A system for conducting measuring corrosion rates in a multiphase environment using electrochemical and weight-loss measurement methods on test coupons is provided. A plurality of inserts is disposed within a test vessel in a vertical arrangement. Each insert is provided with at least one test coupon and at least one working electrode that are exposed to the corrosive test environment within vessel. A test fluid mixture is added to the vessel and the temperature and pressure is maintained such that the mixture exists in a multiphase condition that has a vertical stratification such that each insert is exposed to a different phase of the fluid. Electrical signals from the working electrode is measured to determine the corrosion rate using an electrochemical method. The pre-test weight of the coupon is compared to the post-test weight to determine the corrosion rate using a weight-loss method.