---
type: process
name: "Chemical Conversion Coating"
category: "Finishing"
subcategory: "Additive"
materials: ["Metal"]
tolerances: "Chem film on aluminum adds 0.00001–0.00004 in (0.25–1 µm) and needs no allowance; manganese phosphate on steel can reach 0.0002–0.0004 in (5–10 µm) and should be checked on close fits"
volumes: "1 to millions of parts; immersion lines handle bulk baskets and racked work alike"
lead_time: "1–5 business days at a job shop; commonly same-day when run in line with cleaning"
url: https://manufacturingprocesses.org/processes/finishing/chemical-conversion-coating
---

# Chemical Conversion Coating

Chemical conversion coating reacts the metal surface with a chemical bath to form a thin protective, conductive or paint-adhering film.

- **Category**: [Finishing](https://manufacturingprocesses.org/processes/finishing.md)
- **Family**: Additive
- **Materials**: Metal
- **Typical tolerances**: Chem film on aluminum adds 0.00001–0.00004 in (0.25–1 µm) and needs no allowance; manganese phosphate on steel can reach 0.0002–0.0004 in (5–10 µm) and should be checked on close fits
- **Surface finish**: Conformal and submicron — reproduces the substrate finish exactly, with color from clear through gold iridescent to dark gray
- **Typical volumes**: 1 to millions of parts; immersion lines handle bulk baskets and racked work alike
- **Lead time**: 1–5 business days at a job shop; commonly same-day when run in line with cleaning

## Overview

A chemical conversion coating is formed by reacting the metal surface with a chemical bath so that part of the substrate is converted into an adherent, insoluble inorganic film. Nothing is plated on; the coating grows out of the metal itself, which is why these films are thin, tightly bonded, and add almost nothing to a dimension.

The two families that matter industrially are chromate conversion on aluminum — chem film, Alodine, Iridite, specified by MIL-DTL-5541 — and phosphate conversion on steel, covered by TT-C-490 and MIL-DTL-16232. Chem film on aluminum runs roughly 0.00001–0.00004 in (0.25–1 µm) thick. MIL-DTL-5541 defines two classes with very different purposes: Class 1A for maximum corrosion protection, which must survive 168 hours of ASTM B117 neutral salt spray, and Class 3 for electrical contact, which must hold contact resistance at or below 5000 microhms per square inch. Conversion coatings are used both as a standalone protective finish and as the adhesion base under paint and powder.

## How it works

1. **Clean.** Alkaline soak clean removes oil and shop soil. Every conversion coating failure traces back to cleaning.

2. **Deoxidize or etch.** Aluminum carries a natural oxide that blocks the reaction, so a mild acid deoxidizer strips it and leaves an active surface. Steel is pickled or activated depending on the phosphate type.

3. **React.** The part is immersed, sprayed or brushed with the conversion chemistry. The acid attacks the metal at the surface; the local pH rise at the interface precipitates an insoluble film of chromate, oxide or metal phosphate that is chemically bonded to the substrate. Time and temperature control film weight — typically 1–5 minutes for chem film on aluminum.

4. **Rinse and dry.** Rinsing must be thorough but not aggressive; a fresh chromate film is soft and gel-like until dried. Drying above roughly 140 °F (60 °C) can dehydrate and crack a chromate film, so dry-off temperature is controlled.

### The main chemistries

**Hexavalent chromate (MIL-DTL-5541 Type I)** on aluminum gives the well-known gold-iridescent film and the best self-healing corrosion protection, but hexavalent chromium is restricted under RoHS and REACH. **Trivalent chromium (Type II, and TCP processes)** is the compliant replacement, typically clear to pale blue, and is now the default on new designs.

**Iron phosphate** on steel is a light coating, typically 30–70 mg/ft², applied as a low-cost paint and powder base. **Zinc phosphate** is heavier, typically 150–400 mg/ft² as a paint base, and gives substantially better under-film corrosion performance — it is what separates a powder-coated part that lasts from one that blisters. **Manganese phosphate** is heavier still, dark gray, and is used with oil for anti-galling and break-in on gears, camshafts and firearms components.

