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  1. 1. Molten Salt Corrosion including neutral Salt Pot Performance and fuel ash corrosion Molten salts in general render the protective chromium oxide scale of heat resistant alloys ineffective. Good performance of salt pots depends more upon maintenance than on alloy selection.
  2. 2. MOLTEN SALT CORROSION1 When Ni-20Cr alloys and stainless steels are oxidized while submerged in molten salt (NaCl or NaCl-KCl), readily oxidizable alloy components, such as chromium, and in some cases iron, migrate to the surface to form non-adherent, granular and thus nonprotective oxides. This loss of alloy constituents causes a counter current flow of vacancies which condense into an interconnecting pore network filled with salt. Since the salt penetrates into the structure, the loss of alloying metals does not require intermetallic diffusion over long distances, but instead corrosion products are largely removed by solution of alloying metals as ions in the pre-salt network . . .chromium diffuses down grain boundaries of the grain network to be deposited at grain boundary-pore intersections as chromium ions. The concentration gradient of chromium ions in the salt phase forces chromium diffusion out to the bulk salt where the higher effective oxygen pressure forms Cr2O3 and some chromate ion. A. U. Seybolt, Oxidation of Ni-20Cr Alloy and Stainless Steels in the Presence of Chlorides, Oxidation of Metals, Vol. 2, No. 2, 1970 Hot chloride salts, and particularly salt fumes mixed with air, are very corrosive to heat resistant alloys. In general the higher nickel alloys, such as 600, are preferred, although I have seen encouraging results from the 1.7% silicon grade, 253 MA. Corrosion in Molten Chloride Heat Treat Salts, 1100-2200°F (600-1200°C) testing overseen by Gene R. Rundell, retired Rolled Alloys Depth of Intergranular Attack Grade Nickel, weight Silicon, weight mm inch % % RA85H® * 15 3.5 0.11 0.0044 RA 253 MA® 11 1.7 0.18 0.0069 RA600 76 0.2 0.19 0.0075 RA309 13 0.8 0.32 0.0125 RA330® 35 1.2 0.35 0.0138 *no longer produced Plate samples were exposed in a commercial heat treat salt line. They saw 210 to 252 cycles in preheat salts 700°C (1290°F) and 815°C (1500°F), high heat salt 1200°C (2200°F), quench in 600°C (1100°F) nitrate/nitrite salt, air cool. Preheat and high heat salts were mixtures of potassium, sodium and barium chlorides. Alkali metals in the salt react with the protective chromium oxide scale to fpr, an alkali chromate, which is non-protective and water soluble. As fast as the scale is removed, more chromium diffusing to the surface reforms the scale. Eventually most of the chromium may be removed from the alloy, leaving primarily iron and nickel. 1
  3. 3. Molten Salt Corrosion, continued Fluoride salts are more aggressive than are chloride salts. Molten fluorides are used to flux metals and alloys for brazing operations. Along with fluxing the oxide film on the workpiece, fluorides also attack the chromium oxide film on heat resistant alloy fixturing. A service trial of various alloy fixtures used in aluminum salt bath brazing at 1125°F (607°C) gave the following results: Alloy Total Life, days RA333® 197 (end of test—no failure) 600 112 Nickel 200 51 C-276 40 601 14 Other work has shown the 25% chromium ferritic grade, RA446, to be unsuitable for aluminum salt bath brazing operations. Salt Pots The most common industrial use of molten salts is to heat treat steel. Metallic salt pots used to contain neutral heat-treating salts may last anywhere from 2 days to 18 months, depending upon maintenance and operating procedures. Alloy selection does matter somewhat. However, in my experience, alloy choice may be outweighed in importance perhaps 50 to 1 by how the pot is maintained. The following points are important for good life in a metallic pot for neutral salt heat treating: 1.) Ensure that there is no salt whatsoever in the combustion chamber of a gas-fired pot or about the elements of an electrically heated pot. This is crucial. 2.) Rectify and desludge neutral chloride salts at least daily. 3.) Idle the pot with salt still molten, rather than shutting down completely and letting the salt freeze solid. 4.) Do not put oily work or any foreign matter (no floor sweepings!) into the pot. 5.) In both pots and fixtures, all welded joints must be full penetration welds. Only after these five points have been addressed should alloy selection be reviewed. Let us examine the reasons behind some of these points. 2
