Reference

Chromate Conversion Coating Spec

Chem film / Alodine on aluminum per MIL-DTL-5541: Type I vs Type II, Class 1A vs Class 3, why it stays electrically conductive, and how to call it out on a drawing.

Chromate conversion coating, also called chem film, Alodine, or Iridite, is a thin conversion coating for aluminum that adds corrosion resistance and a paint/primer base while leaving the surface electrically conductive. Unlike anodizing, which grows an insulating oxide, chem film keeps low electrical resistance, so it is the go-to finish for grounding points, EMI enclosures, and parts that need to be painted or bonded. It is governed by MIL-DTL-5541, which splits it into Type I and Type II by chemistry and Class 1A and Class 3 by function.

Type I vs Type II (MIL-DTL-5541)

Type is about the chemistry of the coating. Type II was added so parts can meet RoHS and REACH restrictions on hexavalent chromium.

TypeChemistryNotes
Type IHexavalent chromiumTraditional gold/tan chromate; well proven, but hex chrome is restricted under RoHS/REACH
Type IINo hexavalent chromium (e.g. trivalent)Environmentally friendlier; often near-clear; specified where hex chrome is not permitted

MIL-DTL-5541F covers both hexavalent and trivalent systems with performance verification. Color is not a reliable indicator of type or class.

Class 1A vs Class 3

Class is about function. The same approved chemistries can be run to either class by adjusting immersion time and film thickness.

ClassOptimized forUse for
Class 1AMaximum corrosion protectionPainted or unpainted parts needing the most corrosion resistance and a paint/primer base
Class 3Low electrical resistanceGrounding surfaces, EMI/RFI enclosures, and contacts that must stay electrically conductive while still resisting corrosion

Class 3 is run thinner to keep contact resistance low, so it trades some corrosion life for conductivity. Confirm the class against the part's function.

Chem film vs anodizing, and how to call it out

Anodizing grows a hard, insulating oxide and adds a little thickness; chromate conversion is very thin, electrically conductive, and does not build meaningful dimension. Pick chem film when the part must ground, carry current, or be a paint base with no insulation; pick anodize for hardness and wear. A complete callout names the standard, type, and class, e.g. "Chemical conversion coat per MIL-DTL-5541, Type II, Class 3." Note masked areas and whether the part is subsequently painted.

Values reflect MIL-DTL-5541F and common practice. Type, class, color, and electrical performance vary by process and alloy; confirm what a shop can hold on your aluminum before release.

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Frequently Asked Questions

What is the difference between chem film Type I and Type II?
Under MIL-DTL-5541, Type I chromate conversion coatings contain hexavalent chromium (the traditional gold/tan chem film), while Type II contains no hexavalent chromium and typically uses trivalent chromium. Type II is specified where RoHS and REACH restrict hexavalent chrome. Both are covered by MIL-DTL-5541F with performance verification.
What is the difference between Class 1A and Class 3?
Class is about function. Class 1A is optimized for maximum corrosion protection, painted or unpainted. Class 3 is run thinner for low electrical resistance, so it suits grounding surfaces and EMI enclosures that must stay conductive while still resisting corrosion. The same chemistry can be run to either class by changing immersion time.
Is chromate conversion coating electrically conductive?
Yes. Chem film is very thin and, unlike anodizing, leaves the aluminum electrically conductive, which is why MIL-DTL-5541 Class 3 is used for grounding points, EMI/RFI enclosures, and current-carrying contacts. Anodizing grows an insulating oxide, so it is not used where the surface must conduct.
Should I use chem film or anodize on aluminum?
Use chromate conversion coating (chem film) when the part must ground, carry current, or serve as a conductive paint base, since it is thin and conductive. Use anodize when you need hardness, wear resistance, or a dyed decorative finish and electrical insulation is acceptable. They solve different problems, so match the finish to the part's function.

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