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
  • C23C22/05—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 using aqueous solutions
  • C23C22/06—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 using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/07—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 using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
  • C23C22/08—Orthophosphates
  • C23C22/10—Orthophosphates containing oxidants

Definitions

• the present invention relate to the art of producing a phosphate conversion coating on a ferriferous metal surface and is particularly concerned with that type of conversion coating which is produced by treating the surface of the metal with alkali metal phosphate solutions.
• alkali metal phosphate is to be understood as including both sodium and potassium phosphates as well as ammonium mono and ammonium dihydrogen phosphates.
• the alkali metal phosphate coating baths have been employed as aqueous solutions having pH values ranging from approximately 3.2 to 6.2 although, as a general rule, a somewhat narrower pH range has been employed, say from about 4.7 to 6.0.
• accelerating agents such, for example, as chlorates, nitrates, nitrites and peroxygen compounds which, as is well known, operate to reduce the time required for coating formation.
• bromates as accelerating agents not only yields heavier coating weights than are obtained with nitrites but also permits the use of lower reaction temperatures on the order of 150 to 160 F., although some sacrifice in coating weight results from the use of such lower bath temperatures.
• brornate accelerated baths often produce coatings which are mottled or otherwise uneven in appearance, which uneven appearance frequently affects the appearance of the treated metal surface after it has been given a final siccative finish such as paint or the like.
• the principal object of the present invention may be said to reside in the provision of an improved nitrite accelerated alkali metal phosphate coating process by means of which it is possible to produce, on ferriferous metal surfaces, highly corrosion resistant and extremely effective paint bonding coatings at lower temperatures than has been possible heretofore.
• a concomitant object of this invention is to provision of a solution for and a method of applying phosphate conversion coatings to ferriferous metal surfaces of more uniform appearance and of greater weight than has ever been possible heretofore with any of the nitrite accelerated alkali metal phosphate coating baths familiar to the art.
• the present invention is based upon the discovery that if at least 0.05 gram/liter of a heterocycle compound selected from the group consisting essentially of melamine, 3-amino-l,2,4-triazole, imidazol, succinimide and 2-oxazolidone is added to a nitrite accelerated alkali metal phosphate coating solution having a pH of [from 4.0 to 5.8, such solution can be employed as a coating bath in the treatment of ferriferous metal surfaces to produce an extremely uniform, highly corrosion resistant coating of substantially greater weight than heretofore possible in this art even when customary or normal treating cycles of l to 3 minutes are employed and, furthermore, that these improvements in result can be obtained at appreciably lower coating bath temperatures than have ever been practical heretofore.
• heterocyclic compound So far as the upper limit of heterocyclic compound is concerned it has been found that there is no apparent deleterious effect from the use of as much as 30 grams/ liter of these compounds. Use of even higher amounts thereof are limited in some instances by solubility considerations, but Where a particular heterocyclic compound falling within the class listed hereinabove is soluble in an infinite degree, such as for example, 3-amino-1,2,4- triazone, no limitation on the amount used has been found. In the interests of economy, however, it is preferred not to exceed about 10 grams/liter of heterocyclic compound in the coating solutions of this invention.
• Equations A and B in situations where the quantity of heterocyclic compound lies between 0.05 and 1 gram/liter the following specific example is suggested.
• melamine is the heterocyclic compound employed and is utilized in an amount of from 0.0 5 to 1 gram/liter
• the amount of nitrite ion which is required must lie between the limits determined by substituting the molecular weight of melamine (126.13) in the equations. This will give a range for the nitrite (N0 of 0.02. to 0.36 gram/liter of coating solution.
• the nitrite ion may be introduced into the alkali metal phosphate coating solution as a salt, such, for example, as an alkali or alkaline earth metal salt. Due to commercial availability the sodium salt is preferred. However, the only limitation cn the salt used is that the cation portion thereof exhibit no deleterious effect upon the coating reaction.
