Passivity of Chromium and Stainless Steel In Hydrofluoric Acid

The anodic polarization of chromium and 18Cr-8Ni stainless steel has been investigated in 2 to 48 percent hydrofluoric acid, 40 percent sulfuric acid, and in a solution containing 5 percent hydrofluoric acid and 40 percent sulfuric acid. Chromium can be anodically protected in hydrofluoric acid. The corrosion rate of stainless steel in hydrofluoric acid can be reduced by anodic protection but less than is possible in sulfuric acid. The dissolution rate of passive stainless steel in 5 percent hydrofluoric acid plus 40 percent sulfuric acid is similar to the rate in 5 percent hydrofluoric acid. The breakdown of the passive film on stainless steel probably is accelerated by the formation of stable ferric fluoride complex ions.