6061 / 6082: first choice
The best balance of strength, machinability and hardcoat quality. For most wear parts that do not need 7xxx strength, start here. See 6061 round bar and 6082 round bar.
Type III hardcoat
Hard Anodised Aluminium Bar and Rod: Type III Hardcoat Guide
Hard anodised aluminium bar carries a thick, dense Type III oxide layer, usually 25 – 75 µm, that gives a wear-resistant, electrically insulating surface on a light, easily machined core. The best results come from choosing the right alloy (6061 and 6082 coat most evenly), machining before coating and allowing for the coating build-up on every critical dimension. Metline supplies the bar stock and can deliver it hard anodised to your specification.
- Coating
- MIL-PRF-8625 Type III
- Thickness
- Typically 25 – 75 µm
- Best alloys
- 6061, 6082
- Forms
- Round, hex, square, flat bar
What hard anodising is and how it differs from Type II
Anodising converts the surface of aluminium into aluminium oxide by making the part the anode in an acid electrolyte. Hard anodising (Type III, hardcoat) uses a sulphuric-acid bath held close to freezing, typically around 0 – 5 °C, and a higher current density than decorative anodising. The cold, aggressive conditions slow the dissolution of the oxide and allow a much thicker, denser and harder layer to grow.
Roughly half of the coating grows outward from the original surface and half penetrates inward, so a 50 µm hardcoat changes a diameter by about 50 µm (25 µm per side). The oxide is integral with the metal, not a deposit on top, so it cannot flake like paint or plating. It is, however, harder and more brittle than the substrate, and it lowers fatigue strength, which matters for highly stressed rotating parts.
MIL-PRF-8625 is the specification most often named on drawings. It divides anodic coatings into types by process and into classes by colour: Class 1 is undyed and Class 2 is dyed. AMS 2469 (hard anodic coating) and ISO 10074 (hard anodic oxidation coatings) are common alternatives.
| Item | Type II (sulphuric) | Type III (hardcoat) |
|---|---|---|
| Main purpose | Decoration, colour, general corrosion protection | Wear resistance, functional hard surface, insulation |
| Bath | Sulphuric acid at room temperature | Sulphuric acid near 0 °C, higher current density |
| Typical thickness | About 5 – 25 µm | About 25 – 75 µm, up to about 100 µm on suitable alloys |
| Hardness | Moderate | High; commonly quoted around 350 – 500 HV on 6061, lower on high-copper alloys |
| Colour | Clear or dyed in a wide range | Natural grey to bronze, darker as thickness rises; black is the usual dyed colour |
| Dimensional effect | Small | Significant; must be allowed for in machining |
| Relative cost | Lower | Higher |
Which alloy makes the best hard anodised bar?
The strongest bar and the best bar for hardcoat are rarely the same thing. Alloying elements, especially copper and silicon, form intermetallic particles that disturb oxide growth. The result is thinner, softer, more porous or more uneven coating, and a harder job for the anodiser.
| Alloy / temper | Typical tensile strength | Hardcoat response | Best use |
|---|---|---|---|
| 6061-T6 / T6511 | 310 MPa | Very good: even, hard, predictable colour | General engineering, guide rods, fixtures, automation parts |
| 6082-T6 | About 310 – 340 MPa | Very good | European equivalent route to 6061 |
| 6063-T5 / T6 | 186 – 241 MPa | Very good, smoothest appearance | Visible parts where finish matters more than strength |
| 7075-T6 / T651 | 572 MPa | Good but darker and less uniform; adhesion and brittleness need attention | High-load parts where strength governs |
| 7050-T7451 | About 524 MPa | Similar to 7075 | Thick, stress-corrosion-sensitive high-strength parts |
| 2024-T351 / 2A12 | About 470 MPa | Difficult: copper-rich phases limit thickness and hardness | Only when the design requires 2024 |
Strength values are typical, not specified minimums. Hardcoat response describes typical behaviour; the anodiser should confirm thickness and hardness on test coupons for critical parts.
7075 / 7050: when strength rules
Choose for highly loaded hardware, but expect a darker grey and more variation. Specify thickness and colour tolerance clearly. See 7075 rod.
