Effect of heat treatment process on microstructure and corrosion resistance of Al–10%Si–24%Zn coating

• Published in: Surface & coatings technology Vol. 401; p. 126305
• Main Authors: Yunying, Xie, Wenxin, Hu, Yi, Cheng, Guangxin, Wu, Jieyu, Zhang
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15-11-2020; Elsevier BV
• Subjects: Aluminum; Al–Zn–Si coating; Boron steels; Cathodic coating (process); Cathodic protection; Corrosion effects; Corrosion potential; Corrosion prevention; Corrosion resistance; Corrosion resistant steels; Electrochemical corrosion; Ferrous alloys; Fe–Al intermetallic phase; Grain boundaries; Heat treating; Heat treatment; High temperature; Intermetallic compounds; Iron aluminides; Liquid metal embrittlement; Microstructure; Oxidation resistance; Protective coatings; Silicon; Substrates; Surface treatment; Two-step method; Zinc; Zinc coatings; Al–Zn–Si coating; Two-step method; Fe–Al intermetallic phase; Electrochemical corrosion; Liquid metal embrittlement
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2020.126305

Al–10%Si coating has been widely used in the surface treatment of hot-press-forming steel such as 22MnB5 steel, and it exhibits excellent high-temperature oxidation resistance but poor corrosion resistance; thus, Zn was added to Al–10%Si coating to improve its corrosion resistance. The microstructural evolution and elemental distribution of AS24Z (Al–10%Si–24%Zn) coating before and after undergoing two-step heat treatment (pre-alloying and then austenitizing) were investigated. The results show that the coating was composed of Fe2Al5 and FeAl after holding at 900 °C for 5 min, while it was composed of FeAl and Fe3Al after holding at 1000 °C for 5 min. The location and the chemical state of Zn in the coating after heat treatment were determined. After holding at 900 °C for 5 min, the Zn in the Fe2Al5 phase was expected to accumulate at the Fe2Al5 grain boundaries due to the Zn content (approximately 10 at.%) in the AS24Z coating, which was lower than the solubility limit (approximately 30 at.%) of Zn in Fe2Al5. However, the Zn in the Fe2Al5 phase dissolved to form the Fe2Al5Znx phase in the form of substituted atoms with a Zn content in the range of 3–4 at.%. The content of Zn in the entire coating was very low after holding at 1000 °C for 5 min due to the volatilization of Zn. The effect of Zn on the susceptibility of liquid metal embrittlement was studied. It was difficult for the liquid Zn to penetrate the thick Fe2Al5 layer in the AS24Z coating with relatively high Al content; thus, the Fe2Al5 layer prevented the liquid metal embrittlement of 22MnB5 steel. The corrosion resistances of the Al–10%Si and AS24Z coatings were compared using a polarization curve. Compared with the Al–10%Si coating, the AS24Z coating exhibited a lower self-corrosion potential, which indicates that after austenitization at 900 °C or 1000 °C, the AS24Z coating can still provide excellent cathodic protection for the steel substrate.
•Two-step method was used in heat treatment process.•Position and form of Zn in Fe2Al5 phase were characterized by TEM.•Zn dissolved in Fe2Al5 phase to form Fe2Al5Znx phase after austenitizing.•Fe2Al5 layer prevented liquid metal embrittlement of steel.•Al–10%Si–24%Zn coating can provide excellent cathodic protection to steel.