Patent Publication Number: US-2016237534-A1

Title: Germanium-bearing ferritic stainless steels

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to ferritic stainless steels, in particular, to ferritic stainless steels as a base material before having germanium contained therein. 
     2. Description of Related Art 
     Metals widely used in living goods, tools, and equipments have been indispensable in daily life and industry, as well as in the development of technology. However, metals are inevitably subjected to corrosion in use, resulting in the quality downgrade because of aging and degradation, both of which could be the cause of the inconvenience and even the public-threatening environmental pollution and industrial safety. 
     In order to reduce the corrosion and meanwhile to enhance the anti-corrosion capability, certain approaches such as the use of the anti-corrosive stainless steel, surface coating, cathode protection, and/or anode corrosion prevention have been developed. The fundamental solution may rest on the use of the stainless steel and the selection of different type of stainless steel at different environments and conditions, prompting the development of various stainless steels. 
     Stainless steels could be categorized in terms of the addition elements. Specifically, based on the different amount of the addition of nickel and chromium, stainless steels can be divided into chromium, chromium-nickel, chromium-nickel-manganese, and low chromium-based stainless steels. Characteristics of stainless steels are: 
     (1) chromium: 400 series-based having no nickel or nickel less than 2.5 wt %, including martensitic stainless steels and ferritic stainless steels; 
     (2) chromium-nickel: 300 series-based austenitic and 600 series-based precipitation hardening stainless steels as the most common stainless steels with the addition of nickel to maintain a stable austenitic structure; 
     (3) chromium-nickel-manganese: 200 series-based stainless steels as another kind of relatively inexpensive austenitic stainless steels mainly replacing a part of nickel in the 300 series with manganese; and 
     (4) low chromium: 500 series-based with chromium between 4 to 6 wt % that is not de facto stainless steels and primarily used in the petrochemical industry. 
     Stainless steels may also be categorized into five types including austenitic, ferritic, martensitic, precipitation hardening, and duplex ones in terms of microstructure. Alloy content in stainless steels may differ from one to another, resulting in different corrosion and mechanical properties. The addition of alloying elements to stainless steels plays a critical role in properties. For example, the addition of chromium and nickel can improve the corrosion resistance, the addition of niobium and titanium could reduce the inter-granular corrosion, while the addition of aluminum could enhance the mechanical and elevated temperature corrosion properties. 
     Common stainless steels are austenitic stainless steels containing a large amount of nickel as an FCC stabilizer. With the addition of nickel, stainless steels transform into FCC structure with better mechanical properties, and as a result such stainless steels could be more applicable. For example, because of their superior anti-corrosion capability, ductility, and good weld ability, 304 stainless steel could be used in almost every environment. However, due to the demand of nickel to keep up with the growth in use of the stainless steels, the price of nickel typically dictate the price of stainless steels. Therefore, research in recent years gradually turns to the availability of other elements to replace nickel in stainless steels at no expense of anti-corrosion capability and the weld ability. 
     When it comes to cutting down the cost, the cheaper replacement trace materials should be chosen, which may also help achieve the goals of the superior anti-corrosion performance and weld ability. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a kind of germanium-bearing ferritic stainless steels based on ferritic stainless steel with various trace additions of germanium. 
     The present disclosure provides a kind of germanium-bearing ferritic stainless steels that may help the forming of passivation film, maintain the stability of the same, enhance repairing capability of the passivation film, improve the anti-corrosion capability of alloys, and turn the corrosion within solution having chlorine ions to be a uniform one. 
     The present disclosure provides a kind of germanium-bearing ferritic stainless steels, which, after being immersed within sodium chloride solution, could be uniformly corroded. 
     The germanium-bearing ferritic stainless steels may be prepared from a raw material containing chromium that is 15 to 25 in weight percentage (wt %), manganese that is 0.3 to 0.9 in weight percentage, silicon that is 0.15 to 0.30 in weight percentage, germanium 0.1 to 1.2 in weight percentage, and iron rounding up the remaining of the raw material. 
     The stainless steels may have germanium ranging from 0.1 to 0.3 in weight percentage of the raw material. 
     The stainless steels may have germanium ranging from 0.3 to 0.8 in weight percentage of the raw material. 
     The stainless steels may have germanium ranging from 0.8 to 1.2 in weight percentage of the raw material. 
