- "Caustic embrittlement" of
steel boilers or the SCC of steels in NaOH was one of the first known forms of
stress-corrosion cracking. It is still one of the most common.
Carbon steel is the material of choice for handling caustic solutions at
low temperatures. However, there is a pronounced temperature threshold above
which caustic can cause cracking.
The HSLA steels such as ASTM A 242 show approximately the same SCC
resistance as the common carbon steels
NACE caustic soda service
This information is
from the NACE Corrosion Data Survey (5th edition)
The well known NACE caustic soda service chart (above),
showing the effect of temperature and concentration on SCC susceptibility,
was derived from a NACE survey of field experience in a wide variety of
The temperature threshold curve is most clearly established in the middle
concentration ranges, from about 10 to 50%, since the majority of applications
surveyed were in these concentration ranges. At lower concentrations, particularly below 5%, the chart as published by NACE
is probably conservative since there were relatively few data in the very low
concentration ranges. Laboratory investigators have difficulty producing SCC
on steels in pure caustic solutions with less than 4% NaOH, even at high
temperatures. Field failures in weak caustic solutions can usually be traced
either to concentration to higher caustic percentages (from evaporation or at
liquid-vapor interfaces) or to the presence of impurities such as lead oxide
or copper oxide which tend to accelerate caustic SCC.
- The SCC of steels in several solutions of economic importance (caustic,
nitrates, carbonates and ammonia) apparently takes place by accelerated
corrosion along active paths. These active paths operate in specific ranges of
electrode potential, which vary with the solution (figure 2 below).
Some of the solutions have very wide ranges of cracking potentials, such as
the nitrates and NH3. By contrast, caustic cracking occurs in two rather
narrow (approximately 100 mV wide) potential bands, which would correspond
approximately to the active nose and transpassive regions on the potentiostatic
However, significant uncertainty exists. Practically speaking, this means
that relatively small changes in composition, temperature or concentration
sometimes shift the free-corrosion potential of the system into or out of the
Consequently, both cathodic protection and inhibitor additions (certain
tannins, NaH2PO4, Na2SiO3 or Oxygen)
can inhibit cracking.
- Figure 2:
- Schematic diagram showing potential ranges over which SCC of carbon steels occurs in
- On heat transfer surfaces, such as boiler tubes, crevices or leaks are not
necessary for the concentration of entrained caustic if the temperature
difference between the boiling point of the water and the tubes exceeds certain
levels. Note that the most damaging concentration (33% NaOH) may be produced
with a DT of only 25°C.
- In steam transfer lines, most problems occur at leaks and liquid-vapor
interfaces, where low levels of caustic can concentrate by evaporation to form
deposits very high in caustic. Valve-bonnet leaks, for example, often lead to
SCC of the bonnet bolts.
Low-alloy steels such as AISI 4140 (ASTM A 193
Grade B7) show a parallel susceptibility to the low-strength carbon steels.
However, low-alloy steels are susceptible over a much wider range of potential.
Consequently, a wider spectrum of impurities and operating conditions can
result in cracking. On valve-bonnet bolts, this can be dangerous since a highly
stressed bolt can become a projectile when its elastic strain energy is suddenly
released by SCC.
- On bolting which might be exposed to steam leaks, electroless nickel plating
is widely used to prevent SCC. Nickel is practically immune to caustic SCC, and
the electroless plating process, properly applied, can provide a high-quality
barrier coating over the steel.
Plating is not often economical on carbon steel piping or vessels. In power plants, the standard method of preventing
caustic SCC is by water chemistry control. Nitrates are an effective inhibitor
for caustic SCC in the concentration ranges encountered in steam plants.
Sodium nitrate and sodium sulfate, which is sometimes used for the same purpose,
inhibit caustic SCC by shifting the potential of the solution away from the
cracking range. If operating conditions or other impurities result in a shift
back into the cracking potential range, the mere presence of nitrates or
sulfates in caustic will not inhibit SCC. As a result, NaNO3 and Na2SO4,
although widely used, are referred to by some authorities as "unsafe
inhibitors". True inhibitors of caustic SCC include valonea and quebracho
tannins, NaH2PO4 and (to a lesser extent) Na2SiO3.
- Limited data suggest that as SCC agents, calcium hydroxide and potassium
hydroxide are not as potent as sodium hydroxide. However, since the dangerous
temperature and concentration limits have not been thoroughly established for
any solutions but sodium hydroxide, current industry practice is to use the NACE
caustic soda service chart guidelines for all caustic solutions.
- Slightly alkaline sodium, potassium and ammonium carbonate-bicarbonate
solutions have produced intergranular SCC of steels in buried pipelines and
treating plants removing CO2 from natural gas.
Steels with less than 0.2% carbon are most affected by carbonate SCC.
Therefore, most common carbon steels used in CPI plants would show only mild
susceptibility to this sort of attack were it not for the decarburized layer
often present on hot-rolled plate and pipe. Once cracks initiate in this
decarburized layer, they can propagate more easily into the less sensitive base
material. An exception to the trend noted above is that the Nb-bearing,
low-carbon HSLA steels are comparable in SCC resistance to medium-carbon steels
such as API 5LX-52.
