What’s causing this corrosion?

[Tips Up]

 

[Tips Up]

Aluminum trim along motor well. Only in 1 spot. Boat is kept in driveway. Washed after every use. Don’t seem to have any other issues.
Was going to fill it in with some sikaflex but figure I need to stop what’s causing it.

 

[kingblazer84]

I would sikaflex it and send it, prob just pitted over the years and the coating is coming off allowing the salt to corrode it in the pores possibly?