Volume 13 - Corrosion Part 2 pot

Fig. 19 Typical surface appearance of a stabilized stainless steel (X10CrNiMoTi 15 15) after a 5000- h exposure to flowing sodium at 700 °C (1290 °F). Cavities are formed at the grain corners; coral- like particles of a MoFe phase are on the grain surfaces. Courtesy of H.U. Borgstedt, Karlsruhe Nuclear Center Fig. 20 Corrosion of Inconel alloy 706 exposed to liquid sodium for 8000 h at 700 °C (1290 °F); hot leg of circulating system. A porous surface layer has formed with a composition of 95% Fe, 2% Cr, and <1% Ni. The majority of the weight loss encountered can be a ccounted for by this subsurface degradation. Total damage depth: 45 m. (a) Light micrograph. (b) SEM of the surface of the porous layer. Source: Ref 5 Fig. 21 Corrosion of Nimonic PE 16 exposed to the same conditions described for Fig. 20. A porous coral- like surface layer has formed with a composition similar to that of Inconel alloy 706, but with the addition of corrosion- resistant FeMo particles at the coral tips. Intergranular attack beneath this layer extends to a depth of 75 m. Total damage depth: 135 m. (a) Light micrograph. (b) SEM of the surface of the porous layer. Source: Ref 5 Fig. 22 Deposition of iron-rich crystals on Stellite 6 sheet after 5000 h in flowing sodium at 600 °C (1110 °F). Courtesy of H.U. Borgstedt, Karlsruhe Nuclear Center Liquid lithium systems have been designed for two widely different areas: space nuclear power and fusion reactors. These two applications draw on unique properties of this liquid metal and have led to studies with a wide range of containment materials and operating conditions. Space power reactors require low mass; this in turn demands high-temperature operation. Lithium, with its low melting point/high boiling point and high specific heat, is an ideal candidate heat transfer medium. Refractory metal alloy containment is essential for these reactors, which may have design operating temperatures as high as 1500 °C (2730 °F). Liquid lithium in fusion reactor concepts is selected because here the neutronics allow tritium fuel to be bred from the lithium; this is essential in order to make the economics of the reactor viable. Containment temperatures are below 700 °C (1290 °F); therefore, iron-base alloys can be used for construction. The effects of liquid lithium on stainless steel, nickel, and niobium containment materials are shown in Fig. 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32. Fig. 23 Corrosion of type 316 stainless steel exposed to thermally c onvective lithium for 7488 h at the maximum loop temperature of 600 °C (1110 °F). (a) Light micrograph of polished and etched cross section. (b) SEM showing the top view of the porous surface. Source: Ref 6 Fig. 24 SEM micrographs of chromium mass transfer deposits found at the 460- °C (860- °F) position in the cold leg of a lithium/type 316 stainless steel thermal convection loop after 1700 h. Mass transfer deposits are often a more serious result of corrosion than wall thinning. (a) Cross sec tion of specimen on which chromium was deposited. (b) Top view of surface. Source: Ref 7 Fig. 25 Changes in surface morphology along the isothermal hot leg of a type 304 stainless steel pumped lithium system after 2000 h at 538 °C (1000 °F). C omposition charges transform the exposed surface from austenite to ferrite, containing approximately 86% Fe, 11% Cr, and 1% Ni. (a) Inlet. (b) 7.7 m (25 ft) downstream. Source: Ref 8 Fig. 26 Mass transfer deposits an X10CrNiMoTi 15 15 stainless steel after 1000- h exposure in static liquid lithium at 700 °C (1290 °F). Deposits are of the composition of the capsule steel (18Cr-8Ni). Courtesy of H.U. Borgstedt, Karlsruhe Nuclear Center Fig. 27 Corrosion of a capsule wall of 18 10 CrNiMoTi stainless steel by static lithium in the presence of zirconium foil. A porous ferritic surface layer has formed. Source: Ref 9 Fig. 28 Effects of flowing lithium on the inside surface of a type 316 stainless steel tubing. (a) Pickled surface before exposure. Composition: 65.8 Fe-18.0Cr-9.2Ni-3.3Mn-2.6Mo- 0.9Si. (b) After exposure in flowing lithium (0.3 m/s, or 1 ft/s) for 1250 h at 490 °C (915 °F). Composition: 88.6Fe-7.5Cr-1.7Ni- 0.6Mn. (c) After exposure to flowing lithium (1.3 m/s, or 4.3 ft/s) for 3400 h at 440 °C (825 °F). Taper section used to magnify damaged surface zone in metallographic mount. Courtesy of D.G. Bauer and W.E. Stewart, University of Wisconsin. Fig. 29 Corrosion of nickel in static lithium after exposure for 300 h at 700 °C (1290 °F). (a) Light micrograph. (b) SEM micrograph. Source: Ref 10 Fig. 30 Light micrograph of the polished and etched cross section of niobium containing 1500 wt ppm of oxygen showing the transcrystalline and grain boundary