Aluminium is protected by a tough, instantly self-healing oxide film, and it is at its best in the neutral range. The film forms in milliseconds on exposed metal and is what gives aluminium its genuinely excellent atmospheric and marine durability — architectural and marine aluminium lasts decades with no coating. The film is stable from roughly
pH 4 to pH 9, and outside that window aluminium is
amphoteric: it is attacked by strong acids and, more aggressively still, by strong alkalis. Sodium hydroxide dissolves it rapidly, which is why aluminium must never be used for caustic service and why alkaline cleaners attack aluminium parts. Wet cement, lime and uncured concrete are alkaline enough to etch it. It resists most organic compounds, dry gases and the great majority of foodstuffs, and it is non-toxic and non-sparking.
Corrosion resistance varies enormously across the series, and it tracks copper content. The
1xxx, 3xxx, 5xxx and 6xxx alloys have good to excellent resistance, with the 5xxx magnesium alloys the best of all in seawater. The
2xxx and 7xxx alloys are the problem: the copper that gives them their strength also makes them markedly less corrosion resistant than any other aluminium, prone to pitting and to intergranular attack. That is precisely why
Alclad exists — a thin layer of pure or 7072 aluminium roll-bonded to each face of a 2024 or 7075 core, which is anodic to the core and protects it sacrificially. Where a 2xxx or 7xxx part will see a corrosive environment, specify Alclad material, anodising, or a primer and paint system; bare 2024 and 7075 should not be left exposed.
The 7xxx alloys trade corrosion resistance for strength, and stress-corrosion cracking is the specific hazard. General corrosion resistance is poor by aluminium standards — better than the 2xxx alloys but well below the 5xxx and 6xxx series — and bare 7xxx should not be left exposed. Alclad 7075 with a sacrificial 7072 cladding is available for sheet, and anodising or a primer and paint system is standard for other forms. The more serious issue is stress-corrosion cracking in the peak-aged T6 tempers, especially in the short-transverse direction of thick sections, which has a long and well-documented service history. The T73, T7351 and T76 over-aged tempers exist specifically to solve it.
Stress-corrosion cracking is a real and specific hazard in the 2xxx and 7xxx alloys, and a tightened bolt supplies exactly the sustained tensile stress it requires. Short-transverse stress in thick sections is the dangerous direction, and the peak-aged tempers are the vulnerable ones: 7075-T6 in particular has a poor short-transverse SCC threshold and has a long history of service cracking in thick parts. The industry's answer is the
T73 and T7351 over-aged tempers, which deliberately age past peak strength to coarsen the precipitates and give a dramatic improvement in SCC resistance for roughly a 10 to 15% loss of strength. That trade is why
ASTM F468 specifies 7075 fasteners in the T73 condition and not T6 — a structural bolt is exactly the sustained-tension application in which SCC occurs, and the standard will not accept the peak-aged temper.
Exfoliation corrosion, in which attack follows the elongated grain structure of rolled or extruded product and lifts it in layers like the pages of a book, is the related mechanism in 2xxx and 7xxx; the T76 and T7x tempers also address it.
Aluminium is anodic, and this reverses everything that applies to the other corrosion-resistant metals. Copper, titanium, zirconium and tantalum fasteners are all galvanically noble — they survive and drive corrosion of whatever they are bolted into. Aluminium is the opposite. It sits near the active end of the galvanic series, below carbon steel and far below stainless steel, copper and the nickel alloys, so in a mixed-metal joint
the aluminium is the part that dissolves. The geometry makes it worse rather than better: a small aluminium fastener clamping a large stainless or steel assembly is a small anode against a large cathode, which is the most damaging configuration there is, and the fastener can be consumed while the structure around it is untouched.
An aluminium bolt in a stainless or steel joint, wet, will fail. Copper is the worst partner of all — even trace copper dissolved in water and redeposited on aluminium will initiate pitting, which is why aluminium should not be plumbed downstream of copper. The mitigations are insulating washers, sleeves and gaskets so no electrical circuit exists; a barrier coating or sealant at the faying surface; cadmium, zinc or aluminium-pigmented coatings on the steel part to bring it closer to aluminium's potential; and
anodising, which is particularly effective here because the anodic film is an electrical insulator as well as a corrosion barrier. Where a steel fastener must be used in aluminium structure, specify it hot-dip galvanised, aluminium-coated or 300-series stainless with an isolating washer — never bare carbon steel.
Aluminium loses strength quickly with temperature and has essentially no useful creep strength above about 150 °C. This is a hard limit on the whole family and it is often underestimated, because the loss is not linear: a 6061-T6 part retains most of its strength to 100 °C, has lost a substantial fraction by 200 °C, and is structurally useless by 300 °C. Worse, the loss in the heat-treatable alloys is
permanent, because time at temperature over-ages the precipitate structure that was providing the strength — the part does not recover when it cools. Aluminium bolting in anything that runs warm must be assessed at service temperature and not at room temperature, and prolonged exposure above roughly 120 °C should be treated as a design issue rather than a detail. Conversely aluminium is excellent at low temperature: it has a face-centred-cubic structure with no ductile-to-brittle transition, and both strength and toughness
increase as temperature falls, which is why 5083 is a standard cryogenic tank material down to liquid-helium temperatures.
