Patent Application
20110031126
This disclosure pertains to methods of providing initial coatings on mixed-metal automotive vehicle bodies-in-white that include magnesium alloy surfaces. More specifically, this disclosure pertains to the formation of a conversion coating and an electrocoating on such a mixed-metal vehicle body.
Automotive vehicles may comprise passenger vehicles, trucks, vans, cross-over vehicles and other body variations. The bodies are constructed of load bearing structural members, floor members, closure members and the like. Such body members have been formed of cold rolled steel and galvanized steel and, in more recent years, from aluminum alloys. The respective body members are joined by welding, hemming, clinching, bolting, and like joining practices to form a body structure that is then ready for painting. Such an unpainted vehicle body structure is referred to as a “body-in-white” (sometimes referred to as BIW) because of the appearance of the bare metal elements of the body structure. Such vehicle bodies are then processed through long and sophisticated automotive phosphating and paint lines.
As suggested above, many vehicle bodies-in-white now contain portions that are formed from steel, galvanized steel and various aluminum alloys. A body comprising each of such ferrous, zinc, and aluminum materials is thoroughly cleaned and provided with a phosphate-containing surface conversion coating by immersion in an aqueous bath of phosphating composition. The phosphate conversion coatings chemically formed on the ferrous surfaces include iron (and sometimes zinc) and the phosphate conversion coatings on the aluminum surfaces comprise aluminum, and they are formed as a barrier layer on each exposed surface to provide corrosion resistance. These phosphate-containing conversion coatings have irregular surfaces that provide a tie-in base for a subsequently applied electrocoat paint layer. After phosphating, the vehicle bodies usually receive at least four paint layers to provide additional corrosion protection and color finishes. These paint layers include, in order of application: an electrocoat, a surface primer base coat, a base color coat, and a clear coat.
Now it is desired to make closure panels and other body members using magnesium alloys because of their favorable strength-to-weight ratio and because they can be formed as such body members and attached to complementary body members of magnesium, aluminum, or ferrous-based materials. However, magnesium is very reactive in aqueous solution and subject to galvanic corrosion, especially when coupled with steel alloys or aluminum alloys. When a magnesium body surface is immersed in an aqueous phosphating bath, magnesium dissolves in the bath, contaminates it, and adversely affects the quality of phosphate coating formed on nearby steel or aluminum surfaces.
It is an object of this invention to provide practices for forming conversion coatings and electrocoatings on bodies-in-white that comprise magnesium surfaces and aluminum alloy surfaces and/or steel surfaces, including galvanized steel surfaces.
This invention provides a method for forming a co-extensive electrocoat paint layer on automotive vehicle bodies-in-white that have magnesium alloy surfaces in combination with one or more of steel surfaces, galvanized steel surfaces, and aluminum alloy surfaces. Such body-in-white constructions that have magnesium alloy surfaces in combination with a different metal surface will sometimes be referred to in this specification as mixed-metal assemblies or mixed-metal BIW assemblies.
An example of a magnesium alloy that may be formed into a body member is AZ91D. AZ91D is a magnesium-based alloy that is available in rolled sheet form for shaping into body panels and the like. Its, nominal composition, by weight, is about 9% aluminum, 1% zinc, and the balance magnesium, except for minor amounts of impurities.
Each such mixed-metal BIW is cleaned through spray clean/dip clean/rinse stages. In a preferred embodiment of the invention, each body is conveyed sequentially through a spray cleaning stage, into a dip or full immersion cleaning stage, and then through a spray rinse stage. The first cleaning stage may be an acid cleaner and the second cleaning stage may be an alkaline cleaner to clean and expose the respective metal composition surfaces for the following process step.
