SURFACE ENGINEERING
Post Processing
& Coatings
Nine industrial coating processes for aluminium castings and
machined components — from corrosion protection and wear
resistance through to dry-film lubrication, electrical conductivity,
and thermal barriers. Specification data, application guidance, and
expert selection support.
machined components — from corrosion protection and wear
resistance through to dry-film lubrication, electrical conductivity,
and thermal barriers. Specification data, application guidance, and
expert selection support.
Hard Anodise
Powder Coat
Tin Plating
Alodine
MoS₂
Ceramic
9 Processes
Coating Selection Support
Coating Processes
0
HRC Hard Anodise
0
-70
MoS₂ Friction Coeff.
0
Salt Spray Hours (PC)
0
+
Dim. Change (Alodine)
±
0
COATING 01 OF 09
SOFT ANODISE
MIL-A-8625 Type II · BS EN ISO 7599 · ASTM B580 Type B
Soft anodising — formally Type II sulphuric acid anodising — is an electrochemical process that converts the surface of aluminium into a dense, porous aluminium oxide (Al₂O₃) layer. The electrolyte is a dilute sulphuric acid bath at approximately 18–22°C, and the aluminium part serves as the anode. The resulting oxide grows partly into the surface and partly outward, producing a coating that is integral to the base metal rather than a deposited film. The porous structure accepts dyes readily, making Type II the standard process for coloured aluminium components. Sealing in hot deionised water or chromate solution closes the pores and locks in both colour and corrosion resistance.
Process Specifications
Coating Thickness
5–25 µm
Surface Hardness
250–500 HV
Process Temp.
18–22 °C
Corrosion Resistance
336 hrs salt spray (sealed)
Dimensional Change
~50% penetrates, ~50% builds
Colour Options
Full range (clear, black, red, gold, blue…)
Performance Ratings
Corrosion Resist.
Good
Wear Resistance
Moderate
Colour / Aesthetic
Excellent
Dim. Stability
Good
Typical Applications
Architectural panels
Consumer electronics housings
Automotive trim
Electrical junction boxes
Decorative enclosures
Medical device frames
Lighting fixtures
Paint pre-treatment base
Best for
- Components requiring consistent colour across a batch
- Low-to-moderate wear environments
- Indoor corrosion protection
- Cosmetic surfaces where tight tolerances are not critical
Not ideal for
- High-load sliding or abrasive contact surfaces
- Very tight-tolerance bores or shafts (use hard anodise)
- Electrical contact surfaces (use Alodine or tin plate)
Coating 02 of 09
Hard Anodise
- MIL-A-8625 Type III
- AMS 2468 / 2469
- BS 5599
- ASTM B580 Type A
Hard anodising (Type III) uses the same sulphuric acid chemistry as Type II but with a sharply reduced bath temperature (0–5°C), higher current density, and elevated voltage (up to 100V). These conditions drive a denser, harder oxide crystal structure that builds a coating significantly thicker than conventional anodising — from 25 µm up to 75 µm or beyond for salvage applications. The resulting coating reaches 60–70 HRC surface hardness, approaching tool steel. Because roughly half the coating penetrates inward and half builds outward, engineers must account for approximately 25 µm dimensional addition per surface at a 50 µm total coating thickness. Hard anodise lowers fatigue strength slightly; where fatigue is critical, thin-film alternatives should be evaluated.
Process Specifications
25–75 µm (standard 50 µm)
Coating Thickness
60–70 HRC (500–800 HV)
Surface Hardness
0–5 °C (low temperature)
Process Temp.
~25 µm per face at 50 µm total
Dim. Change (per face)
Excellent (Taber CS-10: <5 mg/1000 cycles)
Abrasion Resistance
Grey to dark grey; black dye available
Colour
Corrosion Resist.
Very Good
Wear / Abrasion
Excellent
Dielectric Strength
Good (800V/mm)
Fatigue Resistance
Reduced
Typical Applications
- Hydraulic cylinder bores
- Pump housings & impellers
- Valve bodies
- Sliding guides & rails
- Gearbox housings
- Aircraft structural components
- Textile machinery
- Medical equipment
- Firearm components
Best for
- Aluminium surfaces in high-wear or abrasive service
- Hydraulic and pneumatic cylinder bores
- Components requiring dielectric insulation
- Sliding surfaces without liquid lubrication
Not ideal for
- Allowance for dimensional build-up must be on drawing
- Reduces fatigue life — review stress applications
- High-silicon alloys (LM6, LM13) anodise to lighter shades
Coating 03 of 09
Powder Coating
- ASTM D3451
- ASTM B117 (salt spray)
- ISO 9227
- BS EN 12206
Powder coating is an electrostatic dry-finishing process in which finely ground thermosetting or thermoplastic polymer powder is sprayed onto a pre-treated surface and cured in an oven (typically 160–200°C). The charged powder particles adhere to the earthed substrate, and oven curing fuses them into a smooth, continuous film. Pre-treatment — degreasing followed by zinc or iron phosphating, or chromate conversion — is critical to adhesion and underpins the corrosion protection performance. Typical industrial coating thickness is 60–120 µm. Epoxy-polyester hybrid powders dominate industrial applications for their balance of hardness, flexibility, and weathering resistance. Pure epoxy powders are preferred for underground or heavily exposed structural parts. For aluminium specifically, a zinc phosphate or Alodine pre-treatment step is standard before application.
Process Specifications
60–120 µm (typical); 40–150 µm range
Coating Thickness
160–200 °C / 15–20 min
Cure Temperature
≥ 1,000 hrs (with phosphate pre-treat)
Salt Spray (ASTM B117)
2H–3H (ASTM D3363)
Pencil Hardness
Pass 80 kg·cm (ASTM D2794)
Impact Resistance
Matte (5°) to High Gloss (90°)
Gloss Range
Corrosion Resist.
Excellent
UV / Weathering
Very Good
Colour Range
Unlimited (RAL)
Chemical Resist.
Good
Typical Applications
- Pump & valve bodies (exterior)
- Electrical enclosures & switchgear
- Generator & motor housings
- Automotive brackets & covers
- Fire equipment housings
- Agricultural machinery
- Industrial frames & panels
- Outdoor architectural hardware
Best for
- Exterior surfaces requiring colour, UV resistance, and corrosion protection
- High-volume, cost-effective finishing
- Components where the coating is a barrier (not structural)
Not ideal for
- Internal bores or surfaces with tight dimensional tolerances
- Thin sections where 160°C cure may affect heat-treated alloys
- Electrical contact surfaces
Coating 04 of 09
Tin Plating
- MIL-T-10727
- ASTM B545
- BS EN ISO 2093
Tin electroplating deposits a thin, bright or matte layer of pure tin (≥99.5%) onto the component surface from an acid sulphate or alkaline stannate bath. On aluminium, tin plating requires careful pre-treatment — degreasing, acid activation, and typically a zincate immersion strike — to remove the insulating native oxide and establish an adherent intermediate layer. The resulting tin coating overcomes aluminium’s inherent problem of a non-conductive surface oxide, providing stable, low-resistance electrical contact that does not degrade with time or temperature cycling. MIL-T-10727 defines five thickness classes from 2.5 µm (Class A, mild conditions) to 30 µm (Class E, severe/continuous dampness), with Class C (8–10 µm) being most common for electrical hardware and Class D (15–20 µm) for harsh industrial or outdoor service.
Process Specifications (MIL-T-10727)
2.5 µm
Class A — Mild
5 µm
Class B — Moderate
8 µm (10 µm on steel)
Class C — Severe
15 µm (20 µm on steel)
Class D — Very Severe
≥ 99.5% Sn
Purity
Very low; stable over life
Contact Resistance
Electrical Conduct.
