Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D13/00—Electrophoretic coating characterised by the process
  • C25D13/20—Pretreatment

Definitions

• the invention relates to a method for electrophoretic dip coating of iron or steel surfaces coated with zinc or cadmium.
• Corrosion protection which is retained even in the event of minor damage to the protective layer, can be provided by galvanizing. Due to the thick zinc layer of typically 50 to 200 ⁇ m, hot galvanizing of steel parts together with the iron-zinc alloy as a transition layer of zinc on the surface to the base material steel provides excellent corrosion protection.
• galvanizing zinc layers are much smaller.
• galvanized steel sheets have no brittle intermediate layers made of iron-zinc alloys.
• Galvanizing is the only cost-effective method a cathodic corrosion protection of the steel, which leads to the fact that the corrosion of the steel is electrochemically suppressed even by scratches up to a width of approx. 0.7 mm by the zinc layer.
• the zinc coating as the sole protection for steel is only a completely inadequate corrosion protection due to the small layer thicknesses that are applied in galvanic processes (10 ⁇ m on average). This applies in particular to shiny, smooth layers. These are not even close to hand.
• the corrosion protection found in the salt spray test according to DIN 50021 is only minimal; after about 6 to 8 hours the zinc layer is corroded away and brown rust appears.
• a significant improvement in corrosion protection can be achieved by chromating or phosphating the zinc surface.
• times of 24 hours are usual for blue chromates and up to about 200 hours for yellow chromates until the first white rust appears.
• Phosphating on galvanic zinc layers also increases corrosion protection. However, these surfaces are rough and, if damaged, do not show the self-healing mechanism that is known from chromating. Therefore, phosphating is only used as a primer for subsequent painting. In the salt spray test, these layers are corrosion-resistant for up to 150 hours of testing.
• the protective effect of chromating can be increased by dipping freshly chromated parts in special aqueous so-called sealing solutions, which has a significant effect in the salt spray test.
• galvanized metal parts are chromated, rinsed and then rinsed wet in a dispersion of Dipped plastics, whereby an elastic, intimately interlocking composite layer of uneven thickness of chromating and plastic is obtained, which provides good protection against corrosion.
• DE-AS 15 21 656 describes the chromating of zinc and zinc alloys. After rinsing and subsequent air drying, the chromated metal surfaces can be given a siccative coating based on so-called drying oils (unsaturated fatty acids) that crosslink with the absorption of oxygen from the air.
• drying oils unsaturated fatty acids
• Electrocoating is particularly suitable since this process produces layers of uniform thickness, so that high demands on the dimensional accuracy of the painted parts can be met.
• critical cavities such as blind holes or inner pipe walls are also continuously painted. These inaccessible areas are usually well protected against injuries from external influences. Electrophoretic dip painting of galvanized steel parts can therefore lead to a significant improvement in corrosion protection.
• the adhesion between zinc and the organically structured paints is very problematic. It has been shown that the paint flakes off within a short time under exposure to weather and / or mechanical stresses. Adhesion mediation is therefore required for the paint application. Adhesion mediation is also necessary in any case if metals with similar properties to zinc, for example cadmium or aluminum, are to be painted. An adhesion promoter is also often used for steel.
• EP-A-0 328 304 (relevant under Article 54 (3) EPC) describes a process for the chromating and subsequent electrophoretic dip coating of galvanized steel.
• JP-A-60082696 relates to a process for the chromating and subsequent anodic dip coating of galvanized iron or iron alloys.
• the object of the invention is to provide a method by which electro-dipping paints on chromated with zinc or cadmium over drawn iron or steel surfaces with much better adhesion than the state of the art can be deposited cataphoretically and by which the mentioned disadvantages of phosphating for promoting adhesion can be avoided.
• chromating layers can be used as adhesion promoters for electrocoat materials, provided that these are kept in the wet state after production until cataphoretic electrocoating.
• freshly produced, still wet chromate layers have a hydrophilic surface which is suitable for electrocoating. This suitability is maintained if the freshly produced chromate layers are kept wet until the electrocoating or are stored in a moist place. After the chromated surfaces have dried, electro-dipping paints can only be deposited with poor adhesion.
• chromating is applied as an adhesive base before the cathodic electrocoating.
• such chromating has a smooth, shiny surface. If the chromated surface is kept wet according to the invention, the surface has such a high surface tension that when wetted with water at the interface, that is, at the edge of the water drop, a contact angle of the order of 0 degrees is produced. If drying takes place, this contact angle is greatly increased, for example to 20 to 50 degrees, which results in poor wettability.
• the high-quality deposition of the electrocoating material on the chromating layer can be ensured by the fact that the lacquer is deposited directly after the chromating without intermediate drying.
• the high-quality deposition of the electrocoating material on the chromating layer can also be ensured by keeping the chromating layers permanently moist until the beginning of the lacquer deposition by spraying with water or by storing in air with a high relative humidity. Spraying with water is particularly suitable if the entire surface can be sprayed.
• a high relative humidity is to be understood as a humidity which is required so that no water can evaporate from the chromated surface and drying out is prevented. It depends on the length of time that must be bridged before the paint deposition begins. In general, for example, a relative humidity of> 90% can be assumed; with long storage it can be up to 100%.
• the process according to the invention can improve the adhesion of coatings deposited by cataphoretic dip coating to iron or steel surfaces coated with zinc or cadmium.
• chromating of the metal surfaces is carried out in the usual manner known to the person skilled in the art. Any known chromating process can be used, as described, for example, in the book T.W. Jelinek, "Galvanic galvanizing", Leuze Verlag 1982, is described.