  4. 4. Salt Pots, continued Mixtures of potassium, sodium and barium chlorides are widely used as heating media that neither oxidize nor decarburize carbon, engineering alloy and tool steels. Regarded as “neutral” salts, any minute amounts of oxygen present will attack the chromium in the Ni-Cr-Fe alloys used for pots and fixtures. While the chloride salts themselves are indeed neutral, the inevitable oxygen content of the bath causes the destruction. Oxygen is present because the surface of the bath is open to the air, and because air is always brought into the bath with the workpieces. The destructive part is that as fast as the alloy forms a protective chromium oxide scale, the alkali chlorides strip or flux that scale, forming potassium, sodium, and/or barium chromates. As fast as chromium from the alloy diffuses to the surface to re-form the oxide scale, the scale is dissolved. The chromium diffuses along grain boundaries orders of magnitude faster than through the grains themselves, and diffusion voids, or pores develop in the grain boundaries2 . Eventually the molten salt physically penetrates the grain boundaries, and permeates the entire thickness of the salt pot wall, until the pot begins to leak through to the outside. This is somewhat more likely to occur in coarse grained regions, such as the fusion line of the weld or in the weld bead itself. Eventual failure in or near a weld does not necessarily mean that weld had been defective. It is the combination of alkali chloride salts and oxygen that attacks the pot. If a new pot is put into a furnace contaminated with leaked salt from the previous pot, that salt will volatilize when heated. Those alkali chloride fumes will attack the chromium oxide scale on the outside of the pot, and the hot air or products of combustion provide more than enough oxygen to scale right through the pot. This occasionally happens in as little as three days. And that is why it is most important to clean out all the spilled salt from the previous pot, when installing a new one. During operation, oxygen builds up in the salt itself. To prevent the steel workpieces from decarburizing the oxygen level of the bath must be kept low. This is referred to as rectifying the salt. It is done by introducing methyl chloride (well away from electrodes and metallic pot sidewalls), which converts the alkali oxides back to chlorides. The solid rectifiers powdered silicon, silica, ferrosilicon or dicyandiamide are also used. If the pot is not rectified well and frequently, the oxygen content will shorten the life of the pot by corrosion from the inside. This happens even at oxygen levels too low to harm the steel workpieces. 3
  5. 5. This photomicrograph, from Rolled Alloys Investigation No. 99-66, shows corrosion attack and corrosion assisted cracking in the fusion line between RA330 plate, top, and the RA330-04-15 weld bead, bottom. In this salt pot the attack along weld fusion lines was so bad that entire lengths of the weld bead could be removed with a few blows of a hammer. The failure occurred simply because the firebox refractory still contained the spilled salt from the previous salt failure. In this case the solution was to replace this blanket insulation each time a new pot is installed. Operating temperature was about 1700-1750°F (930- 950°C) using a non-cyanide carburizing salt. Inorganic rectifiers form a metallic sludge in the bottom of the pot. This sludge must be removed frequently, perhaps twice daily, lest it act as an insulator, causing the bottom of the pot to overheat. Leaving the sludge in can overheat the bottom to the point that it fails, usually in or near a weld, while the sidewalls are still in good condition. Alloy Casting Institute3 studies of salt baths show that corrosion rates in the sludge itself, even without overheating, are increased by nearly a factor of two. 4
