Method of sealing terne sheets
United States Patent Watanabe et al.  
 METHOD OF SEALING TERNE SHEETS Inventors: Takashi Watanabe, Kanagawa;  
 Eiichi Tarumi, Tokyo, both of Japan Assignees: Nippon Steel Corporation, Tokyo;  
 Daido Steel Sheet Manufacturing Corporation, Amagasaki-shi, both of Japan Filed: Dec. 26, 1972 Appl. No.: 318,675  
 Foreign Application Priority Data Apr. 22, 1975 [56] References Cited UNITED STATES PATENTS 1,290,476 1/1919 Allen 148/65 R 1,750,270 3/1930 Jones 148/615 R 2,030,601 2/1936 McDonald 148/616 2,160,061 5/1939 Dolittle 148/6.14 R  
 Primary Examiner-Mayer Weinblatt Assistant E.raminerEdith L. Rollins Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] ABSTRACT A method of sealing terne sheets by treating terne sheets with an aqueous solution containing 0.5l0 percent of orthophosphoric acid.  
 3 Claims, 2 Drawing Figures PATENTEDAPRZZIQYS FIG. I  
 FIG. 2  
 METHOD OF SlEAlLlhlG TERNE SHEETS The main object of this invention is to improve the corrosion resistance of terne sheets and articles thereof by sealing the terne sheets and to improve the heat resistance of the coating.  
  Terne sheets are formed by applying an alloy of lead and tin on steel sheets usually by the hot dip method, thereby the content of tin in the alloy is usually about -20 percent. This alloy is called terne metal.&#34; Although, in some cases, zinc and/or antimony are added to effect further improvements in the properties of the terne metal, such as, the binding strength to the steel sheet, the terne metal itself possesses excellent corrosion resistance, so that if a coating having no pin holes is obtained, very excellent anticorrosion steel sheets will be manufactured. However, techniques, or coating processes for forming terne sheets which are completely free from pin holes have not yet been developed, and thus it will continue to be difficult to form terne sheets having no pin holes so long as improvements are made only in the hot dipping method, because the thickness of the coating is very small e.g. a few microns. Since the terne sheet are formed by the cathode coating process as is well known, it has the disadvantage that when a terne sheet having pin holes is kept in a humid place, the corrosion which occurred in the pin holes proceeds around them, because fine galvanic cells are formed between the base-iron under the pin hole portions and the terne metal of the coating.  
  Therefore, in order to give full play to the characteristic feature of the terne sheets, it is necessary to apply a coating having no pin holes. As an example of this, it may be considered to carry out a sealing treatment after the production of terne sheets. However, there is no effective sealing method which has so far been employed in industry, and the industry has looked forward to establishing a treating method which provides a large economical effect. In practice, the treating method wiil be desirable as a step, if the pin holes can be sealed by a simple immersion or spray treatment.  
  We have conducted various investigations on the sealing of terne sheets, and we have succeeded in making it possible to seal terne sheets by a simple treatment with an aqueous solution.  
  Namely, we have found that when terne sheets are treated with an aqueous solution containing 0.5 ll()% of orthophosphoric acid, most of the pin holes can be sealed to effect a remarkable improvement in their corrosion resistances.  
  This treatment is characterized as having almost no effect on steel sheets, but a marked effect on terne sheets formed by hot dipping steel sheets. This leads us to an understanding that the treatment with said solution not only may suppress the ionization of iron, but may produce a reaction product by the reaction of the treating solution with the terne metal, which has a large effect on the sealing.  
  As the treating method, either the immersion method or spray method may be used to effect adequate results. As for the treating solution, it may be used at room temperature without the necessity of spe ial heating, and further the water-washing after the treatment may be omitted. Treating periods of time ur to a few seconds are sufficient to secure the effect t &#39;zsired, and the short time of the treatment step makes it suitable as a post-treatment.  
  Condensed phosphoric acids, such as, metaphosphoric acid, pyrophosphoric acid, and polyphosphates, such as, sodium metaphosphate have likewise sealing effects as well as orthophosphoric acid, but orthophosphoric acid is advantageously used from an economical point of view.  
  The addition of chromic acid into this sealing solution containing phosphoric acid produces an additional effect. From the treatment with chromic acid alone, some sealing effect may be expected, however, the effect of the treatment with phosphoric acid alone is larger, and the waste solution treatment is easier too. In the case of chromic acid alone, some sealing effect is detectable with concentrations of more than 0.1 percent, but as the concentration exceeds 5 percent, the coloration of surfaces of the terne sheets increases remarkably, so that concentrations ranging from 0.1 to 5 percent are to be preferred.  
