For a Grade 2 Titanium Heating Coil in a 10% Ferric Sulfate Solution at 70°C, What Is the Critical Fe³⁺/Fe²⁺ Ratio Below Which the Passive Film Collapses and Uniform Corrosion Accelerates?

Fe3+ ions in 10% ferric sulphate (Fe2(SO4)3) at 70oC. Fe3+ is an oxidising agent that maintains the titanium passive film. With time or with use , the Fe ^{3+} is reduced to Fe ^{2+} , reducing the oxidising power . The crucial ratio of Fe3+ / Fe2+ for the passive film stability is 0.1. Below this ratio, the passive film breaks down and the uniform corrosion rate increases from 0.02-0.05 mm/yr to 0.20-0.50 mm/yr.

Fe³⁺/Fe²⁺ Ratio vs. Quantitative Corrosion Rate

Fe³⁺/Fe²⁺ Ratio Dominant Mechanism Corrosion Rate (mm/y)1.0 Passive (oxidising) 0.01-0.03
0.5-1.0 Passive 0.02-0.05Transition 0.05-0.15 0.05-0.1 Film collapse 0.15-0.40 <0.05 Active corrosion 0.40-1.00 Fe³⁺/Fe²⁺ Management Guide

Ratio Recommended Action Method of Re-oxidation >0.5 None N/A 0.2-0.5Add H2O2 (10 ppm) or sparge air Weekly Monitor
0.1–0.2 Add oxidiser H2O2 at 50 ppm
<0.1 Add Fe³⁺ or substitute solutionAdd iron sulphate
Engineering Advice

For ferric sulphate solutions, occasional aeration or addition of hydrogen peroxide should be used to maintain Fe 3+/Fe 2+ ratio > 0.2. The engineer controls the ratio to avoid the collapse of passive film.