Influences of Stress-Relief Annealing on Microstructure and Corrosion Behavior of Flux-Cored Zn–2Al Filler Metal

Flux-cored Zn–2Al filler metals were subjected to stress-relief treatment at different temperatures. The salt-spray test was subsequently carried out on the specimens, and the corrosion behavior was analyzed. The findings indicated that Zn–2Al filler metal mainly consisted of α-Al and η-Zn phases. Before stress-relief annealing, α-Al showed a prominent continuous banded distribution. At a stress-relief annealing temperature of 100 °C, recovery and limited recrystallization occurred. This weakened the continuous banded morphology and refined the grain size. Under salt-spray exposure, electrochemical corrosion took place within the filler metal. α-Al acted as the cathode, while η-Zn served as the anode. Stress-relief annealing improved grain-boundary stability and thus enhanced corrosion resistance. In addition, the discontinuous distribution of α-Al impeded corrosion propagation. This further improved the corrosion resistance of the filler metal. After 15 days of salt-spray corrosion, the stress-relieved filler metal retained an elongation of 4.96%. In contrast, elevated stress-relief annealing temperatures promoted Al precipitation at grain boundaries and restored the continuous distribution of α-Al. This created preferential pathways for inward corrosion penetration, which degraded corrosion performance. Accordingly, after 15 days of corrosion, the elongation of filler metal annealed at 250 °C decreased to 2.65%.