After projection welding the nut surface might be damaged and coated with weld residue of unknown chemistry. This coating can act as a sponge and hold humidity and actually contain iron, copper and brass and many other chemical components. Some of these can be targets for rust. Bare metal may have been created during the welding process which is also a target for rust formation in the presence of humidity.
A brief search on the web found that steel can begin to rust within a few hours if in a humid environment. The amount of humidity and bareness of the steel determines the speed that this occurs. Some nuts are coated to retard corrosion. Welding may partially affect this coating. Iron will rust even faster than steel.
To prevent rust reduce the moisture and humidity. Rust preventive coated nuts may reduce rust occurrence.
ASSORTED WELD NUT
References: RWMA - RWMA Resistance Welding Manual 4th Edition