What Is Alodine Finish (Chem Film) – Everything to Know About Chromate Conversion Coating 

Alodine finish is an important metal surface treatment technology that improves corrosion resistance and adhesion for metal surfaces. What is the process of Alodine coating and when do you need it? Let’s get into an overview of the Alodine finish/chem film/chromate conversion coating, covering its types, classes, standards, processes, and comparison with anodizing, etc. Junying is capable of providing Chromate conversion coating surface finishing service for your aluminum machined parts or other components.

What Is Alodine Finish?

Alodine Finish, also known as chem film or chemfilm, is a type of chemical conversion coating mainly used to protect metal surfaces, typically aluminum, and its alloys, to improve their corrosion resistance. The main component of the Alodine coating is chromate, so it is a chromate conversion coating. This surface finishing involves a chemical reaction that transforms the existing metal surface into a protective layer, rather than adding a new layer as in plating.

The coating forms when Alodine dissolves in aqueous solutions, creating a gel that adheres to the metal surface. This treatment preserves some mechanical properties of aluminum, such as its strength, while enhancing its corrosion resistance and aesthetic appeal.

Why Alodine coating is applied for precision machined parts?

– Enhances Corrosion Resistance: Alodine coatings form a protective oxide layer on the metal surface to prevent corrosion. This chromic acid transformation coating significantly enhances the corrosion resistance of aluminum and other alloys.

– Improves Adhesion: the metal parts that have been treated with Alodine surface finishing will be more secure and less likely to peel or flake off.

– Prevent Pollution: Unlike traditional enameling or primers, Alodine coatings do not easily peel or flake off as they do, which avoids contamination of engine lubricating oil. This is especially important for aerospace parts that need to maintain internal cleanliness over a long period of time.

– Applicable to Complex Parts: The Alodine treatment process is applicable to parts of various shapes and sizes, including complex geometric structures. This makes it an ideal choice for precise machined parts.

– Improves Surface Quality: the chem film can form a uniform, thin, and dense protective layer on the part surface, which helps reduce surface defects and holes, thereby improving overall surface quality.

– Electrical Insulation Performance: Alodine coatings also have certain electrical insulation performance, which is useful for some applications that require the prevention of electrical conductivity.

– Easy Maintenance: although the chromate conversion coating involves chemical substances, the application process is relatively simple and has a lower cost, making it suitable for use in large-scale production.

Applications of Alodine Finish (Chem Film)

• Aerospace industry: Used on metal surfaces that require oxidation and corrosion resistance, such as rocket propellant tanks and engine components.
• Military industry: Weapons, equipment, and weapon systems under marine environments.
• Chemical industry: Chemical equipment and pipeline surfaces where needed to prevent corrosion from gases or liquids.
• Marine: Ship bodies and engine parts for preventing corrosion from seawater and extending service life.
• Electrical industry: Surface treatment of wires and outer shells of electronic devices to prevent corrosion-induced short circuits or other issues.
• Food industry: Some packaging and equipment to meet anti-corrosion standards.

Types and Classes of Alodine Finish (Chem Film)

According to the MIL-DTL-5541 (current MIL-DTL-5541F) standard, there are two main types of Alodine finishes: Type 1 and Type 2.  There are other types of Alodine finishes, including AMS-2473 and 2474, MIL-C-81706, and AMS-C-5541, each with their own unique characteristics and applications.

MIL-DTL-5541 Type 1

The first type is characterized by hexavalent chromium, a chemical composition that was widely used in the past but is now tightly regulated due to environmental and health concerns. This type of coating forms a gold or brown film but may appear clear without color. However, its use is restricted in many areas due to the need for special permits and ventilation systems to handle the hazardous chemicals.

MIL-DTL-5541 Type 2

Type 2, on the other hand, is a coating without hex-chromium, uses trivalent chromium, titanium, or zirconium instead of hexavalent chromium. This type of coating is widely accepted across many industries and is considered a safer and more environmentally friendly alternative. It typically forms a faint blue color or appears clear, and its application process is faster, simpler, and more energy-efficient than Type 1.

MIL-DTL-5541 Class 1A

MIL-DTL-5541 Class 1A provides a relatively thicker coating for maximum corrosion protection for both painted and unpainted surfaces. Class 1A is more suitable for applications requiring high protective performance such as aerospace equipment and military hardware.

