Color Variation of Hardcoat Anodized Cylinders

Question:

We have been getting our 6061 pressure cylinders for oceanographic instrumentation hardcoat anodized, but we have had some problems with the appearance of the finish. Sometimes the color varies from gray to green/gold. The color can vary dramatically within a single part or varies among pieces in the same batch. Is it possible to seal after anodizing in order to make the parts uniform in color? M.D.

Answer:

There are two primary reasons for color non-uniformity on your hardcoat anodized cylinders:

Differences in the temper of the metal throughout the cylinder.
Non-uniformity of anodic coating thickness caused by factors other than temper.

You don’t state the temper of the material, but since the alloy is 6061, the cylinders must be extruded. I can’t say for certain how likely the temper issue might be with this type of extrusion. Normally, this would not be an issue with a 6061 extrusion but differences in temper can easily cause coating thickness variance. I would suggest talking with the anodizer first in order to find out what they believe is causing the problem. Thoughtful anodizers usually have an opinion about issues such as this. Have the anodizer measure the coating thickness on the parts to see if the difference in color is associated with differences in coating thickness. There are certain anodizing principles and procedures that should be followed in order to achieve a uniform coating thickness. These are things such as racking the parts with tight contact, racking so that parts don’t shadow each other, racking with uniform spacing between the parts and not too close together. Plenty of air agitation in the hardcoat tank is required, and the temperature of the bath must be uniform and held within a narrow range. Of course, the chemistry must be consistent. Anodizers knows if they are following these basic principles. If the anodizer insists that these procedures are being followed, then you and the anodizer should approach the extruder about the possible temper variance.

Sealing, itself, won’t produce a uniform color appearance. You may be on to something with the black color idea, however. If you want it to be black, the part must be dyed using an organic black dye, which the anodizer should be able to do. Doing that may make the part appear more uniform in color, but it won’t change the coating thickness issue. However, it is possible that if the anodic coating thickness on the parts is widely disparate, the dye may not “take” uniformly. A thicker coating will absorb more dye. Chances are that the coating difference, if that’s the problem to begin with, will not be great enough to matter when dying and the black color will make the part look better. Then it must be sealed in boiling-deionized water or hot-nickel acetate to maintain a lasting color.