Why Does a Titanium Heater Tube with a Slightly Rough Surface (Ra 0.8 µm) Outperform a Mirror-Finished Tube (Ra 0.05 µm) in a Deaerated 2% HCl at 50°C by Promoting Hydrogen Bubble Detachment?

The main cathodic reaction in deaerated 2% HCl at 50°C is the hydrogen evolution. Hydrogen bubbles adhere on the titanium surface, obstructing the active sites and locally increasing the hydrogen partial pressure that can induce absorption and embrittlement. Somewhat rough surface (Ra 0.8 µm) favours early bubble detachment. The bubbles nucleate on the surface peaks and detach easier than on a smooth surface. On a mirror-finished tube (Ra 0.05 µm), bubbles spread and grow into larger, more adhesive bubbles, improving hydrogen absorption.

Quantitative Detachment of Bubbles and Hydrogen Absorption

Surface Roughness (Ra, µm) Bubble Detachment Diameter (µm) Hydrogen Absorption Rate (ppm/1,000h) Corrosion Rate (mm/year)
0.05 (mirror) 500–1,000 80–150 0.15–0.30 0.2 (electropolished) 300–600 50–100 0.12–0.25 0.4 (polished) 200–400 30–60 0.10–0.20 0.8 (somewhat rough) 100–200 15–30 0.08–0.15 1.5 (as-drawn) 80–150 20–40 0.10–0.20
Deaerated HCl Surface Finish Selection Guide

HCl Concentration (%) Temperature (°C) Recommended Ra (µm) Reason
1 50 0.4-0.8 Balance
2 50 0.8 Allow bubble release.
3 40 0.8 Optimum
2 70 0.8 Lower H absorption
Recommendation of Engineering

For deaerated HCl service, provide a slightly rough surface finish (Ra 0.6–0.9 µm) instead of electropolished or mirror finish. The engineer decreases the rate of hydrogen absorption and corrosion by encouraging the escape of hydrogen bubbles.