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topic 4433

Steel screws used in Anodize process.


I have an aluminum part that I did some repairs to for cosmetic purposes. That involved using 2 5-40 steel cap screws to fasten a smaller aluminum block to the part.

The screws were tightened and the holes were plugged with aluminum and this area was remachined so that the defect is not visible.

The question is, When this is black anodized will anything happen to the screws? Or will this area show up after coating? Thanks Don

Don O'Connor
Machine & Tool - North Dighton MA

First of two simultaneous responses-- (2000)

If any of the sulfuric acid electrolyte reaches the steel screws, they will be eaten away. Worse case, some of the aluminum goes with them. We would make you sign a release acknowledging the risks before anodizing your parts. It is almost a certainty that your repair will be obvious after black anodize. You might want to consider paint.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

luke engineering banner

Second of two simultaneous responses-- (2000)

Steel doesn't go into anodize tanks, period. One of many things can and will happen.

A.) The steel will heat up to an extreme temperature and it begin to eat away in the tank. The destroyed steel screw/pin/holder whatever it may be will either stain the aluminum surrounding it, or completely destroy it.

B.) The pin will rust and basically look like junk on a nice black anodized part

C.) They can be masked off to lessen the attacks stated above but your best bet is to remove them completely, even if you have to disassemble the part in question, let the anodizer do all the parts separately and you'll save the both of you a lot of hassle.

Matthew Stiltner
plating company - Toledo, Ohio


This may not be that bad. You plugged the head end of the hole with an aluminum pin. That may work. What about the other end of the screw? Is it thru the aluminum or into a blind tapped hole?

If you used a freeze fit on the pin by using dry ice and methyl alcohol or liquid nitrogen to freeze the plug, the plug can be a thousandths or two oversized and still initially fit in the hole if you are fast and fairly good. This should not leak. The plug has to be from the same lot of aluminum or it will probably show as slightly different after anodized. the anodizer will have to keep etch cycles to a minimum.

The real thing that will happen , 98% of the time, is the prep solutions will wick into the space between the two blocks and will leach out during anodize and or the dye process. This will streak the part at least a tiny bit.

No amount of rinsing will prevent this, but can minimize it. No two pieces will streak the same. The anodizer should charge extra for the special service and not give any guarantee.

James Watts
- Navarre, Florida

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