Short answer: run a Class 3 chromate conversion coating on every surface that has to conduct. Chem film is the shop name; Iridite and Alodine are two makers' labels for the same dip process, and MIL-DTL-5541 Class 3 is the film that holds a joint under 5 mΩ per square inch of contact. Anodize everything that stays dead, because anodizing is a ceramic insulator, and seal the seam edges where a scratch would start corrosion. Keep the joint below about 60 C [140 F]: chromate dries out above that and its resistance climbs. Hotter than that, or on a hex-free RoHS build, electroless nickel is the finish that stays conductive and takes the wear.
What each finish is
Chem film is a conversion coating, so the aluminum surface itself reacts in the bath and becomes a chromium-complex gel a few tenths of a micrometre thick. Nothing builds up, which is why threads and tapped holes keep their dimensions. Iridite and Alodine are two suppliers' names for the same chemistry; order by the standard and the shop's product line stops mattering. Class 1A is the thicker film bought for corrosion protection alone. Class 3 is the thin film the standard's contact-resistance test aims at, the one your ground path depends on.
Anodize is the opposite electrically. It grows an aluminum-oxide ceramic that is hard enough to shrug off handling and thick enough to insulate, which is exactly why joint areas get masked before the anodize tank and chem-filmed after. Electroless nickel is deposited metal, plated with no current, so it covers threads and recesses uniformly and conducts at any temperature your box will see. Tin plate conducts and solders, harder than chem film though softer than nickel.
| Finish | Conductivity at a joint | Wear and handling | Corrosion cover | Where it belongs |
|---|---|---|---|---|
| Chem film, Class 3 | Meets the milliohm-class contact target | Soft; fine scratches re-passivate | Good alone, better under paint | Faying surfaces, ground lands, fastener holes |
| Chem film, corrosion class | No conductivity target | Soft | Best of the conversion films | Non-joint areas when you skip anodize |
| Anodize, Type II | Insulator | Hardest of the group | Excellent | Door faces and exterior skins |
| Electroless nickel | Plated metal path, stable hot and aged | Hard; survives repeated teardown | Very good; uniform in threads | Hot service, wear points, hex-free builds |
| Tin plate | Conductive and solderable | Between chem film and nickel | Good indoors | Terminal lands you solder or crimp |
Where each finish wins
- Default box: Class 3 chem film over the whole enclosure. It is the least costly finish in the group, it protects, and the gel re-passivates its own fine scratches. Chem film also works as the pretreatment under powder coat when you want colour over it.
- Washdown or rough handling: anodize the outside, Class 3 at the joints. Masking costs a drawing note and a little shop time, and you get a door face that survives wiping plus a flange that still bonds; bonding anodized enclosure lids without losing the seal shows the joint detail. When anodize eventually breaks down, the anodized coating loss causes and checks page tells you what you are looking at.
- Frequent teardown: electroless nickel on the flanges. Chem film wears through with repeated re-making of the same seam, and nickel is the wear finish. Its phosphorus content trades conductivity against corrosion cover; the electroless nickel plating grade selection guide compares the grades.
- Shielding seams: an EMI gasket wants the same clean contact, and bare aluminum grows oxide under it fast; see aluminum enclosure EMI gasket contact: oxide and finishes. For a seam you cannot put fasteners through, conductive tape shielding joints and bonding is the other route.
- Grounding: bond the coated seam into the fault-current path the way the enclosure grounding and bonding guide sets out. The coating decides whether that bond reads milliohms or open circuit.
Bare, machined aluminum beats every coating for contact resistance. If you can keep electrolyte off it, a machined land at each bond point is the best joint of all; coat everything around it and seal the edge of the land.
The two limits: heat and RoHS
Heat first. Chromate gels hold water, and the film dries as the enclosure runs hot. Above about 60 C [140 F] the corrosion cover fades and resistance starts to drift upward; designs running near 200 C [390 F] have measured a joint that started in milliohms climbing well past the Class 3 figure. Electroless nickel is plated metal and cannot dehydrate, which makes it the finish for hot boxes.
