For a Titanium Sheath Heater That Experiences Occasional Dry-Out (10 minutes per week) in a 5% Sulfuric Acid Tank, What Is the Maximum Number of Dry-Out Events Before the Oxide Spalls and the Tube Thins Excessively?

Titanium is passivated in 5% sulphuric acid (H 2 SO 4 ) at 80°C by a passive oxide coating. There is occasional dry-out, where the liquid level falls below the heater, exposing the hot titanium to air. The quick temperature increase (from 80°C to >200°C) produces thermal expansion mismatch between the oxide scale and the metal, which results in spalling. Each dry-out removes a very thin layer of oxide and some base metal. Cumulative thinning after around 50 dry-out episodes is 0.1–0.2 mm. Perforation of a 1.2 mm wall may develop after 200 occurrences.

Dry-out events and quantitative thinning

Grade 2 titanium 5% H₂SO₄ at 80°C 10-minute dry-out events:

Dry-Out Events Accumulated Metal Loss (mm) Remaining Wall (1.2 mm initial)Oxide Condition
1.20 Whole 0 0
10 0.02–0.05 1.15–1.18 Minor spalling 25 0.05–0.10 1.10–1.15 Moderate spalling 50 0.10–0.20 1.00–1.10 Significant thinning 100 0.20–0.40 0.80–1.00Severe thinning 200 0.40–0.80 0.40–0.80 Perforation risk 500 1.00–2.00 Perforated Failed Effect of Acid Concentration and Dry-Out Time

H₂SO₄ Concentration (%) Dry-Out Time (min)Metal Loss per Event (µm) Events to 0.5 mm Loss 5 5 2-4 125-250 5 10 4-8 60-125 5 30 8-15 30-60 10 10 6-12 40-80 20 10 10-20 25-50 Dry-Out Management Guide

Maximum allowable per year Expected number of dry-out events per yearSuggested Action 50 (5 years) <10OK
10-25 years 50 years (2-5 years)add low level alarm
25–50 50 (1–2 years) Auto refillNot acceptable Re-design of tank level control
Engineering Recommendations:

For sulphuric acid tanks replace heater after 50 dry-out incidents. Occasional dry-out prior to heater replacement. Install a low level alarm at 20% above heater height to prevent drying out. The engineer manages the dry-out frequency to avoid oxide spalling and excessive thinning.