Why is it so freaking hard to get a weld on a air to not leak?

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I am a diesel/truck mechanic, I do quite a bit of welding also, I am a fairly decent welder but when it comes to welding a pin hole in a steel tank, wether it be a air tank, hydraulic tank or a fuel tank, after welding it and pressurizing it, about 75 percent of the time I end up chasing the pin hole around meanning I weld it and the pin hole moves to the edge of the weld, so I let the pressure out of the tank, I clean it up real good and weld it again and now the pin hole moved to the edge of the new weld, some tiomes it freaking drives me nuts, finely after 4 or 5 times I can get it sealed, I have seen other people have problems also so it is'nt just me.Lately the shop I work at has been doing plumbing on over the road fuel tankers, the piping on those seems to be a problem also.Is it because of the contaminants in the pin hole or what?So, is there any tricks?Doug
Reply:Pin hole on the outside indicates much more corrosion on the inside.The existence of a or scattered pin holes is a BIG RED FLAG for internal bad corrosion,requiring further competent inspection of the tank--before just 'blowing some weldbeads on it and calling it--good to go'Welding on a corroded area, can and does create tight cracking as the weld cools and contracts--due to the existing, unseen inside corrosion. Tight cracks leak under pressure--as you've noticed.IMHO--you're nuts to be welding air brake tanks--replace them. A sudden failure inthose can be bad news. A $low leak detected by DOT i$ RED TAG.Hyd. or fuel tanks with pin holes require further inspection to determine repairabilityand a repair plan. Corroded tanks often have many, many other problems.Blackbird
Reply:Also the weld process adds heat that expands the air inside and if there is any restriction that prevents the air from exiting somewhere else it will seep out the weld site leaving more pinhole leaksTiger Sales:  AHP Distributor    www.tigersalesco.comAHP200x; AHP 160ST; MM350P,  Spoolmatic 30A; Everlast PowerTig 185; Thermal Dynamics 60i plasma.  For Sale:  Cobra Mig 250 w/ Push-pull gun.  Lincoln Wirematic 250
Reply:I usually just braze the pin holes up when they bring leaky fuel tanks into the welding shop. Not my personal favorite method of fixing the problem, but it works. I agree with replacing them but it might take a good 6months to get a replacement over here in Iraq so we make due with what we can so the vehicle can be used until the new tank comes in, minimizing down time.
Reply:I hope you're not using just compressed air to test your non-pressure tanks. If the tank fails, it will do so explosively, possibly killing anyone nearby. And, as already mentioned, a leaking air tank should probably be replaced anyway...
Reply:Weld process will also have something to do with leaks. Mig is notorious for cold starts and leaks. Most seal tight welds are done with tig for this reason. Not to say you can't do it with mig, but it takes more care and prep.To follow up on what others have said about the dangers of air tanks, you might want to look at this thread.http://weldingweb.com/vbb/showthread.php?t=48232Keep this in mind when you weld on pressure vessals. Catastrophic failure is more than just a possibility. A pin hole leak could well be the last straw that unzippers a tank..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Most of what I am talking about on air tanks is having a new tank and having to drill a hole and weld in a threaded nipple for plumbing, the air tanks come with so many different configerations theres no way to be able to stock the exact tanks for all applications, the air tank thing is a pretty much the least of my worries now days.I just mentioned air tanks because the problem we have with fuel tanker plumbing seems like its the same type of problem, it too is with brand new parts, we have been doing alot of plumbing lately and have been sending the work out but we want to become more self suffeciant,( spelling sucks)Doug
Reply:Something you might try is to run a bead about an inch long.  Do not try to just put a tack over the location.  Also immediately take a ball peen hammer and peen the weld as it is cooling from red.  This is a method used to seal up pin hole air leaks in air tight wood stoves.  The pin hole would make a whistling sound that customers did not like.  I think air tight wood stoves have gone the way of the dodo compared to the '70's when every welding shop was making stoves.
Reply:Doug what type of welding are  you doing on these tanks?  Is there any paint or coating on them and are you removing it and then cleaning up the weld area with a sander and a cleaner?Welding Supervisor Department of Corrections.
Reply:On fuel & hydraulic tanks it is critical to get rid of the fluid in the crack. I usually use a torch with a 00 tip going directly over the crack followed with a wire brush after I have properly purged the tank of course. I start at least 1/2" before (more is better) the crack and end at least 1/2" past.Millermatic 200Hobart Handler 120Victor O/A & Ramco BandsawLincoln 225 ACSnapOn AD HoodMiller XMT304/22AHypertherm Powermax 1650 G3Lincoln Idealarc DC600 w/Extreme 12 VSMiller Digital Elite "Joker"
Reply:Are the leaking hydraulic tanks aluminum? Is it the seams that are leaking? This is the area I tend to see leaking. Getting all the oil out of the cracks is tough. Heat, cool and heat again seems to work after the tank is drained (either drained or on end dumps raise the bed but watch the ceiling height). TIG works best also looks the best, no chicken sh*t globs (the customer generally appreciates this). Have found that when these tanks start leaking you need to look at the mounts. Are the rubber isolators still there, are the springs to tight, are they even there, etc."Liberalism is a mental disorder" Dr. Savage
Reply:If you have stops and starts be sure to take a grinder and taper out your stops if there is any hint of cold lap. Same thing with your tacks. Don't assume that you can just burn them in.Tough as nails and damn near as smart