Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
2002-02-25
2003-07-01
Green, Anthony J. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C205S067000
Reexamination Certificate
active
06585812
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is a composition of matter used as an additive to high current density zinc plating, consisting essentially of a zinc salt and an additive, and processes utilizing such composition for reducing high current density dendrite formation, controlling high current density roughness, grain size, and crystallographic orientation of a zinc coating obtained from the bath.
2. Description of Related Art
Zinc corrosion resistant coatings which are applied electrolytically on ferrous metals such as steel are used extensively in industries where corrosion resistance is required, such as in the automotive industry.
Zinc offers sacrificial protection to ferrous metals because it is anodic to the substrate which is protected so long as some zinc remains in the area to be protected. The presence of minor pin holes or discontinuities in the deposit is of little significance. Zinc is plated continuously in most industrial processes such as the electrogalvanic coating of continuous steel substrates employed in the automotive and tubular steel industries. Acid chloride and sulfate baths are used extensively because they are capable of higher plating speeds than cyanide baths.
They have also displaced cyanide baths because of EPA regulations requiring the reduction or elimination of cyanide in effluents. The chloride baths include neutral chloride baths containing ammonium ions and chelating agents and acid chloride baths having a pH of from about 3.0 to about 5.5 that substitute potassium ions for the ammonium ions used in the neutral baths. Acid baths have largely replaced neutral ones in practice.
The ASTM specification for zinc deposits on ferrous metals call for thicknesses of from about 5 to about 25 &mgr;m, depending on the severity of the expected service.
ASTMB633-78. Specification for Electrodeposited Coatings of Zinc on Iron and Steel.
Zinc is deposited from aqueous solutions by virtue of a high hydrogen over voltage since hydrogen would be preferentially deposited under equilibrium conditions.
Typical plating tanks employed in these processes contain anywhere from about 5,000 to about 300,000 gallons and can be employed for plating either zinc or a zinc alloy such as a zinc-nickel alloy. These are continuous plating baths which will accommodate steel rolls about 8 feet in diameter at speeds of anywhere from about 200 to about 850 feet per minute with varying coating weights of from about 20 to about 80 grams/m
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and coating thicknesses from about 6 to about 10 &mgr;m. The solution flow rate is approximately 0.5-5 m/sec.
The steel is drawn over conductive rolls and is pressed against the roll to provide adequate contact. Soluble zinc or insoluble iridium oxide coated titanium anodes are immersed in the baths adjacent the coating rolls. In the case of zinc-nickel alloy plating operations, nickel carbonate is added to the system. Anode current density varies in accord with cathode current density.
Excess buildup of zinc at high current densities, however, can occur. If a relatively narrow steel strip is being coated, there may be excess anodes in the system. It is impossible to remove the excess anodes because the next strip to be coated may be larger in size. Because of the mechanics of the line, it is too cumbersome to remove and add anodes to accommodate the size of the different substrates being plated. Current densities of about 50 to about 100 A/dm
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(amps per square decimeter) or 400-1,000 ASF (amps per square foot) are employed which also contribute to the excessive buildup of zinc on the edge of the steel substrate. Allowances for such high current density plating are made by adjusting the solution conductivity, providing close anode cathode spacing, and providing a high solution flow rate.
Another major concern is that high current density [HCD] produces roughness in the form of dendrites at the edge of the steel strip that is being coated. These dendritic deposits may break off during plating or rinsing. As the electrogalvanized steel is passed over rollers, these loose dendrites become embedded across the coated substrate and subsequently show up as blemishes which are referred to as zinc pickups. The edges of the steel strip that are coated are also non-uniform in thickness, and burned because of HCD processing. Additionally, HCD processes can cause roughness across the width of the steel strip and change the grain size and crystallographic orientation of the zinc coating. Nonetheless, HCD processes are industrially desirable since production speed is directly related to current density i.e., higher coating line speeds can be obtained at higher current densities.
Accordingly, various grain refiners [GR] and antidendritic agents [ADA] are employed to partially offset these problems. Nonetheless, the problems of edge roughness, non-uniform thickness, and edge burn have not been completely overcome and as a result, most industrial processes require that the edges be trimmed from the steel strip after it is coated. Diamond knives are presently used to trim the edges. Other mechanical means may also be employed to remove excess zinc buildup. The GR and ADA additives also do not completely eliminate problems with HCD roughness, grain size and orientation of the zinc coating.
Additionally, applying a protective coating to the edges of the steel strip prior to zinc plating will also minimize or eliminate edge burn as well as excessive build up of zinc on the edge of the steel substrate. The problem of dendritic deposits, grain size and crystallographic orientation of the zinc coating still persists at high current densities.
It has been found with some of the standard GR or ADA materials that the steel strips exhibit considerable HCD burning at lower additive concentrations whereas nodularity or HCD roughness is still seen at higher concentrations.
The surface roughness of the coated steel strip is expressed in “Ra” units whereas the degree of roughness is expressed in “PPI” units or peaks per inch. These parameters are important in that surface roughness promotes paint adhesion and proper PPI values promote retention of oil which is important during forming operations for zinc coated steel that is used in the manufacture of automobile parts or other parts that are subsequently press formed. A rule of thumb is that the Ra and PPI values should be close to that of the substrate. In some instances it is better to have a zinc coating that is rougher than the substrate rather than smoother and vice versa. Accordingly, the Ra value generally should not be less than or exceed 20% of the Ra value for the substrate dependent upon the desired finish and generally should not exceed about 40 micro inches. The PPI value should be anywhere from about 150 to about 225. Additionally, it has been found that of the various crystallographic orientations of the electrodeposited zinc [(002), (110), (102), (100), (101), and (103)] better results are obtained with a randomly oriented deposit.
As noted, production speed can be increased as current density increases and where current densities presently being employed by industry are at about 1,000 ASF (110 A/d m
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) current densities of anywhere from about 1,500 to about 3,000 ASF are being explored in order to obtain higher production rates. Operating at these higher current densities has resulted in unacceptable edge burn, dendritic formation and break off, grain size, problems with obtaining or retention of a given orientation, and unacceptable values for surface roughness.
Additionally, many of the additives to the plating bath employed at about 1,000 ASF do not adequately address the foregoing difficulties.
Korpium et al., U.S. Pat. No. 3,537,959 describes a zinc sulfate electroplating bath and process for producing bright zinc deposits based on a zinc salt in combination with various nitrogen containing compounds and the condensation product of naphthalene sulfonic acids and formaldehyde. The patentee indicates that German Patent
Martyak Nicholas M.
McCaskie John E.
Atotech USA, Inc.
Finnegan Henderson Farabow Garrett & Dunner LLP
Green Anthony J.
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