Decorative chrome is "micro-cracked"....meaning, there are tiny "holes" in the top chromium layer. A bazillion tiny little pores in the protective Chromium layer. Why? This helps to "spread" out the corrosion...kind of neutralizing the rust and preventing a nasty rusty red cancer from forming.
Your wax coating is trapping and preventing corrosion from "spreading" itself out over the part, thus causing a local concentration of corrosion.
Starting from the steel piece that you want to chrome plate: first, you plate down copper (to smooth out the part), then semi-bright nickel (which is very corrosion resistant) and then bright nickel (this gives the part it's "silver" look). Finally, you plate just a VERY thin flash of chromium on top. This gives the nickel that bluish hue that everyone associates with chrome. The chromium is VERY resistant to corrosion. However, platers actually put pores in the chrome ON PURPOSE, otherwise, when a nick does occur in the chromium layer, the corrosion will quickly bore through the nickel and into the steel and then spread laterally....causing a nasty mess.