Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
  • C23C22/76—Applying the liquid by spraying

Definitions

• This invention relates to the art of applylns phosphate coatings to the surface of iron or steel, and particularly to the art of producing these coatings on a succession of pieces by treating them with an aqueous acid phosphate solution such, for example, as zinc phosphate.
• the solution employed in the process of the present invention may comprise a phosphate of a metal such as zinc or manganese.
• a phosphate of a metal such as zinc or manganese.
• I prefer to use zinc phosphate because of its more ready availability and relatively lower cost. Therefore, in the following description, I will refer to zinc phosphate, although it is to be understood that the same principles may very well be applied to other metals of a similar nature.
• the process in its preferred form, comprises subjecting a succession of iron or steel pieces serially to the action of an aqueous zinc phosphate solution, while substantially continuously adding to the solution, regulated quantities of the zinc phosphate solution and of a water solution of a salt of nitrous acid, which additions are made in an amount which is slimcient to maintain the metal content and the pH of the solution substantially constant and also substantially free of dissolved iron.
• the hot solution is supersaturated with respect to the zinc phosphate which precipitates.
• the solution rises in hydrogen ion concentration (falls in pH) due to the greater proportionate loss of zinc. This action continues until the solution has again reached equilibrium at the elevated temperature.
• the highest coating speed is likewise favored by elevation of temperature, up to the boiling point, and for best results the temperature of the solution should be quite accurately controlled.
• elevated temperatures have a tendency to cause the solution to lose its supersaturation, for which reason I prefer a temperature somewhere between F. and F., because my experience has shown that a temperature within this range will yield coatings in a period which is sumciently brief without causing the extremely rapid loss of supersaturation which occurs at higher temperatures.
• Violent agitation of the solution tends to speed up somewhat the rate at which it loses its supersaturation and falls in pH. Nevertheless, rapid renewal of the film of solution actually in contact with the surface of the metal, and the more or less violent agitation which it necessitates, is
• the removal of the iron from the solution may be effected in a number of ways, such, for example, as by the addition to the solution of chemicals capable of precipitating the ferrous iron or of oxidizing the ferrous iron to ferric iron, the latter being practically insoluble in the solution. Furthermore, adequate aeration of the solution may also be employed in removing the iron. I prefer, however, to precipitate the iron by adding a suitable chemical, and the chemical I employ has an added function which will be described below.
• the solution is capable of operating successfully at a given pH over a considerable range of zinc concentrations, but at a given zinc concentration the solution operates correctly only within a narrow pH band. Too high a pH, in general, causes useless precipitation of zinc phosphate, while too low a pH causes the process to become inoperative.
• the precipitating agent which I prefer to employ is a salt of nitrous acid, because such material will function not only as a medium for converting ferrous iron into insoluble ferric iron, but also as an agent for maintaining the pH of the solution substantially constant.
• Sodium nitrite is particularly suitable for the purpose, and I prefer to introduce it as a freshly prepared water solution.
• Barium nitrite or calcium nitrite may also be employed, or some other water soluble salt of nitrous acid.
• stampings as they come from the press, are generally contaminated with oil, grease, and/or drawing compound, and sometimes with rust. Unless the rusting is severe, the parts may be satisfactorily coated without removal of the rust. If severely rusted, the parts may be subjected to pickling or other de-rustlng treatment before entering the cleaning and coating system to be described.
• Grease, etc. should be removed from the articles to be coated by my process. This may be accomplished by any of the well-known conventional means such as wiping or washing with a solvent, vapor degreasing, or alkaline cleaning.
• the stampings After being properly cleaned and prepared for the coating operation, the stampings are subjected to a coating solution such, for example, as the following:
• the diluted solution is placed in a suitable tank which is preferably equipped with means for controlling the temperature of the solution.
• the temperature may vary considerably, but I have found that a temperature in the neighborhood of F. will produce excellent results.
• the above solution has an initial pH of about 2.5.
• any suitable spraying equipment may be provided, the details of which are not illustrated, because they may take various forms, dependent upon individual requirements.
• Circulation of the solution through the tanks to the spraying nozzles or jets is preferably provided for by means of a centrifugal pump, and the material which runs down from the work is collected in suitable troughs and returned to the tank or reservoir from which the solution is pumped.
• stampings to be coated are subjected to the impinging action of the jets of solution at a rate such as will provide for a treatment of from thirty seconds to one minute, although this period may vary considerably, depending upon conditions and the temperature of the solution.
• the pumping system is kept in continuous operation in order to provide for continuous circulation of the solution from the tank to the impinging nozzles or Jets, and from the jets to the work, and then back again to the tank.
• I immediately add to the solution regulated quantitles of the solution of Formula No. l, and also of a water solution of a. salt of nitrous acid, preferably sodium nitrite. These additions are made preferably at a point where the solution is in more or less violent agitation so as to insure thorough mixing with the body of the solution.
• the rate at which these solutions are introduced is regulated so as to maintain the zinc content and the pH of the coating solution substantially constant and also substantially freed of dissolved iron.
• the rate at which these solutions are introduced is regulated so as to maintain the zinc content and the pH of the coating solution substantially constant and also substantially freed of dissolved iron.
• To this and suitable determinations of the zinc content and of the pH may be made in any desired manner known to this art.
• the zinc content of the solution may be determined electrolytically or chemically.
• a convenient method consists in titrating a small sample of the solution with standard potassium ferrocyanide in accordance with the directions given in standard chemical text books.
• the pH may be determined colorimetrically or potentiometrically by any standard method.
• the potential of the glass electrode vs. a saturated calomel electrode furnishes a convenient measure of the pH of the solution.
