Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/08—Electrolytic coating other than with metals with inorganic materials by cathodic processes
  • C25D9/10—Electrolytic coating other than with metals with inorganic materials by cathodic processes on iron or steel

Definitions

• the present invention relates to a process for surface treatment of a stainless steel sheet (including a stainless steel strip) according to which electrochemical treatment is performed using the stainless steel sheet as a cathode for forming a noncorroding film on the surface thereof.
• Chrome plating is known as a process for surface treatment of this type.
• Chrome plating is a kind of electroplating according to which a surface film of pure chrome is formed on the surface of the stainless steel sheet by electrodeposition oi metal chrome.
• This surface film is advantageous in that it is noncorroding and has excellent gloss.
• This film is utilized for treating stainless steel sheets which are inexpensive but are not sufficiently noncorroding, such as sheets according to JIS SUS430.
• this chrome plating adopts as a treatment solution a mixture of sulfuric acid and chromic acid of high concentration. For this reason, the waste solution from the treatment contains a large amount of chromic acid and thus requires costly treatment for prevention of pollution.
• Chrome plating further requires a large current of over several amperes per dm and a long treatment time. This plating process thus results in great power consumption (e.g., integrated current density 6,000 A ⁇ sec/dm 2 ) and higher treatment cost.
• This process is advantageous in that the concentration of residual chromic acid in the solution is low, and the treatment of the waste solution is easy. Furthermore, a corrosion resistance comparable to that obtained with chromic plating may be obtained with a smaller current density and a shorter conduction time.
• the present invention has been made in view of this and has for its object to provide a process for surface treatment of a stainless steel sheet wherein the treatment solution is improved so that gloss and resistance to sulfurous acid are improved.
• a process for surface treatment of a stainless steel sheet characterized by dipping a stainless steel sheet having a BA (bright annealing) film or a passive film in a treatment solution and carrying out cathodic treatment under conditions of 1 to 600 Aosec/dm 2 integrated current density and 10 to 90°C solution temperature, said treatment solution containing 0.1 to 50.0% by weight of phosphoric acid, 0.1 to 10.0% by weight of chromic acid, and 0.1 to 2% by weight of a member selected from-the group consisting of magnesium oxide, sodium silicate, and mixtures thereof.
• BA blue annealing
• a process for surface treatment of a stainless steel sheet characterized by dipping a stainless steel sheet having a BA film or a passive film in a treatment solution containing 0.1 to 70% by weight of phosphoric acid and 0.1 to 10.0% by weight of a molybdate, and carrying out cathodic treatment under the conditions of 1 to 600 A ⁇ sec/dm 2 integrated current density and 10 to 90°C solution temperature.
• a process for surface treatment of a stainless steel sheet characterized by, before or after dipping a stainless steel having a BA film or a passive film in a first treatment solution containing 0.1 to 50.0% by weight of phosphoric acid, 0.1 to 10.0% by weight of chromic acid, and 0.1 to 2.0% by weight of a member selected from the group consisting of magnesium oxide, sodium silicate and mixtures thereof and carrying out cathodic treatment under the conditions of 1 to 600 A ⁇ sec/dm 2 integrated current density and 0 to 90°C solution temperature, dipping the stainless sheet in a second treatment solution containing 0.1 to 70% by weight of phosphoric acid and 0.1 to 10% by weight of a molybdate and carrying out cathodic treatment under the conditions of 1 to 600 Aosec/dm 2 integrated current density and 10 to 90°C solution temperature.
• a process for surface treatment of a stainless steel sheet characterized by dipping a stainless steel sheet having a BA film or a passive film in a treatment solution containing 0.1 to 50.0% by weight of phosphoric acid, 0.1 to 10.0% by weight of chromic acid, 0.1 to 2.0% by weight of a member selected from the group consisting of magnesium oxide, sodium silicate and mixtures thereof, and 0.1 to 10.0% by weight of a molybdate, and carrying out cathodic treatment under the conditions of 1 to 600 Aosec/dm2 integrated current density and 10 to 90°C solution temperature.
