When a Grade 2 Titanium Heater Tube Is Exposed to Humid Ammonia Vapor at 150°C, How Does the Ammonium Hydroxide Concentration in the Condensate Film Modify the Hydrogen Absorption Rate and the Crack Incubation Time?

In humid ammonia vapour at 150 \u00b0C, water condensation occurs on the titanium heater surface, generating a thin coating that absorbs NH3 to generate ammonium hydroxide (NH4OH). The amount of NH_{4}OH in the film of condensate depends on the relative humidity and the partial pressure of NH_{3}. NH4OH is alkaline (pH 10~12) and damages the titanium passive coating, facilitating hydrogen absorption. The hydrogen uptake increases and the crack incubation time decreases with NH4OH concentration.

Hydrogen absorption vs. NH4OH concentration (quantitative)

NH4OH Concentration in Condensate (wt%) pH Hydrogen Absorption Rate (ppm/1,000h) Crack Incubation Time (hours) 0 (clean water) 7 5–15 >10,000 1 10 20–40 3,000–6,000 5 11 50–100 1,000–2,500 10 12 100–200 400–1,000 20 12.5 200–400 150–400 Effect of Temperature on NH4OH Aggressiveness

Temperature (°C) NH₄OH 10% – H Absorption (ppm/1,000h) NH₄OH 10% – Crack Incubation (hours) 100 60–120 800–1,500 150 100–200 400–1,000
200 180-350 200-500 NH4OH concentration control guide

Condensate NH₄OH (%) Recommended Action Life Expected (1.5 mm wall)
<1 None >10 years 1–5 Lower humidity or NH₃ 5–10 years
5–10 Purging with inert gas 2–5 years10 Not acceptable-redesign <2 years
Recommended Engineering

For humid ammonia vapour service, limit relative humidity to <50%, and NH₃ partial pressure to <0.1 atm, to maintain condensate NH₄OH <5%. The engineer restricts ammonium hydroxide concentration to lower hydrogen absorption and increase crack incubation period.