Alodine Finish: A Guide to Conversion Coatings for Precision Machined Parts

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Alodine Finish: A Guide to Conversion Coatings for Precision Machined Parts

One of the leading causes of damage and failure of precision machined parts today is corrosion and surface damage. Parts manufacturers, not to be defeated, have turned to several different measures to combat this, and one of the absolute best is alodine finish.

Here, in this guide, we will walk you through alodine coating for aluminum, its principle, applications, design considerations, and many more important aspects of alodine finish in parts manufacture and precision machining.

What is Alodine?

Alodine, commonly known as Chem Film, is a chromate conversion coating that protects metals, most typically aluminum, from corrosion. A chromate conversion coating is a type of chemical conversion coating, which is a type of coating used on metals. The surfaces of the metals that are covered are turned into coatings with electrochemical or chemical processes.

Alodine finish on aluminum tools

To form alodine aluminum chromate coating, chromium acid and chromium salts interact with aqueous solutions to form an adhesive aluminum coat. The primary significance of this is the addition of an extra layer of protection to the metal surface – a process known as passivation.

Aside from passivating aluminum, alodine is also useful as a base for paint coatings and priming, as well as preserving electrical conductivity. Application of alodine finish is typically by dipping, brushing, or spraying of the precision machined part.

Alodine finish has wide use in parts manufacturing in many industries globally, including the aerospace, electrical, military, and precision machining industries. However, this aluminum finish provides little resistance to abrasion, which may cloud doubt on its use as an effective decorative finish.

Principle of the Alodine finish?

The defining principle of Alodine aluminum finish is the electrochemical reaction of chromate. The Alodine coating process is actually a quite simple chemical process. Aluminum coatings are usually created through passivation, which makes the coated metal surfaces chemically inert. 

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The application of the chromate conversion coating is by immersion in a chemical bath, usually consisting of chromium compounds, including chromic anhydride, sodium fluoride, tetrafluoroborate, etc. Brushing and spraying are also alternatives to immersion. 

The metal remains in the surface bath until a film of the desired thickness forms around the metal surface. Notably, the film formed during the alodine process is not as thick as an anodizing layer. This film is a complex metal-chrome molecule film, and it later forms into a continuous gelatinous coating. It is the continuous gelatinous coating that prevents corrosion from happening by eliminating chemical processes. 

Alodine coating types

The different types of chromate conversion coating usually vary in chemical composition and shape. While there are many different types, the most typical ones are Type 1 and Type 2 of the MIL-DTL-5541 standard. Other standards available today are AMS-C-5541, MIL-C-81706, and AMS-2473 and 2474. Below, we will consider the MIL-DTL-5541 types.

• MIL-DTL-5541 Type 1

The primary distinguishing property of Type 1 is the presence of hexavalent chromium, which is the reasoning behind its name, “hex chrome.” This type was the most used chromate conversion coating for several years before safer coating types were introduced. Now, in some states of the world, different legal processing and regulations guide the use of MIL-DTL-5541 Type 1. This film is usually brown or gold in color but may also be without color, in which case they are “clear.”

• MIL-DTL-5541 Type 2

As with Type 1, MIL-DTL-5541 Type 2 alodine coating contains chromium compounds. However, unlike Type 1, Type 2 contains trivalent chromium in place of hexavalent chromium. This, also, is the reasoning behind the name “hex-free chrome.” This coat usually comes in no color, hence, described as clear.

Not long after its emergence on the alodine finish landscape, the Type 2 standard dethroned Type 1 in becoming the most widely used chromate conversion coating standard. This is not really much of a surprise as technological advancements corresponded to better properties.

Some of these particularly desirable properties of Type 2 over Type 1 are:

  • Better safety profile, making it more of a favorite in the precision machining parts manufacturing industry
  • The application of the alodine aluminum coating is at lower temperatures than Type 1
  • Easier, faster, and a more straightforward application process

Main characteristics of Alodine finish

Thin film 

Aside from alodine coating, anodizing layers are also great ways of protecting aluminum surfaces from corrosion and rust. However, one of the ways alodine aluminum coatings are different is the thinness of the film, which is usually between 0.5 and 4 microns. The film is also soft, porous, has good adsorption properties, and will not alter the part size, justifying its use as base layers for paints.

Application at room temperature 

Manufacturers can apply Alodine films at room temperatures. The implication of this is that parts manufacturers don’t have to stress too much during manufacturing, which is a very desirable property, particularly in precision machining parts. Also, the energy demands of the process are low, as well as the pollution impact on the environment, especially the later forms of alodine coatings.

Alodine solution is toxic

Although alodine is generally safe and doesn’t pollute the environment, it can be quite toxic to the skin. Therefore, it must be very carefully handled and disposed of after use. In fact, the use of some types of alodine coatings is regulated by government bodies. 

