Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
  • B05D7/14—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to metal, e.g. car bodies
  • B05D7/142—Auto-deposited coatings, i.e. autophoretic coatings
  • B05D7/144—After-treatment of auto-deposited coatings

Definitions

• the invention relates to a method for improving the corrosion protection of autophoretically deposited resin coatings on metal surfaces.
• Inorganic pigments can improve the corrosion protection autophoretically on different metal surfaces of deposited coatings. This fact is known from various publications.
• US Pat. No. 4,030,945 discloses a method in which metal surfaces are rinsed after the autophoretic coating with organic resins with dilute aqueous solutions which contain chromium (VI) or combinations of chromium (VI) with reduced chromium compounds.
• chromium (VI) compounds such as e.g. Chromic acid, potassium dichromate, magnesium dichromate, sodium chromate or potassium chromate can be used, the respective chromates also being able to be formed in situ from chromic acid and water-soluble salts of the respective metals.
• the treatment improving the corrosion protection of metal surfaces coated with organic coatings using aqueous chromate solutions is also proposed in US Pat. No. 4,186,226. Sodium dichromate is preferably used in the process according to this document.
• the inorganic pigments which improve the corrosion protection directly to the coating bath and to deposit them with the organic resin layer on the metal surface.
• Barium, strontium, zinc or lead compounds are used as corrosion protection pigments, the chromates of the metals mentioned being preferably used. Without exception, these chromates are poorly soluble in water.
• the autophoretic coating process is based on the fact that the acidic aqueous resin dispersion stains the metal surface to be coated and metal ions of the metal surface to be coated thereby dissolve. These positive charge carriers coagulate the stabilized resin dispersion in the vicinity of the metal surface, which causes a homogeneous coating with the organic resin without current.
• the corrosion protection pigments mentioned Due to the low pH of this coating process (between 1.5 and 4.0), the corrosion protection pigments mentioned are more or less quickly converted into a soluble form in which they are deposited at the same time as the organic resin particles.
• the metal cations present in acidic aqueous solution undesirably contribute to an increased coagulation of the resin dispersion, which under certain circumstances can even lead to a breakdown of the stable resin dispersion through complete coagulation.
• the object of the invention was not only to overcome the disadvantage of using such anti-corrosion pigments directly in autophoretic coating baths, but also to provide a method for introducing metal salts into autophoretically deposited coating layers len, in that metal salts that are readily soluble in neutral aqueous media can be rinsed into the deposited resin layers without incorporating unwanted foreign ions into the organic coatings.
• the invention thus relates to a method for improving the corrosion protection of autophoretically deposited resin coatings on metal surfaces, which is characterized in that the mechanically and / or chemically cleaned metal surfaces which are autophoretically coated with any organic resin and, if desired, water-rinsed with a organic resin are cleaned aqueous solution of 0.5 to 10% by weight of a readily water-soluble metal dichromate or metal hypophosphite and, if appropriate, additionally a well water-soluble metal dihydrogen phosphate, and then drying the resin coating layer containing corrosion protection pigments in a manner known per se and / or baking at elevated temperature.
• Metal surfaces that can be better protected against corrosion with the method according to the invention using metal dichromates or metal hypophosphites are surfaces made of iron, steel, zinc, aluminum or their alloys. Surfaces which are coated with one of the metals mentioned or their alloys are also suitable.
• organic resins A variety of resin materials known from numerous publications can be used as organic resins applied autophoretically to the metal surfaces.
• organic coating resin materials are, for example, polyethylene, polyacrylates, styrene-butadiene copolymers, vinyl chloride / vinylidene chloride copolymers or similar resins.
• these polymeric materials are dispersed in water together with suitable auxiliaries, using known methods and methods, and electrolessly deposited on the metal surfaces which are mechanically and / or chemically cleaned in the usual manner and electrolessly. If desired, the fresh coatings can be rinsed with water immediately after the actual coating reaction.
• the metal surfaces are then brought into contact with an aqueous solution of a readily water-soluble metal dichromate or metal hypophosphite.
• a readily water-soluble metal dichromate or metal hypophosphite a readily water-soluble metal dichromate or metal hypophosphite.
• Calcium dichromate, strontium dichromate, iron (III) dichromate, copper (II) dichromate, zinc dichromate and cadmium dichromate can be used as water-soluble metal dichromates. Of these, calcium dichromate, strontium dichromate and cadmium dichromate are preferably used.
• the dichromate solutions of the so-called motalle used for the process according to the invention are obtained according to J. Schulze, Zeitschrift für inorganicische Chemie 10, 148 (1895) in that the corresponding metal hydroxides or carbonates are mixed with an aqueous CrO 3 solution in a molar ratio of 1 : 2 added, the corresponding metal hydroxides or carbonates thereby dissolved and the aqueous dichromate solutions of the metals mentioned thus obtained immediately used.
• aqueous metal dichromate solutions can be used to improve corrosion protection, but also aqueous solutions of metal hypophosphites.
• Hypophosphites from the group barium hypophosphite, manganese hypophosphite, nickel hypophosphite, zinc hypophosphite and cadmium hypophosphite can be used according to the invention.
• the hypophosphites of nickel and barium are preferably used.
