Silicon vs Silicone

Date 02-01-2007 18:56

I am currently teaching a welding class for the company I work for. I was covering defects the other night and began to talk about different types of slag. I was decribing the silicon produced by the GMAW process when I was corrected by the other instructor. I believe what this instructor was telling me because he has been a CWI for over 9 years now and teaches at a local high school. I have only been a CWI/CWE for about 8 months. Evidently I said silicone (sil-i-kon) instead of silicon (sil-i-kan). I had never thought about it in that much detail before. I tried looking it up in AWS A3.0:2001 standard welding terms and definitions but found nothing. The dictionary difines them as follows:
Silicon-a nonmetallic chemical element found always in combination
Silicone-an organic silicon compound highly resistant to heat, water, ect. and used in resins, lubricants, polishes, ect.
While in the dictionary I also found silica - the dioxide of silicon, SiO2, a hard glassy mineral found in various forms, as in quartz, sand, ect.

Now I am thinking that the slag that forms on a MIG weld is actually silica. I have succeded in thouroughly confusing myself. Anyone know which form of this word is the correct term to use when refering to the slag produced by the GMAW process?

By hogan

Date 02-01-2007 19:04

i hear it commonly referred to as glass

By Logan Mayfield (**)

Date 02-01-2007 19:18

Yes, I use that term loosely. I never considered it to be the technical term.

By G.S.Crisi (****)

Date 02-01-2007 19:13

Being a chemical engineer, I confirm the meaning of "silicon", "silicone" and "silica" posted by Logan.
Chemically, the slag produced by the SMAW process can be neither silicon nor silicone.
Now, I won't put my hand into the fire that it is actually silica, it may be a metallic silicate compound.
Giovanni S. Crisi
Sao Paulo - Brazil

By aevald

Date 02-01-2007 19:16 Edited 02-01-2007 19:26

Hello Logan Mayfield, it has been explained to me in this manner, silica is added during the making of the steel to help with the dispersion of the carbon in the plate, I have not researched this to any extent to verify. Thus when welding takes place the liquification of the weld metal also causes the silica to melt and work it's way to the top of the bead and shows up as "glass". There would also be a certain amount of this present in the filler metal as well, I would assume. I have never known this to be much of an issue other than to say that on multi-pass welds it is preferred that it be removed before making additional passes over the top of any previous beads. The term silica should probably be used in the context of saying " there is a silica deposit which will intermittently show up on the surface of some GMAW beads". Just my $.02. Regards, aevald After reading G.S. Crisi's post I changed the term "silicon" to "silica" from my original post.

By Logan Mayfield (**)

Date 02-01-2007 19:29

You make some good points. The reason I was discussing this in the class was because I was covering our company's weld standard. It states that all slag must be removed. I was explaining to them that they need not clean every weld with a wire brush because our shot blast system would remove it (slag, silicon, glass, whichever it is). I was explaining the differance between the slag produced in SMAW and GMAW. Also, we do require the slag on a GMAW weld to be removed on multipass welds. Most of these student have never welded before this class and have no technical background. I just want to make sure that I am as technical as possible. It's kind of like how most people don't understand the differance between slag and dross. I want to make sure that they do.

By Lawrence (*****)

Date 02-01-2007 20:24

Here is some interesting text from Lincoln discussing examples of the Silicon/Glass deposits in GMAW.
They attribute it to silicon added to the electrode wires as a deoxidzer in varying amounts as determined by the surface of the carbon steel to be welded. ER70S-6 being higher in silicon and ER70S-3 Being lower.
http://www.lincolnelectric.com/knowledge/articles/content/selectingmig.asp

And it's no surprise Ed Craig has about 10,000 words dedicated to the phenomina you are describing.. He goes into great depth about, shield gas selection, mill scale, and solid wire electrodes, describing how each have an effect and how the effects vary in combination. Very good stuff.
http://www.weldreality.com/MIG_welding_gases.htm

As SMAW electrodes go, most core wires for carbon steel applications are the same. The differences are introduced in the fluxes. Sodium and potassium silicates used as binders while Silica is employed as a slagging ingredant. (lifted from Welding Principles and Practices Sacks and Bohnart)

By rebekah (**)

Date 02-01-2007 20:33

Hey Lawerence

I just posted a much more basic question regarding MIG and then read your post about the Silicon/Silicone/Silica issue. I went to Ed Reality website and, oh my lord, can that guy talk! It didnt really answer my much more basic question though. Perhaps you could help clarify...

Rebekah

By Logan Mayfield (**)

Date 02-01-2007 21:03

These links will be helpful. Thanks.

By MBSims

Date 02-03-2007 02:42

The silicon used as a deoxidizer in weld metal does just that - it combines with oxygen in the molten weld metal to form SiO2 (silica) and floats to the surface of the weld.

By MDG Custom Weld (***)

Date 02-03-2007 15:26

I cover this same subject in all of my basic GMAW training classes. I go along the same lines as Marty. Silicon is in the GMAW filler for various reasons, but in the end it combines with oxygen and flats to the surface forming a glass like compound. I encourage the removal of it proir to secondary weld passes, but most compaines I work with are using single pass thin material. One thing I have noticed however is that when the product is getting powder coated, the powder does not stick to the larger pools of silica. I assume this is because the electrical charge is not strong enough to pull the paint through the silica insulator. This does cause some problems for large powder coated assemblies, and most of them must get blasted or wheeled proir to painting.