Alpha often comes across strange and bizarre challenges with some of our customer parts when it comes to the anodizing process. With one medical device manufacturer this was definitely the case. We had been presented with a new part that when we finished the anodizing process it revealed microscopic pits in the surface.
We scratched our heads, came up with a rework process and then tried running it again. The same problem occurred. Now, this particular part was a 2024 alloy which already created a few challenges related to etching. After a conference call and face to face meeting with the customer we tried again on 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. It would be nice if anodizing could cover up machine marks, extrusion lines, or variations in the substrate, but unfortunately it cannot.
Without any solid answers to the problem at that point, we were all frustrated and feeling a little sheepish that we could not find a solution for our customer. Then, it occurred to one of our team members: how were the parts being machined? We asked the customer and discovered that after machining the parts they were belt sanded. Aha! 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! Kind of like what Captain Ramius told Jack Ryan in The Hunt for Red October: “Hey, Ryan, be careful what you shoot at. Most things in here don't react too well to bullets."
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 they did that, we had no more issues with pits showing up in their parts. Nobody particularly likes pits, and we certainly don’t like them in anodizing.
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