Metals are said to be ‘passive’ when they show good corrosion resistance against a specific medium. That is to say that metal A might be passive in liquid 1 but not in liquid 2, whereas metal B might be passive in liquid 2 but not in 1. A passivator is a chemical pre-treatment which optimises the natural passivity of a particular metal. One of the most common uses of a passivator is that of nitric acid used to optimise the oxidation resistance of stainless steel. Typically, the steel component being passivated is immersed in nitric acid of a specified concentration (typically 25-30%) for a period of some minutes, then removed from the acid and washed and dried before being put in to service. Such treatment is used in application where the maximum degree of corrosion is required, as is the case in the manufacture of semi-conductor materials, pharmaceuticals and long-term storage of liquid rocket propellants.
A corrosion inhibitor is a chemical added to a specific medium to achieve a level of corrosion resistance for the metal(s) which come in to contact with that medium. A common example of such inhibitors are the agents present in the coolant solutions (primarily composed of mixtures of glycol and water) used in car engines. Corrosion inhibitors usually react with the surface of the metal to form a protective surface film, either in the form of an adsorbed layer of inhibitor, or a chemical conversion product.
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