After the cleaning stage, each mixed-metal BIW will receive a conversion coating step and an electrocoat step. In a preferred embodiment of the invention these two steps may by combined by immersing the mixed metal body in an aqueous bath of adhesion promoting material composition and electrocoat composition. Upon immersion, the mixed metal body is connected as the cathode in the electrocoating tank. The adhesion promoting material is suitably a composition (for example, cerium trichloride) that will react with magnesium body surfaces and surfaces of the other metal body members upon immersion of the body in the aqueous bath material of the tank. In a preferred embodiment, this mixed-metal body electrocoat process includes adhesion promoter additives in an epoxy-based electrocoat aqueous solution and an applied voltage between −100 to −300V, with the car body being the cathode. Thus, the mixed-metal BIW is cathodically protected and the dissolution of magnesium is mitigated. As the hydrogen gas is evolved from the cathodically charged body, the interface pH increases to cause co-deposition of polymer and adhesion promoter oxides (e.g. cerium, zirconium, vanadium, titanium or silicon-based compounds, etc). Some of the cerium salt (or other adhesion promoter) reacts with the respective metal surfaces to form cerium-containing conversion layers. At the same time, micelles of polymer composition (epoxy in this example) from the bath migrate to the cathodic surface and form a continuous polymer coating over the metal surfaces with their thin conversion layers. The bath composition often contains pigment particles of titanium dioxide, or the like, which become incorporated into the deposited protective coating layer.
The exposure of the mixed metal body-in-white to the adhesion promoter and electrocoating process is about one to three minutes (consistent with painting line speed) with the bath at substantially ambient temperature. As the body is lifted from the bath the respective metal portions each carry a thin conversion coating, 50-500 nanometers thick, which in turn is coated with a more or less fixed polymeric electrocoat layer of thickness 20 to 40 micrometers. And conversion material may be entrained in the newly deposited electrocoat layer from where it can migrate to the underlying metal-conversion coat surface. The polymer layer is suitably fixed to be rinsed with water to remove loosely adsorbed bath material.
The electrocoated mixed-metal body is rinsed with de-ionized water or the like to remove adherent bath material. After removal of extraneous water the electrocoated mixed metal body is conveyed through a paint bake oven to finish polymerization of the electrocoat material.
After baking, this electrocoat will display adhesion and corrosion protection performance comparable to the phosphate/electrocoat combined coatings obtained in vehicle body lines that did not have magnesium-based body surfaces.
Other objects and advantages of the invention will be apparent from a detailed description of preferred embodiments of the invention which follows in this specification.
A first example of a suitable magnesium alloy that may used in door assembly 14 (or other body member) is magnesium alloy AZ31, which has a nominal composition, by weight, of about 3% aluminum, about 1% zinc, about 0.2% manganese, and the balance magnesium. A second example of a magnesium alloy that may be used in making a body-in-white is AZ91D, identified above in this specification.
It should be understood that
In the manufacture of automotive vehicles, like or different metal bodies-in white are continuously constructed according to production schedules. Currently, these bodies are fabricated using steel, galvanized steel, and one or more aluminum alloys. A generally continuous stream of these ferrous and aluminum bodies is then conveyed though a painting line in which each just-made body-in-white is carefully cleaned by spray and immersion processes, provided with phosphate-containing conversion coatings on the respective metal surfaces, and then provided with an electrocoat of paint. Additional painting and vehicle assembly steps follow on a more-or-less continuous basis.
This invention provides a method for including magnesium parts and surfaces in the body-in-white which do not tolerate phosphating and, indeed, damage a phosphating bath to the detriment of adjoining non-magnesium surfaces on the BIW.
In accordance with this invention, magnesium-containing, mixed-metal bodies-in-white are provided with a protective conversion coating (such as a cerium-containing conversion coating) and electrocoated as a cathode at a suitable negative voltage in a suitable aqueous electrocoat composition bath.
As illustrated schematically in
In this embodiment, the body-in-white 10 is immersed in the aqueous alkaline cleaner bath contained in the tank. An example of a suitable alkaline cleaner is an aqueous solution of sodium carbonate containing about 5 percent by weight of sodium carbonate. Again, the line pauses as multi-metal body-in-white 10 is immersed in alkaline cleaner 102. The order and means of application of aqueous acid cleaning and alkaline cleaning is a matter of choice. The body 10 is raised from the alkaline cleaner bath and drains as the body is conveyed through an aqueous spray rinse station 104. For simplicity of illustration, a body 10 is not necessarily illustrated at each stage of the in-line process.