With aluminium the temper matters as much as the alloy, and quoting one without the other is meaningless. The same alloy can differ in strength by a factor of three between tempers, so “6061” on a drawing is an incomplete specification — it must be 6061-T6, or 6061-T4, or 6061-O. The designation system under ANSI H35.1 runs as follows.
F is as-fabricated and
O is annealed, the softest and most formable condition.
H tempers are strain hardened and apply to the non-heat-treatable alloys: H1x is strain hardened only, H2x is strain hardened then partially annealed, H3x is strain hardened then stabilised, and the second digit gives the degree — 2 is quarter hard, 4 half hard, 6 three-quarter hard, 8 full hard and 9 extra hard.
T tempers are thermally treated and apply to the heat-treatable alloys:
T4 is solution heat treated and naturally aged,
T6 solution heat treated and artificially aged to peak strength,
T3 solution treated, cold worked and naturally aged, and
T8 solution treated, cold worked and artificially aged. The
T7x series is deliberately
over-aged past peak strength — T73, T7351 and T76 exist specifically to buy stress-corrosion resistance at the cost of about 10 to 15% of the strength, which is why they dominate thick-section 7xxx work. A trailing
51, as in T351, T651 and T7351, means the product was stress relieved by controlled stretching after solution treatment, which matters greatly for parts that will be heavily machined.
ASTM F468 and F467 cover exactly three aluminium alloys: 2024-T4, 6061-T6 and 7075-T73. Those are the only aluminium alloys for which a certified nonferrous bolt, hex cap screw, stud or nut standard exists, and the specified conditions are part of the designation — F468 will not accept 7075-T6 for the stress-corrosion reasons described below. The F468 strength requirements put aluminium fasteners 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 and 1040 MPa for property class 10.9. Even the strongest aluminium bolt is roughly half the strength of a plain structural steel one, at about a third of the density.
Aluminium is chosen for weight, conductivity, non-magnetic behaviour and corrosion resistance — not for load. Three further practical points follow. Aluminium
threads gall and seize readily, so an anti-seize compound or dry-film lubricant is standard practice and an aluminium nut on an aluminium bolt is poor practice. Aluminium threads also
strip easily — a tapped hole in aluminium needs roughly twice the thread engagement of a steel one, or a steel threaded insert. And aluminium has
low bearing strength under the head, so wide flange heads or large washers are needed to avoid embedment and the preload loss that follows. Torque figures for steel fasteners must never be carried across to aluminium ones.
Anodising is the standard surface treatment and it does several jobs at once. The process grows a controlled, thick, integral aluminium oxide layer that is far more protective than the natural film: sulfuric acid anodising for general corrosion protection and for colour, and hard anodising for a thick, very wear-resistant layer. For fasteners three points matter. First, the anodic film is an
electrical insulator, which makes anodising an effective galvanic isolation measure in its own right — but by the same token an anodised fastener will not provide an earth path or electrical bonding, which is a common and awkward surprise. Second, the film
grows outward as well as inward and adds meaningfully to thread dimensions, so anodised threads must be cut or rolled to allow for the coating thickness or they will not assemble. Third, anodising quality is alloy-dependent: the 5xxx and 6xxx alloys anodise beautifully and give clear, consistent films, while the high-copper 2xxx alloys anodise poorly and give patchy, less protective and discoloured coatings. Chromate and chromate-free conversion coatings are the alternative where electrical conductivity must be retained.
How the series divide. The first digit of the designation names the principal alloying element, and it tells you most of what you need to know.
1xxx —
1050,
1060,
1070,
1080,
1100 and
1145 — is 99% or purer aluminium, bought for electrical and thermal conductivity, corrosion resistance and formability, not strength.
2xxx is aluminium-copper, heat treatable to high strength with poor corrosion resistance and generally no weldability:
2014,
2017 and
2024 are the aerospace alloys,
2011 is the free-machining one and
2090 the aluminium-lithium.
3xxx —
3003 — is aluminium-manganese, the general-purpose sheet alloy.
5xxx is aluminium-magnesium, the marine and cryogenic family:
5052 for general service and
5083 for the highest strength available without heat treatment.
6xxx is aluminium-magnesium-silicide, the structural and extrusion workhorse combining moderate heat-treatable strength with good corrosion resistance and weldability:
6061,
6063,
6066 and
6082.
7xxx is aluminium-zinc, the highest strength of all and the most SCC-sensitive:
7075 and
7050.
