| Zinc-rich organic coatings protect steel from corrosion by blocking corrosive reactants from the steel and by galvanically protecting the steel where it is exposed to corrosive reactants at defects in the coating. Zincrometal('R) is a zinc-rich-organic-coated steel product used in the automotive industry. The Zincrometal coating consists of: (1) a pretreatment layer, called Dacromet('R), comprised primarily of zinc dust and chromic acid; and, (2) a zinc- rich organic primer, called Zincromet('R), comprised of approximately 50 v/o zinc dust in a phenoxy resin binder.;DC and AC electrochemical experiments showed that a 50 v/o zinc-rich primer on Dacromet (i.e., the laboratory version of the Zincrometal coating) was an effective barrier for steel, but the coating was not galvanically active. However, the 50 v/o primer applied directly to steel, with no Dacromet layer, was galvanically active. Auger electron spectroscopy showed that the Dacromet layer consists of zinc particles randomly distributed on the steel, with a thin ((DBLTURN)1000 (ANGSTROM)) chromium oxide film covering the entire surface. The chromium oxide film is non-conducting, and it is a good barrier. However, the film electronically insulates zinc particles in the primer from the steel which limits the galvanic activity.;Corona discharge treatment of the 50 v/o primer on Dacromet formed electronic paths between zinc in the primer and the steel, which made zinc in the primer electrochemically active. The paths were formed by dielectric breakdown of insulating barriers in the coating. However, the effectiveness of the barrier protection provided by the coating to steel was decreased by corona discharge.;treatment. Corrosion of the steel initiated where breakdown of the chromium oxide film of the Dacromet layer had occurred.;In this thesis, the protective mechanisms of the Zincrometal coating were studied as a function of: (1) the pigment volume concentration of zinc in the primer, (2) the Dacromet pretreatment layer, and (3) electrical post-treatment by corona discharge. Electrochemical corrosion experiments were conducted in non-deaerated solutions of 0.5 N NaCl in distilled water at pH = 6.;('R)Zincrometal, Zincromet and Dacromet are registered trademarks of Metal Coatings International, Inc. |
