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
  • C23C22/12—Orthophosphates containing zinc cations
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/36—Phosphatising
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/02—Etching
  • C25F3/06—Etching of iron or steel
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/26—Web or sheet containing structurally defined element or component, the element or component having a specified physical dimension
  • Y10T428/263—Coating layer not in excess of 5 mils thick or equivalent
  • Y10T428/264—Up to 3 mils
  • Y10T428/265—1 mil or less

Definitions

• the invention relates to a phosphatizing treatment for iron or iron alloy products wherein electrochemical phenomena are used as occur through the passage of an electric current through the product.
• the invention also embraces iron or iron alloy products formed by the process described which are characterized by a protective phosphate coating that is high in iron adhering to its metal surface and a superposed zinc phosphate coating adhering to said first coating.
• the process of the invention comprises providing, at the metal-solution interface for conditions that favor the formation of a subcoating of iron phosphate on top of which a second coating of zinc phosphate is being deposited as soon as the iron contents of the solution is substantially reduced.
• the potential difference is maintained during a time suflicient to permit the passage of an amount of current through the electrolytic cell formed which is between 2 and 5 coulombs per cm. of surface to be treated.
• the products of the invention may be unfinished, halffinished or finished and consist of iron or iron-base alloys. They are characterized by a protective corrosion-resistant coating formed by the phosphatizing process of the invention and comprising a protective first coating constituted by a phosphate which is high in iron and adheres directly to the metal and a second superposed coating which adheres to the first coating and is constituted by a zinc phosphate.
• phosphate coatings offer a resistance against corrosion which is clearly higher when the zinc phosphate coating is formed on top of a subcoating of iron phosphate.
• the general principle of the phosphatizing of metals comprises a simple immersion in a phosphate bath.
• the mechanism of the phosphatizing treatment on one hand is an electrochemical process which arises between the metal and the bath and in the course of which the pH of the solution develops in the neighborhood of the metal surface.
• This stage is then followed by a second purely chemical stage which results in the precipitation of the zinc phosphate on the treated surface after the pH of the solution has reached the pH necessary for the phosphate precipitation.
• Formation of the zinc phosphate coating thus depends narrowly on the uncontrolled electrochemical reaction which takes place between certain preferred points of the metal.
• This electrochemical reaction is behind the imbalance of the pH in the solution and consequently the precipitation reaction of the phosphate.
• the thus-obtained protection is due to the presence of a coating which is rich in Zinc phosphate at the surface of the metal.
• the amount of electricity which must circulate in the electrolytic circuit of which the products form the anodes is determined as a function of the initial surface condition of the metal of the products.
• the surface is irregular, oxidized or covered with scale, it may be necessary to effect a more strenuous anodic dissolution in order to obtain a surface condition which is favorable to the formation of regular uniform phosphate coatings i
• the invention and the advantages arising therefrom will be easier understood in the light of the following example and test results comparing products made by the inventions with products made by conventional processes.
• EXAMPLE Phosphatizing tests were carried out with specimens formed in a steel, that had a surface condition as rolled with only a light finishing pass.
• the criterion of the coarseness termed C.L.A. corresponded to 1.2.
• the phosphatizing treatment had been effected in these tests in the conventional phosphatizing bath with a zinc basis which had been accelerated by the addition of nitrates (7 g./ liter (NO Zn) and which bath contained 1.1% of P and 0.7% of ZnO.
• the ratio of free acidity and total acidity in this bath was of the order of 1:4.
• the specimens were immersed-in the bath while the bath was maintained at a temperature of 95 C. and were anodically polarized by means of an external source of current with a voltage of about plus 0.2 volt, measured by relation to a standard satured calomel electrode.
• the current density under the conditions of the test was about 30 to 40 ma./cm.
• the current application was continued for about one minute, which corresponded to the passage of an amount of current between 1.8 and 2.4 coulombs per cm.
