Alodine finish : Process, Types, benefit & Use

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Alodine finish : Process, Types, benefit & Use

Have you ever worked with steel tools or aluminum parts and noticed a golden surface finish on them? Well, that’s alodine coating, and today I will share with you the entire process of alodine finish. But before that, let’s look at what alodine coating is.

What is Alodine Coating?

Adoline finish

Alodine or alodine coating is a surface treatment method that protects the metal surface from corrosion.

Alodine finishing is a chromate conversion coating that applies a conversion coating layer to the aluminum surface, and this passivation processing permits the surface to be corrosion-resistant and improves adhesion properties when painting over aluminum.

Alodine is also called chem film; this chemical process does not alter the mechanical properties of aluminum, like its strength and lightweight. 

Chromate conversion coating might be difficult to understand if you’re newly working with aluminum or looking for ways to improve precision machined parts, so I’ll quickly explain chemical conversion coatings. 

Coating Chromate Conversion

Chromate conversion coating process is a chemical coating process used on bare metals that need protection from corrosion.

Chromate conversion usually involves dipping aluminum parts into a solution containing hexavalent chromium.

Once in the chromate solution, a chemical reaction occurs between the trivalent chromium and hydroxide ions which deposits a layer of chromium oxide on the surface after the water has evaporated. The chem film or yellow chromate coating is not over a few hundred nanometers thick. It provides excellent corrosion resistance and electrical conductivity and aids the primer for painting on the metal substrate.

In addition to this, it is also used as a decorative finish. Alodine coating has many variables during the chemical conversion coating. Adding other chemicals to the solution can affect the surface finish. Here are two common types of alodine coatings.

Alodine Coating Types:

Alodine Finishes

The MIL-DTL standard is a series of military specifications for manufacturing processes. The MIL-DTL-5541 is a specification that has replaced the ISO standard for alodine in commercial applications.

Alodine coating is mostly used on aircraft parts, and because of its great demand in the military and defense industry, there are two types of desirable coating standards. These standards are defined due to the hazardous nature of alodine coatings and environmental effects that led to separating the chemical conversion coating types based on the presence of hexavalent chromium. 

MIL-DTL-5541 Type 1 

The type 1 coating process involves chromate conversion coatings in a hex chrome chemical bath. This is the most common alodine chromate conversion coating and appears in shades of gold and brown on aluminum surfaces.

Type 1 coatings require special working and safety standards because of the hazardous nature of alodine. It is worth mentioning here working with MIL-DTL-5541 Type 1 can require permits as the US EPA treats it as an environmental hazard and carcinogenic chemical.

Furthermore, it is toxic to humans and a fire hazard when combined with clothes and rags and kept for drying. Alodine cannot be mixed with pipe drains or left to seep into the groundwater. For this reason, type 1 coating has additional recycling and disposal costs. 

MIL-DTL-5541 Type 2

The MIL-DTL-5541 Type 2 alodine coating is hex chrome free and instead has trivalent chromium [chromium(III)], which provides the same surface finish and properties. Type 2 alodine coatings have no color or are similar to the original aluminum surface finish.

How Does Alodine Finish Work?

Alodine finishing is one of the final steps in manufacturing aluminum parts. As a surface finish, the alodine aluminum chromate coating process is quite simple. 

Here, I will explain the few simple steps of MIL DTL 5541 standard chromate conversion coatings. 

Type 1 Chromating Process (MIL-DTL-5541 Type 1  )

Time: Depends on part size ( 5 – 30 min)
Steps: At least 6; up to 9
Solution: Hexavalent chromium
Precautions: Yes

Process

The type 1 chromate conversion coating involves preparing the aluminum surface, applying the chem film and finally leaving to dry. The industry uses an immersion process of alodine coating, but spraying and hose-rinsing methods are used in smaller businesses. 

Here are the steps

Step#1: Before using alodine on the aluminum, cleaning the surface for an even finish is essential. The metal part is cleaned using a solution of a general metal cleaner like Alumiprep 33. This solution removes dirt, impurities, or oil from the machining processes and acts on the aluminum alloy’s clad covering.

Step#2: Rinse the metal part and make sure the surface has no detergent or foreign solution. Leave to dry.

Step#3: Almost all aluminum alloys have an oxide layer formed on the surface. A deoxidizing agent removes this oxide layer and prepares the metal for alodine solution dipping. 

Step#4: Cold Rinse.

Step#5: The metal part is immersed in aqueous solutions of alodine and instantly, a layer of chromate deposits on the surface. The time period of immersion in alodine depends on the part size and is usually specified on the technical sheet. The part is removed from the tank after applying chromate conversion coatings. 

Step#6: Rinse. Leave to dry.

Additional Steps

After step 2, if any part geometry needs to be protected from alodine finish, the part can be covered with an inert coating and rinsed before dipping in the alodine solution. 

Type 2 Chromating Process (MIL-DTL-5541 Type 2 )

Hex-free chromate conversion coating has two categories based on a small cleaning difference: acid cleaning and alkaline cleaning process

Acid Clean Process

Step#1: Rinse the part at ambient temperature.

Step#2: Acid wash the aluminum part. In this step, a diluted acid solution agitates the aluminum surface. Nitric and sulphuric acid solutions diluted in a fixed ratio can act as autocatalytic agents and open the pores on the aluminum for the alodine to seep. 