The cleaned and rinsed body-in-white is now ready for immersion in a combined conversion coat and electrocoat bath 106 (also designated as ELPO tank). A larger schematic view of a body-in-white 10 fully immersed in an aqueous conversion coating and electrocoating bath 106 is illustrated in
The conversion coating composition is a dissolved oxidizing composition comprising cations capable of forming a conversion coating with each of the metal surfaces of the body. The resulting conversion coating comprises elements of the cations and oxygen, and often of the underlying metal alloy. The cations of the composition react with each of the mixed-metal surfaces upon immersion of the body 10 in the bath 106. Examples of suitable dissolved oxidizing compositions include one or more of compounds selected from the group consisting of cerium-based compounds, silicon-based compounds, titanium-based compounds, vanadium-based compounds, and zirconium-based compounds. Such conversion coating materials are often used in amounts of about five to about twenty grams per liter of the aqueous bath. Cerium trichloride salt is an example of a preferred conversion coating material. In this example, cerium ions (+3) react with each of the ferrous surfaces, zinc surfaces, aluminum surfaces and magnesium surfaces to form cerium-containing and oxygen-containing layers on the respective metal surfaces. These conversion coatings may also contain elements from the metal surfaces and form thin cratered and irregularly shaped coating layers to which the depositing electrocoat layer adheres. The resulting conversion coatings on the respective metal surfaces are suitably electrically conductive for electrochemical deposition of the electrocoat polymer.
Cathodic electrocoat deposition of water-dispersed organic coatings has gained worldwide acceptance, especially by the automotive industry, because of its numerous benefits, e.g., ability to coat recessed areas, uniform coating thickness, almost complete paint utilization, and reduction of environmental pollution. In the practice of this invention such cathodic coating materials are used in combination with the above-described conversion coating materials to form (preferably in one step or bath; suitably in two steps or successive baths) a combination of conversion coating and electrocoat to a combined thickness of about ten to forty micrometers on the surfaces of each of the multi-metal areas of the immersed body-in-white.
A representative and suitable cathodic electrocoat bath, e.g., DuPont Electroshield™ 21 gray bath comprises 71-82 wt % water, epoxy resin 16-26 wt %, and titanium dioxide 1.3 wt %. The electrocoat emulsion may be prepared and continually replenished using a mixture of a resin feed package and a pigment feed package. In this formulation, the resin feed package include a cathodic electrocoat or electroprimer that is partially neutralized with a weak organic acid (Ra-H), such as acetic acid, and then emulsified in water. The resin package used here is typically composed of an aminoepoxy resin (R—NH2) mixed with a blocked isocyanate cross-linker. In the aqueous bath the resin emulsion stabilizes to contain water soluble polymer coating micelles or particles (R—NH3+), as shown by the reaction: RNH2+Ra—H→RNH3++Ra−. In this embodiment of the invention, the bath also comprises 1.0 wt % (about 10 grams per liter of bath) of cerium chloride for formation of the conversion coating on the mixed-metal body-in-white 10.
The mechanism of the cathodic deposition process includes: 1) hydroxide production at the cathode side and an increase in the local pH value of the paint solution; 2) migration of charged micelles to the cathode; 3) discharge and coagulation of the micelles due to local pH increase and 4) elimination of water from the deposited paint by electro-osmosis. As indicated in
As an example, each body-in-white 10 may be immersed in a bath 106 for a period of two to three minutes to obtain a suitable conversion coating and electrocoat. Indeed, the speed of this paint line may be based on the operation of this coating bath 106.
Body-in-white 10 with its cerium-induced conversion coating and wet, un-cured epoxy-based electrocoat is removed from bath 106 and conveyed through a series of rinses with water and de-ionized water (stage 108 in
In the above illustrative embodiment, the mixed-metal body-in-white was contacted with the conversion coating material and electrocoat material in a common aqueous bath 106 (in
A mixed-metal body-in-white formed of a magnesium alloy surface and at least one of a ferrous metal surface, a zinc-coated ferrous metal surface, and an aluminum alloy metal surface is provided with a conversion coating and an electrocoat. The conversion coating is formed preferably on each of the differing metal surfaces making up the surfaces of the vehicle body. The conversion coating is formed in an aqueous bath containing dissolved cations of at least one oxidizing material. The electrocoat is deposited on each of the metal surfaces of the vehicle body over the conversion coatings and may contain some of the cations of oxidizing material.
While practices of the invention have been described in terms of some illustrative examples, it is clear that other reactive material and practices may be used that are within the scope of the invention.