• aqueous hypophosphate solutions used to improve the corrosion protection of autophoretically coated metal surfaces were prepared by mixing the corresponding water-soluble metal hydroxides or sulfates with an aqueous solution of the easily produced Ea (H 2 PO 2 ) 2 . xH 2 O were implemented and thereby converted into the respective metal hypophosphite.
• slurries of the corresponding metal hydroxides can also be reacted directly with hypophosphorous acid in a molar ratio of 1: 1. This produces aqueous solutions of metal hypophosphite hydrates which were used directly in the process according to the invention.
• the metal surfaces coated with an organic resin coating are optionally rinsed with water after the actual coating reaction and then brought into contact with the aqueous metal dichromate or metal hypophosphite solutions mentioned. This can be done by immersing the metal surfaces in the pigment solutions or spraying the metal surfaces with appropriate pigment solutions, or also by means of combined immersion / spray processes.
• the solutions have a metal dichromate or metal hypophosphite content of 0.5 to 10% by weight. Solutions with a content of 2 to 6% by weight are preferably used.
• the metal surfaces are then treated in a manner known per se.
• the aftertreatment generally consists in drying the metal surfaces provided with a resin-pigment layer and / or baking them in at an elevated temperature. This forms a homogeneous organic layer in which the pigments mentioned are embedded.
• the corrosion protection achieved by adding the pigments mentioned can be further improved if metal dihydrogen phosphates are added to the pigment solutions in addition to the metal dichromates or metal hypophosphites.
• Dihydrogen phosphates of the metals calcium, strontium, barium, manganese, iron, nickel, copper, zinc, cadmium or lead are preferably used.
• These salts are in the pigment solutions in an amount of 0.5 to 10 wt .-%, preferably in an amount of 2 to 6 wt .-%, dissolved, which can further improve the corrosion protection effect of the resulting solutions.
• the metal surfaces rinsed with the pigment solutions described are much better protected against corrosion than metal surfaces whose organic resin layer has been rinsed with a chromic acid solution according to the prior art.
• Another advantage of the method according to the invention is that when using the metal dichromates or hypophosphites in the rinse cycle no foreign ions are introduced into the coating bath and the risk of the dispersion collapsing due to excessive concentration of positive charge carriers does not arise in the first place.
• the corresponding metal hydroxides or carbonates were used as starting materials for the production of various metal dichromate solutions. Metals that were not in the form of hydroxides or carbonates were converted into the corresponding hydroxides.
• the steel surfaces were then held at 160 ° C for 15 minutes.
• the quality of the corrosion protection was determined in accordance with ASTM standard D 1654-74.
• the metal surfaces were scratched diagonally and subjected to a salt spray test for 500 h.
• the extent of the corrosion was then determined on a value scale from 0 to 10, the value 10 standing for complete absence of corrosion.
• a polymer emulsion was prepared according to Example 4 of US Pat. No. 4,313,861, which contained 37% acrylonitrile, 58% butyl acrylate and 5% methacrylic acid, a solids content of 41.6%, a Brookfield viscosity (25 ° C.) of 0.015 Pa . s (15 cps) and had a pH of 4.1.
• Steel sheets were coated with this emulsion according to Example 2, treated with the solutions given in Example 2 or the comparison solution, subjected to the salt spray test (500 h) and evaluated. The results are shown in Table 2 below.
• aqueous solutions containing metal hypophosphites either the corresponding metal hydroxides were used and converted to the hypophosphites with hypophosphorous acid, or water-soluble metal sulfates with the easily produced Ba (H 2 PO 2 ) 2 . xH 2 O converted into the corresponding metal hypophosphite.
• the metal hypophosphites obtained were not isolated from the solution.
• 10 to 20% aqueous metal hydroxide slurries were mixed with hypophosphorous acid in a molar ratio of 1: 2. 63.1 g (0.2 mol) of Ba (OH) 2 .
• hypophosphite hydrates of the manganese, nickel and cadmium were prepared from the corresponding water-soluble sulfates.
• half of the barium hypophosphite hydrate solution prepared as described above was mixed with 28.09 g of NiSO 4 . 7H 2 0 in 100 ml H 2 0 added in small portions.
• the precipitated BaSO 4 was separated off in a cup centrifuge.
• the clear solution contained 5.55% H 3 PO 2 as a salt.
• hypophosphite hydrates of manganese, zinc and cadmium were prepared accordingly.
• the metal surfaces rinsed with aqueous hypophosphite solutions apparently show significantly better corrosion protection values than the metal surfaces rinsed only with water.
• the polymer films treated with the solutions mentioned were baked at 100 ° C. for 30 minutes, the surfaces formed were scored and subjected to a salt spray test for 500 hours.
• the evaluation of the corrosion protection was carried out according to the evaluation criteria given in Example 2; Table 4 shows the corrosion protection values obtained.
• the coatings thus produced exceeded the corresponding solutions containing only metal dichromates (Examples 2 and 3) in the salt spray test over 500 h and were clearly superior to them even after a 1000 h salt spray test.
• the coatings thus produced exceeded the corresponding solutions containing only metal dichromates (Examples 2 and 3) in the salt spray test over 500 h and were clearly superior to them even after a 1000 h salt spray test.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Chemical Treatment Of Metals (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Detergent Compositions (AREA)

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• 1984
  • 1984-11-26 DE DE19843442985 patent/DE3442985A1/de not_active Withdrawn
• 1985
  • 1985-11-18 EP EP85114638A patent/EP0183161A3/fr not_active Withdrawn
  • 1985-11-26 US US06/802,643 patent/US4636265A/en not_active Expired - Fee Related
  • 1985-11-26 JP JP60267060A patent/JPS61133167A/ja active Pending
  • 1985-11-26 CA CA000496170A patent/CA1243564A/fr not_active Expired

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