Excellent
Solderability
Excellent
Corrosion Resist.
Good
Anti-Galling
Good
Typical Applications
- Aluminium busbars & conductors
- Electrical terminal blocks
- Switchgear contacts
- Transformer connectors
- Power distribution hardware
- Motor end-caps (contact faces)
- Food equipment (tin safe for contact)
- RF / EMI shielding chassis
Best for
- Aluminium components used as electrical conductors or contact surfaces
- Bus bars, terminals, and connector bodies in power systems
- Components requiring solderability
Limitations
- Tin whisker growth risk on electroplated thin deposits (mitigated by matte tin or reflow)
- Not a high-wear coating — use hard anodise for abrasion resistance
Coating 05 of 09
Phosphating
- MIL-DTL-16232 (Mn)
- ISO 9717
- ASTM B633 (steel)
- UNI ISO 9717
Phosphating is a chemical conversion process that reacts with the metal surface to form a crystalline, non-metallic, water-insoluble phosphate layer. Three principal types are used industrially: iron phosphate (thin, amorphous, primarily for paint adhesion), zinc phosphate (crystalline, 5–15 µm, good corrosion resistance as paint base or with supplementary oil treatment), and manganese phosphate (heavier, 5–25 µm, crystalline black, best wear resistance and oil retention). The phosphate layer’s porous crystalline structure is highly effective at retaining corrosion-inhibiting oils, waxes, and dry-film lubricants, making it widely used as a base coating under paint, powder coat, or as a final oiled finish on precision steel and iron components. On aluminium, zinc phosphating is the most common type and is standard as a pre-treatment step before powder coating.
Process Specifications by Type
0.3–1 µm · Amorphous · Paint base
Iron Phosphate
5–15 µm · Grey · Corrosion + paint base
Zinc Phosphate
5–25 µm · Black · Wear + oil retention
Manganese Phosphate
10–30 g/m²
Coating Weight (Zn)
400 °C
Max Service Temp. (Mn)
Oil, wax, or paint required
Post Treatment
Paint Adhesion
Excellent
Oil Retention
Excellent
Anti-Galling
Very Good
Corrosion (alone)
Moderate (needs post-treat)
Typical Applications
- Pre-treatment for powder coating on Al
- Gearbox & transmission parts (Mn)
- Fasteners & fixings
- Firearms components
- Engine components (running-in)
- Automotive chassis & brackets
- Sliding / threaded steel parts
Best for
- Steel/iron components requiring anti-galling and oil-retention
- Pre-treatment base coat before paint or powder on aluminium
- Running-in surfaces in mechanical assemblies
Limitations
- Alone (unoiled), corrosion resistance is poor — always use post-treatment
- On aluminium, limited to zinc phosphate type as pre-treatment
Coating 06 of 09
Silver Plating
- ASTM B700
- MIL-QQ-S-365
- AMS 2410 / 2412
Silver electroplating deposits a layer of 98–99.9% pure silver onto the substrate from a cyanide or non-cyanide silver electrolyte. Silver has the highest electrical conductivity of all metals (6.3 × 10⁷ S/m) and the highest thermal conductivity among metals in common engineering use. These properties make silver plating the premium choice for RF/microwave waveguides, high-current terminals, and electrical contact surfaces where contact resistance, signal loss, and conductivity stability are engineering constraints. ASTM B700 classifies silver by purity type, surface appearance grade (bright, semi-bright, matt), and by class based on whether a post-plating tarnish inhibitor treatment has been applied. Minimum recommended thickness for engineering use is 2.5–5 µm; for load-bearing electrical contacts, 12–25 µm is typical.
Process Specifications (ASTM B700)
≥ 98% Ag (engineering grade)
Purity
2.5–25 µm (engineering); up to 50 µm
Thickness Range
6.3 × 10⁷ S/m (highest of all metals)
Electrical Conductivity
429 W/m·K
Thermal Conductivity
90–140 HV (electrodeposited)
Hardness
Bright, semi-bright, or matt
Appearance
Elect. Conductivity
Best of all metals
Thermal Conduct.
429 W/m·K
Solderability
Excellent
Tarnish Resistance
Moderate (requires inhibitor)
Typical Applications
- RF & microwave waveguides
- High-current busbar contacts
- Electrical switchgear contacts
- Aerospace connectors
- Medical device contacts
- High-frequency PCB connectors
- Radar & antenna components
- Heat sink interfaces
Best for
- RF/microwave components where skin-effect conductivity dominates
- High-current contacts where resistance and heat generation matter
- Precision electrical connectors requiring consistent low resistance
Limitations
- Tarnishes in sulphur-bearing atmospheres — specify tarnish inhibitor treatment
- Silver migration risk in DC circuits with high voltage/humidity — specify minimum thickness
- High material cost — justify against requirement
Coating 07 of 09
Alodine
Chromate Conversion Coating
- MIL-DTL-5541
- ASTM B449
- MIL-DTL-81706
Alodine — also known as chem film, chromate conversion coating, or Iridite — is a chemical conversion treatment applied to aluminium by immersion or brush application. Unlike anodising or plating, Alodine chemically reacts with the aluminium surface rather than depositing a foreign layer on it, producing a thin, tightly adherent chromate-aluminium complex. This process is governed by MIL-DTL-5541 and results in either a golden-yellow iridescent film (Type I, hexavalent chromium, Class 1A) or a clear/colourless film (Type II, trivalent chromium, Class 1A/3). Because the coating is extremely thin (0.1–1 µm), it is essentially dimensionally neutral — a critical advantage for close-tolerance machined aluminium components. Class 1A provides maximum corrosion protection; Class 3 is specified where low electrical contact resistance must be maintained alongside corrosion protection.
Process Specifications (MIL-DTL-5541)
0.1–1.0 µm (dimensionally neutral)
Coating Thickness
0.4–2.0 g/m²
Coating Weight (Class 1A)
Gold-yellow; max corrosion protect.
Type I (Hex Chrome)
Clear; RoHS-compliant
Type II (Tri Chrome)
< 5,000 µΩ/in² contact resistance
Class 3 Resistance
200+ hrs ASTM B117
Salt Spray (wrought Al)
Dim. Neutrality
Excellent (0.1–1 µm)
Paint Adhesion
Excellent
Corrosion Resist.
Good
Electrical Conduct.
Good (Class 3)
Typical Applications
- Close-tolerance machined Al components
- Aerospace structural parts
- Paint / primer adhesion base coat
- Electronic chassis & enclosures
- Electrical grounding surfaces (Class 3)
- Multi-alloy assemblies
- Repair of damaged anodise
Best for
- Tight-tolerance aluminium where anodise build-up is not acceptable
- Primer base coat on aerospace and defence aluminium structures
- Grounding surfaces requiring electrical contact continuity (Class 3)
Limitations
- Not suitable as a standalone finish for high-wear or harsh outdoor environments
- Type I (Hex chrome) under REACH/RoHS scrutiny — specify Type II where possible
Coating 08 of 09
MoS₂ Coating
Chromate Conversion Coating
- MIL-PRF-46010
- AS5272
- AMS 2526
Molybdenum disulfide (MoS₂) is a naturally occurring lamellar mineral whose crystal structure — layers of molybdenum atoms sandwiched between sulphur layers — gives it uniquely low friction properties. The inter-layer sulphur-to-sulphur bond is weak, allowing the planes to shear easily under load, acting as a solid lubricating film between sliding surfaces. Unlike conventional liquid lubricants, MoS₂ does not rely on viscosity or replenishment; once applied, it bonds to the substrate and lubricates under conditions where oil or grease cannot function — vacuum, extreme pressure, cryogenic temperatures, and food-contact environments. Friction coefficients as low as 0.03–0.05 are achievable in dry or vacuum conditions (0.15–0.30 in humid air). MoS₂ is thermally stable to 350°C in air and up to 1,100°C in inert atmospheres, and maintains lubricity at loads up to 250,000 psi. Application methods include spray/resin-bonded coatings, burnishing, and PVD sputtering.
