Both screws were put in at the same time. They have the same levels of exposure to water, salt etc. But one has rusted substantially and one hasn’t. Is it as simple as that one maybe got scratched up more when I put it in and it’s gone from there? Or is there possibly something more interesting like galvanic action happening? Steel screw. Steel (chrome plated) bracket. Aluminium alloy bullbar.
My wife is a metallurgical engineer though primarily extractive. She thought that poor manufacturing was the most likely culprit. This screw may not have identical metallurgy to the screw next to it. After that her thought was galvanic ( she said the reaction could me limited enough to impact one screw but not the other) and then lastly damage to the screw that removed the layer that prevents corrosion.
I didn’t prep her with your thoughts at all. No answers but your thoughts seem to be on the right track.
I’m a materials scientist who knows a bit of metallurgy. Both screws and washers appear to be attached to the same materials, so it’s not obvious that one should have more galvanic corrosion than the other, but there could be subtle differences in contact. The ideas already floated about damage to one or poor manufacturing are plausible. The washer on the left looks slightly larger in diameter than the one on the right, which could be a hint that it’s a different, more susceptible, alloy.
Was the as-installed position turned 90-degrees from the picture? Then water would tend to drain OUT of one slot - and INTO the other. Standing water with a bit of road dirt makes a great electrolyte.
Otherwise, I believe that there is a spot of red rust on the right screw, at the top. It is possible that the plating is failing - and once failed quickly goes to red rust. The difference is just normal variation in part performance.
It’s possible the screws came from different melts – that is, the original formulations of the metal might have been different batches of the same alloy number, and have had significant differences though still in specification.
Years ago I worked around the semiconductor industry in very high purity clean room applications. One of the things they would do when using metal tubing systems is require that the tubing and fittings for a job all be formed out of the same melt, the same batch of alloy. This was to reduce the possibility of galvanic action causing metal to enter the water, as nominally identical alloy parts with actually different compositions reacted.