How Does the Inclusions Content (as Measured by ASTM E45, Type A vs. Type C) in a Grade 2 Titanium Heater Tube Correlate with the Pitting Density in a 0.5% Bromine Solution at 50°C?

In grade 2 titanium, pitting initiation sites are inclusions, especially Ti₂Fe particles. Inclusions are classified by kind according to ASTM E45 as follows: kind A (sulphides, rare in titanium), Type B (aluminates), Type C (silicates), and Type D (globular oxides). In titanium, the most prevalent and most harmful inclusions are iron-rich (Type D). The pitting density in the bromine solutions increases with the increase of the inclusion concentration. A severe inclusion rating (E45 rating >2) leads to a pitting density 5–10× higher than a clean material (E45 rating <1).

Pitting Density vs Inclusion Rating (Quantitative)

In 0.5% Br 2 at 50 C for 500 hours

ASTM E45 Inclusion Rating Ti₂Fe Particle Density (particles/mm²) Pitting Density (pits/cm²) Max Pit Depth (µm) <0.5 (extremely clean) <50 5–20 20–50
0.5-1.0 (clean) 50-150 20-60 40-100
1.0-1.5 (moderate) 150-300 50-150 80-180
1.5-2.0 (mod-heavy) 300-600 100-300 120-250 >2.0 (heavy) >600 200-500 200-400
Severity of Pitting vs. Type of Inclusion

Type of Inclusion (ASTM E45)Titanium Pitting Severity (relative) Typical
Type A (sulphides)Low Very rare
Type B (Aluminate) (1)Low Rare
Type C (silicates) Occasional Moderate
Type D (globular oxides/Fe-rich) Common High Inclusion Control Guide

Required Pitting Density (pits/cm2) Maximum ASTM E45 Rating Recommended Manufacturing 20 0.5 VAR melting (vacuum arc remelting)
20-50 <1.0 Double VAR
50-100 <1.5 Single VAR plus clean feedstock >100 >2.0 Not recommended for Br2 service
Engineering proposal

Bromine service at 50°C: Titanium tube with ASTM E45 inclusion grade <1.0 (clean). Use of double vacuum arc remelted (VAR) material for lowest content of inclusions. The engineer controls the inclusion content to lessen the pitting density in harsh bromine solutions.