In a Titanium Electric Immersion Heater for a 10% Sodium Dichromate Solution at 70°C, Why Does the Hexavalent Chromium Ion Not Passivate the Titanium Surface Despite Being a Strong Oxidizer?

Sodium Dichromate (Na2Cr2O7) is a powerfully oxidizing solution, 10% at 70°C. Hexavalent chromium (Cr⁶⁺) is a strong oxidizer, having a standard reduction potential of +1.33 V for Cr₂O₇²⁻/Cr³⁺. Chromate ions are used for passivating several metals, e.g. stainless steel. However, the hexavalent chromium on titanium does not form a stable passive film . The chromium ions are not incorporated into the titanium dioxide lattice. The high oxidation potential actually drives titanium into the transpassive region, resulting in continuous dissolution of the passive film. The corrosion rate of titanium in 10% sodium dichromate at 70°C is 0.05-0.15 mm per year, which is higher than in many neutral chloride solutions. Mechanism of Titanium Behavior in Hexavalent Chromium Solutions A stable TiO2 film passivates titanium. In dichromate solutions, the reduction process Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O happens on the surface of the titanium. Due to the high exchange current density of this reaction, the mixed potential of titanium is shifted to values higher than +1.0 V vs a conventional hydrogen electrode. With such noble potentials titanium reaches the transpassive area in which the TiO 2 layer oxidizes further to soluble Ti 4+ species. The passive film is not stable and dissolves continuously. Chromate ions do not form insoluble compounds with titanium, which may serve as a barrier layer, as they do on iron and chromium, where chromate produces protective spinel oxides. Quantitative Corrosion Rates and Surface Characterization The corrosion behaviour of Grade 2 titanium has been verified by controlled testing for 1,000 hours at 70°C in 10% Na₂Cr₂O₇ as: Corrosion rate is 0.05–0.15 mm/year, i.e. 5–15 times higher than that for fresh water at the same temperature. The surface is of a yellow-brown discoloration due to deposited chrome oxide or hydroxide. X-ray photoelectron spectroscopy investigation shows that the surface coating is a porous, non-protective combination of TiO₂ and Cr(OH)₃. The open-circuit potential stabilizes in the transpassive region at +0.6 to +0.9 V vs. Ag/AgCl . No pitting is visible but uniform thinning is noted. For contrast, the corrosion rate of Grade 2 titanium in 3% NaCl at 70°C is 0.01–0.03 mm/yr with a passive potential of +0.1 to +0.3 V. Effect of Concentration and Temperature on Severity of Corrosion Corrosion rate increases with the increase in dichromate concentration and temperature. Corrosion rates at 5% Na2Cr2O7 and 70°C are 0.03-0.08 mm per year. At 10% and 70°C the rate is 0.05-0.15 mm/year. At 15% and 70°C the rate increases to 0.08-0.20 mm/year. At 10% and 50 °C the rate decreases to 0.02–0.05 mm/year. At 10% and 90°C the rate increases to 0.10-0.25 mm/year. The rate is 0.15–0.35 mm per year at 20% and 90°C. The corrosion is homogenous without pitting, since the transpassive dissolution is uniform over the surface. The activation energy for the process is around 40 kJ/mol indicating mixed kinetic and diffusion control. Corrosion behavior of titanium of various grades in dichromate solutions The table below shows the corrosion rates of several titanium grades in 10% sodium dichromate at 70°C for 1,000 hours and the features of their passive films. Titanium Grade Corrosion Rate (mm/year) Surface Film Composition Open-Circuit Potential (V vs. Ag/AgCl) Pitting Observed Grade 2 (commercially pure) 0.05-0.15 TiO2 + Cr(OH)3 (porous)+0.6 to +0.9 No Grade 7 (Ti-0.15Pd) 0.04–0.12 TiO2 + Cr(OH)3 + Pd +0.7 to +1.0 No Grade 12 (Ti-0.3Mo-0.8Ni) 0.06–0.18 TiO2 + Cr(OH)3 + MoO3 +0.5 to +0.8 No Grade 5 (Ti-6Al-4V) 0.08–0.22 Mixed oxides +0.5 to +0.8 No Mitigation and Material Selection Guide for Dichromate Service The following table is recommended for selection of titanium heaters and operational limits for applications requiring the heating of sodium dichromate solutions. Dichromate Concentration (%) Temperature (°C) Recommended Titanium Grade Expected Corrosion Rate (mm/year) Expected Life (1.5 mm wall) <5 <50 Grade 2 <0.03 >10 years 50–70 5–10 0.05–0.12 3–5 years Grade 7 10–15 70 Grade 7 0.08–0.20 2–3 years >15 >70 Not recommended >0.20 <2 years Engineering Grade Selection Beyond Wall thickness provides a corrosion allowance . A 2.0 mm wall corrodes at a rate of 0.10 mm per year for 20 years . The tube surface finish has little effect on transpassive dissolution. The inclusion of chlorides in the dichromate solution, which may be due to the presence of dirty water, raises the corrosion rate by 2-3 times. The addition of sulphate ions has a small inhibitory effect. Deaeration of the solution for oxygen removal slows the cathodic process and reduces the corrosion rate by 30-50%. Choosing Wisely. For a titanium electric immersion heater in 10 % sodium dichromate at 70 °C, expect a corrosion rate of 0.05 to 0.15 mm per year for Grade 2 or 0.04 to 0.12 mm per year for Grade 7. Use a minimum wall thickness of 2.0 mm for a service life of 10 to 20 years. Dichromate solution can be diluted, or the working temperature lowered for higher concentrations or temperatures. During operation, check the heater for variations in electrical resistance that may indicate thinning. If the solution is polluted with chlorides, lower the temperature to 50 °C. The engineer, knowing that hexavalent chromium does not passivate titanium but transpassively dissolves it, chooses the proper wall thickness and grade for predictable service life in sodium dichromate solutions.