Hydrochloric acid is one of the most difficult corrosives for passive alloys to handle. They are susceptible to serious corrosion problems, including high corrosion rate, localized corrosion and hydrogen embrittlement. Stainless steels tolerate very low concentrations of chlorides. Nickel-based alloys may tolerate high concentrations of chlorides, but the temperature cannot be high. Even with the presence of oxidizing impurities, titanium is not compatible with strong acids. Tantalum is impressive with respect to its resistance to hot, strong acids. It is, however, susceptible to hydrogen embrittlement. Like tantalum, zirconium resists attack by hot, strong acids. Zirconium resists better than tantalum to hydrogen embrittlement when oxidizing impurities are absent. In fact, zirconium prefers reducing conditions. It is the oxidizing condition that may induce corrosion problems in zirconium. The performance of zirconium in HCl solutions is greatly affected by many factors. Zirconium gives optimal performance in HCl environments when affecting factors are properly addressed. This paper discusses metallurgical, design, fabrication, environmental and maintenance issues that affect the performance of zirconium in HCl solutions. Examples will be given on the successes and failures of zirconium in HCl services.
Hydrochloric acid, HCl, is among the most difficult corrosives for passive alloys to handle1. They induce serious corrosion problems, including high corrosion rate, localized corrosion and hydrogen embrittlement. Stainless steels tolerate low concentrations of HCl. Nickel-based alloys may tolerate high concentrations of HCl, but the temperature limit is low. Even with the presence of oxidizing impurities, titanium cannot handle strong acids. Tantalum is impressive with respect to its resistance to hot, strong acids. It is, however, susceptible to hydrogen embrittlement. It lost 33% of its ductility after exposing to 11M HCl at 70°C for 1,000 hours2. Like tantalum, zirconium resists attack by hot, strong acids. Unlike tantalum, zirconium resists well to hydrogen embrittlement under many reducing conditions. Zirconium retained 100% of its ductility after exposing to 11M HCl at 70°C for 1,000 hours2. In fact, zirconium prefers reducing conditions. It is the oxidizing condition that may induce corrosion problems in zirconium. Types of corrosion problems may occur include pitting, intergranular corrosion and stress corrosion cracking (SCC) depending on acid concentration and stress. The performance of zirconium in oxidizing HCl is greatly affected by many factors. When these factors are properly addressed, zirconium may perform acceptable even in oxidizing HCl. This paper will, first, summarize the corrosion properties of zirconium in HCl and, then, discuss metallurgical, design, fabrication, environmental and maintenance issues that affect the performance of zirconium in oxidizing HCl. Readers are encouraged to go to references for more information.
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As indicated in Fig. 1, zirconium resists attack by HCl in all concentrations at temperatures to boiling and above. Very few metals and alloys have the capability to resist this acid so well. Common metals and alloys rely on forming passive oxide films in somewhat oxidizing environments for their corrosion resistance. They have the difficulty of forming passive oxide films in reducing acids like hydrochloric acid and exhibit active corrosion.