     The stainless steels after being immersed within sodium chloride solution may be uniformly corroded. 
     For further understanding of the present disclosure, reference is made to the following detailed description illustrating the embodiments and examples of the present disclosure. The description is only for illustrating the present disclosure, not for limiting the scope of the claim. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings included herein provide further understanding of the present disclosure. A brief introduction of the drawings is as follows: 
         FIG. 1  is the flow chart of preparation of the germanium-bearing ferritic stainless steels according to one embodiment of the present disclosure; 
         FIG. 2  is a table showing CS alloy elements of the germanium-bearing ferritic stainless steels according to one embodiment of the present disclosure; 
         FIG. 3  is a table showing CS alloy elements of a traditional ferritic stainless steel with copper and tin; 
         FIG. 4A  shows linear polarization curves of a traditional ferritic stainless steel with copper and tin (CS200+Cu—Sn) in 0.1 M (molar concentration) sulfuric acid solution; 
         FIG. 4B  shows parameters of the linear polarization curves for a traditional ferritic stainless steel with copper and tin (CS200+Cu—Sn) in 0.1 M sulfuric acid solution; 
         FIG. 5A  shows linear polarization curves of a germanium-bearing ferritic stainless steel (CS200+Ge) in 0.1 M sulfuric acid solution according to one embodiment of the present disclosure; 
         FIG. 5B  shows parameters of the linear polarization curves for a germanium-bearing ferritic stainless steel (CS200+Ge) in 0.1 M sulfuric acid solution according to one embodiment of the present disclosure; 
         FIG. 6  shows variation in hysteresis loop of CS alloy of the germanium-bearing ferritic stainless steels; 
         FIG. 7  shows results of open-circuited potential tests for a traditional ferritic stainless steel (CS200+Cu—Sn) in 0.1 M sulfuric acid solution; 
         FIG. 8  shows results of open-circuited potential tests for a germanium-bearing ferritic stainless steel (CS200+Ge) in 0.1 M sulfuric acid solution according to one embodiment of the present disclosure; 
         FIG. 9  shows a comparison of ion concentration of a traditional ferritic stainless steel (CS200+Cu—Sn) in sulfuric acid solution; 
         FIG. 10  shows a comparison of ion concentration of a germanium-bearing ferritic stainless steel (CS200+Ge) in sulfuric acid solution; 
         FIG. 11  shows a comparison of ion concentration of a traditional ferritic stainless steel (CS200+Cu—Sn) in sodium chloride solution; and 
         FIG. 12  shows a comparison of ion concentration of a germanium-bearing ferritic stainless steel (CS200+Ge) in sodium chloride solution. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     The aforementioned and other technical contents, features, and efficacies will be shown in the following detail descriptions of a preferred embodiment corresponding with the reference Figures. 
     As shown in  FIG. 1  showing a flow chart of steps of preparing a germanium-bearing ferritic stainless steel according to one embodiment of the present disclosure. The germanium-bearing ferritic stainless steels of the present disclosure may be 430 stainless steel-based with iron (Fe), chromium (Cr), manganese (Mn), and silicon (Si) as the main constituent elements along with trace addition of germanium (Ge). The above-mentioned main components of the stainless steels of the present disclosure are shown in  FIG. 2 . The above constituent elements together may be considered as raw material for the germanium-bearing ferritic stainless steels. And the preparation of the germanium-bearing ferritic stainless steels may be through melting. Before any melting could be proceeded, the raw material may be placed in a water-cooled copper mold  101  within a vacuum arc melting furnace. A lid of the vacuum arc melting furnace may be placed at its predetermined position to ensure the chamber of the vacuum arc melting furnace may be properly vacuumed to be at 2.4×10 −2  torr, before having nitrogen flown into the chamber so that the chamber reaches 8 torr. Such vacuuming and nitrogen flowing may repeat for three times, before the melting using the vacuum arc melting furnace could officially start  102 . 
     In the vacuum arc melting furnace, the raw material may be uniformly melted by the vacuum arc melting before being solidified using the water-cooled copper mold to become a bowl-shaped test piece. Such test piece then may be flipped over and over again for four times in one implementation until the alloy component in the raw material is confirmed fully melted and homogeneously mixed  103 . The chamber may be then de-vacuumed for the ingot, which is the CS alloy in the alloyed state, to be removed. Such ingot may then be sliced and polished for the preparation of the germanium-bearing ferritic stainless steel specimens  104 . 
