METHOD FOR TREATING STAINLESS STEEL SURFACE BY HIGH TEMPERATURE OXIDATION

Abstract: This disclosure is concerned with a method for treating a stainless steel surface by high temperature oxidation. The surface of a stainless steel article is cleaned. Optionally, TiO₂ and SiO₂ are mixed as micro­ particles to form a coating agent, and water is added to the mixture to make a slip. The slip is coated on the steel surface to form a coating having a uniform thick­ ness. The coating is dried and the article is subjected to a heat-treatment to form an oxide film. A desirable temperature for the heat-treatment is between 350°C to 700°C. When a coating agent is used, it is later removed by washing, etc.after cooling the article. Finally, a decolorization treatment is applied; that is, the colored oxide film is removed from the surface by dissolution using an acid or an electrolytic treatment.

Description: This invention relates to a method for treating the surface of stainless steel by high temperature oxidation. Conventionally there has been a "metal coloring" method that allows an oxide film formed on the surface of various metals, such as aluminum, titanium or stainless steel, etc., to develop color by utilizing the phenomenon of light interference. Since this method can produce various color tones by controlling the thickness of oxide film without destroying the native brightness of the base metal, the method has been widely used on ornamental or construction materials. The conventional methods for metal coloring comprise: (I) Dipping metallic material in chemical reagents (II) Anodically oxidizing in chemical reagents (III) Oxidizing at elevated temperatures in an oxidizing atmosphere (refer to Japanese Laid Open Pat. Appl. Nos. 48-99047, 49-58035 & 52-134833) Regarding (I) above, since the color tone of an oxide film varies delicately depending on the composition of the reagent and on the dipping time (the color changes with every second and every minute), the color development requires a fine control against degradation of reagents. As to (II) above, inhomogeneities in the electric current density, or generation of oxygen gas can cause an unevenness in the coloring. Therefore, the treated material is limited to metal having simple configurations such as plates or sheets. Colored oxide films obtained by the methods (I) (II) are subject to corrosion or abrasion because of their high porosity, and the film requires a hardening treatment after each coloring. As to method (III) above, the method is widely used for coloring materials such as stainless steels or titanium alloys having high temperature strength, because the method is easy to practice and can give a solid colored oxide film. While this method can form a colored oxide film having a tone corresponding to the heating temperature of the treated metal, it has a drawback in that it causes an unevenness or shading in color, resulting in a poor appearance, because the degree of oxidation differs depending on the location of the metallic surface. Therefore, the use of this method has been limited to the blackening treatment of heat exchanger tubes or to small parts in respect of which there is no concern for aesthetic appearance. In the food or pharmaceutical industry, stainless steel is often used for equipment or factory plant, such as storage tanks, pipes or valves. The corrosion resistance of stainless steel is maintained, in general, by a passive film of Fe-, Cr-, Ni-oxide. However, because the thickness of the coating is only several Å or tens of Å, the dissolution of Fe-ions cannot be avoided. For example, in the brewing industry, sake, wine, beer, etc. contain various kinds of organic acids. In particular, the inner or outer surfaces of storage tanks, ultrafiltration equipment and/or pipes are treated by buffing or pickling to prevent the adherence of germs or sal tartar and to improve their cleanliness. For example, the surface of ultrafiltration equipment used in the manufacture of sake is treated with a No. 400 mirror finish, because of the dissolution of iron into sake and the sanitary standards to be maintained. However, when sake is stored for longer than 10 hours, iron can dissolve from the stainless steel surface into the sake, making the sake colored and lowering its commercial value from the viewpoint of its taste. Accordingly, nowadays materials for piping in such plants or for the modules of ultrafiltration equipment include plastic or plastics-lined materials which are immune to the dissolution of iron. In the pure chemical field, or a field that requires clean water such as a nuclear power station or the electronics industry, there are many processes that need water or solutions free from dissolved Fe-ions. Corrosion-resistant stainless steel is expected to have increased corrosion resistance as a result of a coloring process, but in practice such coloring can decrease the resistance, depending upon the treatment process (Refer to Table 4 herein.) Accordingly, the coloring process can leave some problems for uses where high corrosion-resistance is required. The reason for the deterioration of corrosion-resistance seems to be due to the fact that the oxide film formed by the heat-treatment after mechanical abrasion is not so dense nor so uniform that the base-metal cannot be subjected to crevice corrosion