Chromate conversion coating, often called Alodine after one of the original commercial product names, is one of the oldest and most widely specified surface treatments for aluminium. It shows up constantly in aerospace and defence specifications, and it matters in powder coating too, both as a standalone corrosion protection layer and as a pre-treatment step before paint or powder is applied. Here is what it actually does and why specifications keep calling for it.

What Chromate Conversion Coating Actually Is

The process works by immersing or spraying aluminium parts with a solution containing hexavalent or trivalent chromium compounds. The chemistry reacts with the aluminium oxide surface and deposits a thin, complex layer that is part chromium compound and part converted aluminium oxide. This layer is typically a fraction of a micron thick, sometimes visible as a faint yellow, gold, or iridescent tint depending on the process variant used.

Despite being so thin, this layer does two important jobs. It provides standalone corrosion resistance to bare aluminium, which matters for parts that are never going to be painted or coated. And it dramatically improves the adhesion of any subsequent coating, because it gives the powder or paint a chemically active surface to bond to instead of the relatively inert natural aluminium oxide.

Alodine Versus Generic Chromate Conversion

Alodine is a brand name, originally from Amchem and later Henkel, but it has become something close to a generic term in the industry the way "Sellotape" stands in for adhesive tape. When someone says Alodine, they almost always mean chromate conversion coating in general, regardless of which manufacturer's product was actually used.

There are several chemistry variants in commercial use:

Hexavalent Chromate (Type I)

The traditional, highest-performing chemistry
Excellent standalone corrosion resistance and paint adhesion
Classified as a known carcinogen, restricted under REACH and RoHS
Still permitted for aerospace and defence applications under specific exemptions
Increasingly phased out for commercial and architectural work

Trivalent Chromate (Type II / TCP)

Chromate-free in the regulatory sense, uses trivalent rather than hexavalent chromium
Performance approaching hexavalent chemistry in most tests
Compliant with RoHS and REACH
The standard choice for new commercial and industrial work
Approved for many aerospace specifications including some Boeing and Airbus material standards

Where This Fits Into Powder Coating

For most architectural and industrial aluminium powder coating, chromate conversion coating is one of the recognised pre-treatment options, used alongside zirconium and titanium-based chromate-free alternatives. Qualicoat Specifications permit both chromate and chromate-free conversion coatings, provided the process is validated and meets the required performance tests.

In practice, most commercial powder coating lines have moved to chromate-free zirconium-based systems for everyday architectural and industrial work, simply because they avoid the regulatory and disposal burden of chromium-containing chemistry. Chromate conversion, particularly the hexavalent variant, remains specified mainly where a customer's engineering standard explicitly requires it, which is common in aerospace, defence, and some automotive OEM specifications.

The Process Step by Step

Alkaline Cleaning

Removes oils, fingerprints, and fabrication residues. Mild formulation suited to aluminium to avoid excessive etching.

Rinse

Fresh water rinse removes cleaning chemistry before the next stage.

Deoxidiser / Etch

Removes the natural oxide layer and exposes fresh aluminium, often using a mild acid solution. Critical for consistent conversion coating thickness.

Rinse

Removes etch chemistry before it contaminates the conversion bath.

Chromate Conversion Coating

Parts are immersed in or sprayed with the chromate solution. Reaction time and concentration are tightly controlled, typically 30 seconds to a few minutes depending on the specific chemistry and required coating weight.

Final Rinse and Dry

Deionised water rinse followed by drying. Conductivity is monitored closely as residual chromate or salts on the surface will cause adhesion and corrosion problems downstream.

Performance Standards You Will See Referenced

Standard	What It Covers
MIL-DTL-5541	US military specification for chromate conversion coatings on aluminium and aluminium alloys, defines Class 1A (maximum corrosion protection) and Class 3 (for electrical conductivity applications)
ASTM B449	Standard specification for chromates on aluminium
AMS 2473 / AMS 2474	Aerospace Material Specifications for chemical conversion coatings, commonly referenced in OEM aerospace contracts
BS EN 12487	European standard for chromate conversion treatments

If your project specification calls out MIL-DTL-5541 Class 1A, that means maximum corrosion protection and the part is not intended to be painted or coated afterward, or it is meant to provide corrosion resistance even where the topcoat is damaged. Class 3 is specified where electrical conductivity through the coating matters, such as grounding or bonding applications, since Class 3 coatings are thinner and more conductive than Class 1A.

Why It Still Matters for Powder Coating Specifications

Even on parts that will be powder coated, a chromate conversion coating underneath delivers a meaningful benefit: if the powder coating is ever damaged in service, exposing bare metal, the chromate layer underneath continues to provide corrosion protection at that exposed point. A chromate-free zirconium pre-treatment does not offer the same standalone protection if the topcoat is breached. This is one of the reasons defence and aerospace specifications often still call for chromate conversion even when the part will be painted or powder coated over it.

For most commercial architectural and industrial applications, this level of redundant protection is not required and the chromate-free alternatives perform perfectly well as a pre-treatment for the powder coat itself. The decision usually comes down to what the engineering specification for the project actually calls for, rather than a general performance argument either way.

Factory Story

We had an enquiry a while back from a customer supplying aluminium brackets into a defence-adjacent project. Their drawing called out MIL-DTL-5541 Class 1A, which we do not run on our standard architectural pre-treatment line since most of our aluminium work uses chromate-free zirconium conversion. Rather than trying to force a substitution that would not meet their drawing requirement, we were upfront that this needed a specialist process and pointed them toward an applicator with the right certification for hexavalent chromate conversion. It would have been easy to quietly substitute our standard process and hope nobody checked. We did not, because a substitution like that on a defence specification is exactly the kind of thing that gets caught in an audit and creates a much bigger problem than losing one order.

If you have a specification that calls for chromate conversion coating or a specific MIL standard and you want to understand what pre-treatment options actually meet that requirement, get in touch with our technical team. We will tell you plainly whether it is something we run or whether you need a specialist process.

Chromate Conversion Coating and Alodine for Aluminium Explained

Tags

Have a Coating Challenge?