As a certified welder, and without knowing what the exact environment was, I would venture to say you may be correct. Are you running dual shield? or just flux core? if your running dual shield (flux core AS WELL AS gas) then if the gas gets blown out by wind or something, then you'll get porosity. It can also happen if you have contaminated wire. Check that to see if you have any kind of corrosion on it at all. You could have also missed a piece of slag from a previous pass. Also, you may have gotten some grinder dust in it. And finally, sometimes (although not often) you just get porosity. It happens. And the crappy deal is that if it's on a certification pass or might get x-rayed, you will have to grind the whole pass out. Just make sure that if you grind it out, you wire brush it out like a MOFO after each pass. I always used a wire wheel on a grinder to clean it out after chipping it. Wait until you go for a 6G test. You think gravity sucks now? Upside down and up SUCKS!