MXPA00003362A - Austenitic stainless steel article having a passivated surface layer - Google Patents

Abstract

An austenitic stainless steel article, preferably in the form of a tubing. The article has a passivated surface layer, which in the case of a tubing is on the interior surface of the tubing. The passivated surface layer has an oxide component having Fe2O3 and Cr2O3 and a metal component of Fe with zero valence and Cr with zero valence. The ratio of the oxide component to the metal component is in excess of 8:1.

Description

ARTICLE OF STAINLESS STEEL AUSTENITICO THAT HAS A PASSIVE SURFACE LAYER BACKGROUND OF THE INVENTION Field of the Invention The invention relates to articles of austenitic stainless steel, particularly in the form of pipes, having a passivated surface layer.

Description of the Prior Art In the manufacture of austenitic stainless steel articles and, particularly austenitic stainless steel pipe, it is desirable that the surface thereof be passivated so that, during use, the surface does not rust or react with the environments to which it is subjected during use. Particularly, in the case of austenitic stainless steel pipes, specifically AISI type 316 stainless steel pipe, such as that used in the pharmaceutical industry, during use the inner surface develops a reaction product in the form of an oxide which exhibits a reddish color. This phenomenon is typically called "redness". This reaction product can be a source of contamination for the products that pass through the pipeline during the use thereof, in different industrial applications.

OBJECTS OF THE INVENTION Accordingly, a primary objective of this invention is to provide an austenitic stainless steel article, particularly a pipe, having a passivated surface layer that will not develop redness during exposure to oxidizing environments, during use. SUMMARY OF THE INVENTION According to the invention, a stainless steel article that can be in the form of a pipe, has a passivated surface layer of Cr203 and Fe203 with a metal component of Cr with a valence of zero and Fe with a valence of zero. The ratio between the oxide component and the metal component is greater than 8: 1. Preferably, the stainless steel is an austenitic stainless steel. Preferably, the stainless steel is an austenitic AISI type 316 stainless steel. Preferably, the outer surface of the passivated surface layer will have a total Cr to Fe ratio of at least 1: 1. The passivated surface layer may have, at a depth therein of a maximum oxygen concentration, a total Cr to Fe ratio of at least 1.5 to 1. The passivated surface layer preferably contains an electropolished surface but may also contain a mechanically polished surface, produced for example by webbing or swirling. The reference to "ratio or total ratio Cr to Fe" includes the Fe and Cr present in the oxidizing component. The term "electropolishing" refers to a shiny metal surface created through a combination of electrical action and an acid solution, one component of which is phosphoric acid, the other is normally sulfuric acid. All compositions are given as a percentage by weight unless otherwise indicated.

BRIEF DESCRIPTION OF THE DRAWINGS Figures la and Ib are graphs showing surface composition as a function of passivation time; Figure 2 is a graph showing the metal-to-iron ratio as a function of passivation time; Figure 3 is a graph showing the change in the ratio of Cr203: Cr and Fe203: Fe as a function of passivation time; Figure 4a is a graph that constitutes an energy sweep of the iron junction, showing the relative levels of oxide and free iron; Figure 4b is a graph that constitutes an energy sweep of the iron junction after one minute of passivation, showing the decrease in oxide and the increase in free iron; Figure 5a is a graph that constitutes an energy scan of the chromium binding in the material, without passivation, where the relative levels of oxide to free chromium are shown; Figure 5b is a graph that constitutes a sweep of chromium binding energy of the material, after 60 minutes, where the decrease in free chromium is shown; Figure 6 is a graph that constitutes a sweep of bonding energy of passivated material of 60 minutes, showing significant residual free iron; Figure 7 is a graph constituting a depth profile using Auger Electronic Spectroscopy of an electropolished and passivated surface; and Figure 8 is a graph constituting a depth profile of the three electropolished surfaces stained with different colors, illustrating the color variation as a function of the chromium content.

