In a tight, oxygen-starved crevice — under a gasket, inside a threaded joint, beneath a deposit — commercially pure titanium cannot repassivate and will eventually suffer crevice attack in hot chloride service above roughly 70 to 80°C. The palladium addition changes the cathodic behaviour of the surface so that repassivation occurs even under those conditions, extending useful service into hot brines, wet-process acid plant and reducing acid chloride environments where plain CP titanium would fail. Because a bolted joint is by nature a collection of crevices, this matters more for fasteners than for almost any other product form.
Grade 11 is mechanically identical to Grade 1, so specifying it costs nothing in formability or ductility; the premium is purely the palladium. Where higher strength is also wanted,
Grade 7 gives the same protection on the 345 MPa Grade 2 base, and
Grade 12 gives much of it through molybdenum and nickel at 483 MPa without any precious metal.
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.