or pitting corrosion. One solution for this problem is to dip a stainless steel article having a colored oxide film formed by high temperature oxidation in a nitric acid solution to passivate the base metal at the defective location of the film. This process helps to preserve the corrosion resistance from deterioration to some extent, but it has the risk of causing dissolution of the colored oxide film resulting in a change of color tone. Moreover, in a field of use where extremely rigorous conditions exist, the amount of dissolution of Fe-ions following this prior art process still proves to be too great. Therefore, it is an object of the invention to achieve a drastic reduction in the dissolution from stainless steel of Fe-ions as compared to the prior art. The invention accordingly provides a method for treating a stainless steel surface to decrease Fe-ion dissolution therefrom during subsequent use, in which method a colored oxide film is formed on said surface by high temperature heat-treatment in an oxidising atmosphere, characterised in that before the high temperature heat-treatment the surface to be treated is subjected to a cleaning step that includes electrolytically polishing the surface, and after cooling following the high temperature heat-treatment step the surface is subjected to a decolorizing step in which the colored oxide film is removed. A further procedure that may optionally be employed is as follows: after electrolytic polishing and before the high temperature heat-treatment the surface to be treated has applied to it a coating agent comprising inert micro particles having a high melting point such that they will not be changed chemically or be melted during the high temperature heat-treatment, and after cooling of the surface following the high temperature heat-treatment the layer of coating agent is washed away. In the preferred method, first, the surface of a stainless steel article to be colored is electrolytically polished to improve the characteristics of the polished surface of the base metal suitably for the subsequent formation of the oxide film. Then as an optional step the surface is treated with a coating agent, and afterwards the article is subjected to a heat-treatment in an oxidizing atmosphere, the temperature and time of treatment corresponding to the color tone to be colored. The coating, if employed, is then removed and the decolorizing step is carried out. This process is summarized in more detail as follows: (1) The surface of the stainless steel article to be colored is cleaned by a traditional process, for example, by pickling, buffing and degreasing, to remove oxides or impurities on the surface, and then polished completely by electrolytic polishing. (2) Before the heat-treatment is performed, the surface may be optionally treated with a coating agent consisting of high-melting-point microparticles. (a) The coating agent is composed of materials that do not melt even under the high temperature-heating of this method. As a suitable coating agent powders of TiO₂ and SiO₂ are mixed in a ratio between 100: 0 and 25: 75 in weight and the mixture is pulverized with a crusher, such as a ball mill, etc., and graded by a 150-mesh sieve to achieve a small particle size, and water can then be added to the small-sized microparticles to make a slip. The grading or size adjustment is performed accurately; if the slip contains some coarse particles, the oxidation film becomes uneven at locations where coarse particles contact with the metallic surface and a speckled oxide film is formed during the heat treatment. It has been experimentally confirmed that when the particles are adjusted to sizes smaller than 150-mesh, a slip consisting of such particles causes no unevenness in color. (b) Depositing the coating agent on the exposed metal surface, after the cleaning treatment, is performed by spattering, pouring the slip or dipping the object in the slip; or alternatively by sprinkling the dried coating agent, etc. Among the above methods, spraying the slip is advantageous for the preadjusted slip because it gives a uniform thickness of the coating, like spraying enamel on a glass lining. As already indicated, an optional component of the coating material is SiO₂ which can improve the spraying property of the slip. However, with increasing mixing ratio of SiO₂, the adhesive strength of the dried coating decreases; accordingly, the mixing ratio of SiO₂ is preferably kept under 75%. It is important to coat in such a manner that the coating has a uniformly distributed thickness after completion of the coating. When the thickness of the coating differs depending on the coated location, the difference in the oxidation speed generates different shades of color of the tone of the formed oxide film. A preferable thickness of the slip coating is 0.1 to 1mm. When the thickness is too thin, unevenness in the oxidation grade easily causes irregularities and shades in color, whereas when it is too thick, irregularities in color vanish but the oxidation speed decreases, leading to a longer time required for the heat-treatment. (c) The deposited coating is then dried completely. (3) The heat treatment is then carried out to form the oxide film. This treatment is performed in an oxidizing atmosphere at a temperature and for a time corresponding to the color