Process Specifications (MIL-DTL-5541)
0.03–0.05 (dry); 0.15–0.30 (humid)
Friction Coefficient
5–25 µm (bonded); 0.5–3 µm (sputtered)
Coating Thickness
350–400 °C
Max Temp (in air)
1,100 °C
Max Temp (inert atm.)
Up to 250,000 psi
Load Capacity
Type I (air-cure); Type II (heat-cure)
MIL-PRF-46010 Types
Lubricity (dry)
Exceptional
Load Bearing
Very High
Anti-Galling
Excellent
Wet Lubricity
Poor (use dry conditions)
Typical Applications
- Threaded fasteners & studs (anti-seize)
- Precision sliding mechanisms
- Valve stems & seats
- Aerospace actuators & hinges
- Gearbox bearings
- Cryogenic equipment
- Vacuum / cleanroom assemblies
- Injection mould release surfaces
- Military & ordnance components
Best for
- Sliding or rotating parts where liquid lubricant cannot be replenished
- High-pressure, low-speed contact (anti-galling on threaded assemblies)
- Vacuum, cryogenic, or food-contact environments
- Break-in lubricant for close-fit precision components
Limitations
- Performance degrades in humid / wet environments — avoid wet sliding applications
- Galvanic corrosion risk at MoS₂-to-metal interface in wet conditions
Coating 09 of 09
Ceramic Coating
Chromate Conversion Coating
- Plasma-spray TBC
- Micro-arc oxidation (MAO)
- PVD-Ceramic · AMS 2437
The term “ceramic coating” in industrial engineering covers several distinct processes, each producing a ceramic-phase surface layer with different properties and thickness regimes. For aluminium castings, three are most relevant: Micro-Arc Oxidation (MAO / Plasma Electrolytic Oxidation), which converts the aluminium surface directly into a dense, hard aluminium oxide ceramic layer (up to 200 µm, 1,200–2,000 HV) through a spark discharge process; Plasma-Sprayed Thermal Barrier Coatings (TBC), which deposit thick ceramic oxide layers (yttria-stabilised zirconia, alumina-titania) to insulate hot surfaces; and thin-film PVD ceramic coatings (TiN, AlCrN, DLC) applied to machined aluminium tools and precision components. MAO is the most applicable to cast aluminium parts — it is an evolution of anodising that produces a substantially harder, thicker ceramic oxide that retains excellent adhesion and can withstand higher temperatures than conventional anodise.
Ceramic Coating Types & Key Properties
20–200 µm · 1,200–2,000 HV
MAO / PEO on Al
Up to 1,500 °C (oxide layer)
MAO Max Temp
100–600 µm · Thermal insulation
Plasma-Spray TBC (YSZ)
< 2 W/m·K (vs 160 for Al)
TBC Thermal Conduct.
2–10 µm · 2,300 HV · <0.1 COF
PVD Ceramic (TiN)
Superior to hard anodise
Corrosion Resist. (MAO)
Hardness
1200–2000 HV
Thermal Resistance
Excellent
Corrosion Resist.
Excellent
Wear Resistance
Excellent
Typical Applications
- Engine combustion chamber surfaces
- High-temp pump & valve bodies
- Aerospace structural components
- Hydraulic cylinder liners
- Diesel engine pistons & crowns
- Turbine shrouds & wear rings
- Industrial furnace components
- Extreme-wear sliding surfaces
Best for
- Aluminium components exposed to elevated temperatures (>200°C)
- Extreme-wear surfaces where hard anodise is insufficient
- Thermal barrier requirements (plasma spray TBC for engine / power applications)
Limitations
- MAO / PEO process is specialist and carries a premium cost versus anodising
- Plasma spray TBC requires a metallic bond coat layer — specify full system
- Surface roughness after MAO is higher than hard anodise — post-grinding may be required
COATING 01 OF 09
SOFT ANODISE
MIL-A-8625 Type II · BS EN ISO 7599 · ASTM B580 Type B
Soft anodising — formally Type II sulphuric acid anodising — is an electrochemical process that converts the surface of aluminium into a dense, porous aluminium oxide (Al₂O₃) layer. The electrolyte is a dilute sulphuric acid bath at approximately 18–22°C, and the aluminium part serves as the anode. The resulting oxide grows partly into the surface and partly outward, producing a coating that is integral to the base metal rather than a deposited film. The porous structure accepts dyes readily, making Type II the standard process for coloured aluminium components. Sealing in hot deionised water or chromate solution closes the pores and locks in both colour and corrosion resistance.
Process Specifications
Coating Thickness
5–25 µm
Surface Hardness
250–500 HV
Process Temp.
18–22 °C
Corrosion Resistance
336 hrs salt spray (sealed)
Dimensional Change
~50% penetrates, ~50% builds
Colour Options
Full range (clear, black, red, gold, blue…)
Performance Ratings
Corrosion Resist.
Good
Wear Resistance
Moderate
Colour / Aesthetic
Excellent
Dim. Stability
Good
Typical Applications
Architectural panels
Consumer electronics housings
Automotive trim
Electrical junction boxes
Decorative enclosures
Medical device frames
Lighting fixtures
Paint pre-treatment base
Best for
- Components requiring consistent colour across a batch
- Low-to-moderate wear environments
- Indoor corrosion protection
- Cosmetic surfaces where tight tolerances are not critical
Not ideal for
- High-load sliding or abrasive contact surfaces
- Very tight-tolerance bores or shafts (use hard anodise)
- Electrical contact surfaces (use Alodine or tin plate)
Coating 02 of 09
Hard Anodise
- MIL-A-8625 Type III
- AMS 2468 / 2469
- BS 5599
- ASTM B580 Type A
Hard anodising (Type III) uses the same sulphuric acid chemistry as Type II but with a sharply reduced bath temperature (0–5°C), higher current density, and elevated voltage (up to 100V). These conditions drive a denser, harder oxide crystal structure that builds a coating significantly thicker than conventional anodising — from 25 µm up to 75 µm or beyond for salvage applications. The resulting coating reaches 60–70 HRC surface hardness, approaching tool steel. Because roughly half the coating penetrates inward and half builds outward, engineers must account for approximately 25 µm dimensional addition per surface at a 50 µm total coating thickness. Hard anodise lowers fatigue strength slightly; where fatigue is critical, thin-film alternatives should be evaluated.
Process Specifications
25–75 µm (standard 50 µm)
Coating Thickness
60–70 HRC (500–800 HV)
Surface Hardness
0–5 °C (low temperature)
Process Temp.
~25 µm per face at 50 µm total
Dim. Change (per face)
Excellent (Taber CS-10: <5 mg/1000 cycles)
Abrasion Resistance
Grey to dark grey; black dye available
Colour
Corrosion Resist.
