Effects Of Heat Treatment And Surface Condition On The Corrosion Resistance Of Zirconium In Hydrochloric Acid

When proper precautions are taken, Zr702 Zirconium (UNS R60702) can be an excellent material of construction for a wide variety of hydrochloric acid applications. In some circumstances zirconium can be susceptible to stress corrosion cracking, pitting, and/or intergranular corrosion in these types of environments. The effects of different material conditioning, including heat treatment and surface preparation, on the corrosion resistance of zirconium 702 in 15% and 36% hydrochloric acid will be examined. New corrosion data from the corrosion lab will be presented. Practical recommendations will be given for chemical conditioning and heat treatment of Zirconium Grade 702 to be used in hydrochloric acid environments.

Although few metals can survive chemical environments with high concentrations of chloride ions, reactive metals such as tantalum, niobium, zirconium, and titanium can offer superior corrosion resistance in these media. Tantalum and niobium are often avoided, however, due to their high cost and susceptibility to hydrogen embittlement. Also, tantalum and niobium generally have low tensile strength and are not approved for construction of welded pressure vessels in ASME Code Section VIII, Division I. Commercially pure titanium is lower in cost and is an approved material for pressure vessel construction, but is resistant only in low-to-moderate chloride concentrations, i.e. <12 wt%. Of the reactive metals, only commercially pure zirconium offers both high resistance in concentrated inorganic acidic chloride solutions and has sufficient strength to be approved for construction of welded pressure vessels. Zircadyne^{® (1)} 702 Zirconium is presently being used in many reactors, heat exchangers, and other large equipment for the Chemical Processing Industry (CPI) where hot, concentrated, chloride environments, e.g. 12-36% hydrochloric acid, are present. ¹ Although zirconium is generally successful in these environments ², stress-corrosion cracking (SCC) and intergranular attack (IGA) have occasionally been reported in >20% hydrochloric acid and pitting can occur in <20% hydrochloric acid. Several factors are thought to affect the corrosion resistance of Zr702 in concentrated chlorides including:

• Presence of oxidizing ions in solution, e.g. ferric ion (Fe³⁺) or cupric ion (Cu²⁺)

• Surface roughness of the zirconium

• Internal stresses in the zirconium

• Post-weld heat treatment

• Thickness of passive oxide film

• Impurity levels of base material, i.e. Fe, Ni, Cr, and Sn

Zirconium's resistance in hydrochloric acid where ferric ions are present can be dramatically improved by chemically conditioning or pickling its surfaces in an aqueous solution of HNO₃-HF solution. In addition to HNO₃-HF pickling, stress relief, thickening the oxide layer, using a higher purity zirconium, and high temperature heat treatments (i.e. >760C) have been reported to improve corrosion resistance in concentrated HCl. ³

The purpose of this study is to analyze the interactions and synergistic relationships between these variables in terms of their effects on the corrosion resistance of zirconium in 15% and 36% HCl with and without ferric ions present in the solution. A statistical Design of Experiment with Analysis of Variance (ANOVA) will be used.