Study On Formation Mechanism Of Adhering Tin Defect On The Surface Of Double Reduced Tinplate

The defect of adhering tin on DR materials is a fatal flaw that damages tin coating. It affects directly the corrosion resistance and coating performance of tinplate. In order to improve the tinplate quality, and reduce the incidence of adhering tin defect, the formation mechanism of adhering tin and influencing factors were studied in this paper, which has important significance for improving product competitiveness.The properties and formation reasons of adhering tin defect on tinplate surface were researched through the macroscopic morphology analysis, scanning electron microscopy (SEM) and energy spectrum analysis (EDS). In addition, the binding force between the coating and the substrate was evaluated by using scratch tester. The main results are as follows:(1) There are five types of adhering tin defects for the DR materials found:the first is "white-point type" adhering tin defect, the second is "perforation type" adhering tin defect, the third is "strip tin type" adhering tin defect, the fourth is "blackspot type" adhering tin defect and the last is "impression type" adhering tin defect.(2) The concentrations of both ferrous ion and chloride ion have a great effect on the surface state of DR materials tinplate. When the concentration of ferrous ion in plating solution is above 1.85g/L and the concentration of chloride ion is above 1.2g/L, the phenomena of inhomogeneous coating, surface markings and skip plating on the edge appeared.(3) The compositions and the particles size of tin sludge have a serious impact on the surface state of tinplate for DR materials, and the main composition of tin sludge is SnO2. When the concentration of tin sludge increases to 4g/L, inhomogeneous coating and light yellow markings appears on the surface of tinplate.(4) With the increase of tin sludge concentration, the adhesion strength between the coating and the substrate decreased. The tin coating with small adhesion strength is easy to fall down and adhere to the conductor roller, which will cause the phenomenon of strike fire and form tin sludge defects.(5) In the process of soft melting and water quenching, thermodynamic model is set up according to the model of one dimensional heat transfer. The tinplate temperature after quenching tank is above 85℃, which is calculated by the thermodynamic model under different conditions. With the increase of temperature, the hardness of tin reduces. The high temperature of tinplate coming to 2# conductor roll is one of the reasons of adhering tin defect.(6) Through the orthogonal test, the most influential factor of generation rate of layer of tin-iron alloy is quenching temperature(C), secondary is soft melting temperature(A), and followed by soft melting time(B), and the last is quenching time(D). And the suitable condition was gained:soft melting temperature is 285℃, soft melting time is 120 seconds, quenching temperature is 70℃ and quenching time is 75 seconds.(7) The binding force of tin-iron alloy layer is measured by the use of scratch hardness tester under different current density. With the increase of current density, the binding force of tin-iron alloy layer is on the rise, so the electroplating current density should be controlled as large as possible in the allowed range. This results in the increasing binding force to reduce the incidence of adhering tin defect on the 2# conductor roll.(8) The content of plating solution in quenching tank has a great effect on the tinplate surface state. With the increase of the content of plating solution in quenching tank, the surface state of tinplate becomes bad. In order to obtain tinplate of good surface state, the volume ratio of quenching water vs. plating solution is as large as possible.