Sand is a BIG NO for cornsnakes!
Corn Snakes are not desert animals, and sand can cause them lots of problems. As a corn snake moves through the sand, it can get in their mouths and their nares (nostrils). When it gets in their mouths and is accidently ingested, sand can cause impactions which can kill the snake. And it can be quite a pain to flush the sand out of a snake's nares. If not kept out of the nares, it can lead to other priblems such as difficulty breathing and respiratory infections.
Corns don't need quite as high humidity levels as do other snakes, but the sand is really drying to them which can lead to difficulty shedding, dehydration, etc.
So, in my opinion, sand is definitely NOT a good substrate for corns!