Inhibitors for low alloyed chromium steels in hydrofluoric acid solutions

Some classes of organic substances were tested as inhibitors of 3% HF attack on low alloyed chromiums teels (ASTM A335 type steels containing 1.35-10% chromium). Only S-containing additives (phenylthiourea, dibenzylsuphoxide, etc.) showed inhibiting efficiency values of 99% at both 45 and 60 °C, regardless of steel chromium content. These additives acted by inhibiting both the anodic and the cathodic processes. Alloyed chromium hinders the diffusion of atomic hydrogen into the iron lattice (hydrogen permeation) which is also influenced by S-containing additives. Phenylthiourea increased both the diffusivity and the permeability of hydrogen, while Dibenzylsuphoxide decreased both.