tone to be achieved. The preferred temperature for the heat-treatment is 350° to 700°C. At temperatures lower than 350°C, formation of the oxide film becomes incomplete. At temperatures higher than 700°C (heat-resisting temperature of stainless steel being assumed to be 800°C), the oxide film becomes too thick which results in it being too brittle. Stainless steel can undergo precipitation of chrome-carbide at temperatures between 450° and 750°C depending on the type, leading to a risk of pitting corrosion or stress-corrosion cracking. Therefore, when the equipment or apparatus is to be used under severe corrosive conditions, it is recommended that the temperature for the heat-treatment be limited to lower than 450°C. At each heating-temperature, the growth in thickness of the oxide film is retarded at the expiry of the heating time. Since the heating time differs depending on the circumstances of the process, it is recommended to determine a desirable heating time matched to a stable thickness of the film, in accordance with the result of an experiment performed with some test pieces to become familiar with the formation behaviour of the oxide film. These heating temperatures and times are to be determined by considering the type of steel and the behaviour of the coating, and by cross-reference to the examples to be described later and the accumulated data of pretrials. Since the oxide film is formed under the coating of the coating agent, it cannot be distinguished visually during the process. (4) Afterwards, the coating agent, if employed, is removed by washing or other means after cooling. (5) Finally, the decolorizing treatment is performed, that is, the colored oxide film is dissolved and removed as by acid or by an electrolytic treatment. Though each step described above is a separate one, a preceding step affects closely a following step. For example, practising the cleaning treatment of the first step with electrolytic polishing will affect the last process profitably. And at the second step, a uniform application of the coating agent, consisting of high melting point microparticles, before the high temperature heating will facilitate the practice of the third step thereby preventing a possible adverse result. The electrolytic polishing is physically different from mechanical polishing and since the electrolytic polishing is a type of chemical polishing, the surface of the stainless steel subjected to the electrolytic polishing reveals some characteristic chemical change. When a desired colored film is formed on the electrolytically polished stainless steel article by keeping it at a predetermined temperature and for a predetermined time in the oxidizing atmosphere, the film is more dense, has a better appearance and has a better corrosion resistance property as compared with an oxide film formed under similar conditions after only a mechanical polishing. The reason seems to be due to the fact that metallic components of the stainless steel surface are changed by the electrolytic polishing, and it is assumed, correctly it is believed, that the chrome content has been condensed 1.5 to 2 times compared with the content before the polishing. Since chrome has more corrosion resistance than iron, the surface condensed to increase the chrome content seems to have improved corrosion resistance. When the surface of stainless steel is heat-treated without a layer of a coating agent, the surface has a color unevenness due to the difference in oxidation gradation. If the coating agent including TiO₂, SiO₂ is applied uniformly before the heat-treatment, the colored oxide film is formed uniformly with no color unevenness or shading. A relatively long period of heat-treatment makes the operation easier and serves to produce a stable result. The colored oxide film formed by heat-treating the stainless steel in an oxidizing atmosphere appears to consist of Fe₂O₃, Cr₂O₃, NiO and compounds combined with them. Because the oxidation speeds among Fe, Cr and Ni are different from each other, it is assumed that within the colored oxide film the relative amount or content of the Fe component is larger, whereas at the interface between the colored oxide film and the base metal underneath the relative contents of the Cr and Ni components become larger and the content of Fe becomes relatively less. Accordingly, by removing the colored oxide film having more Fe on its surface, the interface having more Cr and Ni components is exposed. This exposed surface seems to act effectively to decrease the amount of Fe-ions dissolving into a contacting liquid during use. According to an experiment on SUS 304, the color of the colored oxide film formed by the heat-treatment of the above process step (3) depends on the temperature of the heat-treatment; for example, a heating temperature of 350° to 400°C produces a golden color, a temperature of 500°C produces a red color and 800°C produces a blue color. On the other hand, the decolorized surface of stainless steel subjected to the above process step (5), after heat-treatment at a temperature of 500°C, maintains the original metal brightness with no change in color; at 600°C heat-treatment it becomes a light golden color and at 800°C it begins to bear a light blue color. These phenomena show that the composition of the stainless steel surface subjected