Our steam sterilizer heating plate is made of FDA approved food grade 304 stainless steel, under instructed use, there shouldn't be any chance for the stainless to oxidize and corrode. If you started to see pinhole size rust with a small indentation on it, this is an indication that you have not done enough fresh water rinse after using citric acid or vinegar for descaling.
While Vinegar and citric acid are great for descaling, it can also cause a reverse effect such corrosion, pitting or dullness to the heating plate if left unattended for over a long period of time.
Just as all stainless steel appliances, any descaling process should not take longer than 1-2 hours, also, also be sure to do sufficient fresh water rinse after each descaling solution usage. Any small bit of acidic solution left behind can cause permanent damage to the heating plate, and it's not reversible.
If you see brown residues where there is no indentation, it may be just telling you that the feeding gears may need more washes.