Patent Publication Number: US-3881039-A

Title: Process for the treatment of amorphous carbon or graphite manufactured articles, for the purpose of improving their resistance to oxidation, solutions suitable for attaining such purpose and resulting product

Description:
United States Patent Baldieri et al.  
 1 1 Apr. 29, 1975 Francesco Baldlerl; Carlo Borgiannl. both of Rome. Italy Assignce: Snam Progetti S.p.A.. Milan. Italy Filed: Aug. 21. 1973 Appl. No.: 390.196  
 Related U.S. Application Data Division of Ser. No. [011.739.1un. 22. 1971. Put. No. 31114.69).  
 Inventors:  
 Foreign Application Priority Data Feb. 23. 1971 Italy 19661/71 U.S. Cl. 427/294: 14106.15: 204/294.  
  252/397 Int. Cl 844d 1/20 Field of Search 117/228. 109 R; 204/294;  
 [56] References Cited UNITED STATES PATENTS 504.105 8/1893 Corleis et a1 117/2211 1.610.362 12/1926 Coslctt 148/61 2.500.673 3/1950 Gibson 148/615 7 2.964.434 12/1960 Coleman 148/615 R 3.288.655 11/1966 Harwell ct 14106.15 3.553.010 1/1971 Riebisch 117/169 R 3.592.701 7/1971 Pekur Mil/6.15 R  
 FOREIGN PATENTS OR APPLICATIONS 398.353 9/1933 United Kingdom 14106.15 7.  
 Primary E.\&#39;urriiri0rWi1liam E. Schulz [57] ABSTRACT A process is disclosed for improving the oxidative resistance of amorphous carbon or graphite articles. The process comprises applying a solution of water; an oxide or hydroxide of a group 2 metal: a member of the group consisting of nitric acid; sodium nitrite and sodium carbonate; and horic acid or sodium tetruhorate.  
 5 Claims. I Drawing Figure 1 PROCESS FOR THE TREATMENT OF AMORPHOUS CARBON OR GRAPHITE MANUFACTURED ARTICLES, FOR THE PURPOSE OF IMPROVING THEIR RESISTANCE TO OXIDATION, SOLUTIONS SUITABLE FOR ATTAINING SUCH PURPOSE AND RESULTING I PRODUCT This is a division, of application Ser. No. 108,739, filed Jan. 22, i971 and now US. Pat. No. 3,8l4,699.  
  The present invention refers to a process designed to give amorphous carbon or graphite manufactured articles greater resistance to oxidation at high temperature&#39;, reference is made. in addition. more particularly to a series of solutions designed to make the said manufactured articles more resistant to oxidation at high temperature, and to the said manufactured articles which are obtained by such processes and solutions. it is known that carbon materials whether amorphous or crystalline are, in view of their particular electric. mechanical and thermal properties, used in many types of industries as electrodes for furnaces in the iron and steel industry, or. again as electrodes, in the production of aluminum, or as refractories, crucibles, resistors, etc.  
  Despite their wide field of use, carbon materials have always found a limit in their easy oxidability at high temperatures.  
  Proposals have been made for coverings protecting the jackets on the surface of the manufactured article, with a base of silicons, polysiloxans, colloidal silica, carbides,- nitrides and silicides of refractory metals, etcx, these systems are not the best, both because of the complexity of the operations and equipments required for the coating, and because the protective layer comes away fairly easily, and also owing to the addition of elements which are sometimes undesired, which may happen when manufactured articles protected in this way are used as electrodes or refractories in the iron and steel industry or in metallurgy in general,  
  Trials have also been made with impregnation with solutions having a phosphoric base, either alone or in combination with compounds such as ammonium phosphate, magnesium or zinc phosphate, chlorides of alkaline metals, sodium tetraborate, etc. But also these methods do not ensure good protection against oxidation except at relatively low temperatures.  
  The purpose of the present invention is to supply a process and solutions capable of making carbon or graphite manufactured articles resistant to the oxidation caused by air, molten metals, or other high temperature oxidating means, in a wider temperature-range than that hitherto covered. Another. purpose, equally important, of this invention is to supply carbon or graphite manufactured articles capable of resisting high temperature oxidation better than those so far known. The ways to achieve the above objectives and the advantages deriving from the present invention will be iilustrated in the description given hereunder.  
