UNS R56404Titanium exposure to warm, naturally-aerated, dilute, HF-containing, fresh acid mixtures does result in unacceptable, elevated etch rates. Several common inorganic salts such as aluminum, molybdate, and/or borate salts, inhibit this attack.
Ronald W. Schutz / Heath W. Walker
Increasing interest in utilizing hydrofluoric acid (HF) for acid stimulation of offshore wells to improve hydrocarbon productivity has raised acid compatibility concerns with titanium alloy riser system components such as Titanium Stress Joints (TSJs). Recent published lab test findings by the authorsconfirmed that UNS R56404Titanium exposure to warm, naturally-aerated, dilute, HF-containing, fresh acid mixtures does result in unacceptable, elevated etch rates, which dramatically increase with temperature and/or acid concentration. These lab studies, however, also revealed that several common inorganic salts such as aluminum, molybdate, and/or borate salts, can effectively inhibit this attack if added in sufficient molar ratios to free fluoride in these fluids.
Corrosion of titanium in these well stimulation acids may also induce significant absorption and/or penetration of byproduct hydrogen, raising additional concern for hydrogen damage to a stressed component if not properly inhibited. Continued lab acid testing of UNS R56404(1) and UNS R56407 titanium coupons confirms that substantial hydrogen uptake and microstructure-dependent penetration can occur during severe HF-containing acid etching. Primary influencing factors are identified from assessment of the effects of CO2 gas saturation; acid formulation, pH, and temperature; fluid flow; and fresh versus spent acid condition; and are considered with respect to inorganic salt inhibitor effectiveness.
Key words: titanium, well acidizing, acid stimulation, hydrofluoric acid, corrosion inhibitor, hydrogen, corrosion