Corrosion of Co-Cr-W alloy in liquid zinc
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11/8/03
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Corrosion of Co-Cr-W Alloy in Liquid Zinc WANG XIBAO The corrosion of Co-Cr-W alloy in the presence of both static and dynamic liquid zinc has been investigated at 465 °C. It was shown that the Co-Cr-W alloy is severely attacked by liquid zinc especially under dynamic condition. The corrosion rate of this alloy in the moving liquid zinc was about 1.5 times higher than in static liquid. Corrosion of the Co-Cr-W cast alloy in liquid zinc can be characterized as alloying (of Co-Zn and Fe-Zn) and dissolution (of Cr and W atoms from Co-base phase into liquid zinc). The static corrosion of Co-Cr-W alloy is characterized as preferential corrosion in eutectic groups and uniform corrosion of Co-base phase. The dynamic corrosion of this alloy is characterized as frequent break of the corroded surface layer and preferential corrosion in grain boundaries besides uniform corrosion. The Fe content of this alloy is another important factor affecting its corrosion rate in liquid zinc. Based on these results, it is suggested that the amount of eutectic groups and the elemental Fe content in the Co-Cr-W alloy should be limited in order to improve its corrosion resistance in liquid zinc. Moreover, the refinement of its grain structure will also be useful for the improvement of its corrosion resistance.
I. INTRODUCTION
ZINC, due to its excellent corrosion-proof performance in air surroundings, has been used as an anticorrosion coating material for iron and steels for nearly 100 years. Many articles on the research of corrosion of various materials by liquid zinc were published in the 1960s and 1970s.[1–4] With the recent increasing demands on the galvanized steel strips by automobile and home electric apparatus industries, the research of corrosion-resistant materials against liquid zinc is attracting attention again in the materials research field.[5,6] It has been acknowledged that molten zinc has a high affinity for most of the metals and alloys. In modern steel strip hotdip galvanizing line, one of the most difficult problems may be the corrosion of machinery parts such as agitators, sink, and stable rolls in the liquid zinc. In fact, the attack of a solid material by a liquid metal is found in various engineering fields such as casting, brazing, hot-dip coating, and heat transfer using liquid metals as coolants. For about 40 years, research in corrosion of liquid metals has never ceased.[7–13] Some advanced corrosion-proof materials in liquid zinc such as ceramics and C/C composites have been reported.[14,15,18] Usually, these materials exhibit excellent corrosion resistance in liquid metals; however, most of them are difficult to process. Therefore, the scale of these materials adopted in the hot-dip galvanizing industry will not be enlarged until the cost of parts made of these materials gets less expensive. The Co-Cr-W alloy, due to its good mechanical performance at elevated temperature and corrosion resistance in liquid zinc, has been one of the important materials in building the parts tha
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