The case for Grade 23 over Grade 5 is damage tolerance. Interstitial oxygen strengthens titanium but embrittles it, so capping oxygen at 0.13% and hydrogen at 0.0125% raises fracture toughness and ductility measurably — which matters in two situations. The first is fracture-critical structure: airframe parts and offshore tubulars where crack growth and residual strength, not yield, set the design. The second is surgical implants, where ASTM F136 specifically requires the ELI chemistry, and where the lower interstitial content also improves biocompatibility consistency.
The cost is strength: 828 MPa minimum tensile against Grade 5’s 895, and 759 MPa yield against 828. Where strength governs and toughness does not, Grade 5 remains the right specification and is cheaper.
Corrosion behaviour matches Grade 5 — excellent in marine and offshore environments and in oxidising acids, but less resistant than the CP grades in reducing media, so it is not a substitute for them on corrosion duty.
Titanium alloys divide into four classes, and the class tells you more than the grade number does. Alpha and near-alpha alloys are weldable and creep resistant and run hottest, but are not heat treatable to high strength. Alpha-beta alloys — of which Ti-6Al-4V is the archetype — are heat treatable, strong and the general structural choice. Near-beta and metastable beta alloys are deeply hardenable, can be cold formed in the solution-treated condition and then aged to very high strength, which makes them the natural choice for fasteners and springs. Commercially pure and lean corrosion grades trade all strength away for corrosion performance.
Three absolute exclusions apply to every titanium grade: it must not be used in dry chlorine, with which it reacts violently; not with red fuming nitric acid; and it is rapidly attacked by hydrofluoric acid at any concentration. Fine titanium swarf and grinding dust are a fire hazard and are handled wet.
Titanium galls, and for a threaded fastener this is the dominant practical issue. A titanium thread run against another titanium thread will cold-weld and seize, often badly enough that the fastener has to be cut off. Every titanium bolted joint needs one of three mitigations: an anti-seize compound or dry-film lubricant such as molybdenum disulphide; a plated or coated thread, commonly silver or solid-film lubricant; or a nut in a dissimilar material. Specify which at order stage.
Titanium is galvanically noble. It will not corrode itself, but a titanium fastener in an aluminium, carbon steel, zinc or magnesium assembly drives accelerated corrosion of the structure around it, and a bolted joint’s small-anode, large-cathode geometry is the worst case. Insulating washers and sleeves or a barrier coating should be specified in those assemblies.
