US 3151050 A
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P 1964 D. K. WILBURN 3,151,050
LAMINATED ANTI-CORROSIVE PAINT SYSTEM Filed Feb. 15, 1963 D C POWER 1 T INPUT VOLTAGE OPPOSING GALVANIC CORROSION SEMI-CONDUCTING PAINT CONDUCTING PAINT FINISH COAT REGULAR PAINT FIG. I
BASE METAL OF VEHICLE I'I.5 MIL THICK' CONDUCTIVE COAT PRIMER I OLIVE DRAB FINISH SOLDERED CONNECTION TO CONDUCTIVE COAT FIG. 2
DAVID K. WILBURN BY 5W .A i/WEMQL mmw wishes This invention described in the specificationatitl' claims may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
The present invention provides a process for the practical application of an" electrolytic paint system; for reducing atmospheric corrosion of vehicles instoragef.
The techniques of cathodic protection are well known and extensively used in industry. Underground pipe lines and tanks can be so protected as well as marine structures in salt water environments. in practice the metal to be protected is made the cathode or negative terminal of a DC. power source. The essential components of a cathodic system are: (l) the electrolyte, (2) cathode, or item to be protected, (3) a sacrificial or anodic electrodeand (4) a' power source; Underground and submerged sea water structures are therefore easily protected' by cathodictefchhiques since both cases soil" or sea water acts as iiicie'nt electrolyte. Mtall stmctures inf the atmos pher'e, however, have' not been cathodic'ally protected since air is a non conductor and is not an elctrolyte.
To overcome this problem; a special semi conducting electrolytic paint was' formulated to act as an electrolyte for metal structures in airenvironment's; c
Until comparativelyrecently the only methods" for the prevention of corrosion were protective coatings, applicw tion of expensive corrosion resistanf materials, and utilization of insulating coatings and sealers, c
In contradistinction thereto, the present invention for thefirst time provides a; method of cathodic coating protectioh for vehicles and components in storage. Fundamentally, cathodic protection consists in impressing electromot'ive forces in the structure to be protected, through auxiliary anodes, in such a way as tom'ake the entire strueture" cathodic with respect to the environmental medium. Although this procedure does not eliminate corrosion, it transfers the corrosion from the protectedj structure to the auxiliary anodes, which are more easily and economically replaceable.
Theoretically, it should be possible to stop electrolyte or galvanic corrosion by bringing the item to be protected to such a potential with respect toth'e g roundthat no current flows. Practically, however, neither ground, nor the item to be protected, can be inatle equipotential, hence, the application of a cathodic charge to the item to be protected.
The most difficult problem, in connection with the application of cathode protection, is that of determining whether or not the environmental medium is adept to current flow; c p
With increasing numbers of vehicles and components subjected to prolonged periods of storage, problems of atmo'spheric and galvanic corrosionsare becoming of prime importance. ln'terior compartments of vehicles in' field storage are exposed to severe corrosion attack due to: high concentrations or water vapor resulting from abrupt changes in temperature and pressure. According to the process of the present invention, cathodic coating protection will eliminate such corrosion by simple electrolytic "ice 2 cells associated with each unit tobe protected; Because of the small power requirements, a vehicle could employ a solar cell system which would adequately meet daytime power demands. i i
It is thereforeone of the outstanding obj cts of this invention to produce a laminated cathodic coating for iniproving the corrosion resistance of rnetals aiidalloys.
Another object of this invention isto produce a sandwich-liite cathodic coating for metal structures in the atsp r H .H
A further object oi the invention is to materially nhance the operating life of stored vehicles and components by reducing atmospheric cor rosion a sandwich cathodic coating appliedto the base metal.
