US3785940A - Method for electrolytically treating the surface of a steel plate with a chromate solution - Google Patents

Method for electrolytically treating the surface of a steel plate with a chromate solution Download PDF

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Publication number
US3785940A
US3785940A US00321823A US32182373A US3785940A US 3785940 A US3785940 A US 3785940A US 00321823 A US00321823 A US 00321823A US 32182373 A US32182373 A US 32182373A US 3785940 A US3785940 A US 3785940A
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electrolyte
layer
steel plate
paint
chromium
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US00321823A
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English (en)
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H Yamagishi
H Takano
M Takeuchi
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JFE Engineering Corp
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Nippon Kokan Ltd
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    • 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
    • C25D11/38—Chromatising

Definitions

  • This invention relates to a method for electrolytically treating the surface of a steel plate with a solution of chromate so as to coat the surface of steel plate with a duplex layer having an excellent property of rendering the steel surface very resistant to corrosion and elevating the adhesivity of paint thereto.
  • Japanese Pat. No. 269,930 proposes aromatic sulfonic acid or salts thereof, Japanese Pat. No. 524,872 (the corresponding US. Pat. No. 3,484,347) thiocyanic compounds, and Japanese patent publication No. 2,768/68 sulfuric acid or salts thereof.
  • a steel plate whose surface is electrolytically treated with a solution of chromate by the known methods has an excellent appearance and a strong corrosion resistance, but is still handicapped by the slightly low adhesivity of paint to said surface. Where a steel plate is subjected to very deep drawing after its surface is coated with paint, part of the coated paint unavoidably comes off.
  • the present invention has been accomplished for further improvement of the foregoing method (US. Pat. No. 3,679,554).
  • the present invention is characterized in that a second promoter consisting of aluminium sodium fluoride AlF '3NaF or aluminium potassium fluoride AIF '3KF is substituted for that of the aforesaid hydrosilicofluoric or borofluoric compounds, thereby offering the advantage of permitting the growth of a thinner, more uniform and compact layer of hydrated chromium oxide than has been possible with immediately preceding invention, substantially preventing the depletion of the anode, attaining the higher electrolytic efficiency of forming a base layer of metallic chromium than has been realized by any of the former inventions and reducing the time of electrolysis with the resultant elevation of productivity.
  • FIG. 1 is a curve diagram showing the relationship of the thickness of a base layer of metallic chromium (said thickness is expressed in the amount of metallic chromium per unit surface area of steel material) included in a duplex layer deposited on said steel surface and the concentrations of the first and second promoters, as combined between the case Where the method of this invention was used and the other case;
  • FIG. 2 is a curve diagram showing the relationship of the thickness of a layer of hydrated chromium oxide constituting the upper portion of the duplex layer and the concentration of the second promoter, with that of the first promoter kept unchanged;
  • FIG. 3 is a curve diagram showing the relationship of the results of the screw cap test for the peel off of paint coated on the surface of steel plate as compared between the case where the method of this invention was applied and the other case.
  • the method had the drawbacks that absence of compounds having a thiocyanic group in an electrolyte caused a layer of metallic chromium constituting the base portion of a duplex layer deposited on the surface of iron and steel material to have a very small thickness, but instead, a layer of hydrated chromium oxide forming the upper portion of said duplex layer to have an unduly large thickness, and that, though the surface treated product displayed an elevated corrosion resistance, yet the paint coated on the surface of said product indicated too low adhesivity to effect the deep drawing of the product under a satisfactorily coated condition.
  • An electrolyte according to the method of this invention should preferably contain to 150 g./l. of chromium VI oxide. Where the concentration of the chromium VI oxide exceeds said range, a layer of hydrated chromium oxide is not ubstantially formed. Where said concentration grows smaller than said range, said layer becomes exclusively thick, causing the surface of steel material to be undesirably tinted yellow or orange.
  • the proportion of a thiocyanic compound contained in an electrolyte according to this invention is chosen to account for to of the main agent of chromium oxide. Formation of a layer of metallic chromium on the surface of steel material presents considerable difliculties where the electrolyte only consists of chromium oxide whose concentration falls within the aforesaid range of 10 to 150 g./l. Deposition of a layer of metallic chromium is made possible only when the electrolyte contains the aforesaid thiocyanic compound.
  • the electrolytic etficiency of depositing a layer of metallic chromium varies with the ratio of the con centration of the thiocyanic or SCN group to that of the chromium oxide CrO
  • a maximum electrolyzing efliciency is about percent on an average under the electrolyzing conditions where the temperature is set at 50 C. and the current density at the cathode is a./dm. Said efficiency little varies from that of ordinary electrolytic plating of chromium metal.
  • the electrolyte contains only a thiocyanic compound as a promoter
  • a layer of hydrated chromium oxide deposited on the surface of steel material is very compact and strongly resistant to corrosion.
  • said layer of hydrated chromium oxide is unduly thick to be tinted blue, preventing paint-coated steel material from being subjected to deep drawing under a satisfactory condition, namely, without the peel off ofthe paint.
  • the present inventors searched for a new second promoter substituting the above-mentioned compounds which would not corrode the anode.
  • aluminium sodium fluoride AlF -3NaF
  • aluminium potassium fluoride AlF -3KF
  • the concentration of both double salts should preferably account for 3 to of that of the chromium VI oxide used as a main agent. If each of said double salts is added to the electrolyte at higher concentration than then not only said addition will fail to indicate an increased effect, but also there will be presented ditficulties in the dissolution of the double salt in the electrolyte.
  • FIG. 1 is a curve diagram showing the relationship of the concentrations of thiocyanic compounds and aluminium sodium fluoride in an electrolyte and the deposited amount of metallic chromium, Where electrolysis was carried out under the following conditions:
  • the curve A represents the case where the electrolyte did not contain thiocyanic compounds.
  • the curves B, C and D denote the cases where the electrolyte contained 0.3, 0.6 and 0.9 g./l. of sodium thiocyanate respectively.
  • the concentration of the thiocyanic compound is fixed at any of the above three values, then the aluminium sodium fluoride can be added to the electrolyte at such optimum concentration as allows metallic chromium to be deposited at a maximum rate.
  • the optimum concentration of said aluminium sodium fluoride varies with the concentration at which said thiocyanic compound is added to the electrolyte.
  • FIG. 1 also shows that in the absence of thiocyanic compounds, the deposition of metallic chromium would be extremely small, regardless of the addition of the aluminium sodium fluoride.
  • the curve B shows variation in the content of chromium in a deposited layer of hydrated chromium oxide, where electrolysis was carried out 3.5 seconds at a current density of 25 a./dm. and a temperature of 50 C., using various kinds of electrolyte prepared by adding diflFerent amounts of aluminium sodium fluoride to a solution in which there were dissolved 100' g./l. of CrO and 0.3 g./l. of sodium thiocyanate.
  • the curve F presents in comparison with the curve B the data obtained where electrolysis was carried out under the same conditions, excepting that the electrolyte did not contain sodium thiocyanate.
  • a layer of hydrated chromium oxide formed by the method of this invention had a thickness of less than 0.2 mg./dm. as expressed in chromium, namely, far smaller than the thickness (0.2 to 0.4 mg./dm. of a layer of hydrated chromium oxide electrolytically deposited by the method of the immediately preceding invention (U.S. Pat. No. 3,679,554), using an electrolyte containing hydro silicofluoric or borofluoric compounds as the second promoter. Therefore, the present invention enables a paintcoated steel plate to be deep drawn with the peel off of the paint more assuredly prevented by that extent. Further, a steel plate treated by the method of the present invention presents a substantially transparent surface.
  • FIG. 3 is a curve diagram showing the plotted results of the screw caps tests conducted on the samples of coated steel materials whose surfaces were subjected to chromate treatment using an electrolyte according to the present invention containing thiocyanic compounds as a first promoter and aluminium sodium fluoride as a second promoter, and the samples of coated steel materials whose surfaces were similarly subjected to chromate treatment using an electrolyte only containing aluminium sodium fluoride as a promoter.
  • the curves G and H of FIG. 3 represent the cases where the electrolyte consisted of 100 g./1. of CrO 0.3 g./l. of NaSCN and 0 to 5 g./l.
  • FIG. 3 clearly shows that a steel plate whose surface was electrically treated with an electrolyte prepared by the method of this invention attained the prominent adhesivity of paint to the surface of said steel plate.
  • the surface came ofi prominently, giving rise to irregularities over the entire steel surface.
  • the method of this invention can be applied, as is customarily practiced, at an electrolyzing temperature of 30 to 70 C. and a cathode current density of 10 to 100 a./dm.
  • the more elevated the electrolyzing temperature the more decreased the deposition of metallic chromium and hydrated chromium oxide, and the more increased the cathode current density, the higher the electrolytic efficiency of forming a layer of metallic chromium.
  • EXAMPLE 1 A cold-rolled steel strip used for manufacturing a tinplate was electrolytically degreased 5 seconds at a temperature of C. and current density of 10 a./dm. using an electrolyte consisting of an aqueous solution of 30 g./l. of ortho-sodium silicate. After washed with water, the steel strip was electrolytically pickled 5 seconds at room temperature and a current density of 3 a./dm. using an electrolyte containing 10 g./l. of sulfuric acid. Thereafter the steel strip was subjected to electrolytic chromate treatment with an electrolyte consisting of 50 g./l. of CrO 0.5 g./l. of NaSCN and 3 g./l. of
  • EXAMPLE 2 The steel strip was subjected to the same pretreatments as in Example 1. Thereafter, the steel strip was subjected to an electrolytic chromate treatment with an electrolyte consisting of g./l. of CrO 0.3 g./l. of NaSCN and 5 g./l. of AIF -SKF, with the anode prepared from an alloy of 99% Pia-1% Ag. Electrolysis was carried out under the same conditions as in Example 1. The chromate treatment formed on the surface of the steel strip a colorless transparent duplex layer, the upper portion of which consisted of a layer of hydrated chromium oxide having a thickness of 0.12 mg./dm.
  • EXAMPLE 3 The same pretreatments as in Example 1 were applied to the steel strip, which was later subjected to the electrolytic chromate treatment with an electrolyte consisting of g./l. of CrO 0.5 g./l. of NaSCN and 3.0 g./l. of AlF -3NaF. Electrolysis was carried out under the same conditions with the anode prepared from the same material as in Example 1. Said chromate treatment deposited on the surface of the steel strip a duplex layer, the upper portion of which consisted of a colorless transparent layer of hydrated chromium oxide having a thickness of 0.08 mg./dm.
  • No. 2 was too thinly coated with a layer of metallic chromium because the electrolyte contained an excessively large amount of NaSCN. Accordingly, a layer of hydrated chromium oxide substantially disappeared.
  • N0. 3 was thinly coated with a layer of metallic chromium due to the absence of NaSCN, and in consequence excessively coated with a layer of hydrated chromium oxide, resulting in the coloration of the surface of said sample, and the failure of its deep drawing under a satisfactory coated condition.
  • No. 4 had its surface tinted, because a layer of hydrated chromium oxide became too thick due to the absence of AIF;, 3NaF in the electrolyte.
  • No. 5 was coated with an unduly thick layer of hydrated chromium oxide because of the too small content of CrO in the electrolyte, causing the failure of the deep drawing of said sample under a satisfactory coated condition.
  • No. 6 was treated with an electrolyte containing too much CrO and in consequence was plated with chromium to an ordinary extent of chromium plating.
  • No. 7 was treated with an electrolyte containing HBF as a second promoter according to the immediately preceding invention (US, Pat. No. 3,679,554), but was coated with a layer of hydrated chromium oxide more thickly than when the method of this invention was applied, leading to the slight coloration of the surface of said sample.
  • the electrolyte consists of a chromic acid compound as the main agent, thiocyanic compounds as the first promoter and a second promoter selected from the group consisting of aluminium sodium fluoride and aluminium potassium fluoride.

