The same nut weld part plated and unplated is welding with good strength but the plated version is failing the impact test. Without any other input one can only assume the plating has changed the welding parameters and the part is not resulting with the same final weld joint. It is either weak or brittle.
PROJECTION WELD PARTS
The first thought is that some heat energy is being absorbed by the coating being melted. That energy would normally have gone into the weld projection. Thus they have not heated up as much as in an uncoated part. This may lead to less potential set down and strength.
The second issue is the coating may be in the joint and acting as a braze joint. That is OK but it is not as strong as the desired projection weld and is likely to contain weld contaminants. In short it is not a desirable braze joint.
A schedule change is probably needed to ensure more projection weld set down and involvement on the coated part projections.
References: RWMA – Resistance Welding Manual 4th Edition