to the process step (5) differs from that of the original stainless steel, and support the theory that the Fe component at the surface has been decreased while the Cr, Ni components have been increased. During the electrolytic polishing applied in the cleaning treatment of step (1), Fe dissolves selectively leaving the Cr more concentrated, so that the process step (1) further enhances the drastic reduction in dissolution of Fe-ions from the stainless steel surface that is achieved by application of process step (5). Furthermore, since practising the application of a coating agent according to step (2) before the heat-treatment enables a colored oxide film of uniform thickness to be achieved, the decolorizing treatment of step (5) can then be carried out smoothly, without producing unevenness. In general, a passive film formed on the surface of stainless steel comprises oxides of Fe, Cr, Ni (in the form of Fe⁺⁺⁺, Cr⁺⁺⁺, Ni⁺⁺⁺) several Å in thickness. On the other hand, the film formed by the method of the present invention seems to comprise (CrFe)₂O₃. (FeNi)O · xH₂O having a 300 to 500 Å thickness and a stable state. As a result it is presumed that the amount of iron that can be dissolved from the surface of the stainless steel, as ions of Fe⁺⁺ or Fe⁺⁺⁺, is very small. Though the mechanism of dissolution of Fe into a liquid in equipment or apparatus during use is not known accurately, the result of one experiment using test pieces is given as follows, compared with the result when using the prior art methods of buffing and pickling. The material used in the test was SUS 304. Test conditions _ with sake, normal temp., 20 hr. dip (electrolytic polish + high temp. oxidation) In the above table, the dissolution amount of Fe is equal to the measured amount minus the Fe concentration inherently contained in sake. Amount of liquid per cm² contact area of the test pieces was taken as 0.16ml. Experiment I The surfaces of SUS 304 stainless steel pipe and SUS 316 stainless steel sheet were first buffed and then degreased with a ketone or alcohol. Equal amounts of TiO₂ and SiO₂ were mixed together, pulverized to form particles less than 150-mesh and dispersed in water to form a slip. The slip was applied on the surface of the steel pieces by spraying to make a uniform coating having about 0.2 mm in thickness. After drying of the coating, heat-treating the coating in a heating furnace under conditions as described in Table 1 produced various kinds of colored oxide film having various tones without color unevenness or shading, as set out in Table 1. In the case of stainless steel, the color of the oxide film varies with the heating temperature, as already described. With increasing time, the color concentration increases and remains stable after 30 minutes. Experiment 2 As test pieces of stainless steel, pipes of 1 inch (2.54 cm) in diameter and sheets having the dimension of 30 mm × 40 mm × 1 mm, made of SUS 304 and SUS 316, were used. The treating method was as follows: The surfaces of the stainless steel test pieces to be colored were buffed to remove solid foreign substances from the surfaces, and degreased by a ketone or alcohol; this was followed by the electrolytic polishing. The polishing was performed by using an acidic electrolyte under conditions of a current density of 5 to 30A/dm² and an energizing time of 15 min. The test-pieces were made completely free from the electrolyte by washing them, and then dried and placed in a heating furnace to be subjected to the heat-treatment under the conditions described in Table 2 to form the colored oxide film. The color tone is also described in Table 2. Because they had a small surface area, the test pieces for the above corrosion test were heat-treated without a coating. However, the test-pieces exhibited colored oxide films having the tones given in Table 1, without color unevenness or shading. Test pieces having the dimensions and treatment as described in this example were subjected to a corrosion test to compare them with test pieces treated according to the prior art. The results are given hereunder. (1) SUS 304 stainless steel A corroding solution having a pH of 3 was formed by adding 1cc of 85% lactic acid to 3 l. pure water treated by ion-exchange. Each test piece was dipped in 250cc of the solution for 48 hrs. at 50°C. The results are shown in Table 3. Each test piece was dipped in 180cc of pure water, deaerated with nitrogen gas, for 250 hr. The results are shown in Table 4. Experiment 3 Test pieces made of SUS 304 stainless steel were subjected to various treatments according to this invention, and according to the prior art, to compare their corrosion resistance. The results were as follows: Test (I) Treatment conditions Sample 1 mechanical polishing with #600 Sample 2 only electrolytic polishing Sample 3 electrolytic polishing and heat-treatment at 450°C for 30 min. Sample 4 mechanical polishing with #600, heat-treatment at 450°C for 30 min., and oxide film removed with 1N-HCl. Sample 5 (this invention) electrolytic polishing, heat-treatment at 450°C for 30 min. and oxide film removed with 1N-HCl. Corrosion test conditions A corrosive solution having a pH of 3 was formed by adding 1cc of 85% lactic acid to 3 l. pure water treated by ion-exchange. Each test piece having the dimensions 30 mm × 40 mm × 1 mm was dipped in 