MIG Welding Gas from your Cars Exhaust

In article , mark writes

Mark,

What I don't know about welding would fill a very large book, but I do know something about combustion chemistry.

First of all, most of the exhaust gas would be nitrogen - it represents about 79% of the air going in (or over 70% of the mixture) and, apart from a trivial amount converted into NOx, it ain't going anywhere except out the exhaust. AIUI, MIG welding uses CO2 or argon, the latter being preferred, and since the latter is several times as expensive as nitrogen, I deduce there must be a reason nitrogen is not used, probably as it will form nitrides with some metals and hence local changes in hardness. If I understand you correctly, you quote the exhaust figures in g/km; in order to understand what you have got, you really need to look at the gas concentrations in volume %. That would highlight the glaring hole which is mostly the nitrogen.

Second, there will be some oxygen in the exhaust, from incomplete combustion even if the mixture is exactly stoichiometric. This will obviously be bad news for the weld.

Third, the mixture of water and NOx in the exhaust gas will not do the inside of your pressure bottle a lot of good.

Fourth, the CO in the exhaust would not do you any good if you were welding in a confined space. Some would get oxidised in the arc, but not all.

Sorry to rain on your parade - and give it a go (with suitable precautions), it might work...

David

I was just going on what those results in the table ..said to me .. co2 there is the dominant gas ... ...so is the table wrong ?

and to get rid of some of the nasties ...i could filter the whole lot through those carbon granules you get from aquarium shops.

and wasn't planning on doing it in a confined area ..it would be well ventilated ....more so, than most MOT garages that just have the front doors open ......

The process of welding gives off lots off CO and other poisonous gases anyway.

As for the stuff, that could rot the cylinder ...there's not much sulphur in the gases from an engine run on lpg.......would the carbon granules filter get rid of what there is left of any potential rust agent . ?

all the best.markj

No (at least I have no reason to think it is), just incomplete - exhaust analysis is looking at pollutant levels; nitrogen and oxygen are not considered pollutants. (Though, with bureaucrats who consider stones dug up from farmers' fields as taxable waste, you never know.)

Well, I was talking about NOx - which with water would produce nitric acid and other nasties. Yes, carbon granules would probably absorb both

- until exhausted.

You would probably do better making your own CO2 from marble chippings + acid though.

David

I'm not going to take issue with the chemists here but CO2 is a pretty miserable Active gas in the Mig/Mag welding scene. In my experience, it needs something like Argon to produce a smooth weld.

There were others who increased their alcohol consumption to get CO2 bottles from their hostelries. The wiggly runs were from the alcohol but the excess spatter was from the CO2.

All the rest- used ArgoShield

Norm

On or around Sun, 30 Nov 2008 05:38:35 -0800 (PST), mark enlightened us thusly:

where's the hydrogen oxide gone then?

Sounds like a NoNo for welding but is it usable as a non oxydising atmosphere in a small muffle?

I don't know ... No ones proved otherwise ...it could give good results ..

May be forced to try it when the next rental period is due

Looked up nitrogen and the exhaust gases .......seems that most of the nitrogen in the air pases strait through the cars combustion process unharmed unchanged .. and nitrogen is as much as a semi inert gas as co2...thats according to what research i could do .

and nitrogen is used as a MIG welding gas ..........particularly as a a gas to fill tubing that is to be welded .

yes, i would like to carry on using argoshield ...but cant justify the expense it costs when i only use one bottle a year.

I'm not arguing ..or saying to people.... do it my way ... I'm just after constructive critisim...........which Ive had so far .so alls well and good. I didn't realize that the nitrogen was the dominant gas ....Dave put me right on this ...and i did a bit more looking on google with that in mind.

information for all

i don't know if people realize now ...........you cant do unrented bottle swaps any more...since the new owners of boc's bottle business (air-liquide) put into place a fool proof system.

so anyone that used to do this is in for a big surprise when or if they task there bottle to be swapped.

all the best.markj

'Man in North Wales found in garage with car engine running and rubber pipe on exhaust'

Just a thought, fellas- just a thought.

I was dying to write that

Norm

A friend pointed this company out to me the other day ... anybody used them ? (See Hobby Welding Gas)

-adrian

yup, looked into that ..and it works out the even more expensive, i would use a few of those small bottles a year ...they hold 137 bar and

where as my boc bottle holds 11.71 m3 at 230 bar

he has his prices on ebay

here

What about letting the car run with the exhaust hose directed to the weld nozzle no tank no regulator just a 2 inches rubber hose near your weld in a well vented area of course.