  The sealing effect decreases rapidly as the concentration of phosphoric acid decreases from 0.1 percent, so that concentrations of more than 0.5 percent are preferable.  
  The upper limit of the concentration of phosphoric acid should not be made too high, being determined by taking into consideration the appearance of the treated terne sheets, the loss prevention of the treating solution and the prevention of blockings caused by stack of the articles and at the time of packing. Of course, there will be obtained a sufficient sealing effect, even if the concentration is more than 10 percent, but when the concentration is as high as 10 percent, the surfaces of the treated terne sheets are whitened to damage their appearances, so that concentrations of less than 10 percent are preferable.  
  Further, we have found that when terne sheets are treated with a compound solution containing O.51O percent, preferably O.55 percent of orthophosphoric acid, and 0.0140 percent of either alumina sol or silica sol, or a mixture of the both sols in order to seal the pin holes of the terne sheets, the pin holes disappear and their corrosion resistance improves remarkably.  
  This sealing film is so excellent in heat resistance that even when the treated terne sheets are allowed to stand in an atmosphere heated to lOO-200C for a few minutes through several tens of minutes, no lowering of the sealing effect can be detected, and the sealing film does not deteriorate.  
  This proves that the sealing effect owing to orthophosphoric acid interacts properly with the sealing maintenance effect owing to the colloidal particles. Particularly, it is to be understood that the water combined with the colloid does not easily undergo dehydration at a temperature of 200C to suppress the dehydration of the reaction product produced by the reaction of the terne metal with phosphoric acid, or the reaction product produced by the reaction of the baseiron with phosphoric acid and also seems as a supply of water to the hydration.  
  Other polyphosphoric acids, such as, metaphosphoric acid, pyrophosphoric acid than orthophosphoric acid, or polyphosphates, such as, sodium hexametaphosphate may be used likewise to produce the sealing effect, but orthophosphoric acid is advantageously used from several points of view.  
  On the other hand, instead of alumina sol or silica sol to be added in combination with orthophosphoric acid, colloidal titanic acid, or other positive sols such as,  
 metal hydrosols may be used likewise to produce the effect.  
  But since alumina sol and silica sol are superior from several points of view, it is preferable to use the combination of orthophosphoric acid and alumina sol or silica sol as the sealing agent when preparing the treating solution.  
  As for the concentration of alumina sol or silica sol added in combination with orthophosphoric acid, it should be more than 0.01 percent (reduced by A1 or SiO because the sealing film effected by the application of the treating solution with a concentration of less than 0.01% is so poor in heat resistance that the combination with orthophosphoric acid produces little enhanced effect.  
  The upper limit of alumina sol or silica sol should be percent (reduced to A1 0 or SiO When the concentration exceeds 10 percent, the treating solution will be applied with difficulty and it will be liable to be applied in excess. Within this concentration range, the combination of orthophosphoric acid with a mixture of alumina sol and silica sol will produce a similar effect.  
  Still further, we have found that when terne sheets are treated with a treating solution prepared by adjusting the pH value of an aqueous solution containing more than 0.1 percent of one or more biphosphates to less than 3 with orthophosphoric acid or a condensed phosphoric acid in order to seal the pin holes of the terne sheets, the number of pin holes is decreased remarkably to improve their corrosion resistances to a considerable extent. The reason why orthophosphoric acid or a condensed phosphoric acid is used for the pH adjustment is based on the fact that the adhering property, heat resistance and sealing effect of the treating solution are very good as compared with other acids. As the pH value increases from three, the reactivity of the treating solution to the terne sheet decreases. The biphosphates defined in this invention refers to aluminum biphosphate, and magnesium biphosphate, barium biphosphate, calcium biphosphate.  
  The terne sheets subjected to the sealing treatment are so excellent in heat resistance too that their corrosion resistances cannot be deteriorated by heating to 150C for as long as 30 minutes. This means that the sealing films formed on the surfaces of the terne sheets by applying the treating solution thereon have such high resistance to heat that they are hardly susceptible to thermal deterioration. This sealing method is characterized by its very large effectiveness on terne sheets formed by hot dipping steel sheets, although such a large improvement on the anticorrosion of the naked steel sheet base cannot be expected from this method. As the treating method, either the immersion method, or spray method may be used to effect adequate results.  