Carbonate SCC is observed in the pH range 8-10.5. Cracking has occurred in
concentrations as low as 0.25N. Several silicate and phosphate compounds are
effective inhibitors for carbonate SCC.
Cracking can occur from the freezing point up to 90°C. However, most
field failures take place above 60°C. Increasing temperature exponentially
increases the crack growth rate and the probability of failure; the crack growth
rate follows an Arrhenius law with increasing temperature such that every 10°C
increase in temperature will approximately double the crack velocity.
Threshold stresses for carbonate SCC are fairly high, and field failures
are extremely rare at design stresses below 60% of the specified minimum yield
strength. Consequently stress-relief is an effective preventive measure.
The range of electrode potentials over which cracking occurs averages about
100 mV, but the width of the range varies with pH.
- Some corrosion engineers consider monoethanolamine (MEA) and diglycolamine
(DGA) as stress-cracking agents on steels and refineries routinely
stress-relieve equipment which handles them.
However, there is some confusion over the circumstances of cracking. No
stress-cracking normally takes place in steel equipment used to manufacture and
store a wide variety of ethylene and ethanol-based amines. It could be that the
stress-cracking in MEA and DGA service in refineries is related not to the amine
itself but rather to CO2, CO or CN- compounds in the gas, or to NaOH from
regeneration systems. More research is needed on this point.
- The 18-8 type stainless steels, exemplified by Types 304 and 316, have a
useful low corrosion rate in caustic at up to 50% concentration to about 70°C.
In 40-50% NaOH from 80 to 100°C, the 18-8 austenitic stainless steels have
unstable passivity and may exhibit severe general corrosion if activated.
Note that Diagram 1 below (based on Copson's work; LaQue &
Copson, Corrosion resistance of metals and alloys, Reinhold Publ. Corp., NY,
1963) is somewhat more optimistic, showing less than 1 mpy up to about 95°C.
This is probably due to dissolved oxygen or traces of oxidizing species (cf.
infra) because it is known that Type 304 can go active in 40% caustic at about
80°C and in 50% caustic at 70°C. In the active state, the 18-8 alloys
corrode faster than C-steel!
Wrought austenitic stainless steels also suffer rapid SCC in caustic
solutions above certain threshold temperatures related to solution composition.
Below these temperature limits, however, the austenitics are
serviceable in caustic and offer an alternative with better corrosion resistance
than carbon steel and lower cost than nickel.
Three Diagrams provided below (Copson 1963; INCO Databooks 1973; and Swandby, Chemical
Engineer, Vol. 69, Nov.1972) differ slightly in the boundaries at both
very low or very high concentrations. Moreover, the INCO limits are significantly more conservative in the 50-75%
- Diagram 1
- Isocorrosion chart for Type 304 and Type 316 stainless steels in NaOH, with
SCC boundary superimposed (Copson 1963 / Schillmoller for NiDI, 1988).
- Diagram 2
- Isocorrosion chart for austenitic Cr-Ni stainless steels in NaOH (INCO Databook, 1973)
- Diagram 3
- Caustic soda service chart for austenitic stainless steels in NaOH (Swandby 1972)
- After passing through the temperature range where activation can take place,
passivity is apparently once more assured and SCC may occur. The bottom
temperature range for SCC is still uncertain, but rapid cracking will occur in
the 300-350°C range. Cracking of as-welded Type 304 (but not Type 304L) has
been reported in 3-5% caustic at 50°C - presumably intergranular in
nature. It is also reported that at 150°C, SCC occurred on U-bend
With austenitic stainless steels, the morphology of cracking is often
transcrystalline, indistinguishable from that of chloride cracking. However,
intergranular cracking is also observed. At high temperatures (e.g., 300°C)
a characteristic "blueing" (similar to the color of gun metal) is
observed on austenitic stainless steel cracked by caustic.
Oxygen & oxidizers
- Oxygen is reportedly not required for environmental cracking of stainless
steels by caustic. Data from laboratory tests in 10-30% sodium hydroxide at 330°C
showed that the presence of chromate ions prevented SCC of stainless steels.
- Although some authors] state that anhydrous caustic does not cause SCC of
stainless steel, very severe and rapid (i.e., in less than 24 hours) SCC has
been observed in molten sodium at 330°C under a high-pressure hydrogen
atmosphere. This was occasioned by the presence of parts per million of water in
an organic reactant. Thus, the resulting environment was 100% anhydrous caustic
dissolved in molten sodium under anaerobic conditions.
Other alkali metal
- Although sodium salts are more commonly encountered, other alkali metal
compounds cause similar problems. Potassium and lithium hydroxides, carbonates
and bicarbonates are capable of inducing environmental cracking in a manner
directly analogous to that of their sodium counterparts.
- Many of the high-chromium ferritic stainless steels
are outstanding in their resistance to hot caustic soda. 26-1 is widely used in
caustic evaporators as heat exchanger tubing, in 177°-204° C (350°-400°
F), 45% NaOH. Although general corrosion, erosion-corrosion and intergranular
attack are sometimes observed, no SCC has been encountered below 290° C
(550° F). 29-4 and 29-4-2 alloys look good in laboratory tests. As with
any stainless material, their general corrosion resistance is improved by the
presence of small amounts of oxidizing species such as chlorates or oxygen.
