CHROMIUM-CONTAINING LOW-CURE COATING COMPOSITION

Abstract: A coating composition is now provided which can have a great number of desirable coating characteristics, when the composition is applied and cured on a metal substrate. Moreover, the coating composition has applicability to a great variety of metal substrates. Such substrates include base metal such as steel as well as metal coated substrates such as zinc electroplated steel. Furthermore, the coating compositions can be useful when applied to substrates sensitive to elevated cure temperature, e.g., annealed steel. In one important aspect the composition can contain hexavalent-chromium-providing substance in mixture with cobalt compound. Particulate metal may be present in the compositon. A composition furthermore most desirably contains a special organic acid/alcohol reducing agent combination for the hexavalent chromium. The composition can provide a low-cure coating on a metal substrate, which can then translate into higher production line speeds as when used for coating of metal coils.

Description: BACKGROUND OF THE INVENTION Chromium-containing "bonding coating" compositions for metal substrates that are applied prior to painting typically impart corrosion resistance to the surface of the substrate. Such compositions, when applied and cured on a metal substrate, exhibit excellent adherence to the substrate and offer many further desirable characteristics. These include substrate corrosion resistance, the advantage of metal formability without significant coating loss, and retention of substrate weldability. Much of the early efforts with bonding coatings involved developing combinations of hexavalent-chromium-containing substances, often supplied by chromic acid, with a wide variety of reducing agents. For example, U.S. Patent No. 3,382,081 discloses combinations of organic reducing agents that can include aliphatic dicarboxylic acids in the combination. Coating characteristics including corrosion resistance can be augmented by including in the pre-paint coating composition a particulate metal, such as finely divided zinc. For example, in U.S. Patent No. 3,671,331 there are disclosed pulverulent metal-containing chromium bonding coating compositions. These contain, in addition to pulverulent metal such as zinc, the hexavalent-chromium-containing substance plus reducing agent for such substance. Zinc as a metallic coating has also been widely used to reduce the tendency of iron or steel surfaces to corrode. In the past, the principal methods of applying such coatings were hot-dipping, also known as galvanizing and the electroplating of a zinc layer onto the steel. Zinc has been electroplated on the steel surfaces from various plating baths, preferably from acid plating baths, for providing protection of steel surfaces for various uses. It has also been known as in the U.S. Patent No. 4,500,610 to improve the corrosion resistance of the coating layer by topcoating the metallic zinc with a bonding coating. Moreover, it has been shown, as in U.S. Patent No. 3,990,920, that desirable coating characteristics for these bonding coating compositions can be improved by exercising control over the pH of the coating composition. This is done by introducing a pH adjusting agent into the composition. The agent can enhance compositional stability. But such compositions must perform a wide variety of functions, and perform them well. It has been found to be extremely difficult to upgrade a particular coating characteristic without downgrading some others. Or without downgrading the coating composition itself, as by retarding shelf life, since these formulations are becoming increasingly complex. It would nevertheless be desirable to enhance features of the coating composition such as augmented protection for the base metal where a cut or nick may penetrate the coating and expose such metal. It would moreover be desirable to have the composition useful as a coating over a wide variety of substrates including not only steel surfaces but electroplated surfaces such as metallic zinc coated substrates. It would be most desirable if the coating from the composition could furthermore protect the base metal, e.g., steel substrate or the like, where the electroplated layer has been penetrated. Such wide application of the coating composition to substrate metals could furthermore be most desirably extended to include annealed steel substrate metal. However, such metal can be sensitive to applied coating compositions where elevated temperature curing is then needed. SUMMARY OF THE INVENTION It has now been found that a coating composition can be obtained which has a wide range of applicability to a variety of substrate metals. For example, the composition can be successfully applied and cured on annealed steel, where a low cure temperature can be critical. Additionally the composition can be readily applied to surfaces such as those containing elemental zinc and provide a desirably adherent coating of good coating characteristics, e.g., weldability for the coated substrate. Moreover the coated substrate can exhibit highly desirable corrosion resistance for nicks and cuts, which penetrate not only through the coating but also through a metal sublayer, such as zinc electroplate, where a substrate or base metal, e.g., steel, is exposed. Moreover these features for the composition can be attained through substitutions in the coating composition constituency, but without deleterious effect on the compositional stability. In a broad aspect, the present invention is directed to a method of obtaining a heat curable coating providing corrosion resistance to a metal substrate, which coating is phosphate-free and at least substantially resin-free and comprises a pulverulent metal, chromium from a hexavalent-chromium-providing compound and optionally a reducing agent for said hexavalent chromium which method is characterized in that there is applied a phosphate-free, at least substantially resin-free and pulverulent-metal-containing composition containing the hexavalent chromium providing compound in an amount sufficient to provide a coating of at least 107.6 mg of chromium per square meter and that it further contains a cobalt compound in an amount sufficient to provide a coating having a weight ratio of chromium to cobalt of greater than 1.5:1. In another aspect, the present invention is directed to a method of obtaining a heat curable coating in the sense above but modified in that the applied composition contains hexavalent-chromium-providing compound, cobalt compound, carboxylic acid and polyhydric alcohol. Other aspects of the invention include components which may also be useful for obtaining coating compositions, which components contain pH modifier at least about 50 weight percent of the modifier being replaced by cobalt compound. In a yet further aspect, the method according to the invention uses components or low-cure coating