Backing gas for stainless steel

Why I need to maintain an inert gas backing for stainless steel welding but on other hand we do not need that for carbon steel welding?

An inert gas backing to prevent air contamination and keep it from oxidizing rood bead.
Inert gases are most used for light alloys and those sensitive to oxidation.

Photo: welding with backing gas
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Photo: welding without backing gas
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Read more standard: NORSOK standard M-601 Annex B, Acceptable oxidations of welds in stainless steels

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Many thanks