Step#3: Rinse the metal substrate.

Step#4: Dip the aluminum part in a trivalent chromium solution depending on the time specified in the technical sheet or according to the part of size. 

Step#5: Rinse gently with deionized water as the chem film coat is still forming. Leave to dry, or use compressed air to remove water.

Alkaline Clean Process

Step#1: Rinse the part at ambient temperature.

Step#2: Use a moderately alkaline pH solution to act on the aluminum alloy. Aluminum alloys with a rich magnesium layer on the surface can be treated with an alkaline solution leaving an aluminum oxide layer behind.

Step#3: Rinse. 

Step#4: Deoxidize the aluminum part because of the alkaline solution treatment.

Step#4: Dip the aluminum part in a trivalent chromium solution depending on the time specified in the technical sheet or according to the part of size. A chemical film conversion coating forms on the surface. 

Step#5: Rinse gently with deionized water as the chem film coat is still forming. Leave to dry, or use compressed air to remove water.

Benefits of Chemical Conversion Coatings

Adoline Sheet Metal

Chemical conversion coatings especially, alodine coat has many advantages for parts used in conductive or fluid applications. Let’s look at these highly sought-after alodine properties for metal substrates.

Corrosion Resistance

Alodine finish protects aluminum by depositing a few hundred nanometers thick protective layer. The corrosion resistance works by covering tiny microscopic cracks and pores where water molecules could enter to weaken the aluminum alloy. 

Electrical Conductivity

Aldine finish is one of the few surface treatment methods that not only prevents corrosion but also offers a great conductive surface. Chemical conversion coatings can significantly alter electrical properties. Alodine chemical conversion coatings have different classes and class 3 chem film makes preserving electrical conductivity possible because the alodine finish is so thin it doesn’t interfere with the aluminum conductivity. The low electrical resistance allows them to be great ground connections.

Thermal Conductivity

Alodine surface offers great thermal conductivity and alodine-coated parts are used in heat sinks.

Paint Adhesion

Paint can also be used on metal parts for corrosion protection. An alodine surface provides great paint adhesion. Paint coatings on airplanes are also a reason for alodine finish on airplanes. 

Simple Chemical Process

The chromate conversion coating is a simple process without any complexity. The thin film coating means an overall quick process and fast application. The process can be done at room temperature, compared to costly furnaces of other surface treatment processes, especially for iron substrates.

Alodine Coating Applications

The MIL-DTL-5541 standard alodine finish is used in many military and commercial applications. I’ve compiled some of the most common uses of alodine according to their industry.

CNC Machining Parts

Alodine coating improves the corrosion protection of CNC precision machining parts. The precision parts have close tolerances, and an alodine chemical bath doesn’t affect the tolerance as much as some other surface treatment operations. CNC machining parts, generally aluminum parts, are coated in an alodine finish.

Sheet Metal Fabrication

Alodine Finished

Metal sheets are prone to rust and corrosion. Usually made from steel, sheet metals are galvanized to protect from moisture. While iron and steel sheets cannot be directly alodine-finished, zinc-plated steel can undergo an electrolytic process for an alodine finish. This chemical film prolongs the metal’s life, and the application process is not different from the galvanizing setup.

Aerospace industry

Aluminum has a wide application in the aerospace industry. Aircraft hulls, landing gear, and shock absorbers have a chemical film for corrosion protection. Many airplane bodies are made from aluminum alloys. Alodine finish also helps the paint adhere to the aluminum surface of airplanes. 

Military and Defense industry

Alodine MIL-DTL-5541 is called chemical film in commercial manufacturing shops. This standard of chromate conversion coating has wide applications in the defense industry, like alodine finish on guns, airplane parts, automotive wheels, etc.

Design Considerations for Alodine

The design considerations for alodine aren’t very extensive because it’s one of the final steps in manufacturing. The alodine layer is less than 1 micron thick, not interfering with any clearance fit in common applications. For intricate geometries, keeping a 1-micron tolerance addition to your design is better.

Precautions and Environmental Effects of Alodine

Alodine is a toxic chemical that can lead to chrome sores when spilled on the skin. People regularly working with alodine without proper precautions and protective gear are at risk of kidney failure through overexposure. 

Precautionary measures:

  • Avoid contact with the body; wear impermeable gloves

  • Avoid breathing mist and atmospheric gases; face shield and goggles

  • Wear impervious PPE suit

  • Sufficient ventilation

  • Provide exhaust

  • Dispose of according to federal regulations

Environmental Effects:

  • Toxic to aquatic life

  • Don’t let it run to drain or ground; causes long term adverse effects

 

FAQs

Q: Alodine Vs. Anodizing: What’s The Difference

A: Anodizing and alodine finishing are both surface finishes. The primary difference between the passivation process is anodizing uses an electrolytic process. Anodizing involves taking primarily aluminum and leaving it in an electrolytic acid medium. The chemical reaction starts, and oxygen ions react with the aluminum metal depositing a layer of aluminum oxide on the surface, preventing corrosion.

Q: Is Alodine Toxic?

A: Alodine is a toxic chemical that irritates the skin and respiratory system. If spilled on the skin, it can cause chrome sores, and longer exposure can lead to kidney failure or cancer.

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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