Removed the stem from the handlebars and fork. Both interfaces are AL to AL. No corrosion at all. There are steel to AL interfaces between the steerer tube and cartridge bearings. I was thinking sweat might get into those interfaces, especially the top one, because sweat gets all over the top of the steerer tube and runs down the sides, over the spacers carbon fiber spacers. There was a fair amount of grease on the outsides of those bearing cartridges. That grease might have made the difference. The reason I know sweat gets onto the steerer tube is it gets all over my phone when it is mounted in the middle of the handlebars, right next to the stem/handlebar interface. The sweat come from my head dripping.