     Thereafter, in order to reduce the influence of the pores and trace deviation associated with the alloy, germanium-bearing ferritic stainless steel specimens may be processed at 1100 degrees Celsius. Such process may include having the melted specimens in the as-cast state sealed inside a quartz tube, heating the tube at the increasing rate of 4.5-degree Celsius until reaching the 1100-degree Celsius, and maintaining the tube at the same degree for six hours. When the six-hour mark is reached, the quartz tube may be retrieved for water quenching treatment. As the temperature of the specimens is at the room temperature, the tube is broken so that the CS alloy specimens in the homogeneous state may be obtained. 
     Germanium-bearing ferritic stainless steel specimens may go through electrochemical experiments such as linear polarization scanning, impedance spectroscopy, cyclic voltammetry, and/or open-circuited potential tests and different corrosion solution tests, for the evaluation of corrosion properties of the CS alloys with the trace-added Cu—Sn and Ge. Tests of an optical microscope (OM) for the analysis of microstructure of the CS alloy, inductive coupled plasma (ICP) for the analysis of the composition of the CS alloys soaked within the test solution alloy composition analysis, and X-ray photoelectron spectroscopy (XPS), and Auger electron energy spectrometer alloying (AES) for the analysis of the passive film structure and composition of the CS alloys may be followed. Plus, the present disclosure may analyze the data corresponding to the trace Cu and Sn so as to differentiate the trace addition of Ge, Cu, and Sn as shown in  FIG. 3 . 
     The corrosion property test may use sulfuric acid solution before the below result could be obtained: 
     Linear Polarization: 
     (a) Based on  FIGS. 4A and 4B , after the homogenization, the addition of Cu—Sn may not necessarily increase the corrosion current density as well as the critical current density. Additionally, from the perspective of the current density at the active region the extent of the corrosion for the post-homogenization alloy may not be necessarily alleviated. However, the post-homogenization alloy may be associated with increased passive film density as the passive current density may have been effectively lowered; 
     (b) In view of  FIGS. 5A and 5B , after the homogenization, the addition of Ge may necessarily reduce the corrosion current density. Similarly, the critical current density and the passive current density of the post-homogenization alloy may be significantly reduced. Consequently, the addition of Ge into the CS alloy may help improve the anti-corrosion capability by lowering the corrosion current density before the alloy becomes passivated; and 
     (c) After the homogenization, the addition of the Cu—Sn to the CS alloy has little impact on the trend of parameters in the active region despite significantly increasing the passive current density. On the other hand, the addition of Ge to the CS alloys could improve the anti-corrosion capability in both the active region and the passivation region. Of the improved, the addition of CS212Ge could result in the most significantly reduced passive current density. 
     (2) Impedance Spectroscopy (IS): 
     (a) IS has been often used as an electrochemical tool for the corrosion as well as the change to the corrosion property on basis of which the anti-corrosion of surface passivation layer may be evaluated using the corresponding simulation software; and 
     (b) After IS analysis, the passivation film of the CS alloys may be single-layered subjected to extreme dissolving of iron (Fe) ions. With the addition of Cu—Sn increasing, the thickness of the passivation film may decrease and the resistance may be reduced. With the addition of Ge increasing, the thickness of the passivation may decrease also but the resistance tends to increase. The structure of the passivation film may be affected because of the Ge addition the passivation film may take longer period of time to form. 
     (3) Cyclic Voltammetry: 
     (a) The pore suppression and the repair of the passivation film of the CS alloys in 0.1 M sulfuric acid may be evaluated with the cyclic voltammetry-based approach. Negative hysteresis loop area versus the amount of the trace addition may be plotted as shown in  FIG. 6  where the negative hysteresis loop area may vary in accordance with the trace addition. Specifically, whether the trace addition is Cu—Sn or Ge the hysteresis loop area may become larger before reducing as the amount of the trace addition increases; and 
     (b) The negative hysteresis loop may indicate that the CS alloys with the addition of Cu—Sn or Ge in the sulfuric acid solution may be less susceptible to porous corrosion and with superior passivation film repairing capability. 
