Nited States Patent O?Ice Patented Nov
Total Page:16
File Type:pdf, Size:1020Kb
3,007,818 nited States Patent O?ice Patented Nov. 7, 1961 1 2 3,007,818 which often impairs the operation of certain types of PROTECTIVE COATINGS 0N METALS equipment such as relays and switches. The chromate Kurt Erich Schimkus, Chicago, Ill., assignor to Acme coatings referred to have been employed in an effort to Steel Company, Chicago, 111., a corporation of Illinois overcome this di?iculty. No Drawing. Filed Mar. 31, 1958, Ser. No. 724,794 It has been the usual practice to apply chromate coat 44 Claims. (Cl. 148-616) ings by an immersion process although electrolytic meth ods have sometimes been employed. The compositions This invention relates to protective coatings on metals of these baths are largely made up according to secret and to coating baths and coating methods for applying formulae but it is known that they all contain two basic the coatings. ' ingredients, viz., hexavalent chromium ions and mineral Many coatings and coating methods have been utilized acid with the addition, in some cases, of one or more heretofore to protect base metals, i.e., metals which cor organic acids. On immersion, the zinc coated or cad rode and discolor upon exposure to high humidity condi mium plated article is attacked by the mineral acid with tions and to salt water and the like. In certain instances an attendant rise in the pH of the solution adjacent to it is desirable that such coatings be transparent whereby 15 the metal. At the same time some hexavalent chromium to leave substantially unchanged the appearance of the is reduced to the trivalent state. When the pH reaches surface coated. In other instances it is desirable to color a critical value, the trivalent chromium and some hexa the metal surface to be coated and, accordingly, the valent chromium are deposited on the metal surface. coating must either be colored or readily receive coloring agents. The chromate films which are thus deposited are gen 20 erally noncrystalline, nonporous and gel-like. They are One speci?c example of an application in which it is quite susceptible to being damaged by abrasion immedi desired to impart a particular color to a metal surface ately after being formed and should, therefore, be al is in the case of galvanized steel strapping. It has been lowed to age from twelve to twenty-four hours before found advantageous to color such steel strapping black. being subjected to normal handling operations in the The methods which have heretofore been employed for 25 shop. When the chromate coatings are applied by elec providing black coatings on strap steel or the like have trolytic methods these precautions need not be taken. not been entirely satisfactory. If the steel is painted to The chromate ?lms which give maximum protection provide the desired color and protective coating, it is are those which contain both trivalent and hexavalent not possible to provide an even and uniform coating chromium. The nonporous nature of the ?lm enables when the metal is moved at high speed during the coating 30 it to exclude moisture from the metal to a very great process and small pin holes are often present in the coat extent and, if there are any discontinuities in the ?lm, ing with the result that the strip rusts at these bare spots the hexavalent chromium, being slightly soluble, exerts and the rusty solution bleeds out onto the surface of a well known inhibiting action. It has been found that the paint. Another prior method comprises the step of the formation of the undesirable white corrosion product passing a galvanized steel strip through a nickel salt solu 35 is inhibited as long as there is a small amount of hexa tion which must be maintained at a high temperature, valent chromium in the ?lm. but this method has the disadvantage that the resulting Although chromate coatings have found. wide industrial black nickel oxide coating rubs off easily so that it is use, it appears that, because of the above mentioned li necessary to provide an additional coating of lacquer to ability to abrasion, they have been employed only on protect it. Also, when this method is carried on with 40 parts which are substantially static when. in use as dis the metal strip moving at high speeds, a uniform coating tinguished from moving parts and parts having contact cannot be insured and there are resulting bare spots with each other or with other members, such as continu where the metal is protected only by the zinc coating ously moving metal strips which are acted upon during provided by the galvanizing process. In general, prior their movement by forming or shearing members. Fur color coatings have had the disadvantage that there has 45 thermore, chromate coatings cannot be applied by con been no chemical union between the coating and the tinuous strip processes since the coating being gel-like metal base. would be readily removed by the strip feeding mecha Special problems are encountered when the metal nism immediately after formation and before it had an part is to be exposed to high temperature and high opportunity to age. humidity conditions which may also include salt Water. 50 Accordingly, it is an important object of the present Such conditions are often encountered in the tropical invention to provide improved protective coatings for regions. The protection of metals under such conditions base metal surfaces and improved coating baths and coat is most difficult. In one practice heretofore a coating ing methods to apply the coatings to the base metals. of zinc has been provided on steel products as in the 55 Another object of the invention is to provide improved usual galvanizing processes. Such zinc coated parts as coatings for base metals which are substantially colorless well as cadmium plated parts and aluminum and zinc and transparent whereby to leave unimpaired ‘the desir base die cast parts have heretofore further been provided able surface characteristics of the base metal while pro with chromate conversion coatings to provide additional vidin g a maximum protection therefor. > protection therefor. Furthermore, these coatings in turn 60 Yet another object of the invention is to provide coat have sometimes been covered with lacquer or paint to ing methods and baths which can apply protective coat improved both the protection and the appearance of the ings of the type set forth to the surfaces of coating metals articles. such as zinc with substantially less depletion of the zinc In the absence of the additional protective coatings as compared with prior coating methods and »baths. set forth above zinc, for example, when exposed to 65 Still another object of the invention is to provide pro moisture develops a bulky white corrosion deposit. This tective coatings of the type set forth which are more deposit is particularly troublesome in tropical climates readily colored to provide pleasing and durable colored where the daily temperature variations cause an inter protective coatings. ’ mittent condensation of moisture in the nature of dew. In connection with the foregoing object it is another The storage of zinc coated parts in unheated warehouses 70 object of the invention to provide improved coating meth or the packaging of them in unseasoned Wood containers ods in which a colored protective coating can be pro or damp paper also causes this type of corrosion deposit duced on a base metal surface in a single step process. Yet another object of the invention is to provide a 3,007,818 3 4 protective coating which can serve as a bonding agent be 1 N aqueous solution of sodium hydroxide was added tween a base metal surface and other protective surfaces until the pH of the coating solution was about 1.5. The such as paints, lacquers, oils, greases and the like. steel strip was then immersed in the coating solution for Still another object of the invention is to provide im ten seconds to form a coating thereon. The coated strip ‘ proved coatings formed of a combination of protective was then taken from the solution and any excess coating compounds and coating methods which can deposit these solution removed. After drying, the coated steel strip combinations of protective compounds simultaneously in was examined and found to have a colorless transparent a one step process. phytate coating thereon. It was determined that between Yet another object of the invention is to provide a gal 35 and 40 mg. per square foot of coating had been vanized steel strip having a black coating which is chemi 10 formed on the strip. In addition to being colorless the cally united with the zinc and thereby tightly ‘bonded coating exhibited ‘good paint adhesion and provided good thereto, the color coating being uniformly and evenly protection against humidity and salt water when subjected distributed so that there are no bare spots and no addi to standard humidity and salt water corrosion tests. The tional protective coating is required. strip so prepared exhibited better corrosion resistance Still another object of the invention is 1to provide an 15 properties than a like steel strip having 120 to 150‘ mg. improved method of applying a black coating to base per square foot of protective phosphate coating thereon. metal surfaces in which the coating is effected by im The method of Example 1 above has been operated mersing the base metal in a coating solution at room successfully when the concentration of phytic acid in the temperature, thereby avoiding the necessity of provid coating solution is increased to as much as 0.2% by ing the expensive heating equipment which has been re 20 weight.