What is a chemical conversion coating (chem film) on aluminum?
Summary
A chemical conversion coating on aluminum is a thin film, usually chromate or trivalent chromium, formed by immersion, spray or brush, that resists corrosion and stays electrically conductive. MIL-DTL-5541F (2006) divides it into Type I with hexavalent chromium and Type II without, each in Class 1A for maximum corrosion protection or Class 3 for low electrical resistance.
If no type is specified, MIL-DTL-5541F calls for Type I, and Type I and Type II may not be substituted for each other unless the contract or order allows it (MIL-DTL-5541F, 2006).
Coated test panels must show no more than 5 isolated spots or pits per specimen, none larger than 0.031 inch (0.79 mm) in diameter, and no more than 15 across all five specimens after 168 hours of 5 percent salt spray (MIL-DTL-5541F, 2006).
Unpainted conversion coatings start losing corrosion resistance when exposed to 60 °C (140 °F) or higher (MIL-DTL-5541F, 2006).
Touch-up of mechanically damaged areas, where the contract permits it, is limited to 5 percent of the item surface area unless the acquisition activity approves more (MIL-DTL-5541F, 2006).
How does a chromate conversion coating form on aluminum?
A chromate conversion coating forms on aluminum through a redox reaction: hexavalent chromium in the bath is reduced to trivalent chromium as the aluminum is oxidized, and the resulting hydroxides deposit on the metal as a gel (Wikipedia, 2026). For aluminum the process is rapid, 1 to 5 minutes in a single ambient-temperature tank with a rinse (Wikipedia, 2026).
The film is soft and gelatinous when first applied, then hardens and becomes hydrophobic as it dries, typically in 24 hours or less (Wikipedia, 2026). As it dries it shrinks and cracks into a "dried mud" pattern, and trapped solution keeps reacting with metal exposed in the cracks, so the final coating covers the whole surface (Wikipedia, 2026).
Trivalent chromium process (TCP) films form differently: a zirconium-chromium mixed oxide outer layer sits over an aluminum oxide or oxyfluoride layer at the metal, and no hexavalent chromium was found in the coating (Guo and Frankel, 2012). Both routes are compared in how trivalent and hexavalent chromium coatings protect aluminum.
Why does chem film keep aluminum electrically conductive?
Chem film keeps aluminum electrically conductive because the film is very thin, and MIL-DTL-5541F defines Class 3 for exactly this use: electrical and electronic applications that need lower contact resistance than Class 1A or anodic coatings (MIL-DTL-5541F, 2006). Coating thickness is varied by immersion time, so the same material can be qualified for both classes, and Class 3 films are thinner and therefore more susceptible to corrosion than Class 1A (MIL-DTL-5541F, 2006).
The qualification limits are stated in the specification's notes. Under a nominal electrode pressure of 200 psi (about 1.38 MPa), Class 3 coatings are qualified under MIL-DTL-81706 at no more than 5,000 microhms per square inch as supplied and 10,000 microhms per square inch after 168 hours of salt spray (MIL-DTL-5541F, 2006). For production parts, the contract or order sets the test method, test frequency and required resistance values (MIL-DTL-5541F, 2006).
The measurement itself can mislead. Rough test panels give lower resistance because the coating fractures under the contact electrode, and surface roughness and electrode flatness may affect the value more than coating thickness does (MIL-DTL-5541F, 2006).
How thick is a chemical conversion coating on aluminum?
A chemical conversion coating on aluminum is tens to hundreds of nanometers thick: a dry hexavalent chromate film is typically 200 to 300 nm (Wikipedia, 2026), while measured trivalent and zirconium films are about 30 to 120 nm (Guo and Frankel, 2012; Adediran et al., 2019). MIL-DTL-5541F sets no thickness value. It controls the coating through salt spray, paint adhesion and, for Class 3, contact resistance (MIL-DTL-5541F, 2006). ASTM B449-93(2022) classifies chromate coatings on aluminum into Classes 1 to 4 by coating thickness (ASTM, 2022).
The table below shows how the three film chemistries differ in thickness, color and conductivity.
Chemistry | Film thickness | Color | Conductive? |
|---|---|---|---|
Hexavalent chromate (MIL-DTL-5541 Type I) | 200 to 300 nm typical dry film; 200 to 1000 nm for one commercial bath (Wikipedia, 2026) | Light gold after 1 min, golden-brown after 3 min in that bath (Wikipedia, 2026) | Yes as Class 3, the thinner film (MIL-DTL-5541F, 2006) |
Trivalent chromium, TCP (Type II) | 40 to 120 nm on AA2024-T3 (Guo and Frankel, 2012); about 40 nm plus a 50 nm interface layer on AA3003 (Mujdrica Kim et al., 2019) | No obvious color change in earlier trivalent coatings; one dyed product (eTCP) is silver to blue or purple (Finishing and Coating, 2020) | Yes when qualified as Type II Class 3 (MIL-DTL-5541F, 2006); one dyed product averaged 522.5 microhms per square inch before salt spray, against a 5,000 limit (Finishing and Coating, 2020) |
Non-chrome zirconium film | 30 nm after 60 s and 50 nm after 900 s immersion (Adediran et al., 2019) | Not reported in the cited study | Only if the product is qualified as Type II Class 3 (MIL-DTL-5541F, 2006) |
The key row is trivalent chromium: in these measurements it is thinner than a typical hexavalent film, and without a dye its color gives little proof that the coating is there. A supplier comparison states that chem film adds no measurable change to part dimensions, so parts can be coated after final machining (ProleanTech, 2023).