The application process is fast

The application process of the alodine coat usually lasts mere minutes. The exact time depends on the method in use. Generally, immersion in a chemical bath will not last more than an hour at the utmost.

Great conductive surface

One of the characteristics of alodine chromate conversion coating is that it creates an excellent conductive surface, which is also useful as a base for priming and paints. 

Alodine vs. Anodizing 

Anodizing and Alodine are two of the commonest aluminum finishes available today. They share the same goal in CNC machining, which is making the metal more resistant to corrosion and rust. The difference is in how they do it.

Anodized parts

Let’s briefly consider what anodizing is to get a clearer view of the picture. Anodizing is the process of coating a metal with an oxide layer through electrolytic processes to make the metal more resistant to corrosion and wear. We’ll assume you already know what alodine coating is!

One of the major differences between the two, as you may have noticed, alodine aluminum coating process does not use electrical currents. Rather, it relies on a simple chemical coating procedure.

Again, the impacts they have on the final dimensions of the metal substrate also differ. With alodine finish, there are very little to negligible effects on the final dimensions, as the film formed is a very thin one. Whereas anodizing layers are thicker than alodine films and the impact on the final dimensions may be as much as tens of microns. 

Functionally, they have very similar applications. However, you can use anodizing for decorative purposes, unlike alodine aluminum coating because it is more prone to abrasion and wear. 

A similarity between the two is in their production processes. Both typically involve immersion of the metal substrate in the appropriate chemicals and letting the film develop. The chemicals involved differ, though.

In terms of cost impact, alodine finish is generally cheaper than anodizing. However, it is worth noting that the cost implication will vary from one CNC machining part to another. Not to mention that there are also various types of anodizing and alodine types and forms which can also impact the price. 

Applications of Alodine finish

Alodine finishing is a vital metal finishing process for aluminum and aluminum alloy in several different industries and fields. 

CNC precision machining parts 

The most relevant to us is in CNC precision machining parts. Aluminum is one of the most widely used metals in CNC machining parts because of its several desirable qualities. That said, it only makes sense that manufacturers be well-versed in the best methods for preserving the metal. This is where alodine conversion coatings enter the discussion. 

They make the metals resistant to corrosion without any significant alterations to the final dimensions. Other benefits in CNC machining parts are improving adhesion during priming and changing the conductivity of coatings.

These, coupled with the fact that the whole process is easy and straightforward, make alodine aluminum finish essential in precision machining parts.

The aerospace industry

Many parts in the aerospace industry contain aluminum, perhaps because of its lightweight and sturdiness. As a result, alodine coating is vital as a surface finishing for these parts. Some of these parts are aircraft hulls, landing gear, shock absorbers, etc.

Military and Defense industry

Well, this is quite something, isn’t it? But militaries of the world usually put a lot of faith in aluminum. Given the gravity of the assignment, the aluminum used needs to be able to resist rust and corrosion under the harshest of conditions. 

Pros and cons

Pros

  • Good corrosion protection properties
  • Doesn’t significantly alter the weight of the metal
  • Useful as a base for paintings and priming
  • Good conductive properties

Cons

  • Cannot be used for decorative purposes
  • Irritating to the skin

RapidDirect’s Alodine Surface Finish Services

Alodine is a chromate conversion coating that passivates and protects aluminum from corrosion. Given the widespread use of aluminum today in many CNC machining parts and in other industries and fields of the world today, it becomes necessary to protect the metal properly. There are fewer better ways to do this than with alodine finish. And there are fewer better organizations that will attend to your alodine finish needs than RapidDirect.

We are always ready and prepared for you. Our team of experts capable of solving several different alodine aluminum problems. Reach out to us today, upload your files, and get an instant quote.

FAQs

What is alodine finish?

Alodine finish is a chemical conversion coating used for metals, particularly aluminum, to improve its resistance to rust and corrosion.

Why is alodine finish important for CNC machining parts?

CNC machining parts rely heavily on aluminum and aluminum alloy as their primary constituents. Thus, the metal needs to be properly protected from rust, as well as retain its dimensions. Alodine finishes assure of all of these.

What are the types of Alodine coatings?

The commonest standard of alodine finish used today is MIL-DTL-5541, and its two types are  –  MIL-DTL-5541 Type 1 and  MIL-DTL-5541 Type 2.

Conclusion

Alodine coating is an aluminum metal surface finish that helps to protect the metal from corrosion. Because of how common the use of Aluminum is today in industry parts, it is essential to have basic knowledge about alodine, especially when dealing with precision parts. Aside from being a fantastic corrosion-resistant finish, the application process of alodine is also very straightforward and convenient. Amid all the benefits and advantages of this wonderful finish, you must be careful one handling it, still, because of its tendency to be toxic to humans and also pollute the environment.

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