Influence of Halides in Phosphoric Acid on the Corrosion Behavior of Stainless Steels

The corrosion and electrochemical behavior of austenitic stainless steels in pure H3PO4, in the presence and absence of fluoride, chloride, and bromide, have been investigated in the 20 to 120 C (68 to 248 F) temperature range. Significant corrosive attack of Types 304 and 316 steels in pure H3PO4 starts to occur at about 60 C (140 F) and 90 C (194 F), repsectively. Up to these temperatures, these steels exhibit passivity, but with the increase of temperature, breakdown of passivity occurs, the potential becomes active, and an intense corrosive attack is established. The values of electrochemical parameters indicate corrosion, e.g., critical current density, change from system to system, in accordance with their respective corrosion rates. The presence of halide ions in H3PO4 affects the corrosion rate, the change in rate being dependent upon the free halide ion concentration, its chemical reactivity, and the ionic size. Values of the critical current density, passive current density, and corrosion potential change accordingly. The corrosion activity of halides in 30% and 50% H3PO4 follows the sequence: F− >Cl− >Br− and: Cl− > F− >Br− in 70% and 85% H3PO4.