What to Do about Stains, Scratches, and Blemishes on Anodized Aluminum

What to Do about Stains, Scratches, and Blemishes on Anodized Aluminum

January 12, 2016
By Dave Diblasio

“Can You Fix This?”

The real question here is: can we avoid “having to” fix this? We often get asked if we can fix problems such as stains, scratches, and blemishes on products we are anodizing.

As we’ll discuss below, there are remedial steps we can take for some issues. The most effective approach, though, is to have your products anodize-ready before they even get to the anodizer. Let’s talk a bit more about the processes and operations involved that need to be considered when anodizing.

Product Prep

Can-You-Fix-it-Blog_crop

Prior to products being sent to a finishing house, they often go through many operations including—but not limited to—machining, heat treating, bead blast, tumbling, grinding, polishing etc. In addition, parts are sometimes stored in wet environments, exposed to the elements, or retain machine oils and cutting fluids that can stain.

The result to the products of the prior operations and handling can be staining or oxidation in the aluminum, scratches, uneven bead blasting, embedded finger prints, wheel marks in extrusions, or machine marks and tooling lines, just to mention just a few.

As the anodize process is often the last step in the production cycle, the customers expect the anodizing house can make these issues disappear. The potential loss of product due to the aforementioned conditions is usually not considered until it happens.

Pre-Treatments

There are pre-treatments that the anodizer can consider prior to the final finish, but these may come with risks of their own. Some of the common processes to prepare the aluminum substrate for anodize include:

  • Soaping
  • Etching (both heavy and light)
  • Bright dip
  • De-oxidation

Note that etching and bright dip are metal removing chemistries and will effect part size but may also help remove or hide the offending condition prior to finishing. This should not be the finisher’s call as to what to do to achieve cosmetic and in-tolerance end product. The product should be anodize-ready on incoming.

Tolerance Considerations

However, if tolerances are not a major concern, chemical preparation can remove anywhere from a few ten thousandths of an inch to several thousandths. This will depend on what preparation is used and how much material must be removed to eradicate/hide the defect.

Tolerance issues must be considered when determining if the anodizer can try to remove the conditions causing concern. If scratches are several thousandths deep, trying to remove the condition will definitely affect final tolerances and may even begin to create other unseen conditions such as grain boundary exposure, extrusion imperfections, and others. While anodizing can rebuild some small tolerance, hard coat is best if this is necessary. Either way, it’s important to remember that a defect in the material at the time of anodizing will likely be seen after anodizing.

Machining, tumbling, bead blast, polishing, etc., create a thin skin of aluminum smeared across the surface of the part. This condition can be good for anodizing. Etching through this layer to try to remove scratches, stains, or fingerprints can expose grain boundaries, tooling marks, coring from uneven heat treat, orange peel texture, silica in castings, and other undesirable conditions.

Sometimes, after metal removal operations and prep, it may appear the offending condition is removed. However, after anodize and dye processes are completed, some differences may still appear on the part finish. Leftover oxidation that was too severe to remove, or other contaminants, can cause the aluminum oxide structure to build uneven.

What You Need to Know for Sure

While a good anodizing house can often work wonders salvaging your parts when these conditions are present, it is prudent to be aware of what is, and is not, possible when deciding whether to send your parts to the finisher. Making the finisher choose between cosmetic acceptance and functional acceptance is a lose-lose situation. The optimal circumstances for you, your products, and your anodizer exist when you provide anodize-ready product.

At DCHN, we know that many questions arise when considering aluminum anodizing, hardcoat, and other metal finishing jobs. 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 12 Proven Tips Now

Anodized Aluminum, Stained Aluminum Parts