Duplex high-temperature corrosion

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

DUPLEX Disks extend corrosion resistance to highly oxidizing agents, halogens except free fluorine, and virtually all other corrosives. A sheet of PIPE is used as a barrier on the service side of the disk. See page 12. Additionally, these disks are processed to accommodate temperatures to 392E without insulation. 

ASTM A 351/A 351M-05 Standard Specification for Castings, Austenitic, Austenitic-Ferritic (Duplex), for Pressure-Containing Parts (contains most of the stainless grades, mostly corrosion, but some high-temperature)... 

T.M. Devine and B.J. Drummond, Use of Accelerated Intergranular Corrosion Tests and Pitting Corrosion Tests to Detect Sensitization and Susceptibility to Intergranular Stress Corrosion Cracking in High Temperature Water of Duplex 308 Stainless Steel, Corrosion, Vol 37, 1981, p 104-115... 

When the aluminum content exceeds 8 %, the alpha-beta duplex structures appear. The beta phase is a high-temperature phase retained at room temperature on fast cooling from 565°C (1050°F) or above. Slow cooling or long exposure at temperatures from 320 to 565°C (610 to 1050°F) tends to decompose the beta phase into a britde alpha + gamma-2 eutectoid having either a lamellar or modular structure. The beta phase is less resistant to corrosion than the alpha phase, and eutectoid structures are even more susceptible to attack. 

The most critical parts, the contact parts of the rotating assembly, are most frequently made in one of the many stainless steels. Nevertheless some manufacturers continue to supply howls in carbon steel. The 316-type stainless steel is a common material for the smaller decanters, but for the larger machines at higher g-forces, materials with higher strength, such as the duplex stainless steels, must be used. Where high temperatures, or extra high speeds, or corrosive materials are in use, special corrosion-resistant or temperature-resistant materials may be used. 

Stress corrosion cracking (SCC) is a special corrosion form, which is seen as fine-branched transgranular cracks in the material. SCC is caused by a combination of mechanical stress, the presence of chlorides and elevated temperature. At high temperature and high mechanical stress. SCC of 316-type stainless steel can occur at chloride levels even below 1000 ppm. The answer to this is to use a more corrosion-resistant stainless steel or a duplex stainless steel. Duplex stainless steels normally do not show SCC at temperatures below 100°C. 

Today different high-quality grades are assured by improved metallurgy. Table 1-6. Corrosion resistance can be improved by addition of Mo, Cu, W and N. Nitrogen as an austenite stabilizer reduces precipitation of the a-phase. Overall, the precipitates in Duplex steels can be very complex. Beside the high temperature phases (a-phase, T-phase, Cr2N-nitride, M23C6) a 475 °C embrittlement can occur (Schlapfer and Weber, 1986 ... 

At a lower mean stress duplex stainless steel has very good resistance to corrosion fatigue of 20 to 220 MPa. Only at the very high temperature of 150 °C does increased corrosivity reduce the corrosion fatigue. Fig. 1-25 (Schmitt-Thomas et al., 1986, 1987). 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

Materials for FGD pumps with suspension temperatures up to 70 °C are subjected to extreme corrosive conditions. To discover whether duplex steels can withstand these conditions, corrosion tests were performed on two types of alloy - standard alloy 1.4517 (sample No. 5,5.1,21,28), with slight variations in Cr, Mo, Cu, and N content, and the material 1.4471 (G-X3CrNiMoCuWN26-6-3-1-1, sample 12 and 23), a 1% W alloyed cast duplex stainless steel. The good corrosion resistance of this duplex stainless steel is illustrated in Fig. 1-49. In a fluoride-free medium, the critical breakthrough potential is 940 mVscE. With increasing fluoride content, it decreases to ca 200 mV, but with a value of 720 mVscE for a fluoride content of 500 mg L the resistance is quite high. 

Due to the high chromium contents, duplex alloys are sensitive to 885 (475°C) embrittlement. This generally limits their usage to 600°F (SIS C) maximum for pressure vessels. Due to the presence of nickel, chromium, and molybdenum they are also susceptible to the formation of affect mechanical properties and corrosion resistance due to alloy depletion. The temperature range of 1100°F (593 C)-1600°F (882°C) and most rapidly at about 1450°F (788°C). The deleterious effects of phase formation are not obvious at the elevated temperature but can become a factor at room temperature. The formation of a phase in these alloys is sufficiently rapid to have an effect on properties due to slow cooling (air) after anneal. A measurable effect as a result of exposure in this temperature range due to welding has been demonstrated. 

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