• a process for surface treatment of a stainless steel sheet characterized by dipping a stainless steel sheet having a BA film or a passive film in a 0.1 to 70% by weight aqueous solution of phosphoric acid, and carrying out cathodic treatment under the conditions of 1 to 600 A ⁇ sec/dm 2 integrated current density and 10 to 90°C solution temperature, utilizing the stainless steel as a cathode and metal molybdenum as an anode.
• a BA (bright annealing) film or a passive film must be formed on the surface of the stainless steel sheet to be treated.
• the BA film or the passive film is a film which renders the surface of the stainless steel sheet inactive.
• the BA film may be obtained by bright heat- treatment according to which the sheet is heated in a vacuum, an inert gas, a reducing gas, or a hot salt.
• the passive film may be obtained by leaving to the sheet in air or dipping the stainless steel sheet in a 20% (by volume) aqueous solution of nitric acid at room ' temperature for 30 minutes or at 65°C for 10 minutes.
• the reason why the BA film or the passive film should be formed before the treatment according to the present invention is that if such a film is not formed in advance,-corrosion resistance may not be improved even if cathodic treatment is conducted according to this invention. This has been confirmed by experiments to be described later (Example 1).
• a stainless steel sheet having a BA film or a passive film formed thereon undergoes cathodic treatment in a treatment solution containing phosphoric acid, chromic acid, magnesium oxide and/or sodium silicate.
• Phosphoric acid. and chromic acid contained in this treatment solution are necessary components for forming a noncorroding film on the surface of the stainless steel sheet.
• the lower limit of the amount of phosphoric acid to be used has been set at 0.1% by weight since with smaller amounts of phosphoric acid, satisfactory corrosion resistance may not be obtained.
• the upper limit of phosphoric acid to be used has also been set to 50% by weight, since larger amount of phosphoric acid result in loss of gloss at the surface of the stainless steel sheet and a greater waste of solution.
• the preferable range of the amount of phosphoric acid to be used herein is 5 to 20% by weight.
• the range for the amount of chromic acid has also been set between 0.1 and 10.0% by weight for the same reason as in the case of phosphoric acid.
• the amount of chromic acid is not in this range, satisfactory corrosion resistance may not be obtained.
• the amount of chromic is out of this range, surface gloss is lost and the amount of the waste solution will be great.
• the preferable range'of the amount of chromic acid is 0.5 to 1.8% by weight.
• Magnesium oxide and sodium silicate are components for preventing loss of gloss at the surface of the stainless steel sheet.
• the range of the amount of magnesium oxide has been limited since satisfactory gloss may not be obtained below 0.1% or above 2.0% by weight.
• the preferable range of the amount of magnesium oxide is 0.5 to 1.5% by weight.
• the amount of sodium silicate has been limited between 0.1 and 2% by weight. When this amount is less than 0.1% by weight, satisfactory gloss may not be obtained. When the amount of sodium silicate exceeds 2% by weight, it precipitates in the form of gel from the solution, so that it may not exhibit the advantageous effects of sodium silicate.
• the preferable range of sodium silicate is 0.1 to 0.8% by weight.
• the amounts of magnesium oxide or sodium silicate have been limited to within the range of 0.1 to 2% by weight. When the amount of either of these is below 0.1% by weight, the satisfactory effects provided by addition of these members may not be obtained. When the amount exceeds 2% by weight, the surface gloss may not be obtained.
• the preferable range of the amount of magnesium oxide or sodium silicate is 1.0 to 1.8% by weight.
• the treating conditions for cathodic treatment in such a treatment solution are 1 to 600 Aosec/dm2 integrated current density and 0 to 90°C solution temperature.
• the integrated current density has been limited to the range of 1 to 600 Aosec/dm2 since a lower integrated current density than 1 A ⁇ sec/dm 2 results in insufficient corrosion resistance and a higher integrated current density than 600 A ⁇ sec/dm 2 results in unsatisfactory surface gloss and higher cost.
• the preferable range of integrated current density is 60 to 360 Aosec/dm 2 .