Type 430, Type 439 and 18-2 stainless steels are also resistant to caustic
SCC. However, these 16-19% Cr grades should be used with caution. Their general
corrosion resistance is poor in 10% NaOH in the range of temperatures
encountered in boiling water reactors 315° C (600° F).
alloy" ferritics (12-14% Cr) also show high rates of general corrosion in
hot concentrated caustic solutions and should be approached with caution as a
- The duplex stainless steels are a relatively new class of alloys; their
caustic corrosion and SCC performance have not yet been clearly defined. Some
tests on AISI 329 suggest that it can be used in up to 50% NaOH to at least 135°
C without SCC.
- SCC of hardenable stainless steel turbine components
is a serious source of forced outages in power plants. The alloys normally
used are 12 Cr martensitic grades such as 403 and 410. NaOH is used in power
plants to control feedwater pH and can concentrate due to carryover into
drum-type boilers or misoperation of condensate polishers. In addition, pH
control with trisodium phosphate can give rise to locally high hydroxyl ion
concentrations. If the boiler water treatment system allows the presence of free
hydroxide ion (OH-), carryover from the steam drum can cause SCC of downstream
Cracking is worst in deoxygenated solutions; the presence of oxygen tends
to retard the attack. The higher the metal hardness, the shorter the time to
failure. In up to 10% NaOH solutions, fully-annealed material and material with
hardness less than about Rc30 show good resistance to cracking up to 315°C. However, the material can be made to crack in U-bends. At
higher concentrations, the 12 Cr alloys show high rates of general corrosion and
are not suitable for use.
- In caustic of 70% concentration or more, even the high-nickel alloys will
crack over prolonged periods of time at temperatures above 300°C and at
high stress levels. In ascending order of merit are Alloy 400, Alloy 600, Alloy
200 and Alloy 201.
- Alloy 400 can suffer SCC at concentrations in excess of 80% if
temperatures exceed 175°C.
- Alloy 600 is more readily cracked
by hot concentrated caustic when oxygen is present.
- Nickel Alloy
200 fails only under very high stress at temperatures above 300°C after prolonged periods.
- Chlorides are a common contaminant in caustic and many SCC failures in
alkaline environments have been ascribed to them.
It is important, although
admittedly difficult, to differentiate their influence, as this can profoundly
affect the selection of alternative materials. For example, when both chlorides
and caustic are present, Alloy 600 is to be preferred to Alloys 800 and 825,
which can be cracked by caustic more easily than Alloy 600.
- Oxidizing contaminants, such as chlorates or hypochlorites, can affect the
corrosivity of caustic streams in an unpredictable fashion, either aggravating
or inhibiting specific phenomena.
- Although some sources indicate that sodium salts other than halides do not
cause SCC, this allegation must be tempered with the knowledge that some sodium
salts can and do suffer either hydrolysis or thermal decomposition with
attendant liberation of caustic, which does cause SCC. The thermally unstable
bicarbonates and carbonates are frequent offenders.
- Steam is a common environment for the SCC of stainless steel, due to
carryover from the boiler drum of either chlorides or caustic or both. Parts per
million of free caustic may accumulate or develop in crevices (e.g., from sodium
carbonate) to concentrations that will cause SCC in a matter of hours. Gasket
areas, welds lacking complete penetration, and bellows-type expansion joints are
frequent victims. Steam at 300 psia (215°C) and 400 psia (230°C) has
caused SCC of both 18-8 stainless steel and Alloy 400.
- Copper (ETP, DLP or DHP) shows useful general
corrosion resistance even in 50% NaOH at temperatures up to 130°C. Although copper equipment is not widely used for handling concentrated
caustic, it is used in process streams where caustic is used as a neutralizing
agent, inhibitor, etc., with good success. Stress-corrosion cracking has not
In the laboratory, 70-30 brasses have been cracked in NaOH solutions with
pH greater than 12. Cracking apparently does not take place unless the
potential is displaced at least 100 mV less negative than the free corrosion
potential. At a pH of 12 or less, cracking of brass in NaOH has not been
observed. These laboratory data would seem to confirm a long-held CPI tradition
against the use of high-Zn brasses such as CA 270 and 280 in caustic solutions.
- General corrosion rates of aluminum alloys in caustic
solutions are so high that stress-corrosion cracks never get a chance to
initiate. Even mildly alkaline solutions (pH>10.5) should not be handled in
- Strong, hot caustic can strip the oxide film off
titanium, leading to rapid general corrosion. However, stress-corrosion has not
- Zirconium shows good corrosion and stress-corrosion
cracking resistance in tests in 50-90% NaOH up to 260°C.
Since the installed cost of zirconium equipment has historically ranged from 50
to 100% higher than that of comparable nickel equipment, zirconium has not been
widely used for caustic service. However, changing prices and availability
problems with nickel may result in more widespread use of zirconium in caustic.