You basically got i, except that zinc doesn't rust. What you see is the steel itself rusting through, the same way is does through chrome on (old) car bumpers, and painted bridges.

Depending on the extent of the rust, you may be surprised at how long these wheels last. Before stainless steel became dominant for better spokes, spokes were all like yours - plated steel, and though they didn't age well cosmetically, they held up for decades.