Then chemistry. Hexavalent chromium is a RoHS-restricted substance, so the classic hex chromates are out for RoHS builds. The trivalent chem films are the compliant route and still meet Class 3 conductivity, at the cost of some salt-spray life. Where the spec is strict or the box runs hot, skip the whole question with electroless nickel or tin.
Joint resistance across a 10-screw lid flange
Work it for a typical drawing: a custom aluminum enclosure 12 x 10 x 6 in [305 x 255 x 150 mm], lid closed with 10 M5 stainless screws, each carrying a standard M5 flat washer 10 mm OD over a 5.3 mm hole, and a Class 3 film at 5 mΩ per in² of contact. Inputs come from the hardware drawing and the coating spec; the rule is that joint resistance equals the film's resistance-area figure divided by true contact area, and screws in parallel divide it again.
The washer annulus is π/4 × (10² − 5.3²) = 56 mm², call it 0.09 in² of real contact per screw. Per screw: 5 mΩ-in² ÷ 0.09 in² = 56 mΩ. Ten in parallel: 56 ÷ 10 = 5.6 mΩ lid to body. Against a 5 mΩ joint budget you need 56 ÷ 5 = 12 screws, and 12 screws gives 4.6 mΩ. A through-bolted lid adds a second coated interface at the nut, so double the per-screw figure there and either add fasteners or fit toothed washers that bite through the film. Above budget, the fix is fastener count or electroless nickel flanges.
What to write on the drawing
| Where | What to write |
|---|---|
| Joint surfaces | Chem film per MIL-DTL-5541, Type II (hex-free), Class 3: all faying surfaces, ground lands, tapped holes |
| Exterior | Anodize per MIL-A-8625, Type II, clear; masked at the areas listed above |
| Fasteners | 300-series stainless screws, toothed washer under the head at every bonded screw |
| Seam | Sealant fillet over the seam edge after final torque |
| Hot or hex-free build | Electroless nickel per MIL-C-26074, 0.0005 in [13 μm] minimum on flanges and lands |
An aluminum box built for hose-down service carries its type as a manufacturer rating — rated NEMA 4X / IP66 by the manufacturer, not UL listed — and the coating spec is part of what that rating assumes. We build custom aluminum enclosures to your drawing, coating spec and cutouts included, delivered in 10 business days counted from your approval of the drawing. If the box itself is still open, the enclosure selector narrows size and rating first.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Are Iridite and Alodine different coatings?
No. They are chromate conversion coatings from different makers, the same dip chemistry under two trade names, and chem film is the generic shop name. Order by the standard instead: MIL-DTL-5541 Class 3 for a conductive joint and the thick corrosion class where only cover matters. Any plating shop runs it on whichever product line it stocks.
How do you ground through an anodized lid?
Mask before anodizing. Leave faying surfaces, tapped holes and ground lands bare, run Class 3 chem film on those spots, and anodize the rest. If parts are already anodized, a toothed washer under the screw head cuts through a 0.0002 to 0.0004 in [5 to 10 μm] Type II layer, while a plain washer rides on top of it and insulates the joint.
Which conductive coating survives 200 C service?
Electroless nickel. Chromate films dehydrate as they heat and their resistance rises; at 200 C [390 F] users measure drift far past the milliohm class of a fresh film. Electroless nickel is plated metal, so its conductivity moves little with temperature, and it deposits uniformly into threads and recesses that current-driven plating misses.
Will a scratch through chem film break the bond?
Rarely. A Class 3 film is only about 0.00002 in [0.5 μm] thick, and chromate re-passivates fine scratches on its own, so joint resistance normally stays under the Class 3 limit. A scratch is more of a corrosion site than a resistance problem, so seal seams where water can sit and wipe standing electrolyte off the flange.
Is chromate conversion RoHS compliant?
The trivalent version is. Hexavalent chromium is a RoHS-restricted substance, while the trivalent films covered by MIL-DTL-5541 Type II are generally compliant and still meet the Class 3 contact-resistance target, giving up a little salt-spray life against hexavalent. Electroless nickel and tin sidestep the question entirely.
