Someone told you to specify a coating. Before you pick one, decide which family you are in. This page covers the four conversion and brightening processes Gleco runs in Texas — chem film, zinc phosphate, dry film lubricant and bright dip — and where each stops being the right answer.
Plating, paint and powder all deposit a foreign material on top of the metal. A conversion coating does not. The surface of the part is chemically reacted — converted — into a compound of the metal it was already made of. Aluminum becomes an aluminum chromate or oxide film. Steel becomes an iron-zinc phosphate crystal layer grown out of the steel itself.
Three consequences follow, and they are the honest reason engineers reach for these processes.
Negligible build. There is no deposit to accumulate. A chromate conversion film on aluminum is typically well under a ten-thousandth of an inch, thin enough that the industry does not measure it by thickness at all but by coating weight in milligrams per square foot. Compare that with electroplating, where the deposit adds its full thickness to a flat face and twice that to a diameter, or with Type III hardcoat anodize, where roughly half the oxide grows outward and a two-thou coating can move a shaft diameter by about two thou.
No dimensional consequence to design around. Because the build is negligible, a conversion coating does not close a bore, does not change a pitch diameter enough to fail a gauge, and does not need a pre-plate undersize. On a machined part with tight features that is often the whole argument.
Adhesion from chemistry, not mechanical key. The film is not sitting on the surface hoping to stay; it is continuous with the substrate because it was made from it. Nothing can peel off a boundary that is not there. That is why conversion coatings are the standard base under primer, and why skipping one is the most common cause of paint delaminating in the field.
Anodize is the edge case. Anodic oxide is also grown from the aluminum rather than deposited, so it is a converted surface in the strict sense. It is separated out here because it is electrolytic and it builds measurably — which is the difference an engineer actually has to design around. See types of metal plating and finishing for the full map.
Chemical conversion coating, chromate conversion, chem film, and the brand names used as shorthand for it. The point of it is that it protects aluminum from corrosion while still conducting — the one thing anodize cannot do. Two fields control the callout. Type I contains hexavalent chromium and is the classical iridescent gold film; Type II is hex-free and usually near colorless. Class 1A is maximum corrosion protection, painted or unpainted; Class 3 is corrosion protection where low electrical resistance is required. Both classes face the same 168-hour neutral salt spray exposure to ASTM B117; only Class 3 carries a contact resistance limit. The chemistry itself is qualified separately under MIL-DTL-81706, which is why the shop, not the print, names the product. Details at chemical conversion coating, and the full field-by-field breakdown at the MIL-DTL-5541 specification reference.
A crystalline zinc phosphate layer grown out of ferrous surfaces. It does three unrelated jobs and the drawing rarely says which one it wants. As a paint base it gives primer a chemically bonded, high-surface-area anchor — the pretreatment specification for that duty is TT-C-490, where Type I zinc phosphate is called out at 150 to 500 mg/ft² and Type V at 500 to 1,100 mg/ft². As a mild corrosion surface it is only protective once it is sealed, because the crystal structure is porous by design; MIL-DTL-16232 classes exist precisely to say whether oil, wax or nothing follows. As a break-in and forming surface the same porosity holds lubricant, which is why it prevents galling in cold extrusion and deep drawing and why it turns up on gears and bearing faces. Where it does not belong: as a substitute for a plated deposit. Phosphate is not sacrificial and adds no meaningful barrier on its own — if the part needs to survive salt spray unpainted, you want zinc plating or zinc-nickel, not phosphate. See zinc phosphate coating.
Molybdenum disulfide in a thermosetting resin binder, sprayed on and heat cured until it is a bonded solid film. You specify it when oil is not available to you: vacuum, sustained heat, dust and grit that would turn grease into lapping compound, sealed assemblies nobody will ever re-lubricate, and cleanliness-critical hardware that must not be greased. It gives low friction, anti-galling behavior on threads and stainless-on-stainless contact, and repeatable torque-tension on fasteners — a solid lubricant does not migrate, drip or attract contamination. It is a coating, so it does build, and it is not a corrosion coating in its own right; the specification gets its corrosion performance from the pretreatment underneath. See dry film lubricant.
A mixed-acid immersion that removes oxide and surface haze and levels the metal itself, leaving a smooth reflective surface. What it is not: a coating. Nothing is deposited, nothing is converted, and it offers no ongoing protection — a bright-dipped copper busbar will tarnish again. It fits in two places. As a finish in its own right where appearance at time of shipment is the requirement and the part will be enclosed or sealed. And as preparation ahead of tin, nickel or silver plating, where a bright, oxide-free substrate is what makes the deposit look right. It is for copper and brass; brightening aluminum ahead of a decorative finish is a mechanical polishing job before Type II anodizing, not this. See bright dip.
