(a) Metals react with the things like moisture, oxygen, carbon dioxide to form a layer of substance like rust in iron which hides their shiny surface. This is what we call corrosion.
(b) Nitric acid is a strong oxidising agent and Zinc is not a very reactive metal according to the reactivity series. Hence the nitric acid oxidises the hydrogen gas which is produced may be in the first step if reaction between acid or metal had taken place. So the hydrogen gas does not evolve.
(c) Metal sulphides are insoluble in water and halides are soluble so they get washed away in spite of getting deposited over the crust.