• nitrite titration is immediately taken and, if such titration shows the presence of nitrite ion, it is only necessary to add at least 0.05 gram/liter of a heterocyclic compound in order to restore the bath to its original and desired coating ability.
• nitrite titration shows a deficiency of this accelerator, it is preferred practice to add nitrite without adding heterocyclic compound, since results with this process show that the heterocyclic compounds are not consumed in the coat ing reaction, and that the only loss thereof is through entrainment or drag-out during coating operations.
• the alkali metal phosphate coating solutions of this invention must be maintained within a pH range of 4.0 to 5.8. Where pH values of less than 4.0 are employed the solution exhibits an undesirable etching action on the ferriferous metal surfaces thereby impairing coating formation. Conversely, where the solution pH is permitted to rise above about 5.8, the coatings produced will be found to be thin and powdery, while still further pH increases will result in no coating being produced upon the metal surfaces.
• a preferred pH range for operating the process of this invention has been found to be from 4.5 to 5.7 since optimum coatings are produced within this narrower pH range.
• Adjustment of the coating solution pH may be made, where needed, by the addition of small increments of either phosphoric acid or sodium hydroxide according to well established art practices.
• One of the outstanding improvements derived from the process of this invention is the ability to obtain coatings on ferriferous metal surfaces at temperatures of from to F.
• prior usage of alkali metal phosphate coating solutions necessitated the employment of coating temperatures of to 180 F. in order to obtain satisfactory results.
• the reduced operating temperatures made possible by the improvements of this invention represent considerable economic advantages to the coating industry.
• the 120 to 150 F. operating temperature range applies to dip, spray or roller coating applications and, while no harmful results are obtained by use of coating temperatures above 150 F., that is temperatures of 170 to 180 F., or even higher, excessive temperatures have been found to result in reduced coating weights, and such temperatures are completely unnecessary and represent an economic waste when utilizing the present process. Temperatures below the minimum of 120 F. should not be used since it has been found that the desired coating weights will not be obtained at such low temperatures within reasonable operating cycles.
• the coating cycle will provide the desired coatings when operated for as little time as 30 seconds to as much time as 5 minutes.
• the pre ferred operating cycle is from 1 to 3 minutes utilizing a temperature of from about 135 to 145 F. Use of lower treating temperatures will necessitate longer contact times, While, conversely, use of slightly higher treating temperatures will require shorter contact cycles.
• a tap water solution was prepared containing, per liter, 10 grams of monosodium phosphate (NaH PO The pH of this solution was 5.2.
• each panel was subjected to an immediate water rinse after which a dilute chromic acid rinse was utilized in accordance with well established art practice.
• a baking enamel was then applied to these panels, and following a curing cycle they were subjected to standard salt spray tests (ASTMB11757T). A resume of the corrosion resistance of these panels is presented below in Table II.
• nitrite ion (calculated as N0 within the range determined by the following equations:
• heterocyclic compound selected from the group consisting of melamine, 3-amino-l,2,4-triazole, imidazol, succinimide and 2-oxazolidone and (2) a quantity of nitrite ion (calculated as N0 which is not less than 0.05 Molecular weight of heterocyclic compound used maintaining the pH of said solution at from 4.0 to 5.8; and maintaining the temperature of the solution at not less than 120 F.
• a bath for use in forming a phosphate conversion coating on a ferriferous surface consisting essentially of:
• a bath for use in forming a phosphate conversion coating on a ferriferous surface said bath consisting essentially of:

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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)
• Chemically Coating (AREA)

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Cited By (1)

Citations (3)

• 1962
  • 1962-04-04 US US184960A patent/US3129122A/en not_active Expired - Lifetime
• 1963
  • 1963-03-21 GB GB11329/63A patent/GB963129A/en not_active Expired
  • 1963-04-01 BE BE630412D patent/BE630412A/fr unknown
  • 1963-04-02 DE DEP1269A patent/DE1269451B/de active Pending
  • 1963-04-02 FR FR930177A patent/FR1352567A/fr not_active Expired
  • 1963-04-03 NL NL291056A patent/NL123242C/nl active
  • 1963-04-03 NL NL291056D patent/NL291056A/nl unknown
  • 1963-04-04 FI FI630648A patent/FI44506C/fi active

Patent Citations (3)

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