Hard Anodize Chipping/Corroding

Question:

We are a small manufacturer of food processing equipment. All of our aluminum food contact parts are hard anodized. The 6061-T6 alloy is anodized to MIL- A-8625, Type III, Class 1 to 2.0 mils (0.002 inch) thickness. After 3 to 4 years in service, the edges of the mixing blades are showing severe pitting or erosion. Our customer is mixing various muffin batters in the machine and is concerned that metal is flaking off into the muffins. We believe that the mixing blades should give 10 or more years of service without exhibiting any pitting. Do we need to change alloys or anodizing processes or both? I have attached some pictures of this problem with my e-mail so you can see a typical example. G.H.

Answer:

Upon reviewing the pictures it appears that the pitting or “erosion” is occurring in areas that have relatively sharp corners. At a sharp corner (perhaps less than 0.062-inch radius) there will be little, if any, anodic coating. This is because the coating builds perpendicularly to the aluminum substrate. At a sharp corner there is, theoretically, no substrate to build on. Imagine a sharp right angle with coating building perpendicular to each surface. At a very small area along the corner there is virtually no anodic coating thickness. This area is vulnerable to corrosion from the product. The corrosion also undermines the adjacent coating, and you get the “progressive erosion” seen in the pictures. If you put a small radius on the edges of your part, the anodic coating will build uniformly around the corner and will most likely give the corrosion resistance you need. Of course, the larger the radius, the better the coating integrity. If a 0.062-inch radius can be used that should be sufficient.

I also see some general corrosion of the parts. Sealing after anodizing might help this situation, but the gain in corrosion resistance would come at a loss of up to approximately 30% in the coating hardness. Since the coating is so hard, the amount of hardness loss may not affect the useful quality of the coating in your application. Perhaps it would be a good tradeoff that would enhance the overall quality of your equipment. Sealing in boiling deionized water or in a so-called “mid-temp” proprietary nickel acetate bath would give good corrosion resistance.