Anodizing

Anodizing is an electrochemical process that thickens the natural oxide layer on a metal, most often aluminum, by making the part the anode in an electrolytic cell. The resulting aluminum oxide layer is hard, porous when first formed, and integral with the metal. It can be left clear, dyed, sealed, or used as a base for primers and adhesives. Titanium and magnesium can also be anodized, but aluminum accounts for most industrial use.

How the oxide forms

The part is immersed in an acid electrolyte and connected to the positive side of a DC power supply. Oxygen generated at the part surface reacts with the aluminum to form aluminum oxide. Because the electrolyte slowly dissolves the oxide as it grows, the layer develops a structure of tiny, regular pores perpendicular to the surface. Pore size and layer thickness depend on the acid, temperature, current density and time.

Part of the oxide grows into the metal and part builds outward, so anodizing changes dimensions slightly. Machined parts with tight tolerances must allow for this.

Main types

Industry and military specifications classify anodic coatings by process. The most cited specification in North America is MIL-A-8625, which uses “Types”.

Chromic acid anodizing (Type I)

Produces a thin, flexible film with good corrosion resistance and minimal effect on fatigue strength. It has been used on aerospace parts, but hexavalent chromium regulations have driven a shift to alternatives such as thin-film sulfuric and boric-sulfuric processes (Type IC and IB).

Sulfuric acid anodizing (Type II)

The most common general-purpose process. It forms a moderately thick, porous layer that accepts dyes well. It is used for architectural extrusions, consumer products and many industrial parts.

Hard anodizing (Type III)

Run in colder sulfuric electrolyte at higher current density, hard anodizing produces a thick, dense, very wear-resistant layer. It is used for pistons, valve bodies, sliding parts and tooling. The layer is usually darker and less suitable for bright colors.

Other processes

Phosphoric acid anodizing produces a very open oxide that is used as a surface treatment for adhesive bonding in aerospace. Proprietary processes exist for decorative and specialty finishes.

Sealing

Freshly anodized aluminum has open pores that can absorb stains and allow corrosion. Sealing closes the pores, typically by:

• Hot water or steam sealing, which hydrates the oxide and swells it shut.
• Nickel acetate or mid-temperature seals.
• Cold seals based on nickel fluoride or newer nickel-free chemistry.

Sealed layers give better corrosion resistance and color retention. Unsealed layers give better adhesion for paints and bonding, which matters for the next section.

Painting or bonding over anodized aluminum

Anodizing can be an excellent base for organic coatings if the process is chosen for that purpose.

• Unsealed or lightly sealed anodize is preferred under primers and adhesives because the pores provide mechanical anchoring.
• Fully sealed anodize has a smooth, low-energy surface. Paint adhesion can be poor unless the surface is abraded or a suitable primer is used.
• Field coating of existing anodized architectural metal generally requires cleaning, light abrading and a primer approved by the coating manufacturer. Test adhesion on a small area first (see adhesion testing).

Compare this with chemical conversion coatings, which are thinner and used mainly as paint pretreatments.

Properties and limitations

Strengths:
– Integral with the metal, so it does not peel like paint.
– Hard and wear resistant, especially Type III.
– Electrically insulating.
– Accepts dyes for durable color, particularly when sealed.

Limitations:
– Can crack if the part is bent after anodizing, because the oxide is brittle.
– Some alloys, especially high-copper and high-silicon grades, anodize poorly or unevenly.
– Dyed colors may fade outdoors; inorganic and electrolytic coloring is more lightfast.
– Strong acids and alkalis attack the oxide. Alkaline cleaners can etch it.

Quality control

Common checks include:

• Thickness, typically by eddy-current gauge on non-ferrous metals (see dry film thickness measurement).
• Seal quality, using dye-spot tests or acid dissolution tests.
• Corrosion resistance, often by neutral salt spray (see salt spray and cyclic corrosion testing).
• Abrasion resistance for hard coats.

Applications

• Architectural window frames, curtain walls and trim
• Aerospace structures, often with primer and topcoat
• Cookware, consumer electronics and sporting goods
• Hydraulic components and wear parts (hard anodize)
• Marine hardware, where sealing quality is critical

Key terms

• Type II anodizing. Sulfuric acid anodizing that produces a moderate oxide layer suited to dyeing and general corrosion protection.
• Type III (hard) anodizing. Run at lower temperatures and higher current density to produce a thicker, harder layer for wear resistance.
• Sealing. A post-treatment, often in hot water or nickel acetate, that closes the pores of the oxide to improve corrosion resistance.
• Pore structure. The hexagonal cells and central pores of anodic oxide that allow dyes and primers to penetrate.
• Rack marks. Small unanodized spots where parts contact the electrical racks, usually specified in drawings so they fall in hidden areas.

Frequently asked questions

Is anodizing a coating or a surface treatment?

Both terms are used. It is a conversion of the metal surface into oxide, so it is often grouped with conversion processes, but it creates a much thicker layer than chemical conversion coatings.

Can steel be anodized?

Not in the usual sense. Steel is protected by other processes, such as galvanizing, phosphating or painting.

Can damaged anodizing be repaired in the field?

Not by re-anodizing in place. Field repairs are usually made with touch-up coatings, and the appearance will differ from the original finish.

Related entries

• Coating aluminum
• Aerospace coatings
• Adhesion science
• Galvanic corrosion
• Conversion coatings (new)
• Filiform corrosion (new)