For a Grade 7 Titanium Heater Tube Exposed to 1,000 ppm Hydrogen Sulfide (H₂S) in 3% NaCl at 80°C, How Does the Sulfide Ion Compete with Chloride for Surface Sites and Change the Pitting Potential from +0.3V to -0.1V vs. Ag/AgCl?

The pitting potential for Grade 7 titanium in 3% NaCl at 80°C is +0.3 to +0.5 V vs. Ag/AgCl. Adding 1000 ppm hydrogen sulfide (H 2 S) lowers the pitting potential to -0.2 to 0.0 V. The negative shift of 400-500 mV is associated with the stronger adsorption of sulfide ions (S 2- ) on the titanium surface than chloride ions. The adsorbed sulfide produces a titanium sulfide (TiS) layer which is less protective than TiO2. Pitting initiates at much lower potentials and the time to pitting is reduced from >1,000 hours to 50-200 hours.

Mechanism of Sulfide Induced Depression of Pitting Potential

Sulfide ions are soft bases and they make strong covalent interactions with titanium surface atoms . They replace absorbed oxygen and hydroxide and hence impede the creation of a stable TiO2 passive film. It is coated by a mixed oxy-sulfide coating with enhanced electronic conductivity and reduced corrosion resistance. For more positive potentials than -0.2 V local breakdown of this film occurs and pitting starts.

H₂S Concentration versus Quantitative Pitting Potential

For Grade 7 titanium, the following has been confirmed from controlled testing at 80 C with different H2S levels in 3% NaCl: At 0 ppm H 2S the pitting potential is in the range of +0.3 to +0.5 V and the time to pitting at +0.2 V is > 1000 hours. The pitting potential is +0.2 to +0.3 V at 10 ppm H2S, and the time to pitting at +0.2 V is 500 to 1,000 hours. 50 ppm H2S Potential: 0.0 to +0.2 V Time to pitting: 200-500 hours. The potential is -0.1 to +0.1 V at 100 ppm H2S and the duration to pitting is 100 to 300 hours. Potential is -0.2 to 0.0 V and time to pitting is 50-150 hours at 500 ppm H2S. At 1,000 ppm H₂S, the potential is -0.2 to -0.1 V and the time to pitting is 20-80 hours.

Grade 7 Chloride Service Sulfide Tolerance Guide

H 2 S Concentration (ppm) Pitting Potential (V vs. Ag/AgCl)Pitting Time at Open Circuit (hours)Sour Service Recommendation <10 >+0.2 >1,000 Yes 10-50 0.0 to +0.2 300-1,000 Limited 50-200 -0.1 to +0.1 100-500>200 <100 <-0.1Not advised
Developing an Informed Specification

Grade 7 titanium is not recommended for sour service with H₂S >50 ppm. Use Grade 12 or higher alloy. The engineer selects acceptable materials by knowing the depression of sulfide-induced pitting potential.