Here are a couple pictures of the failure on Aluminum FWIW
Good shots of those failure areas. I'm sticking to my story on priming aluminum if you go with touching up. Ironically, "Stix" is available from Home Depot.. An auto paint supply will have a rattlecan of zinc chromate for about ten bucks. Also one of those paints by BM is a heat resistant paint containing alumimum.
Back to square one, or in this instance circle one. See where that powdercoating is coming off? IMO, that is from moisture running down that reverse curve and laying in a puddle on the bottom of the circle for who knows how many cycles. The moisture is going through the powdercoat, the aluminum is oxidizing, outgassing and pushing the powdercoat off the substrate. I don't see any evidence of any sort of primer only the oxidation.
I'd still be talking to the outfits that did the work originally for a complete re-do. The gates look fairly new. Even a small hole at the bottom of each circular element would have allowed drainage.
Famous aluminum story.. A car mag got a sample of some aluminum from a famous 1930's GP car. Arranged to have it analyzed by an eminent metallurgist..(story says he was from MIT) They shove a mike in the guys face and ask him about the wonderment of the alloy. He tells them the alloy is about the same as they would find on lawn chairs from Kmart. (or Home Depot)