Austenitic stainless steels, corrosion

Rhodes, P. R., Mechanism of Chloride Stress-corrosion Cracking of Austenitic Stainless Steel , Corrosion, 25, 462 (1969)... 

Gizhermo, R. and Khristo, E., Effect of the Deoxidation Method on the Intercrystalline-corrosion Tendency of Cr-Ni Austenitic Steels , Melalurgiye, 5, 17 (1972) C.A., 80, 98898y Joshi, A. and Stein, D. F., Chemistry of Grain Boundaries and its Relation to Intergranular Corrosion of Austenitic Stainless Steel , Corrosion, 28, 321 (1972)... 

Brigham, R. J., Pitting of Molybdenum Bearing Austenitic Stainless Steel , Corrosion, 28, 177... 

R.J. Brigham and E.W. Tozer, Effect of Alloying Additions on the Pitting Resistance of 18% Chromium Austenitic Stainless Steel, Corrosion, Vol 30, 1974, p 161-166... 

M.O. Speidel, Stress Corrosion Crack Growth in Austenitic Stainless Steel, Corrosion, Vol 33, 1977, p 199-203... 

MR Louthan and RG Derrick, Hydrogen transport in austenitic stainless steel . Corrosion Science, vol. 15, 1975, pp. 565-77. 

R.J. Brigham, E.W. Tozer, Effect ofalloying additions on the pitting resktance of 18% of Cr austenitic stainless steel. Corrosion 30 (1974) 161—166. 

P.R. Rhodes, Mechanism of chloride stress corrosion cracking of austenitic stainless steels. Corrosion 25 (1969) 462-467. 

Gamer, A., The Effect of Autogenous Welding on Chloride Pitting Corrosion in Austenitic Stainless Steels, Corrosion, Vol. 35, No. 3, 1979, pp. 108-113. 

Rooyen, D. V, Qualitative Mechanism of Stress Corrosion Cracking of Austenitic Stainless Steels, Corrosion, September 1960, p. 93. 

Lu, Y.C., Ives, M.B.(1995). Chemical treatment with cerium to improve the crevice corrosion resistance of austenitic stainless steels. Corrosion Science, Vol. 37, No. 1 pp.145-155, ISSN 0010-938X. 

Armijo, J. S. (1968X Intergranular Corrosion of Non-sensitized Austenitic Stainless Steels, Corrosion Vol. 24, No. 1, 24-30. 

Although Hitec is nonflammable, it is a strong oxidizer and supports the combustion of other materials. Consequendy, combustible materials must be excluded from contact with the molten salt. Hitec is compatible with carbon steel at temperatures up to 450°C. At higher temperatures, low alloy or austenitic stainless steel is recommended. Adding water to Hitec does not appreciably alter its corrosion behavior. 

Standard Wrought Steels. Steels containing 11% and more of chromium are classed as stainless steels. The prime characteristics are corrosion and oxidation resistance, which increase as the chromium content is increased. Three groups of wrought stainless steels, series 200, 300, and 400, have composition limits that have been standardized by the American Iron and Steel Institute (AlSl) (see Steel). Figure 8 compares the creep—mpture strengths of the standard austenitic stainless steels that are most commonly used at elevated temperatures (35). Compositions of these steels are Hsted in Table 3. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

The highly aHoyed austenitic stainless steels are proprietary modifications of the standard AISI 316 stainless steel. These have higher creep—mpture strengths than the standard steels, yet retain the good corrosion resistance and forming characteristics of the standard austenitic stainless steels. Nickel-Base Superalloys. 

Nickel—Iron. A large amount of nickel is used in alloy and stainless steels and in cast irons. Nickel is added to ferritic alloy steels to increase the hardenabihty and to modify ferrite and cementite properties and morphologies, and thus to improve the strength, toughness, and ductihty of the steel. In austenitic stainless steels, the nickel content is 7—35 wt %. Its primary roles are to stabilize the ductile austenite stmcture and to provide, in conjunction with chromium, good corrosion resistance. Nickel is added to cast irons to improve strength and toughness. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Materials of Construction and Operational Stress. Before a centrifugal separation device is chosen, the corrosive characteristics of the Hquid and soHds as weU as the cleaning and saniti2ing solutions must be deterrnined. A wide variety of materials may be used. Most centrifuges are austenitic stainless steels however, many are made of ordinary steel, mbber or plastic coated steel. Monel, HasteUoy, titanium, duplex stainless steel, and others. The solvents present and of course the temperature environment must be considered in elastomers and plastics, including composites. 

The most widely used austenitic stainless steel is Type 304, known as 18—8. It has excellent corrosion resistance and, because of its austenitic stmcture, excellent ductihty. It may be deep-drawn or stretch formed. It can be readily welded, but carbide precipitation must be avoided in and near the weld by cooling rapidly enough after welding. Where carbide precipitation presents problems. Types 321, 347, or 304L may be used. The appHcations of Types 304 are wide and varied, including kitchen equipment and utensils, dairy installations, transportation equipment, and oil-, chemical-, paper- (qv), and food-processing (qv) machinery. 

Corrosion. Copper-base alloys are seriously corroded by sodium thiosulfate (22) and ammonium thiosulfate [7783-18-8] (23). Corrosion rates exceed 10 kg/(m yr) at 100°C. High siUcon cast iron has reasonable corrosion resistance to thiosulfates, with a corrosion rate <4.4 kg/(m yr)) at 100°C. The preferred material of constmction for pumps, piping, reactors, and storage tanks is austenitic stainless steels such as 304, 316, or Alloy 20. The corrosion rate for stainless steels is <440 g/(m yr) at 100°C (see also Corrosion and corrosion control). 

M. O. Speidel, in R. W. Staehle and M. O. Speidel, eds.. Stress Corrosion of Austenitic Stainless Steels, ARPA report, 1979. 

When austenitic stainless-steel tubes are used for corrosion resistance, a close fit between the tube and the tube hole is recommended in order to minimize work hardening and the resulting loss of corrosion resistance. 

Other Considerations Autoignition can occur if combustible fluids are absorbed by wicking-type insulations. Chloride stress corrosion of austenitic stainless steel can occur when chlorides are concentrated on metal surfaces at or above approximately 60°C (140°F). The chlorides can come from sources other than the insulation. Some calcium sihcates are formulated to exceed the requirements of the MIL-I-24244A specification. Fire resistance of insulations varies widely. Calcium sihcate, cellular glass, glass fiber, and mineral wool are fire-resistant but do not perform equally under actual fire conditions. A steel jacket provides protection, but aluminum does not. 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, fresh water, and organic exposures). 

Austenitic stainless steels are the most corrosion-resistant of the three groups. These steels contain 16 to 26 percent chromium and 6 to 22 percent nickel. Carbon is kept low (0.08 percent maximum) to minimize carbide precipitation. These alloys can be work-hardened, but heat treatment will not cause hardening. Tensile strength in the annealed condition is about 585 MPa (85,000 Ibf/in"), but workhardening can increase this to 2,000 MPa (300,000 Ibf/in"). Austenitic stainless steels are tough and ducdile. 

Figure 2.20 Austenitic stainless steel plate from plate-and-frame heat exchanger. The orange oxide was formed from corrosion product originating at the regularly spaced pits. Pits are present near points of contact between adjacent plates. Corrugations run at right angles on adjacent plates.

---