The present invention pertains to method and apparatus for determining corrosion rate or loss of metal, due to chemical reaction with a corrosive testing fluid under defined shear rate and varied pressure and temperature conditions. This apparatus features a coaxially positioned cylindrical shaped rotating cup and a static cylindrical coupon, or a static cylindrical cup and a rotating coupon located in a test chamber. This defined shear rate corrosion tester can rotate various speeds through a magnetic drive mechanism. Due to its configuration, the corrosion tester is easy to couple with different kinds of testing chambers on demand, and it requires low maintenance and ease of cleaning.