Anolyte cells of electrocoating paint. I: Anodes, impurities and dissolution

The occasionally occurring sudden destruction of the used stainless steel anodes during the electrocoating process and the regular consumption of their thickness are due respectively to local and non-local anodic dissolutions. The aim of this paper is to identify the operating conditions that protect the anode from the sudden destruction of local dissolution, and also to assess the non-local dissolution rate in order to predict the mean life of these anodes. Both types of dissolution are known in the field of corrosion, but this field concerns passivation and trans-passivation potential and not higher range, i.e. oxygen evolution reaction potential. Specific tests are performed with stainless steels that contain different concentrations of molybdenum and typical weak acids of anolyte solution that normally also contain traces of chloride, sulphate phosphate and nitrate as impurities. The results of these tests show that the anodes operate in non-safeguard conditions with respect local dissolution when: (i) traces of nitrates are absent from the anolyte solution and (ii) low molybdenum concentration stainless steels are used as anodes. When the safeguard conditions are ensured, the rate of non-local dissolution is easily detected and the results obtained show that its value increases with (i) the increase in molybdenum in the stainless steel, (ii) the decrease the pH value of the anolyte solution (iii) the increase in the temperature and (iv) the decrease in anolyte recycling in the cell. The results obtained show that sudden destruction and the mean life of anodes are the result of added chemicals in the paint bath and from the pre-treatments adopted. In other words, the interactions between pre-treatments, electrocoating and accessories (anodes), is required when the optimization of the electrocoating process is proposed.