I have an old brick wall (made of common brick ) it is deteriorated the brick is starting to flake off & the mortar is crumbling is there some kind of product I can coat & seal it with? Is it possible just to coat it with cement?

Normally you could "parge" it with a cement/sand mix. But it will fall off if brick and mortar are sloughing off. You might be able to sandblast the brick and repair the mortar joints and then parge. I would suggest you have it checked by a masonry contractor or several to see what they recommend.