## Design guidelines

### Use chem film where anodize would insulate

Anodize is a dielectric. A chromate conversion coating to MIL-DTL-5541 Class 3 protects aluminum while holding contact resistance at or below 5000 microhms/in², which is why it is specified on chassis grounding pads, RF enclosure flanges, connector mounting faces and bonding straps. A common arrangement is Type II anodize on the visible exterior with Class 3 chem film on the internal grounding surfaces, which requires masking and two operations.

### Treat it as dimensionally free

At 0.00001–0.00004 in (0.25–1 µm), chem film changes no fits and needs no allowance on threads, bores or gage surfaces. Phosphate coatings are heavier — a manganese phosphate can reach 0.0002–0.0004 in (5–10 µm) — and on close-fitting parts that build should be checked.

### Specify class and type, not just "chem film"

The drawing note must state the specification, type (hexavalent or trivalent) and class (1A or 3). A part called out only as "Alodine" leaves the shop to choose, and Class 1A and Class 3 are not interchangeable: the corrosion-optimized film is a poorer conductor and the conductive film is a weaker corrosion barrier.

### It is a base coat as much as a finish

Under paint or powder, the conversion layer is what the organic film mechanically keys into and what limits under-film creep from a scratch. Choosing iron phosphate over zinc phosphate to save money at the pretreatment stage is the most common reason a powder-coated steel part fails salt spray.

### Do not use it to hide surface defects

The film is submicron and completely conformal. Machining marks, weld discoloration, casting texture and blast profile all remain. Where appearance matters, set it with the mechanical prep before the bath.

### Handle it gently until sealed or painted

A fresh chromate film is soft and can be wiped off by handling before it dries and hardens. It is also degraded by drying above roughly 140 °F (60 °C). Parts should be racked, not stacked, and coated late in the routing.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Chem film thickness (aluminum) | 0.00001–0.00004 in (0.25–1 µm) | — | Conversion film, dimensionally negligible |
| MIL-DTL-5541 Class 1A | 168 h ASTM B117 minimum | — | Corrosion-optimized film |
| MIL-DTL-5541 Class 3 | ≤ 5000 µΩ/in² contact resistance | — | Electrical bonding surfaces |
| Iron phosphate coating weight | 30–70 mg/ft² | Paint base only | Light film; limited corrosion benefit |
| Zinc phosphate coating weight | 150–400 mg/ft² | — | Substantially better under-paint corrosion resistance |
| Drying temperature (chromate) | ≤ 140 °F (60 °C) | — | Higher temperatures dehydrate and crack the film |

## Cost drivers

Conversion coating is a low-cost, short-cycle tank or spray operation, usually the cheapest line on a finishing routing. Cost is driven by handling and by what else has to happen around it.

- **Application method.** Immersion in a line tank is cheapest. Spray application on large fabrications, and brush touch-up on individual machined faces, are hand operations priced by the hour.
- **Chemistry.** Trivalent and non-chrome systems generally cost more per gallon than hexavalent but avoid the hazardous-waste and compliance burden, which usually nets out in their favor.
- **Masking and dual finishes.** Anodizing the exterior and chem filming an internal ground pad on the same part means masking, two setups and two inspections — often several times the cost of either finish alone.
- **Testing.** Salt spray to 168 hours for Class 1A and contact resistance measurement for Class 3 are real costs on qualification lots.
- **Pretreatment level for paint base.** Upgrading from iron to zinc phosphate adds bath stages and sludge handling, and is the main reason one coater's price differs from another's.

Cost-reduction tactics:

1. Specify one class across the whole part where the requirements allow, and avoid dual-finish masking.
2. Use conversion coating rather than anodize where the part is painted afterward — it is cheaper and a better paint base.
3. Batch to fill the line; the tank cycle cost is fixed.
4. Where the design permits, put the ground path on a fastener or a machined boss that can be masked simply rather than on a large interrupted face.
5. Do not over-specify zinc phosphate on interior parts that will never see moisture; iron phosphate is materially cheaper.