compositions which contain reducing agent for hexavalent-chromium compound, the reducing agent including in combination especial carboxylic acid and polyhydric alcohol together with a liquid medium. In other aspects, the invention is directed to the coated metal substrates obtainable according to the above method, where the metal substrates can be base metal or metal coated base metal, as well as being directed to resulting coated metal articles. DESCRIPTION OF THE PREFERRED EMBODIMENTS The metal substrates obtained by the present invention are exemplified by a great number of metals, and these can include alloys and intermetallic mixed metals. For example, such metal substrates may be aluminum and its alloys, zinc and its alloys, copper and cupriferous, e.g., brass and bronze. Additionally, exemplary metal substrates include cadmium, titanium, nickel, and its alloys, tin, cobalt, lead, chromium, magnesium and alloys thereof, and for weldability, preferably a ferrous metal substrate such as iron, stainless steel, or steel such as cold rolled steel or hot rolled and pickled steel. All of these for convenience are usually referred to herein simply as the "substrate" or "base" metal. Such substrate may receive a coating of elemental metal, e.g., of nickel, copper or zinc as a metal plate or cladding, and be referred to herein for convenience as a "plated" metal substrate. Thus a steel substrate might contain a zinc layer such as a galvanized or electrodeposited zinc coating. Although many such coating metals, including the foregoing and hereinabove mentioned metals, are contemplated, particularly serviceable metals in addition to the nickel, copper and zinc can be cadmium, cobalt and chromium. Alloys may also be useful, e.g., zinc-iron, zinc-aluminum, zinc-cobalt and zinc-nickel. According to the process of the present invention, the hexavalent chromium is usually supplied to the coating composition as chromic acid or its equivalent, for example, chromium trioxide or chromic acid anhydride. It is also possible to use salts to supply the hexavalent chromium. These include the calcium, barium, magnesium, zinc, cadmium and strontium dichromates. Generally, the sodium or potassium salts, if present, are used sparingly as they may detract from the coatings corrosion resistance. Occasionally present in minor amounts are also substances such as ammonium dichromate or other hexavalent chromium compounds in addition to those already mentioned. The composition is preferably free from trivalent chromium for best compositional stability, although it is to be understood that some exceedingly minor amount of trivalent chromium, e.g., less than about 0.05 gram per liter, may form in composition that is subjected to long shelf storage before use. Although the chromium may be present in the coating composition in an amount of as little as 10 grams per liter, expressed as CrO₃, it is more typically present in an amount from 20 to 60 grams of CrO₃ per liter. Substantially concentrated amounts can be as much as 80-100 grams per liter of coating composition although greater than 200 grams per liter is avoided for economy. Advantageously for best coating characteristics, the coating composition applied according to the invention, which may also be referred to herein for convenience as the "bonding coating" composition or the "pre-paint" coating composition, will contain reducing agent for the hexavalent-chromium. Preferably for best low-cure coating composition characteristic as well as composition stability, the reducing agent will be a combination agent comprising carboxylic acid and polyhydric alcohol. For both the acid and the alcohol it is advantageous for efficient composition preparation that if such substance is insoluble or only sparingly soluble in water, that it at least be completely miscible with water. Preferably for best efficiency, such substance is completely water soluble. It is to be understood that particularly where the acid or alcohol is miscible but not soluble, such can be added to other composition ingredients during complete coating composition preparation, although these ingredients are more usually packaged. The carboxylic acid will generally be selected from one or more of a C₂-C₁₄ acid. More usually such acid will be supplied by some to all of a C₂-C₆ hydroxyl-containing carboxylic acid. Where dicarboxylic acids are used, these will most always include the C₃-C₆ dicarboxylic acids. Usually when the carboxylic acid reducing agent is present, it will be present in an amount between 0.5-50 grams per liter of total coating composition. Less than an amount of 0.5 gram per liter can be inefficient for providing reducing effect for the hexavalent-chromium, while greater than 50 grams per liter can be uneconomical. Preferably for best hexavalent-chromium reduction as well as economy, the carboxylic acid, when present, will be present in an amount from 1-20 grams per liter of the coating composition. Generally the carboxylic acid will be an aliphatic acid, either straight chain or branched chain, but such may also be a cycloaliphatic acid, or any of such acids in mixture. The acids will most always be saturated acids for efficient composition stability, but if unsaturated acids are present, they will only be present in mixture with saturated acids and then only in very minor amount, e.g., less than 5 to 10 weight percent, basis weight of the saturated acids. Preferably the composition is totally free from unsaturated acids. Acids which can or have been used include malonic, malic, citric, glycolic, lactic, tartaric, tartronic and succinic acids as well as mixtures thereof. Preferably for most desirable chromium reduction as well as a broad range of augmented coating characteristics, the carboxylic acid used will be malic acid. The carboxylic acid will almost always be present as a reducing agent in combination with polyhydric alcohol. The alcohol can desirably supplement the reducing capability of the acid. For economy, the polyhydric alcohol is advantageously a C₂-C₈ alcohol. The most useful alcohols are the aliphatic alcohols with the aliphatic being straight, branched or cyclic. The alcohol can be saturated or unsaturated aliphatic alcohol and may contain more than one hydroxyl group, e.g., two such groups, but for economy, will usually contain no more than 3 hydroxyl groups. The polyhydric alcohol, when present, should be supplied in an amount between 0.5-50 grams per liter of total composition. Less than 0.5 gram per liter can be insufficient for providing an efficient reduction of the hexavalent-chromium. Greater than 50 grams per liter, on the other hand, of the polyhydric alcohol can be uneconomical. Preferably for best chromium reduction, coupled with economy, the polyhydric alcohol will be present in an amount of from about 2 to about 20 grams per liter of total coating composition. Useful polyhydric alcohols include glycerol, ethylene glycol, 1,3-propanediol, 