Comparison of methods for chromium determination in tinplate passivation film

摘要

Passivation treatment is the final stage of the whole manufacture process of tinplate, if oiling is excluded. The passivation treatment which is based on the electrolysis of an aqueous solution of hexavalenf chromium between two electrodes (tinplateand carbon steel), causes a reduction of the oxidised tin compounds formed in the previous fusion process and a deposition of chromium-based compounds. Quantitative determination of chromium compounds is of paramount importance for characterising some tinplate properties such as resistance to oxidation, resistance to sulphur-staining and lacquer adhesion. In this work various analytical methods (chemical and electrochemical, used for the determination of total chromium amount in the passivation film) were compared. In particular, in the SSICA laboratories the chemical method was applied which is described in the Euronorm 10202/February 2002 and which is reported as official method of analysis; it uses diphenylcarbazide as main reagent. Various kinds of materials were considered: finplates passivated, on an industrial line, at different charge density values in Na_2Cr_2O_7 solutions (25 g/L, T= 50 deg C) with different acidity levels (pH = 2.5, 4.5 and 5.5); finplates passivoted in the laboratory, atdifferent charge density values in a Na_2Cr_2O_7 solution (25 g/L, T = 50 deg C, pH = 4.5), using a particular electrochemical cell which simulated industrial passivation treatment; commercial tinptafes, Important differences were found between the results obtained with the different analytical methods. In particular, the chemical diphenylcarbazide method yielded, in all cases, chromium values higher than those provided by the other techniques.