A study of the film formation kinetics on zinc in different acidic corrosion inhibitor solutions by quartz crystal microbalance

Chromates conversion coatings provide very effective corrosion protection for many metals. However, the high toxicity of chromate leads to an increasing interest in using non-toxic alternatives such as molybdates, silicates, rare earth metal ions and etc. In this work, quartz crystal microbalance (QCM) was applied as an in-situ technique to follow the film formation process on zinc (plated on gold) in acidic solutions containing an inorganic inhibitor, i.e. potassium chromate, sodium silicate, sodium molybdate or cerium nitrate. Using an equation derived in this work, the interfacial mass change during the film formation process under different conditions was calculated, indicating three different film formation mechanisms. In the presence of K₂CrO₄ or Na₂SiO₃, the film growth follows a mix-parabolic law, showing a process controlled by both ion diffusion and surface reaction. The apparent kinetic equations are 0.4t = −17.4 + 20Δm_f + (Δm_f)² and 0.1t = 19.0 + 8.4Δm_f + 10(Δm_f)² respectively (t and Δm are in seconds and μg/cm²). In solutions containing Na₂MoO₄, a logarithmic law of Δm_f = −24.7 + 6.6 ln t was observed. Changing the inhibitor to Ce(NO₃)₃, the film growth was found to obey an asymptote law that could be fit into the equation of Δm_f = 55.1(1 − exp(−2.6 × 10⁻³t)).