2024: handle with care
High copper makes the coating slower to grow, softer and more prone to burning. Thinner coatings and close process control are needed. See 2024 bar.
Alloys above about 2 % copper or 7 % silicon, including most die-casting alloys, are the hardest to hardcoat well. MIL-PRF-8625 recognises this by allowing a higher Taber abrasion weight loss for high-copper alloys than for the rest.
Hard anodised bar forms and sizes
| Form | Common alloy-temper | Size range | Type III thickness | Length |
|---|---|---|---|---|
| Round bar / rod | 6061-T6 / T6511, 6082-T6, 7075-T651, 2024-T351 | About 5 – 200 mm (3/16 – 8 in) | 25 – 75 µm typical | Cut to length |
| Hex bar | 6061-T6 / T6511, 7075-T651 | Across flats to order | 25 – 75 µm typical | Cut to length |
| Square bar | 6061-T6 / T6511, 6063-T5 | To order | 25 – 75 µm typical | Cut to length |
| Flat bar | 6061-T6 / T6511, 6063-T5, 2024-T351 | To order | 25 – 75 µm typical | Cut to length |
| Precision ground rod | 6061-T6, 7075-T6, 2017-T451 | Tight-tolerance OD to order | To drawing | Cut to length |
Hardcoat on long bar is applied after cutting, because the coating cannot be cut without exposing bare metal at the ends. For shafts and rods with a fit tolerance, start from precision ground rod or turn the OD to a pre-anodise size.
Coating build-up, machining allowance and masking
Hard anodising is not dimensionally neutral. Plan every toleranced feature around the build-up, and state on the drawing whether dimensions apply before or after coating.
| Type III thickness | Growth per side (approx.) | Change in diameter (approx.) |
|---|---|---|
| 25 µm (0.001 in) | 13 µm (0.0005 in) | 25 µm (0.001 in) |
| 50 µm (0.002 in) | 25 µm (0.001 in) | 50 µm (0.002 in) |
| 75 µm (0.003 in) | 38 µm (0.0015 in) | 75 µm (0.003 in) |
| 100 µm (0.004 in) | About 50 µm, alloy dependent | About 100 µm; confirm by trial |
- Machine firstTurn, mill and drill to pre-coat dimensions calculated from the target coating thickness. Finish machining after hardcoat means grinding through a very hard layer.
- Break sharp edgesGive edges a radius of at least about 0.5 mm. Oxide grows perpendicular to the surface, so sharp corners form voids and chip easily.
- Mask what must stay bareThreads, bearing bores, electrical contact points and press-fit seats are usually masked. Identify them on the drawing.
- Specify the finished sizeQuote the final OD, hole sizes and thread class after coating, plus the coating thickness and its tolerance.
- Inspect after coatingCheck thickness by eddy current, and confirm critical diameters and threads with gauges.
Threads are the most common problem. A 50 µm coat on a thread flank changes pitch diameter by roughly four times the growth per side, so unmasked threads usually bind. Either mask them, or cut them oversize (internal) or undersize (external) to allow for the coating.
Colour, sealing and PTFE-impregnated hardcoat
Natural (Class 1)
Undyed hardcoat ranges from light grey to dark bronze-grey depending on alloy and thickness. 6061 gives the most consistent shade; 7075 and 2024 run darker and more varied.
Black (Class 2)
Dyed black is the usual coloured hardcoat. Bright colours are hard to achieve on thick Type III because the base film is already dark.
Sealed
Sealing (hot water, nickel acetate or dichromate) closes the pores and improves corrosion resistance, at a small cost in surface hardness and wear resistance.
Unsealed
Often specified for sliding and wear surfaces, where maximum hardness matters more than salt-spray performance.
PTFE-impregnated
Fluoropolymer drawn into the pores lowers friction and improves release. Used for pistons, slides and food or packaging machinery parts.
Colour approval
If colour matters, specify a tolerance or approve a sample before production, because shade varies between alloys, lots and thicknesses.
Limitations and common mistakes
- It does not make the bar stronger. Hardcoat improves the surface only; the tensile strength of the core is unchanged.
- It reduces fatigue strength. The hard, brittle layer can initiate cracks. Avoid thick hardcoat on highly stressed rotating or cyclically loaded parts, or confirm by test.
