Laboratory testing was used to demonstrate the differences in corrosion rate of typical zirconium 702 (R60702) containing approximately 2400 ppm tin, and low-tin zirconium 702 containing approximately 1400 ppm tin. Sample material was heat-treated at 770ºC for 1 hour. Standard immersion testing was conducted in 60-70% sulfuric acid at boiling temperatures. Autoclave testing was performed at the same concentrations of acid, but at temperatures up to 180°C. Updated 5 mpy isocorrosion curves for zirconium in sulfuric acid are presented. The effect of heat treatment on corrosion resistance in sulfuric acid applications is discussed.
Zirconium has been used for handling sulfuric acid at elevated temperatures for over 35 years. Along with tantalum, zirconium is one of the few materials capable of handling sulfuric acid above the boiling point. Zirconium has had an almost continuous evolution, starting in the early 1950?s with the development of the nuclear grades of zirconium (Zircaloy-2 and Zircaloy-4). The study and modification of nuclear grades of zirconium continues to this day in several laboratories around the world. This development continued with the introduction of Zr702 (here after referred to by its generic name Zr702) for the chemical processing industry (CPI) in the mid 1960?s. Many of these efforts are in applications aimed at improving its performance in sulfuric acid as well as other mineral acids. In the past, even though the Zr702 met all ASTM requirements, the chemistry of zirconium was quite variable in tin content and other elements.
Fabrication methods, including improved methods of heat treatment, have also evolved as new technologies and techniques have become available and have been accepted into common practice.
With few exceptions, zirconium has served well in severe sulfuric acid applications. One example occurred in the late 1960?s. Rohm and Haas experienced accelerated corrosion of the weldments in their Zr702 process equipment handling sulfuric acid. 1,2 In response, Rohm and Haas was among the first to develop a heat treatment for Zr702 welds. Their study revealed that heat-treating zirconium weldments at 774ºC for one hour per inch (2.54 cm) of thickness reduced this tendency for preferential corrosion in the weld and heat affected zone (HAZ) metal. Wah Chang has continued this study and determined much of the current information available on heat treatments for zirconium.
Celanese Chemicals also contributed to current understanding of the effects of chemistry on the corrosion behavior of zirconium. In 1996, Celanese found that the high tin level in the Zr702 (R60802), which includes both Zircaloy-2 and Zircaloy-4, was detrimental in the production of acetic acid.3
Over the years, as a result of the constant push for higher efficiencies in CPI production units and reducing waste streams, acid concentration and operating temperatures have increased. In sulfuric acid service in the 60-70% concentration range, it was observed that in some cases zirconium exhibits low corrosion rates and, in some cases, the corrosion rate was at a higher level. From studies undertaken in the late 1980?s, it was known that certain intermetallic compounds, including tin, have an adverse effect on the corrosion resistance of zirconium, particularly in sulfuric acid service.4,5
This current study of the effects of tin content on the corrosion behavior of Zr702 in sulfuric acid is a continuation of this evolutionary process. Table 1 gives representative alloy compositions for Zr702 and Zircadyne®-704 (here after referred to its generic name of Zr704) material.
TABLE 1 CHEMICAL COMPOSITION OF COMMERCIAL AND NUCLEAR GRADES OF ZIRCONIUM