Another object of the invention is to provide a method for the application whereby a base metal is coated to produce a more stable structure to resist corrosion when exposed to the atmosphere. l 5 u i i Other objects of this invention will become rtdn ap} parent from the following detailed description which merely illustrates the invention and does not limit my invention. i i i i FIG. 1 illustrates a grossly enlarged but proportional illustration of a portion of the protected r'netal wired for protection shown in cross-section and treated in accordancewith this invention; I H V e FIG. 2 is another enlarged crosssection showing details of the laminated paintsystem. v
Corrosion is the chemical combination of a nietal with certain non-metals such as oxygen, ni ti ogen, sulfur, etc. It is characterized by a degradation of the metal and loss of its metallic properties. Since iron and its alloys are used extensively as structural and mechanical members in motor vehicles, it is important to reduce the atmospheric corrosion of these metals, especially during long periods of outside storage. i A i Corrosion in iron occurs when the metal loses two electrons from each atom forming a ferrous compound;
Fe Fe 2e Iron (l i electrons ions The metallic iron goes into solution as ferrous ions, and the metal takes a negative charge from the electrons which remain on it.
This results in' the passage of an electric current from the metal into the electrolyte. Atmospheric corrosion of iron is possible through the collection of moisture on metal surfaces from rain, etc, and the subsequent chemical reaction between the metals and gases entrapped in the moisture. In general, paints simply act as barriers between the metal they are to protect and an environmental electrolyte. Simple cells develop as porosity in the protctive coating allows the electrolyte to come in contact with the metal. Cathodic protection is then the reversal of the normal electron current fiow from metal to positive metal ion. This is achieved in practice by making the item to be protected more negative with respect to a com mon anode through an electrolyte. Protection can be achieved under most conditions of polarization and in the presence of sulfate-reducing bacteria;
The electrolyte must, however, be: (1) conductive to electrical current, and (2) able to supply ions in electrical chemical reactions. In electrolytic conduction, molecules are separated into two oppositely charged parts calletfions. Although the dissociation of the molecule produces positive and negative ions, there is no tendency for them to move unless conduction takes place. By making the metal to be protected cathodic, or negative, the negative ions are then attracted toward the anode. This is a reverse of the normal ion flow produced in iron by the chemical corrosion process.
The conducting electrolytic paint used in this invention to impress a negative charge on the vehicle body exhibits the required characteristics of an electrolyte. It is conductive and supplies ions when dissociated electrically. The electrical characteristics of resistivity can be formulated to provide the desired current transfer. For example, a 2 /2 mil thick coating of the electrolytic paint affords a resistivity of approximately 4,500 ohm-cm. This corresponds to a soil electrolyte to silt loam. Sea water has a resistivity of about 100 ohm-cm.
The potential necessary to reverse the Work function of normal corrosion of steel has been found to be -0.80 to 0.85 volt with respect to the anodic electrode. The current density to achieve complete protection was found by the test hereinafter described to be between 0.0064 and 0.96 milliamp per square inch of surface area. Such results depend upon the corrosiveness of the atmosphere and Will vary from desert to coastal regions and industrial to rural areas.
By way of a specific example of an embodiment of my present invention the electrolytic-cathodic paint system was applied on body panels of a conventional ton test vehicle. Prior to the application of the system of FIG. 2, the body was removed from the vehicle, disassembled, and the original paint removed down to clean base metal. To arrive at comparative results, individual body panels were selected for treatment as follows:
(1) A FIG. 2 paint system With cathodic potential applied.
(2) A FIG. 2 paint system without cathodic potential applied.
(3) A'standard exterior finish consisting of a red oxide primer with a finish coat of olive drab enamel.
The outside exposure of the test vehicle was accomplished in a light industrial area in the Great Lakes region of the United States.
All body members, after removal of old paint, were phosphate metal conditioned. The key to the paint system is explained in FIG. 2. Paint was applied using conventional paint-spraying equipment and the thickness was measured with a magnetic thickness gauge. The electrolytic paint was controlled to a 1 /2 mil thickness. To eliminate electrical shorts between the conductive coating and the base metal, all bearing edges and other areas of impact were not coated with the conductive paint. A No. gauge electrical wire was affixed to the conductive coating by means of a low temperature printed circuit solder. The soldered connections were sealed in place with an epoxy cement. Each body panel to be protected by the electrolytic paint system had several electrodes attached to the anodic coating.