  The objectives of the present invention&#39;are achieved by impregnating. or covering in some other way, the carbon or graphite manufactured articles, using a solution composed of:  
 water phosphoric acid one or more compounds selected in one or more of the groups formed by the salts. oxides, and hydroxides of a metal of the 2nd group of the periodic system one or both the components of the group formed by boric acid and sodium tetraborate decahydrate one or more compounds selected from among nitric acid, sodium nitrite, and sodium carbonate. v 5 The advantages of using such a composition with regard to protection from oxidation will be evident in the examples which follow. Other immediate advantages of these solutions consist in not using hydrochloric acid as solvent and in providing for relatively low concentrations of phosphoric acid, thus making the preparation, handling and application of the solutions easier; furthermore the process selected for the preparation of the manufactured articles does not require their bnklng. so that it is simpler and more economical thun other processes, which do require it.  
  According to the general idea of the present invention, the solutions inhibiting oxidation may have a percentage composition by weight varying within the ranges indicated below:  
  water 57.0 76.0; one or more compounds selected from among the salts, oxides and hydroxides ofa metal ofthe 2nd group (H 6.0; one or both the components of the group formed by sodium metaborate decuhydmte and boric acid 16.9 2.07: phosphoric acid 26.0 10.0; one or more compounds selected from amongnitric acid. sodium nitrate. and sodium carbonate 0.0 6.0;  
  A study of such solutions has shown that they can be divided into two groups, according to whether sodium tetraborate decahydrate or boric acid is used.  
  in the two cases the composition of the solutions can vary within the following limits, again expressed as a percentage composition by weight:  
 WHICI&#39; ho 57.0-67.0; phosp rscac one or more compounds selected from among the salts. oxides. and hydroxides of: metal ofthe 2nd group boric acid one or more compounds selected from among nitric acid, sodium nitrite. and sodium carbonate The results obtained by using the solutions described in the present invention as protective solutions are set forth in the following examples.  
 EXAMPLE 1 A sample of graphite weighing 0.450 gr. taken from an Elettrocarbonium brand electrode diameter 350 mm was placed in an emptiable container and subjected to degassing with a vacuum up to 5.10&#34; mm Hg; once the sample had been degassed, the impregnating solution was introduced into the container, still under vacuum, at a temperature of C, so that it covered the sample; the pressure is raised to a maximum of 5 atmospheres. The sample. removed from the solution.  
 &#39; was dried and seasoned at room pressure and temperature for about 48 hours.  
 The impregnating solution had the following percent- 5 age composition by weight:  
 water 62.8% phosphoric acid lR.0% calcium oxide 0.4% sodium tetruborate decuhydrate l4.h% nitric acid 2.9% sodium nltnue 0.3% sodlum carbonate L0;  
 Total Percentage Temperature Decrease in Weight up to l00OC 8.2%  
 up to I250C 22.8%  
 up to l400&#39;C 39.2%  
 up to IGOO&#39;C 63.3%  
 EXAMPLE 2 A sample having&#39;the samecharacteristics as the one in example 1 and obtained from the same zone of the electrode was treated. on&#39;the basis of the technique described above. with a solution having the following composition by weight:  
 water 62.8% phosphoric acid 18.0% zinc oxide 0.4% sodium tetrabomte decahydrate l4.6% nitric acid 2.9% sodium nitrite 0.3% sodium carbonate 1.0%  
  The solution was prepared in the way described in example l. The thermobalance test. carried out with the same procedure as in example I. gave the following results:  
 Total Percentage Temperature Decrease in Weight up to l000C 8.0% up to IZSO&#39;C 21.6% up to I400&#39;C 42.8% up to I600&#39;C 7l.4%  
 EXAMPLE 3 A sample having the same characteristics as the one in example i and obtained from the same zone of the electrode was treated, on the basis of the technique described above. with a solution having the following composition by weight: a  
 water 65.5% phosphoric acid l8.8% calcium oxide 0.4% sodium tetruborate decuhydrnte l5.3%  
  The thermobalance test. carried out with the same procedure as in example i, gave the following results:  
 Total Percentage Temperature Decrease in Weight up to IOOO&#39; C 4.5% up to l250&#39;C 25.4% up to l400C 43.8% up to l600C &#39;ll.0%  
 EXAMPLE 4 A sample having the same characteristics as the one in example I and obtained from the same .&#34;one of the electrode was treated, on the basis of the tee --que described above. with a solution having the following composition by weight:  
 wuter 65.5% phosphoric acid 18.8% cadmium oxide 0.4% sodium tetruborate decahydrute ISJVZ The thermobalance test. carried out with the same procedure as in example i, gave the following results:  
 Total Percentage Temperature Decrease in Weight up to IOOOC 6.8% up to 1250C 20.4% up to l400C 37.8% up to l600C 65.2%  
 EXAMPLE 5 A sample (sample D) of the type already used was treated, onthe basis of the technique described above, with a solution having the following percentage composition by weight:  
 water 74.2% phosphoric acid 20.4% calcium oxide 0.4% horic acid 5.0%  
  By means of the thermobalance test, carried out with the same procedure as in example I. the following were determined:  
 a. temperature of start of reaction b. speed of reaction expressed as percentage reduction in weight of the sample per C (percent C&#34;) c. the total percentage decrease in weight of the samrpteurpttntbeeterrmnmtmmtflfi. The results oT&#39;the &#39;testare *shown in&#39;tables l :anli 2.  