     (4) Open-Circuited Potential: 
     (a) The post-homogenization CS alloys, as shown in  FIG. 7 , for example, CS200, may maintain its potential in the passive region after 18,000 seconds of immersing, suggesting the passivation film may be less likely to compromise at low concentration (e.g., 0.1 M) of sulfuric acid corrosive environment. However, with the addition of Cu—Sn the CS alloy may see its potential to enter into the active region rapidly (nearly ninety degrees) less than 5000 seconds of immersing, representative of the reduced stability of the passivation film. With the increase of Cu—Sn, the occurrence of the compromise of the passivation film may be less than 1500 seconds from one to the next one. The compromise of one passivation film corresponding to the increased addition of Cu—Sn may not necessarily take longer for more than 1500 seconds compared with that of the passivation film corresponding to less Cu—Sn addition. The above indicates that, at the low concentration (0.1 M) sulfuric acid corrosive environment, the addition of Cu—Sn may have the undesired impact on the passivation film of the CS alloys; 
     (b) The homogenized CS alloys with Ge addition, as shown in  FIG. 8 , at the low concentrated sulfuric acid-based corrosive environment, may maintain above the passive region after 18,000 seconds of immersing, suggesting the low concentrated sulfuric acid is insufficient to compromise the passivation film. Therefore, the increase Ge may have little impact on the stability of the passivation film; and 
     (c) It can be inferred that the addition Cu—Sn may compromise the stability of the passivation film at the low concentrated sulfuric acid environment. The CS200 alloy may not endure a high concentrated sulfuric acid environment and the addition of Cu—Sn may not do any change to such result. On the other hand, the addition of Ge may prevent the passivation film from being compromised in the low concentrated sulfuric acid condition disregarding whether the addition of Ge increases or not. In the high concentrated sulfuric acid environment, it is evident that the addition of Ge may help maintain the endurance of the passivation film. 
     (5) Post-Corrosion Metallurgical Performance: 
     (a) With the addition of Cu—Sn in the CS alloys, the corrosion occurs rapidly and fairly conspicuous. Such corrosion despite partially may worsen with increased amount of Cu—Sn being added; and 
     (b) With the addition of Ge in the CS alloys, no apparent corrosion may occur despite some minor deformation at the pores. Of those CS alloys, CS203Ge may be with the most unaffected surface though CS203Ge may not stand out from others after being immersed for 120 minutes. 
     (6) ICP Component Comparison Before and After Immersing: 
     (a)  FIGS. 9 and 10  show the comparison between the solution concentration after having the CS alloys immersed in the 0.1 M sulfuric acid solution and the component concentration of the alloy according to one embodiment of the present disclosure. Interactive table can be found in the CS alloys according to the component percentage of the comparison table, the CS alloys may be corroded uniformly and the anti-corrosion capability of chromium oxide in the passivation film is better than that if the iron oxide in the same passivation film. With the increased trace addition, the uniform corrosion generally may take place without significant selective corrosion. In other words, the impact of the trace addition and the increase thereof may be insignificant on the anti-corrosion capability; and 
     (b) On basis of the dissolved, the CS alloy with the addition of Ge may be in possession of a better anti-corrosion capability than that of the addition of Cu—Sn, minimizing the compromise (or corrosion) of the chromium oxide film. 
     (7) ESCA and AES Analysis of the Passivation Film: 
     (A) The results of using electron spectroscopy for chemical analysis (ESCA) to scan the whole spectrum suggest the CS alloy with the trace addition may enhance the signal strength associated with Fe and Cr despite not enhancing the signal strength of other elements. After the analysis of the chemical shift, the passivation film of the alloy whose primary oxide includes Fe 3 O 4 , FeO/Fe 2 O 3 , Cr 2 O 3 , CuO, SnO 2 , and GeO 2  may see its thickness to decrease when the amount of the Cu—Sn addition increases. However, the thickness of the passivation film may decrease much more with the increased addition of Ge. As the result, it may be safe to conclude that the trace addition may reduce the thickness of the passivation film and even alter the structure of the passivation film (to be more concentrated or loose); and 
     (B) Based on the nanoscale Auger electron spectroscopy (AES) to scan the distribution of the elements, which may vary in accordance with the change in the thickness, the thickness curve of the passivation film may rapidly decline at the outset, indicative of the structure change to the passivation film. Specifically, the thickness of the passivation film of the CS alloy may decrease when Ge is added. 
