The dissolution reaction of Zr in HF is: Zr + 4H+ Zr++ → 2H2. The overall rate of dissolution V at 25 C and under the conditions of the experiments is much faster than that of Ti, and is a linear function of the HF concentration (up to N = 0.5): V = 2000 N mm3 cm−2 min−1. A black hydride film is formed on Zr during its dissolution.

Fluoride additions (NE4F, KF, NaF) to the acid initially increase the rate of dissolution. When added in greater amounts, passivation of Zr is caused which is quite complete for NaF additions. The order of increasing effectiveness is NH4F → KF → NaF, apparently caused by the decreasing solubility of fluozirconate salt film formed on the metal.

Potential measurements of Zr in HF showed an approach to a steady potential of about —0.74 volt with increasing concentration of the acid. The potential time curves had in general a logarithmic shape, the potential becoming more positive with elapsing time. Fluoride additions caused a breakdown of the initial film, formed during dissolution in pure HF, and a sudden drop of potential down to —1.112 volt (in 6.5 M NH4F). The potential gradually became more positive, because of the salt film formation on the surface of Zr, until a steady potential was reached. Of the three additions, NaF produced the most positive potential (—0.528 volt) and a complete passivation of Zr. 6.3.20

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Copyright 1959 by the National Association of Corrosion Engineers.
1959
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