Cr-free Passivation For Tin-plating And Performance Testing

Tinplate is the low carbon steel plate which surface is coated with thin layer of tin, it is widely used in industry of tinned food, industry of packaging materials and other industries. In order to enhance the corrosion resistance and sulfur resistance performance of tinplate, passivation treatment is needed. Chromate passivation is generally used in the traditional tinplate, but it harmful to the human body and environment, so it is important to find a way to replace chromate passivation.The formula of molybdate/phytic passivation solution and process conditions were determined by contrast experiment in this paper:(NH4)6Mo7O24·4H2O 20~30g·L-1, phytic acid 5~9ml·L-1, hexamethylenetetramine 2~3 g·L-1, p H 4.0~5.0, current density 150~200m A·dm-2, passivation time 5~15s, and the passivation temperature 20~50℃, the passivation film under this condition obtained was uniform, bright and delicate, and the corrosion resistance performance detected by electrochemical test was good. The corrosion rate of molybdate/phytic passivation film in brine soaking was 0.035g·m-2·h-1, and it appeared yellow rust corrosion in 48 h in neutral salt spray test. The surface of molybdate/phytic passivation film observed by SEM was smooth, and there were some cracks, but the cracks were not deep. The EDS results indicated that Fe, Sn, Mo, O were simultaneously existed in the film surface, and Fe and Sn were the reflection of tinplate.The formula of molybdate/Ce(NO3)3·6H2O passivation solution and process conditions were determined by contrast experiment in this paper:(NH4)6Mo7O24·4H2O 20~30g·L-1, Ce(NO3)3·6H2O 3~5g·L-1, citric acid 4~6g·L-1, tartaric acid 4~7g·L-1, p H 4.0~5.0, current density 100~200m A·dm-2, passivation time 10~15s, and the passivation temperature 20~50℃, the passivation film under this condition obtained was uniform, bright and delicate, and the corrosion resistance performance detected by electrochemical test was good. The corrosion rate of molybdate/phytic passivation film in brine soaking was 0.012g·m-2·h-1, and it appeared yellow rust corrosion in 84 h in neutral salt spray test. The surface of molybdate/Ce(NO3)3·6H2O passivation film observed by SEM was smooth, and there were some cracks, but the cracks were not deep. The EDS results indicated that Fe, Sn, Mo, O, Ce were simultaneously existed in the film surface, and Fe and Sn were the reflection of tinplate.The tin plate, molybdate/phytic passivation sample, molybdate/Ce(NO3)3·6H2O passivation sample and chromate passivation sample were detected by 5% salt immersion test, neutral salt spray test and electrochemical test, through those test it was found that the corrosion resistance of tinplate after passivation has been improved greatly.Cathodic polarization curve of tin plate in molybdate/phytic solution and molybdate/Ce(NO3)3·6H2O solution were tested.