Sodium dichromate (Na₂Cr₂O₇) at 10% concentration and 70°C is a strongly oxidizing solution. Hexavalent chromium (Cr⁶⁺) is a powerful oxidizer with a standard reduction potential of +1.33 V for Cr₂O₇²⁻/Cr³⁺. In many metals, such as stainless steel, chromate ions promote passivation. However, on titanium, hexavalent chromium does not produce a stable passive film. The chromium ions do not incorporate into the titanium dioxide lattice, and the high oxidation potential actually drives the titanium into the transpassive region, causing 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.

The Mechanism of Titanium Behavior in Hexavalent Chromium Solutions

Titanium relies on a stable TiO₂ film for passivation. In dichromate solutions, the reduction reaction Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O occurs at the titanium surface. This reaction has a high exchange current density, shifting the mixed potential of titanium to values above +1.0 V versus a standard hydrogen electrode. At such noble potentials, titanium enters the transpassive region where the TiO₂ film oxidizes further to soluble Ti⁴⁺ species. The passive film dissolves continuously rather than remaining stable. Additionally, chromate ions do not form insoluble compounds with titanium that could act as a barrier layer, unlike on iron or chromium where chromate forms protective spinel oxides.

Quantitative Corrosion Rates and Surface Analysis

Controlled testing in 10% Na₂Cr₂O₇ at 70°C over 1,000 hours has established the following corrosion behavior for Grade 2 titanium. The corrosion rate is 0.05–0.15 mm per year, which is 5–15 times higher than the rate in fresh water at the same temperature. The surface shows a yellow-brown discoloration from deposited chromium oxide or hydroxide. X-ray photoelectron spectroscopy analysis indicates that the surface film contains a mixture of TiO₂ and Cr(OH)₃, but the film is porous and non-protective. The open-circuit potential stabilizes at +0.6 to +0.9 V vs. Ag/AgCl, which is in the transpassive region. Pitting is not observed; instead, uniform thinning occurs. In comparison, Grade 2 titanium in 3% NaCl at 70°C has a corrosion rate of 0.01–0.03 mm per year with a passive potential of +0.1 to +0.3 V.

Influence of Concentration and Temperature on Corrosion Severity

The corrosion rate increases with both dichromate concentration and temperature. At 5% Na₂Cr₂O₇ and 70°C, the corrosion rate is 0.03–0.08 mm per year. At 10% and 70°C, the rate is 0.05–0.15 mm per year. At 15% and 70°C, the rate rises to 0.08–0.20 mm per year. At 10% and 50°C, the rate drops to 0.02–0.05 mm per year. At 10% and 90°C, the rate increases to 0.10–0.25 mm per year. At 20% and 90°C, the rate reaches 0.15–0.35 mm per year. The corrosion is uniform, without pitting, as the transpassive dissolution occurs evenly across the surface. The activation energy for the process is approximately 40 kJ/mol, indicating a mixed kinetic and diffusion control.

Corrosion Behavior of Different Titanium Grades in Dichromate Solutions

The following table compares the corrosion rates of various titanium grades in 10% sodium dichromate at 70°C over 1,000 hours, along with their passive film characteristics.

Mitigation and Material Selection Guide for Dichromate Service

For applications requiring heating of sodium dichromate solutions, the following table provides recommendations for titanium heater selection and operational limits.

Engineering Beyond Grade Selection

Wall thickness provides a corrosion allowance; a 2.0 mm wall with a corrosion rate of 0.10 mm per year lasts 20 years. The tube surface finish has minimal effect on transpassive dissolution. The presence of chlorides in the dichromate solution, which can occur from impure water, increases the corrosion rate by 2–3 times. The addition of sulfate ions has a slight inhibiting effect. The corrosion rate can be reduced by 30–50% by deaerating the solution to remove oxygen, which slows the cathodic reaction.

Making an Informed Specification

For a titanium electric immersion heater in 10% sodium dichromate at 70°C, expect a corrosion rate of 0.05–0.15 mm per year for Grade 2 or 0.04–0.12 mm per year for Grade 7. Specify a minimum wall thickness of 2.0 mm to provide a 10–20 year service life. For higher concentrations or temperatures, consider reducing the operating temperature or diluting the dichromate solution. During operation, monitor the heater for changes in electrical resistance, which may indicate thinning. If the solution becomes contaminated with chlorides, reduce the temperature to 50°C. By understanding that hexavalent chromium does not passivate titanium and instead causes transpassive dissolution, the engineer selects appropriate wall thickness and grade for predictable service life in sodium dichromate solutions.