5-4. Controls. Now that you know what corrosion is and what causes it, you must know how to
control it. Basically, the same principle that causes corrosion is used to counteract it. This is done in
several ways, but the most common are known as passivation and cathodic protection.
a. Passivation.
(1) A passivator is an inhibitor that changes the potential of a metal to a more cathodic value.
An inhibitor is a chemical substance or mixture which, when added to an environment, usually in small
concentration, effectively decreases corrosion. Table 5-3 lists several mixture types for passivating
metal surfaces. The term passivity may be defined as the property by which certain metals become
inactive in a specific environment. Metals that do not form protective films under service conditions are
protected by the method of immersion in a chemical bath containing inhibitors. The passivity of a
metal, such as stainless steel, is gained by the method of a protective-film formation (electroplate) on the
metal's surface through its absorption of atoms or ions.
(2) By either of the methods (immersion or protective film), the passivation of the metal's
surface serves to make the surface more resistant to corrosion by either physical or chemical treatment.
One is based on the corrosive behavior of the metal or alloy, and the other is based on the
electrochemical behavior of the metal or alloy.
EN0562
5-14
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