Why does the corrosion of the zinc coating initially occur faster than later on?

The newly hot-dip galvanized zinc coating lacks a protective layer on its surface, which results in a relatively fast corrosion rate. However, once a layer of corrosion products forms, it acts as a protective barrier for the underlying zinc coating, functioning as a protective film. As a result, the rate of continued corrosion of the zinc coating slows down significantly compared to the initial stage.