Passivating Inhibitors

Passivating inhibitors (passivators) cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range. There are two types of passivating inhibitors:

• Oxidizing anions, such as chromate, nitrite and nitrate, that can passivate steel in the absence of oxygen

• Non oxidizing ions such as phosphate, tungstate and molybdate that require the presence of oxygen to passivate steel.

These inhibitors are the most effective and consequently the most widely used. Chromate based inhibitors are the least expensive inhibitors and were used until recently in a variety of applications, e.g. recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers. Sodium chromate, typically in concentrations of 0.04-0.1% was used for these applications.

At higher temperatures or in freshwater with chloride concentrations above 10 ppm higher concentrations are required. If necessary, sodium hydroxide is added to adjust the pH to a range of 7.5-9.5. If the concentration of chromate falls below a concentration of 0.016% corrosion will be accelerated. Therefore it is essential that periodic colorimetric analysis be conducted to prevent this from occurring. In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential that monitoring of the inhibitor concentration be performed. (reference)