Very Good
Wear / Abrasion
Excellent
Dielectric Strength
Good (800V/mm)
Fatigue Resistance
Reduced
Typical Applications
- Hydraulic cylinder bores
- Pump housings & impellers
- Valve bodies
- Sliding guides & rails
- Gearbox housings
- Aircraft structural components
- Textile machinery
- Medical equipment
- Firearm components
Best for
- Aluminium surfaces in high-wear or abrasive service
- Hydraulic and pneumatic cylinder bores
- Components requiring dielectric insulation
- Sliding surfaces without liquid lubrication
Not ideal for
- Allowance for dimensional build-up must be on drawing
- Reduces fatigue life — review stress applications
- High-silicon alloys (LM6, LM13) anodise to lighter shades
Coating 03 of 09
Powder Coating
- ASTM D3451
- ASTM B117 (salt spray)
- ISO 9227
- BS EN 12206
Powder coating is an electrostatic dry-finishing process in which finely ground thermosetting or thermoplastic polymer powder is sprayed onto a pre-treated surface and cured in an oven (typically 160–200°C). The charged powder particles adhere to the earthed substrate, and oven curing fuses them into a smooth, continuous film. Pre-treatment — degreasing followed by zinc or iron phosphating, or chromate conversion — is critical to adhesion and underpins the corrosion protection performance. Typical industrial coating thickness is 60–120 µm. Epoxy-polyester hybrid powders dominate industrial applications for their balance of hardness, flexibility, and weathering resistance. Pure epoxy powders are preferred for underground or heavily exposed structural parts. For aluminium specifically, a zinc phosphate or Alodine pre-treatment step is standard before application.
Process Specifications
60–120 µm (typical); 40–150 µm range
Coating Thickness
160–200 °C / 15–20 min
Cure Temperature
≥ 1,000 hrs (with phosphate pre-treat)
Salt Spray (ASTM B117)
2H–3H (ASTM D3363)
Pencil Hardness
Pass 80 kg·cm (ASTM D2794)
Impact Resistance
Matte (5°) to High Gloss (90°)
Gloss Range
Corrosion Resist.
Excellent
UV / Weathering
Very Good
Colour Range
Unlimited (RAL)
Chemical Resist.
Good
Typical Applications
- Pump & valve bodies (exterior)
- Electrical enclosures & switchgear
- Generator & motor housings
- Automotive brackets & covers
- Fire equipment housings
- Agricultural machinery
- Industrial frames & panels
- Outdoor architectural hardware
Best for
- Exterior surfaces requiring colour, UV resistance, and corrosion protection
- High-volume, cost-effective finishing
- Components where the coating is a barrier (not structural)
Not ideal for
- Internal bores or surfaces with tight dimensional tolerances
- Thin sections where 160°C cure may affect heat-treated alloys
- Electrical contact surfaces
Coating 04 of 09
Tin Plating
- MIL-T-10727
- ASTM B545
- BS EN ISO 2093
Tin electroplating deposits a thin, bright or matte layer of pure tin (≥99.5%) onto the component surface from an acid sulphate or alkaline stannate bath. On aluminium, tin plating requires careful pre-treatment — degreasing, acid activation, and typically a zincate immersion strike — to remove the insulating native oxide and establish an adherent intermediate layer. The resulting tin coating overcomes aluminium’s inherent problem of a non-conductive surface oxide, providing stable, low-resistance electrical contact that does not degrade with time or temperature cycling. MIL-T-10727 defines five thickness classes from 2.5 µm (Class A, mild conditions) to 30 µm (Class E, severe/continuous dampness), with Class C (8–10 µm) being most common for electrical hardware and Class D (15–20 µm) for harsh industrial or outdoor service.
Process Specifications (MIL-T-10727)
2.5 µm
Class A — Mild
5 µm
Class B — Moderate
8 µm (10 µm on steel)
Class C — Severe
15 µm (20 µm on steel)
Class D — Very Severe
≥ 99.5% Sn
Purity
Very low; stable over life
Contact Resistance
Electrical Conduct.
Excellent
Solderability
Excellent
Corrosion Resist.
Good
Anti-Galling
Good
Typical Applications
- Aluminium busbars & conductors
- Electrical terminal blocks
- Switchgear contacts
- Transformer connectors
- Power distribution hardware
- Motor end-caps (contact faces)
- Food equipment (tin safe for contact)
- RF / EMI shielding chassis
Best for
- Aluminium components used as electrical conductors or contact surfaces
- Bus bars, terminals, and connector bodies in power systems
- Components requiring solderability
Limitations
- Tin whisker growth risk on electroplated thin deposits (mitigated by matte tin or reflow)
- Not a high-wear coating — use hard anodise for abrasion resistance
Coating 05 of 09
Phosphating
- MIL-DTL-16232 (Mn)
- ISO 9717
- ASTM B633 (steel)
- UNI ISO 9717
Phosphating is a chemical conversion process that reacts with the metal surface to form a crystalline, non-metallic, water-insoluble phosphate layer. Three principal types are used industrially: iron phosphate (thin, amorphous, primarily for paint adhesion), zinc phosphate (crystalline, 5–15 µm, good corrosion resistance as paint base or with supplementary oil treatment), and manganese phosphate (heavier, 5–25 µm, crystalline black, best wear resistance and oil retention). The phosphate layer’s porous crystalline structure is highly effective at retaining corrosion-inhibiting oils, waxes, and dry-film lubricants, making it widely used as a base coating under paint, powder coat, or as a final oiled finish on precision steel and iron components. On aluminium, zinc phosphating is the most common type and is standard as a pre-treatment step before powder coating.
Process Specifications by Type
0.3–1 µm · Amorphous · Paint base
Iron Phosphate
5–15 µm · Grey · Corrosion + paint base
Zinc Phosphate
5–25 µm · Black · Wear + oil retention
Manganese Phosphate
10–30 g/m²
Coating Weight (Zn)
400 °C
Max Service Temp. (Mn)
Oil, wax, or paint required
Post Treatment
Paint Adhesion
Excellent
Oil Retention
Excellent
Anti-Galling
Very Good
Corrosion (alone)
Moderate (needs post-treat)
Typical Applications
- Pre-treatment for powder coating on Al
- Gearbox & transmission parts (Mn)
- Fasteners & fixings
- Firearms components
- Engine components (running-in)
- Automotive chassis & brackets
- Sliding / threaded steel parts
Best for
- Steel/iron components requiring anti-galling and oil-retention
- Pre-treatment base coat before paint or powder on aluminium
- Running-in surfaces in mechanical assemblies
Limitations
- Alone (unoiled), corrosion resistance is poor — always use post-treatment
- On aluminium, limited to zinc phosphate type as pre-treatment
Coating 06 of 09
Silver Plating
- ASTM B700
- MIL-QQ-S-365
- AMS 2410 / 2412
Silver electroplating deposits a layer of 98–99.9% pure silver onto the substrate from a cyanide or non-cyanide silver electrolyte. Silver has the highest electrical conductivity of all metals (6.3 × 10⁷ S/m) and the highest thermal conductivity among metals in common engineering use. These properties make silver plating the premium choice for RF/microwave waveguides, high-current terminals, and electrical contact surfaces where contact resistance, signal loss, and conductivity stability are engineering constraints. ASTM B700 classifies silver by purity type, surface appearance grade (bright, semi-bright, matt), and by class based on whether a post-plating tarnish inhibitor treatment has been applied. Minimum recommended thickness for engineering use is 2.5–5 µm; for load-bearing electrical contacts, 12–25 µm is typical.
Process Specifications (ASTM B700)
≥ 98% Ag (engineering grade)
Purity
2.5–25 µm (engineering); up to 50 µm
Thickness Range
6.3 × 10⁷ S/m (highest of all metals)
Electrical Conductivity
429 W/m·K
Thermal Conductivity
90–140 HV (electrodeposited)
Hardness
Bright, semi-bright, or matt
Appearance
Elect. Conductivity
Best of all metals
Thermal Conduct.