The electrolytic paint composition layer having a 3 mil thickness for coating the steel surfaces exposed to the atmospheric elements comprises the following ingredients:
The semi-conducting paint air dries to a given electrical value in two hours or it can be baked 10-15 minutes at 150-250" F. in a drying oven to accelerate drying as was the case in this example.
The low electrical resistance anodic colloidal copper paint layer is a dispersion of metallic copper in a lacquer solution. The composition has been sold as dag Dispersion No. 235 by the Acheson Colloids Company and contains solids of metallic copper pigment and lacquer type resin, the solids content being 66% by Weight of the solution. The composition is diluted in a toluol solution, said composition diluted in the ratio of two parts of composition to one part of thinner.
The anodic paint is a low loss electrical conductor having a resistance of 6 ohms per square for a one mil thick coat and 1.5 ohms per square for a 2 mil thick coat.
The test vehicle was placed in the test area for a total of 33 months, 9 days exposure. A DC. voltage was supplied between a main ground terminal on the vehicle chassis, and to individual printed-circuit electrodes on the copper anode. This caused a current flow from the anodic layer to the base metal making it more cathodic. Sufficient current liowed to raise the base metal potential above its corroding potential.
The total current required to maintain an 0.85 volt potential difierence between anode and cathode ranged from 340 milliamps at the start of the test to 460 milliamps at the conclusion.
Corrosion failures as related to surface area under protection for the three systems are as follows:
Estimated Area Percent of Protected, System Total Area Sq. n. Exhibiting Corrosion, Percent Problems involved in completely removing the old paint from the vehicle body resulted in several areas where the electrolytic paint did not have good adhesion to the base metal. Lifting of the paint caused high resistance areas to form, and resulted in loss of protecting potentials in these locals. Similarly, bearing and impact areas which did not receive the anode coating exhibited an accelerated corrosion of the base metal and subsequent loss of paint adhesion. It is therefore important that the electrolytic paint exhibit good adhesion to the base metal, and the anodic coating adhere tightly to the electrolytic paint.
Where the anode touches any part of the cathode a short develops. These shorts cause excessive power drains, and under some circumstances a power failure. Shorts in the system can sometimes be eliminated by discharging a ca pacitor into the shorted circuit. This in eifect opens or burns out the immediate short, thus placing the circuit block in operation. Partial shorts are difficult to detect since it is not possible to actually measure the flux density of the system. That is, it is not known if all the power into a given electrode is being distributed to the cathode evenly. A low resistance area of a few square inches could easily have a flux density of several milliamps, thus reducing the distribution of the surrounding cathode to near zero.
While the electrolytic-cathodic paint system did not provide total protection from corrosion, it did exhibit superior corrosion protection as compared to the conventional paint system and the system of FIG. 2 without cathodic potential applied.
Although a specflic embodiment of the invention has been illustrated and described, it will be understood that various alterations in the details of the paint system may be made without departing from the scope of the invention as indicated in the following claims:
1. A method for improving the corrosion resistance of ferrous metals comprising the steps of applying to a ferrous metal base a coating of an electrolytic paint comprising a suspension of'carbon, manganese dioxide, ammonium chloride and an organic filler in a solvent in an amount to insure said suspensions in said solvent being approximately 20% by weight, applying a conducting coating of a resin lacquer solution in a suitable thinner con 2. The method of claim 1 wherein the thickness of said taining a suspension of finely divided colloidal metallic electrolytic coat is approximately 3 mils and the thickcopper in an amount to insure said copper being approxiness of said conductive coat is approximately 11.5 mils.
mately 66% by Weight of said lacquer solution, applying a top coat of protective paint or enamel and lastly apply- 5 References Clted m the file of thls patent ing a DC. voltage between the conducting paint and the UNITED STATES PATENTS metal base thus causing a current to flow from said con- 1,281,108 Vaughn Oct. 8, 1918 ducting paint to said metal base in an amount sufiicient to 1,867,984 Pistor July 19, 1932 raise the metal base above its corroding potential. 2,491,225 Stearns Dec. 13, 1949