MIL-DTL-5541 Class 3

MIL-DTL-5541 Class 3 typically provides a thinner coating with lower electrical resistance. This type of coating may appear lighter or less obvious due to its thickness. Class 3 coatings are widely used in the aviation industry, such as for the protection of fasteners to prevent fuel leakage from affecting the coating. It is also commonly used in electronic devices to provide electromagnetic interference shielding and ensure good performance of coatings in high-frequency electrical performance.

Aluminum Conversion Coating Standards

Standard	Description
MIL-DTL-5541	A military specification for chemical conversion coatings on aluminum and aluminum alloys, commonly used for chem film coatings.
MIL-DTL-81706	A military specification for chemical conversion materials used for coating aluminum and aluminum alloys, intended for manufacturers of the chemicals.
ASTM B449	A standard specification for chromates on aluminum, similar to MIL-DTL-5541 Type 1.
ASTM B921	A standard specification for non-hexavalent chromium conversion coatings on aluminum and aluminum alloys, similar to MIL-DTL-5541 Type 2.
AMS 2473	An aerospace specification for general-purpose chemical film treatment for aluminum alloys, similar to MIL-DTL-5541 Class 1A.
AMS 2474	An aerospace specification for low electrical resistance chemical treatment for aluminum alloys, similar to MIL-DTL-5541 Class 3.
ISO 8081	An aerospace process standard that describes requirements for producing and testing chemical conversion coatings for aluminum alloys.
BAC 5719	A Boeing Process Specification for chemical conversion coatings for aluminum and aluminum alloys, similar but not identical to MIL-DTL-5541.
MIL-C-81751	A military specification, mainly involving metal-ceramic coatings, is used to provide corrosion and oxidation protection for metal substrates.
SJ 20813-2002	A military specification for chemical conversion coatings formed on the surfaces of aluminum and aluminum alloys.

Alodine Surface Treatment Process – How Is Chem Film Applied?

Here are the detailed processing steps for the two main types of Alodine coating:

MIL-DTL-5541 Type 1 Chromating Process

Stage 1 – Parts undergo cleaning to remove any oils, greases, dirt, or other contaminants introduced during prior manufacturing or assembly. Cleaning ensures optimal adhesion of the subsequent coatings.
Stage 2 – Parts are rinsed to remove any residues from cleaning chemicals.
Stage 3 – Any areas where the coating is not desired are etched. This prepares the surface for precise coating application. Etching chemicals need to be rinsed away to terminate the etching reaction.
Stage 4 – Oxidation is removed from the surface through deoxidation to clean the substrate.
Stage 5 – Deoxidizer is rinsed off the parts.
Stage 6 – Parts are immersed in a chromate solution where the conversion coating forms on the metal surface. Immersion time can vary based on part size and desired coating properties.
Stage 7 – Rinsing removes unreacted coating chemicals. Parts receive a final warm water rinse before drying to achieve a smooth coated surface.

MIL-DTL-5541 Type 2 Chromating Process

• Acid Clean
  Parts are rinsed at ambient temperature. Aluminum parts then undergo acid etching where a dilute acid solution agitates the surface, using nitric and sulfuric acids in fixed proportions as a self-catalyzing etchant to open pores in the aluminum and allow chromate penetration. The metal substrate is rinsed and then immersed in a trivalent chromium solution for a time specified in the technical data sheet depending on part size. While the chemical film forms, parts receive a light rinse with deionized water, and then are dried with compressed air.
• Alkaline Clean
  Parts are rinsed at ambient temperature. An alkaline solution at a pH appropriate for the aluminum alloy treats the surface, leaving an aluminum oxide layer on alloys with a magnesium-rich layer. Rinsing follows an alkaline treatment for deoxidation of the aluminum parts. Parts are then immersed in trivalent chromium solution for a time specified in the technical data sheet depending on part size, forming a chemical conversion coating. While the film forms, a light rinse with deionized water is applied before drying with compressed air.