429 W/m·K
Solderability
Excellent
Tarnish Resistance
Moderate (requires inhibitor)
Typical Applications
- RF & microwave waveguides
- High-current busbar contacts
- Electrical switchgear contacts
- Aerospace connectors
- Medical device contacts
- High-frequency PCB connectors
- Radar & antenna components
- Heat sink interfaces
Best for
- RF/microwave components where skin-effect conductivity dominates
- High-current contacts where resistance and heat generation matter
- Precision electrical connectors requiring consistent low resistance
Limitations
- Tarnishes in sulphur-bearing atmospheres — specify tarnish inhibitor treatment
- Silver migration risk in DC circuits with high voltage/humidity — specify minimum thickness
- High material cost — justify against requirement
Coating 07 of 09
Alodine
Chromate Conversion Coating
- MIL-DTL-5541
- ASTM B449
- MIL-DTL-81706
Alodine — also known as chem film, chromate conversion coating, or Iridite — is a chemical conversion treatment applied to aluminium by immersion or brush application. Unlike anodising or plating, Alodine chemically reacts with the aluminium surface rather than depositing a foreign layer on it, producing a thin, tightly adherent chromate-aluminium complex. This process is governed by MIL-DTL-5541 and results in either a golden-yellow iridescent film (Type I, hexavalent chromium, Class 1A) or a clear/colourless film (Type II, trivalent chromium, Class 1A/3). Because the coating is extremely thin (0.1–1 µm), it is essentially dimensionally neutral — a critical advantage for close-tolerance machined aluminium components. Class 1A provides maximum corrosion protection; Class 3 is specified where low electrical contact resistance must be maintained alongside corrosion protection.
Process Specifications (MIL-DTL-5541)
0.1–1.0 µm (dimensionally neutral)
Coating Thickness
0.4–2.0 g/m²
Coating Weight (Class 1A)
Gold-yellow; max corrosion protect.
Type I (Hex Chrome)
Clear; RoHS-compliant
Type II (Tri Chrome)
< 5,000 µΩ/in² contact resistance
Class 3 Resistance
200+ hrs ASTM B117
Salt Spray (wrought Al)
Dim. Neutrality
Excellent (0.1–1 µm)
Paint Adhesion
Excellent
Corrosion Resist.
Good
Electrical Conduct.
Good (Class 3)
Typical Applications
- Close-tolerance machined Al components
- Aerospace structural parts
- Paint / primer adhesion base coat
- Electronic chassis & enclosures
- Electrical grounding surfaces (Class 3)
- Multi-alloy assemblies
- Repair of damaged anodise
Best for
- Tight-tolerance aluminium where anodise build-up is not acceptable
- Primer base coat on aerospace and defence aluminium structures
- Grounding surfaces requiring electrical contact continuity (Class 3)
Limitations
- Not suitable as a standalone finish for high-wear or harsh outdoor environments
- Type I (Hex chrome) under REACH/RoHS scrutiny — specify Type II where possible
Coating 08 of 09
MoS₂ Coating
Chromate Conversion Coating
- MIL-PRF-46010
- AS5272
- AMS 2526
Molybdenum disulfide (MoS₂) is a naturally occurring lamellar mineral whose crystal structure — layers of molybdenum atoms sandwiched between sulphur layers — gives it uniquely low friction properties. The inter-layer sulphur-to-sulphur bond is weak, allowing the planes to shear easily under load, acting as a solid lubricating film between sliding surfaces. Unlike conventional liquid lubricants, MoS₂ does not rely on viscosity or replenishment; once applied, it bonds to the substrate and lubricates under conditions where oil or grease cannot function — vacuum, extreme pressure, cryogenic temperatures, and food-contact environments. Friction coefficients as low as 0.03–0.05 are achievable in dry or vacuum conditions (0.15–0.30 in humid air). MoS₂ is thermally stable to 350°C in air and up to 1,100°C in inert atmospheres, and maintains lubricity at loads up to 250,000 psi. Application methods include spray/resin-bonded coatings, burnishing, and PVD sputtering.
Process Specifications (MIL-DTL-5541)
0.03–0.05 (dry); 0.15–0.30 (humid)
Friction Coefficient
5–25 µm (bonded); 0.5–3 µm (sputtered)
Coating Thickness
350–400 °C
Max Temp (in air)
1,100 °C
Max Temp (inert atm.)
Up to 250,000 psi
Load Capacity
Type I (air-cure); Type II (heat-cure)
MIL-PRF-46010 Types
Lubricity (dry)
Exceptional
Load Bearing
Very High
Anti-Galling
Excellent
Wet Lubricity
Poor (use dry conditions)
Typical Applications
- Threaded fasteners & studs (anti-seize)
- Precision sliding mechanisms
- Valve stems & seats
- Aerospace actuators & hinges
- Gearbox bearings
- Cryogenic equipment
- Vacuum / cleanroom assemblies
- Injection mould release surfaces
- Military & ordnance components
Best for
- Sliding or rotating parts where liquid lubricant cannot be replenished
- High-pressure, low-speed contact (anti-galling on threaded assemblies)
- Vacuum, cryogenic, or food-contact environments
- Break-in lubricant for close-fit precision components
Limitations
- Performance degrades in humid / wet environments — avoid wet sliding applications
- Galvanic corrosion risk at MoS₂-to-metal interface in wet conditions
Coating 09 of 09
Ceramic Coating
Chromate Conversion Coating
- Plasma-spray TBC
- Micro-arc oxidation (MAO)
- PVD-Ceramic · AMS 2437
The term “ceramic coating” in industrial engineering covers several distinct processes, each producing a ceramic-phase surface layer with different properties and thickness regimes. For aluminium castings, three are most relevant: Micro-Arc Oxidation (MAO / Plasma Electrolytic Oxidation), which converts the aluminium surface directly into a dense, hard aluminium oxide ceramic layer (up to 200 µm, 1,200–2,000 HV) through a spark discharge process; Plasma-Sprayed Thermal Barrier Coatings (TBC), which deposit thick ceramic oxide layers (yttria-stabilised zirconia, alumina-titania) to insulate hot surfaces; and thin-film PVD ceramic coatings (TiN, AlCrN, DLC) applied to machined aluminium tools and precision components. MAO is the most applicable to cast aluminium parts — it is an evolution of anodising that produces a substantially harder, thicker ceramic oxide that retains excellent adhesion and can withstand higher temperatures than conventional anodise.
Ceramic Coating Types & Key Properties
20–200 µm · 1,200–2,000 HV
MAO / PEO on Al
Up to 1,500 °C (oxide layer)
MAO Max Temp
100–600 µm · Thermal insulation
Plasma-Spray TBC (YSZ)
< 2 W/m·K (vs 160 for Al)
TBC Thermal Conduct.
2–10 µm · 2,300 HV · <0.1 COF
PVD Ceramic (TiN)
Superior to hard anodise
Corrosion Resist. (MAO)
Hardness
1200–2000 HV
Thermal Resistance
Excellent
Corrosion Resist.
Excellent
Wear Resistance
Excellent
Typical Applications
- Engine combustion chamber surfaces
- High-temp pump & valve bodies
- Aerospace structural components
- Hydraulic cylinder liners
- Diesel engine pistons & crowns
- Turbine shrouds & wear rings
- Industrial furnace components
- Extreme-wear sliding surfaces
Best for
- Aluminium components exposed to elevated temperatures (>200°C)
- Extreme-wear surfaces where hard anodise is insufficient
- Thermal barrier requirements (plasma spray TBC for engine / power applications)
Limitations
- MAO / PEO process is specialist and carries a premium cost versus anodising
- Plasma spray TBC requires a metallic bond coat layer — specify full system
- Surface roughness after MAO is higher than hard anodise — post-grinding may be required
COATING 01 OF 09
SOFT ANODISE
MIL-A-8625 Type II · BS EN ISO 7599 · ASTM B580 Type B
Soft anodising — formally Type II sulphuric acid anodising — is an electrochemical process that converts the surface of aluminium into a dense, porous aluminium oxide (Al₂O₃) layer. The electrolyte is a dilute sulphuric acid bath at approximately 18–22°C, and the aluminium part serves as the anode. The resulting oxide grows partly into the surface and partly outward, producing a coating that is integral to the base metal rather than a deposited film. The porous structure accepts dyes readily, making Type II the standard process for coloured aluminium components. Sealing in hot deionised water or chromate solution closes the pores and locks in both colour and corrosion resistance.