2-butene-1,4-diol, cyclohexanedimethanol, 2-butyne-1,4-diol and their mixtures. Preferably for economy of composition formulation as well as best coating characteristics, the polyhydric alcohol used will be glycerol. It will be understood that particularly where combinations of reducing agents are to be used, additional reducing agent material may be present in the coating composition. Such additional substances can be exemplified by the dicarboxylic acids as disclosed in U.S. Patent No. 3,382,081, in addition to the succinic acid mentioned hereinbefore, as well as the amino-, tri- or polycarboxylic acids that have been taught in U.S. Patent No. 3,519,501. It is advantageous for ease in coating preparation, that all reducing agent substances present, as in combinations of reducing agents, be water soluble. Thus substantially all the coating compositions contemplated in the present invention are simply water-based for economy. It is to be understood however that ingredients such as reducing agent can contribute to the composition liquid medium. For example, polyhydric alcohol may provide up to 5 weight percent of such medium. But, for additional substances for supplying minor amounts of liquid medium, there have been taught, as in U.S. Patent No. 3,437,532 blends of chlorinated hydrocarbons and tertiary alcohols including tertiary butyl alcohol. Another useful component most always present for the coating composition is pH adjusting agent, or "modifier". The use of such agent, most notably zinc oxide, has been shown to be heretofore useful for adjusting the pH of aqueous chromic acid solutions. It is critical that when pH adjusting agent is present that some to all of the pH adjusting agent be cobalt compound. Although the method of the present invention is substantially focussed on the use of low-cure coating compositions, the compositions demonstrating enhanced corrosion resistance, may be low-cure or conventional or high-cure compositions. It is advantageous for ease in preparation of the coating composition that the cobalt compound be water soluble. However, particularly where several compounds may be employed, it is not necessary that all the compound be soluble so long as they can be well dispersed, e.g., dispersible compound could it be added separately as the bath is being prepared. Where the cobalt compound supplies the pH adjusting agent, it should be present in an amount from 2 to 100 grams per liter of total coating composition. Less than 2 grams per liter of cobalt compound can be insufficient for providing best compound characteristics to the coating composition. On the other hand, greater than 100 grams per liter of compound can be uneconomical. For best economy provided with best coating characteristics, it is preferred that there be present in the coating composition from 3 to 30 grams per liter of compound, basis total coating composition. Where the cobalt compound is supplemented by additional pH adjusting agent, e.g., zinc oxide, it will be most useful if the cobalt compound supplies at least 50 weight percent of total pH adjusting agent. The use of less than 50 weight percent of cobalt compound for the total pH adjusting agent can be insufficient for providing augmented coating characteristics, such as corrosion resistance for substrate metal exposed by cuts or nicks. Such exposed metal protection is often hereinafter referred to as "cut edge" protection. Preferably for best cut edge protection, the cobalt compound will supply from 75 to 100 weight percent of the pH adjusting agent. The suitable cobalt compound used for preparing the coating composition can be selected on the basis of the anion introduced into the composition. For example, for a great variety of coating characteristics including best weldability, the composition should be phosphate-free. Thus care should be taken not to introduce phosphate anion into the composition when selecting cobalt compound. Serviceable cobalt compounds that can or have been used include cobalt chromate, carbonate, acetate, sulfamate, naphthenate, propionate and mixtures thereof. Preferably for ease in formulating the coating composition as well as for economy, the cobalt compound is cobalt carbonate. Such compound when present in an amount from 2 to 100 grams per liter of total coating composition will supply from 1 to 50 grams per liter of cobalt. A greater amount than 50 grams per liter can be uneconomical, while less than the 1 gram per liter can be insuffcient for best coating composition characteristics. Usually the cobalt compound will be present in an amount from 10 to 60 grams per liter of coating composition. The pH adjustment agents are typically initially present together in an individual package. This package can contain chromic acid and initially have, from the acid for example, a pH of 1 or below, e.g., less than 0.8. The reducing agent may also be present in such package. The amount of pH adjusting agent will be sufficient to elevate the pH of the package to above 1, and typically to a pH in the range of 1.1-4, but most always not above 5. In usual practice then, the agent is blended into the package to provide for adjusted pH of the package of above 1 and up to 5, inclusive. As a result, the total composition will typically have a pH within the range of from 4 to 5.5. The amount of agent actually used will depend upon the concentration of the chromic acid in the package and, additionally, for example, on the concentration of reducing agent and on the neutralizing strength of the particular pH adjusting agent. Such adjusted pH packages have been discussed for example in U.S. Patent No. 3,970,482. A further component that is present in the coating composition used in the method of the invention is the pulverulent metal. The presence of pulverulent metal in such compositions has been previously discussed in U.S. Patent No. 3,671,331. Most usually, the particulate metal will be pulverulent zinc, although pulverulent aluminum is contemplated as well as blends, including pulverulent aluminum and particulate zinc and alloys thereof. The particulate metal portion of the coating composition when present should be present in an amount sufficient to provide at least 50 grams per liter of the pulverulent metal. There further should be sufficient of the chromium component to provide in the coating composition a weight ratio of chromium, expressed as CrO₃ to pulverulent metal of not substantially less than 0.08:1. A ratio of less than that may not provide sufficient chromium in the subsequent coating to achieve augmented bonding of the pulverulent metal to the metal substrate. On the other hand, a ratio greater than 0.4:1 may detract from the most enhanced corrosion resistance for the coated substrate. Thus, for an exemplary composition containing 200 grams per liter of pulverulent metal, the chromium component should be sufficient to provide in the coating composition between 15-80 grams of CrO₃ per liter of