Aluminum will only corrode 1 or 2 molecules deep before aluminum oxide will stop the process. It's the black stuff you get on your hands when holding to an aluminum hand rail.
The white corrosion you have if the metal is aluminum is the result of electrolysis between two metals, in this case steel and aluminum with the added catalyst of saltwater. The only way to stop it would be to replace the steel with stainless, or aluminum. If you weld on the metal that is chrome plated you will cause the chrome to peel off. I have welded steel, aluminum, galvanized, stainless, in my old line of work as a sign installer doing large outdoor commercial signs. Never a car bumper, but the issue if electrolysis with steel and aluminum is a big problem in Florida. Mainly because of aluminum rivets being used to fasten aluminum sign panels to a steel frame. Near the beaches all the rivets would rot out in about 2 years. Stainless steel rivets would hold up, but were a pain to use. You could try cleaning the white powder off with a vinegar bath and then paint with a good two part primer (Mathews Primer, the greenish-yellow stuff) and top coat with urathane paint.