Metal surface phosphatization : a comparison of air-dried and thermal-cured self-phosphating coatings

Includes bibliographical references (pages [88]-91)Corrosion of metal is commonly a chemical or electrochemical process. For years, the best way to protect metals has been with organic coatings. To obtain the best protection from the coatings, metal phosphate conversion with chromic acid rinsing is used prior to the application of organic coatings. Because of the environmental concerns about the disposal of the phosphate bath waste and the use of chromium, an in-situ phosphating paint system was recently developed in our lab. The phosphating reagent is predispersed into the paint system. When the paint is applied on the metal, the phosphating reagent diffuses to the metal surface and reacts with metal to form a metal surface layer with the coating on top of the metal phosphate; this can improve surface adhesion and corrosion protection. Chromic acid rinsing is not needed in this technology and the problem of phosphate bath waste is also eliminated. The use of both air-dried and thermal-cured coating systems on metal have been studied, including air-dried alkyd, acrylic latex and thermal-cured polyester. FTIR was to used probe the metal phosphate interface produced by the in- situ phosphating paint system. Also, the coating film's dielectric and corrosion resistance were measured by Electrochemical Impedance Spectroscopy (EIS) and the salt-water spray test. In both the air-dried and the thermal-cured systems, the formation of interface metal (iron) phosphate was observed by FTIR spectroscopy. Coating performance was tested by salt (fog) spray and salt-water immersion and Electrochemical Impedance Spectroscopy (EIS). These tests showed the solvent- borne air-dried alkyd has a better corrosion protection when an adequate amount of phosphoric acid is predispersed into it, although the drying time became longer. Enhanced corrosion protection was not obtained from the water-borne air-dried alkyd and acrylic latex due to the water sensitivity caused by adding phosphating reagent and ammonium hydroxide. The performance of thermal-cured polyester was very promising in this study. Coatings with phosphonic acid and with vinyl phosphonic acid showed performance comparable to the multi-step coating process.M.S. (Master of Science