Explanation

A 1% volume solution of hydrochloric acid (HCl) is commonly used as a test for the porosity of cadmium and zinc coatings. The test involves applying a drop of the HCl solution to the coating, and observing if the acid causes bubbles to form on the surface. If bubbles form, it indicates that the coating is porous and that the underlying metal is not fully protected from corrosion. 

The test can also be used to detect defects in the coating such as pinholes, blisters or scratches.