250cc of the solution for 48 hrs. at 50°C. Test result The amount of dissolved Fe-ion and Cr-, Ni-ion in the solution is shown in Table 5. Test (II) Treatment conditions Sample 6 electrolytic polishing, heat-treated at 450° for 30 min. Sample 7 (this invention) electrolytic polishing, heat-treated at 450°C for 30 min., oxide film removed with 1N-HCl. Corrosion test conditions Test pieces having the same dimensions as for Test (1) were dipped in 250cc of a 0.1 wt% sulfuric acid solution at 50°C for 96 hrs. Test result The amounts of dissolved Fe-ion and Cr, Ni-ion in the solution are shown in Table 6. The decolorizing treatment conditions of the oxide film differ depending on the parameters obtaining, such as the thickness of the oxide film, the type of acid, the concentration and temperature of the acid, etc. Therefore, before industrial use, it is desirable to determine each condition by means of the result of an experiment performed on some test-pieces, to become familiar with the decolorizing behavior. The behavior upon removal of the oxide film can be confirmed visually by the experiment. The surface treatment in accordance with this invention of various kinds of typical parts of brewery equipment or apparatus made of stainless steel are described as follows: (I) Examples of simple configurations (I-1) Tanks The surface of a stainless steel tank was cleaned by the electrolytic polishing method. A coating agent of SiO₂, mixed if desired with TiO₂ in an amount by weight from 0 to 25%, was formed and the mixture was sieved and processed so that all particles would pass through a 150-mesh sieve. This mixture was used as the coating agent which, after mixing with water, was coated on the surface of the metal so that the coating had a uniform thickness between 0.1 to 0.2 mm. Then the coating was dried and the surface heated at a predetermined temperature between 350° to 450°C in an oxidizing atmosphere to form the oxide film. After cooling to room temperature the coating agent was washed away and removed. Now, the removal treatment for the oxide film may be performed. (I-2) Pipes The inner surfaces of stainless steel pipes were cleaned by electrolytic polishing and the coating agent described above was coated on the surfaces by spraying or casting. Then the coating was dried and the surface heat-treated to form the film under similar conditions as described above. Next the coating agent was removed by washing. Now, the oxide film can be removed. The advantages of the method of this invention are summarized as follows. (a) In the prior art, coloring a metal surface with a high temperature oxidation treatment causes color unevenness or shading which lowers the value of the product, whereas the method according to the present invention produces a uniform coloring with no unevenness. Further, compared with the method of the prior art with accompanying reagent treatment, the method of the present invention provides the product with an improved corrosion resistance. (b) By practising the high temperature oxidation coloring of this invention with the use of a heating furnace capable of good temperature control, metals having complex configurations can be successfully treated or large numbers of articles can be treated in quantity at a time. Thus the method of this invention provides the advantage of widening the scope of the process as well as of mass-producing pieces at a reduced cost. (c) Further, since the method of this invention reduces the dissolution of Fe-ions to a very small amount, equipment and pipes used for pharmaceuticals or in the food industries, which conventionally require high corrosion resistant alloys or nonmetallic materials such as glass linings, can be fabricated in ordinary stainless steels treated according to the invention. Wherever a reference is made herein to heating an article in an oxidizing atmosphere, the ambient air present in a heating oven can serve as the oxidizing atmosphere.

Claims: 1. A method for treating a stainless steel surface to decrease Fe-ion dissolution therefrom during subsequent use, in which method a colored oxide film is formed on said surface by high temperature heat-treatment in an oxidising atmosphere, characterised in that before the high temperature heat-treatment the surface to be treated is subjected to a cleaning step that includes electrolytically polishing the surface, and after cooling following the high temperature heat-treatment step the surface is subjected to a decolorizing step in which the colored oxide film is removed. 2. A method according to Claim 1, wherein after electrolytic polishing and before the high temperature heat-treatment the surface to be treated has applied to it a coating agent comprising inert micro particles having a high melting point such that they will not be changed chemically or be melted during the high temperature heat-treatment, and after cooling of the surface following the high temperature heat-treatment the layer of coating agent is washed away. 3. A method according to Claim 1 or Claim 2, wherein the high temperature heat-treatment is carried out at a temperature in the range 350°C-700°C.

Inventors: GOTO, SHIGEO, HATA, TOMIHIRA, SHIBATA, MITSUAKI, TAKAHASHI, HARUJI, TAKATA, SYUICHI
Applicants: SHINKO PANTEC CO., LTD.
Application year: 1985
Main classification: C23C8
IPC codes: C23C8