• the solution temperature has been limited to the range of 0 to 90°C since the solution temperature below 0°C results in a longer treating time and difficulty in maintaining the temperature of the treatment solution; a higher solution temperature than 90°C results in degradation in quality of the treatment solution.
• the preferable range of solution temperature is 50 to 70°C.
• a stainless steel sheet with a BA film or a passive film formed thereon undergoes cathodic treatment or dipping treatment in a treatment solution containing phosphoric acid and molybdate.
• the phosphoric acid contained in this treatment solution is a necessary component for forming a noncorroding film on the surface of the stainless steel sheet as in the case of the first aspect of the present invention.
• the lower limit of the amount of phosphoric acid has been set to 0.1% by weight since a smaller amount of phosphoric acid results in unsatisfactory corrosion resistance.
• the upper limit of phosphoric acid has been set to 70% by weight since larger amount results in loss of surface gloss at the surface of the stainless steel sheet and a greater solution loss.
• the preferable range of the amount of phosphoric acid is 5 to 50% by weight.
• the molybdate is effective in improving resistance to sulfurous acid by forming the corrosion resistant film on the surface of the stainless steel sheet and in maintaining excellent gloss.
• the molybdate may be included in the form of ammonium molybdate, sodium molybdate and so on.
• the amount of the molybdate to be used herein has been set as described above since a smaller amount of the molybdate does not provide satisfactory effects and a larger amount results in loss of surface gloss.
• the preferable range of the amount of the molybdate is 1 to 3% by weight.
• the treating conditions for this treatment are the same as those as have been described with reference to the first aspect of the present invention except that the lower limit of the solution temperature may be lowered to 0°C in this case.
• the third aspect of the present invention provides three options:
• the mixing ratio of the respective components in the first treatment solution is the same as that described before with reference to the first aspect of the present invention.
• the mixing ratio of the respective components of the second treatment solution is the same as that described with reference to the second aspect of the present invention.
• the cathodic treatment conditions in the first treatment solution are 1 to 600 A ⁇ sec/dm 2 in integrated current density, and 0 to 90°C in solution temperature, preferably 10 to 30°C.
• the integrated current density has been limited to this range for the reasons as described below. When the integrated current density is below 1 Aosec/dm 2 , the gloss may be maintained but the resistance to corrosion, such as resistance to sulfurous acid, salt damage (table 1) and so on, is not satisfactory.
• the integrated current density exceeds 600 A ⁇ sec/dm 2 , the gloss may not be maintained and the process is uneconomical.
• it. is preferable to set the integrated current density of 10 to 80 A ⁇ sec/dm 2 .
• the temperature of this treatment solution should be controlled between 0 and 90°C for the reasons to be.described below. When the solution temperature is below 0°C, the treatment time becomes longer and maintenance of the temperature of treatment solution becomes difficult.
• the solution temperature exceeds 90°C, the effects of improving resistance to corrosion may not be obtained.
• the treatment time may be shortened and the power consumption may be reduced with an increase in temperature of the treatment solution.
• a solution temperature ranging from 70 to 90°C, a film of sufficient corrosion resistance may be formed on the surface of the stainless steel sheet with an integrated current density of below 1 Aosec/dm 2 , or even at 0 A ⁇ sec/dm 2 (dipping in the solution with no electric current flowing through the stainless steel sheet).
• the cathodic treatment conditions in the second treatment solution are 1 to 600 A ⁇ sec/dm 2 integrated current density, and 10 to 90°C solution temperature, preferably 40 to 60°C.
• the integrated current density has been set within the range of 1 to 600 A ⁇ sec/dm 2 for the reasons to be described below. When the integrated current density is below 1 A ⁇ sec/dm 2 , the gloss may be maintained but the corrosion resistance may not be sufficient. When the integrated current density exceeds 600 A ⁇ sec/dm 2 , the surface gloss may not be obtained, discoloration may occur, and the process is uneconomical. Particularly for improving the resistance to sulfurous acid, it is preferable to set the integrated current density within the range of 60 to 200 A ⁇ sec/dm 2 .
• the integrated current density is preferable to set within the range of 40 to 80 Aosec/dm 2 .