Alodine Chromate Conversion Coating Materials

• Alodine 1200: a powdered product used to form a conversion coating on aluminum and its alloys, ranging in color from light gold to tan. This coating provides excellent corrosion resistance and ensures superior adhesion for subsequent paint and plastic coatings. It is primarily used for general-purpose coatings, such as corrosion protection and improving coating adhesion.
• Alodine 1200S: a variant of Alodine 1200, also a powdered chemical used to generate a protective film ranging from light iridescent gold to tan. This coating not only reduces corrosion but also enhances the bonding between the substrate and subsequent coatings. Additionally, Alodine 1200S complies with the MIL-C-5541 standard, making it suitable for military applications.
• Alodine 407: a chromic acid phosphate treatment used to improve paint adhesion, maintain the electrical resistivity of the treated alloys, and increase the adhesion of adhesives and inks. The color of its coating varies from colorless to deep green depending on operating conditions and customer requirements.
• Alodine 1500: a liquid chemical used to produce a colorless, transparent protective coating on aluminum or aluminum alloys. This coating not only provides protection for aluminum but also serves as an excellent bonding agent for organic coatings. It is particularly suitable for applications where preserving the natural color of aluminum is desired.
• Alodine 600: a powdered chemical used to produce a chromate conversion coating on aluminum and its alloys, with colors ranging from light iridescent gold to tan. This coating is free from complex cyanides and is recommended for applications requiring low dielectric impedance coatings. Alodine 600 can be applied by immersion or spray methods and is a QPL-81706-approved material.

Best Alodine/Chem Film Coating Practice at Junying

As a leading provider of chromate conversion coating services, we understand the factors that determine coating quality. Our highly trained technicians will select the optimal material type and purity level for your specific application. Every step of the coating application receives close attention, from effective rinses that prevent tank contamination to precisely monitored acid activation and pH levels. Partner with Junying Alodine/Chem Film/Chromate Conversion metal finishing service and experience the benefits of an optimized production line. We’ll process your parts with care to deliver a flawless surface, then apply the protective chromate conversion coating through a process tailored for maximum performance.

FAQs

Alodine vs Anodize, What Are the Differences and Which One to Choose?

Alodine (Chromate Conversion) is a chemical treatment used to prepare the surface of a metal for painting or other finishing processes. It produces a layer of silicon dioxide on the surface of the aluminum, which helps to protect the material from further corrosion and lengthens the lifespan of the surface.

Anodize is a process that thickens the natural oxide layer on the surface of a metal to protect it from corrosion. It is an electrolytic process that applies electrical current to the metal while it is immersed in an electrolyte solution. This process can also be used to improve lubrication or allow dyeing (coloring) of the aluminum surface.

Key Differences
– Application: Alodine is typically used on aluminum parts, while Anodize can be applied to steel and other metals as well.
– Durability: Anodize is a more durable finish that can withstand more wear and tear, while Alodine is not as durable and is not as resistant to wear and tear.
– Cost: Alodine is generally cheaper than Anodize.
– Metal Compatibility: Clear Alodine can be applied to a wider range of metals, while Anodize is more suitable for aluminum and aluminum alloys.
– Permanence: Anodize is a more permanent finish than Alodine.
– Tarnish and Scratch Resistance: Anodize is more resistant to tarnish and scratch than Alodine.
– Surface Preparation: Alodine requires a surface that is free of oils and greases, while Anodize does not have this requirement.
– Electrical Conductivity: Alodine allows aluminum to retain electrical or thermal conductivity, while Anodize does not.

Which surface finish should you choose?
– Application Requirements: If the aluminum part needs to be electrically grounded, Alodine may be the better choice. If the part requires a more durable finish, Anodize may be the better option.
– Cost and Technical Expertise: If budget is a concern, Alodine may be the more cost-effective option. However, if the application requires more technical expertise, Anodize may be the better choice.
– Environmental Impact: If sustainability is a concern, Type II Alodine and anodizing aluminum may be more worth considering compared to Type I.

Is Alodine electrically conductive?

Alodine itself is not electrically conductive – it is a chromium oxide coating that forms an insulating barrier layer. However, the Alodine coating permits the underlying aluminum substrate to remain electrically conductive. This is because the thin Class 3 Alodine coating is not thick enough to block the natural conductivity of the bare aluminum metal. It is more accurate to say that a thin Class 3 Alodine coating allows the retention of conductivity in the aluminum part by not completely insulating the metal substrate.