Alodine / Chem film / Chromate Conversion Coating

What is Alodine^®? Also known as chem film or chemfilm, Alodine^® is a chromate conversion coating that protects aluminum and other metals from corrosion. Related products include Iridite^®, TCP-HF, Bonderite^®, and Chromicoat^®. The process of applying a chromate conversion coating is referred to as chromating.

Chromate conversion coatings are a type of chemical conversion coating. In chemical conversion coatings, the metal undergoes a chemical reaction at the surface. Unlike plating, which adds a new layer to the metal surface, a chemical conversion coating transforms the existing metal surface into a protective layer via chemical reaction.

The primary benefit of an Alodine coating is aluminum corrosion protection. But chemical film coating with Alodine offers other benefits. The chemical film or Alodine coating leaves a base that provides better adhesion for organic coatings, and it can protect against the loss of electrical conductivity.

Chromate conversion coatings are used for heat sinks, automotive wheels, and everyday aluminum hardware and components. In aerospace applications, chromate conversion coatings are used on aircraft hulls, including shock absorbers, side and torsion struts, landing gear, and flight control systems such as rudder systems and wing parts.

Chem film is applied by dipping, brushing or spraying, and the chromate conversion coating thickness does not change the dimensions of the part. The layer of transformed metal is very thin: Alodine coating thickness is typically only 0.00001-0.00004 inches (0.25-1 μm). Aluminum conversion coating improves the adhesion of both paint and primer if it is applied to an aluminum surface prior to the primer.

Alodine Equipment for Applying Chromate Conversion Coatings

Chromate Conversion Coating Types

The commonly used MIL-DTL-5541 specification distinguishes between different types and classes of conversion coating for aluminum. The Type of a chem film coating depends on the chemical composition of the coating material, while the Class relates to its intended purpose.

For Types, the MIL-DTL-5541 standard defines two types of chemical conversion coating for aluminum, based on the presence or absence of hexavalent chromium in the coating.

MIL-DTL-5541 Type 1 contains hexavalent chromium, also known as hex chrome. This was the customary process for many years, and was used until safer chemicals were introduced.

Exposure to hexavalent chromium is now tightly regulated in the United States by both the EPA, as an environmentally hazardous chemical, and by OSHA, as a carcinogenic chemical. If using Type 1 for MIL-DTL-5541 spec work, be sure to check with your state or county for processing permits that pertain to proper venting and disposal of hex chrome. The need for additional ventilation can add significantly to the cost of a Type 1 chem film system.

MIL-DTL-5541 Type 2 is known as hex-free chromating. In this type the hexavalent chromium has been replaced with trivalent chromium, titanium or zirconium, and it is typically accepted as the standard across many industries. Chemeon, an industry leader in hex-free trivalent chromium coating chemistries, offers a hex-free product named TCP-HF (hexavalent free) that has approvals for many aerospace and military applications.

The hex-free coating can result in a faint blue color appearing almost unchanged from bare material. eTCP is a colored version of TCP-HF developed to exhibit visual identification for quality control purposes.

Advancements in trivalent chromate conversion coating technology allow the process to have many benefits over Type 1:

1. Faster to apply
2. Room temperature to apply. This saves on electrical energy that would otherwise be needed to heat the tanks. Lack of heated tanks also eliminates concerns about evaporative losses.
3. Fewer process steps (typically 5 steps vs. 9 steps for hexavalent chromium, Type 1)

Classes of Chromate Conversion Coatings

The Class of a chemical film coating depends on its intended purpose. The MIL-DTL-5541 spec describes two classes that are commonly used in chemical film applications.

Class 1A: This class of chromate coating tends to be much thicker than other classes, as well as darker, which provides a high grade of corrosion protection for unpainted items. It also improves adhesion of paint finish systems on aluminum and aluminum alloys. The military uses chemical agent-resistant coating (CARC) paint on a wide variety of vehicles and equipment as added protection against chemical agents.

Class 3: This class of chromate coating has two objectives: It is designed not only to protect against corrosion, but also to reduce interference with electrical conductivity. These coatings are usually thinner than Class 1A, and they have a lighter coating weight. Alodine conductivity is technically an incorrect term as the coating itself is not conductive; rather, the Class 3 coating is thin enough to permit the aluminum to retain its conductivity.

The following chart summarizes the key differences between the Types and Classes of chromate conversion coatings according to the MIL-DTL-5541 standard. This standard is also sometimes referred to as MIL-C-5541.

MIL-DTL-5541 Types and Classes

Alodine Chemical Conversion Coating Tank

Alodine Aluminum Treatment Process

The process steps for chemical film applications vary for Type 1 vs. Type 2. The Type 2 chemical conversion coating process allows for use of either an alkaline or acid cleaner. Though the time varies according to the part and process type, it typically takes each part ten minutes to a half-hour to go through the chromate coating process.