• the integrated current density is preferably set within the range of 60 to 80 Aesec/dm 2 .
• the solution temperature of the second treatment solution has been set to be within the range of 0 to 90°C for the same reasons described with reference to the second aspect of the present invention.
• the treatment time may be shortened and the power consumption may be reduced with a higher solution temperature.
• a solution temperature ranging from 70 to 90°C
• a corrosion resistant film may be formed on the surface of the stainless steel sheet with an integrated current density of below 1 Aesec/dm 2 , or even at 0 A ⁇ sec/dm 2 (dipping in the solution with no electric current flowing through the stainless steel sheet).
• the cathodic treatment conditions with a solution mixture of the first and second treatment solutions may be substantially the same as those described with reference to the cathodic treatment with either the first or second treatment solution, and may be appropriately selected accordingly.
• a stainless steel sheet having a BA film or a passive film formed thereon is dipped in an aqueous solution containing 0.1 to 70% by weight of phosphoric acid, preferably 5 to 50% by weight, and cathodic treatment of the stainless steel sheet is carried out under the treatment conditions of 1 to 600 A'sec/dm2 integrated current density, and 10 to 90°C solution temperature, preferably 40 to 60°C, using the stainless steel sheet as a cathode and metal molybdenum as an anode.
• the reasons for setting the phosphoric acid amount and the cathodic protection conditions are the same as those described with reference to the second aspect of the present invention.
• Metal molybdenum is used as the anode in this process for electrolytically eluting the metal molybdenum during electrolysis.
• This metal molybdenum improves the resistance to sulfurous acid of the noncorroding film formed on the surface of the stainless steel sheet. It also maintains excellent gloss.
• Metal molybdenum of higher purity is preferable for improving the corrosion resistance to thereby prevent degradation of the treatment solution.
• the metal molybdenum as the anode since the metal molybdenum as the anode is electrolytically eluted, the molybdate need not be incorporated in advance, thus improving the workability of the treatment. Furthermore, since the metal molybdenum does not contain a large amount of impurities, it prevents degradation in quality of the treatment solution and prolongs the life of the treatment solution.
• the treatment time may be shortened with a higher solution temperature and a greater current density.
• a corrosion resistant film may be formed, generally, in 10 seconds to 10 minutes.
• a film having corrosion resistance may be formed on the surface of a stainless steel sheet, and its corrosion resistance will not be degradated for a long period of time after treatment.
• This film is rich in gloss and improves the resistance to corrosion of a stainless steel sheet having insufficient resistance to corrosion, thus improving its product value.
• JIS SUS 430 No. 1
• JIS SUS 434 No. 2
• JIS SUS 304 No. 3
• the treatment solution contained 9.25% by weight of phosphoric acid, 1.68% by weight of chromic acid, 0.78% by weight of magnesium oxide, and 0.10% by weight of sodium silicate.
• the stainless steel sheets were subjected to cathodic treatment for 3 minutes at a current density of 2.0 A/dm 2 and a solution temperature of 20°C.
• Stainless steel sheet raw materials were used which were obtained by bright annealing the sheets according to JIS SUS 430, JIS SUS 434, and JIS SUS 420J2 to form BA films, and by hair-line treating (forming a passive film on) the sheet according to JIS SUS 304.
• the treatment solution contained 9.37% by weight of phosphoric acid and 1.34% by weight of sodium molybdate.
• a stainless steel sheet as in the above example was similarly subjected to cathodic treatment with a treatment solution which contained phosphoric acid but which did not contain a molybdate.
• the results of the corrosion resistance test on this stainless steel sheet are also shown in Table 4.
• Example 3 The surface gloss of the stainless steel sheet (JIS SUS 430) obtained in Example 3 was compared with that of the stainless steel sheet having the BA film formed thereon. The presence or absence of gloss was evaluated. The obtained results are shown in Table 5.
• Stainless steel sheets were used which were obtained by bright annealing sheets according to JIS SUS 430 to form BA films thereon.
• the first treatment solution ' contained 9.25% by weight of phosphoric acid, 1.68% by weight of chromic acid, 0.78% by weight of magnesium oxide, and 0.10% by weight of sodium silicate.
