White Spots on Anodized and Dyed Parts

Ask an Expert From: Products Finishing, from Anodizing Technologies

Posted on: 2/1/2006

Question: After Type II anodizing our parts are dyed using organic dyes and then sealed in hot nickel acetate solution.

Question:

After Type II anodizing our parts are dyed using organic dyes and then sealed in hot nickel acetate solution. Often times we get white corrosion-like stuff on the parts. It can be wiped off quite easily but underneath the “corrosion” there are tiny pits. Sometimes these pits are too small to be seen with the unaided eye, but they can be seen under low power magnification. Why does this happen and how can we fix this problem? D.K.

Answer:

This sounds like galvanic corrosion. This is a fairly common occurrence in organic dye baths. Galvanic corrosion has the potential to occur whenever two dissimilar metals come in contact with one another in the presence of an electrolyte. In this case, the electrolyte is obviously the dye solution which allows a very small galvanic current to flow between the processed aluminum parts of higher electrical potential and the titanium racks, of lower potential, used to hold the parts, assuming you use titanium racks.

The presence of bath contaminants such as chlorides, sulfates, aluminum and dirt, dust and other debris that falls into the tank over time are some things that can exacerbate the situation. Each one of these is a subject in itself, so I won’t elaborate now. There are other things which should be checked and which are more likely to be the cause of the problem.

If the dye tank is metal, make sure the load bar or rack is insulated from the tank by a plastic or rubber insulator. Check the tank and the tank liner to be sure they are properly grounded to a good ground such as a metal water supply line. If the tank is plastic, keep the edges of the tank clean where the load is resting. Make sure the load pick up point on the hoist, if you are using a hoist, is insulated from the hoist cable or chain. If none of these helps, it is sometimes possible to “transfer” the corrosion by using a sacrificial anode made of magnesium. Hang the magnesium bar or rod directly on the load bar with the parts or attach it to the rack in some way and it may corrode instead of the aluminum parts.

If all of the above efforts fail, it probably means that there is too much titanium on the load in relation to the amount (surface area) of aluminum on the load. If you are using all-titanium racks, try building the rack frames of aluminum and attach titanium leaves or fingers to the aluminum frame to hold the parts during anodizing and dying. Or make the racks entirely of aluminum. Hopefully, this will cure the problem.

Incidentally, if you are already using all aluminum racks or aluminum frames with titanium leaves, the problem is probably not caused by too much titanium. Then one, or a combination, of the above steps should cure your problem.

 


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