Introduction of Sodium Lignosulphonate in Zinc Phosphate Coatings on Mild Steel to Enhance Corrosion Resistance
Haowei Huang, Jing Mo, Jieming Xu, Daocong Lin, Wenzhi Li, Huiran Yang, Menglin Zhao, Minchun Xu, Jiawei Li, Feng Liu, Yanwu Chen, Xinya ZhangAbstract
Sodium lignosulphonate (SLS), a renewable bio-based polymer, was introduced as a green accelerator into a low temperature zinc phosphating bath for mild steel. The effect of SLS concentration on the microstructure, phase composition and corrosion resistance of the phosphate coating was systematically investigated. Scanning electron microscopy and X-ray diffraction results revealed that SLS did not alter the dual-phase structure of the coating consisting of hopeite and phosphophyllite, but significantly refined the crystal grains due to the abundant heterogeneous nucleation sites provided by its functional groups. The coating modified with 1.5 g/L SLS exhibited the most uniform and compact morphology, with carbon and sulphur elements uniformly co-deposited within the film. Electrochemical measurements demonstrated that the SLS modified coating achieved a corrosion current density of 1.29 × 10−5 A/cm2 and a corrosion rate of only 1.89 mpy, which is 65.3% lower than that of the unmodified coating. Moreover, the charge transfer resistance increased by nearly three orders of magnitude. These results confirm that SLS is an efficient, low cost and eco-friendly accelerator for low temperature phosphating, significantly enhancing the corrosion resistance of zinc phosphate coatings on mild steel.