DESCRIPTION OF THE PREFERRED MODALITIES Preferably, according to the invention, the desired passivated surface layer is achieved by an electropolishing operation, electropolishing with an oxidizing acid or a mechanically polished surface treated with an oxidizing acid. Therefore, the passivation process for producing the passivated surface layer according to the invention is achieved, by exposing the surface to an oxidizing acid after it has been preferably electropolished or has been abraded, for example with an operation polishing with sand. In this operation, the surface is specifically altered by increasing the chromium to iron ratio, by removing the surface roughness, providing a greater depth of oxygen penetration, removing the contamination, for example the occluded iron or removing martensite transformed by tension; remove inclusions, especially manganese sulphides and remove visible manufacturing defects. During the passivation process, which can occur partially in air within several hours after the surface of the stainless steel has been abraded or otherwise altered, for example by electropolishing, the chromium combines with the oxygen and forms a Waterproof chrome oxide barrier to react additionally with the material below the barrier or passivation film. It has been determined that as the chromium content increases, the film becomes a better barrier. During the electropolishing, the iron elements and others on the surface are preferably removed in order to increase the chromium on the surface, consequently, after electropolishing, the chromium to iron ratio increases significantly on the passivated surface layer. The average depth of oxygen penetration, as seen in Figure 7, is a measure of the depth of the passivated layer. In general, the deeper the penetration of coarser oxygen, the more passivated the layer, and the greater the corrosion resistance of the material. This happens effectively, but only if the components are essentially Cr203 and Fe203 in combination with the metal components Cr and Fe, both with zero valence, the Cr203 to Fe203 ratio being relatively high. This can be achieved by subjecting the surface to an oxidizing acid such as for example nitric acid (HN03) or citric acid for a determined period of time and adequate to complete the reaction of Cr203 and Fe203. The change in composition can be observed as a function of the depth of the passivated layer of Figure 7. Passivation has a profound effect on the chromium-to-iron ratio in chemically polished type 316L stainless steel tubing. The pieces of the same tube were subjected to hot nitric acid for several times of passivation and the passive layer was analyzed using XPS (X-ray Photoelectron Spectroscopy). The changes in surface chemistry, especially with respect to the amount of elemental iron in the passive layer, were quite measurable. There were significant differences in the Cr: Fe ratio and in the ratio of elemental chromium to chromium oxide. Other elements that exhibited normal behavior were silicon and molybdenum. More elemental iron and elemental chromium exists in the passive layer of the mechanically polished pipe than in the equivalent electropolished pipe, suggesting a more easily corrodible surface for the mechanically polished pipe. Type 316L stainless steel is the material of choice for most high purity (HP) and injectable water (WFI) water systems in the pharmaceutical industry. Two surface finishing conditions are used for these systems: Electropolishing and mechanical polishing. Piping is normally ordered for the ASTM A 270 specification, which in its current form requires mechanical polishing regardless of the smoothness of the P1049 existing surface. Mechanical polishing takes one of two forms, polished in a whirl or polished by longitudinal band. Whirlwind grinding uses a rotating knocking wheel that moves up and down the length of the tube, removing only a thin surface layer of the material and creating a "rubbed surface". The longitudinal band grinding uses an abrasive band that moves along the length of the tube, while the length rotates and uses an air blade to pressurize the band in order to remove the surface material. This technique removes a measurable amount of material, 0.0006-0.0008 inches (0.015-0.020 mm) and is a precursor of electropolishing at low Ra (<8μ-inch or 0.2 μm). The two methods remove the passive layer of normal depth that develops during the production of the stainless steel strip from which the pipe is made. Occasionally, the discoloration of the mechanically polished surface is obtained as a result, especially in hot and humid climates. This is observed with the two types of mechanically polished pipe. This surface discoloration varies from light yellow to light red. It is easily removed by immersion in hot nitric acid followed by rinsing with water. Once the tube is treated with acid it does not discolor another P1049 once, as long as the treatment is carried out at an elevated temperature for a sufficiently long period of time. A study was started to determine what changes occur on the surface of the mechanically polished pipe at the different passivation times with nitric acid. The acid