Stainless steel water markers won't stay polished

I am an architect working on a new water intake structure in a highly visible area of the Sacramento River. We've embedded multiple type 316 stainless steel water markers flush to the concrete structure. We've ground them to a fairly reflective polish twice now, only to have them (oxidize?) turn gray as concrete almost uniformly after a rain.

Is there a way to keep this stainless steel reflective w/o constant polishing? Are we using the wrong sandpaper? Is there a chemical to both remove the gray film and maintain the polish?

Thank you,

As long as you keep grinding, the parts will keep corroding. What you need to do, is have the parts passivated to produce a finish that will be resistant to the water. Passivating will leave an oxide-rich layer that is more resistant to the attack of the water than the iron-rich surface that is created by grinding. You may also look at electropolishing to accomplish this.