Chemical Conversion and Anodizing Processes

Both chemical conversion and anodizing processes are surface treatment to enhance the oxidation process especially for aluminum alloy. Aluminum is usually reactive with water or air to form a stable passive layer of aluminum oxide. Actually, this anti-corrosion protective layer can stop the rest of aluminum metal from oxygenating. In the previous articles, you can learn about:

  1. How to avoid the blotchy and uneven anodizing
  2. Anodizing surface treatment for cosmetic purpose
  3. Anodizing surface can sometimes fail


Aluminum alloy is becoming the most common used raw material for many industrial application. So, apparently, there are more and more CNC machining services projects in which our customer prefer to use aluminum alloy due to its advantages. Generally speaking, properties which make aluminum alloy popular include:

  1. Light weight but still strong enough: The density of aluminum alloy is about 1/3 of steel. High strength to weight ratio make aluminum alloy good material for many application, such as structural, .
  2. Corrosion resistance: Aluminum alloy can naturally generate a protective oxide coating layer on its surface.
  3. Heat and electric conductivity: Aluminum alloy is an excellent conductor and commonly used material in power transmission system.

What is Chemical Conversion?

Sometimes, we use Alodine as an alternative name for chemical conversion coating which applies chromate to the metal substrate. In the past, hexavalent chromium was mostly used in the immersion bath process for coating, but now it has been prohibited by RoHS Directive. Nowadays, trivalent chromium based coating processes are commonly available for commercial application.

Chemical conversion coating:

1.) Creates an anti-corrosive, durable and electrically conductive surface.

2.)Serves a better surface treatment option for aluminum chassis components due to its electrical conductivity, compared with anodizing

3.) Is a pre-treatment to improve paint adhesion prior to powder coating. The paint can mechanically bond to the conversion coating, but not just sit on top of the surface. Otherwise, the paint will be likely to undergo flaking or delamination from the metal surface.

4.) Can also be a primer prior to anodizing treatment.

We offer MIL-DTL-5541(*Ref.1) chemical conversion coatings which form protective films by chemical reaction with aluminum alloy. Generally speaking, these conversion coatings are categorized by the following types and classes:

Type I: Containing hexavalent chromium. Typically, it appears to be gold or yellow in color, which can be optionally specified as “clear” color (as an alternative to no color). However, Type I chemical conversion coating has been prohibited by RoHS Directive.

Type II: Containing no hexavalent chromium, but trivalent chromium. Typically, it appears to be “clear” color (as an alternative to no color).

Class 1A: Providing increased corrosion resistance and adhesive properties for painted or non painted workpieces.

Class 3: Providing increased corrosion resistance and adhesive properties and maintains electrical conductivity. Color ranges from light to dark yellow or gold.

Chemical Conversion Electrical Mount Base Adapter Plate
Parts treated with Chemical Conversion MIL-DTL-5541 Type 1, Class 1A

*Ref.1: See more about MIL-DTL-5541

Aluminum Anodize Coating Failure

Aluminum anodizing is to produce oxide layer on the surface of aluminum parts to improve the capability of anti-corrosion. Actually, we had talked about this topic in previous case study. But, in this topic, we are talking about aluminum anodize coating failure. The failures caused by various issues on a aluminum CNC machining part generally fall into some of the following categories:

1.) exposure to chlorine based solution

Chlorine is very reactive and causes pitting corrosion by removing the oxide layer. Be careful if Chloride ion in a sulfuric acid electrolyte exceeding a critical level of chloride of 80 ppm. Please also notice that chlorine based solvents are for degreasing which may also cause acid pitting.

2.) exposure to very acid or alkaline solution

Solutions, pH lower than 4 or higher than 9, has the ability to break down the oxide layer and make the underlying aluminum susceptible to corrosion. Generally speaking, to rinse completely the aluminum anodize part is the most satisfactory method of eliminating this problem.

Blind holes v.s. anodizing failure

The anodizing failure rate can be high if your CNC aluminum parts featured with blind holes. If the high acidity anodization solution are not well removed from the blind holes, the liquid will flow out and damage the surface of CNC aluminum parts resulted in flaking-off and spots on its anodiizing surface.


Below photo showed an aluminum CNC milled part etched by acid residues. The white, dirty, flaking off oxide powder surrounding the threaded hole is gradually getting worse if no further action against it.

Aluminum anodize can be failure on occasion.
Aluminum anodize failure because of acid.


Preventive action to against failure of aluminum anodize

As described above, to rinse and clean the blind hole on a CNC aluminum anodize part is the best way to remove acid solution. It is a MUST to take this preventive action to against failure of anodizing.


Should you have any questions about aluminum anodize or other RFQ, feel free to send us RFQ for project reviewing.

Aluminum Anodizing Surface Appearance

As for aluminum surface treatment, we recommend anodizing for cosmetic appearance purpose. There are generally two types of aluminum anodizing. One is conventional anodizing (MIL-A-8625 Type II), the other one is hard coat anodizing (MIL-A-8625 Type III). Both anodizing treatment do not only build up the aluminum oxide on the surface 0.005 – 0.0008 mm thick, but also penetrate into the raw material the same depth.



What is aluminum anodizing?

The anodizing layer is achieved by using direct current through an electrolytic solution, with an aluminum object served as an anode. Aluminum anodizing is to build up the thickness of oxide layer on its surface to improve the capability of anti-corrosion and wear. The aluminum oxide layer is with improved adhesion of paint primers and glues than just on bare metal.

The common method, Type II anodize, apply an aluminum oxide layer that is generally 0.005 – 0.025 mm thick. While, hard coating, Type III anodize, calls for 0.050 mm (+/-20%) thickness. Compared to Type II anodize, the hard anodizing items usually have a bronze, gray or black finish. The color can vary depending on the purity of the aluminum substrate. Furthermore, hard anodizing will make the aluminum alloy more resistant to corrosion. The thicker oxidized layer protects the finished part from moisture, oxygen, and other factors.



Quality of Aluminum Anodizing

As time goes by, the quality of anodizing layer might likely get worse. The main reasons are not only anodized parts stored in high salty and humidity environment, but also poor quality of anodize solutions used in coating process. As for good aluminum anodizing,  the surface appearance should be glossy and silk like.


Surface Appearance after Black Anodizing
Black Anodizing on CNC Milling Part



Below photo showed the comparison of bad anodizing(left one) and good(right one). We can easily tell the difference from color, surface quality. In general, the defect anodizing one is with dirty grey, slightly brown color, scratched surface appearance.


Surface Appearance after Aluminum Anodizing
Different Surface Appearance after Anodizing



Teflon impregnate after anodizing

Instead of anodizing, we can also provide Teflon impregnate process service. Our Teflon impregnate process produces a continuous layer of Teflon bonded to the anodized part surface. The Teflon layer is to provide significantly great lubricity and corrosion resistance.


Taiwan OEM Customized CNC Milling Aluminum 6061 T6 Black Hard Anodized Teflon Impregnate Mount Bracket Slide Lock
Teflon Impregnated Aluminum Part with black hard anodizing



Send us email if you have any questions about surface treatment on the CNC aluminum parts. Also feel free to send us RFQ for free project reviewing.