js55,
You have not read my post correctly, silicon on the surface of a weld in any significant amount would be classed as slag. Slag is a no no both by AWS and ASME in the most part.
Check the AWS A3.0 definition of Flux:
'
flux. A material used to hinder or prevent the formation
of oxides and other undesirable substances in molten
metal and on solid metal surfaces, and to dissolve or
otherwise facilitate the removal of such substances.
See also active flux, neutral flux, and slag.'
Now check ESAB's description of it's Spoolarc 29S welding wire, this consumable was just picked out of numerous descriptions of GMAW wire, any make or brand have the same propaganda:
http://esabna.com/us/en/products_catalog.cfm?Product_ID=67"As part of the Elite series, Spoolarc 29S is a general purpose copper-coated solid wire suitable for many carbon steel welding applications using either the MIG/Gas Metal Arc Welding (GMAW) or the Submerged Arc Welding (SAW) processes. Spoolarc 29S contains moderate amounts of manganese and '
silicon to provide sufficient deoxidation 'over light mill scale. Shielding gas choices for Spoolarc 29S in the GMAW mode are 100% carbon dioxide, argon/carbon dioxide mixtures, argon/oxygen mixtures, and other argon based mixed gas blends. In the SAW mode, Spoolarc 29S can be combined with a variety of active and neutral bonded fluxes to achieve better welding performance over Spoolarc 81 wire. Spoolarc 29S is used in a wide variety of applications including heavy equipment, automotive parts, railcars, agricultural equipment, and sheet metal welding."
Flux is a material added to a welding consumable to do something, in this case de-oxidate, the resultant left over is a slag......slag in this case being the undesirable stuff that was removed from the weld i.e oxidants ........Slag left on the surface of a weld is unacceptable, (it prevents complete visual inspection of the weld for a start). Unacceptable levels of slag left inside a weld are unacceptable, the amount which is subject to what standard is being used).
I think you have gotten confused with the part about using UT for determining surface phenomena? I don't think this is viable? I am no UT person so am unsure about this.