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
During the anodizing process, the aluminum oxide film grows perpendicular to the surface of the part. Because of this there will be less coverage on the sharp corners of parts leaving a thin spot in the anodic coating. That thin spot will then be a site of low dielectric strength and reduced corrosion resistance. In the case of parts being hard anodized, because it is a site of low electrical resistance, an increased amount of current will flow at that spot, frequently causing the part to overheat and produce a discoloration or “burn”. In some cases the temperature becomes so hot that the anodic coating and the underlying aluminum are actually dissolved in the sulfuric acid electrolyte and large amounts of material are removed from the part.
This phenomenon very rarely causes a problem in conventional anodizing. However, in the hardcoat anodizing process where power densities can approach 3000 watts per square foot, sharp corners or points can initiate a “burn” which can destroy portions of the part. Being aware of this potential problem, Alpha Metal Finishing has developed procedures and computer control technology that will minimize this possibility. However it is much better practice to design parts without sharp points or edges.
A different but equally undesirable phenomenon occurs with sharp inside corners. The two surfaces growing perpendicular to each other impinge upon each other at the corner creating a seam. The seam is not contiguous like normal anodizing and can expand and contract with temperature allowing the environment access to the base aluminum at its root. MIL-A-8625 recommends a minimum radius of .030 in. on both inside and outside corners.