Corrosion resistant coatings

#The invention relates to corrosion resistant coatings which are produced by forming a zinc phosphate conversion coating on a metal surface and thereafter autodepositing a resin layer over the zinc phosphate conversion coating.

#Autodeposition compositions are used to lay down a film of water resistant resin on a substrate without the use of electricity. One important feature of an autodeposition coating is that the thickness of the film increases over time as the substrate remains in the treatment bath. Contact times within the range of ten seconds to four minutes are commonly used.

#Autodeposition compositions are generally in the form of an acidic liquid comprising dispersed resin particles, an oxidizing/and or etching agent, and a surfactant in water with various other additives. Such formulations are thought to follow a deposition mechanism and process by which metal cations are liberated from the substrate and destabilize the dispersion. Once destabilized, the resin particles coalesce and deposit from solution onto the metal substrate to form a coating that is sufficient to withstand gravity and moderate spraying. The wet, uncured coating is typically rinsed, dried, and cured at temperatures above 100° C. to form the final coating.

#Panels made of different metals were treated with various phosphate pretreatments, followed by applications of autodeposited resinous films. In some cases, a further coating was applied by electrodeposition.

#Materials

#Test panels fabricated of cold-rolled steel (hereinafter referred to as “CRS”), electro-galvanized steel (“EG”), and aluminum 6111 T4 alloy (“AL”) were purchased from ACT Laboratories, Hillsdale, Mich. PARCO Cleaner (“PCL”) 1523, FIXODINE (“Fix”) Z-8, BONDERITE (“B”) 958, PARCOLENE (“PLN”) 90, AUTOPHORETIC Coating Chemical (“ACC”) 901 (composition containing dispersed epoxy resin particles) and AUTOPHORETIC Starter 300 (all products of the Henkel Surface Technologies Division of Henkel Corporation, Madison Heights, Mich.) were taken from laboratory stock. Electrodeposition primer ED-5050B was obtained from PPG Industries. A second epoxy resin dispersion was prepared according to Formula A: 2 TABLE 1 Formula A Epoxy Resin Dispersion Parts By Ingredient Weight Description Manufacturer PB-8990 2100.9 Epoxy resin Henkel Surface Technologies VESTAGON B1530 254.3 Blocked isocyanate Huels crosslinker TEXANOL 101.2 Coalescesing Eastman solvent Methylisobutylketone 1277.4 Dispersing solvent RHODAPEX CO 436 76.6 Surfactant Rhone Poulenc Deionized Water 5189.6 Solvent Total 9000.0 17.8% solids