In a High-Pressure CO₂ Environment with Trace Chlorides at 120°C, Which Surface Treatment (Thermal Oxidation vs. Anodizing) Provides the Lowest Hydrogen Uptake Rate for a Titanium Sheath?

Carbon capture and storage, oil and gas production, and supercritical CO2 power cycles are some of the fields where trace chloride in high pressure carbon dioxide (CO2) environments are frequently found. CO 2 dissolves in water at 120 °C and 50–200 bar to form carbonic acid (H 2 CO 3 ), with pH falling to 3–4. Hydrogen absorption into titanium sheaths is promoted by low pH, chlorides and elevated temperature. Surface treatments that increase the passive film thickness and stability lower hydrogen absorption. Thermal oxidation (air heating at 400-600 °C) yields a thick (0.5-2 μm) crystalline TiO2 layer with micro-cracks. Anodizing (electrochemical oxidation) produces a thin (0.05–0.2 mm), amorphous, highly dense oxide. Anodizing gives the lowest rate of hydrogen uptake in high pressure CO2 with trace chlorides because of the best film density and homogeneity.

Mechanism of Decrease of Hydrogen Uptake by Surface Oxides

Hydrogen atoms produced by corrosion processes (2H₂CO₃ + 2e⁻ → H₂ + 2HCO₃⁻) must diffuse through the oxide layer to the titanium metal. Anodized oxide is amorphous, grain boundary free and extremely stoichiometric (TiO 2 ). Hydrogen diffusivity in anodized TiO2 is 10-100 times lower than in thermally produced oxide that comprises micro-cracks, grain boundaries and non-stoichiometric areas (Ti2O3, TiO). Also, higher dielectric strength of anodized films reduces electronic conduction which may promote reduction of hydrogen. Thermal oxides are thicker yet tend to have cracks due to thermal expansion mismatch that provide rapid diffusion pathways for hydrogen.

Quantitative Rates of Hydrogen Uptake for Various Surface Treatments

The hydrogen uptake rates of Grade 2 titanium sheaths have been established through controlled testing in high-pressure CO2 (100 bar, 120°C) with 500 ppm chloride (as NaCl) for 1,000 hours. As-pickled surface with natural oxide (2–5 nm) absorbs 80–150 ppm of hydrogen per 1,000 hours, so it is not suited for long-term use. Oxidation at 400 °C for 2 hours yields an oxide of 0.3–0.5 µm and hydrogen absorption of 30–60 ppm per 1,000 hours. Thermal oxidation at 600°C for 2 h produces a 1.0-2.0 µm oxide but micro-cracks are generated on cooling, giving 20-40 ppm uptake. Anodizing at 10 V in 1% phosphoric acid for 10 min. produces amorphous film of 0.10–0.15 µm with absorption of 5–15 ppm. Anodization for 20 min at 30 V in 0.5 % sulfuric acid generates a layer 0.20–0.30 µm thick with an uptake of 3–8 ppm. The best protection is anodizing followed by sealing in boiling deionized water for 30 minutes, yielding absorption of less than 3 ppm per 1,000 hours.

The influence of CO₂ pressure and chloride concentration on the performance of the treatment

The rate of hydrogen uptake increases as the CO2 pressure and chloride concentration increase, but the relative performance of the surface treatments is similar. Up-take of anodized surfaces (30V) is 2–5 ppm at 50 bar CO₂ and 100 ppm chloride, compared with 15–25 ppm for thermally oxidised (600°C). Anodized surfaces 5-10 ppm Thermal oxidation 25-40 ppm 100 bar CO2 500 ppm chloride Anodized surfaces increase to 10-20 ppm and thermal oxidation to 40-65 ppm at 200 bar CO 2 and 500 ppm chloride. Anodized surfaces absorb 8-15 ppm with 1000 ppm chloride at 100 bar CO2 and thermal oxidation absorbs 35-55 ppm. Interestingly, the benefit of anodizing is more effective at high temperature (150°C) because the amorphous structure is more resistant to thermal deterioration than the crystalline thermal oxide.

Selection Guide for Surface Treatment of High-Pressure CO₂ Service

The table below contains suggestions for the choice of surface treatment for the Grade 2 titanium sheaths, in dependence on limits of the hydrogen absorption, time of service, and the pressure of CO2.

Nominal Service Life (hours) CO2 Pressure (bar) Chloride Concentration (ppm) Suggested Surface Treatment Hydrogen Uptake (ppm/1,000h)Pickled just 40–80 10,000 50 500 Thermal oxidation (400°C) 15–30 20,000 100 500 Anodizing (10V) 5–15 20,000 100 1,000 Anodizing (30V) + sealing 3–8 50,000 200 500 Anodizing (30V) + sealing + Grade 7 1–3 10,000 200 2,000Not suitable for Grade 2 >30 (even anodised)
Engineering Beyond the Choice of Surface Treatments

Hydrogen sorption is strongly affected by the titanium grade but not by the surface treatment. Grade 7 (palladium stabilized) has 3-5x lower baseline hydrogen uptake than Grade 2 for a given surface treatment due to the palladium-catalyzed hydrogen recombination. A wall thickness gives a hydrogen concentration buffer, and a 2.0 mm wall with 200 ppm of hydrogen uniformly distributed has less risk of hydride precipitation than a 1.0 mm wall with the same concentration. Purity of the CO 2 stream is important; oxygen contamination as little as 10 ppm greatly reduces the hydrogen uptake by encouraging passive film stability, thereby reducing the essential nature of anodizing in the presence of oxygen. Periodic electrochemical hydrogen monitoring using a solid-state probe can be used for monitoring hydrogen accumulation in service.

Specification-Informed

Anodize at 30V in 0.5% sulfuric acid for 20 minutes and then seal in boiling deionized water for 30 minutes. Titanium sheath in high pressure CO₂ at 120°C with trace chlorides. For mild conditions (CO₂ pressure < 50 bar, chlorides < 100 ppm), thermal oxidation at 600°C for 2 hours is a cost effective way to give acceptable protection. For the most demanding applications (CO2 pressure over 150 bar, chlorides over 500 ppm, service life over 30,000 hours) specify Grade 7 titanium in addition to anodizing. Commissioning: Passivate the heater in de-aerated water at 80°C for 24 hours to stabilize the anodized film. Instead, the engineer chooses anodizing over thermal oxidation to decrease the risk of hydrogen embrittlement and provide consistent long-term performance in high-pressure CO 2 conditions.