That nipple corrosion is very complex to work out root cause(s). I would not want to live my next life as a spoke nipple that is for sure!! It is exposed to lots of different muddy dirty salty and wet conditions over lifetime. And the galvanic corrosion between spoke, nipple and rim materials is quite nasty setup. If the protective anodised surface of the alloy nipple is removed the corrosion will accelerate and become very obvious. Also the quality of the alloy is likely to vary between manufacturers. Using a good protective/lubricating coating between nipple/spoke thread and consider rim washers. Also try to get very well anodised nipples, use suitable length spoke and avoid scratching nipple surface. Yes I know that sealant gets the blame. However it is not likely to be the only/main cause. As I said I would hate to be a spoke nipple!! The level of ammonia in latex sealants has dropped a lot in last 10 years.
I have found the some sealants have a corrosion issue with aluminium rims and spoke nipples if it gets to within the rim cavity.
