Intermetallic spike growth mechanisms in 316L stainless steel in contact with molten 55%Al-Zn metal coating alloy

BlueScope Product Innovation and Technology, Port Kembla, NSW 2505, Australia
e-mail: nega.setargew@bluescopesteel.com

Received: 22 December 2015
Accepted: 11 March 2016

Abstract

Immersed pot hardware used in 55%Al-Zn metal coating pots is fabricated from austenitic stainless steel grade 316L. This alloy is selected for immersed pot hardware applications because of its good corrosion performance in contact with the metal coating bath. The steel however, can be susceptible to excessive corrosion damage after prolonged service at elevated temperatures. The types of corrosion damage observed in immersed pot hardware include intergranular and pitting corrosion and the formation of cone shaped intermetallic alloy spikes, often referred to as horn growth defects. When 316L reacts with molten 55%Al-Zn coating bath, two distinct interfacial alloy layers are formed. The alloy layers provide a diffusion barrier and minimise excessive dissolution of the steel and prevent the formation and growth of the alloy spikes. Laboratory immersion experiments show that when the two interfacial diffusion barrier alloy layers are absent, alloy spike is promoted. It is also found that the diffusion of aluminium into the substrate steel leads to γ to α-ferrite transformation in the steel resulting in a self-sustaining growth process for continued intermetallic alloy spike growth. The microstructures of the laboratory produced alloy spikes are compared with those observed in sink rolls and other immersed pot equipment. Alloy spike growth mechanisms are discussed together with experimental verification of the proposed alloy spike growth mechanisms.