uniform “sponge like” appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part.

Chlorides are problematic with austenitic stainless steels like 304 as they can cause pitting, crevice corrosion, and stress corrosion cracking. Temperatures above ambient and cycling between hot and cold temperatures can make corrosion worse as concentration of chlorides due to evaporation can occur. The generally recommended maximum chloride level for 304 stainless steel is only 200 ppm (1000 ppm for 316 stainless steel). The low free chlorine levels of typical potable water systems will not affect austenitic stainless steels. However, free chlorine concentrations of as little as 25 ppm can have a detrimental effect on them. The subject of chloride induced corrosion of austenitic stainless steels is very complex and depends on many things such as concentration, temperature, pH, etc.

A clear epoxy power coating is a tough, durable clear protective coating that protects stainless steel from corrosion, salt air pitting and provides excellent fingerprint and smudge prevention. Stainless steel surfaces protected with a clear epoxy powder finish will be much easier to keep clean and will never darken as it will with oily protectants.


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