Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc

Definitions

• the present invention relates to a method for producing a zinc-chromium alloy-plated steel sheet having excellent corrosion resistance and excellent plate adhesion.
• Galvanized steel sheets are widely used as rust-preventive steel sheets for automobiles, household electric appliances, construction materials and the like. This is effective because since a pure zinc layer is less noble relative to iron of the steel sheet, the zinc layer has a sacrificial anticorrosion effect in that the zinc is first corroded in formation of plating defects such as pinholes or the like and portions where the matrix iron is exposed by processing and these portions are covered by corrosion products, thereby preventing rusting of the steel sheet. However, the zinc layer has a fault that because pure zinc is active, the corrosion thereof very rapidly develops in a corrosive environment such as a spray of salt water or the like.
• the method disclosed in Japanese Patent Laid-Open No. 57-67188 uses an electroplating bath containing 70 to 370 g/l sulfate ion, 45 to 60 g/l nickel ion, 0.5 to 13 g/l chromium ion and 10 to 80 g/l boric acid, the bath being kept at a pH value of 1.4 to 2.
• the amount of chromium contained in the plating bath used in this method is 1.0 wt% at most, and any anticorrosion effect of chromium can hardly be expected.
• the chromium content must be further increased for improving the corrosion resistance.
• Japanese Patent Laid-Open No. 64-55398 discloses a method of producing a zinc-chromium-plated steel sheet with excellent surface quality and corrosion resistance, wherein plating is effected with a current density of at least 50 A/dm2 by using an acid plating bath containing zinc ions, trivalent chromium ions and 0.01 to 20 g/l of polyoxyalkylene derivative.
• This method permits the Cr content in the plating to be increased to about 40 wt%.
• the plated layer exhibits poor adhesion, and is thus easily peeled off from a steel sheet in both the adhesion tests below.
• Japanese Patent Laid-Open No. 1-309998 discloses a method of producing an electroplated steel sheet with excellent corrosion resistance and surface glossiness, wherein electroplating is performed by using an acid plating bath containing Cr ions and a cation polymer and having a ratio of Cr6+ ion/ Cr3+ ion of 0.1 or less.
• the specification also discloses that a quaternary amine polymer is used as the cation polymer.
• this method is capable of producing a Zn-Cr alloy-plated steel sheet, the method has the problems that the concentration of the cation polymer cannot easily be kept constant because the cation polymer is easily entrapped in the plated layer, and that although the adhesion of the layer plated with a low current density (50 A/dm2) is good, the adhesion of the plated layer obtained by plating with a current density of more than this value abruptly decreases. Further, although both Japanese Patent Laid-Open Nos. 64-55398 and 1-309998 take the amount of Cr deposition into consideration, improvements not only in corrosion resistance but also in adhesion are important problems. However, both specifications fail to describe improvement of adhesion.
• an object of the present invention is to provide a method of producing a zinc-chromium alloy-plated steel sheet having excellent plating adhesiveness and corrosion resistance after processing.
• a method of producing a zinc-chromium alloy-plated steel sheet having excellent plating adhesiveness by plating the surface of the steel sheet using an acid plating bath containing zinc ions (Zn2+) and chromium ions (Cr3+) at a molar concentration ratio of about 0.1 ⁇ Cr3+/(Zn2+ + Cr3+) ⁇ 0.9 in a total amount of at least about 0.5 mol/l within the dissolution range, and about 0.1 to 30 9/l of at least one nonionic organic additive having at least a triple bond, at a bath temperature of about 25 to 70°C and a pH of about 1.0 to 4.0 with a current density of about 50 to 200 A/dm2.
• the nonionic organic additive having at least a triple bond is expressed by either of the following formulas: wherein the number of carbon atoms which form a molecule is within the range of from about 10 to 800, wherein R1, R2, R3 and R4 each being at least one selected from a group consisting of phenyl group, naphthalene group, anthracene group, phenol group, naphthol group, anthranol group, alkyl-group adducts and/or alkylene-group adducts and/or sulfonic acid-group adducts of these groups, hydrogen, hydroxyl group, alkyl group, alkylene group, alkoxy group or its polymer, and sulfonic acid group, and wherein R is at least one selected from a group consisting of hydrogen, alkoxy group or its polymer.
