Anodizing Issues for Cast Aluminum

Posted September 08, 2017
By Jack Tetrault

Understanding the Chemistry and Options for Anodic Coating on Medical Devices

Before choosing a casting and anodic finish, it is important to understand some of the basic chemistry behind why some anodic coating options work better than others on medical devices. Knowing the reasons behind why challenges occur allows you to choose your aluminum material and finish more wisely.

Cast Aluminum Blog_Die Cast Anodized.jpgLet’s discuss anodizing on cast aluminum medical devices and why you can run into production problems with it. Producing visually unappealing product using standard anodize on cast aluminum is a common issue. That is due, in part, to the silica content—but there is more to it.  

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Three-Pronged Problem

The problem with anodizing cast aluminum is layered—as none of the variables below will allow a complete and proper anodize coating to occur: 

  • The silica does not form into an anodic coating
  • The porosity (air spaces) caused by the casting method leaves voids
  • The relatively low aluminum-to-other-metals ratio (only the aluminum converts to aluminum oxide) shows as surface defects

The silica, porosity, and alloying metals prevent the anodic coating from being uniform and homogenous, and therefore, create defects in the coating. Taken together, this makes for a rather poor coating where the defects become pathways for corrosion.

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Options and Considerations

Generally speaking, there are some better cast materials and methods that may contain less silica and help reduce other impurities, but this is only to a degree. To improve the anodic coating, there are some specialty cast etching chemistries that remove surface silica but these can never produce the conditions needed to form a quality anodic coating. These chemistries do help produce a better anodic coating, but never as good—from a functional or cosmetic perspective—as starting with a wrought alloy. Typically, cast aluminum is used only when aesthetics are not important. 

Customers have asked us if our patented MICRALOX® is a viable option in this instance. While it is an exceptional product, the answer is, simply: no. Cast materials are not good candidates for MICRALOX® coatings because they do not allow the formation of a complete and even anodic film required to attain the expected degree of chemical and corrosion resistance. As such, we do not recommend cast materials for MICRALOX® coating, or where chemical and corrosion resistance or aesthetics are important. However, it is important to realize that a MICRALOX® coated casting will be superior to traditional anodized castings regarding corrosion resistance.

Here’s a short explanation of MICRALOX® if you are unfamiliar with it. MICRALOX® is an anodic coating: aluminum is electrochemically converted to aluminum oxide. The difference between MICRALOX® and Type II or Type III anodizing is that we partially convert the amorphous coating into partially crystalline coating, resulting in significantly increased chemical resistance. The process also makes for a more complete barrier layer to stop corrosion at the interface between the coating and the substrate. It produces a long lasting, virtually indestructible surface that delivers dramatically superior chemical corrosion resistance and eliminates color fading due to super-heated steam.  

At DCHN, we know that many questions arise when considering aluminum anodizing, hardcoat, and other metal finishing projects. Our white paper, “12 Proven Tips to Save Time & Money for Aluminum Anodizing, Hardcoat, and Other Metal Finishing Services,” is a guide full of great tips to help you save time and money. Download it now.  

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