MIL-DTL-5541 Chem Film: Types, Classes and the Hex-Free Question

The consequence is not that one type passes and the other fails — both have qualified materials meeting Class 1A and Class 3. The difference shows at the edges: Type I is credited with better performance on deep scratches and filiform corrosion, because nothing in a hex-free film migrates to heal damage. Type II films are thinner and lighter, which is a visual problem before it is a technical one.

Migration is driven by regulation, not preference. Hexavalent chromium is restricted in electrical and electronic equipment under RoHS at 0.1 percent by weight in a homogeneous material, and chromium trioxide sits under REACH authorisation in the EU. Defense and space hardware falls outside RoHS scope and aerospace programs have continued under authorisations — which is why the same enclosure can be Type I on the defense variant and Type II on the commercial one.

The most common rejection of good chem film is “it doesn’t look yellow, so it isn’t coated.” MIL-DTL-5541 does not specify a color. It requires the coating to be continuous, visibly discernible in daylight, and free of powdery or loose coating, voids, scratches and flaws.

“Visibly discernible” is not “yellow”. Hexavalent films are strongly colored because chromate is colored. Hex-free films are thin and largely colorless; a Type II Class 3 film on bright machined 6061 can be nearly invisible under a QC lamp. Nothing is wrong with the part.

Two habits prevent the rejection. Do not write color into the drawing as an acceptance criterion unless you intend to restrict the chemistry — and if you do, specify the type, which is the field that controls it. And verify by certification and process control rather than by eye: the certification records the qualified material, type and class, and the periodic corrosion and resistance testing behind them. Where visual confirmation matters, a hex-free film can be checked chemically rather than by shade.

The reverse error costs money too: accepting a heavy yellow film where Class 3 was required. That one is caught by resistance measurement, not by looking.

Prints treat them as alternatives, which they are not. Chem film is a reaction film measured in millionths of an inch: it conducts, adds no meaningful dimension, and is soft. Anodizing is an electrochemical oxide that grows into and out of the surface: it insulates, it builds, and it is hard.

Under paint either can work. Chem film under primer is the dimensionally neutral path and keeps the substrate conductive where paint is later removed for bonding; anodize under paint gives a tougher base at the cost of growth and an insulating layer. The combination that confuses shops is the correct one on most airframe and enclosure hardware: anodize the general surfaces, chem film the faying and grounding surfaces. That means masking, two operations, and a drawing that says which surfaces get which — a flag note and a surface list, not a general note. Running both processes in one building is why we handle this under one roof.

A fresh conversion coating is a soft, gelatinous layer until it dries and hardens. Handled wet or bagged too early it takes fingerprints, glove marks, rack witness and rinse-water tracks that will not polish out. Heat is the other trap: corrosion resistance falls off with elevated-temperature exposure, so force-drying, downstream bakes and paint cures all matter. If a bake follows, let the drawing state the order of operations.

MIL-DTL-5541 permits touch-up of mechanically damaged areas when the contract calls for it, using the same qualified material by brush, swab or pen, and caps how much of an item may be repaired that way. Touch-up covers handling damage and surfaces exposed after coating — a drilled hole, a trimmed edge, a machined-off masked area. It is not a way to salvage a poorly processed lot; widespread damage means recleaning and recoating. Say on the drawing whether field touch-up is permitted, to the same type and class.

Gleco Plating has finished aluminum in Texas since 1979. Yellow chem film runs at our Rowlett plant, 2220 Grisham Dr, alongside the anodize line and our other metal coating processes — so a part needing conversion coating on the faying surfaces and anodize elsewhere is masked and routed inside one building. Rowlett serves the DFW corridor directly, including chem film work out of Fort Worth; plant details are on the Rowlett facility page.

Tell us the type and class at RFQ, with alloy and temper. If your program has moved to hex-free, say so on the purchase order as well as the print — that is the field most often left off, and the one that decides the chemistry. Most of this work comes from aerospace and defense customers, so flow-downs, first article and lot traceability are routine. We hold AS9100D and ISO 9001:2015, are ITAR Registered, DFARS compliant and RoHS compliant; scope is on the certifications page. For the process itself rather than the specification, see chemical conversion coating.