Re: White fluffy powder/salt forming at seams.
A picture would greatly help the many knowledgeable folks here help determine the source of your problem. I would go along with the dissimilar metals theory and Iowasculptor's suggestion of sealing the piece from moisture. If your 'salt' formations are 'fluffy', that sounds like a condition particular to aluminum. I can't recall ever encountering a powder coated bronze but it is possible I suppose. If you are not getting orange rust as well it is likely not steel but wouldn't hurt to give it the magnet test. If the problem lies in the welding, brazing or soldering alloy, the cost effective remedy again is sealing out moisture. Powder coating has the potential to be corrupted and might need to be redone.