Effect of Phosphating and Post-sealing on the Corrosion Resistance of Electro - galvanized steel

Abstract

To improve the corrosion resistance of electro-galvanized (EG) steel, the EG steel was initially treated by utilizing the phosphating with phosphate and then sealed by the technique of post-sealing with silicate solution. The surface morphology and elemental composition of the three samples were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The effect of the phosphating and post-sealing treatment on the corrosion resistance of EG steel was investigated by neutral salt spray test (NSS), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The results showed that, after phosphating, the porous phosphate crystals formed on the surface of the electro-galvanized steel, and additionally, due to the existence of sodium silicate which adequately filled into the phosphate pores, a complete layer of silicate film formed on the phosphate coating, the corrosion resistance ability of EG steel was enhanced further. The phosphating and post-sealing treatment reduced the corrosion current density of EG steel by one order of magnitude, besides, the impedance values were increased obviously. In particular, the anodic oxidation and the cathodic reduction of zinc were inhibited during corrosion process, the active dissolution of zinc was much lower, improving remarkably the corrosion of the EG steel.

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“…Anode passivation and transpasition were detected on all thicknesses (5-25 μm). Also, in the case of alkaline and acidic galvanizing, the production of white corrosion that is easily deposited on the surface of the sample due to the high electrochemical reactivity of zinc, as confirmed by the work of the author An, et al, 2017. In the acid electrolyte coating diagram 10, we see that a good sample protection is also performed, and that the same sequence of polarization resistance increases as the thickness of the coating is increased by the scanning velocity of 0.166 mV/s.…”

Effect of different sealers on the properties of phosphate conversion coatings on 30CrMnSi alloys

“…At present, most of the chromium-free sealers are based on organic resins, emulsions and other environmentally friendly and non-polluting substances as the main components, and their mechanism of action is to form a layer of organic film on the surface of the specimen to be sealed (Yunxia and Yajuan, 2014;LIN Bilan et al, 2008), and the organic film can effectively resist corrosive substances from destroying the phosphate film immersed in the surface of the substrate, thus improving the corrosion resistance. There is also a chromiumfree sealer is a nanoscale material as the main substance (Lin and Lu, 2015;Ran and Jie., 2023), through the chemical reaction so that the nanomaterials in the sealer filled into the surface of the phosphating film, effectively reducing the surface holes, to improve the effect of corrosion resistance. The research on the chromium-free sealing under the composite system of "film-forming 1 hole-filling" is not complete, which is worthy of further study.…”