In phosphating solutions, sodium nitrite performs as an accelerator and oxidizer, serving to reduce processing times and control buildup of ferrous ions in solutions, respectively.  Phosphate coatings are applied to steel as a base coating prior to painting.  Sodium nitrite is added to the phosphate solution to accelerate the process and aid in oxidization of the metal. 

In gold-sulfite-plating baths, sodium nitrite functions in the formation of a gold-sulfite-nitrite complex.  From this complex, the gold can be electrolytically deposited.  This bath is considered to be safer than the poisonous cyanide baths traditionally used for gold plating.