Are phosphate floor screws suitable for subfloor which will be covered in thinset and cement board?

I am installing hardiebacker concrete board with thinset to a plywood subfloor.

I am putting down the new subfloor and want to be sure the screws that I use to secure the plywood to the joists will not corrode from the layer of thinset over the subfloor.

All the floor screws I could find in the box stores are black phosphate screws. Are these black phosphate screws alkali resistant and will they be OK in contact with a layer of thinset and cement board on top of the subfloor?