- Thicker is not always better. Beyond about 50 – 75 µm the outer layer becomes softer and more porous, edges chip more easily and dimensional control worsens.
- It can craze when heated. The oxide expands much less than aluminium, so heating coated parts well above about 100 °C can cause fine surface cracking.
- It cannot be touched up. Unlike plating, damaged hardcoat cannot be repaired locally; the part must be stripped and recoated, which removes metal.
- Welding and forming come first. Weld, bend and heat-treat before anodising. Coated bar cannot be formed without cracking the layer.
Hardcoat is an excellent electrical insulator when dry and sealed, which is useful for insulated shafts and fixtures. If a part also needs an earth path, mask a contact area.
Where hard anodised aluminium bar is used
Fluid power
Pneumatic cylinder barrels, pistons and rods where a hard, low-friction sealing surface is needed at low weight.
Automation and linear motion
Guide rods, slide rails, rollers and lightweight shafts for pick-and-place and packaging machines.
Semiconductor and vacuum
Insulated fixtures, chamber parts and wafer-handling components where clean, hard surfaces are required.
Medical and laboratory
Instrument components and fixtures that need wear resistance and repeated cleaning.
Marine hardware
Sheaves, fittings and parts where sealed hardcoat adds corrosion and wear protection.
Defence and sporting equipment
Rails, mounts, housings and hardware where light weight and a durable surface are both required.
What to include in your enquiry
- Alloy and temperFor example 6061-T6511, 6082-T6, 7075-T651 or 2024-T351.
- Form and sizeDiameter, across-flats or width and thickness, and cut length, in mm or inches.
- CoatingSpecification (MIL-PRF-8625 Type III, AMS 2469 or ISO 10074), thickness and tolerance, Class 1 or 2, colour.
- Sealing and extrasSealed or unsealed, PTFE impregnation, masking areas.
- Dimensions after coatingFinal OD, hole and thread requirements, and whether machining is required before coating.
- DocumentsMill test certificate (EN 10204 3.1) for the bar, coating thickness report and certificate of conformity.
References and standards
- MIL-PRF-8625 – Anodic Coatings for Aluminum and Aluminum Alloys
- SAE AMS 2469 – Hard Anodic Coating Treatment of Aluminum and Aluminum Alloys
- ISO 10074 – Anodizing of aluminium and its alloys: Specification for hard anodic oxidation coatings
- ASTM B221 – Extruded bars, rods, wire, profiles and tubes in aluminium alloys
- ASM Handbook Vol. 5 – Surface Engineering: Anodizing
Hard anodised aluminium (aluminum) bar: frequently asked questions
Is hard anodised aluminum better than regular anodised aluminum?
Only for the right job. Type III is far more wear-resistant and insulating, but it costs more, changes dimensions more and reduces fatigue strength. For colour, appearance and light corrosion protection, Type II is usually the better choice.
Is Type III the same as hardcoat anodising?
Yes. "Hardcoat", "hard anodising" and "Type III" all describe the low-temperature sulphuric-acid process defined as Type III in MIL-PRF-8625. Class 1 is undyed and Class 2 is dyed.
Which aluminium alloy is best for hard anodising?
6061 and 6082 give the most uniform, hardest coatings with predictable colour. 7075 can be hard anodised well but runs darker and less uniform. 2024 and other high-copper alloys are the most difficult.
Should bar be machined before hard anodising?
Yes, in almost every case. Machine to pre-coat dimensions that allow for about half the coating thickness growing outward on each surface, break sharp edges and mask threads and bearing seats.
Does hard anodised aluminium corrode?
Aluminium does not rust, and a sealed hardcoat gives good resistance to atmospheric and salt exposure. Damage that exposes bare metal can corrode locally, particularly in chloride environments, and the coating cannot be touched up.
How thick should a Type III coating be?
MIL-PRF-8625 uses 50 µm (0.002 in) as the nominal thickness when none is specified. 25 – 50 µm suits most wear parts; thicker coatings are reserved for severe abrasion and need alloy-specific review.
Need hard anodised aluminium bar?
Send the alloy, size, length, coating specification, colour and quantity. We confirm the route and reply with a delivered price.