• the zinc content of the solution may be varied over a considerable range without appreciably affecting the operation of the process, but for each zinc concentration there exists an optimum pH from which only narrow departures are allowable.
• the minimum pH (maximum hydrogen ion concentration) which is allowable to secure good coatings in the time allowable, varies a little with the type of metal to be coated, and the method of cleaning employed.
• the correct pH likewise varies with the temperature at which the coating solution is maintained.
• F'or solvent-cleaned steel values about 0.1 pH unit lower my be used to obtain excellent coatings.
• my experience has shown that a solution which operates perfectly with alkalicleaned steel will likewise operate perfectly with solvent cleaned steel, if both are reasonably free of grease and uniformly cleaned.
• stampings After the stampings have been coated they may be washed and then dried, whereupon they are ready for the application of any desired finish, such as paint, varnish, lacquer, Japan, or the like.
• the zinc restoring solution may be the concentrated solution of Formula No. l, and the sodium nitrite solution may contain about 60 grams of sodium nitrite per liter of water.
• the zinc-containing solution used for maintaining the zinc concentration of the coating solution may likewise be varied considerably in composition and in concentration. It is convenient, however, to use the same solution as was originally used for making up the original coating solution.
• the pH and the manganese content of the coating solution are maintained constant at appropriate values in a manner entirely analogous to the maintenance of the pH and the zinc content of the coating solution of the given example.
• the content of coating metal in the Working solution is determined by standard chemical methods forming no part of the present invention. Rapid methods of chemical analysis, such as titrations, are naturally to be preferred.
• My improved process is capable of giving the finest-grained and smoothest coatings on surfaces which have been alkali-cleaned and rinsed, without drying, wiping, brushing or mechanical treatment either prior to or during the coating operation, which is a distinct and marked advantage over previous processes known to me, which, for the production of the smoothest coating, require that the work be solvent-cleaned and wiped or brushed before coating, or, at least, if it be alkalicleaned, that it be dried and solvent-wiped before being coated, or that it be mechanically rubbed during the coating operation.
• This advantage makes it possible to combine the apparatus stages, in which the pre-cleaning is performed, into one continuous unit with the coating stage, which, of course, effects a corresponding saving in floor space and a marked economy in construction and equipment costs.
• the substantially continuously operable method which comprises spraying a succession of pieces with a zinc phosphate solution; collecting the run-oil for re-use in the spraying operation; and substantially continuously adding to the solution regulated quantities of zinc phosphate and of a soluble salt of nitrous acid suflicient to maintain the sprayed solution substantially constant as to zinc content and pH and also substantially free of dissolved iron.
• substantially continuously operable method which comprises spraying a succession of pieces with a zinc phosphate solution; collecting the runof! for re-use in the spraying operation; and substantially continuously adding to the solution regulated quantities of zinc phosphate solution and of an aqueous solution of sodium nitrite suflicient to maintain the sprayed solution sub stantially constant as to zinc content and pH and also substantially free of dissolved iron.
• the substantially continuously operable method which includes spraying a succession of pieces with an aqueous acid phosphate coating solution which is substantially free of dissolved iron, collecting the run-off for re-use in the spraying operation, and substantially continuously adding to the solution regulated quantities of the acid phosphate and of a soluble salt of nitrous acid sufficient to maintain the sprayed solution substantially constant as to its metal content and pH and also substantially free of dissolved iron.
• the substantially continuously operable method which includes spraying a succession of pieces with an aqueous acid phosphate coating solution which is substantially free of dissolved iron, collecting the run-off for re-use in the spraying operation, and substantially continuously adding to the solution regulated quantities of aqueous acid phosphate coating solution and of a water solution of a salt of nitrous acid sufficient to maintain the sprayed solution substantially constant as to its metal content and pH and also substantially free of dissolved iron.
• the substantially continuously operable method which comprises spraying a succession of pieces with a manganese phosphate solution; collecting the run-01f for re-use in the spraying operation; and substantially continuously adding to the solution regulated quantities oi manganese phosphate and of a soluble salt of nitrous acid sufficient to maintain the sprayed solution substantially constant as to manganese content and pH and also substantially free of dissolved iron.
• the substantially continuously operable method which comprises spraying a succession of pieces with a manganese phosphate solution; collecting the run-oflf for re-use in the spraying operation, and substantially continuously adding to the solution regulated quantities of manganese phosphate solution and of an aqueous solution of sodium nitrite sufficient to maintain the sprayed solution substantially constant as to manganese content and pH and also substantially free of dissolved iron.
• the substantially continuously operable method which comprises spraying a succession of pieces with a manganese phosphate solution; collecting the run-01f for re-use in the spraying operation; and substantially continuously adding to the solution regulated quantities oi manganese phosphate and of a soluble salt of nitrous acid sufficient to maintain the sprayed solution substantially constant as to manganese content and pH and also substantially free of dissolved iron.
• the substantially continuously operable method which comprises spraying a succession of pieces with a manganese phosphate solution; collecting the run-oflf for re-use in the spraying operation, and substantially continuously adding to the solution regulated quantities of manganese phosphate solution and of an aqueous solution of sodium nitrite sufficient to maintain the sprayed solution substantially constant as to manganese content and pH and also substantially free of dissolved iron.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)

Priority Applications (5)

Applications Claiming Priority (1)

Publications (1)

Family

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Family Applications (1)

Country Status (5)

Cited By (11)

Families Citing this family (1)

• 0
  • DE DENDAT863889D patent/DE863889C/de not_active Expired
  • BE BE427074D patent/BE427074A/xx unknown
• 1937
  • 1937-04-30 US US139947A patent/US2132883A/en not_active Expired - Lifetime
  • 1937-08-30 GB GB23669/37A patent/GB501739A/en not_active Expired
• 1938
  • 1938-04-30 FR FR847128D patent/FR847128A/fr not_active Expired

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