The Rate of Dissolution and the Passivation of Titanium in Acids with Ammonium Fluoride Added 1

Although Ti is very resistant o the action of all acids, except HC1, H2SO4, and es-pecially HF, its resistance breaks down if soluble fluorides are added to the acidic solu-tions. It was found that the HF l iberated by acids part ia l ly dissolves the protect ive film that is always present on the surface of Ti. Hydrogen ions then are discharged at the local cathodes, which are now exposed to the acids through the pores of the film. In agreement with this concept, the rate of dissolution of Ti increases only sl ightly with increased concentrat ion of a strong acid (HC1, H2SO4), at a constant concentra-t ion of NH4F, but it increases greatly with increased concentrat ion of NH4F (at a con-stunt concentrat ion of the strong acid). If the concentrat ion of NH4F is increased still further, the Ti becomes passive, and simultaneously its potent ia l decreases to-0.94 volt (hydrogen scale). This passiva-t ion could be explained by formation of a part ia l salt film on the surface of the dis-solving Ti, and by increase of hydrogen overvoltage on local cathodes, because of the NH4F present