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

4 Things That Will Impact Your Manufacturing Costs (Part 1)

So far, to exactly predict machining time and manufacturing costs can be very difficult. For instance, factory location, facility or even politics do have impact on the quotes of manufacturing. Sometimes, to request a quote for your manufacturing project feels like looking for a needle in a haystack. In fact, you can still have an initial concept of cost even without 2D and 3D CAD drawings.

Various factor which have impact on manufacturing cost for CNC machining services
Top-end CNC machinery can manufacture high quality parts.


 What is included in Manufacturing Costs?

Generally speaking, there are various factors which directly have impact on the cost of CNC machining services. For instance, materials used, dimensions of the part, quantity per batch, manufacturing process applied, post processing used. Furthermore, auxiliary processing equipment can also affect the cost of manufacturing project such as customized cutting tools, jigs and tooling. In spite of above, there are still some basic guidelines which will have impact on your project. Hopefully, this can be grateful for your evaluation in early planning stage.

1.) Material Costs

Obviously, different type of material costs are largely various from each other. Nowadays, material cost is getting more transparency of pricing due to freely global trading. You can get a roughly idea of material cost by looking for local price for raw material. Furthermore, parts made of cheaper material is likely lack of durability or poor quality compared to high-end material. However, parts made of high-end material usually means more manufacturing lead time for good quality.

Manufacturing costs is relative to many factors.
Various factors may affect your manufacturing costs.


2.) Surface Treatment Costs

Surface treatments are generally for cosmetic and anti-rust purpose. In the past, the chemical solution used in most of surface treatment process are usually with toxic hazards. Actually, the proportion of surface treatment cost in total manufacturing cost were not so high. Nowadays, the directive of RoHS(*Ref.1) has strictly restricted these hazardous substances. At the same time, surface treatment costs are getting higher due to stricter environment policy. For a roughly estimation, the heavier the weight, the more expensive the surface treatment cost.

Surface treatment may cost more nowadays.
Surface treatment: Anodizing


Learn more: 4 Thing That Will Impact Your Manufacturing Costs (Part 2)


*Ref.1: See more about the directive of RoHS

Cross Knurling Profile DIN 82-RGV

Standard specification of DIN 82 (link)


After CNC fabrication processing, the workpiece is usually with smooth metal surface. Knurling is a manufacturing process to feature straight, crossed, angled, diamond-like lines or pattern onto the CNC components. Generally speaking, knurling can perform better grip for finger/hand operation, plastic injection insert or decoration purpose. Sometimes, we also machined multiple shallow slots or polygonal for same above purpose. DIN 82 is most commonly used knurling spec in CNC turning machining field. For example, DIN 82-RGV is with cross knurling pattern. DIN 82-RBR/RBL is with right/left hand spiral. DIN 82-RGE is with diamond-like 30° cross male knurling.


Failure Cross Knurling

Our Belgium textile industrial customer had sent us the original sample made of stainless steel 304. The sample made by Belgium local prototyping manufacturer was with failure knurling surface. See the shorter item in below photo. The measurement of sample was precise and within tolerance, but with bad knurling which was with improper length, too light pattern, and also lack of lead in chamfer at the threaded hole.

Cross Knurling DIN 82-RGV
Cross Knurling DIN 82-RGV Compare Zoom-in


The improper length of knurling do bad for its appearance, a customized full length cross knurling tool can solve this issue. Too light patter may result from the CNC programming or limitation of CNC lathe. Moreover, our CNC production lines can easily machine the lead in chamfer on the edge of threaded hole.

Improved knurling

Therefore, we ordered a customized full length knurling tool to meet the required pattern spec of DIN 82-RGV 0.8. An automatic Japaneses CNC turn-mill machine also helps improving the quality of surface and dimensions. Above photo shows the longer workpiece machined by APPORO is with improved knurling pattern.


We can offer CNC precision parts with several types of knurling surface, such like straight knurling, cross knurling, diamond-like knurling. Our production lines are also capable of knurling on precision plastics parts. Should you have any inquiry for above knuring surface on your CNC workpieces, kindly send us RFQ for free project reviewing without hesitation.

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.