I have a sheetmetal part that has some rust and pitting that I'd like to braze. I've ground the surface as much as I can and treated what's left with rust converter. My question is, just how clean does a piece of steel have to be before braze will stick to it? Is there any way to braze to a treated rust surface?
It should not have any rust at all. We have to sand blast anything that even shows a slight layer of rust. Braze will not stick to anything with rust.
Hello Dave, when you are considering brazing anything it is best to have cleaned the part as completely as possible. If you don't have the means to do the type of cleaning that MDG Custom Weld talked about you will certainly run the risk of a possible early failure of the brazing that you do. The application and what you are trying to repair or join will have a lot to do with your success or failure. At the same time that you are paying attention to how clean you get the parts you don't want to make the mistake of making the surface too smooth, you want the surface to be free of rust, dirt, grease, and any other surface contaminants, but if you make it too smooth, as could be the case with using say 80 grit or higher sanding type pads, it might hinder the ability of the brass to obtain proper adhesion to the surface of the metal. A simplified way of describing brazing would be to look at the surface of the metal in the way of a sponge, by being able to melt the brass at a lower temperature than the steel you allow it to flow into the cracks,crevices, and pockets of the surface and when it has cooled it forms a mechanical bond. The fluxes typically serve the purpose of protecting the brass from oxidation and contamination in it's molten state and also help to float surface impurities out of the brazing zone. To give the short answer to your original question, you can certainly braze the parts even though you may still have some pits and rust pockets showing, the quality of the astetics of the job may suffer some and how long the repair lasts might be shorter than if you were able to get it totally clean. Good luck and regards, aevald
Try this web site!
http://www.esabna.com/EUWeb/OXY_handbook/589oxy14_3.htm
Dave,
You have probably completed the job by now and hope it worked out. If so let my remarks speak to the after-action critique, adding to anything the excellent link to the ESAB site may have left to your imagination....
The conversion coating you said you used actually tends to contaminate the metallic surface. I dont know how to remove it other than by abrasives- grinding, sanding, sandblasting, etc. whatever the case I avoid the conversion unless prepratory to organic coating (paints)
I have also used oxyacetylene torch flame to remove rust, I think with a slightly oxidizing or reducing flame... I think that was the flame but be assured flame cleaning is a technique that works and is documented, otherwise acid is widely available as rust removal agent active in gells and liquids ("Naval Jelly", "Army Preserve" and other brnd names available even at your local supermarket if not your hardware store) amd could be used if heat from flame cleaning is an issue (doubtful an issue though before heat of brazing is applied) However many try to leave a rust inhibitive deposit of phosphate; I dont know how braze will behave on much of that so I also avoid those containing phosphoric acid when I read the ingredients on the label. I dont know what the result would be and rarely opt for acids; if anyone else does know, drop me the answer please. I generally only use the acid for cleaning neglected tools whin it becomes unavoidable, to restore them and prevent personal embarassment.
Regards
d
Thorough cleaning is extremely critical for soldering, and silver soldering and technically also brazing, although one can flame clean the connection prior to actual brazing and achieve good results.