Passivation of Titanium Alloy with Aminopolycarboxylic Acid-Based HF Acid Fluid

Abstract

Abstract Exposure of a titanium (Ti)-based alloy to a hydrofluoric (HF) acid-containing fluid can result in corrosion of the alloy and potentially complete dissolution, depending on the temperature and HF acid concentration. The demanding conditions encountered in harsh environments, such as geothermal and high-pressure/high-temperature (HP/HT) wells, require suitable treatment fluids that can be used to remove wellbore scale or accumulated minerals in the near-wellbore (NWB) area. The use of HF acid fluids can then preclude the effective treatment of such formations unless corrosion can be managed. There is no known effective corrosion inhibitor that can be used in conjunction with HF acid on Tialloys. This paper presents findings that can provide advantages for the design of appropriate corrosion inhibitors for these alloys, and similarly, HF acid-sensitive metallurgies. A HF acid-containing fluid based on an aminopolycarboxylic acid (APCA)chelant has the ability to significantly prevent and even minimize the corrosive effects of such fluid on Ti. The chemical combination of chelant/HF acid and conventional corrosion inhibitor plays a functional role in the prevention of the complete corrosion of Ti alloys in the presence of HF acid. Suitable chelant concentrations range up to 0.6 mol/L, corrosion inhibitor loadings up to 4% w/v, up to 2%HF acid concentration, and a pH of 1 to 4. Corrosion loss screening spanned 250 to 300°Fwith APCA and HF acid and up to 375°F with low to no HF acid in the fluid, and specimens treated using an inhibitor suitable for hydrochloric (HCl) acid-based fluids were protected for up to 4 hr. Reasonable corrosion rates less than 0.05 lbm/ft2 were obtained based on the pH, temperature, and HF acid concentration, and to a lesser degree, the amount of chelant. The Ti specimens suffered from pitting, and this was the major negative effect observed. Notably, the use of a conventional yet inhibited HCl/HF acid fluid corroded and dissolved the specimen, unlike all the tests where the chelant/HF acid combination was used. The specimens were analyzed using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS) and surface elemental mapping, and the spent fluids were analyzed for dissolved ions (ICP); such analyses serve to elucidate the effects of the chelant and HF acid role in passivating the surface of the alloy.