Effect of Al on phase evolution in Prior-Ni coated galvanized steel: Thermodynamic and kinetic assessment

Abstract

Coating of advanced high strength steel and ultra-high strength steel is a burning issue due to the problem of surface oxidation of alloying elements and liquid metal induced embrittlement (LMIE). In last decade, prior coating of Ni before galvanizing has gained much interest as it has the potential to suppress the surface oxidation of advanced high strength steel with added benefits. Recently, the galvanized ultra-high strength steel with a prior-Ni coating also has been proved as a solution for hot stamping application. It has been observed that prior-Ni reacts with Zn at the stage of galvanizing and forms different Ni–Zn alloy phases with high melting temperatures and resist the LMIE at the time of hot stamping. Al is one of the most important alloying element in commercial galvanizing bath. However, effect of Al on the Ni–Zn phase formation in such coating has not been researched in detail. In the present study, phase formation in prior-Ni coated galvanized steel in the presence of Al in Zn bath has been thoroughly investigated. The influence of Al on the Ni–Zn phase formation has been assessed by thermodynamic and kinetic calculation and then validated by experiments. Al3Ni2 with dissolved Zn precipitated in the Zn overlay and of Ni–Zn alloy phases such as NiZn8 and gamma phase formed on prior-Ni layer in a Zn–Al bath up to 0.01 mass fraction of Al. However, Ni–Zn alloy phases such as NiZn8 and gamma phases were absent in coating achieved by Zn–Al bath above 0.01 mass fraction of Al. Instead a thin compact layer consisting high amount of Ni, Al and Zn formed on prior-Ni layer and hindered the formation of Ni–Zn alloy phase in coating. The mechanism of the phase formation in the prior-Ni coated galvanized steel in presence of Al has been described with the help dissolution, phase fraction, driving force calculations and experimental observations.