my first mig welds (on the exahust silent box)

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this are one of my first welds with mig my mig its a miller 172  in tap #2 .30 solid wire  rinning CO2 as shielding gas  and the wire vel its 65-70and some body could tell me why i get a thin "brownish slag"  on the welds Attached Images
Reply:Are you cleaning the steel before you weld? Down to bare metal?axehind
Reply:the grinder or even the cup wheel wire on it may be i finished with out sheet metal but it happen also on clean material  (bare material)
Reply:It looks like there is something in or on the metal that is cooking out when it is heated during the welding. Could be that it's released and  drawn toward the weld when heated. Try grinding and cleaning a bigger area around where the weld will be.Do you have any new metal to try a weld on. It may look clean but is it bare. Some old metal things will just be that way and you can't help it. If you can brush it off and the weld is good under it, that's about all you an hope for. You might try increasing the gas feed a little to see if that helps.
Reply:poor quality gas?  are the gas lines clear?Move toward the light...
Reply:Just by looking at the color of the smoke stains also around the weld area-----------could it be that this old muffler is a galvanized or similarly coated muffler??But hey, the slag is on the 'outside' of the weld. So what the heck, There are ideal conditions and then there are the conditions we have.  On older exhaust systems just be glad there is enough good metal to weld to. Exhaust systems and radiators are about the same, by the time you get them good and clean the metal is gone."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt
Reply:The "brownish slag" you are talking about is silicon. Silcon is in the electrode and it works as a cleaner and floats impurites to the top of the puddle. An ER70S-6  or ER70S-3 electode contain high amounts of silicon which is good for welding on dirty base metals. Still always clean the base metal. The silicon can easily be removed with a wire brush or wire wheel however you can weld right over top of it and it will not become entrapped in the weld like slag from a stick electorde. The silcon will break back down and work out to the top. Good luck.
Reply:not bad for your first time. i know people in this forum dont like to give praise. maybe more people would post and be more interesting forum??? i check into this forum now and then, and some people always jump on someone  because they didnt buy the best welder or didnt weld like they would. isnt it better to get a welder, try it out and if you like it down the road upgrade...i know my first car wasnt a hotrod. im sure some may wont to blast me for the comments. but not bad for your first time with mig!
Reply:Originally posted by bail76 The "brownish slag" you are talking about is silicon. Silcon is in the electrode and it works as a cleaner and floats impurites to the top of the puddle. An ER70S-6  or ER70S-3 electode contain high amounts of silicon which is good for welding on dirty base metals. Still always clean the base metal. The silicon can easily be removed with a wire brush or wire wheel however you can weld right over top of it and it will not become entrapped in the weld like slag from a stick electorde. The silcon will break back down and work out to the top. Good luck.