• nonionic organic additives each having at least a triple bond include acetylene alcohols, acetylene glycols and derivatives thereof.
• a method of producing a zinc-chromium alloy-plated steel sheet of the present invention is described in further detail below.
• the plating bath used for Zn-Cr alloy plating in the present invention comprises Zn2+ ions and Cr3+ ions as main metal ions, which are prepared in various known ways as by dissolving as sulfates, etc.
• the total concentration of these Zn2+ ions and Cr3+ ions is at least about 0.5 mol/l within the dissolution range. Namely, with a total concentration of less than about 0.5 mol/l, the surface is easily burnt deposited. On the other hand, with a total concentration beyond the dissolution range, a solid is produced, and significant improvement of appearance color tone and uniform electrodeposition properties is not achieved.
• the Zn content in the plated layer is controlled to be about 60 wt% to 95 wt%, and the molar ratio of Cr3+/(Zn2+ + Cr3+) in the plating bath is set to a value of about 0.1 to 0.9.
• the ratio of less than about 0.1 the amount of chromium contained in the plated layer obtained cannot be increased, and thus a plated layer having excellent corrosion resistance cannot be obtained.
• the Zn content in the plated layer cannot be easily controlled to be at least about 60 wt%, thereby deteriorating the adhesion between the plated layer and the steel sheet.
• the plating bath may contain as a conductive auxiliary at least one member selected from the group consisting of K2SO4, Na2SO4, (NH4)2SO4, CaSO4 and MgSO4.
• the plating bath preferably contains at least about 10 g/l of such an auxiliary.
• the conductive auxiliary is added for improving the conductivity of the plating solution, decreasing the consumption of electric power and decreasing the burnt depositing of the surface.
• the current density is about 50 to 200 A/dm2, preferably about 70 to 150 A/dm2. With a current density of less than about 50 A/dm2, the deposition of Cr is hardly obtained, and with a current density of more than about 200 A/dm2, the surface is easily burnt deposited, thereby deteriorating the adhesion of the plated layer.
• the bath temperature is preferably about 25 to 70°C. At less than about 25°C, the adhesion between the plated layer obtained and the steel sheet deteriorates, and at more than about 70°C, the appearance tends to become black.
• the pH value is preferably about 1.0 to 4.0. With a pH value of less than about 1.0, not only the efficiency of cathodic deposition is decreased, but also the apparatus used is significantly corroded. With a pH value of more than about 4.0, precipitation of zinc hydroxide significantly occurs.
• At least one nonionic organic additive having at least a triple bond is added to the plating bath in order to obtain a Zn-Cr alloy-plated layer having excellent adhesion and a uniform alloy composition.
• the nonionic organic additive having at least a triple bond is a compound expressed by the following formulas: wherein R1, R2, R3 and R4 each being at least one selected from a group consisting of phenyl group, naphthalene group, anthracene group, phenol group, naphthol group, anthranol group, alkyl-group adducts and/or alkylene-group adducts and/or sulfonic acid-group adducts of these groups, hydrogen, hydroxyl group, alkyl group, alkylene group, alkoxy group or its polymer, and sulfonic acid group, and wherein R is at least one selected from a group consisting of hydrogen, alkoxy group or its polymer.
• the number of carbon atoms which form a molecule of the nonionic organic additive is preferably within the range of about 10 to 800, more preferably about 10 to 250.
• a carbon number of less than about 10 the formation of a complex with the metal ions contained in the plating bath becomes unstable, and a eutectoid of both metal ions cannot be easily formed due to a large change in polarization.
• a carbon number of more than about 800 a portion near the triple bond exhibits high steric hindrance, and the adhesion on the surface of the steel sheet thus significantly deteriorates, thereby causing difficulties in obtaining a plated layer with glossiness.
