Skip to content
Material World
Process · Changes the form, not the material

Anodising

Grow the oxide deliberately instead of letting it happen — a surface that is part of the metal rather than a coating on it.

The part is made the anode in an acid electrolyte, and the oxide that aluminium would form anyway is grown thick, ordered and deliberately — typically five to twenty-five micrometres, against the two or three nanometres air produces.

The important distinction is that this is not a coating. The oxide is converted from the metal itself, so it cannot chip or peel the way plating and paint can; it grows partly into the surface and partly out of it. It is hard, electrically insulating, and porous as formed, which is what lets it take dye — and the pores must then be sealed in hot water or the colour and the corrosion protection both leach away.

It is also brittle, and it makes the part very slightly larger. A thick anodised layer on a fatigue-loaded component can initiate cracks, which is why aerospace specifications control anodising thickness rather than maximising it.

Uses

Architectural aluminium — window frames, curtain walling, cladding — where a fifty-year finish is expected. Consumer products, where the coloured anodised finish of a phone or a laptop case is the visible surface. Cookware. Aerospace components for corrosion protection before painting.

Hard anodising, run cold and thick, produces a wear surface used on hydraulic components and pistons. Titanium and magnesium anodise too; titanium's is the basis of the colours seen on surgical implants and jewellery, which are interference effects from oxide thickness rather than dye.

History

Developed in the 1920s, initially to protect aluminium seaplane parts from seawater. The sulfuric acid process that dominates today dates from the 1920s and 1930s and has changed remarkably little.

The chromic acid process, long standard in aerospace for its thinner and less fatigue-damaging layer, is being displaced by regulation rather than by technology: hexavalent chromium is a recognised carcinogen and is restricted under REACH.

Economic significance

Cheap, fast and continuous, which is a large part of why aluminium became the standard material for architectural glazing systems. The process is well suited to extruded profile — the material arrives in long uniform lengths, which is exactly what a treatment line wants.

It is one of the few surface treatments that adds essentially no mass and no separate material to recycle, which makes anodised aluminium substantially easier to return to the melt than painted or plated metal.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

  • 6000 series aluminium alloy — alloy · and it anodises better than the copper- and zinc-bearing families, which is the other half of why architectural aluminium is this one
  • Aluminium — element · the oxide is grown from the metal rather than applied to it, which is why an anodised surface cannot chip off
  • Titanium — element · and the colours are interference from oxide thickness rather than dye, which is why anodised titanium jewellery is coloured without pigment

is used in

  • Construction — industry · architectural aluminium, where a fifty-year finish is expected of a window frame

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Where it comes from, and what it becomes

Follow Anodising back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Anodising → takes as input (the oxide is grown from the metal rather than applied to it, which is why an anodised surface cannot chip off) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
  • Anodising → takes as input (and the colours are interference from oxide thickness rather than dye, which is why anodised titanium jewellery is coloured without pigment) → Titanium → is produced by (as sponge, which must then be crushed, melted and cast before it is usable metal) → Kroll process → takes as input (the inert atmosphere, without which the titanium would take oxygen from the air) → Argon → is produced by (drawn from an intermediate height in the column, between nitrogen and oxygen) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Anodising → takes as input (and it anodises better than the copper- and zinc-bearing families, which is the other half of why architectural aluminium is this one) → 6000 series aluminium alloy → is produced by (and the composition window is narrow: the magnesium-to-silicon ratio decides whether the precipitate forms usefully) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Anodising → takes as input (the oxide is grown from the metal rather than applied to it, which is why an anodised surface cannot chip off) → Aluminium → is produced by (collects at the cathode while the carbon anodes are consumed) → Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt
  • Anodising → takes as input (and the colours are interference from oxide thickness rather than dye, which is why anodised titanium jewellery is coloured without pigment) → Titanium → is produced by (as sponge, which must then be crushed, melted and cast before it is usable metal) → Kroll process → takes as input (converts the ore to a distillable tetrachloride, which is how the purification is done) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Anodising → takes as input (and the colours are interference from oxide thickness rather than dye, which is why anodised titanium jewellery is coloured without pigment) → Titanium → is produced by (as sponge, which must then be crushed, melted and cast before it is usable metal) → Kroll process → takes as input (the reducing agent, recovered afterwards by electrolysing the magnesium chloride by-product) → Magnesium → is produced by (from molten magnesium chloride, including the chloride returned by the Kroll process) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite

These are the most distinct paths back. Anodising can be traced through others besides.

Downstream — what it becomes

  • Anodising → is used in (architectural aluminium, where a fifty-year finish is expected of a window frame) → Construction