Corrosion of Stainless Steel in Crude Phosphoric Acid

In order to qualify stainless steel as structural material for transport containers to carry crude phosporic acid, corrosion tests were conducted on various types of stainless steel.The results obtained from the tests indicated that the alloying metals most effective in increasing corrosion resistance against crude phosphoric acid are Mo, Cr and Cu. Of these three, Mo and Cr enhancing the passivation of stainless steel are particulary effective.Based on this finding, stainless steels of high Mo and/or Cr content-317L and 25Cr-4Ni-2Mo steel-were tested to determine the influence of impurities contained in crude phosphoric acid on the corrosion behavior. The impurities found to accelerate corrosion were H2SO4 and F-; impurities, on the other hand, that inhibited corrosion were Fe3+, Mg2+, Ca2+, Al3+. The action exerted by these impurities on the corrosion can be explained electrochemically, based on the results of measurements of anodic polarization curve in a solution containing these impurities singly or in combination: In solutions containing both H2SO4 and F-, the critical current density for passivation and the passive current density were found to be markedly raised, whereas the solution containing Fe3+ was found to have the corrosion potential of stainless steel shifted from the active to the passive region.Better corrosion resistance was found to be provided by the 25Cr-4Ni-2Mo than by the 317L steel.