Posted by Valerie on September 24, 1997 at 14:21:33:

In Reply to: Flux interferring with metal paint after the weld posted by Tami Schwerin on September 23, 1997 at 20:09:47:

:after we weld an object, we wire brush (by hand) the flux off, then we prime and paint.

: No matter how well we brush off the flux, it breaks

: through the paint after a week or two.

Are you welding with braze welding (oxyfuel), Stick welding (SMAW) or Wire-feed with flux core (FCAW)?

A flux used in braze welding or brazing is made to be corrosive to the metal so that it breaks down oxide films to make the welding easier. If the flux is just brushed off, there is some chemical residue left. This can interfere with electrical connections, paint and the metal itself (can keep corroding after the fact). Flux residues can usually be removed with hot water. This is assuming the part is small! Some fluxes are oxide-saturated and take more than rinsing to remove. You usually do not want to put a braze weld in water right after welding. But if the metal and joint design can stand quenching, flux can be removed by quenching the brazement from an elevated temperature. This doen't mean red-hot, but still warm. The metal and flux shrink at different rates making the flux pop off. AWS Handbook also recommends that if the flux is stubborn to remove, you may use a warm acid solution such as 10% sulfuric acid or one of the commercially available flux-removal compounds. Nitric acid should not be used on alloys containing copper or silver.

There are basic-type fluxes and acid-type fluxes. The acid ones are obviously more reactive. The distinquishing point is made mostly for the solders. Perhaps a different flux coating would make a difference. I would experiment and see if just a baking soda wash would make a difference on your parts after brazing.

If you are not talking about brazing fluxes but the coating left from stick welding, this is a different story. If there is much slag left from the coating after you have chipped it from the weld, it usually means that you have too much space between the stick electrode and the base metal. Try this so that the slag comes off easier. Slag should comes off relatively easily. Then brush with wire brush that is only used for that type metal. If you use the same wire brush for your mild steel and your stainless, you create a problem for your stainless.

If this doesn't completely answer your question, please post back with the type of metal you are welding on, welding process you are using (gas, stick..etc) and the filler material. We can go from there!

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