Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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66 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS SELECTION & DESIGN tion. For a material to possess adequate corrosion resistance, it must develop a sta- ble barrier film to isolate the metallic com- ponent from the aggressive environment. A widely used engineering alloy, carbon steel, is susceptible to severe oxidation in contact with oxygen for temperatures >450 °C; 3 hence, it is ruled out for use as a con- struction material for the second casing of the UCG wells. Most of the high-temperature engineer- ing alloys currently in use are iron, nickel, or cobalt-based alloys. These alloys acquire corrosion resistance by the formation of oxides of chromium (Cr 2 O 3 ), silicon (SiO 2 ), or aluminum (Al 2 O 3 ). 5 Alloying require- ments for the production of specific oxide scales have been translated into minimum levels of scale-forming elements or a com- bination of elements, depending on the base alloy composition and the intended temperature of service. Chromium plays an important role in high-temperature oxida- tion corrosion studies, not as a metal for high-temperature service application but as an important alloying element that pro- vides a protective Cr 2 O 3 layer. Chromium oxidizes in the presence of oxygen by out- ward diffusion of Cr 3+ ions through chromic oxide. The addition of a small percent of chromium results in the formation of a chromium-ri ch oxi d e alon g w ith iron oxides. With an increase in the concentra- tion of chromium, iron-chromium spinels are formed and the iron(II) oxide (FeO) layer correspondingly becomes thinner relative to iron (II,III) oxide (Fe 3 O 4 ) because Fe 2+ ions are blocked by the spinel oxide. With a further increase in Cr, a mixed spi- nel of Fe(Fe,Cr) 2 O 4 is produced , w hich decreases the oxidation rate significantly. Since these ions are more mobile through this layer than Cr 3+ ions, the outer layer can still consist of the iron oxide, especially after long oxidation times. It has been established that the high-temperature oxi- d a t i o n re si st a n c e i n c re a s e s w i t h t h e increase in chromium content of the steel (Figure 1). 3 According to Figure 1, at a temperature of 600 °C, 5Cr-0.5Mo steel has excellent oxi- dation corrosion resistance. Hence, the second casing for the UCG injector well can be fabricated of 5Cr-0.5Mo steel conform- ing to UNS K41545 as described in ASTM DS-56E. 7 Nickel is generally not alloyed with iron for the purpose of improving the high-tem- perature properties of iron. The main pur- pose of alloying Ni to Fe-Cr alloys is to transform the Fe from a ferritic to an aus- tenitic phase, which has a face-centered cubic structure and is more stable at high temperatures. 3 Thus, a combination of chromium and nickel synergises the oxida- tion corrosion resistance of a Fe-Cr alloy (i.e., 18Cr-8Ni-Mo alloy). At a 900 °C tem- perature, 18Cr-8Ni-Mo steel alloy (Type 316 stainless steel [SS] [UNS S31600]) has ade- quate oxidation corrosion resistance. When the temperature exceeds 900 °C, however, molybdenum present in Type 316 SS st eel w i l l form moly bd enum oxi d e (MoO 3 ), which volatilizes and leads to rapid oxidation of steel and severe corrosion. Furthermore, the chromia begins to con- vert to volatile chromium trioxide (CrO 3 ) as the temperature exceeds 1,000 °C. 8 Thus, the benefit of the protective scale forma- tion from chromium will be lost as the tem- perature exceeds 1,000 °C. Hence, Type 316 SS is not adequate for an environment with temperatures of 1,200 °C. Above 950 °C, chromium is not suitable for providing corrosion resistance. Instead, aluminum provides excellent oxidation resistance by forming Al 2 O 3 scale. It is ther- modynamically more stable than Cr 2 O 3 and less prone to vaporization effects. When both Al and Cr are present within an alloy, they will compete to form surface scale, and with >4% Al content, alumina forma- tion is favored. Thus, oxidation resistance of Fe-Ni-Cr steel is increased significantly by alloying it with an additional 4-5% alu- minum. 6 Furthermore, at a high temperature such as 1,200 °C, adhesion of Al 2 O 3 scale is enhanced by the inclusion of rare earths (yttrium, cerium, and lanthanum), which assist in the development of a more resil- ient scale that delays oxidation and other high-temperature corrosion processes such as sulfidation. Thus, Alloy MA 956 con- forming to UNS S67956 described in ASTM DS-56E has been reported 7 to form thermo- dynamically stable Al 2 O 3 scale on the alloy when temperatures exceed 1,000 °C. 9 An FIGURE 1 Oxidation resistance of carbon, low-alloy, and SS.

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