Electrochemical Properties of Zn-Based Coatings on Direct Press Hardened Steels

Abstract

The rise of Zn-coatings on direct press hardened steels for body-in-white passenger safety applications over the widely used Al-Si coatings is due to its lower cost, compatibility with Zn-based paint systems, and offers sacrificial cathodic protection in addition to barrier protection. Manufacturing the complexly-shaped high strength automotive parts using the direct hot press forming method (DHPF) transforms the Zn-based coating into a mixture of Γ-Fe3Zn10 and α-Fe(Zn). Previous literature has determined that a minimum of 15 vol% Γ-Fe3Zn10 is required within the coating to provide robust cathodic protection of the steel substrate. This assumed the mixed potential theory is valid for modeling the electrochemical properties of the mixed phase coating; however, the interwoven coating phase morphology results in varying volume fractions of Γ-Fe3Zn10 and α-Fe(Zn). Potentiodynamic polarization scans of GI70 coated 22MnB5 steel annealed at 890°C for various annealing times revealed that Γ-Fe3Zn10 + α-Fe(Zn) coatings with at least 11 vol% Γ-Fe3Zn10 exhibit electrochemical properties insignificantly different from those comprising pure Γ-Fe3Zn10, and behaves similarly to pure α-Fe(Zn) for coatings with less than 11 vol% indicating that the Γ-Fe3Zn10 + α-Fe(Zn) coatings behave as a homogeneous single phase, thus validating the use of the mixed potential theory. Scanning vibrating electrode technique analysis of various galvanic couples determined that Γ-Fe3Zn10 provides strong cathodic protection for the 22MnB5 steel and moderate protection for α-Fe(Zn), while the 22MnB5 steel is only weakly protected by α-Fe(Zn). Separation of the 22MnB5 steel and Γ-Fe3Zn10 by an intermediary α-Fe(Zn) layer reduces the cathodic protection of the 22MnB5 steel since the α-Fe(Zn) layer acts as an electron receptor and limits the macroscale throwing power of Γ-Fe3Zn10.