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 ø5c (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.