Process Specifications
Coating Thickness
5–25 µm
Surface Hardness
250–500 HV
Process Temp.
18–22 °C
Corrosion Resistance
336 hrs salt spray (sealed)
Dimensional Change
~50% penetrates, ~50% builds
Colour Options
Full range (clear, black, red, gold, blue…)
Performance Ratings
Corrosion Resist.
Good
Wear Resistance
Moderate
Colour / Aesthetic
Excellent
Dim. Stability
Good
Typical Applications
Architectural panels
Consumer electronics housings
Automotive trim
Electrical junction boxes
Decorative enclosures
Medical device frames
Lighting fixtures
Paint pre-treatment base
Best for
- Components requiring consistent colour across a batch
- Low-to-moderate wear environments
- Indoor corrosion protection
- Cosmetic surfaces where tight tolerances are not critical
Not ideal for
- High-load sliding or abrasive contact surfaces
- Very tight-tolerance bores or shafts (use hard anodise)
- Electrical contact surfaces (use Alodine or tin plate)
Coating 02 of 09
Hard Anodise
- MIL-A-8625 Type III
- AMS 2468 / 2469
- BS 5599
- ASTM B580 Type A
Hard anodising (Type III) uses the same sulphuric acid chemistry as Type II but with a sharply reduced bath temperature (0–5°C), higher current density, and elevated voltage (up to 100V). These conditions drive a denser, harder oxide crystal structure that builds a coating significantly thicker than conventional anodising — from 25 µm up to 75 µm or beyond for salvage applications. The resulting coating reaches 60–70 HRC surface hardness, approaching tool steel. Because roughly half the coating penetrates inward and half builds outward, engineers must account for approximately 25 µm dimensional addition per surface at a 50 µm total coating thickness. Hard anodise lowers fatigue strength slightly; where fatigue is critical, thin-film alternatives should be evaluated.
Process Specifications
25–75 µm (standard 50 µm)
Coating Thickness
60–70 HRC (500–800 HV)
Surface Hardness
0–5 °C (low temperature)
Process Temp.
~25 µm per face at 50 µm total
Dim. Change (per face)
Excellent (Taber CS-10: <5 mg/1000 cycles)
Abrasion Resistance
Grey to dark grey; black dye available
Colour
Corrosion Resist.
Very Good
Wear / Abrasion
Excellent
Dielectric Strength
Good (800V/mm)
Fatigue Resistance
Reduced
Typical Applications
- Hydraulic cylinder bores
- Pump housings & impellers
- Valve bodies
- Sliding guides & rails
- Gearbox housings
- Aircraft structural components
- Textile machinery
- Medical equipment
- Firearm components
Best for
- Aluminium surfaces in high-wear or abrasive service
- Hydraulic and pneumatic cylinder bores
- Components requiring dielectric insulation
- Sliding surfaces without liquid lubrication
Not ideal for
- Allowance for dimensional build-up must be on drawing
- Reduces fatigue life — review stress applications
- High-silicon alloys (LM6, LM13) anodise to lighter shades
Coating 03 of 09
Powder Coating
- ASTM D3451
- ASTM B117 (salt spray)
- ISO 9227
- BS EN 12206
Powder coating is an electrostatic dry-finishing process in which finely ground thermosetting or thermoplastic polymer powder is sprayed onto a pre-treated surface and cured in an oven (typically 160–200°C). The charged powder particles adhere to the earthed substrate, and oven curing fuses them into a smooth, continuous film. Pre-treatment — degreasing followed by zinc or iron phosphating, or chromate conversion — is critical to adhesion and underpins the corrosion protection performance. Typical industrial coating thickness is 60–120 µm. Epoxy-polyester hybrid powders dominate industrial applications for their balance of hardness, flexibility, and weathering resistance. Pure epoxy powders are preferred for underground or heavily exposed structural parts. For aluminium specifically, a zinc phosphate or Alodine pre-treatment step is standard before application.
Process Specifications
60–120 µm (typical); 40–150 µm range
Coating Thickness
160–200 °C / 15–20 min
Cure Temperature
≥ 1,000 hrs (with phosphate pre-treat)
Salt Spray (ASTM B117)
2H–3H (ASTM D3363)
Pencil Hardness
Pass 80 kg·cm (ASTM D2794)
Impact Resistance
Matte (5°) to High Gloss (90°)
Gloss Range
Corrosion Resist.
Excellent
UV / Weathering
Very Good
Colour Range
Unlimited (RAL)
Chemical Resist.
Good
Typical Applications
- Pump & valve bodies (exterior)
- Electrical enclosures & switchgear
- Generator & motor housings
- Automotive brackets & covers
- Fire equipment housings
- Agricultural machinery
- Industrial frames & panels
- Outdoor architectural hardware
Best for
- Exterior surfaces requiring colour, UV resistance, and corrosion protection
- High-volume, cost-effective finishing
- Components where the coating is a barrier (not structural)
Not ideal for
- Internal bores or surfaces with tight dimensional tolerances
- Thin sections where 160°C cure may affect heat-treated alloys
- Electrical contact surfaces
Coating 04 of 09
Tin Plating
- MIL-T-10727
- ASTM B545
- BS EN ISO 2093
Tin electroplating deposits a thin, bright or matte layer of pure tin (≥99.5%) onto the component surface from an acid sulphate or alkaline stannate bath. On aluminium, tin plating requires careful pre-treatment — degreasing, acid activation, and typically a zincate immersion strike — to remove the insulating native oxide and establish an adherent intermediate layer. The resulting tin coating overcomes aluminium’s inherent problem of a non-conductive surface oxide, providing stable, low-resistance electrical contact that does not degrade with time or temperature cycling. MIL-T-10727 defines five thickness classes from 2.5 µm (Class A, mild conditions) to 30 µm (Class E, severe/continuous dampness), with Class C (8–10 µm) being most common for electrical hardware and Class D (15–20 µm) for harsh industrial or outdoor service.
Process Specifications (MIL-T-10727)
2.5 µm
Class A — Mild
5 µm
Class B — Moderate
8 µm (10 µm on steel)
Class C — Severe
15 µm (20 µm on steel)
Class D — Very Severe
≥ 99.5% Sn
Purity
Very low; stable over life
Contact Resistance
Electrical Conduct.
Excellent
Solderability
Excellent
Corrosion Resist.
Good
Anti-Galling
Good
Typical Applications
- Aluminium busbars & conductors
- Electrical terminal blocks
- Switchgear contacts
- Transformer connectors
- Power distribution hardware
- Motor end-caps (contact faces)
- Food equipment (tin safe for contact)
- RF / EMI shielding chassis
Best for
- Aluminium components used as electrical conductors or contact surfaces
- Bus bars, terminals, and connector bodies in power systems
- Components requiring solderability
Limitations
- Tin whisker growth risk on electroplated thin deposits (mitigated by matte tin or reflow)
- Not a high-wear coating — use hard anodise for abrasion resistance
Coating 05 of 09
Phosphating
- MIL-DTL-16232 (Mn)
- ISO 9717
- ASTM B633 (steel)
- UNI ISO 9717
Phosphating is a chemical conversion process that reacts with the metal surface to form a crystalline, non-metallic, water-insoluble phosphate layer. Three principal types are used industrially: iron phosphate (thin, amorphous, primarily for paint adhesion), zinc phosphate (crystalline, 5–15 µm, good corrosion resistance as paint base or with supplementary oil treatment), and manganese phosphate (heavier, 5–25 µm, crystalline black, best wear resistance and oil retention). The phosphate layer’s porous crystalline structure is highly effective at retaining corrosion-inhibiting oils, waxes, and dry-film lubricants, making it widely used as a base coating under paint, powder coat, or as a final oiled finish on precision steel and iron components. On aluminium, zinc phosphating is the most common type and is standard as a pre-treatment step before powder coating.