 EXAMPLE 6 A sample (sample C) of the type described above was treated, on the basis of the technique described above. with a solution having the following percentage composition by weight:  
  6 After impregnation the sample was dried and then suhjemedatmesieatiom an H1536, and; bakingah500C Tfhetrestswere thencnariietiruutwtthtthemmomu ance in accordance with example 5 and with the same procedure as in example I. The results obtained arc shown in tables I and 2.  
 water 64.8; phosphoric acid 18.6; calcium oxide 0.4; sodium tetraborate decahydrate l5.l; sodium carbonate Ll;  
  By means of the thermobalance test. the same determinations as in sample 5 were made. The results obtained are shown in tables 1 and 2.  
  in order to emphasize the advantages of the series of solutions covered by the present invention, a comparison was made between samples treated according to examples 5 and 6 and other samples, of which one not treated (sample A) and another treated with a solution and a method already known in technique (sample B). All the samples were of course obtained from the same zone of the same electrode.  
  A non-treated sample (sample A) was tested directly; on the thermobalance, with the procedure set forth in example I; the same determinations as in sample 5 were carried out. The results are shown in tables 1 and 2.  
  The other sample (sample B) was treated with the solution and the method which seem to have given the best results in known technique. Such solution has the following percentage composition by weight:  
 In order better to evidence the properties of the solutions covered by the present invention, other experiments were carried out, the results of which are shown hereunder.  
 Table 3 shows the percent losses in weight after a stay by the samples of 4 hours at the temperature t.  
 The tests were made in a muffle with an air circulation obtained by natural draught and the samples. about 160 g, were placed in a magnesite crucible when the muffle had reached the working temperature.  
 The percentage increase in weight of the samples.  
 due to impregnation, was always at least 5 percent after seasoning.  
 Tables 4 and 5 show the results obtained on the thermobalance when using the solution of example 5. but varying once the calcium oxide and once the phosphoric acid respectively. and maintaining the other components constant in order to demonstrate the importance of the composition of the solutions for purposes of good protection; it is found that the percentages used in the solution of example 5 are the most suitable ones.  
  water 21.9; phosphoric acid 63.3; Table 3 zinc oxide 13.8; hem: and Percentage los afier 4 hours at temperature tC t&#39;C 900&#39; 1200&#39; 1400 The impregnation vessel containing the sample was h&#39;t 7 36 0 degassed and then filled, still under vacuum, with the fi f i 3 3 above solution. lmpregnatron under pressure was then cxarnpe :0. g :3 36 exampe o. 4| carried out. with a maximum pressure of 5 atmo ample Na 6 H 25 37 spheres.  
 Table 4 Percentage of calcium oxide 0.] 0.4 3.8  
 Total decrease in weight up to IOOOC 9 5.3 9 lZSOC 2b 19 2X HOOC 44 34 45 I600&#39;C 72 56 64 Table Percentage of phosphoric acid I09 NH 23 Total &#39;1 decrease in weight up to HXXPC 9 5.3 7 I250C 25 19 22 l-ttXlC 39 34 36 l6(l(l&#39;C 63 $6 60 The attached graph shows the results of some experiments which give a comparison between non-treated graphite (curve a); graphite treated with the solution already known to technique. and used in description after example 6 (curve b); graphite treated according to example 5 (curve c); graphite treated according to example 6 (curve d). The ordinates of the graph represent the percentage losses in weight of the sample when heated up to temperature tC. as shown in the abscissae with heating speed of C/min in air flow equal to 6.4 litres/h. From the foregoing examples and comparisons it can be seen that there is a clear improvementin the properties of resistance to oxidation that the solutions proposed as new by the present invention are capable of giving to graphite or carbon bodies. as compared with the. results which can be obtained with solutions already known in technique.  
