Alodine Finish

Alodine Finish

We offer complete Alodine finishing services as a one-stop shop to our clients, enabling them to capture faster turn-arounds on their precision parts and peace of mind that the high tolerance specifications for the final parts are met.

Many design engineers specify Aluminum with the assumption that it has good corrosive resistance as it does not rust like steel.  While this is true for some applications, in others exposure to oils, salt water (even from human skin) or conducting electricity, the part will perform better if a coating is applied to the finished piece.

Alodine is a coating process that improves the corrosion resistance of the part without impacting the final dimensions of the part made from Aluminum.  Unlike Anodizing process that has the same goal as Alodizing, an Alodine finish functions as a primer and allows the part to be painted if required.  This is due to the type of process, passivation, simply applying coating applied to the part.  It is important to remember this is not a cosmetic surface finish and is focused on improving 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

When specifying this process our customers 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 studied the above standards and focused our process on meeting the MIL-DTL-5541 standard as it is the most robust and is 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
null

Tips

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

First which type, Type 1 or Type 2 Chromate coating as both deliver the same performance?  Type 1 is done with Hexavalent Chromate which is a known toxicant.  Type 2 is a done with more environmentally friendly chemicals making it a greener process.

How do we determine the class and thereby the performance of the coating?  The Class (think of class in place of a thickness of coating) defines the durability of coating.    If the focus is on corrosion resistance, then determining the necessary desired number of hours before corrosion appears in a Salt Spray Test – the MIL standard is for 168 hours.  The number of hours can be reduced based on application requirements of your specific part.  If the coating is being applied a primer for paint, then the wet tape method provides a better measure of coating performance.  Please be aware both tests are destructive testing methods, if required to be done on your parts, plan on additional parts with the first order, as the parts are destroyed during the testing process.

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

  • Coating is continuous and a color change to raw part
  • No surface imperfections like dust and powder, voids or scratches, and loose coating
  • Contact marks from other parts or tooling shall be minimal and show controlled process practices

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

Learn more about Macfab's specialty finishing services.