Process Specifications by Type
0.3–1 µm · Amorphous · Paint base
Iron Phosphate
5–15 µm · Grey · Corrosion + paint base
Zinc Phosphate
5–25 µm · Black · Wear + oil retention
Manganese Phosphate
10–30 g/m²
Coating Weight (Zn)
400 °C
Max Service Temp. (Mn)
Oil, wax, or paint required
Post Treatment
Paint Adhesion
Excellent
Oil Retention
Excellent
Anti-Galling
Very Good
Corrosion (alone)
Moderate (needs post-treat)
Typical Applications
- Pre-treatment for powder coating on Al
- Gearbox & transmission parts (Mn)
- Fasteners & fixings
- Firearms components
- Engine components (running-in)
- Automotive chassis & brackets
- Sliding / threaded steel parts
Best for
- Steel/iron components requiring anti-galling and oil-retention
- Pre-treatment base coat before paint or powder on aluminium
- Running-in surfaces in mechanical assemblies
Limitations
- Alone (unoiled), corrosion resistance is poor — always use post-treatment
- On aluminium, limited to zinc phosphate type as pre-treatment
Coating 06 of 09
Silver Plating
- ASTM B700
- MIL-QQ-S-365
- AMS 2410 / 2412
Silver electroplating deposits a layer of 98–99.9% pure silver onto the substrate from a cyanide or non-cyanide silver electrolyte. Silver has the highest electrical conductivity of all metals (6.3 × 10⁷ S/m) and the highest thermal conductivity among metals in common engineering use. These properties make silver plating the premium choice for RF/microwave waveguides, high-current terminals, and electrical contact surfaces where contact resistance, signal loss, and conductivity stability are engineering constraints. ASTM B700 classifies silver by purity type, surface appearance grade (bright, semi-bright, matt), and by class based on whether a post-plating tarnish inhibitor treatment has been applied. Minimum recommended thickness for engineering use is 2.5–5 µm; for load-bearing electrical contacts, 12–25 µm is typical.
Process Specifications (ASTM B700)
≥ 98% Ag (engineering grade)
Purity
2.5–25 µm (engineering); up to 50 µm
Thickness Range
6.3 × 10⁷ S/m (highest of all metals)
Electrical Conductivity
429 W/m·K
Thermal Conductivity
90–140 HV (electrodeposited)
Hardness
Bright, semi-bright, or matt
Appearance
Elect. Conductivity
Best of all metals
Thermal Conduct.
429 W/m·K
Solderability
Excellent
Tarnish Resistance
Moderate (requires inhibitor)
Typical Applications
- RF & microwave waveguides
- High-current busbar contacts
- Electrical switchgear contacts
- Aerospace connectors
- Medical device contacts
- High-frequency PCB connectors
- Radar & antenna components
- Heat sink interfaces
Best for
- RF/microwave components where skin-effect conductivity dominates
- High-current contacts where resistance and heat generation matter
- Precision electrical connectors requiring consistent low resistance
Limitations
- Tarnishes in sulphur-bearing atmospheres — specify tarnish inhibitor treatment
- Silver migration risk in DC circuits with high voltage/humidity — specify minimum thickness
- High material cost — justify against requirement
Coating 07 of 09
Alodine
Chromate Conversion Coating
- MIL-DTL-5541
- ASTM B449
- MIL-DTL-81706
Alodine — also known as chem film, chromate conversion coating, or Iridite — is a chemical conversion treatment applied to aluminium by immersion or brush application. Unlike anodising or plating, Alodine chemically reacts with the aluminium surface rather than depositing a foreign layer on it, producing a thin, tightly adherent chromate-aluminium complex. This process is governed by MIL-DTL-5541 and results in either a golden-yellow iridescent film (Type I, hexavalent chromium, Class 1A) or a clear/colourless film (Type II, trivalent chromium, Class 1A/3). Because the coating is extremely thin (0.1–1 µm), it is essentially dimensionally neutral — a critical advantage for close-tolerance machined aluminium components. Class 1A provides maximum corrosion protection; Class 3 is specified where low electrical contact resistance must be maintained alongside corrosion protection.
Process Specifications (MIL-DTL-5541)
0.1–1.0 µm (dimensionally neutral)
Coating Thickness
0.4–2.0 g/m²
Coating Weight (Class 1A)
Gold-yellow; max corrosion protect.
Type I (Hex Chrome)
Clear; RoHS-compliant
Type II (Tri Chrome)
< 5,000 µΩ/in² contact resistance
Class 3 Resistance
200+ hrs ASTM B117
Salt Spray (wrought Al)
Dim. Neutrality
Excellent (0.1–1 µm)
Paint Adhesion
Excellent
Corrosion Resist.
Good
Electrical Conduct.
Good (Class 3)
Typical Applications
- Close-tolerance machined Al components
- Aerospace structural parts
- Paint / primer adhesion base coat
- Electronic chassis & enclosures
- Electrical grounding surfaces (Class 3)
- Multi-alloy assemblies
- Repair of damaged anodise
Best for
- Tight-tolerance aluminium where anodise build-up is not acceptable
- Primer base coat on aerospace and defence aluminium structures
- Grounding surfaces requiring electrical contact continuity (Class 3)
Limitations
- Not suitable as a standalone finish for high-wear or harsh outdoor environments
- Type I (Hex chrome) under REACH/RoHS scrutiny — specify Type II where possible
Coating 08 of 09
MoS₂ Coating
Chromate Conversion Coating
- MIL-PRF-46010
- AS5272
- AMS 2526
Molybdenum disulfide (MoS₂) is a naturally occurring lamellar mineral whose crystal structure — layers of molybdenum atoms sandwiched between sulphur layers — gives it uniquely low friction properties. The inter-layer sulphur-to-sulphur bond is weak, allowing the planes to shear easily under load, acting as a solid lubricating film between sliding surfaces. Unlike conventional liquid lubricants, MoS₂ does not rely on viscosity or replenishment; once applied, it bonds to the substrate and lubricates under conditions where oil or grease cannot function — vacuum, extreme pressure, cryogenic temperatures, and food-contact environments. Friction coefficients as low as 0.03–0.05 are achievable in dry or vacuum conditions (0.15–0.30 in humid air). MoS₂ is thermally stable to 350°C in air and up to 1,100°C in inert atmospheres, and maintains lubricity at loads up to 250,000 psi. Application methods include spray/resin-bonded coatings, burnishing, and PVD sputtering.
Process Specifications (MIL-DTL-5541)
0.03–0.05 (dry); 0.15–0.30 (humid)
Friction Coefficient
5–25 µm (bonded); 0.5–3 µm (sputtered)
Coating Thickness
350–400 °C
Max Temp (in air)
1,100 °C
Max Temp (inert atm.)