  It should be borne in mind that. although the samples tested in the various experiments were all obtained from electrodes. the present invention refers to-the treatment of any type of manufactured articles made from amorphous carbon or graphite, such for example as crucibles. refractory bricks. resistors and also materials treated with carbon products. Furthermore. although the examples have mentioned only one particular technique of impregnation. it is clear that the present invention also refers to any other technique of application (varnishing. immersion. etc.) of the solutions forming the subject of the present invention.  
 What we claim is:  
  1. Process for giving resistance to oxidation to manufactured articles made of amorphous carbon or graphite. characterized by the fact that:  
 a. said manufactured articles are degassed and immersed. under vacuum. in a solution obtained by mixing: water; phosphoric acid; one or moreicompounds selected from the group of salts. oxides and hydroxides of a metal of the 2nd group ofthe periodic system; one or both components of the group formed by sodium tetraborate decahydrate and boric acid; one or more components-selected from the group formed by nitric acid. sodium nitrite, and sodium carbonate; the components of the said solution are mixed in accordance with percentage ratios by weight ranging between 57.0 26.0 0.1 16.9 0.0 and 76.0 10.0 6.0 2.0 6.0 respectively;  
 b. the said manufactured articles are impregnated with said solution;  
 c. the said manufactured articles are allowed to dry and lemon at room temperature.  
  2. Process for giving resistance to oxidation to manufactured articles made of amorphous carbon or graphite. characterized by the fact that:  
 a. said manufactured articles are degassed and immersed. under vacuum. in a solution obtained by mixing: water; phosphoric acid; one or more components selected from the group of salts. oxides and hydroxides of a metal of the second group of the periodic system; sodium tetraborate decahydrate; one or more compounds selected from the group formed by nitric acid. sodium nitfite. and sodium carbonate; the components of the solution are mixed in accordance with percentage ratios by weight ranging between 57.0 20.0 5.0 12.0 6.0 and 67.0 16.0.: 0.4 16.6 0.0 respectively;  
 b. the said manufactured articles are impregnated 30 &#39;with said solution;  
 c. the said manufactured articles are allowed to dry and season at room temperature. 3. Process for giving resistance to oxidation to manufactured articles made of amorphous carbon or graphite.-characterized by the fact that:  
 a. said manufactured articles are separated and immersed. under vacuum. in a solution obtained by ;mixing: water; phosphoric acid: one or more compounds selected from the group of salts. oxides and hydroxides of a metal of the-second group of the periodic system; boric acid. one or more components selected from the group formed by nitric acid. sodium nitrite. and sodium carbonate; the components of the solution are mixed in accordance with percentage ratios by weight ranging between 70 .O 2L0 2.0 2.0 5.0 and 76.0 16.6 0.4 :&#39;7.0: 0.0 respectively;  
 b. ,the said manufactured articles are impregnated with said solution;  
 c. the said manufactured articles are allowed to dry and season at room temperature.  
  4. Process in accordance with claim 1. in which the metal of the 2nd group of the periodic system is calcium.  
  5; Process in accordance with claim 1, in which the metal of the.2nd;group of the periodic system is cadmium.  
 i l t t UNITED STATES PATENT OFFICE CETIFICATE 0F CORRECTION PATENTNO. 3,881,039 DATED April 29, 1975 |NVENTOR(S) Francesco Baldieri and Carlo Borgianni it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 On the Title page, under designation [30] &#34;Foreign Application Priority Data&#34;, please delete &#34;19661/71&#34; and insert 19661 A/70 Signed and Sealed this second D ay of September I 9 75 [SEAL] Arrest:  
 RUTH C. MASON C. MARSHALL DANN Arreslmg Officer (mnmissiuncr uj&#39;lalents and Trademark-x UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO. 3,881,039 DATED April 29, 1975 |NVENTOR(S) Francesco Baldieri and Carlo Borgianni it is certified that error appears in the ab0veidehtified patent and that said Letters Patent are hereby corrected as shown below:  
 On the Title page, under designation [30] &#34;Foreign Application Priority Data&#34;, please delete &#34;19661/71&#34; and insert 1.9661 A/7O Signed and Sealed this second D 3) Of September 19 75 [SEAL] Arrest:  
 RUTH C. MASON C. MARSHALL DANN Altesling Officer (umml&#39;rsinner uflalenrs and Trademarks