In a Titanium Immersion Heater for a 15% Sodium Chromate Solution at 70°C, How Does the Chromate Ion (CrO₄²⁻) Raise the Pitting Potential from +0.2V to +0.8V vs. Ag/AgCl and Passivate the Titanium Surface?

CrO 4 2− in 15 % sodium chromate at 70 °C : The chromate ion is a strong oxidizer, and raises the pitting potential of Grade 2 titanium from +0.2 V (in chloride) to +0.6-0.9 V. The corrosion rate is 0.005 to 0.015 mm/yr. It aims to incorporate chromate ions into the titanium oxide coating to produce a combined Ti-Cr oxide which is more stable and protective than TiO2 alone. The passivation remains even in presence of chlorides up to 1,000 ppm.

Mechanism of Chromate Passivation

Chromate ions are converted to Cr 3+ which is incorporated into the TiO 2 lattice. The mixed oxide has a lower defect density and better resistance to attack by chlorides. The film thickness is 5-10nm.

Pitting Potential vs Chromate Concentration : Quantitative

Na2CrO4 concentration (%) Pitting potential (V vs. Ag/AgCl) Corrosion rate (mm/year)
0 +0.1 to +0.3 0.01-0.03 1 +0.3 to +0.5 0.008-0.015 5 +0.5 to +0.7 0.005-0.010 10 +0.6 to +0.8 0.003-0.008 15 +0.7 to +0.9 0.002-0.005
Creating an Informed Specification

For titanium heaters in chloride containing solutions add 5-10% sodium chromate as a corrosion inhibitor. The engineer increases the pitting potential so that pitting does not occur. Chromates are toxic and regulated. Look for alternatives such as molybdates.