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.
One of the primary goals for autodeposition coatings is an enhanced resistance to corrosion, as measured by conventional tests. Such tests measure, among other things, the continuity of the deposited film and its adhesion to the substrate.
Substrates that have been previously treated with certain types of phosphate conversion coating (e.g., a zinc phosphate conversion coating) have not performed well with autodeposition coatings. To achieve a satisfactorily rapid rate of autodeposition, it is generally believed necessary to maintain a relatively high concentration of a strong acid. Typically, however, the acid levels of autodeposition compositions are sufficiently aggressive that the zinc phosphate coating is etched from the metal substrate. This stripping effect eliminates the beneficial effects of the zinc phosphate coating and makes it difficult to obtain the cumulative benefits that might be obtained from a zinc phosphate conversion coating that is further protected by an autodeposition coating. Additionally, the zinc and other cations present in the zinc phosphate conversion coating represent a significant source of contaminant ions that can destabilize the autodeposition composition prematurely or cause other undesired effects in the formation or performance of the autodeposited coating.
It would be desirable to have an autodeposition composition and method of application that would permit zinc phosphate conversion-coated substrates to be coated with autodeposition processes. Such an additional coating could also help to form a corrosion resistant surface that was smooth enough to be painted with little or no additional treatment.
It would be particularly beneficial if the corrosion resistance from an autodeposition coating was sufficient that the process did not require the use of a reactive chromium rinse before the film of autodeposited resin was cured. Reactive rinses containing hexavalent chromium were common in the earlier disclosures of the technology but are now disfavored for environmental concerns.