penetration that o ccurred after exposure to isothermal lithium for 100 h at 500 °C (930 °F). Source: Ref 11 Fig. 31 Intergranular attack of unalloyed niobium exposed to lithium at 1000 °C (1830 °F) for 2 h. Light micrograph. Etched with 25% HF, 12.5 HNO 3 , 12.5% H 2 SO 4 in water. Source: Ref 12 Fig. 32 Iron crystals found in a plugged region of a failed pump channel of a lithium processing test loop. Multifaceted platelike crystals are 0.4 mm (0.015 in.) across. Composition: 86 to 93% Fe, 7 to 14% Ni, 0 to 1% Mn. (a) SEM. 70×. (b) Iron x-ray scan. 70×. (c) SEM. 90×. Source: Ref 13 Liquid mercury, potassium, and cesium have also been used for space and terrestrial applications. In some cases, these have involved two-phase systems in which the corrosion consideration became significantly altered. Lead, lead-bismuth, and lead-lithium alloys (Fig. 33) have received attention for topping cycle heat extraction systems, heat exchangers, reactor coolants, and, more recently, fusion reactor designs. Fig. 33 Light micrograph of the polished cross section of a type 316 stainless steel exposed to thermally convective Pb-17at.% Li at 500 °C (930 °F) for 2472 h. Source: Ref 14 There are many other combinations of containment and liquid metals that have contributed to the knowledge of corrosion behavior; some have proved to be benign, while others have resulted in short-term catastrophic failures. In the discussion "Safety Considerations" in this section, some brief notes are given regarding safety precautions for handling liquid metals, operating circulating systems, dealing with fire and spillage, and cleaning contaminated components. Forms of Liquid-Metal Corrosion The forms in which liquid-metal corrosion are manifested can be divided into the following categories. • Dissolution from a surface by (1) direct dissolution, (2) surface reaction, involving solid- metal atom(s), the liquid metal, and an impurity element present in the liquid metal, or (3) intergranular attack • Impurity and interstitial reactions • Alloying • Compound reduction All the variables present in the system play a part in the form and rate of corrosion that is established. There are ten key factors that have a major influence on the corrosion of metals and alloys by liquid-metal or liquid-vapor metal coolants. These are: • Composition, impurity content, and stress conditionof the metal or alloy • Exposure temperature and temperature range • Impurity content of the liquid metal • Circulating or static inventory • Heating/cooling conditions • Single or two-phase coolant • Liquid-metal velocity • Presence/control of corrosion inhibition elements • Exposure time • Monometallic or multialloy system components. These factors have a varied influence, depending on the combination of containment material and liquid metal or liquid- metal alloy. In most cases, the initial period of exposure (of the order of 100 to 1000 h, depending on temperature and liquid metal involved), is a time of rapid corrosion that eventually reaches a much slower steady-state condition as factors related to solubility and activity differences in the system approach a dynamic equilibrium. In some systems, this eventually leads to the development of a similar composition on all exposed corroding surfaces. High-nickel alloys and stainless steel exposed together in the high-temperature region of a sodium system will, for example, all move toward a composition that is more than 95% Fe. Compatibility of a liquid metal and its containment varies widely, as is illustrated in Fig. 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, and 33. For a pure metal, surface attrition may proceed in an orderly, planar fashion, being controlled by either dissolution or a surface reaction. For a multicomponent alloy, selective loss of certain elements may lead to a phase transformation. For example, loss of nickel from austenitic stainless steel exposed to sodium may result in the formation of a ferritic surface layer (Fig. 17, 18a, 25, and 27). In high-nickel alloys, the planar nature of the corroding surface may be lost altogether, and a porous, spongelike layer may develop (Fig. 20 and 21). A more insidious situation can produce intergranular attack; liquid lithium, for example, will penetrate deep into refractory metals if precautions are not taken to ensure that the impurity element oxygen is in an oxide form more stable than Li 2 O or LiO solutions, and is not left free in solid solution. Figure 31 illustrates intergranular attack in niobium. Three factors surface attrition, depth of depleted zone (for an alloy), and the presence of intergranular attack should be evaluated collectively in any liquid-metal system. This evaluation will lead to an assessment of total