  The following examples will serve to illustrate the sealing method.  
 EXAMPLE 1 A number of terne sheets of 1 mm thick having 0.35 oz/ft of coated terne metal were treated with aqueous phosphoric acid solutions, and the sealing effect was examined. The dependence of the effect on the concentrations was investigated using a series of solutions with different concentrations ranging from 0.01 to 10.0 percent. The terne sheets were immersed in their respective treating solutions at room temperature for 10 seconds, and immediately after the treatment they were dried in hot air. The sealing effect is evaluated according to both of the salt spray test (118 Z 2371) and the formalin test. The test results are shown in Table 1, from which it is understood that the effect appears when the concentration of phosphoric acid exceeds 0.5 percent, and it becomes remarkable with concentrations of more than 1.0 percent.  
 The criterion used in these tests are as follows.  
 00 no rust 0 rust point l/dm A rust point 2-9/dm x rust point 1O-2O/dm xx rust point more than 2l/dm On the other hand, the formalin test is carried out in such manner that test pieces are placed on the middle plate of a desiccator charged with formalin containing 37 percent formaldehyde, and the lid is sealed with silicone rubber. The desiccator is then placed in a con stant temperature chamber heated to 50C, and the formation of rust points is observed.  
  According to this method, the formaline vapor which fills the desiccator accelerates the formation of rust, so that it is suited for testing the sealing effect in acceleratlon.  
 EXAMPLE 2 A number of terne sheets are treated with each of the solutions with the concentrations used in Example 1, and then stacked one upon another, on which a pressure is applied by use of a hydraulic press to carry out a number of stack tests. The terne sheets pressed under kg/cm were allowed to stand for one month in a chamber of percent humidity at 20C, but upon separation, nothing unusual happened on the surfaces of the terne sheets, and no blocking between the adjacent sheets occurred.  
  As mentioned above, when terne sheets are treated with an aqueous phosphoric acid solution, the pin holes present on the surfaces of terne sheets are scaled to improve their corrosion resistance remarkably, and moreover, the adhesiveness to the coatings is favorably improved. Terne sheets formed by cathode coating may also have their qualities improved by sealing the pin holes. Therefore, the significance of the treatment for sealing terne sheets on the basis of the method of this invention may be said to be great.  
 EXAMPLE 3 Terne sheets 1 mm thick having 0.35 oz/ft of coated terne metal were treated with a series of solutions containing orthophosphoric acid, alumina sol, silica sol, and a mixture of both sols, and the effect of the sealing treatments were examined.  
  The sealing treatments were carried out for 10 seconds by the spray method, thereby the solutions were kept at ordinary temperature. Immediately after the treatment, they were dried in hot air. Before the sealed terne sheets were subjected to the salt spray test (.115 Z 2371), they had been heated to C for 30 minutes.  
  The results of the examination of whether their corrosion resistances were secured from the heating carrried out after the sealing treatment are shown in Table 2.  
  The table shows that the effect appears when the Table 1 concentration of phosphoric acid exceeds 0.5 percent,  
 and alumina sol silica sol, and a mixture thereof are rk ff h h The sealing treatment of terne sheets with i ewise e ective w en t eir concentrations are more aqueous phnsphmic acid (Dependence on the than 0.01 percent (reduced to A1 or S10 5 phosphoric acid concentration) The criterion of this test 15 as follows. valuation of the Sealing effect Phosphoric acid 00 no rust concentration /r) Salt spray test Formalin.test  