concentration was that specified in the MIL STD QQ-P-35 method and the ASTM A 967 - Nitric Acid 3 method, especially 20% at the specified temperature of 120-140 ° F (50-60 ° C). This concentration and temperature provided the best results with the standard salt spray test. In this study, the time at that temperature varied and the surfaces were analyzed using X-ray Photoelectron Spectroscopy (XPS). The results of the passivation study are presented below. The reactive grade nitric acid was diluted with deionized water to a volumetric percent of 20 (v / o) and heated to a constant temperature of 136 ° F (58 ° C). Five samples of the mechanically polished pipe were immersed in this solution, each for 1, 5, 15, 30 and 60 minutes, respectively. A sample was analyzed in the "polished" condition. After rinsing and drying, each of the mechanically polished, treated samples were evaluated using XPS. There was no visual difference between the six samples. All had identical surface luster. X-ray photoelectron spectroscopy is one of the most novel analytical tools available and is known as Electronic Spectroscopy for chemical analysis or ESCA. During the XPS, a sample is irradiated with soft monoenergetic X-rays and the emitted photoelectrons are analyzed to determine the energy response. For this experiment, monochromatic X-rays Al K at 1486.7 electron volts were used. These X-rays interact with the atoms on the surface and emit photoelectrons. These photoelectrons are generated at approximately 30-50A from the surface with a resultant kinetic energy that is expressed as; KE = hv - BE - Fs where: KE is the kinetic energy; Hv is the energy of the photon; BE is the binding energy of the atomic orbital from which the electron originates; and Fs is the function of the spectrometer work. Each element and compound has a unique set of binding energies. Therefore, XPS can be used to identify the concentration of the elements on the surface being analyzed and determines the binding energy of the surface species. From this binding energy, inferences can be made to identify the chemical state of the element. This is an extremely useful function since changes in the composition of the passive layer can be identified as a function of passivation time. After sweeping each surface the surface was bombarded ("crackling") with ionized argon to remove approximately 25A of material (or approximately 8 atoms of depth), subsequently, the new surface was analyzed again. This continued until the maximum depth of oxygen penetration was reached or until there were no further changes in composition. For each sample in each depth, a recognition sweep was carried out in the energy range of 1200-0 eV to determine the elemental composition. Subsequently, for each element of interest, a narrow window of approximately 20 eV was analyzed around the central peak in a high energy resolution mode to determine the binding energy of the surface species. The displacement of the peaks in the XPS can be considered as a measure of covalence and the more ionic compounds, for example intermetallic compounds, may or may not move significantly from the peak value of the pure element. The binding energy obtained for each element is compared either with the values published in the literature for known standards or with the theoretical standards that are based on chemical bonding. The presence of multiple and overlapping binding energies can make identification difficult. Data from the Handbook of Photoelectron Spectroscopy, J.F. Moulder et al., Physical Electronics, Inc., Eden Prarie, Minnesota, 1995 and Practical Surface Analysis by Auaer and X-Ray Photoelectron Spectroscopy, D. Briggs et al., J. Wiley & amp;; Sons, Chichester, England, 1983, were used to assign the binding energy to the compounds. The XPS system used for the analyzes was a Physical Electronics Model 5700 system. The values of the bonding energy were calibrated with an internal standard. Carbon from atmospheric exposure, established at 284-7 eV. The quantitative values for the data were obtained by the use of sensitivity factors established in the publication D. Briggs mentioned above, which is based on the yields calculated for pure elements. Analytical information should at best be taken as semi-quantitative and more appropriately used only for comparisons. As all samples were taken from the same tube and within one inch (25 mm) of separation from each other, only one of the samples was analyzed in the condition as received, after rinsing with isopropanol to remove the contamination produced by its management Each surface of the samples treated with acid was analyzed with XPS. In addition, samples as received and passivated samples of 30 and 60 minutes were sputtered to determine the elemental composition and the oxidation state as a function of depth. Table 1 summarizes the surface chemistry of Type 316L Stainless Steel samples after different times in hot nitric acid. The data represent the atomic percentage composition of the elements above the atomic number 3 within 40A (12 atoms) of the surface. Figures la and Ib are graphs of metals only with atomic surface concentration as a function of passivation time.