• the thickness of the protective coatings thus obtained with the process of the invention was about 15 while in the conventional method the coatings had a thickness between 20 and 25p.
• specimens which had been treated by the process of the invention and control specimens which had been phosphatized by simple immersion were subjected to exhaustive corrosion tests. It should be emphasized that these tests were carried out without subjecting the specimens to any supplemental protective coatings such as a rinsing and passivation with chromic acid, oil impregnation or application of protective paints.
• ELECTROCHEMICAL CORROSION TESTS This test comprised a measurement of the potential at which cracking of the phosphate coating would occur using a specimen which had been immersed in an electrolyte of sodium phosphate and chlorine ions. The cracking potential measurement permitted to determine the resistance to the corrosion of the phosphate coating.
• the electrolyte used for this test contained 0.1 M of Na P-O H, l2 H 0 and 0.1 M of NaCl.
• the amount of current passing through the specimen in a test period of 30 minutes was found to be 3 coulombs for a conventionally phosphated steel and 0.6 coulomb, that is five times less, for specimens which were phosphatized with a preceding polarization. This confirms the results earlier given.
• the process of the invention has numerous advantages. For instance, among others, there is the advantage that the preliminary polarization treatment 'by its superficiary dissolving of the metal of the treated product causes a pickling which extends to the entire surface and thus permits to treat the product irrespective of scale or previous rusting directly by the method of the invention.
• Such immediate treatment would not be possible with the conventional methods which would require a preparation of the surfaces intended to be phosphatized because zones which are not sutficiently clean from contamination points of lower resistance and thus of lowered protection against corrosion.
• a preceding chemical pickling treatment results in the formation of phosphate coatings having a coarse grain and that the protective value of these coatings is rather mediocre.
• Another advantage of the anodic polarization is the activation of the surface to be protected.
• the active centers where the precipitation reaction is started oil? are increased and, likewise, a much finer crystallization and more regular crystallization is obtained in the phosphate coatings.
• the corrosion resistance is thus appreciably increased.
• the coating which has a finely crystalled structure has a much better adherence for outer coatings like pains and has a rather weak absorptive power for coloring agents.
• the improved behavior against corrosion in the coatings of the invention is found to be obtained irrespective what the composition may be of the phosphatizing bath.
• the process of the invention also permits to phosphatize metals which have undergone a surface passivation treatment, particularly alloy steels and light alloys.
• a process of forming a protective coating of a metal phosphate on iron or iron alloy products comprising the steps of immersing the product in a phosphatizing bath containing zinc in solution; applying a sustained potential ditference between the product as anode and a counterelectrode as cathode and thereby exerting a superficiary dissolving action on the iron in said product so as to increase the number of iron ions at the metal productsolution interface; then terminating the said potential difference and thereby causing a first phosphate coating to form at the surface of the product by virtue of the iron ions initially being present at said interface, the said first phosphate coating being high in iron content; a second phosphate coating forming on top of said first phosphate coating after substantial reduction of the number of iron ions due toformation of said iron phosphate coating, the said second coating being high in zinc content by virtue of the zinc initially introduced in said phosphatizing bath.
• a finished, semi-finished or unfinished article consisting essentially of iron and an iron-base alloy and having a protective phosphate coating that is high in iron adhering to its metal surface and having a superposed zinc phosphate coating adhering to said first coating, the said article being produced by the process of claim -1.

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

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Citations (7)

• 1967
  • 1967-12-04 FR FR130842A patent/FR1554824A/fr not_active Expired
• 1968
  • 1968-11-22 GB GB55526/68A patent/GB1245286A/en not_active Expired
  • 1968-11-26 BE BE724457D patent/BE724457A/xx unknown
  • 1968-11-27 LU LU57403D patent/LU57403A1/xx unknown
  • 1968-12-03 US US780893A patent/US3544390A/en not_active Expired - Lifetime

Patent Citations (7)

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