Are these grates okay to cook on?

I'm having trouble with some enamel-coated cast iron grill grates. I've only had this grill for a few years, I don't live on the beach (salt), we don't get much rain, and I store it outside but keep it covered when not in use. However, at this point I strongly suspect that the discoloration is rust, not old grease. It never goes away no matter how much I scrub (before and after every use), and over the last couple weeks I've tried:

• bathing them in Simple Green BBQ cleaner,
• hosing them off while scrubbing,
• running them through the dishwasher, and
• putting them in the oven on self-cleaning mode.

Nothing seems to help, and it looks like they're actually crumbling. I've read all sorts of conflicting information about what's considered normal, how to handle cast iron, and so on, and I'm just not sure whether these are in the "a few specks of oxidized iron won't affect you in the slightest" territory, or "you need to throw those away today and stick with stainless steel from now on" territory.

Stainless steel grates are not available for this model of grill (Coleman Roadtrip LXE), and the cast-iron replacements are about half the cost of the entire thing, so if these are no good I'll probably get a new, stainless steel grill.

What do I do about these things?

Update: After getting the answers below, I've been doing regular, minor grilling. After each session, I lightly scrub the grates, put a bit of oil on them, and lightly wipe down the oil with a paper towel. Over time, the rust spots darkened and started to look more like proper cast iron, which I suppose just means properly-seasoned cast iron. I grilled for a get-together a couple days ago, and it all seemed to work out fine. No flakes or other debris from the grates, no problems with sticking or anything, no apparent issues at all. From now on, I'll try to use it more regularly and wipe it down with a thin coating of oil after use.