coating composition. The coating composition may additionally contain a variety of additives. Such can include the viscosity modifiers including thickening agents, most typically the xanthan gum hydrophilic colloids. Additional thickeners include other gum thickeners which can be used in mixture, especially with the xanthan gum. Cellulose ethers may also be employed in thickening. Additional agents include wetting agents and suspending agents and the like. These may also be dispersing agents, and those typically used may be hydroxyl-containing hydrocarbon ethers. Such ethers more particularly include the alkyl ethers of alkylene glycols such as butyl ether of propylene glycol and the oxyalkyl ethers of alkylene glycols. Further additives may include preservatives. Some to all of these agents, together with the particulate metal, may be preblended. Preblended compositions of zinc flake and thickener have been discussed, for example, in U.S. Patent No. 3,940,280. These other compounds may further include inorganic salts often typically employed in the metal coating art for imparting some corrosion resistance or enhancement in corrosion resistance for metal surfaces. Such materials include zinc chloride, magnesium chloride and various molybdates, or the like, and these are all preferably avoided, but if present, are most usually employed in the liquid composition in a total maximum combined amount of less than 5-10 grams per liter. Further, as mentioned hereinabove, the composition is phosphate-free. Also, it should be substantially resin-free, e.g., contain less than 5 grams per liter of resin, and is preferably completely resin-free. For storage considerations, the particulate metal may often be packaged individually. As mentioned hereinbefore, it may be pre-blended with composition additives such as thickening agent, as well as with preservative and dispersing agent. Other ingredients may also be available as pre-blended packages of ingredients. For example, as discussed hereinbefore in regard to pH adjustment, the hexavalent-chromium-providing compound and cobalt compound may be present in a pre-blended package. This package may further contain any additional pH adjustment agent. So that there will be no deleterious package reduction of hexavalent-chromium during storage or handling, this package will usually not contain reducing agent. Thus the reducing agent will almost always be pre-blended as a component by itself in liquid medium, e.g., water. This pre-blend can however contain both dicarboxylic acid as well as polyhydric alcohol reducing agent. Usually both the acid and the alcohol will be present in the package in an amount from 5 to 20 weight percent for each, basis total package weight. It is contemplated that where various additives will be used, they may be included in the various packages, e.g., a dispersing agent and reducing agent package. Exemplary specific packages and their utilization in preparing a coating composition will be more fully discussed hereinbelow in connection with the examples. For the metal substrates containing applied coating composition, the subsequent curing of the composition on the substrate will usually be a hot air oven cure, although other curing procedures can be used, e.g., infrared baking. For economical curing, the special combination of carboxylic acid and polyhydric alcohol will have been used as reducing agent in the composition. This will permit a low temperature cure of applied coating composition. Such low cure can be dependent upon the subsequent application of a heat-cured topcoat. For example, where a topcoat will be applied that will be heat-cured at elevated temperature, e.g., on the order of 177°C (350°F) or more, the low temperature cure for the pre-paint coating may generally be in the range of from 149°C (300°F) to 177°C (350°F) peak metal temperature. Without such subsequent topcoating the low temperature cure will typically provide a peak metal temperature within the range of from 177°C (350°F) to 232°C (450°F). Thus, although oven air temperatures may be more elevated, e.g., on the order of 260°C (500°F), for economy the metal temperature need not exceed 149°C (300°F) or 177°C (350°F), depending on subsequent processing, e.g., topcoating. More elevated cure temperatures may be employed, as with compositions containing differing or additional reducing agents. The cure temperatures, i.e., the peak metal temperatures, however do not often exceed above about 371°C (700°F). Curing, such as in a hot air convection oven, can be carried on for several minutes. More typically, and particularly with a low-cure composition, cure times may be less than 5 minutes and more typically on the order of from 0.5 to 3 minutes. It is to be understood that cure times and temperatures can be effected where a subsequently applied, heat-cured topcoating will be used. Thus shorter time and lower temperature cures can be employed followed by a topcoating that proceeds through a more elevated temperature bake at a longer cure time. The resulting weight of the coating on the metal substrate may vary to a considerable degree, but will always be present in an amount supplying greater than 107.6 mg/m² (10 milligrams per square foot) of chromium, measured as chromium and not as CrO₃. A lesser amount will not lead to desirably enhanced corrosion resistance. Advantageously, greater than 1.61 mg/m² (15 milligrams per square foot) of coated substrate of chromium will be present for best corrosion resistance, while most typically between 215.2 - 5380 mg/m² (20-500 milligrams per square foot) of chromium, always expressed as chromium and not CrO₃, will be present. Moreover, for augmented corrosion resistance there will always be present in the coating above 53.8 mg/m² (5 milligrams per square foot) of cobalt. It is preferred for best corrosion resistance that the cobalt will be present in the coating in an amount above 107.6 mg/m² (10 milligrams per square foot). Most usually the cobalt in the coating will be present in an amount from 107.6 to 2152 mg/m² (10 to 200 milligrams per square foot) of coated substrate. Coating weight amounts for the cobalt exceeding 2690 mg/m² (250 milligrams per square foot) can be uneconomical. Also, when particulate metal is present the coated metal substrate should contain between 538 and 53,800 mg/m² (50 and 5,000 milligrams per square foot) of pulverulent metal. Furthermore, when cobalt is present the coating should have a weight ratio of chromium, expressed as chromium, to cobalt of greater than about 1.5:1, e.g., 1.75:1. At the low coating weights, as for example at 107.6 - 215.2 mg/m² (10-20 milligrams per square foot) for chromium and 53.8 - 107.6 mg/m² (5-10 milligrams per square foot) for cobalt, such ratio might approach 2:1. Thus a ratio in the range of 1.5:1-3:1 or more can be readily expected to provide for enhanced corrosion resistance of the coating. At the most elevated coating weights, a 2:1 