P1049 The data illustrates that the chromium and oxygen concentrations reach a maximum after 30 minutes of passivation and that the iron has its lowest value. When the data are compared as the metal-to-iron ratio, as in Table 2 and Figure 2, the maximum Cr / Fe ratio occurs after 30 minutes of passivation. For an inexplicable reason, the passivation of 15 and 60 minutes shows a decrease in the Cr / Fe ratio. The Ni / Fe and Mo / Fe ratios reach a maximum at 15 minutes and begin to decrease after 30 minutes of passivation.

The passivated samples of 0, 30 and 60 minutes were sputtered with ionized argon and the elemental composition was determined as a function of depth. The data is summarized in Table 3 for the samples as received. Table 4 for the passivated sample of 30 minutes and Table 5 for the passivated sample of 60 minutes.

P1049 H s.

The examination of the specific binding energy peaks for each element indicates that both the oxide and the metal are present, that is, the metal with a valence of zero. In the case of iron, oxide and elemental iron are present in significant quantities. This happens especially for the elemental iron in passivation times less than 30 minutes. Table 6 and Figure 3 show the iron-chromium relationships with respect to their respective oxides. These data indicate that the iron oxide decreases abruptly after one minute and continues to move down until the chromium oxide reaches near a saturation point, between approximately 15 and 30 minutes. After 30 minutes, both ratios increase, although the speed of the increase is greater for the chromium oxide than for the iron oxide. This would indicate that the surface is becoming more passive with a longer exposure to hot nitric acid.

P1049 s > A passivation treatment of mechanically polished Type 316L stainless steels seems necessary to improve their resistance to corrosion. Mechanical polishing destroys the passive layer formed during the manufacture of the strip and tube. The passive layer is quite thin, on the order of 50-400Á or 12-150 atomic thicknesses. Although vortex polishing does not remove a measurable amount of metal, the passive layer is destroyed as surface oxidation proves. When these oxidized surfaces are immersed in hot nitric acid, the colors disappear, indicating removal of the iron oxides. Therefore, passivation after polishing is a necessary operation. The most dramatic change in surface chemistry occurs after only one minute has been taken in hot nitric acid, during which time the Cr / Fe surface ratio changes from 0.26: 1 to 1.1: 1. These proportions may vary according to the type of analytical instrument used: Auger Electronic Spectroscopy (AES) tends to give lower values than XPS. Most of these changes appear to be due to the dissolution of the surface iron oxide, as seen in Figures 4a and 4b. A careful examination of the binding energy curves for iron and chromium shows that metallic chromium (zero valence), Figures 5a and P1049 5b, decays constantly with increasing passivation time and chromium oxide increases. However, the metallic iron remains a significant species, even after a passivation of 60 minutes, as seen in Figure 6. By comparison, the electropolished material exhibits very little metallic iron, which suggests that it will have better resistance to corrosion. The mechanism for passivation seems to be related to the progressive oxidation of chromium, as a first step. Once the free chromium is essentially consumed, the iron begins to form its oxide. The iron oxide formed by the atmosphere, which was dominant in the material as received, it dissolves quickly in the hot nitric acid and the metallic iron remains as the dominant species until 30 minutes, where the amount of oxide finally exceeds that of the metallic iron. Effective passivation does not appear to occur until the metallic elements all essentially become oxide. For mechanically polished materials this will be a passivation of more than 60 minutes in hot nitric acid. The following was concluded from this experimental work. 1. There were dramatic changes in the surface chemistry of the Type 316L sample mechanically P1049 polished, during passivation. The iron decreased as did silicon, nickel and molybdenum. Oxygen and chromium both decreased. The Cr / Fe ratio increased with the passivation time. 2. The passivation mechanism appeared to be controlled by the oxidation of chromium metal in trivalent oxide. The iron does not start to form appreciable trivalent oxide until the chromium is satiated. 3. Even after passivation of 60 minutes in hot nitric acid, there is still a defined metallic iron peak, indicating that more passivation could occur. Electropolishing has not been recognized as a means to produce an improved finish, except within a very limited area, namely in the pharmaceutical and semiconductor industries. Electropolishing is recognized as a means to produce an extremely smooth adventitious iron contamination surface, essentially free of surface stains, with a highly lustrous surface that resembles chromium plating. Electropolished surfaces having improved corrosion resistance on the mechanically polished surface are also recognized. With the advent of specialized analytical equipment, it was possible to determine exactly what is P1049 that happened on the surface. Auger Electronic Spectroscopy (AES) was the first of these techniques that made its debut only three decades ago. A little later, the "bombardment or sizzle" with ionized argon was developed, allowing the AES to determine the composition as a function of distance from the surface. Figure 7 represents a typical AES study of depth profile of an electropolished surface. The main problem with the AES is that only the elements are reported and not their molecular form. Another very useful analytical technique developed almost at the same time was Dispersive Energy Spectroscopy (EDS). This is also an elementary analytical method and can be used in conjunction with electron scanning microscopy as a microprobe to identify the composition of small particles, such as the inclusion of steel. A novel technique, Electronic Spectroscopy for Chemical Analysis (ESCA), also known as X-ray or XPS Photoelectron Analysis uses X-rays instead of electrons. This method has the advantage of the identification of molecular species. There are some differences between the values reported for XPS, AES and EDS. The reasons for this are not fully understood, but in general they are attributed to the difference in the P1049 depth of the analysis, the size of the point and the type of spectrum generated. A comparison of these three analytical techniques is provided in the following Table 7.

