It has always been my understanding the zink blocks keep from eating away the aluminum in boat motors and not rust. Electrolisis is what causes the aluminum to be eaten away and electrolisis eats the zink before the aluminum. Never heard of this preventing rust. I had zink all over my inboard/outboard motor, and the exhause still got rust and had zink blocks all over the place, but outdrive was protected as it was aluminum and electrolisis is much worse in salt water, thus, zink is much more important on boats and motors in salt water. We also use zink in heat exchangers to prevent erosion to metals from electrolisis in the chemical plant I work at.