ratio is desirable for economy. On the other hand, a weight ratio of about 4:1 for chromium to cobalt will not generally be exceeded to insure the best coating characteristics imparted by the cobalt. Before starting the treatment of the present invention it is, in most cases advisable to remove foreign matter from the metal surface by thoroughly cleaning and degreasing. Degreasing may be accomplished with known agents, for instance, with agents containing sodium metasilicate, caustic soda, carbon tetrachloride, trichloroethylene, and the like. Commercial alkaline cleaning compositions which combine washing and mild abrasive treatments can be employed for cleaning, e.g., an aqueous trisodium phosphate-sodium hydroxide cleaning solution. In addition to cleaning, the substrate may undergo cleaning plus etching. After preparation of the resulting coated substrate, such can be further topcoated with any suitable paint, i.e., a paint or primer, including electrocoating primers and weldable primers, such as the zinc-rich primers that may be typically applied before electrical resistance welding. For example, it has already been shown in U.S. Patent No. 3,671,331 that a primer topcoating containing a particulate, electrically conductive pigment, such as zinc, is highly serviceable for a metal substrate that is first treated with a bonding coating composition which itself contains a pulverulent metal such as finely divided zinc, with the zinc being supplied in a pre-paint coating from a composition containing hexavalent-chromium-providing substance, such as chromic acid and a reducing agent for the substance. Other topcoating paints may contain pigment in a binder or can be unpigmented, e.g., generally cellulose lacquers, rosin varnishes, and oleoresinous varnishes, as for example tung oil varnish. The paints can be solvent reduced or they may be water reduced, e.g., latex or water-soluble resins, including modified or soluble alkyds, or the paints can have reactive solvents such as in the polyesters or polyurethanes. Additional suitable paints which can be used include oil paints, including phenolic resin paints, solvent-reduced alkyds, epoxys, acrylics, vinyl, including polyvinyl butyral and oil-wax-type coatings such as linseed oil-paraffin wax paints. The following examples show ways in which the invention has been practiced but should not be construed as limiting the invention. In the examples, the following procedures have been employed. PREPARATION OF TEST PANELS Unless otherwise specifically described, test panels are typically 10.16 x 20.32 cm (4 X 8 inch) cold rolled, low carbon steel panels. Steel panels can be prepared for coating by first immersing in a cleaning solution typically containing 37.45 kg/m³ (5 ounces, per gallon) of water, of a mixture of 25 weight percent tripotassium phosphate and 75 weight percent potassium hydroxide. This alkaline bath is maintained at a temperature of 66°C - 82°C (150°F.-180°F). Panels having a surface such as electrogalvanized steel can be cleaned by immersion in an aqueous cleaner combining silicate and phosphate cleaning agents and typically having a pH of approximately 10-12. This cleaner is likewise maintained at elevated temperature. Following the immersion, the panels are scrubbed with a cleaning pad which is a porous, fibrous pad of synthetic fiber impregnated with an abrasive. Thereafter, the scrubbed panels are water rinsed and again immersed in the cleaning solution. Following removal from the solution, the panels are rinsed with tap water and preferably dried. PRIMER TOPCOATING AND APPLICATION When pre-painted panels are primer topcoated, unless otherwise specified, the primer used is a commercially available primer which is a zinc-rich weldable primer having a weight per liter of 1.821 kg (per gallon of 15.2 lbs), a solids volume of about 27%, and containing about 62 weight percent of nonvolatiles. The binder component is prepared from a high molecular weight epoxy resin. The primer has a typical viscosity of about 80 seconds as measured on a No. 4 Ford cup. This primer is typically applied to pre-painted panels by drawing the primer down over the panel with a draw bar to provide a smooth, uniform primer coat, generally of about 7.62 - 12.70 µm (0.3-0.5 mil) thickness. Resulting coated panels are usually cured for about 0.5-3 minutes in an oven at 260-288°C (500°-550°F). ADHESION/CORROSION TEST The corrosion resistance of the coating system on the panel under extreme conditions can then be measured by first subjecting the coating to an adhesion test. In this test, cleaned and coated panels, cleaned in a manner such as above-described, are pressed with a cup-like impression. In the test, more specifically, there is used a cup ductility machine of the motor driven hydraulically actuated type. In general, an unlubricated, coated test panel is held firmly between male and female dies, by grippers at the perimeter of the die covering. The coated side of the panel faces the female die. A ball is forced in the aperture of the male die, resulting in the pushing and stretching of part of the panel into the cup-like impression of the female die. The test employs a female die cup diameter of 2.54 cm (1 inch), a ball diameter of 2.22 cm (7/8-inch), and provides a cup or "dome" depth, from the under surface of the flat panel to the under surface of the cup, of 0.762 cm (0.3-inch). The resulting "domed" panel is then subjected to the below described corrosion resistance test. CORROSION RESISTANCE TEST (ASTM B-117) AND RATING Corrosion resistance of coated parts is measured by means of the standard salt spray (fog) test for paints and varnishes ASTM B-117. In this test, the parts are placed in a chamber kept at constant temperature where they are exposed to a fine spray (fog) of a 5% salt solution for specified periods of time, rinsed in water and dried. The extent of corrosion on the test parts can be expressed as percent of red rust. For a test panel portion containing a dome, the dome can be rated according to a photographic scale. Initially following coating, a pressure sensitive tape is applied to the dome. The tape is then quickly removed from the dome. This is done to determine coating adhesion. The panel is then subjected to the corrosion resistance test. In the following examples the efficacy of the dome corrosion resistance is quantitatively evaluated by rating domes against photographic standards using a numerical scale from 0 to 8, with 0 best. In general, 0 represents no red rust and retention of film integrity. An 8 represents a substantial showing of red rust. Where test panels are scribed, the scribing is performed before corrosion resistance testing by cutting an "X" configuration on the face of the panel. The scribe lines are made through the coatings to expose the base metal. The extent of corrosion along the scribe lines is also made by visual inspection and through comparison among test panels. For edge