In a Titanium Tube Bundle Heater for a 20% Ammonium Nitrate Solution at 80°C, How Does the pH (4.5 vs. 6.5) Change the Passivation Current Density and the Risk of Stress Corrosion Cracking?

The pH of the solution (20%) has a great influence on the passivation behaviour of titanium in ammonium nitrate (NH4NO3) at 80°C. At pH 6.5 (close to neutral), the passive film is stable with a passivation current density of 0.05–0.1 µA/cm² and little SCC risk. At pH 4.5 (acidic from hydrolysis) the passivation current density increases to 0.5 – 2 µA/cm2 and the probability of stress corrosion cracking (SCC) dramatically increases due to the aggression of nitrate ion at lower pH.

Quantitative Passivation Current Versus pH

For Titanium grade 2 in 20% NH4NO3 at 80 deg C:

pH Passivation Current Density (µA/cm2) SCC Initiation Time (hours) SCC Risk
6.5 0.05-0.10 >5,000 Low 6.0 0.08-0.15 3,000-5,000 Low-Moderate 5.5 0.15-0.30 1,000-3,000 Moderate 5.0 0.30-0.70 500-1,500 High 4.5 0.50-1.50 200-800Severity 4.0 1.0–3.0 100–400Very bad, but
Interaction Effect of Chloride and Nitrate

pH Chloride (ppm) SCC RiskSuggested Grade 6.5 0 Low Grade 2 6.5 100 Moderate Grade 2 5.5 0 Moderate Grade 2 5.5 100 High Grade 7 4.5 0 High Grade 7 4.5 100Severe Grade 7 + isolation pH Control Guide

Operating pH Recommended actionExpected SCC Free Life ( Grade 2)6.0 None >10 years
5.5–6.0 Weekly pH monitoring 5–10 years
5.0-5.5 Adjust pH using NH4OH 3-5 yrs
4.5–5.0 Use Ti Grade 7 5–8 years (Ti Grade 7)
<4.5 Redesigning process <2 years
Recommendation of Engineering

For ammonium nitrate service keep pH > 5.5 for Grade 2 or > 5.0 for Grade 7. Add ammonium hydroxide to neutralise acidity. The engineer manages pH to minimise passivation current density and to minimise stress corrosion cracking.