 o rust point l/dm A rust po nt 2-9/dm fee 8 t x rust point 10-20/dm 01 xx A 2 0.5 A 0 xx rust point more than 2l/dm 1.0 A 00 1.5 00 00 As stated above, when terne sheets are treated with a treating solution containing orthophosphoric acid m, 0.0 or) no and either alumina sol, or silica sol, or a mixture of the 1 both sols, the pin holes present on the surfaces of the Sammy test 00 hours terne sheets can be sealed, and their corrosion resist- Fornialin test 100 hours ances i p k b]y (The sealing treatment is carried out at room temperature for 10 seconds) Since the films formed by the sealing treatment have 0 good heat resistances, they are insusceptible to the heat Table 2 generated at the time of baking the paint coating, thus favorably maintaining a constant sealing effect. valuation f the sealed tame Shem after heating to 150C for 30 minutes EXAMPLE 4 Salt spray test on Terrie sheets Of 1 mm thick having 0.30 OZ/fll Of Composition of treating solution the sealing coated terne metal were treated with treating solutions Qrthophosghoric (48 hours) prepared by adjusting the pH values of aqueous soludud (VOL I) so] ((72) M) tions containing aluminum biphosphate (acid alumi- 0 0 0 xx Control num phosphate), magnesium biphosphate and barium 3 l g 1, biphosphate with phosphoric acid by the spray method for 10 seconds to examine the sealing effect. 0 0 ff P The treating solutions were maintained at the ordi- 8&#39; 8 nary temperature, and hot air-drying is applied after 1.0 0.1 0.1 m1 the treatment. After the drying, the sealed terne sheets g were heated to 150C for 30 minutes and then sub- 8 Z r jected to the salt spray test (JlS Z 2371). The results of the examination whether their corrosion resistances sel 0 0 Eximple cured from the heating carried out after the sealing L0 0 (mm x Comm treatment are shown in Table 3. 4O 0 0. 5 A  
  The table shows that when terne sheets are treated L0 0 0m 0 Example with a treating solution prepared by adjusting the pH H) 00 M05 value of an aqueous solution containing more than 8- g 0.1percent of one or more biphosphates, such as, alu- L0 00 minum biphosphate, magnesium biphosphate, etc. to 0 v less than 3 with orthophosphoric acid or a condensed 8 i8 phosphoric acid, such as, pyrophosphoric acid, there is 3.0 00  
 obtained a sealing film which is hard and susceptible to thermal deterioration, and the corrosion resistance of M20&#34; terne sheets improves remarkably. &#34;$10.,  
 The criterion of this test is as follows.  
 Table 3 00 no rust 2 0 rust l l/dm Composition of treating solution A rust point 2-9/dm 2 Salt s ray X lUSt P0lf l 1 2O/dm pH (with Aluminum Magnesium Barium tes t xx rust point more than Zl/dmphosphoric biphosbiphosbiphos- 14s acid) phate(7i) phate(7r) phateU/Z) hours) As stated above, it is clarified that when terne sheets comm, L 0&#39;05 0 U A are treated with a treating solution containing biphos- Example 1.85 0.1 0 0 0 phate and having a decreased pH value with phos- 1-32 9&#39;3 8 8 phoric acid, the pin holes present on the surfaces of the Comm] 150 0 m A terne sheets are sealed to effect a remarkable improvel 0 0 A 1. Example 1 50 0.05 0.05 0 0 ment of their corrosion resistances. Since the sea ng 65 50 005 005 0405 film has such high resistance to heat, it is preferably in- Control 1.85 0 0 0 X A r d tb dth k Example 2.10 1.0 0 0 0 susceptible to the heat ue to pain a mg an e e, 250 0 L0 0 0 while maintaining the sealing effect constant. 3.00 0 0 1.0 u  
 Table 3-Continucd Composition of treating solution Salt spray pH (with Aluminum Magnesium Barium test phosphoric biphosbiphosbiphos- (48 acid) phateU/z phate(72 phateW: hours) Control 3.10 1.0 0 A Example 3.00 0.5 0.5 0.5 0 2.00 0 0 0.1 0 2.00 0 0.1 0 o 2.00 0.1 0.1 0.1 00 Control 3.25 0.5 0.5 0.5 A Example 1.85 0.1 0 0 0 1.85 0 0.1 O 0 1.85 0 0 0.1 0 Control 3.10 1.0 0 0 X A Example 3.00 0 1.0 0 0 2.00 0.1 0.1 0.1 on Control 1.50 0.05 0 0 A 1.50 0 0.05 0 A 1.50 0 0 0.05 A Example 1.50 0 0.05 0.05 o 2.50 0.1 0.1 0.1 0 Control 1.85 0 0 0 .r A  
 Control Untreated original terne sheet xx What is claimed is:  
  l. A method of sealing terne sheets which comprises treating terne sheets with an aqueous solution consisting essentially of 0.5-.0 percent of orthophosphoric acid.  
 . 2. A method of sealing terne sheets which comprises treating terne sheets with a treating solution prepared by adjusting the pH of an aqueous solution containing more than 0.1 percent of at least one biphosphate to less than 3 with an acid selected from the group consisting of orthophosphoric acid and condensed phosphoric acids.  
  3. A method of sealing terne sheets which comprises treating terne sheets with an aqueous solution consisting essentially of 0.5-10 percent of orthophosphoric acid and 0.01-10 percent of a material selected from the group consisting of alumina sol, silica sol, and mixtures thereof.