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. 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.
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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.
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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. 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. 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.
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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.
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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
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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.
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