Anodising and Surface Treatment for Aluminium Fittings: What It Does and Why It Matters

Here is something that confuses a lot of buyers: aluminium actually forms its own natural oxide layer — aluminium oxide (Al₂O₃) — spontaneously on exposure to air. This natural oxide layer is why aluminium does not rust the way steel does. But the natural oxide layer is thin, relatively soft, and does not provide strong protection against mechanical wear or corrosive environments.

Anodising takes this natural process and dramatically amplifies it — creating a thick, hard, controlled oxide layer that transforms aluminium's surface performance. Understanding anodising helps you specify the right treatment for each application.

How Anodising Works

Anodising is an electrochemical process — not a coating applied on top of aluminium, but a controlled conversion of the aluminium surface itself into aluminium oxide. The part becomes the anode in an electrolytic bath (typically sulphuric acid). Current is passed through the bath, causing oxygen ions to migrate to the aluminium surface and react to form aluminium oxide.

The oxide grows both outward from the surface (approximately 50% of thickness) and inward into the aluminium (approximately 50%). The resulting layer is an integral part of the part — it cannot be peeled or scratched off like a paint or plating. The porous columnar structure of anodised aluminium can be sealed (by hot water or chemical sealing) to close the pores and improve corrosion resistance.

Anodising Types — What Each Delivers

Type II (Standard Anodising)

The most common treatment for industrial aluminium fittings. Produces a 10–25 μm oxide layer that provides good corrosion resistance for most indoor and sheltered outdoor applications. The porous structure accepts dyes — black anodised aluminium pneumatic fittings are almost universally Type II anodised, dyed black, and sealed.

Type III (Hard Anodising)

Produced at lower temperature and higher current density than Type II, hard anodising creates a much thicker (25–75 μm), denser, and harder oxide layer. Hardness reaches 350–500 Vickers — comparable to hardened steel. Hard anodised surfaces are highly wear-resistant and provide excellent corrosion protection in demanding environments.

For aluminium fittings in marine environments, outdoor industrial applications, or anywhere physical abrasion is likely, hard anodising is the specification of choice. It costs more than standard anodising, but the service life improvement is substantial.

Thread Protection in Anodised Parts

Threads are typically masked before anodising or threaded after anodising — both approaches have trade-offs:

Mask before anodising: Threads are protected during anodising and retain their original dimensions. But the thread root/crest have no corrosion protection from anodising.

Thread after anodising: The body has full corrosion protection. But threading through the anodised layer can crack it at thread roots, potentially reducing corrosion protection in that zone.

For high-specification aluminium fittings, thread inserts (stainless steel Helicoil) are often used — these provide steel thread strength and durability while the aluminium body benefits from anodising protection.

Alternative Surface Treatments

Chromate Conversion Coating (Alodine / Iridite)

A chemical conversion process that creates a thin chromate film on aluminium — excellent for corrosion protection and paint adhesion, but contains hexavalent chromium (Cr⁶⁺), a carcinogen and RoHS/REACH restricted substance. Being phased out in favour of trivalent chromium (Cr³⁺) alternatives. Not a wear-resistant treatment — purely corrosion protection and adhesion primer.

Powder Coating

Electrostatic application of polymer powder, cured at 180–200°C. Provides excellent colour options, good corrosion resistance, and reasonable wear resistance. Not suitable for precision-tolerance surfaces (adds 60–100 μm). Good for valve bodies and large fittings where appearance and general corrosion resistance matter more than dimensional precision.

PTFE / Teflon Coating

Applied over anodised aluminium, PTFE coatings provide low friction, non-stick properties, and excellent chemical resistance. Used in fluid handling fittings where contamination prevention is critical and in food-grade applications. Not a structural coating — provides chemical protection, not mechanical reinforcement.

Specifying Surface Treatment Correctly

When ordering anodised aluminium fittings, your specification should include:

• Anodising type (II or III)
• Minimum oxide thickness (e.g., "15 μm minimum Type II" or "40 μm minimum Type III")
• Colour (natural/clear, black, or specific RAL colour for Type II)
• Sealing requirement ("sealed with hot water/nickel acetate" for Type II)
• Thread treatment (mask or post-machine, or thread insert)

An incomplete surface treatment specification leaves the supplier to make assumptions — and assumptions in manufacturing mean variability in what you receive.