Type 1 Chromating Process

1. Clean: A general washing in a cleaning solution takes place as a pre-process step. This ensures that dirt, oil, and any residue formed from prior manufacturing or packaging processes is removed before the chromate coating begins. Then a rinse and dry takes place.
2. Rinse: Ensure any alkaline detergents are removed from the part. A water-break-free test is often performed.
3. Etch: If there are any areas that the chem film coating should not be applied to, the etching process covers those areas.
4. Rinse: Ensure any etching chemistry is rinsed to stop the etching process.
5. Deox: Deoxidation of the part occurs next, to remove any excess materials and oxides, contaminants and chemicals.
6. Rinse: Remove any deox solution.
7. Chem film coat: The surface conversion process takes place when the aluminum is immersed in the tank. The chromate conversion coating gets applied immediately. The period of time that it remains in immersion varies based on the method of the application, the composition of the chemical film and the composition and size of the part itself.
8. Rinse.
9. Warm rinse.

Video: Chem Film/Chromate Conversion Alodine and Passivation 14-Tank Line

Type 2 Chromating Process

The Type 2 hex-free chem film surface conversion process with TCP-HF has two variations, Acid Clean and Alkaline Clean.

Acid Clean process

1. Acid Clean
2. Rinse
3. Trivalent chromium, hex-free chrome coat
4. Rinse
5. Second Rinse is used in the Type 2 method of surface conversion coating known as TCP-HF.

The alternative Type 2 hex-free chem film conversion process is known as Alkaline Clean process.

Alkaline Clean process

1. Alkaline Clean
2. Rinse
3. Deox
4. Rinse
5. Trivalent chromium, hex-free chrome coat
6. Rinse
7. Second Rinse is used in the Type 2 method of surface conversion coating known as TCP-HF.

It’s important to deoxidize the part prior to trivalent chrome processing. While deoxidizing is inherent in the Type 2 Acid Clean process, the Type 2 Alkaline Clean process requires 2 additional steps to deoxidize and rinse the part prior to trivalent chrome processing.

Best Technology can build equipment for Type 1 or Type 2 chromate conversion coatings.

Type 2 Chem Film / Chromate Conversion Coating Tank System

Testing per MIL-DTL-5541 specification

The U.S. Department of Defense was the first to use TCP-HF (trivalent chromium, hex-free) and this trivalent chromate conversion coating has become such an accepted standard that a detailed specification, MIL-DTL-5541, included it. Test cycles according to this standard usually run 168 hours in a salt-fog chamber, using a five-percent salt spray. Tests are usually performed by companies on a monthly basis, with test panels, because the test is a destructive procedure.

Anyone wishing to do work with the U.S. government and military will need its product to undergo two years of Naval testing and validation, and, meeting or exceeding requirements, will become QPL (Quality Parts List) certified. Another certification to look for is Nadcap (National Aerospace and Defense Contractors Accreditation Program), which has very stringent requirements to get certified.

The TCP-HF (hexavalent free) conversion coating equipment offered by Best Technology and TCF-HF chemistry by Chemeon are both QPL-listed and Nadcap-certified.

Whether your needs are Type 1 or Type 2 (TCP-HF), call on Best Technology for all of your chemical conversion coating equipment needs!

Standards and specifications

What industries can benefit from chemical film coating? What is a conversion coating used for?

Aerospace: Aircraft travel at very high rates of speed. The high speed coupled with atmospheric gases at extremely high altitudes contributes to corrosion of aircraft parts. As a result maximum protection is necessary for as many parts as possible. Even passing through clouds can cause corrosion of aircraft parts. Aluminum corrosion protection is critical to aerospace metal finishing applications and an Alodined or aluminum chromate coated aerospace part can prevent surface exposure to such elements.

Maritime/Naval: Watercraft, large and small, need to be as resistant to corrosion as possible. Boat interiors and mechanical parts can be impacted by water damage, especially boats meant to be in saltwater. Conversion coating allows parts to withstand corrosive saltwater environments and pass salt-spray testing methods. The anti-corrosive properties of chemical films are in high demand for this purpose. The chromate coating’s qualities as a paint primer can be useful here too, for custom colors on watercraft.

Military and defense: The U.S. Department of Defense uses chemical film in a variety of ways. Such a large number of military parts are primed and protected with Alodine chem film coating, that the U.S. military has created a standard for the finishing method, with detailed specifications. The standard MIL-DTL-5541 regulates the chem film chemical coating treatment and places chemical conversion coatings into categories based on their applications. Brushed-on chem film is also used to repair surfaces and components that have been anodized.

Electrical industry: Electricians appreciate the corrosion protection when they apply a surface conversion coating to wire housing, which holds and/or protects wiring. This helps to prolong the life and performance of the wires. Alodine coatings in the electrical industry provide not only corrosion protection to wire housings and panels but also reduction of electrical conductivity losses.