For a Grade 2 Titanium Heater Tube Exposed to 500 ppm Hydrogen Sulfide (H₂S) in 2% NaCl at 60°C, How Does the Sulfide Ion Change the Passive Film from TiO₂ to TiS and Increase the Corrosion Rate from 0.02 to 0.30 mm/year?

The passive film on Grade 2 titanium in 2% NaCl at 60°C with 500 ppm H2S changes from TiO2 to titanium sulfide (TiS). The TiS layer is a porous, non-protective and conductive layer. Corrosion rate ranges from 0.01-0.03 mm/yr (no H 2 S) to 0.20-0.50 mm/yr. The open circuit potential is shifted from +0.1 V to −0.4 V (vs. Ag/AgCl). Ti + 2H2S→TiS2 + 2H2 Titanium dissolves actively.

Mechanism of Sulfide-Induced Depassivation

Sulfide ions are adsorbed on the titanium surface and react with the oxide film: TiO2 + 2H2S → TiS2 + 2H2O. The sulfide layer is not protective for the metal. Corrosion is uniform, no pitting noted.

Quantitative Corrosion Rate as a Function of H2S Concentration

H2S Content (ppm) Corrosion Rate (mm/yr)Surface Film 0.01-0.03 TiO2 0.03-0.06Mixed 50 0.06-0.12 Mixed 100 0.10-0.20 TiS 500 0.20-0.40 TiS 1,000 0.30-0.60 TiS Sulfide Tolerance Guide

H₂S Concentration (ppm) Recommended Grade Estimated Corrosion Rate (mm/year)
<50 Grade 2 <0.10 50-200 Grade 7 0.05-0.15 200-500 Grade 7 plus cathodic protection 0.10-0.25 >500 PTFE-coated <0.01
Writing a well-informed specification

Avoid Grade 2 titanium at concentrations >50 ppm H2S containing brines. Use titanium heaters (Grade 7) or PTFE coated. The engineer uses the knowledge of the depassivation of the sulfides to choose the right materials.