Eliminating Grain Exposure in Aluminum Anodizing What’s Behind Coring and Alligator Skin on Anodic Coatings

Posted December 10, 2014
By Jack Tetrault

“What is this? The part was processed and returned and I took one look and said, ‘How did you guys cause this issue?’” This was a question from one customer, but is voiced from time to time in the industry. His confusion at seeing product with coring returning from the anodic coating process came from not understanding that the grain boundaries on some aluminum alloys can show themselves when the metal is prepared for anodic coating.

Customer’s Perspective
Eliminating Grain Exposure in Aluminum AnodizingThe customer’s assumption was that we etched too much or that we had contaminated tanks or that we stained the parts by poor rinsing; anything, but not the aluminum. How could it be the aluminum? The parts look great upon leaving their building and the aluminum manufacturer had specifications and certification to prove everything was perfect! Then why did these parts look as they did when returned?

In this case, the aluminum was made from round stock of 6061 alloy. The parts were sliced from the bar and machined to dimensional specifications. They had great cosmetics and were sent in as-is from the machining operation. The “simple” task of anodizing should have yielded perfect parts. Then why was coring present in this customer’s material?

Simple Chemistry
It’s simple. Aluminum is dissolved and transformed into aluminum oxide through an electro-chemical process. The anodic coating operation inherently consumes aluminum as it forms aluminum oxide. This process will dissolve aluminum from the surface inward. As it does, it can expose the grain boundaries that are formed by differential cooling rates from outside to inside. Inside grain boundaries are referred to as coring. These will exhibit themselves, if present, if the aluminum is sliced like a piece of pepperoni. Outside grain boundaries have no name, but exhibit the same “alligator skin” or “fish scale” look as coring. This outside effect normally exhibits itself on the outer 1/16” to 1/4” of the round part as a scaly look. It is not necessarily on the very outside for various reasons, but rather just inside the OD.

Common Assumptions
These grain boundaries are not apparent before anodizing and they are not to be “assumed” present by the anodize supplier. Customers who buy stock do not know if this condition exists and usually do not understand why it is seen only AFTER anodizing. This isn’t to say that there is a plot by aluminum extruders to “hide” this information, because this issue is well documented and understood by them. The problem is that often distribution houses or suppliers may not be as well versed. Also, because certifications are supplied, it is assumed that all is well with the aluminum stock. The fact is that certifications only cover the metal make up and temper, but not necessarily manufacturing processes or cooling methods or results.

Easy Solution
So how can you avoid these issues? Well, one thing is to understand that they can exist and are more common in round stock. Then, the aluminum vendor can question the aluminum supplier/vendor and put them on alert that this condition will not be acceptable. If you have stock in your possession already, take a slice (or two) off the bar stock and send it to us so we can check for this condition before you manufacture parts.

If confronted with this issue, there are things that an anodizer can do to help prevent it from reappearing, but only after there is some mechanical remediation. For more information on this topic, please contact DCHN.

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