     Corrosion test using sodium chloride solution: 
     (1) According to the linear polarization method, the addition of Cu—Sn may not significantly improve the anti-corrosion of the CS alloys. On the other hand, the addition of Ge to the CS alloys may increase the cross voltage and the potential at corroded pores and reduce corrosion current density along with the passive current density, all of which may indicate that the addition of Ge enhances the corrosion endurance of the CS alloys; 
     (2) According to the impedance spectrum method, the impedance of the CS alloys with the Cu—Sn addition may not increase significantly. That is to say, the addition of Ge may increase the impedance of the passivation film of the CS alloys with ion diffusion occurring when certain amount of Ge has been added; 
     (3) According to the cyclic voltammetry, a positive hysteresis may be present for the CS alloy, suggesting the occurrence of the corroded pores and inferior passivation film repairing. The increased addition of Cu—Sn may undermine the repairing capability of the passivation film of the CS alloys while the addition of Ge may, on the other hand, improve the repairing capability of the passivation film of the CS alloys; and 
     (4) Based on inductively coupled plasma ICP composition analysis method for the evaluation of the difference before and after the immersing, the CS alloy may selectively corrode and such corrosion may start at the iron oxide. From  FIGS. 11 and 12 , which show the comparison of the solution concentration after having the CS alloys immersed in the 0.1 M sodium chloride solution and the component concentration the original CS alloys (pre-soaking), Cu of the CS alloys may corrode first after the addition of Cu—Sn and corrode significantly while Sn may be free of significant corrosion. In short, the addition of Cu may have the undesired corrosive consequence in the chlorine sodium solution. But the addition of Ge may turn the selective corrosion to the uniform one (minimize the selective corrosion of the iron oxide), representative of the change to the corrosion mechanism for the passivation film of the CS alloy may be realized with the addition of Ge. 
     with sodium hydroxide solution for the corrosion test: 
     (1) According to the linear polarization method, the CS alloys with Cu—Sn may improve their anti-corrosion capability by lowering the critical current density and the corrosion current density. The CS alloys with Ge, in addition to reducing the current density in the active region, may further reduce the passive current density and grow the size of the actual passive region, showing the added Ge may further enhance the anti-corrosion capability of the CS alloys in the sodium hydroxide solution even such enhancing may not be significant enough; and 
     (2) According to the impedance spectrum method, the CS alloy with the trace addition of Cu—Sn or Ge itself may be good at anti-corrosion in the sodium hydroxide environment. The increased addition of Cu—Sn may increase the impedance of the passivation film and the increased addition of Ge may further increase the impedance of the passivation film in a more significant way. 
     Based on the above three solutions for the corresponding corrosion tests in the sulfuric acid, the addition of Cu—Sn or Ge may reduce the corrosion of the active region, but the addition of Cu—Sn may have the undesired impact on the passivation film, which is not the case when Ge is added to the CS alloys. The addition of Ge may help the forming of the passivation film and improve the stability of the same. In the sodium chloride environment for the pitting corrosion, the addition of Cu—Sn may not buck the trend of the corrosion when the addition of Ge could alter the corrosion mechanism and improve the repairing capability of the passivation film. In the sodium hydroxide, both Cu—Sn and Ge additions could improve the anti-corrosion capability of the CS alloys no matter how slightly the improvement may be. 
     Compared with the traditional art, the germanium-bearing ferritic stainless steel provided in the present disclosure may be with the following advantages: 
     1. The ferritic stainless steel-based raw material may be used in the present disclosure having iron, chromium, manganese, and silicon as the main constituent elements, and may have the trace addition of germanium, for the preparation of the germanium-bearing stainless steels; 
     2. The relevant analysis conducted for the germanium-bearing ferritic stainless steels suggests the trace addition of germanium may improve the stability of the passivation film, improve the forming of the same, the repairing capability of the passivation film, and the anti-corrosion capability of the same, and even turn the corrosion to be a uniform one; and 
     3. The germanium-bearing ferritic stainless steels after being immersed in the sodium chloride solution may result in a uniform corrosion (general corrosion). 
     Some modifications of these examples, as well as other possibilities will, on reading or having read this description, or having comprehended these examples, will occur to those skilled in the art. Such modifications and variations are comprehended within this disclosure as described here and claimed below. The description above illustrates only a relative few specific embodiments and examples of the present disclosure. The present disclosure, indeed, does include various modifications and variations made to the structures and operations described herein, which still fall within the scope of the present disclosure as defined in the following claims.