damage, which may be presented either as a rate or as a cumulative allowance that must be made for the exposure of a given material over a given time. A large body of literature exists in which rate relationships for numerous liquid metal/containment combinations have been established. The more basic principles of liquid-metal corrosion are outlined in the article "Fundamentals of High-Temperature Corrosion in Liquid Metals" in this Volume and in the Selected References that follow this article. One vitally important aspect of liquid-metal corrosion that is often overlooked is deposition. Corrosion itself is very often not a factor of major concern because surface recession rates in regions of maximum attack are often of the order of microns per year. The formation of compounds in the circulating liquid metal and the accumulation of deposits in localized regions where there is a drop in temperature, a change in flow rate or flow direction, or an induced change in surface roughness can, however, be very serious. If these deposits do not succeed in restricting flow channels completely, their nature is often such that they are only loosely adherent to deposition surfaces and may be dislodged by vibrations or thermal shock in the system, thus creating a major coolant flow restriction in a high-temperature region. Most deposits have a very low packing density; therefore, deposit growth can proceed at a rate that outstrips corrosion by several orders of magnitude. Examples of loosely adherent deposits are shown in Fig. 18(b), 22, 24, and 26. Figure 32 shows iron crystals that restricted flow in a pump channel of a lithium processing test loop. Liquid-metal corrosion, as in other forms of corrosion, involves an appreciation for the source of corrosion in any system and an understanding of how potential sinks will operate on the corrosion burden, particularly if the liquid metal is not static but is circulated in a heat-transfer system either by pumping or by thermal convection. Safety Considerations The extensive work with the alkali and liquid metals has shown that such materials can be safely handled and used, provided certain precautions are needed. The requirements for the safe use of liquid metals are in essence those of good industrial or laboratory practice, involving protection from contamination, chemical reactions, exposure to toxic or irritating substances, and protection from high temperatures. More specific information and details on safe operation and handling are available in Ref 15 and 16 and in the Materials Safety Data sheets issued by the Manufacturing Chemists' Association. Chemical Reactivity. All the liquid metals react with oxygen and moisture to some degree; with the alkali metals, the reaction is vigorous enough to be potentially hazardous, particularly with potassium, rubidium, and cesium. Use of inert gas covers and the exclusion of moisture are the best defenses. Even with the nonalkali metals where the reactions with water are slow, water, such as that found on equipment that is not completely dry, must not be brought in contact with liquid metals because of the danger of a steam explosion that can scatter liquid metal over a considerable area, damaging equipment and inflicting severe burns on unprotected workers. It goes without saying that workers in the vicinity of liquid-metal systems should wear appropriate protective clothing (Ref 15). Potassium, rubidium, and cesium form higher oxides than the monoxide when exposed to air; these compounds are powerful oxidizing agents, often shock sensitive, and definitely hazardous in the proximity of organic materials. They will form at room temperature in contact with the solid metals. The practical application of this information is the need for extreme caution when handling the metals or compounds of the heavier alkali metals, particularly when they have been stored for extended periods under less-than-ideal conditions, or when cleaning spills or fire residues. It is noted that the sodium-potassium eutectic alloy (NaK) will form potassium super-oxide when exposed to air or oxygen. Because NaK is liquid at normal room temperature, small leaks at low temperature do not necessarily freeze and self-seal. Circulating Systems. It is usually convenient to provide a closed circulating system, or loop, in which to perform liquid-metal corrosion experiments. Reference 16 describes a simple system; the principles involved are the same for even very complex specialized devices. The loop will provide the means for circulation of the liquid metal; it will contain devices for on-line purification (if needed), while the inert cover gas will protect against chemical contamination. Insulation and an enclosure will protect against high