The reactivity of 316L stainless steel in 55%Al-Zn bath

BlueScope Steel Limited, Australia

Received: 8 April 2013
Accepted: 4 June 2013

Abstract

Static immersion experiments were conducted to assess the reactivity of austenitic stainless steel grade 316L in contact with molten 55%Al-Zn alloys. The static immersion tests were carried out at bath temperatures of 600 to 650 °C and continuous immersion times of 24 to 645 h. The level of reactivity was assessed by measuring the alloy layer thickness that formed on the test coupons after the immersion experiments. The reaction of 316L in contact with molten 55%Al-Zn bath results in the formation of two distinct alloy layers on the surface of the steel: one alloy layer forms adjacent to the steel surface (designated “alloy layer 1”) and the other formed between alloy layer 1 and the bath (designated “alloy layer 2”). Characterization of the nature of the alloy layers was carried out by optical and scanning electron microscopy. Analyses of the chemical composition of alloy layer 1 by SEM/EDS (and taking into account the Al/(Fe + Cr + Ni) ratio) suggest that the alloy layer is most likely based on FeAl₂ intermetallic phase. Alloy layer 2, on the other hand, consists of modified ø_5c (the normal dross intermetallic phase modified by the substitution of Fe by Cr). After 27 days of continuous immersion at 600 °C, the 316L sample immersed in 55%Al-Zn bath had a total alloy layer thickness of 423 ¯m. The alloy layer growth is diffusion controlled and follows a parabolic rate law: at bath temperatures of 600 to 650 °C and continuous immersion times of up to 27 days, the alloy layer thickness increases approximately with the square root of immersion time. The parabolic rate constants for the total alloy layer thickness growth were: 2.84 × 10^-7 m.s^−1/2 at 600 °C, 3.34 × 10^-7 m.s^−1/2 at 615 °C and 6.76 × 10^-7 m.s^−1/2 at 650 °C. Growth of the alloy layers takes place predominantly at the interface between alloy layer 1 and alloy layer 2, and in part at the substrate/alloy layer 1 interface. The growth of alloy layer 2 is dominated by the inward diffusion of aluminium, while both inward diffusion of aluminium and outward diffusion of iron are believed to play a role in the formation of alloy layer 1.