H/E Rusting

I grabbed this pic from the internet to explain to clients what happens when the evaporator pan drain line gets blocked.
My question is this. Will the H/E really rust externally as indicated if it gets wet? I thought they were made of something higher grade than just carbon steel.

Yes they will rust!!! Back in the day when furnaces had standing pilot lights we would recommend leaving the pilot burning all summer long as it produced just enough heat to dry any condensate from the heat exchanger during the off cycle of the A/C unit, but those days are long gone with electronic ignition

Thanks Charley but does that still apply today with the materials used in a modern high efficiency furnace? I thought they were SS.

Not SS to my knowledge and if you allow condensate to drip down on them they will rust