## FAQ

### What is chem film and how thick is it?

Chem film — also called Alodine or Iridite — is a chromate conversion coating on aluminum specified by MIL-DTL-5541. It is roughly 0.00001–0.00004 in (0.25–1 µm) thick, thin enough to need no dimensional allowance on threads, bores or gage surfaces.

### What is the difference between MIL-DTL-5541 Class 1A and Class 3?

Class 1A is optimized for corrosion protection and must survive 168 hours of ASTM B117 neutral salt spray. Class 3 is optimized for electrical contact and must hold contact resistance at or below 5000 microhms per square inch. They are not interchangeable, so the drawing must state which one applies.

### Should I specify chem film or anodizing on an aluminum enclosure?

Anodize where you want durability, wear resistance and color; chem film where the surface must stay electrically conductive, where the part will be painted afterward, or where no dimensional change is acceptable. Many enclosures use both — anodize outside, Class 3 chem film on internal grounding pads — which requires masking and two operations.

### Is hexavalent chromate still allowed?

It remains specified in some aerospace and defense work but is restricted under RoHS and REACH, so trivalent chromium (MIL-DTL-5541 Type II and TCP processes) is the default for new designs. Trivalent films are typically clear to pale blue rather than gold iridescent and offer somewhat less self-healing behavior.

### Iron phosphate or zinc phosphate under powder coating?

Iron phosphate at roughly 30–70 mg/ft² is cheaper and adequate for interior parts. Zinc phosphate at roughly 150–400 mg/ft² gives substantially better under-film corrosion resistance and much less creep from a scratch. Choosing iron phosphate to save money is the most common reason a powder-coated steel part fails salt spray.

### What is manganese phosphate used for?

It is a heavy, dark gray phosphate typically 0.0002–0.0004 in (5–10 µm) thick, used with oil as a break-in and anti-galling surface on gears, camshafts, fasteners and firearms components. The porous coating holds oil, which is what provides the lubricity and the corrosion resistance.

## Alternative processes

- [Anodizing](https://manufacturingprocesses.org/processes/finishing/anodizing.md): Anodizing grows a hard porous oxide layer on aluminum electrochemically, which can then be dyed and sealed for wear and corrosion resistance.
- [Black Oxide](https://manufacturingprocesses.org/processes/finishing/black-oxide.md): Black oxide converts the steel surface to magnetite in a hot alkaline bath, giving a black finish that adds essentially no thickness.
- [Passivation](https://manufacturingprocesses.org/processes/finishing/passivation.md): Passivation removes free iron from a stainless steel surface with an acid bath so the chromium oxide layer can reform unbroken.
- [Electroplating](https://manufacturingprocesses.org/processes/finishing/electroplating.md): Electroplating deposits a metal coating from solution onto a conductive part by passing current through an electrolyte bath.
- [E-Coating (Electrophoretic Deposition)](https://manufacturingprocesses.org/processes/finishing/e-coating-electrophoretic-deposition.md): E-coating deposits paint electrically from a bath so it plates evenly into every recess, then cures to a uniform film.

## Related processes

- [Anodizing](https://manufacturingprocesses.org/processes/finishing/anodizing.md): Anodizing grows a hard porous oxide layer on aluminum electrochemically, which can then be dyed and sealed for wear and corrosion resistance.
- [Powder Coating](https://manufacturingprocesses.org/processes/finishing/powder-coating.md): Powder coating sprays electrostatically charged dry polymer powder onto an earthed part, then bakes it into a tough continuous film.
- [Spray Painting](https://manufacturingprocesses.org/processes/finishing/spray-painting.md): Spray painting atomises liquid coating onto a prepared surface, building a film that cures into a decorative and protective layer.
- [E-Coating (Electrophoretic Deposition)](https://manufacturingprocesses.org/processes/finishing/e-coating-electrophoretic-deposition.md): E-coating deposits paint electrically from a bath so it plates evenly into every recess, then cures to a uniform film.
- [Black Oxide](https://manufacturingprocesses.org/processes/finishing/black-oxide.md): Black oxide converts the steel surface to magnetite in a hot alkaline bath, giving a black finish that adds essentially no thickness.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/finishing/chemical-conversion-coating)*

*Last updated: August 11, 2026*