Nature of the inhibition layer in GA baths^⋆

¹ ArcelorMittal Global R&D - Automotive Products Centre, Voie Romaine BP 30320, 57283 Maizières-Lès-Metz, France
e-mail: jean-michel.mataigne@arcelormittal.com
² École Centrale Paris, Grande Voie des Vignes, 92295 Châtenay-Malabry Cedex, France

Received: 6 December 2013
Accepted: 22 January 2014

Abstract

The nature of the intermetallic layer which forms on the steel surface during immersion in typical galvanizing baths for galvannealed (GA) sheets production has been investigated on two commercial Titanium-stabilized Interstitial-Free (Ti-IF) steel substrates galvanized in baths with different Al contents. Results from this study show that in both cases the inhibition layer is biphasic and composed of a very thin Al-rich phase layer, identified as Fe₂Al₅Zn_x, and a thicker Zn-rich phase layer on top of it, identified as δ. Experimental results also show that the Fe₂Al₅Zn_x phase layer becomes discontinuous when decreasing the bath Al content. Discussions about the mechanisms of formation and the final microstructure of this inhibiting layer are also tackled in this paper by means of the Al-Fe-Zn ternary phase diagram at 460 °C and assumptions to justify any deviation from thermodynamic equilibrium are as well proposed.