Study of the corrosion behaviour of phosphatized and painted industrial water heaters

A technology for producing industrial water heaters exists, where the inter nal protective layer is an organic coating. This technology was largely ado pted in the last decades and different methodologies for surface preparatio n or coating application were proposed and used; as an example bare steel i s 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 fai lure occurs by the detachment of large coating areas. This phenomenon is du e to a fast paint delamination caused by a cathodic disbonding induced by t he magnesium anode. In order to obtain a better coating adhesion, also unde r the typical water heater working conditions, a proper preparation of surf ace steel must be chosen. The aim of this work is the evaluation of differe nt types of surface preparation, including degreasing, sand blasting and th e use of iron or zinc phosphate conversion coatings. The samples prepared w ere coated using epoxy powder resins following the same application procedu re. Adhesion was evaluated using classical methods like the pull-off test a nd the cathodic disbonding test. In addition, electrochemical impedance mea surements were carried out on defect free samples and on others having an a rtificial defect. Coating delamination was studied by following the evoluti on with time of the electrical parameters measured from the equivalent elec trical circuit. Both electrochemical tests and cathodic delamination tests showed that zinc phosphate treatments, and in particular phosphatization ba ths containing manganese greatly improve the metal-paint adhesion under the very stressing condition typical of the water heaters, whereas chromium pa ssivation of iron phosphates had a very limited action. Electrochemical imp edance measurements were shown to be very helpful in evaluating coatings pe rformances, and in particular coating adhesion. (C) 2001 Elsevier Science B .V. All rights reserved.