Aluminium

Aluminium is anodic — in a mixed-metal joint, the aluminium is the part that dissolves

This reverses everything that applies to the other corrosion-resistant metals we supply. Copper, titanium, zirconium and tantalum fasteners are all galvanically noble — they survive and drive corrosion of whatever they are bolted into. Aluminium sits near the active end of the series, below carbon steel and far below stainless, copper and the nickel alloys. The geometry makes it worse: a small aluminium fastener clamping a large steel or stainless assembly is a small anode against a large cathode, the most damaging configuration there is, and the fastener can be consumed while the structure around it is untouched. An aluminium bolt in a wet stainless joint will fail. Copper is the worst partner of all — even trace copper dissolved in water and redeposited on aluminium initiates pitting. Use insulating washers and sleeves, barrier sealant at the faying surface, or anodising, whose film is an electrical insulator as well as a corrosion barrier.

Only three aluminium alloys have a certified fastener standard, and aluminium bolts are not strong. ASTM F468 and F467 cover 2024-T4, 6061-T6 and 7075-T73 and nothing else. The required strengths put the material in perspective: 6061-T6 is 255–359 MPa full-size tensile, 2024-T4 is 379–483 MPa and 7075-T73 is 421–524 MPa, against 830 MPa for an ordinary ASTM A325 structural steel bolt. Even the strongest aluminium bolt is about half the strength of a plain steel one, at a third of the density. Three further practical points: aluminium threads gall and seize, so anti-seize is standard and an aluminium nut on an aluminium bolt is poor practice; aluminium threads strip easily, so a tapped hole needs roughly twice the engagement of a steel one or a steel insert; and aluminium has low bearing strength under the head, so wide flanges or large washers are needed. Never carry steel torque figures across to aluminium.

F468 specifies 7075 in T73, not T6 — and that is deliberate. The 2xxx and 7xxx alloys are susceptible to stress-corrosion cracking, and 7075-T6 in particular has a poor short-transverse SCC threshold with a long service history of cracking under exactly the sustained tension a bolt applies. The over-aged T73 temper trades 10 to 15% of the strength for a large improvement in resistance, which is why the standard will not accept peak-aged material. Specifying "7075-T6 bolts" is a specification error, not a strength upgrade.

Two service-temperature limits. Aluminium loses strength quickly with temperature and has essentially no useful creep strength above about 150 °C; in the heat-treatable alloys the loss is permanent, because time at temperature over-ages the precipitate structure and the part does not recover on cooling. Separately, 5xxx alloys above about 3% magnesium sensitise: 5083 must not be used in sustained service above 65 °C, where grain-boundary β-phase precipitation makes it susceptible to intergranular corrosion and cracking. Conversely aluminium is excellent cold — face-centred cubic with no ductile-to-brittle transition, and 5083 is a standard cryogenic alloy.

Corrosion resistance tracks copper content, and aluminium is amphoteric. The protective oxide is stable from roughly pH 4 to pH 9; outside that window aluminium is attacked by strong acids and, more aggressively, by strong alkalis — sodium hydroxide dissolves it, alkaline cleaners attack it, and wet cement and lime etch it. Across the series, 1xxx, 3xxx, 5xxx and 6xxx have good to excellent resistance while the copper-bearing 2xxx and 7xxx are markedly poorer and should not be left bare in a damp or marine environment; Alclad, anodising or paint is required. The 2xxx and 7xxx alloys are also not practically arc weldable — they hot crack — and are joined by riveting, bolting, bonding or friction stir welding instead.