Up to 250,000 psi
Load Capacity
Type I (air-cure); Type II (heat-cure)
MIL-PRF-46010 Types
Lubricity (dry)
Exceptional
Load Bearing
Very High
Anti-Galling
Excellent
Wet Lubricity
Poor (use dry conditions)
Typical Applications
- Threaded fasteners & studs (anti-seize)
- Precision sliding mechanisms
- Valve stems & seats
- Aerospace actuators & hinges
- Gearbox bearings
- Cryogenic equipment
- Vacuum / cleanroom assemblies
- Injection mould release surfaces
- Military & ordnance components
Best for
- Sliding or rotating parts where liquid lubricant cannot be replenished
- High-pressure, low-speed contact (anti-galling on threaded assemblies)
- Vacuum, cryogenic, or food-contact environments
- Break-in lubricant for close-fit precision components
Limitations
- Performance degrades in humid / wet environments — avoid wet sliding applications
- Galvanic corrosion risk at MoS₂-to-metal interface in wet conditions
Coating 09 of 09
Ceramic Coating
Chromate Conversion Coating
- Plasma-spray TBC
- Micro-arc oxidation (MAO)
- PVD-Ceramic · AMS 2437
The term “ceramic coating” in industrial engineering covers several distinct processes, each producing a ceramic-phase surface layer with different properties and thickness regimes. For aluminium castings, three are most relevant: Micro-Arc Oxidation (MAO / Plasma Electrolytic Oxidation), which converts the aluminium surface directly into a dense, hard aluminium oxide ceramic layer (up to 200 µm, 1,200–2,000 HV) through a spark discharge process; Plasma-Sprayed Thermal Barrier Coatings (TBC), which deposit thick ceramic oxide layers (yttria-stabilised zirconia, alumina-titania) to insulate hot surfaces; and thin-film PVD ceramic coatings (TiN, AlCrN, DLC) applied to machined aluminium tools and precision components. MAO is the most applicable to cast aluminium parts — it is an evolution of anodising that produces a substantially harder, thicker ceramic oxide that retains excellent adhesion and can withstand higher temperatures than conventional anodise.
Ceramic Coating Types & Key Properties
20–200 µm · 1,200–2,000 HV
MAO / PEO on Al
Up to 1,500 °C (oxide layer)
MAO Max Temp
100–600 µm · Thermal insulation
Plasma-Spray TBC (YSZ)
< 2 W/m·K (vs 160 for Al)
TBC Thermal Conduct.
2–10 µm · 2,300 HV · <0.1 COF
PVD Ceramic (TiN)
Superior to hard anodise
Corrosion Resist. (MAO)
Hardness
1200–2000 HV
Thermal Resistance
Excellent
Corrosion Resist.
Excellent
Wear Resistance
Excellent
Typical Applications
- Engine combustion chamber surfaces
- High-temp pump & valve bodies
- Aerospace structural components
- Hydraulic cylinder liners
- Diesel engine pistons & crowns
- Turbine shrouds & wear rings
- Industrial furnace components
- Extreme-wear sliding surfaces
Best for
- Aluminium components exposed to elevated temperatures (>200°C)
- Extreme-wear surfaces where hard anodise is insufficient
- Thermal barrier requirements (plasma spray TBC for engine / power applications)
Limitations
- MAO / PEO process is specialist and carries a premium cost versus anodising
- Plasma spray TBC requires a metallic bond coat layer — specify full system
- Surface roughness after MAO is higher than hard anodise — post-grinding may be required
Coating Processes
0
HRC Hard Anodise
0
-70
MoS₂ Friction Coeff.
0
Salt Spray Hours (PC)
0
+
Dim. Change (Alodine)
±
0
Coating Selection Guide
What Does Your Application Need?
Quick Reference
Coating Comparison
All nine processes compared across the properties most relevant to engineering decisions.
● High ◑ Medium ○ Low / Not applicable
| Coating | Thickness (mm) | Corrosion | Wear/Hardness | Electr. Conduct. | Lubricity | Thermal | Dim. Change | Key Standard |
|---|---|---|---|---|---|---|---|---|
| Soft Anodise | 5–25 | Moderate | MIL-A-8625 T.II | |||||
| Hard Anodise | 25–75 | Significant | MIL-A-8625 T.III | |||||
| Powder Coating | 60–120 | High (60–120 µm) | ASTM D3451 | |||||
| Tin Plating | 2.5–30 | Low | MIL-T-10727 | |||||
| Phosphating | 0.3–25 | Low | ISO 9717 | |||||
| Silver Plating | 2.5–25 | Low | ASTM B700 | |||||
| Alodine | 0.1–1 | Negligible | MIL-DTL-5541 | |||||
| MoS₂ Coating | 0.5–25 | Low | MIL-PRF-46010 | |||||
| Ceramic (MAO) | 20–200 | Significant | AMS 2437 |
Not Sure Which
Coating is Right?
Coating selection depends on alloy grade, service environment, dimensional tolerances, and downstream assembly requirements. Tell us what your component does, where it operates, and what your drawing specifies — our engineering team will advise on the most appropriate coating process and specification.
- We'll review your component drawing, alloy, and service conditions
- Recommend the right coating type, class, and thickness specification
- Flag any dimensional allowances needed on the machining drawing
- Advise on pre-treatment requirements and multi-coat systems
- No-obligation technical review — response within 1 business day
FAQ
Coating Questions
Technical questions about coating selection, specifications, dimensional effects, and process suitability for aluminium castings and machined components.
Soft anodise vs hard anodise — what is the difference?
Soft anodise (Type II) runs at 18–22°C in dilute sulphuric acid, producing a 5–25 µm porous coating at 250–500 HV — suitable for colour, corrosion protection, and light wear. Hard anodise (Type III) runs at 0–5°C with higher voltage, producing a denser 25–75 µm coating at 60–70 HRC (500–800 HV) — for hydraulic cylinders, sliding guides, and surfaces where wear resistance is the primary requirement.
Which coating for marine / wet environment aluminium?
For exterior surfaces with corrosion and UV exposure, epoxy or polyester powder coating over zinc phosphate pre-treatment gives the best cost/performance balance (1,000+ hrs salt spray). For engineered surfaces requiring abrasion and corrosion resistance together, hard anodise (Type III) is preferred. Alodine (MIL-DTL-5541) is suitable as a base for paint systems where near-zero dimensional change is needed.
Does hard anodising change part dimensions?
Yes. Hard anodise grows roughly 50% into the surface and 50% outward from the original surface. A 50 µm total coating therefore adds approximately 25 µm per coated face outward. On a bore, the bore diameter reduces; on a shaft, the OD increases. Engineering drawings must include pre-anodise machining allowances on all critical features — typically specified as a “pre-plate” size on the drawing.
What coating is recommended for aluminium busbars and electrical terminals?
Tin plating per MIL-T-10727 or ASTM B545 is the industry standard for aluminium busbars and electrical terminals. It overcomes aluminium’s non-conductive oxide layer, providing stable low contact resistance, solderability, and protection against galvanic corrosion at dissimilar metal joints. Class C (8–10 µm) is standard for general electrical hardware; Class D (15–20 µm) for demanding outdoor or industrial service.
What is Alodine and when should it be specified?
Alodine is a chromate conversion coating per MIL-DTL-5541 that reacts with the aluminium surface to form a 0.1–1 µm adherent chromate layer — essentially dimensionally neutral. Specify it when: (a) close tolerances make anodise build-up unacceptable; (b) maximum paint adhesion is needed before priming; or (c) electrical grounding continuity must be maintained alongside corrosion protection (specify Class 3). Specify Type II (trivalent) where RoHS/REACH compliance is required.
What is MoS₂ coating and when is it used?
MoS₂ (molybdenum disulfide) is a dry film lubricant coating per MIL-PRF-46010. Its lamellar crystal structure allows layers to shear at very low force, giving friction coefficients as low as 0.03–0.05 in dry/vacuum conditions. Specify it for sliding or rotating assemblies where liquid lubricants cannot be replenished, for anti-galling on precision threaded assemblies, and for components in vacuum, cryogenic, or food-contact environments. Performance degrades in wet conditions — avoid for wet sliding applications.