• the plated layer obtained by the above-described production method has a Zn content of about 60 to 95 wt%, and exhibits a uniform color tone of a white gray to silver white and more excellent plate adhesion, without forming a stripe pattern.
• the Zn-Cr alloy plating method of the present invention can be applied to Zn-Cr binary alloy electroplating and electroplating of an alloy mainly consisting of Zn and Cr, for example, Zn-Cr-P, Zn-Cr-Ni, Zn-Cr-Al2O3, Zn-Cr-Ti and Zn-Cr-Fe alloy plating.
• Electrolysis was effected by using a steel sheet as an anode in an aqueous solution containing 30 g/l of sodium hydroxide and 1 g/l of surfactant at a temperature of 60°C for 10 sec. with a current density of 20 A/dm2.
• a steel sheet was pickled in an aqueous solution of 10 g/l of sulfuric acid at a temperature of 30°C for a dipping time of 5 sec.
• a reverse OT test was performed by bending a steel sheet at 180° so that the test surface to which a cellophane tape was applied was on the inside without producing a gap in the bent portion, and was then returned to a substantially flat state.
• the plated layer rising was peeled by a cellophane tape, and the amount of the plated layer peeled was measured by fluorescent X-rays.
• the powdering resistance was evaluated on the basis of the following criteria: Peeling amount Evaluation Symbol 10 mg/m2 or less o 10 to 100 mg/m2 ⁇ 100 to 1000 mg/m2 ⁇ 1000 mg/m2 or more X
• a zinc-chromium alloy-plated steel sheet was cut in a size of 75 x 150 mmand was subjected to phophating, electrodeposition coating, intermediate coating and final coating.
• the time taken until rust occurred was examined by a composite cycle corrosion test (CCT) comprising spraying salt water for 4 hr, drying at 60°C for 2 hr and humidity at 50°C for 2 hr.
• CCT composite cycle corrosion test
• the corrosion resistance was evaluated on the basis of the following criteria: Time to occurrence of rust Evaluation symbol 100 days or more o 50 to 100 days ⁇ 20 to 50 days ⁇ 20 days or less X
• the zinc-chromium alloy-plated steel sheet obtained was visually evaluated on the basis of the following criteria: Color tone Evaluation symbol White ⁇ Gray ⁇ Black or at least two tones X
• a zinc-chromium alloy-plated steel sheet was produced by plating the same steel sheet as that used in Examples 1 to 37 under the same conditions with the exception that Fe2+, Ni2+, Co2+, Al2O3, SiO2 or TiO2 was added in an amount shown in Tables 4-1, 4-2, 5-1 and 5-2 to produce a zinc-chromium alloy-plated steel sheet with a plated layer containing one of the above substances.
• the powdering resistance and the corrosion resistance after processing were evaluated under the above-described conditions. The results obtained are shown in Tables 4-1, 4-2, 5-1 and 5-2.
• TMDD indicates 2,4,7,9-tetramethyl-5-decyne-4,7-diol
• TMDDE an ethylene oxide addition product of TMDD
• the use of the organic additive disclosed in the present invention permits the formation of a zinc-chromium alloy-plated steel sheet having excellent plate adhesion and excellent corrosion resistance.
• the method of the present invention uses a plating bath with excellent stability, and thus permits stable production of a plated steel sheet on an industrial scale. It is very significant that the present invention enables the industrial production of a zinc-chromium alloy-plated steel sheet having excellent plate adhesion and excellent corrosion resistance.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
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• Organic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Electroplating Methods And Accessories (AREA)

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• 1993
  • 1993-04-15 EP EP93106155A patent/EP0566121B1/de not_active Expired - Lifetime
  • 1993-04-15 DE DE69311833T patent/DE69311833T2/de not_active Expired - Fee Related
  • 1993-04-16 KR KR1019930006489A patent/KR960001036B1/ko not_active IP Right Cessation

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