  6. 6. 3/8” (9.5mm) RA309 salt pot bottom 0.8 scale The photo above is a classic example of pot failure due to metallic sludge build- up, in this case over two inches (50mm) deep in the bottom. This sample came from a Western US heat treat shop. Neutral salt pots here were failing by leaks at the bottom weld in 4 to 6 weeks of operation, some after as little as 2 weeks. Operation was neutral salt at 1400—1600°F (760—870°C) 24 hours per day, 6 days per week. The pot was idled at 1200°F (650°C) on Sundays. Etchant: Oxalic Acid Magnification: 25X Crack near the fusion line, inner weld bead of the RA309 pot above. When salt freezes it contracts in volume. If a salt pot is shut down and allowed to freeze solid, that salt will go through a volume increase when remelted on start- up. This increase can be about 3/8 to 1/2 inch per foot (31 to 42 mm/metre) of pot depth. 5
  7. 7. Salt Pots, continued If the pot is full of solid salt, there are a couple of unpleasant possibilities. One is that, as the salt melts first on the bottom, it expands and cracks open the pot, either in a weld or along the knuckle radius of a dished head. Alternately, once sufficient salt has melted, the volume expansion may cause it to explode through the remaining frozen layer on top. If one plans a shutdown, it is a good idea to ladle out most of the salt before it freezes. Floor sweepings can do interesting things to salt pots. Aluminum foil left over from someone’s lunch, for example, will melt and go right through the bottom of the pot. Sulfur, from machining oils, will attack the nickel in the pot, the higher the nickel alloy, the worse the attack. One alloy fabricator related an incident in which short life of their 309 pots was indeed traced to the practice of disposing of floor sweepings in the heat treat pots at night. Salt Pot Alloy Selection Since the 1930’s the most popular alloys have been the wrought alloys 309, RA330 and 600, or the cast grades HT (17Cr 35Ni) and HW (12Cr 60Ni). The higher chromium content of 310, or of cast HK, is regarded to be disadvantageous in salt. Both laboratory studies and observations of fixtures are reasonably consistent in showing that the higher nickel grades usually have better resistance to alkali chloride salts. If that were all there were to it, all metallic salt pots would be Inconel® 600 or HW (N08001), and all electrodes in ceramic pots would be either 600 or commercially pure nickel. In practice the majority of metallic pots today are RA330 or 309, with 600 a distinct minority. Almost all submerged or over-the-top heating electrodes are RA446, with a very, very few being RA330 or 600. Almost none are pure nickel. It is the case that the various ills that befall metallic pots obscure any theoretical advantages of higher nickel to the extent that 35% or 13% nickel grades are considered more cost-effective. With respect to fixtures for automated salt lines, performance often follows the alloy’s resistance to chloride salts. Either 600 or RA330 is, in my opinion, superior to 309. In some shops 600 has the advantage over RA330, in others there is no clear difference. In all cases, full penetration welds are necessary. Some fairly reliable testing, given on page 1, suggests that alloy 253 MA is better than the 35% Ni grade RA330, and may be comparable to alloy 600. 253 MA is both stronger and less expensive than either of these higher nickel alloys and may be worth considering for fixturing in molten salt operations. Neither 253 MA, nor any other alloy, is likely to solve existing problems with alloy salt pots. 6