temperatures and the spread of reaction products in case of a containment breach. Such systems provide a safe, convenient environment for handling liquid metals. Detailed operating procedures, involving common-sense principles such as maintaining the inert cover gas at all times and melting frozen metals by directionally heating away from a free surface, must be worked out for each system. The tens of millions of hours of safe operation of such systems, ranging from 1-L capacity test rigs to 4000-MW (thermal) nuclear reactors, validate the concept. References 17 and 18 describe many such test systems and the experiments performed in them. Recovery From Spills and Accidents. It must be remembered that spills of the nonalkali metals, even though the chemical reactivity hazard may not be great, must be handled with care because of the toxic nature of many of the metals and their vapors. Leaks and spills of the alkali metals, particularly when some of the leaked material has burned, present a special hazard because the spilled material often contains finely divided unreacted metal mixed with combustion products. Such mixtures can react vigorously with moist air, water, and alcohol. The products of the heavier alkali metals are the most reactive in this respect, but mixtures containing sodium and lithium are certainly not immune to violent reactions if carelessly handled. Cleanup of these residues must be approached with extreme care. Removal of Residual Metals From Corrosion Specimens. Nonalkali metals can often be removed from corrosion specimens by draining, forming an amalgam or solution with an alkali metal, and then removing the mixture by a technique discussed below. Alkali metals can be removed from specimens by reaction with water or alcohols; the most vigorous reaction is with water, and the rate decreases as one progresses to heavier weight alcohols. The reaction becomes more vigorous with increasing atomic weight of the alkali metals. Cesium/water reactions are definitely explosive. Use of ethanol and methanol is generally safe for sodium reaction, but one must remember the flammability hazard with alcohol vapors. The glycol ethers, such as butyl cellosolve, can also be used; they react more slowly than water, present less of a fire hazard than ethanol or methanol, but have toxic liquid and vapors. The water, alcohol, and glycol ether reactions all generate hydrogen; adequate ventilation must be provided to prevent the buildup of the hydrogen and the attendant danger of an explosion. Anhydrous liquid ammonia forms a true solution with the alkali metals and can be used to remove adherent alkali metals from corrosion specimens. Precautions against the hazards of liquid ammonia must be taken; the alkali metal in solution with ammonia is then usually reacted with water or alcohol before the ammonia mixture is discarded. The conditions must be maintained truly anhydrous in order to avoid hydrogen generation and contamination of the samples. Hydrogen can become implanted in refractory metal samples and embrittle them even at subzero temperatures. If the proper equipment is available, evaporation of the residual metal from the surface can be done with excellent results. Removal of alkali materials from pipework, if hydrogen generation is not a problem, can be accomplished with alcohol or gycol ether reaction, optionally followed by water rinsing to wash away the reaction products. It must be remembered that these reagents react very slowly, if at all, with oxides of the alkali metals. Another successful method in use is reaction with water vapor/inert gas (argon or nitrogen) mixtures, or water spray in an inert carrier gas, followed by water rinsing. Evaporation has also been successfully used and could be considered where hydrogen generation is not permitted. Ammonia-base systems have also been used for refractory metal pipework where hydrogen generation was prohibited. References 16, 17, and 18 contain more detailed information. Fire Protection and First Aid. Firefighting and medical treatment should, of course, be left to the professionals. There are, however, several factors to keep in mind. An alkali metal fire does not expand, as does, for example, a petroleum fire. It does produce vast quantities of caustic smoke that react with moisture in or on the body, and this produces severe burns. The smoke must be avoided unless respiratory protection and protective clothing are worn. Lithium presents a special hazard because of the toxic nature of some of its compounds and because it reacts with nitrogen. The combustion products of a lithium fire contain nitride and acetylide, which react with water to form ammonia and acetylene, respectively. [...]