Chromate Conversion Coating (Alodine / Chem Film) for Die Cast Parts

Chromate conversion coating is the thinnest, cheapest corrosion protection you can put on an aluminum die casting - and the most misunderstood. Buyers meet it under three names (Alodine, chem film, chromate) that all mean the same family of processes, and under a military specification (MIL-DTL-5541) whose types and classes actually decide what you get.

This guide explains what the coating is, how the naming maps to the spec, and when it is the right call for die cast parts.

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The Naming Confusion, Cleared Up

Name you will hear	What it actually is
Chromate conversion coating	The generic process name - a chemical reaction that converts the aluminum surface into a protective chromate layer
Chem film	Shop-floor shorthand for the same process ("chemical film")
Alodine	Henkel's brand name for its chromate conversion chemicals - like calling all tissues "Kleenex"
Iridite	MacDermid's competing brand name
Yellow chromate / clear chromate	Appearance descriptions that map to spec classes (below)

When a drawing says "Alodine," the engineer almost always means "chromate conversion coating per MIL-DTL-5541" - the brand name is just habit. What matters is the type and class called out.

What the Coating Is (and Is Not)

The part is immersed in (or sprayed with) an acidic chromate solution. The bath dissolves a microscopic amount of aluminum and re-deposits it as a gel-like chromium-containing film, typically 0.5 to 2.5 um thick. That film:

• Blocks corrosion by passivating the surface
• Anchors paint and powder coat (it is the standard adhesion pretreatment)
• Stays electrically conductive - unlike anodize or powder coat
• Adds essentially no dimension - threads, bores, and mating faces are unaffected

What it is not: decorative, thick, or particularly scratch-resistant. Fresh chromate films are soft and mark easily; they harden over about 24 hours of aging. Nobody wins a design award with a chem film finish - that is not its job.

MIL-DTL-5541: Types and Classes That Matter

The current specification (superseding MIL-C-5541) splits coatings two ways:

By chemistry - Type:

• Type I: hexavalent chromium (Cr6+). The traditional chemistry - best-established corrosion performance, but hex chrome faces tightening environmental regulation.
• Type II: trivalent chromium or chrome-free (TCP and alternatives). RoHS-compliant; the default for new commercial programs and any part sold into the EU.

By performance - Class:

• Class 1A: maximum corrosion protection. Thicker, usually golden/yellow to brown appearance. Used as a final finish or under paint where corrosion is the enemy.
• Class 3: corrosion protection with low electrical resistance. Thinner, clear to faintly iridescent. The class you specify for grounding pads, EMI contact surfaces, and electrical bonding points.

So "MIL-DTL-5541, Type II, Class 3" decodes to: RoHS-compliant clear chromate that protects against corrosion while staying electrically conductive - the most common callout on electronics housings.

Alodine vs Anodizing

Buyers mix these up constantly because both are "aluminum surface treatments that start with A." They are opposites in almost every property:

Property	Chromate conversion (Alodine)	Anodizing (Type II)
Thickness	0.5-2.5 um	5-25 um
Electrical	Conductive (Class 3 especially)	Insulating
Hardness/wear	Soft, marks when fresh	Ceramic-hard
Appearance	Clear to gold iridescent	Uniform metallic, dyeable
Corrosion alone	Moderate (better under paint)	Good (better when sealed)
Dimension change	Negligible	Small but real
Cost	Low	Moderate

Rule of thumb: anodize is a finish; chem film is a foundation - or the answer when conductivity or zero dimension change rules out everything else. For the full comparison with powder coating, see anodizing vs powder coating.

Chromate on Die Castings: What to Watch

High-silicon alloys affect appearance, not function. On A380/ADC12 (8-12% silicon), the coating still protects, but the color comes out darker and less uniform than on wrought aluminum. If consistent gold appearance matters, run a sample first.

It is the standard pretreatment under powder coat. The classic stack - clean, etch, deoxidize, chromate, powder coat - is how outdoor die cast enclosures survive. Skipping the conversion layer to save pennies is how you get adhesion failure and underfilm corrosion in year two. (Chrome-free pretreatments now fill this role where RoHS or REACH applies.)

Magnesium needs its own chemistry. Magnesium die castings (AZ91D, AM60B) are too reactive for standard aluminum chromate baths; they get dedicated magnesium conversion treatments before painting. The principle is the same; the bath is different.

Zinc usually goes another way. Zamak parts more often take plating (nickel, chrome) or go straight to powder coat. Chromate on zinc appears mainly as a post-plating sealer - the iridescent or black finish on plated hardware.

Specifying It on Your Drawing

A good callout takes one line:

Chromate conversion coat per MIL-DTL-5541, Type II, Class 1A (or Class 3 where electrical contact is required). Mask threads M6 and grounding face as noted.

Then confirm with your supplier before tooling release:

1. Which type/class the finishing line actually runs (Type I lines are not RoHS; assume nothing)
2. Whether masking of threads, bores, or contact pads is included in the quote
3. Salt-spray or adhesion test expectations if the coating sits under paint
4. Sample approval for appearance on your specific alloy

Finish decisions interact with tooling: once the die is cut, adding "oh, and it needs to survive 500 hours of salt spray" gets expensive. Our die casting surface finishing guide covers how finishing choice feeds back into gate design and machining allowance.

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Frequently Asked Questions

Is Alodine the same as anodizing?

No. Alodine (chromate conversion) is a sub-micron conductive chemical film; anodizing is a much thicker, hard, insulating oxide grown by electric current. They solve different problems and are sometimes even used together on different faces of the same part.

Is chromate conversion coating RoHS compliant?

Type I (hexavalent chromium) is not. Type II - trivalent chromium (TCP) or chrome-free - is RoHS compliant and is what you should specify for new programs unless a legacy military contract demands Type I.

How much corrosion protection does chem film give on its own?

Enough for indoor and mild-service aluminum parts, and it meaningfully slows filiform corrosion. For outdoor or marine exposure, treat it as a pretreatment under paint or powder coat rather than a standalone barrier.

Does chromate conversion coating change dimensions?

Effectively no. At 0.5-2.5 um, it is the only corrosion finish you can apply to threaded holes and precision bores without reworking tolerances - which is exactly why engineers specify it.

Can die castings with porosity be chromate coated?

Yes - porosity does not block the process the way it ruins decorative anodizing. Visible pits remain visible (the film is transparent-thin), but corrosion protection still works. Severe porosity is a casting problem to fix at the source; see die casting porosity testing.

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Related Reading

• anodizing vs powder coating - the decorative finish decision
• die casting surface finishing guide - every finish option on one table
• bead blasting - mechanical surface prep before coating
• die casting plating services - plating options for zinc castings
• die casting quality standards - specs and inspection language that belongs on drawings

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Have a MIL-DTL-5541 callout on your drawing? Send the spec to yaoqingpu1983@gmail.com and we will confirm the finishing route for your alloy before tooling release | +86 138 1403 4409 | No.6, Rungu Road, Nanjing, China