Study On Factors Affecting Corrosion Resistance Of Low Tin Steel Sheet

Tinplate is widely used as packaging materials for food and beverages.However,in recent years,the development of alternative materials such as aluminum cans and plastics has promoted the development of tinplate with higher strength and thinner thickness.The amount of tin has been reduced from 11.2 g·m^-2,5.6 g·m^-2 to the current 2.8 g·m^-2,1.1g·m^-2.In the future,a tin plate of 1.0 g·m^-2 or less should be developed,which poses a bigger challenge for the corrosion resistance of tinplate.As the tin plating layer is thinned,the tin plating layer has weakened protective performance on the tinplate.Therefore,it is necessary to study the influence of the surface characteristics of the substrate and the passivation process on the low tin steel sheet,and finally control the surface characteristics of substrate and passivation film layer to improve the protective performance of the low tin steel sheet.In this paper,electrochemical tests,scanning electron microscopy?SEM?,chemical microscopy,X-ray diffraction techniques were used to characterize the morphology,roughness,crystal orientation,and true surface area of the primary cold rolled sheet of Bao gang,Mei gang,Wu gang,and secondary cold rolled sheet of Bao gang.The initial nucleation and growth process of the four substrates in the methane sulfonate solution were characterized by chronoamperometry,SEM,etc.,and the morphology and roughness of 0.7 g·m^-2,1.1 g·m^-2,2.8 g·m^-2 tin-plated layer were tested.The electrochemical process of rotating disk electrode+cyclic voltammetry was used to study the growth process of tin plating on the substrate.It was found that the initial nucleation type was affected by the potential.So the initial nucleation of-0.65 V--0.73V?vs SCE?potential range is continuous nucleation and the-0.75 V--0.80 V potential is instantaneous nucleation.The randomly distributed initial tin grains on the four substrates are hemispherical,and adjacent grains grow together to cover the entire substrate surface.And then as the tin layer continues to grow,the tin grain size becomes larger and the number decreases,and the plating layer is flatter.The porosity and corrosion resistance of un-reflowed plates and reflowed plates prepared by four kinds of substrates were tested and compared.It was found that the properties of the coatings were mainly affected by the dissolution activity,roughness,real surface area and surface defects of the substrate.Using iron plate and Sn-Fe alloy plate to simulate the exposed Fe and exposed Sn-Fe area in the pores of tinplate,and using impedance potential,cathodic polarization test,XPS analysis,etc.to characterize cathodic passivation behavior,and the content and composition of the passivation film on the surface of the passivated Sn-Fe alloy plate,Fe plate.It showed that the chromium content in the passivation film on the iron was 0.067C·dm^-2,which was lower than that on the reflowed plate and the Sn-Fe alloy plate,respectively,0.076 C·dm^-2,0.073 C·dm^-2,the former is considered to have a low chromium content,which is mainly due to the positive hydrogen evolution potential during the passivation of the iron plate,which is easy to decompose hydrogen and inhibit the formation of trivalent chromium in the passivation film.Finally,electrochemical technology and neutral salt spray test were used to study the effect of passivation process on the performance of passivation film of low tin steel sheet.It was found that the passivated charge was in the range of 2 C·dm^-2-60 C·dm^-2 and the chromium content increases first and then decreases with the increase of the passivation charge.The optimum passivation amount is 40 C·dm^-2,and increasing the current density can increase the chromium content.