  8. 8. Salt Pot Alloy Selection, concluded This alloy selection discussion is for pots containing chloride salts, in which some steel piece will be austenitized. Tempering salts, which are mixtures of sodium nitrate and sodium nitrite, may be made either of carbon steel or of 304 stainless. More is not better. High nickel alloys in molten tempering salts may corrode very quickly. The reason is that the nitrate is an outstanding source of oxygen. Sodium nitrate was, for example, the oxygen source in old-fashioned black gunpowder meant for blasting. The combination of oxygen and sodium will quickly corrode the chromium in the alloy to the point that molten salt will begin leaking through what had been solid metal The same pot should not be used to hold both tempering salts for one operation, and neutral chloride salts for another. The combination of the two salts, one being residual, may corrode intergranularly through the pot wall in a rather short time. Fuel Ash Corrosion Equipment fired with residual fuel oil suffers corrosion wherever the fuel ash deposits on hot metal. Heavy oils such as No. 6 or “Bunker C” may contain both sulfur and vanadium. When this oil is burned, the vanadium forms vanadium pentoxide, V2O5. This vanadium pentoxide, along with sodium sulfate, makes a molten compound which is aggressively corrosive. It will eat away most heat resistant alloys in less than a year. A high level of sulfur in the oil might be 2 or 3%, while only 0.05% (or, 500 parts per million) of vanadium is “high” enough to be destructive. Venezuelan crude oil is particularly high in vanadium and is often used in the Northeastern U.S.A. The alloys with good resistance to fuel ash corrosion are usually cast compositions that are both weak and brittle. 50Cr-50Ni cast IN-657 (UNS R20501) is the best, while HE (28Cr 9.5Ni) is said to be reasonable. IN-657 is expensive and readily embrittled, and HE is particularly weak and brittle. Available wrought alloys are not at all as resistant to fuel ash corrosion but are used for their much better ductility. RA333, RA625, RA330, RA 253 MA and RA310 have all been used or are on trial. Frankly, we have no good comparative field data for these wrought alloys. Mostly based on rumor, I might suggest either RA333 or RA 253 MA as worth trying, but they definitely will not be as good as 50%Cr-50%Ni cast. February 1, 2015 8
  9. 9. References 1. James Kelly, Neutral Salt Pot Alloy Life: Maintenance is the Key, Heat Treating, April 1990 2. A.U. Seybolt, Oxidation of Ni-20Cr Alloy and Stainless Steels in the Presence of Chlorides, Oxidation of Metals, Vol. 2, No. 2, 1970 3. J. H. Jackson and M. H. LaChance, Resistance of Cast Fe-Ni-Cr Alloys to Corrosion in Molten Neutral Heat Treating Salts, Transactions of the ASM Vol. 46, 1954, pp 157-183. DISCLAIMER: The data and information in this manual are believed to be reliable. However, this material is not intended as a substitute for competent professional engineering assistance which is a requisite to any specific application. James Kelly Metallurgist makes no warranty and assumes no legal liability or responsibility for results to be obtained in any particular situation. Improvements and additions to this work may be made from time to time. Typographical errors may be present. 9
  10. 10. Alloys, nominal chemical composition alloy UNS EN/W.Nr. Cr Ni Si Mn C Fe Other 446 S44600 - - 25 - - 0.5 0.7 0.05 73 0.1N HE J93403 - - 28 9 1.5 1.5 0.4 60 - - 253 MA S30815 1.4835 21 11 1.7 0.6 0.08 65 0.17N 0.04Ce 309S S30908 1.4833 23 13 0.8 1.6 0.06 62 - - RA85HA S30615 - - 18.5 14.5 3.5 0.5 0.20 61 1Al 310S S31008 1.4845 25 20 0.5 1.6 0.05 52 - - HT J94605 - - 35 17 1.7 0.9 0.5 44 - - RA330 N08330 1.4886 19 35 1.2 1.5 0.05 43 - - RA330-04 N08334 - - 19 35 0.8 5.2 0.25 39 - - RA333 N06333 2.4608 25 45 1 1.5 0.05 18 3Co 3Mo 3W IN-657 R20501 2.4680 50 47.5 0.4 0.2 0.06 0.5 0.1N 0.1Al 0.3Ti 1.5Cb 671 R20500 - - 50 48 0.5 0.5 0.05 0.5 0.3Ti 0.2Al C-276 N10276 2.4819 15.5 57 0.05 0.5 0.005 5.5 2V 4W15.5Mo HW N08001 - - 12 60 1.5 1.5 0.5 24 - - 601 N06601 2.4851 22.5 61.5 0.2 - - 0.05 14Fe 1.4Al 600B N06600 2.4816 15.5 76 0.2 0.3 0.08 8 0.1Al 0.2Ti Ni 200 N02200 2.4068 - - 99.5 0.2 - - 0.02C 0.2 - - A no longer in production. Developed and patented by G.R. Rundell, retired Rolled Alloys B one bar producer chose to add a little Nb instead of Al & Ti in their 600 alloy. Vacuum heat treat baskets made from that bar appeared to deform more than did those fabricated of 600 bar produced by the inventor of the grade. ©2015 James Kelly Metallurgist LLC Michigan, USA 10