Alodine Finish

We offer our clients complete alodine finishing services, enabling them to capture faster turnarounds on their precision parts and giving them peace of mind that the high tolerance specifications for the final parts are met.

Many design engineers specify aluminum, assuming it has good corrosive resistance as it does not rust like steel. This is true for some applications. But in others, with exposure to oils, saltwater (even from human skin), or the need for electricity conductivity, the part will perform better if a coating is applied to the finished piece.

uniformed woman in the laboratory

Alodine is a coating process that improves the part’s corrosion resistance without impacting the final dimensions of the aluminum part. Unlike the anodizing process, which has a similar effect as allodizing, an alodine finish functions as a primer and allows the part to be painted if required. This is due to the passivation process, which applies a coating to the part.

It is important to remember this is not a cosmetic surface finish and is meant to improve part performance, as seen in the table below:

IndustryBenefits to final productProduct Example
Consumer and Medical GoodsReduces corrosion and primer for paintAluminum handles
Electrical ComponentsReduces electrical conductivity loss between partsHeat sinks
Marine and Port MachineryImproved corrosion resistance to salt waterCover plates
AerospaceImproved part longevityFlight controls and landing gear components

Our customers that specify this process typically come from these recognized international standards.

Organization IssuingStandard Number
United States Department of DefenseMIL-DTL-5541
International Standards Organization (ISO)8081
Society of Automotive Engineers (SAE)AMS 2474, AMS 2473
American Society for Testing and Materials (ASTM)B449, B921

At Macfab, we have focused our process on meeting the MIL-DTL-5541 standard, as it is the most robust and offers the best foundation for meeting our customer requirements. The table below provides a quick overview of the standard.

Type 1
(Hexavalent Chromate)
Type 2
(Hex-Free Chromate – Trivalent Chromate)1
Class 1AMaximum corrosion protectionMaximum corrosion protection
Class 3Less corrosion protection, improved electrical conductivityLess corrosion protection, improved electrical conductivity
Process StepsClean PartsAcid or Alkaline Clean
RinseRinse
EtchCoat
RinseRinse
Deoxidations (Deox)2nd Rinse if needed by coating material
Rinse
Coat
Rinse
Warm Rinse
Note 1: Type 1 is tightly regulated environmental protection agencies, and Type 2 was developed as a more environmentally friendly process
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Tips

Here are a few tips for specifying the alodine coating for your parts. At a high level, the type determines the environmental friendliness of the process. The performance of the coating is defined by class.

For example, Type 1 or Type 2 chromate coatings offer the same performance. However, Type 1 is done with Hexavalent Chromate, a known toxicant. Type 2 is done with more environmentally friendly chemicals, making it a greener process.

The class is determined by the thickness, durability and performance of the coating. If the focus is on corrosion resistance, then determining the desired number of hours before corrosion appears in a salt spray test – the MIL standard is 168 hours. The time required can be reduced based on the application requirements of your specific part. If the coating is applied as a primer for paint, then the wet tape method provides a better measure of coating performance. Please be aware both tests are destructive methods. If required, plan on requesting additional parts with the first order, as the parts are destroyed during the testing process.

Macfab focuses on visually controlling parts from our suppliers to ensure they conform to these performance expectations. Visual inspection of parts is the most reliable method to determine the consistency of the allodizing process. A few of the visual checks include:

  • Confirming that the coating is continuous and reflects a colour change from the raw part
  • No surface imperfections observed, like dust and powder, voids or scratches, or loose coating
  • Seeing minimal contact marks from other parts or tooling, showing controlled process practices

Macfab invests in its relationship and experience with its allodizing suppliers to provide additional value to our customers. This partnership allows Macfab to deliver finished parts to our customers.

Our alodine finishing services complement our full turnkey manufacturing approach, exclusively available when you leverage Macfab for CNC machining. It's important to note that these services are an inherent part of our comprehensive manufacturing solution and are not provided as standalone offerings.