Why do published chem film thickness values differ?
Published chem film thickness values differ by tenfold or more because they describe different chemistries, immersion times and measurement methods. One supplier comparison gives 0.025 to 0.1 µm (25 to 100 nm) for chem film without naming the chemistry (ProleanTech, 2023), while Wikipedia gives 200 to 300 nm for a typical dry hexavalent film and 200 to 1000 nm for one commercial hexavalent bath (Wikipedia, 2026).
Chemistry is the first cause. Trivalent chromium process films measured 40 to 120 nm on AA2024-T3 (Guo and Frankel, 2012), which overlaps the supplier range; this suggests the lower figure describes trivalent or non-chrome films, not hexavalent ones. Immersion time is the second: MIL-DTL-5541F states that coating thickness is varied by immersion time (MIL-DTL-5541F, 2006), and the commercial hexavalent bath went from light gold at 1 minute to golden-brown at 3 minutes (Wikipedia, 2026).
Measurement is the third. The high vacuum of a scanning electron microscope dehydrates a TCP coating and creates mud-crack artifacts (Guo and Frankel, 2012), and a coating of around 100 nm is thinner than the analysis depth of energy dispersive X-ray spectroscopy (Mujdrica Kim et al., 2019). A chromate film also shrinks as its gel dries (Wikipedia, 2026). On a drawing, specify type and class, which MIL-DTL-5541F tests, not a thickness.
What do chem film colors mean on aluminum parts?
Chem film color on aluminum shows that a coating is present and gives a rough idea of how heavy it is, but it does not prove type, class or corrosion protection. MIL-DTL-5541F notes that qualified materials produce coatings that range from clear or colorless to iridescent yellow, brown, gray or blue (MIL-DTL-5541F, 2006).
Color may vary from alloy to alloy in the same bath, is less uniform on welds and castings, and dark spots can come from chemical dripping as parts leave the tank (MIL-DTL-5541F, 2006). For one hexavalent bath, color tracked immersion time, from light gold after 1 minute to golden-brown after 3 minutes (Wikipedia, 2026). A darker hexavalent film is therefore usually a heavier one; this is an inference from the immersion-time rule, not a specification test. Heavier is not always better, because excessively thick coatings may cause paint adhesion problems such as blistering (MIL-DTL-5541F, 2006).
Trivalent films are harder to judge. Earlier trivalent coatings lacked the obvious color change of hexavalent chromium, which made it hard to confirm that parts were coated (Finishing and Coating, 2020). For a clear Type I coating, a spot test in ASTM B449 confirms the coating; for Type II, the coating maker's method applies (MIL-DTL-5541F, 2006). Visual examination cannot show whether contamination or overheating has impaired the coating (MIL-DTL-5541F, 2006).
Chemical conversion coating vs anodizing on aluminum
Chemical conversion coating and anodizing on aluminum differ in process, thickness and electrical behavior. MIL-DTL-5541F applies chem film nonelectrolytically by spray, brush or immersion (MIL-DTL-5541F, 2006), while anodizing is an electrochemical process that grows a thick aluminum oxide layer (ProleanTech, 2023). One supplier comparison lists anodic layers of 0.5 to 2.5 µm for chromic acid Type I, 5 to 25 µm for sulfuric acid Type II and 25 to 100 µm for hard anodize Type III (ProleanTech, 2023), against at most about 1 µm for chem film in the measurements above.
Electrical behavior follows thickness. MIL-DTL-5541F intends Class 3 chem film for contacts that need lower resistance than anodic coatings to MIL-A-8625 (MIL-DTL-5541F, 2006), and the supplier comparison notes that anodic aluminum oxide is an insulator, so contacts may need masking before anodizing (ProleanTech, 2023).
The choice by service condition is set out in chem film vs anodizing on aluminum, and the type and class rules in MIL-DTL-5541 chem film requirements. When chem film parts fail salt spray or a contact resistance check, Lubinpla can organize the process records, data sheets and test results into one cited document for your review.
Related terms
Type I and Type II: MIL-DTL-5541 coatings with and without hexavalent chromium.
Class 1A and Class 3: MIL-DTL-5541 classes for maximum corrosion protection and for low electrical resistance.
MIL-DTL-81706: the qualification specification for the conversion chemicals, with qualified products on QPL-81706.
Trivalent chromium process (TCP): a conversion coating with trivalent chromium and zirconium and no hexavalent chromium.
Alodine and Iridite: trademarked names for chromate conversion processes, also used informally for chem film.
Anodizing: electrochemical growth of a thicker, insulating aluminum oxide layer.