Study On Pb Content Regulation,Microstructure And Properties Of Tinplate Coating

Tinplate is widely used in food and beverage packaging industry with non-toxic,beautiful,corrosion resistance and other characteristics.However,the tin used in the tinplate contains trace amounts of Pb,resulting in the solution containing trace Pb.The standard potential of the Sn/Sn2+and Pb/Pb2+ are close,respectively,-0.136 V and-0.126 V vs.NHE,they can be easily co-deposited in the electroplating process.With the food industry for packaging materials increasing environmental requirements,the downstream users of tinplate coating of lead content requirements have become more stringent,more and more users were asked coating lead content = 0.01%.As people increasingly pay attention to food safety,in the near future,more stringent lead content tin plate coating national standards will also be introduced,so the coating of Lead control is imminent,the deposition mechanism of Pb and tinned products strategies has become a top priority.In this paper,a laboratory-made equipment was used for electroplating operations,effects of technological conditions in PSA tin plating coating of MR low carbon aluminum killed steel on the trace Pb was studied.Atomic absorption spectrometry(AAS),glow discharge spectrometry(GDS),scanning electronic microscopy(SEM),X-ray diffractometer,X-ray Photoelectron Spectroscopy and Electrochemical workstation were used to analyze the microstructure,composition,the distribution of elements,phase behavior of the coatings.Simultaneously laboratory findings has been applied industrial production lines and develop a PSA system of the industrial production line plating bath of lead content stability control technology.The results indicated that trace Pb in the tin coating reduced along with bath temperature.When the temperature exceeded 40 ?,the grains in the tin coating were coarse and loose,and its corrosion resistance decreased.Thus,taking into account the quality of the coating and the coating of lead content,bath temperature maintained at 40?50 ? in the electroplating process.When current density increased,trace Pb increased rapidly.However,low current density plating could lead to drain regions.The plating rate had no obvious effect on trace lead in tin coating.Ensuring other factors in the same conditions,we study the bath composition on Pb content.The results indicate that SO42-and Fe2+have no influence on the Pb content in the coating.With Sn2+ increasing,the lead content decreases.while the Sn2 + is 32 g/L,continuing to increase Sn2+,the lead content of the coating is almost unchanged.Within a certain range of concentrations,EN and ENSA additives have a similar impact on the trend of trace Pb,the lead content of the coating is increasing and then decreases with the concentration increased,With the increase of Pb content in the bath,its content increases significantly.With the increase of PSA,the lead content of coating decreases.While the bath of PSA in 18 g/L,the tin coating has the lowest level of lead,as 102 ppm.As PSA continues to increase,the lead content the coating of also increases.the order of the effect on Lead content by orthogonal experiment is Sn2>bath temperature>current density,the optimum process parameters is that bath temperature of 50 ?,current density of 15A/dm2,Sn2+ concentration of 32g/L.By analyzing 20 ?,30 ?,40 ? bath temperature of cathodic polarization curves shows that as the temperature rises,tin deposited on the cathode potential positive shift,limiting current density increases and the cathodic polarization weakened.By analyzing the cathodic polarization curves before and after the organic additives can be drawn that the ability to add an additive significantly enhances the polarization of the bath,the bath stability improves significantly,which has an important role to improve the effciency of tin plating and tinplate surface quality.By analyzing the amount of tinplate 2.8 g/m2,we can get that lead content is relatively high in the surface coating.With the increasing depth,Pb trace decreased.The oxygen content of the tinplate surface and alloy layer is the highest,where lead enrichment is the most serious,because of the reaction of oxygen with tin,the increasing of tin oxide reduces the cathodic polarization voltage and offset to Pb deposition,which could easily lead to lead enrichment.By adding various concentrations of barium hydroxide,barium and strontium in PSA bath,it can be draw the conclusion that the lead removal effect of barium hydroxide and barium is remarkable.When the bath was added 0.5g/L of Ba(OH)2,the lead content of the bath decreased sharply to less than lmg/L.When added to 0.5g/L of strontium carbonate,the lead ion content reduce significantly,with increasing strontium,the content of lead ions decreased,but the concentration remains high.Although the effect of removing lead is significant after addition of barium carbonate,the process will make a large amount of gas,there are some security risks when used in industrial production.Within a certain temperature range,the lead ion content decreases with increasing temperature,when the temperature continues to rise to 60 ?,the effect of removing lead is slightly lower.With increasing aging time,the remaining lead ions decrease after precipitation and adsorption reaction.By using Ba(OH)2 as the precipitant,the XRD analysis show that the precipitate is the solid solution(Ba,Pb)SO4,not containing PbSO4.By Hull cell test the added of Ba(OH)2 does not cause changes in PSA bath each active component concentrations and has no adverse effects on the performance of the bath.The industrial production line applications shows that by adding 2g/1 Ba(OH)2 in the bath and standing for 6 hour,it can achieve a better result to remove the lead in the bath and still have no significant effect on the quality of the bath.The online experiments show that lead ion is controlled at 16ppm or less,Sn2+ at 28-32g/1,temperature in 43?-45?,the current density of 16-20A/dm,which can be stably controlled lead content of the tin coating weight 2.8g/m2 in the product within 100ppm.After adding Ba(OH)2,it have no effect on surface morphology,surface composition,surface corrosion resistance,surface iron dissolution value and surface of the crystal structure,what's more there is no residual Ba elements in the coating.