• the stainless steel sheets were subjected to cathodic treatment by varying the treatment conditions within the ranges according to the present invention.
• the second treatment solution contained 9.37% by weight of phosphoric acid and 1.34% by weight of sodium molybdate.
• the stainless steel sheets were subjected to cathodic treatment again by varying the treatment conditions within the ranges according to the present invention.
• the stainless steel sheets surface-treated in this manner were subjected to the corrosion resistance test according to the method shown in Table 1 (the resistance to sulfurous acid was evaluated according to the sulfurous acid gas corrosion test as defined in Dln, and the resistance to salt damage was evaluated according to the Dip and Dry method (GM conditions)).
• the obtained results are shown in Table 6.
• Observations were made on gloss and discoloration of the surfaces of the sheets, and the obtained results are also shown in Table 6.
• stainless steel sheets were subjected to various treatment methods. Some stainless steel sheets were subjected to the first and second treatments wherein the treatment conditions deviated from the ranges according to the present invention (Nos. 9 to 12). A stainless steel sheet was subjected to cathodic treatment in the first treatment solution but not to the second treatment (No. 13). Stainless steel sheets were subjected to the first treatment but not to the second treatment (Nos. 14 and 15). A stainless steel sheet which did not have a BA film was subjected to the first and second treatments (No. 16). A stainless steel sheet was subjected to the second treatment first and to the first treatment thereafter (No. 17). A stainless steel sheet was subjected to a treatment with a solution mixture of the first and second treatment solutions (No. 18). These stainless steel sheets were subjected to the corrosion resistance test and were evaluated for gloss and discoloration. The results are shown in Table 6.
• the composition of the solution mixture was 9.37% by weight of phosphoric acid, 1.68% by weight of chromic acid, 0.78% by weight of magnesium oxide, and 1.34% by weight of a molybdate.
• the treatment conditions were 1 A/dm 2 current density, 60 second treatment time, and 50°C solution temperature.
• stainless steel sheets were subjected to dipping (without conduction of current) instead of cathodic treatment in the first and second treatment solutions (Nos. 19 to 21). These stainless steel sheets were subjected to the corrosion resistance test and were evaluated for gloss and discoloration. The results are shown in Table 6.
• Stainless steel sheets were obtained by bright annealing the sheets according to JIS SUS 430 to form BA films thereon. A treatment was carried out using these sheets as cathodes and metal molybdenum plates of 99.0% by weight purity as anodes in a treatment solution containing 10% by weight of phosphoric acid.
• the stainless steel sheet having a BA film was also subjected to the corrosion resistance test and the obtained results are shown in Table 7.
• the stainless steel sheet having neither the BA film nor the passive film was subjected to cathodic treatment under the same conditions as in the example.
• the results of the corrosion resistance test of this sheet are also shown in Table 7.
• a treatment solution containing sodium molybdenum also results in good corrosion resistance of the resultant sheet. However, as will be made clear in Example 8, degradation in quality of the treatment solution is significant with this composition.
• the surface gloss of the stainless steel sheet (JIS SUS 430) obtained in Example 6 was compared with that of the stainless steel sheet having a BA film formed thereon. Evaluations were made as to the presence or absence of gloss. The obtained results are shown in Table 8.
• Example 6 To the treatment solution used in Example 6 was added 0.5% by weight of sodium molybdate. Cathodic treatment was carried out using a ferrite anode. Degradation in quality of the treatment solution was evaluated. Degradation in quality of the treatment solution was first observed after 25 hours.
• a belt-shaped plate of a sample 100 mm in width was continuously treated in 1 liter of the electrolytic solution.
• the solution was judged to have been degraded when the rusting rate according to the sulfurous acid resistance test exceeded 10%.
• corrosion resistance particularly resistance tc sulfurous acid is excellent and excellent gloss may be maintained, providing an excellent process for surface treatment of stainless steel sheets for automobiles.
• preparation for the treatment is easy and degradation in the treatment solution may be prevented for a long period of time.

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• 1981
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