... mils/d NaCl 0.3 3-0 .53 1 3- 21 Acetic acid 0.5 Copol NaCl Lacquer Filament width mm mils 6 5-8 5 0. 1-0 .3 4-1 2 20 86 0.15 6 0.03 1 .2 6 0-9 4 Acetic acid 0.85 33.5 0. 1-0 .5 4 -2 0 Linseed oil NaCl 0.0 4-0 .08 1. 6-3 .1 0.0 5-0 .1 2- 4 Alkyds NaCl 0.50 20 80 0. 1-0 .5 4 -2 0 Acetic acid 0.1 4 85 FeCl2 0 .2 6-0 .43 1 0-1 7 80 0 .25 10 Steels Varnish Alkyd urea Epoxy urea NaCl/FeCl2 0.0 1-0 .46 0. 4-1 8 80 0 .25 10 Epoxy Acetic... NaCl 0.1 9-0 .86 7. 5-3 4 80 0 .25 10 Acetic acid 0.1 4 85 NaCl 0.1 6-0 .50 6. 3 -2 0 90 0. 1-0 .3 4-1 2 Acetic acid 0.09 3.5 85 Acetic acid 0.08 3.1 85 Polyurethane Polyester Aluminum Alloys Alkyds HCl vapor 0.1 4 85 0. 5-1 .0 2 0-4 0 Acrylic HCl vapor 0.1 4 85 0. 5-1 .0 2 0-4 0 Polyurethane HCl vapor 0.1 4 7 5-8 5 0. 5-1 .0 2 0-4 0 Polyester HCl vapor 0 .2 4 85 0. 5-1 .0 2 0-4 0 Epoxy HCl vapor 0.09 3.5 85 0. 5-1 .0 2 0-4 0 Alkyds... coal gasifier environment (pO2 = 3 × 1 0 -2 0 atm and pS2 = 1 × 1 0-7 atm) at 870 °C (1600 °F) for 100 h (a) and (b) Macrograph and micrograph, respectively, of a test coupon with a 0 .25 4-mm (0.01-in.) diam grain size (c) Micrograph showing external sulfides, sulfide scale, and intergranular sulfidation of a test coupon with a 0. 02 2- to 0.0 3 2- mm (0.000 8- to 0.0 01 3- in.) diam grain size (a) 1.5× Courtesy... Austenitic Stainless Steel, Microstruct Sci., Vol 12, 1985, p 21 3- 22 6 7 P.F Tortorelli and J.H DeVan, Mass Transfer Deposits in Lithium-Type 316 Stainless Steel Thermal Convection Loops, in Proceedings of the Second International Conference on Liquid Metal Technology in Energy Production, CONF-800401, National Technical Information Service, 1980, p 1 3- 56 to 1 3- 62 8 C Bagnall, A Study of Type 304 Stainless... 1979, Lithium Spill and Fire in the Lithium Processing Test Loop." ANL-8 1 -2 5, Prepared for the U.S Department of Energy under Contract W-3 1-1 09-Eng-38 Argonne National Laboratory, Dec 1981 14 P.F Tortorelli and J.H DeVan, Corrosion of Ferrous Alloys Exposed to Thermally Convective Pb-17 at.% Li, J Nucl Mater., Vol 14 1-1 43, 1986, p 59 2- 5 98 15 O.J Foust, Ed., Liquid Metals Handbook, Sodium and NaK Supplement,... Brehm, "Grain Boundary Penetration of Niobium by Lithium," Ph.D thesis, Report HYO- 322 8-1 1, Cornell University, 1967 • C.F Cheng and W.E Ruther, Corrosion, Vol 28 (No 1), 19 72, p 2 0 -2 2 • M.H Cooper, Ed., Proceedings of the International Conference on Liquid Metal Technology in Energy Production, CONF-760503, P1 and P2, National Technical Information Service, 1977 • J.M Dahlke, Ed., Proceedings of the... sulfur partial pressures of 3 × 1 0 -2 4 atm and 1 × 1 0-8 atm, respectively (b) and (d) Tested at 650 °C ( 120 0 °F) and pO2 = 3 × 1 0 -2 4 atm and pS2 = 1 × 1 0-9 atm SEM micrographs show sulfide scale (c) and an external sulfide formation (d) (a) and (b) 2 Courtesy of G.R Smolik and D.V Miley, E.G & G Idaho, Inc Fig 42 Sulfidation attack of Alloy 800 test coupons exposed to a coal gasifier environment (pO2 =... Fig 45 Fig 45 Ni -2 0 Cr-2ThO2 after simulated Type I hot corrosion exposure (coated with Na2SO4 and oxidized in air at 1000 °C, or 18 32 °F) A, nickel-rich scale; B, Cr2O3 subscale; C, chromium sulfides Courtesy of I.G Wright, Battelle Columbus Division Very small amounts of sulfur and sodium or potassium in the fuel and air can produce sufficient Na2SO4 in the turbine to cause extensive corrosion problems... the function of the fastener was not diminished On the other hand, Fig 10 shows crevice corrosion beneath the water box gasket of an alloy 825 (44Ni -2 2 Cr-3Mo-2Cu) seawater heat exchanger that allowed sufficient leakage to warrant shutdown and replacement after only 6 months Fig 9 Crevice corrosion at a metal-to-metal crevice site formed between components of type 304 stainless steel fastener in seawater... CONF-800401, National Technical Information Service, 1980 Selected References General References • C.P Dillon, Ed., Forms of Corrosion Recognition and Prevention, National Association of Corrosion Engineers, 19 82 • M.G Fontana and N.D Greene, Corrosion Engineering, 2nd ed., McGraw-Hill, 1978 • H.H Uhlig and R.W Revie, Corrosion and Corrosion Control, 3rd ed., John Wiley & Sons, 1985 Atmospheric Corrosion . and its containment varies widely, as is illustrated in Fig. 17, 18, 19, 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30, 31, 32, and 33. For a pure metal, surface attrition may proceed in an orderly,. ( 120 0 °F) and oxygen and sulfur partial pressures of 3 × 10 -2 4 atm and 1 × 10 -8 atm, respectively. (b) and (d) Tested at 650 °C ( 120 0 °F) and pO 2 = 3 × 10 -2 4 atm and pS 2 = 1 × 10 -9 . Fig. 18(b), 22 , 24 , and 26 . Figure 32 shows iron crystals that restricted flow in a pump channel of a lithium processing test loop. Liquid-metal corrosion, as in other forms of corrosion, involves