• the chromating is preferably carried out using a chromating solution which consists only of inorganic components.
• a chromating solution has, for example, a concentration of 1 to 10 g / l chromic acid (H2CrO4), in particular about 4 g / l chromic acid at a pH of 0 to 3, preferably 2.3 to 2.7. It is cheap but not necessary if the chromating solution contains one or more salts of the metal to be chromated. Examples of such salts are chlorides, nitrates and / or fluorides.
• the concentration of such salts which may be present is, for example, 0.001 to 0.1 mol / l, preferably 0.05 mol / l.
• the pH of a chromate solution to be freshly prepared can be adjusted, for example, with an oxide or hydroxide of the metal to be chromated. It can be checked during operation by measurement, for example with a glass electrode or by conductivity measurement, and set again to the desired value by adding acid, oxide or hydroxide.
• chromations represent the last layer, the so-called "finish" for chromatable metals and are optimized for this purpose; ie they offer some protection against corrosion (with zinc: yellow and olive chromating) or improve the appearance (with zinc: blue and black chromating).
• finish for chromatable metals and are optimized for this purpose; ie they offer some protection against corrosion (with zinc: yellow and olive chromating) or improve the appearance (with zinc: blue and black chromating).
• other aspects such as environmental friendliness, long service life, regenerability etc., which do not directly affect the quality of the chromating layer, are currently hardly considered.
• the chromation is only required to impart adhesion, the requirements for the appearance and, in part, also for the corrosion protection are met by the paint or by the combination of galvanically deposited zinc layer / paint layer.
• chromating solutions that contain only inorganic components are particularly suitable, since they can be regenerated by the process known from DE-PS 31 38 503.
• disruptive degradation products are removed from the chromating solutions with the aid of ion exchangers, electrodialysis, electrolysis or chemical oxidation, the pH or conductivity are measured during operation, the Cr6+ - and Cr3+- Concentrations are determined photometrically and supplementary solutions are added in accordance with these analysis values and the flow through the ion exchangers or the separation, exchange or reaction devices are regulated in such a way that the composition of the chromating solution is kept within a predetermined concentration range.
• Very low-concentration chromating solutions are sufficient to promote the adhesion between galvanically deposited zinc and electrocoat. Because the chromate solution is carried away with the chromated metal parts, undesired degradation products in the chromate solutions cannot be concentrated to disturbing values. On removal of the unwanted degradation products, as described in DE-PS 31 38 503 e.g. is provided with the help of ion exchangers, can therefore be dispensed with; it is sufficient to supplement the compensation of the chromate solution that has been carried out.
• the known chromating processes can also be used which work without hexavalent chromium, that is to say without chromate.
• These methods are also familiar to the person skilled in the art and are described, for example, in the above-mentioned book by TW Jelinek.
• Such Chromating is badly suited for corrosion protection purposes, but it gives good adhesion and has the advantage that the baths used are environmentally friendly because they do not contain hexavalent chromium.
• Such baths contain chromium (III) salts, such as potassium chromium sulfate; they can contain acids such as nitric acid and salts such as fluorides, for example ammonium hydrogen fluoride. As described above, they can be regenerated simply by adding supplementary solution.
• Rinsing with water can be beneficial to remove excess chromating solution immediately after chromating. Whether such a rinsing process is carried out depends on the concentrations of the chemical compounds and ions used in the chromating solution and on the procedure. With electrophoretic paint deposition, as few ions as possible should be carried over into the paint bath. If the chromated metal parts are to be introduced into the electrodeposition bath without delay, a rinsing process can be carried out before the, regardless of the composition of the chromating solution used Electrocoating can be particularly cheap. If, on the other hand, the chromated metal parts are kept wet by spraying with water until they are introduced into the electrodeposition bath, an additional rinsing process can be dispensed with if the chromating solution is suitably composed.
• metal surfaces pretreated according to the invention by chromating and wet holding can be coated or painted by conventional cataphoretic dip coating. All customary cataphoretically depositable coating agents or electrophoretic dip coating processes which are familiar to the person skilled in the art are suitable.
• lacquer layers produced in the manner according to the invention are glossy, smooth and non-porous and provide excellent protection against corrosion.
• These lacquer layers can serve, for example, as primers, on which conventional further processing with, for example, fillers and topcoats can take place.
• the chromating layer is almost transparent and leads to very good adhesion of the lacquer layer.
• the paint layer is shiny, smooth, even, non-porous and provides good protection against corrosion.
• the chromating solution shows no self-decomposition.
• decomposition products do not accumulate to a disruptive concentration; cleaning of the solution using a cation exchanger is therefore unnecessary.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)
• Paints Or Removers (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Electrochromic Elements, Electrophoresis, Or Variable Reflection Or Absorption Elements (AREA)
• Devices For Indicating Variable Information By Combining Individual Elements (AREA)

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• 1988
  • 1988-09-29 DE DE3833119A patent/DE3833119A1/de active Granted
• 1989
  • 1989-09-28 BR BR8904920-9A patent/BR8904920A/pt not_active IP Right Cessation
  • 1989-09-29 DE DE58909586T patent/DE58909586D1/de not_active Expired - Fee Related
  • 1989-09-29 AT AT89118116T patent/ATE133723T1/de not_active IP Right Cessation
  • 1989-09-29 ES ES89118116T patent/ES2093611T3/es not_active Expired - Lifetime
  • 1989-09-29 MX MX017770A patent/MX173144B/es unknown
  • 1989-09-29 CA CA000614601A patent/CA1339915C/en not_active Expired - Fee Related
  • 1989-09-29 EP EP89118116A patent/EP0366941B1/de not_active Expired - Lifetime
• 1994
  • 1994-09-28 US US08/314,612 patent/US5707505A/en not_active Expired - Fee Related

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