Since the XPS can identify the chemical status of the element and can be used with crackling to obtain a depth profile, it allows the evaluation of surface treatments that improve the corrosion resistance. For this reason, XPS was used as the primary evaluation tool. The middle P1049 primary comparison was the Cr / Fe ratio. Other relationships of interest included the ratio of the Cr203 / Cr °: Fe203 / Fe ° oxides. This last relation is probably the one that best describes the passivation techniques since it allows to follow the relative oxidation speed for different metals. Additional experimental work was performed to examine both electropolishing and passivation as a means of improving corrosion resistance. In addition, the effect of orbital welding on surfaces with improved properties was considered. As discussed and demonstrated before, mechanically polished surfaces, as they are, have very low Cr / Fe ratios. This is demonstrated by the data presented in Table 8. Also as analyzed and demonstrated before, "passivation with air" does not improve the Cr / Fe ratio. By putting air-passivated surfaces into service without passivation, accelerated "reddening" can be obtained in high purity water applications. The adequate passivation will greatly improve the Cr / Fe ratios in all cases.

P1049 Electropolishing is simply an electroplating in reverse. The process involves pumping a solution of the concentrated sulfuric and phosphoric acids through the inside of the tube, while direct current is applied. The metal dissolves from the tube (anode) and the cathode will be plated if the chemistry of the solution is not balanced to dissolve the metals as quickly as they are plated to the surface. As oxygen is released at the surface of the pipe, the resulting passive layer has a high Cr203 / Fe203 ratio. This result is a very smooth surface with a high luster. A complete description of this process is established in "Electropolished Stainless Steel Tubing", J.C. Tverberg, TPJ - The Tube and Pipe Journal, September / October 1998. Normally, the surface finish is measured with a profilometer and is usually expressed as Ra or average roughness. However, the roughness by itself P1049 is not enough to describe the true nature of the surface. The use of an electronic scanning microscope together with the profilometer gives a better analysis of the surface. The industry normally acquires electropolished tubing at a maximum surface roughness of 10 μ-inches (0.25 μm) or 15 μ-inches (0.38 μm). There is a difference in how these two finishes are obtained, depending on how the surfaces are prepared before electropolishing. Generally, as discussed above, two methods of mechanical polishing are used to prepare the surface. To make the surface smoother, the interior surfaces of the pipe are polished using a longitudinal strip. This removes most of the metal from the surface of the internal diameter, below the depth of the manufacturing-induced defects. When surfaces are electropolished, surface finishes of 2-5 μ-inches (0.05-0.12 μm) are not common. The other mechanical polishing method uses turning rotating wheels that produce a whirlwind finish. When subjected to electropolishing, surface finishes in the range of 8-13 μ-inches (0.20-0.33 μm) can be achieved. The whirling polish removes very little metal, producing a "rubbed" surface, so that very few are removed P1049 surface defects. A bright, highly cold-worked, hydrogen-annealed surface will essentially produce the same surface finish. In both cases, the Cr / Fe ratios will be practically the same. As demonstrated by the experimental work discussed above, passivation has the effect of introducing oxygen into the surface layer and dissolving other elements, leaving chromium and iron as the two primary surface metals. Both carbon and oxygen are in high concentration. Some of the carbon and oxygen come from the occluded carbon dioxide. Carbon appears to be at a higher concentration on mechanically polished surfaces than electropolished surfaces. These investigations to date have involved 20% nitric acid at 50 ° C and at 25 ° C and 20% nitric acid, 1% hydrofluoric acid at both 50 ° C and 25 ° C. Passivation times vary with solutions and temperature. Two additional passivation treatments are planned for further study: 10% citric acid + 5% EDTA and 5% orthophosphoric acid. The effect of electropolished surfaces stained with color on the composition of the passive layer was studied. The golden tints seem to have all the P1049 Cr ° and Fe0 oxidized to the trivalent oxides and show extremely high Cr / Fe ratios. When the color changes to blue, the iron begins to form Fe304, which is also expressed as Fe203FeO, and the content decays as seen in the depth profile of Figure 9. The latest passivation studies involve whirling polishing, plus electropolishing and longitudinal band + electropolished surfaces. The highest Cr / Fe ratio, 4.04, was achieved over vortex polishing only in nitric acid after 20 minutes and that Cr / Fe ratio decreased after 30 minutes to 3.15. Table 9 compares the different passivation treatments for different starting materials.

