Pitting After Anodizing


Medical Device Manufacturer:

A new part from a medical device manufacturer revealed pits in the surface after the anodizing process. The alloy was 2024 and was a crucial new component to an assembly.

Alpha came up with a rework process and then tried processing the part again, but the same problem occurred. We checked and rechecked our processes but no other parts from the customer, or others for that matter, revealed the pitting issue that we encountered.

After a conference call and face-to-face meeting with the customer we processed some new, raw parts. The pits came through the substrate again! The substrate was inspected each time and it was initially thought that perhaps there were extrusion lines or possibly bad material in the original stock of the alloy that could not be visibly seen.

During the anodizing process if there are “junk” elements in the alloy they can fall out in the process, potentially causing pits. This is one of the reasons we ask our customers at times where they actually purchased their aluminum stock. If the stock or extrusion has any issues the anodizing process will only reveal the imperfections in the substrate. After reviewing the material we determined that was not the root cause.

We then asked: how were the parts being machined? We discovered that after machining the parts they were belt sanded. With a few more questions we further discovered that the belt sander was also used on steel parts as well. Any steel particles embedded in the aluminum substrate will not react well with the anodizing process.

With this new knowledge we were able to inform the customer that they needed to change the belt prior to sanding the aluminum parts. Once the change on the customer’s end was effected we did encounter any more issues with pits showing up in the substrate of their parts.

For more on this issue we recommend Anodic Coating Defects, Their Causes & Cure by Arthur W. Brace. Also, see Robert Probert’s response to this question at http://www.finishing.com/481/64.shtml