Discoloration Behavior of Stainless Steels by Crude Phosphoric Acid

In order to shed light on a phenomenon (blackening) occurring on the surface of tanks of chemical tankers used to carry crude phosphoric acid, corrosion tests and surface analyses were conducted on various types of stainless steels in crude phosphoric acid. The tests and analyses revealed that when stainless steels were exposed to crude phosphoric acid, the surface of the metal blackened in the gas phase, while no corrosion or discoloration occurred in the liquid phase. The overall corrosion, resembling light surface roughening occurring directly below the part discolored in the gas phase, resulted in no problem with all stainless steels because the corrosion rate was lower than 0.1 mm/year. When the practical use of chemical tanks is considered, however, it is necessary to clean the tanks because blackening might possibly contaminate other chemicals through the mixing of impurities. Blackening results primarily from the fluoride gas liberated from the crude phosphoric acid that forms a black film as the product of corrosion by reacting with the metal surface. The black film is considered to comprise either or both of oxides or fluorides of such metals as iron, chromium and nickel. In order to prevent or inhibit the discoloration, it is necessary to remove or dilute the fluoride gas from the crude phosphoric acid. This can be effectively achieved by replacing the gas phase with inactive gases.