Start from the requirement, not the finish. Nearly every argument about which coating to specify is really an argument about which of these rows the part is in.
| What the part has to do | Right family | Why the others lose |
|---|---|---|
| Conduct or ground, on aluminum | Chem film, Class 3 | Anodize is a dielectric — it insulates by design. Paint insulates. Plating conducts but adds steps and build. |
| Conduct, on steel or copper alloy | Tin, silver or nickel plating | No conversion coating on steel is conductive. Phosphate is not a substitute for a plated contact surface. |
| Cannot grow — tight fits, fine threads, gauged features | Conversion coating | Plating adds its thickness to a face and twice that to a diameter. Anodize moves a diameter by about the coating thickness. Paint is thicker than both. |
| Survive salt spray, bare, on steel | Zinc or zinc-nickel plating | Phosphate alone is porous and only protective once sealed. It is not sacrificial. |
| Survive salt spray, bare, on aluminum | Anodize, or chem film Class 1A | Chem film is the lighter, cheaper, conductive answer. Anodize is the harder, thicker, non-conductive one. |
| Resist wear, abrasion or galling | Type III hardcoat, electroless nickel, or dry film lube | Chem film and phosphate have effectively no hardness. Paint has less. |
| Hold paint or powder | Conversion coating first, then the organic layer | Nothing else gives chemically bonded adhesion. Blasting alone gives mechanical key that is not corrosion protection. |
| Look a specific color | Powder, paint or dyed Type II anodize | Conversion coating color is a by-product of the chemistry, not a specification. Do not use it as a color control. |
| Look bright and reflective | Bright nickel, bright silver, or bright dip on copper alloys | Conversion coatings do not level or brighten. Anodize records the surface it was given. |
| Cost the least per part | Conversion coating | No metal in the bath, no current, short cycle. It is the cheapest thing in this table and the least capable of everything except adhesion and conductivity. |
Two rows deserve a caveat. Color is the one people get wrong most often: on a chem film part, color comes from the chemistry, not the class, so a hex-free film and a hexavalent film to the same class can look nothing alike. And cost only looks decisive until a coating fails a requirement it was never able to meet. If corrosion, wear and conductivity are all on the drawing at once, the answer is usually a stack, not a single process — and if that stack crosses families, running it in one company avoids the handoff. That is the point of under one roof.
Most conversion coating work is one layer of something larger. Three stacks cover the majority of it.
Chem film under primer, on aluminum. Chem film to MIL-DTL-5541, then epoxy primer, then topcoat. The chromate film is what the primer chemically bonds to and what limits corrosion creep back from a scratch or a fastener hole. Class 1A is the normal base under paint; if the part also has ground pads, mask them and run them Class 3 rather than downgrading the whole part. This stack is the default on aerospace and defense enclosures.
Zinc phosphate under paint, powder or CARC, on steel. Clean, blast if the drawing calls for a profile, phosphate, then the organic layer. If the part is going to CARC or powder, the phosphate is not optional decoration; it is the adhesion and undercreep control. Note the interaction with blasting: media blasting is a profile operation, not a pretreatment, and it comes before the phosphate, never after.
Dry film lubricant over phosphate, on steel. MIL-PRF-46010 does not go straight onto bare metal. On steel the specification calls for abrasive blast followed by a phosphate coat to MIL-DTL-16232, Type M or Type Z, Class 3 — Class 3 meaning no supplementary oil, because oil under a bonded film prevents it from bonding. On aluminum the pretreatment is anodize to MIL-A-8625; on stainless it is blast and passivate. Then the film is sprayed and heat cured, and until it is cured it is not a lubricant.
The clock. Conversion coatings are gel-like and reactive when freshly formed, and they cure and dehydrate over hours. Coat too soon and you trap moisture under the primer; wait too long, handle the parts, or let them pick up fingerprints and shop oils, and adhesion falls off. A dwell window between conversion coating and paint belongs on the traveller, and it is one of the practical reasons to run both operations in the same company rather than shipping wet-sensitive parts between two. The design guide covers how to write the sequence into a drawing so the shop is not guessing.
A chromate film is heat sensitive. If the part requires an embrittlement relief bake, the bake happens before the conversion coating, not after.
Gleco runs two plating plants and a separate coatings facility. Conversion coating work is not spread evenly across them, so route the part before you quote the program.
Yellow chem film, zinc phosphate and dry film lubricant all run at the Rowlett plant, in the Dallas–Fort Worth metro. So do alkaline zinc, black zinc, matte tin, citric passivation and the full anodize color range. If your stack is phosphate plus dry film lube, or chem film plus paint, it lands here. Local work also comes through chem film in Fort Worth and metal finishing in Garland.
Paint, powder and Cerakote are run at the coatings facility, also in Rowlett. That is what makes the phosphate-then-paint and chem-film-then-primer stacks a single routing rather than an outside shipment. Everything about the finished-coating side is at capabilities.
The McAllen plant carries a different process list — cadmium, Type III hardcoat anodize, gold, sulfamate nickel, and bright and semi-bright silver. If a part needs a McAllen process and a Rowlett conversion coating in the same routing, say so on the RFQ and we will sequence it. Certification scope, including AS9100D, ISO 9001:2015 and ITAR Registered status, is listed at certifications and approvals.
The specification. MIL-DTL-5541, MIL-DTL-16232, TT-C-490, MIL-PRF-46010 — and the revision if your flow-down names one. A pre-2006 MIL-C-5541 drawing has no Type field at all, which means the shop picks unless you say.
