309L - a stainless rod, but not the best for welding mild steel to mild steel. Typically we coat the new weld with a zinc rich coating. ZRC is a good product but pricey.
https://www.gmesupply.com/zrc-cold-galvanizing-compound-1-quart?gclid=EAIaIQobChMIwN-V7I2i6QIVEovICh3-DgrwEAQYASABEgIHm_D_BwE