corrosion, after coating the panel is cut to expose a fresh edge. The edge corrosion is usually measured, in millimeters, as the distance of corrosion from the edge. Or the parts can be rated for edge corrosion objectively by comparing parts, one with the other, all by visual inspection. PRE-PAINT COATING COMPOSITION Unless otherwise specified, compositions are formed from separately prepared constituents. The Part A constituent contains 495 grams of water, 2.5 grams of heteropolysaccharide dispersing agent, 2.3 grams of formalin, and 0.5 gram of a wetter which is a nonionic, modified polyethoxide adduct having a viscosity at 25°C of approximately 0.8-0.9 Pas (800-900 centipoises) and a density at 25°C of 1 kg/ℓ (8.4 lbs. per gallon). For particulate metal containing coating compositions, these will often include a Part B zinc dust. The zinc dust has all particles finer than 18 µm and an average particle size of 3-8 µm. Moreover about 95 percent of the particles have size less than 10 microns and 99% are finer than 15 µm. For control purposes, usually a Part C will be used which contains 483 grams of water, 32 grams of chromic acid, 5.3 grams of succinic acid, 12 grams of zinc oxide and 1.3 grams of succinimide. For invention compositions, unless otherwise specified, there will be used a Part C of 448 grams water, 31.5 grams chromic acid and 20 grams cobalt carbonate. There will also be used a Part D of 30 grams water, 3.5 grams malic acid and 4.2 grams glycerine. To prepare a coating composition from all these components, the usual order of addition of the parts to one another, is as follows: Part B is dispersed in Part A. Then Part C is added to the dispersion followed by the addition of Part D thereto, when such is used. APPLICATION OF COATING TO TEST PARTS AND COATING WEIGHT Clean panels are typically coated by dip coating at 51 cm/min (20 inches/minute) into coating composition, removing and then immediately baking, or air drying at room temperature until the coating is dry to the touch and then baking. Baking proceeds in a hot air convection oven at temperatures and with times as specified in the examples. Coating weights for coated articles, as chromium, and not as CrO₃, and as particulate metal, e.g., zinc, both being typically in weights in milligrams per square foot of coated substrate, have been presented in the examples. Such weights are determined by a portable x-ray fluorescence spectroscope. The lithium fluoride analyzing crystal is set at the required angle to determine chromium, and at the required angle to determine zinc. The instrument is initially standardized with coatings containing known amounts of these elements. The machine is adapted with a counter unit and the count for any particular coating is translated into milligrams per square foot by comparison with a preplotted curve. EXAMPLE 1 Both control and invention coating compositions were used for this test. The control composition was as described hereinbefore. The initial, or control, composition contained no cobalt and all zinc oxide while the final (an "invention") composition had all zinc oxide replaced by cobalt carbonate. For the invention composition the Parts C and D which were initially control compositions as described hereinbefore were combined and made suitable for invention purposes by replacing the zinc oxide with 13.4 grams of cobalt carbonate. The formulation contained 416 weight parts of Part A, 160 weight parts of Part B, 418 weight parts of Part C and 31.4 weight parts of Part D. The substrate material used for this test was a 2-sided electroplate steel coil. This was a commercially available electroplated steel coil using a draw quality, special killed base steel. The electrogalvanized zinc coating had a weight of 65 grams per square meter (g/m²) of coated base steel on the one side of the coil and a zinc coating weight of 15 g/m² of the coated base steel on the reverse coil side. This electrogalvanized steel coil was coated on the light zinc coating weight side on a 2-roll reverse roll coater coating line having a urethane cover applicator roll and a steel pick-up roll. Before coating the coil was hot-alkali spray cleaned with abrasive brushing, all on-line, followed by on-line fresh water rinsing and blowing dry. The coil procedes through cleaning, coating and curing at a line speed of 122 m (400 feet) per minute. Coil samples coated with the pre-paint coating compositions, both control and invention, were then topcoated with zinc-rich weldable primer as described hereinbefore but having 65% solids by volume and a weight of 2.1 kg/ℓ (17.5 pounds per gallon). For the coil sample containing control pre-paint coating, curing proceded in the usual manner, including pre-paint curing at 154°C (310°F) peak metal temperature and weldable topcoating curing at 257°C (495°F) peak metal temperature. For the invention samples, the portion of the coil was cured after pre-paint coating at a peak metal temperature of 157°C (315°F) and after topcoating at a peak metal temperature of 196°C (385°F). Samples for both control and invention compositions, these being 10.16 x 30.48 cm (4 X 12-inch) samples, were domed and then subjected to the above-described corrosion resistance (salt spray) test. For some of the test panel samples, panel edges were taped to retard or eliminate red rust run-down over the face of the panel during testing. For other test panels, the edges were untaped. Results of this testing are shown in Table 1 hereinbelow. Panel Corrosion Resistance: Dome Ratings Untaped Panels 664 Hours Edge Taped 664 Hours Panels 808 Hours Control 4.7 1.1 2.2 Invention 0.9 0.1 0.6 EXAMPLE 2 The substrate material used in this experiment is a 2-sided electroplated steel coil. This is a 60.96 cm (24 inch) wide, commercially produced steel coil using an aluminum killed, draw quality base steel. The electrodeposited layer is a zinc/iron alloy, coated both sides, with the alloy containing about 18-19 weight percent iron with a balance of zinc. There was an alloy coating weight of 25 g/m² on the one coil side and an alloy coating weight of 50 g/m² on the reverse side. The invention composition of Example 9 was used for this test. The coil was coated in the manner of Example 9. The coil proceeds through cleaning, coating and curing at a line speed of 73 m/min (240 feet per minute). The pre-paint coating is cured at a peak metal temperature of 149°C (300°F). The coating contained 204.51 mg/m² (19 mg/ft²) of cobalt and had a weight ratio of chromium to cobalt of about 1.8:1. Prepainted coil portions are topcoated with the weldable primer used for the invention panels of Example 1. This topcoating is cured at a peak metal temperature of 177°C (350°F). Test panels, being 10.16 x 30.48 cm (4 X 12-inch) test panels, were then domed and subjected to the above-described corrosion resistance test. After 1,896 hours of testing, panels showed no percent red rust on panel domes. EXAMPLE 3 The coating composition