The type and class. Type I or Type II. Class 1A or Class 3. Type M or Type Z, and which class of supplementary treatment. These are the fields that change what runs, and they are the fields most often left blank.
The alloy and temper. Conversion coating performance is alloy sensitive. High-copper alloys behave differently from 6061, and castings behave differently from wrought material. Tell us what it is rather than letting the line find out.
Whether the surface has to conduct. If any surface is a ground path, bond strap land or gasket seat, flag it on the face of the drawing with a surface list. Do not put the whole part in Class 3 because one pad needs it — that quietly downgrades corrosion protection everywhere else.
What comes next. If the part is going to primer, powder, CARC or dry film lube afterwards, say so. It changes the class, the sealing, and how long the parts can sit. Conductivity-critical assemblies should also flag any EMI shielding requirement now rather than at first article.
Not in any way you need to design around. A chromate conversion film on aluminum is typically well under a ten-thousandth of an inch, and MIL-DTL-5541 sets no thickness requirement at all — it is a performance specification, and the industry controls the process by coating weight in milligrams per square foot rather than by thickness. Contrast that with anodize, where roughly half the oxide grows outward from the original surface, so a diameter moves by about the coating thickness. If a feature genuinely cannot grow, chem film is the reason conversion coatings exist.
Chem film is a thin chemical film that stays electrically conductive and adds essentially no thickness. Anodize is an electrolytic oxide grown into and out of the aluminum, measurably thicker, much harder, and a dielectric — it insulates. Pick chem film when the surface has to conduct or carry ground, when the part cannot grow, or when cost matters most. Pick anodize when you need wear resistance, abrasion resistance, a dyed color or a harder surface. The Type II versus Type III comparison covers the anodize side of that choice.
Class 3 is specified to be. MIL-DTL-5541 caps Class 3 contact resistance at 5,000 microhms per square inch as coated and 10,000 microhms per square inch after 168 hours of salt spray, measured at 200 psi electrode pressure. Class 1A carries no such limit and should not be assumed conductive — it is the heavier film, and film thickness and conductivity trade directly against each other. Flag grounding and bonding surfaces as Class 3 by surface, not in a general note.
On steel, in almost every case where the paint has to survive anything. The phosphate crystal layer is what primer bonds to chemically and what limits corrosion creeping back from a chip or a scratch; without it you have mechanical key and nothing else. TT-C-490 is the pretreatment specification for exactly this duty, with Type I zinc phosphate at 150 to 500 mg/ft² and Type V at 500 to 1,100 mg/ft². The equivalent step on aluminum is chem film. Skipping the pretreatment is the most common root cause of coating adhesion failures we are asked to look at.
Because that is what a hex-free film looks like. Color in a Type I film comes from hexavalent chromate, which is intensely colored. Type II chemistries are trivalent or chromium-free, and the films are thinner and largely colorless — sometimes with a faint blue or straw iridescence, sometimes effectively invisible on bright machined 6061. Nothing is wrong with the part, and color is not an acceptance criterion for the class. If your inspection procedure says “yellow”, it is inspecting for Type I, not for compliance.
That is the specified stack, not a workaround. MIL-PRF-46010 calls for steel to be abrasive blasted and then phosphate coated to MIL-DTL-16232, Type M or Type Z, Class 3 before the lubricant goes on. Class 3 matters: it means no supplementary oil or wax, because a preservative under a bonded film stops it bonding. The phosphate gives the resin binder a keyed, high-surface-area substrate and supplies the corrosion performance the lubricant on its own does not have. Both operations run at Rowlett, so the part does not leave the building between them.
No. Nothing is deposited and nothing is converted — it is a mixed-acid immersion that dissolves oxide and haze and levels the copper or brass surface underneath. It leaves the part bright, not protected, and a bright-dipped copper part will tarnish again in service. Use it where appearance at shipment is the requirement, or as preparation ahead of tin, nickel or silver plating. If you need brightness that lasts, you need a plated deposit or a clear organic topcoat over it.
Yellow chem film, zinc phosphate and dry film lubricant run at the Rowlett plant, and paint, powder and Cerakote run at the Paint and Powder facility, also in Rowlett. That keeps the common stacks — phosphate then paint, chem film then primer, phosphate then dry film lube — on one site. McAllen carries a different process list, including cadmium, Type III hardcoat anodize, gold, sulfamate nickel and bright and semi-bright silver. Tell us the whole routing on the RFQ and we will sequence it rather than discovering the split mid-job.
Specification, type, class, alloy, and which surfaces have to conduct. We will confirm what we can run against the callout, and which plant runs it, before you release the order.
Precision metal finishing since 1979. Plating, anodizing, coatings and passivation from three Texas facilities.
Our paint, powder & Cerakote division: Gleco Paint & Powder Coating
Gleco Rowlett
2220 Grisham Drive
Rowlett, TX 75088
972-475-4300
Gleco McAllen
3800 W. Ursula Ave
McAllen, TX 78503
956-800-4069
