Aluminium vs Magnesium
Sikorsky gear box housings have traditionally been made from magnesium. It was determined that the housings were a weak element in the in the system. Gear boxes returned for repair or overhaul required housing replacements well short the overhaul cycle due to corrosion. In response to this Sikorsky began coating the outer surfaces to protect the housing against corrosion. This began in the early 1990s. I believe that it was either a substance called sermatech or rockhard. It was baked onto the outer surface as part of the manufacturing process. Aluminum would have a similar problem with corrosion in the salt environment. Any change may be driven by NAVAIR.