A technology for producing industrial water heaters exists, where the internal protective layer is an organic coating. This technology was largely adopted in the last decades and different methodologies for surface preparation or coating application were proposed and used; as an example bare steel is coated by an epoxy powder resin layer obtained by spray technique after a phosphatization treatment in an iron phosphate bath. However, when defects and organic coating discontinuities are present, the main water heater failure occurs by the detachment of large coating areas. This phenomenon is due to a fast paint delamination caused by a cathodic disbonding induced by the magnesium anode. In order to obtain a better coating adhesion, also under the typical water heater working conditions, a proper preparation of surface steel must be chosen. The aim of this work is the evaluation of different types of surface preparation, including degreasing, sand blasting and the use of iron or zinc phosphate conversion coatings. The samples prepared were coated using epoxy powder resins following the same application procedure. Adhesion was evaluated using classical methods like the pull-off test and the cathodic disbonding test. In addition, electrochemical impedance measurements were carried out on defect free samples and on others having an artificial defect. Coating delamination was studied by following the evolution with time of the electrical parameters measured from the equivalent electrical circuit. Both electrochemical tests and cathodic delamination tests showed that zinc phosphate treatments, and in particular phosphatization baths containing manganese greatly improve the metal–paint adhesion under the very stressing condition typical of the water heaters, whereas chromium passivation of iron phosphates had a very limited action. Electrochemical impedance measurements were shown to be very helpful in evaluating coatings performances, and in particular coating adhesion.