How does Autodeposition work?

Autodeposition is a waterborne process which depends on chemical reactions to achieve deposition.  This process has been in commercial use since 1975.  Since then, autodeposition has grown and matured.  Since its inception, this unique process has been employed to coat over one billion square feet of surface.  Presently, there are numerous installations in operation which coat a variety of fabricated steel parts for the automotive and general industry makers.

An autodeposition bath consists of a mildly acidic latex emulsion polymer, DI water and other proprietary ingredients.  The bath solids are normally between 3% and 5% by weight.  The bath viscosity is close to that of water with little or no organic solvents in the coating bath.  Another characteristic of the autodeposition bath is its acidic nature; the pH is typically between 2.5 to 3.0.  The autodeposition coating process is quite simple.  The mild acid bath liberates a small amount of iron from the steel parts being immersed, causing an immediate surface reaction that releases iron ions.  These ions interact with the latex in the solution causing coating deposition on the surface of the steel parts.  The newly deposited organic film is adherent yet quite porous.  This allows the chemical activators to diffuse rapidly in the film and reach the surface of the metal.  The continuous formation of coating provides a film with a high degree of surface protection and corrosion resistance.

The coating thickness of the auto deposited film is time and temperature related.  The film thickness continues to grow as long as ionic species are being produced at the coating/metal interface.  Initially, the deposition process is quite rapid, but slows down as the film increases in thickness.  Typically, film thickness is controlled from 0.6 to 1.0 mil.

A unique feature of the autodeposition process is the formation of a very uniform film over the entire surface of the work piece, even in difficult to reach areas,  this allows the coating dispersion to flow into and around the most complex shapes.  Unlike coating processes that require a charge to deposit the coating (i.e. where electrical energy is required to “throw” the coating into recess areas), autodeposition will coat tubular, assembled, or intricate-design areas uniformly.  Autodeposition coating film thickness is controlled by diffusion of reactants in the bath and through the coating already formed on the surface of the substrate.  This diffusion control leads to a uniform thickness on complex shapes and in recessed areas. 

The film formed by the autodeposition process is more unique for a latex vehicle.  Latex films are formed by coalescence.  However, the degree to which the film forms in the autodeposition process via the coalescing action is the basis for its uniqueness.  Parts coated via autodeposition can be water rinsed immediately after leaving the bath with very little material loss.  On leaving the bath, the coating consists of two layers: one is a very cohesive reaction layer; while the other is composed of under deposited excess polymer and activator from the bath.  The chemical reaction continues in a second layer which results in film deposition and increased film thickness instead of solids loss to drag out.  This unusual feature strongly limits carryover of coating chemicals into the rinse tank and greatly reduces the demand for waste treatment. 

It should be mentioned that the autodeposition process does not require a chemical conversion coating stage or in the case of autodeposition process using Autophoretic 800 series coating chemicals, even a heavy metal final rinse stage.  The elimination of these stages leads to considerable savings in required floor space, energy and operating costs, as well as the elimination of toxic metals from the process.  Considerably lower temperatures are required to cure the autodeposited parts.  These are just some of the many advantages of using the autodeposition process.

A typical process sequence of an autodeposition finishing line using Autophoretic 800 series coating chemicals is as follows:

Stage No. 1 – Alkaline Spray Clean
Stage No. 2 – Alkaline Immersion Clean
Stage No. 3 – Plant Water Rinse
Stage No. 4 – Water Rinse
Stage No. 5 – Autodeposition with Autophoretic 800 Series coating chemical
Stage No. 6 – Plant water rinse
Stage No. 7 – DI water rinse
Stage No. 8 – Curing oven, low temperature cure