P1049 ? or In each case where the Cr / Fe ratio decreases with a prolonged passivation time, there is an increase in the amount of free iron with respect to iron oxide and chromium metal with respect to chromium oxide. This suggests that the surface layers dissolve and the substrate struggles to regain the proper Cr / Fe equilibrium. This is logical with the use of hydrofluoric acid since it is a halogen acid that easily attacks chromium. Orbital welding on a swirl polished type 316L stainless steel tube was analyzed using XPS. In this study we analyzed the weld flange, a slag deposit that forms on the welding flange and the dark oxide on the heat affected area. The data is presented in Table 10.

P1049 These results show that the non-passivated solder has a very low Cr / Fe ratio. Ideally, the Cr / Fe ratio should be 1.0 or more to have reasonably good corrosion resistance. The depth profiles using XPS in these areas were not performed, but were based on EDS analysis, the chromium content increased with depth. Chromium was highly variable from sample to sample, preferably depending on whether the electronic probe analyzed austenite or delta ferrite. The results are consistent with other EDS analytical work where the welded surfaces normally showed high manganese content and low chromium content. Similarly, the slag patch is consistent with other findings. The slag appeared as an accumulation of the inclusions in the steel or an incomplete gas cover that allows the oxidation of the weld cluster. In this case, the slag point seems to come from the inclusions in the steel and from the steel being deoxidized with calcium and aluminum. The area of dark rust on the affected area with heat had the highest chromium level and the lowest iron level in the analyzes performed. When compared to the actual corrosion failures in the field, the dark rust seemed P1049 remains intact and acts as a crack-former, cracking corrosion under the dark rust. This suggests that the high chromium content makes this dark oxide quite resistant to corrosion, thus allowing galvanic corrosion to bond to the surface under the oxide. Several significant observations mark the difference between a mechanically polished, an electropolished and a passivated surface and the surface of the orbital welds. These are: 1. The mechanically polished surface has essentially all the elements present in the alloy and in the same approximate ratios. 2. The electropolished and passivated surfaces do not show molybdenum and show very little nickel.

