What causes air pockets when welding?

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I use a flux core welder and I keep getting air pockets popping out of the weld right before it cools.   Does anyone know what causes this?
Reply:Yep, oxygen. What are you welding? Essentially you are losing gas coverage, either due to moisture in the flux core wire (and I don't use FC much at all, so it may be other things as well), or poor technique, and air is getting to your molten puddle of metal.
Reply:Pretty much anything I weld is giving me air pockets.  I've tried holding the gun different ways and stuff but air still pops out of the puddle several times during a project.
Reply:Try holding at the end for a sec and then backing up a 1/4".  Sealing up a tube will cause the puddle to pop too.My name's not Jim....
Reply:It's caused by moisture, oil, dirt, or organic material getting into your welds, one way or another. It can also be caused by excessive rust and/or mill scale.  Basically, either dirty welds, or bad wire.The first thing you do is grab a grinder and make your joint area shiny before you tack-weld it together.  Grinding before fit-up is a good habit no matter what you're welding.With self-shielded flux core, moisture being absorbed by the flux is a recurring problem. If grinding your joints doesn't help, then that's where your problem lies. This is just one of a long list of reason I stopped using that crap.Last edited by Joshfromsaltlake; 06-02-2012 at 11:56 PM.
Reply:What is the filler metal designation? Is it a gas shielded or self-shielded? What polarity are you using?What shielding gas, if any, are you using? What positions are you welding ?
Reply:It most likely is hydrogen, a breakdown product of water. It is soluble in molten iron but not in the solid, so the bubbles form as the metal cools and the solubility drops. Rust, one form of iron oxide, is hydrated so the water can come from it when heated. Oxygen combines with the iron and remains combined as iron (or the oxide) cools; it takes much higher temperatures to break the chemical bond, as in a blast furnace.