What is a passivator and when should I use one?
A passivator, also known as an anodic inhibitor, is a type of corrosion inhibitor that reduces or blocks reactions at the anode, thus shifting the potential of the metal to a more electropositive value. In other words, the chemical reactivity of the metal substrate is minimized such that it exhibits a substantially reduced corrosion rate.
Passivators typically work by using a series of chemical treatments to remove foreign reactive ions from the metal's surface that inhibit the formation of protective oxide films. Removing these ions renders the surface inert and encourages the development of an oxide film when the surface is exposed to the atmosphere.
For example, under ideal conditions stainless steel forms a protective oxide film when exposed to air and moisture, which protects it from further corrosion. (Learn how this process works in the article Why is Stainless Steel Corrosion Resistant?) However, in actual practice foreign material can make its way to the stainless steel surface during manufacturing, which can inhibit the oxide layer from forming. Therefore, these contaminants need to be chemically removed down to the grain boundaries so that the oxide film can form.
The process of removing foreign matter and encouraging oxide film growth is called passivation. Some passivators will also react with the metal’s natural corrosion products to further enhance the passive oxide layer to make it “sturdier.” Chromates, nitrates, tungstate and molybdates are just a few examples of passivators.
Passivators are especially useful for preventing corrosion damage after wet abrasive blasting. After wet abrasive blasting, salt ions (or other ion contaminants) can remain lodged in microscopic crevices on the metal's surface and promote flash corrosion. To solve this problem, a passivator is added to the blast water. The passivator chemically removes these ions, thus changing the surface of the metal to a less reactive (passive) state. As a result, flash rusting on the substrate is slowed, providing a longer protection time before a final coating is applied.