Synergy of ball-milling and pre-oxidation on microstructure and corrosion resistance of hot-dip zinc coating of nodular cast iron

Abstract

In this study, nodular cast iron parts, grinding balls, and NaHCO3 powder were placed together in a ball milling cylinder to perform the coordinated pretreatment of ball-milling and pre-oxidation. And then the cast iron parts were hot-dipped galvanized to study the synergy of ball-milling and pre-oxidation on the microstructure and corrosion resistance of the coating. The results showed that after pre-oxidation at 300 °C, Si on the surface of nodular cast iron is oxidized to form SiO2, Fe is not oxidized, and the substrate surface is selectively oxidized. After the non-oxidization sample is pickled, its surface Si content is 2.88 wt. %. After the pre-oxidized sample is pickled, its surface Si content is 2.07 wt. %. After the synergy of ball-milling and pre-oxidation, the surface Si content on the cast iron is 1.52 wt. %. Compared with the sample without pre-oxidation, the surface Si content on the cast iron after pre-oxidation decreases by 28%, and the surface Si content on the cast iron with the synergy of ball-milling and pre-oxidation decreases by 47%. In the hot-dip galvanized coating, as the surface Si content on the cast iron decreases, the ζ grains are refined, and the thickness of the Fe–Zn alloy layer decreases. The sample with the synergy of ball-milling and pre-oxidation has the finest ζ grains and the thinnest alloy layer thickness. In electrochemical corrosion, as the Si content on the cast iron surface decreases, the corrosion potential of the ζ layers are consistent, the corrosion current density decreases, the thickness of the passivation film increases, and the polarization resistance increases. The corrosion resistance of the ζ layers obtained under different pretreatment conditions is in order non-oxidization and pickling sample < pickling after pre-oxidation sample < synergy of ball-milling and pre-oxidation sample.

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