They are not actually zinc, but zinc-coated steel, also known as galvanized steel.
The treated-wood industry recommends the use of hot-dip galvanized or stainless steel fasteners for use with treated wood. Electroplated/electro-galvanized and mechanically-galvanized coatings should not be considered to be hot-dip galvanized. (Class 55, or higher, mechanical galvanizing provides galvanizing equivalent to the hot-dip galvanizing used on connectors and fasteners.)
Source: Strong-Tie's pressure-treated wood FAQ, which has additional information about various kinds of pressure-treated wood, galvanized steel, and stainless steel.