Characteristics of Tantalided and Hafnided Samples in Highly Corrosive Electrolyte Solutions

An extended feasibility study was made of the use of the metalliding process to the application of highly impervious diffusion coatings on inexpensive conductive metals which normally corrode in acid electrolytes. Tantalum and hafnium, which proved highly corrosion resistant in acid media, are expensive for large scale use in components and cannot be deposited as acid impervious coatings onto inexpensive metals by way of conventional deposition techniques. It was demonstrated that the metalliding process was uniquely suited for the deposition of Ta and Hf as acid resistant diffusion coatings on stainless steel and nickel. The chemical and electrochemical corrosion rates of tantalided and hafnided samples of nickel and AISI Type 316 stainless steel in strong electrolyte solutions were determined. The electrolyte media included sulfuric, nitric, hydrochloric, phosphoric acid and sodium chloride. Electrolyte concentrations ranged from 5 to 105% by weight and temperatures from 25 to 150 C (77 to 302 F). The behavior of the metallided samples in the strong electrolyte solutions was found to be related to the nature of the coating which is controlled by the positions of the "iding” ion and major substrate ions in the fluoride or complex fluoride electromotive series, as well as the alloying properties of the "iding" metal and substrate. The thermodynamic and alloying properties of nickel and tantalum are such that the plating parameters were optimized to give an alloy coating with a surface which was nearly pure tantalum and impervious to acid. The tantaliding process was explored for applications to acid fuel cell current collectors and electrochemical machining.