Essentially the only two elements of certain significance are chromium and iron, although silicon is variable and in the case of electropolished surfaces it can change its valence form. 3. Electropolished surfaces tend to have a greater depth of oxygen penetration than passivated surfaces. 4. Surfaces passivated by adequate time appear to have higher Cr / Fe ratios, but not depth of penetration P1049 oxygen 5. The Cr203 / Cr ° ratios seem to control the passivation process. 6. The Fe203 / Fe ° ratio can have a greater impact on the passivation and, therefore, on the corrosion resistance, with respect to the Cr / Fe ratio. The smaller the Fe0, the more stable the passive layer. 7. Orbital welds have a very low chromium and high iron surface. Similarly manganese is in high amount. 8. Dark rust on the heat-affected area of the orbital welds is very high in chromium, low in iron and is generally associated with field crack corrosion. 9. Slag deposits that occasionally appear on the surface of the orbital weld appear to be low melting point refractory compounds that arise from inclusions in the steel or oxidation of the weld cluster. These observations suggest that the passive layer may actually be crystalline in nature. The closest crystalline form is that of chromite spinel, which has the general formula (Fe, Mg) 0. (Cr, Fe) 203. East P1049 crystal has the oxygen atoms arranged on a cubic network centered on the face (Dana et al., A Textbook of Mineralocry, John Wiley &Sons, New York, 1951), thus coinciding with the crystal lattice of austenitic stainless steel . Also, due to the composition of the glass, the lack of certain elements in the surface layers of the passivated and electrodeposited materials would be explained and a reason is provided why the passivation process requires time in an oxidizing solution to allow it to form the glass . A surface with a high iron content will not form the proper glass and, therefore, will lack chemical stability. Since the composition of an orbital weld is low in chromium, the resulting surface crystal will be either hematite (Fe203) or magnetite (Fe304), none of which has corrosion resistance. Therefore, the surface must be passivated with acid to first dissolve the excess iron and then allow the chromium to become the dominant element. The dark oxide on the affected area with heat has the general composition of chromite, FeCr204 or FeO.Cr203. The composition may vary considerably, but in all cases, it is very high in chromium. This gives the glass excellent resistance to P-.049 corrosion in oxidizing media, probably much more than the metal it covers. This will lead to galvanic corrosion (crack corrosion) conditions and explains the type of corrosion observed in systems that have had poor gas coverage during welding. The only rectification is to chemically dissolve the oxide, usually with a nitric acid + hydrofluoric acid, which should passivate the whole system. However, this treatment can distribute an electropolished surface. The following was concluded and, in addition, is established from an additional experimental work: 1. The interior of the stainless steel pipe can be upgraded to increase the service life. The two most common systems are electropolishing and acid passivation. In either case the Cr / Fe ratio needs to approach or exceed 1.0 to achieve the best corrosion resistance. 2. The amount of free iron in the passive layer is critical to the stability of the layer. If the free iron exceeds the iron oxide, then the film will not be stable, which can lead to a break during service. 3. Passivation achieves an optimal Cr / Fe ratio within a relatively short time, after P1049 seems to reverse itself. 4. Some characteristics of the passive layer suggest that this may be of a crystalline nature, taking the characteristics of the chromite spinel. 5. Orbital welding surfaces are of high iron and manganese content, but very low in chromium, suggesting that the surfaces in their welded form are of poor resistance to corrosion. 6. The dark oxide that can cover the affected area with heat in the welding is of very high content of chromium and low iron content. This suggests that the oxide is chromite, which has a very good resistance to corrosion. 7. The slag points that sometimes appear on the surfaces of the welds are inclusions accumulated from the steel. Under conditions of poor gas coverage, these slag points can be the oxidation of silicon, iron and chromium in the molten solder cluster. Other embodiments of the invention will be apparent to those skilled in the art from the consideration of the specification and practice of the invention set forth herein. It is intended that the specification and examples be considered as only exemplary and P1049 that the true spirit and scope of the invention is indicated by the following claims. P1049

Claims (14)

1. CLAIMS: 1. A stainless steel article having a passivated surface layer, the surface layer consisting essentially of an oxide component having Cr203 and Fe203 and a metal component having Fe with a valence of zero and Cr with a valence of zero and the ratio of the oxide component to the metal component is greater than 8: 1.
2. 2. The article according to claim 1, wherein the stainless steel is an austenitic stainless steel.
3. 3. The article according to claim 2, wherein the stainless steel is AISI Type 316 steel.
4. 4. The article according to claim 2 or 4, wherein the passivated surface layer is an electropolished surface.
5. 5. The article according to claim 1 or 2, wherein the exposed surface of the passivated surface layer will have a total Cr: Fe ratio of at least 1: 1.
6. 6. The article according to claim 1 or 2, wherein the passivated surface layer, at a depth thereof having a maximum oxygen concentration, has a total Cr: Fe ratio of at least 1.5: 1.
7. 7. The article according to claims 1 or P1049 2, wherein the passivated surface layer is a polished surface.
8. The article according to claim 7, wherein the polished surface is a mechanically polished surface.
9. 9. A stainless steel pipe having a passivated inner surface layer, the surface layer consists essentially of an oxide component having Fe203 and Cr203 and a metal component having Fe with a valence of zero and Cr with a valence of zero where the ratio of the oxide component to the metal component is greater than 8: 1 as determined by XPS, where an exposed surface of the surface layer has a total Cr: Fe ratio of at least 1: 1 and the surface layer , at a depth of the same that has a maximum concentration of oxygen, has a total Cr.-Fe ratio of at least 1.5: 1.
10. The pipe according to claim 9, wherein the stainless steel is an austenitic stainless steel.
11. The pipe according to claim 10, wherein the stainless steel is AISI Type 316L steel.
12. The pipe according to claims 10 or 11, wherein the passivated surface layer is an electropolished surface. P1049
13. 13. The pipe according to claims 9 or 10, wherein the passivated surface layer is a polished surface. The pipe according to claim 13, wherein the polished surface is a mechanically polished surface. P1049