This type of corrosion is called Galvanic, the electrical potential is significantly different between the SS bolt and the aluminum. In this instance the Stainless steel bolt is the cathode and the aluminum metal is the anode, in the presence os a electrolyte such as salt water and warm temps the aluminum will always corrode. The warmer the temps and the higher the chlorides the faster this will happen -

In the future - 1. paint / coat the stainless steel bolt as washers to reduce the exposed surfaces 2. try to sleeve the bolt shank to electrically isolate the bolts from the aluminum, 3. Use a fiber non electrically washer under the stainless washer to you again electrically isolate the SS. 4. Use a magnesium or zinc anode bolted to the surface to minimize this, the less noble metals will then sacrifice to the SS cathode

What to do now - Remove the tabs, wash the surfaces in clean fresh water, and or steam clean, then wash again with salt remover such as salt away, blast clean the surfaces ( all) wash again with salt away / chlorride or similar - then recoat using powder coat or conventional wet epoxy urethane systems.- you must remove invisable salts before recoating or you will have osmotic blisters very soon - this is normal stuff - the exposed stainless is the problem here

Too much beer in the cooler up front:drink:

thats a trim tab used as a example

Same thing on the out drive - corrosion most likely is around a SS tie bar bracket or bolt - If corrosion is out in the middle of the drive then "maybe" the drive is a used/ rebuilt drive and the salts were not removed before the coating operations- the fix is still the same get a 24 pack of ice cold beer, swear a lot -take the chit apart and start over - everyone knows if you have corrosion on your drive, you have zero chance to get laid