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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)
  • Electroplating And Plating Baths Therefor (AREA)
  • Electroplating Methods And Accessories (AREA)
  • Application Of Or Painting With Fluid Materials (AREA)
  • Electrochemical Coating By Surface Reaction (AREA)
US00321823A 1972-01-14 1973-01-08 Method for electrolytically treating the surface of a steel plate with a chromate solution Expired - Lifetime US3785940A (en)

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JP47006313A JPS5230461B2 (en)van) 1972-01-14 1972-01-14

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JP (1) JPS5230461B2 (en)van)
AU (1) AU473353B2 (en)van)
BR (1) BR7300289D0 (en)van)
CA (1) CA1007593A (en)van)
FR (1) FR2177720B1 (en)van)
GB (1) GB1371590A (en)van)
IT (1) IT983045B (en)van)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4455355A (en) * 1979-03-30 1984-06-19 Toyo Kohan Co., Ltd. Tin-free steel can body
EP2922983B1 (en) * 2012-11-21 2019-02-20 Tata Steel IJmuiden BV Chromium-chromium oxide coatings applied to steel substrates for packaging applications and a method for producing said coatings

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5464161U (en)van) * 1977-10-13 1979-05-07
JPS583037B2 (ja) * 1979-08-24 1983-01-19 キヤノン株式会社 被覆法
JPH01125345U (en)van) * 1988-02-16 1989-08-25

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4455355A (en) * 1979-03-30 1984-06-19 Toyo Kohan Co., Ltd. Tin-free steel can body
EP2922983B1 (en) * 2012-11-21 2019-02-20 Tata Steel IJmuiden BV Chromium-chromium oxide coatings applied to steel substrates for packaging applications and a method for producing said coatings

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JPS5230461B2 (en)van) 1977-08-08
AU473353B2 (en) 1976-06-17
DE2301601A1 (de) 1973-07-19
IT983045B (it) 1974-10-31
FR2177720B1 (en)van) 1976-04-30
BR7300289D0 (pt) 1973-09-27
FR2177720A1 (en)van) 1973-11-09
JPS4875432A (en)van) 1973-10-11
GB1371590A (en) 1974-10-23
AU5073473A (en) 1974-07-04
CA1007593A (en) 1977-03-29
DE2301601B2 (de) 1977-04-07

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