used herein was all invention composition, and all as described in Example 1. A variety of substrate materials were used for this test. All were electroplated steel coils. All of the steel coils had been electrogalvanized with a metallic zinc coating. The first coil was a commercially available electroplated steel coil having a base steel designated as aluminum killed, draw quality. The electrodeposited layer was a zinc layer having an electroplate layer coating weight of 30 g/m² on the one coil side with a layer coating weight of 20 g/m² on the reverse coil side. The second coil was likewise a commercially available electroplated steel coil having the same quality substrate steel. For this coil, the one coil side was bare steel and on the reverse thereof was an electrodeposited layer of zinc having a plating coating weight of 30 g/m². The third coil was an electrogalvanized steel coil as described in Example 1. The pre-paint coating composition of Example 1 used for this test for the first coil was coated on the heavier plated side for a portion of the coil run and on the lighter plated side for an additional portion of the coil run. For the second coil, the invention composition was coated only on the plated side. For the third coil, the invention pre-paint coating composition was coated on the light 15 g/m² plated side. The coating had 204.51 mg/m² (19 mg/ft²) of cobalt and a weight ratio of chromium to cobalt of about 1.75:1. For all coils, the coating apparatus was a 2-roll reverse roll coater coating line having a urethane cover applicator roll and a steel pick-up roll. All coils were hot-alkali spray cleaned with abrasive brushing, all in-line, followed by in-line fresh water rinsing and blowing dry. The coils proceded through cleaning, coating and curing at a line speed of 122 m/min (400 feet per minute). For the first coil the pre-paint coating composition was cured at a peak metal temperature of 160°C (320°F). For the second coil, such curing was at 157°C (315°F) for one portion of the coating, at 160°C (320°F) for a second, and at 163°C (325°F) for a third. For the third coil, the coating was cured at a 157°C (315°F) peak metal temperature. All panels were topcoated with the zinc-rich weldable topcoating described in Example 1. For the first and third coils, these topcoatings were cured at a 196°C (385°F) peak metal temperature. For the second coil, the lowest pre-paint composition cure temperature (157°C) (315°F.) was topcoat cured at a peak metal temperature of 202°C (395°F). The second coil portion (pre-paint cured at 160°C (320°F) was topcoat-cured at 177°C (350°F) peak metal temperature, and the final portion (pre-paint cure at 163°C (325°F) peak metal temperature) was topcoat-cured at 193°C (380°F) peak metal temperature. Panels from all these coils, all being 10.16 x 30.19 cm (4 X 12-inch) test panels, were then domed and subjected to the above-described corrosion resistance (salt spray) test. Results of this test are shown in Table 2 below. Test Coil Pre-Paint Chromium Coating Weight Topcoat Thickness µm (Mils) Dome Rating 1,120 Hours No. 1 30 Side 35 8.89 (0.35) 3.1 No. 1 20 Side 36 10.668 (0. 42) 3.5 No. 2 157°C (315°F) Cure 26 9.652 (0. 38) 5.6 No. 2 160°C (320°F) Cure 36 10.414 (0. 41) 5.3 No. 2 163°C (325° F) Cure 36 9.652 (0.38) 6.2 No. 3 15 Side 33 9.652 (0.38) 4.1 Coating weights for the pre-paint coatings were determined for sample panels in accordance with the method as described hereinbefore in connection with the examples. Primer topcoating thicknesses were determined by eddy current measurement. Additionally, for control purposes a coil was coated with control pre-paint coating composition as described hereinbefore in connection with the examples. This control coil had the control pre-paint composition cured at a peak metal temperature of 157°C (315°F). This coil had a zinc-rich weldable topcoating as described in Example 1 which was cured at 260°C (500°F) peak metal temperature. This control coil was coated in the manner of the other coils. These control panels can be expected to have a coated chromium weight for the pre-paint coating of 269.1 mg/m² (25 mg/ft²) and have a topcoating thickness of 14.732 µm (0.58 mil). For control purposes, these test panels from this coil were domed and likewise subjected to the above-described corrosion resistance test. These panels showed an average rating on the dome of 5.8 after only 760 hours of testing, (compared to a range of 0.2-1.7 for the invention panels), at which time the test was terminated for these control panels.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GB, IT, LI, LU, NL, SE A method of obtaining a heat curable coating providing corrosion resistance to a metal substrate, which coating is phosphate-free and at least substantially resin-free and comprises a pulverulent metal, chromium from a hexavalent-chromium-providing compound and optionally a reducing agent for said hexavalent chromium characterized in that there is applied a phosphate-free, at least substantially resin-free and pulverulent-metal-containing composition containing the hexavalent chromium providing compound in an amount sufficient to provide a coating of at least 107.6 mg of chromium per square meter and that it further contains a cobalt compound in an amount sufficient to provide a coating having a weight ratio of chromium to cobalt of greater than 1.5:1. The method according to claim 1 characterized in that said pulverulent metal is selected from the group consisting of magnesium, aluminum, manganese, zinc, their alloys and mixtures including intermetallic mixtures. The method according to one of the preceeding claims characterized in that the hexavalent-chromium-providing compound is supplied by one or more of chromic acid or a salt thereof including dichromate salts providing not in excess of 200 grams of chromium per liter of coating composition, said chromium being expressed as CrO₃. The method according to one of the preceeding claims characterized in that said cobalt compound is selected from the group consisting of cobalt chromate, carbonate, acetate, sulfamate, naphthenate, propionate and mixtures thereof, and said cobalt compound is present in said composition in an amount from 2 to 100 grams per liter of coating composition. The method according to one of the preceeding claims characterized in that said cobalt compound supplies from above 1 gram per liter to not in excess of 50 grams per liter of cobalt basis each liter of coating composition. The method according to one of the preceeding claims characterized in that said reducing agent includes saturated carboxylic acid selected form the group consisting of straight chain aliphatic, branched chain aliphatic and cycloaliphatic acids or mixtures thereof, and said carboxylic acid is present in said coating composition in an amount between 0.5-50 grams per liter. The method of claim 6 wherein said carboxylic acid includes one or more of a C₂-C₁₄ acid. The method of claim 7 wherein said carboxylic acid includes one or more of a C₂-C₆ hydroxyl-containing carboxylic acid. The method of claim 7 wherein said carboxylic acid includes one or more of a C₃-C₆ dicarboxylic acid. The method of claim 6 wherein said carboxylic acid is selected from the group consisting of malonic, malic, citric, glycolic, lactic, tartaric, tartronic and succinic acids as well as mixtures thereof. A modification of the method of applying a coating composition according to one of the preceding claims characterized in that the applied composition contains reducing agent and the reducing agent contains a carboxylic acid and a polyhydric alcohol and wherein the pulverulent metal is omitted. The method according to claim 11 characterized in that said reducing agent includes polyhydric alcohol selected from the group consisting of saturated or unsaturated straight chain aliphatic, branched chain aliphatic and cycloaliphatic alcohols or mixtures thereof, and said polyhydric alcohol is present in said coating composition in an amount between 0.5-50 grams per liter. The method of claim 11 or 12 wherein said polyhydric alcohol includes one or more of a C₂-C₈ alcohol. The method of claim 11 - 13 wherein said polyhydric alcohol contains no more than three hydroxyl groups per molecule and is selected from the group consisting of glycerol, ethylene glycol, 1,3-propanediol, 2-butene-1,4-diol, cyclohexanedimethanol, 2-butyne-1,4-diol and their mixtures. The method according to one of the preceeding claims characterized in that it further contains one or more substances selected form the group consisting of pH modifiers, viscosity modifiers, dispersing agents and preservatives. The method according to one of the preceeding claims characterized in that it is provided in a liquid medium comprising water, alcohol, halogenated hydrocarbon solvent and mixtures therof. The method of claim 16 characterized in that the composition is cured at a temperature between 148°C - 371°C and preferably between 148°C - 232°C. The method according to claim 16 or 17, characterized in that the resulting coated metal substrate is subsequently topcoated. Coated metal substrate obtainable according to the method of claims 1 to 18. Claims for the following Contracting States : ES, GR A method of obtaining a heat curable coating providing corrosion resistance to a metal substrate, which coating is phosphate-free and at least substantially resin-free and comprises a pulverulent metal, chromium from a hexavalent-chromium-providing compound and optionally a reducing agent for said hexavalent chromium characterized in that there is applied a phosphate-free, at least substantially resin-free and pulverulent-metal-containing composition containing the hexavalent chromium providing compound in an amount sufficient to provide a coating of at least 107.6 mg of chromium per square meter and that it further contains a cobalt compound in an amount sufficient to provide a coating having a weight ratio of chromium to cobalt of greater than 1.5:1. The method according to claim 1 characterized in that said pulverulent metal is selected from the group consisting of magnesium, aluminum, manganese, zinc, their alloys and mixtures including intermetallic mixtures. The method according to one of the preceeding claims characterized in that the hexavalent-chromium-providing compound is supplied by one or more of chromic acid or a salt thereof including dichromate salts providing not in excess of 200 grams of chromium per liter of coating composition, said chromium being expressed as CrO₃. The method according to one of the preceeding claims characterized in that said cobalt compound is selected from the group consisting of cobalt chromate, carbonate, acetate, sulfamate, naphthenate, propionate and mixtures thereof, and said cobalt compound is present in said composition in an amount from 2 to 100 grams per liter of coating composition. The method according to one of the preceeding claims characterized in that said cobalt compound supplies from above 1 gram per liter to not in excess of 50 grams per liter of cobalt basis each liter of coating composition. The method according to one of the preceeding claims characterized in that said reducing agent includes saturated carboxylic acid selected form the group consisting of straight chain aliphatic, branched chain aliphatic and cycloaliphatic acids or mixtures thereof, and said carboxylic acid is present in said coating composition in an amount between 0.5-50 grams per liter. The method of claim 6 wherein said carboxylic acid includes one or more of a C₂-C₁₄ acid. The method of claim 7 wherein said carboxylic acid includes one or more of a C₂-C₆ hydroxyl-containing carboxylic acid. The method of claim 7 wherein said carboxylic acid includes one or more of a C₃-C₆ dicarboxylic acid. The method of claim 6 wherein said carboxylic acid is selected from the group consisting of malonic, malic, citric, glycolic, lactic, tartaric, tartronic and succinic acids as well as mixtures thereof. A modification of the method of applying a coating composition according to one of the preceding claims characterized in that the applied composition contains reducing agent and the reducing agent contains a carboxylic acid and a polyhydric alcohol and wherein the pulverulent metal is omitted. The method according to claim 11 characterized in that said reducing agent includes polyhydric alcohol selected from the group consisting of saturated or unsaturated straight chain aliphatic, branched chain aliphatic and cycloaliphatic alcohols or mixtures thereof, and said polyhydric alcohol is present in said coating composition in an amount between 0.5-50 grams per liter. The method of claim 11 or 12 wherein said polyhydric alcohol includes one or more of a C₂-C₈ alcohol. The method of claim 11 - 13 wherein said polyhydric alcohol contains no more than three hydroxyl groups per molecule and is selected from the group consisting of glycerol, ethylene glycol, 1,3-propanediol, 2-butene-1,4-diol, cyclohexanedimethanol, 2-butyne-1,4-diol and their mixtures. The method according to one of the preceeding claims characterized in that it further contains one or more substances selected form the group consisting of pH modifiers, viscosity modifiers, dispersing agents and preservatives. The method according to one of the preceeding claims characterized in that it is provided in a liquid medium comprising water, alcohol, halogenated hydrocarbon solvent and mixtures therof. The method of claim 16 characterized in that the composition is cured at a temperature between 148°C - 371°C and preferably between 148°C - 232°C. The method according to claim 16 or 17, characterized in that the resulting coated metal substrate is subsequently topcoated.

Inventors: DORSETT, TERRY E., GUHDE, DONALD J.
Applicants: METAL COATINGS INTERNATIONAL INC.
Application year: 1988
Main classification: C23C22
IPC codes: C09D5, C23C22