Method of coating metal using low temperature plasma and electrodeposition

#This invention relates to the use of low temperature plasma technology for the corrosion protection of metals. Our novel process involves pretreatment of the metal with a plasma gas, followed by plasma deposition of a thin polymer film, and finally application of an electrocoat primer.

#The corrosion protection of steel substrates is important for many industries, including the automotive and steel industries. Currently the most common methods of corrosion protection of steel substrates are galvanizing, application of zinc phosphate, application of primer materials by electrodeposition, conventional spray or dip priming, oil coating and combinations thereof. However, especially in the automotive industry, these methods are associated with pollution in the form of volatile organic compounds (VOC), (2) excessive waste disposal, (3) inadequate coverage of recessed areas, and (4) inadequate retention or performance of corrosion protection.

#It is generally known that plasma deposition of thin films gives a very dense layer of film, with uniform deposition, no "pin holes", and good edge coverage. Furthermore, such a process does not require solvents, so there is no VOC problem. However, most of the work in the plasma deposition area has been restricted to small objects (e.g. microelectronic components). Plasma processing for larger objects has been used primarily for plastic substrates.

#FIG. 1 is a schematic diagram of one embodiment of a plasma deposition system for carrying out the present process.

#FIG. 2 is a frontal view of one of the anodes employed in the plasma deposition system of FIG. 1.

#FIG. 2A is an enlarged side view of the anode in FIG. 2.

#It has been found that a simple coating system which involves plasma pretreatment, plasma deposition, and electrocoat application is an effective way of protecting a metal surface, either ungalvanized or galavanized, from corrosion. Our novel system is usable with bare steel, cold rolled steel, stainless steel, galvanized steel, aluminum, copper, and brass. Our system is also applicable to metal substrates of different sizes and shapes, including stamped or unstamped metal. Suitable metal substrates are preassembled autobodies, autobody parts, rolls, coils, sheets, and so forth. However, the corrosion protection of ungalvanized cold-rolled or bare steel is the most advantageous and important use of the present invention at this time. In fact, the corrosion protection obtained by the present invention with ungalvanized steel is surprisingly as good or better than with galvanized steel.

#Our novel system represents three basic steps. The first step is pretreatment of the steel substrate with a particular kind of plasma gas. the second step is plasma deposition of a thin film. The third step is the application of an electrocoat primer over the plasma film. An optional fourth step is the addition of further coatings, including primer, monocoat and basecoat, clearcoat systems.

#If the metal substrate to be coated is oiled or otherwise contaminated, as sometimes occurs when the material is received from a metal manufacturer, then it should preferably be cleaned prior to the above mentioned plasma pretreament. Cleaning may be readily accomplished by conventional methods, for example with solvents and/or detergents. Alternatively, plasma treatment with an oxidizing gas such as oxygen can be used to remove organic contaminants. Such a plasma treatment, for the purpose of cleaning, may be anodic or cathodic, employing AC or DC voltage.