Hard anodic coatings

Type III hard anodise.

Thick, dense anodic coatings to MIL-A-8625F — built for wear. Film thicknesses of 10–60 μm on aluminium alloys, verified by eddy current.

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FIG. 01Hard anodising line · Wetherill ParkMIL-A-8625F · TYPE III · CL 1 & 2

The process

An oxide layer that outwears the tool steel it runs against.

Type III hard anodising grows a denser oxide at freezing bath temperatures and high current density. The coating hard enough to serve as a bearing and wear surface, competing with tool steels for hardness.

Current density is the whole game. Our in-house software programs the rectifier per load — surface area calculated in square decimetres, ramp and hold profiles matched to the alloy — and logs the current profile against the batch.

Coatings run unsealed where maximum abrasion resistance is the goal, or sealed when the drawing calls for corrosion resistance at some cost to wear. Dimensional growth is predictable and quoted with the job — critical bores and threads are masked or machined with anodise allowance.

SEALING · MIL-A-8625F §3.8.2SELECTED TO THE DUTY

UNSEALED

Maximum wear resistance

The default for wear applications — full as-grown hardness. Wear index ≤1.5 mg/1000 cycles (≤3.5 on alloys with ≥2% copper).

SEALED

Corrosion resistance

Sealed in boiling deionised water, dichromate or acetate chemistry when the drawing specifies exterior corrosion duty — traded against some abrasion resistance.

Quality control

Verified the way the spec demands.

№TestMethodAcceptanceFrequency

01Visual & dimensionalMIL-A-8625F §3.13 · §3.9Continuous, uniform coating free of breaks and powdery areas; drawing dimensions held after coatingEVERY LOT

02Coating thicknessASTM B244 · eddy currentNominal per drawing · ±20% up to 50 μm · ±10 μm aboveMONTHLY · ≤35 DAYS

03Abrasion resistanceTaber abraser · CS-17 · 1 kgWear index ≤1.5 mg/1000 cycles · ≤3.5 for alloys ≥2% CuMONTHLY · ≤35 DAYS

04Coating weightASTM B137 · in lieu of thickness≥4320 mg/ft² per 25 μm of coatingAT PROCURING OPTION

05Salt spray corrosionASTM B117 · sealed coatings onlyPit criteria per MIL-A-8625F §3.7.1.2WHEN SEALED

06Solution analysisPer process control documentAll line solutions within PCD control ranges · full bath history retainedFORTNIGHTLY

PROCESS CONTROL FAILURE HALTS PRODUCTION · TRACEABLE WORK QUARANTINED UNTIL CAUSE IS CORRECTED

Applications

Where Type III is called out.

SLIDES, RAILS & WEAR SURFACESABRASION RESISTANCE

PISTONS, CYLINDERS & VALVE BORESHYDRAULIC & PNEUMATIC

WEAPON & ORDNANCE COMPONENTSHARD USE · LOW REFLECTANCE

GEARS, CAMS & ACTUATION PARTSSURFACE HARDNESS

THERMAL & DIELECTRIC BARRIERSELECTRICAL INSULATION

SUB-SEA & UNDERWATER SYSTEMSTHICK FILM · SALT WATER BARRIER

SALVAGE & DIMENSIONAL RESTORATIONCONTROLLED BUILD-UP

FIG. 02 · COATING GROWTH · TYPE III AT 50 μm

Dimensional change

50 μm of film moves every dimension.

№FeatureWhat happensChange at 50 μm filmDesign allowance

01Flat surface~25 μm grows outward per faceFace position moves +25 μmMACHINE −25 μm PER FACE

02External diameter / ODGrowth acts on both sides of the diameterOD grows +50 μm on diameterMACHINE −50 μm ON Ø

03Bores / IDGrowth on both walls closes the holeID shrinks −50 μm on diameterBORE +50 μm · OR MASK

04External threadsGrowth acts on both face of thread pitch ~4× per-surface growth on pitch diameterPitch Ø grows ~+100 μmCUT UNDERSIZE · OR MASK

05Internal threadsThread growth closes on all faces of threadPitch Ø shrinks ~−100 μmTAP OVERSIZE · OR MASK

06Sharp edgesCoating won't bridge a knife edge—RADIUS and Break all sharp edges

RULE OF THUMB: DIAMETRAL CHANGE ≈ FILM THICKNESS ON PLAIN FEATURES, ≈ 2× FILM THICKNESS ON THREAD PITCH DIAMETERS · TIGHT-TOLERANCE FEATURES ARE MASKED OR MACHINED WITH ANODISE ALLOWANCE — FLAG THEM ON THE DRAWING

Good to know

Design for the growth.

Call out anodise allowance on critical dimensions, or we mask. Alloys above 5% copper or 8% silicon need approval before processing.

SPECMIL-A-8625F · TYPE III · CL 1 & 2

FILM10–60 μm TO DRAWING CALLOUT

ENVELOPE2600 × 1100 × 900 MM · 500 KG

RECORDSCOC · CURRENT PROFILE · THICKNESS MAP