CO2 Purge for brazing?
does anyone have any experience brazing with CO2 as the shielding gas?
I've always used nitrogen but have read in a couple of specs and text books that say you can use CO2. Just curios if anybody knows of a good reason not to.
First of all you need an inert gas. co2 is not inert, it is a molecule. CO2 is often contaminated with oil to some degree. Nitrogen is the most popular due to the price. You can use helium or argon also. I have used helium in the past and it works wonderfully. If you use helium for leak testing your leaks will show a pressure loss much faster and smaller leaks can be found since it weighs so much less.
CO2 IS an inert gas and can be used for a purge. All gasses are comprised of molecules, even nitrogen. That has nothing to do with their chemical reactivity.
Originally Posted by yorktek37
If you can get CO2 of the proper grade. It MUST be dry.
I guess I have to add this...
Originally Posted by dprimrose
Get your post count up to 15 and apply for Pro membership if you are in the trade. It's free.
There is actually a thread on this very topic in the "pro technical" forum.
First of all a molecule is made from atoms that have bonded together.Did you not remember science in middle school. Look at a periodic table of the elements and tell me where you see CO2 in the noble gasses column. CO2 is not an inert gas and cannot be made inert. It has two oxygen atoms(oxidizer) and 1 carbon atom(metalloid). It however does not react much but there is some. Now to the next comment. Gee I want to save some money and instead of using nitrogen I want to use CO2. I will take a 10 LB bottle. Oh but wait, I need a real high purity. Do you have any 99.9997 Percent purity Now what would that cost. CO2 by nature has oil in it. That is why they use it in the gas and oil industry. And thats where some of it comes from. Ever heard of CO2 Re-injection. CO2 will absord hydrocarbons. Now show me an inert gas that will do that.
If you are not sure it is sometimes better to keep it to yourself.
And that is my opinion but I could be wrong.
I did make a mistake in my post. Nitrogen is not inert. It's close but not inert. Nitrogen will bond with oxygen easily to get Nitrogen oxide (nox) Nitrogen tetroxide(oxidizer used for rockets) And everyones favorite Nitrous oxide. And everyones least favorite Nitric oxide(Highly deadly dead before you hit the ground gas) and also Nitric Acid. There is very little reactivity with copper though and thats why it does well with brazing. I personally use heluim alot for brazing and for leak checking. If it passes with helium there is no leak!
Let's no go head-to-head on science. Atoms bond to form molecules, I know my science. As I said, ALL gasses, ALL gasses are comprised of molecules. Nitrogen (N2) is a molecule (not an atom) made up of 2 Nitrogen atoms. I never claimed that CO2 was an element, nor a noble gas (which is different from being inert) and neither did you.
As far as CO2 being inert, here is a pdf from the other thread that I mentioned regarding the use of CO2 in medical gas brazing. See page 2. http://img170.imageshack.us/i/medicalgases.pdf/
If it is good enough to use for a purge in a medical gas brazing application, then it's probably OK for HVAC.
After all, this isn't life and death most of the time.
Under the right conditions, anything can be made to react. Nothing is perfectly inert.
Originally Posted by yorktek37
What would the problem with with using a CO2 purge be? What reaction would take place that would be detrimental to the clenliness and operation of the system?
I have done it and gotten nice, clean joints, internally. Yes, I inspected them.
After doing the work, I pulled a good vacuum, thus ensuring that no CO2 would be left behind, just the same as you would using any other purge gas.
Yes, Nitrogen, Argon, Helium, all make good purge gasses.
So does CO2.
Is the fact that it can contain oil and it can be "wet". I wouldn't do it. Plus if you are purging and let's say the cylinder tips over you could get liquid co'2 in there and if anthing is still hot it just crystallized. Reactivity is very low, If it was high or moderate how could it be used for SMAW welding. The hydrocarbon content is what the problem is. On a medium or large system it would most likely do no harm but on small low temp and very low temp systems there could be an issue.
on the other post "nothing is perfectly inert". Inert atoms are "full" they have an equal number os electrons, protons and neutrons. There is no room for anything to "bond". However a couple of them can be a "hexaflouride" for a very short time and is almost impossible to do but for all practical purposes inert will not combine to create a molecule. I remember seeing a clip on mythbusters about sulfur hexafloride and what it does to your voice. Just the opposite of helium. It makes it very deep since it is so dense. It also makes a great insulating gas for High voltage switching
check out the 500kv switch. This is really cool!
Sorry if I "copped" an atitude about that but I kinda get that way when I know I am correct on something and someone else says I am wrong. Teachers hated me in school but they made sure of all of the facts on their material. I am just like that I don't take BS too well. And there is alot of it in this buisness. Inspectors really hate me too. They don't get much past me.
Last edited by yorktek37; 08-19-2009 at 01:44 AM.
If you are using the correct grade of CO2, then oil and water aren't a problem. Same as nitrogen. There are also oil and water issues with some grades of nitrogen.
If you are using a pressurized gas cylinder, it should be properly secured, right? Therefore it shouldn't tip over.
I have spent a lot of time studying chemistry,it was my major in college. I understand about valence electrons and how they work in chemical reactions. I will stand by my statement that nothing is perfectly inert.
We have the same attitude. When I know I'm right, I will not back down. The idea that CO2 makes a poor purge gas is an old wives tale.
Here in Texas you can only use UHP Nitrogen on medgas. Nothing else.
Originally Posted by jpsmith1cm
Dang, all this science talk makes me want to go make a volcano out of baking soda and vinegar! Interesting discussion guys, my heads starting to hurt!