Self-employed consulting corrosion engineer retired from U.S. Steel
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN.
(9 printed pages)
The following article is a follow-up to "Industry Study Indicates Stress
Corrosion Cracking in Anhydrous Ammonia Storage Vessels," which appeared in the
October 1988 BULLETIN. This recapitulation of experiences with
stress-corrosion cracking of steel in liquefied ammonia summarizes the subject
over the last three decades. From the many reports reviewed, an effort was made
to concentrate on those that would lead to a better understanding of the
problem facing the industry today and suggest a solution to the problem. The
review is aimed toward a summary of the conclusions from various sources;
specific details are available in the referenced reports.
Stress-corrosion cracking of steel in liquefied ammonia is traced from its
early recognition in the 1950s, from experiences in agricultural and industrial
use from NACE (National Association of Corrosion Engineers) recommended DOT
(Department of Transportation) regulations to prevent damage, and from
laboratory studies to explain the factors involved. Control measures are
discussed; including control of air contamination, inhibition by water
addition, postweld heat treatments, influence of the strength of the steels
used and the applied stress, and periodic inspection techniques.
Soon after World War II the Mississippi State College proposed a method of
injecting liquefied ammonia directly into the soil as a chief source of
nitrogen fertilization (ammonia contains about 82 percent nitrogen). Because
excellent results were achieved, very soon a large network of ammonia
distribution and application facilities developed, which used many pressure
vessels constructed primarily of carbon steels. Although liquefied ammonia had
been used without significant difficulties for several decades in the
refrigeration, chemical and steel heat-treating industries, inexplicable
ruptures of ammonia containers in the agricultural service occurred soon after
the introduction of ammonia into this service. A survey indicated that about 3
percent of the pressure vessels failed within 3 years of service life **.
Thereupon a research committee was formed by the Agricultural Ammonia Institute
to determine the causes of equipment failure and to recommend palliative
measures. By evaluating the storage and handling facilities and practices for
liquefied ammonia and by conducting controlled tests, it was determined that
stress-corrosion cracking (SCC) was the most likely cause of the failures .
The probability of SCC increased with increased stress level as well as with
increased yield strength of the steel. Of the many contaminants that could have
been present in the ammonia, air was identified as the specific agent which
caused SCC. (Although at that time the normal carbon dioxide content of air was
believed to be essential to the cracking process, later studies modified that
concept.) It was also shown repeatedly that at least 0.1 percent of water in
the liquefied ammonia inhibits SCC of constructional steels used in the
fabrication of the pressure vessels for agricultural service. As the result of
the studies the research committee recommended  that:
The report  also remarked that, inasmuch as each of these recommendations
has limitations when considered individually, all three should be followed to
avoid SCC. The adoption of these recommendations was very effective in
practically eliminating SCC in carbon-steel vessels used in the agricultural
Pressure vessels should be either fully stress relieved or fabricated with
heads that are hot-formed or stress relieved.
Extreme care should be used to eliminate air from the ammonia systems; new
vessels must be thoroughly purged to eliminate air contamination.
Ammonia should contain at least 0.2 percent water to inhibit SCC.
Outside the agricultural industry SCC began to occur in the mid-1960s,
primarily involving DOT Specification truck-transport cargo vessels. This onset
of SCC may have been due to improvements in ammonia production technology which
resulted in a gradual increase of the ammonia purity, mostly by a reduction of
the water content which inhibits SCC . An examination of 19 ammonia vessels,
that was performed at that time, included several steels representing a wide
range of strength levels. Sectioning eight of these vessels that had been in
ammonia service for 8 to 23 years and conducting a detailed inspection by a wet
fluorescent magnetic particle technique of the entire interior surface did not
reveal any indication of cracks. However, two additional DOT Specification 3AA
vessels contained stress-corrosion cracks. The hardness of the steels, obtained
in lieu of tensile strength, which did not contain SCC ranged from 134 to 272
HV, and the hardness of the cracked vessels was 207 HV and 241 HV. The reason
for this lack of specificity with respect to the hardness (or strength) level
is not known. Perhaps the applied stress of the cracked vessels was high in
relation to their strength, or the contamination of the ammonia was at a
detrimental level. The specific grade of ammonia that was contained in any of
these vessels is not known, but it can be assumed that it was metallurgical or
refrigeration grade ammonia, which do not contain appreciable amounts of
inhibiting water (33 ppm max or 150 ppm max , respectively). SCC was also
found in three refrigeration system ammonia condensers fabricated of ASTM A515
Grade 70 steel . The condensers had not been postweld heat treated and the
cracks were located in the base metal near welds. Two refrigeration system
ammonia storage vessels of this steel also showed SCC in cold formed vessel
heads near welds that were not postweld heat treated.
An ASTM A202 Grade B hot-rolled steel truck-transport vessel contained SCC in
and adjacent to repair welds . Although the vessel had been postweld heat
treated after fabrication, subsequent repair welds, however, were not heat
treated and the residual stresses caused stress corrosion to occur. Note that
Compressed Gas Association guidelines for the repair of truck-transport cargo
vessels  permit "minor repairs" without subsequent heat treatment; this
aspect will be discussed in a later section. SCC in ASTM A202 Grade B steel
ammonia transport vessels that were heat treated after welding, however, was
conspicuous by being essentially absent.
In 1967 DOT reported about 8000 truck-transport cargo vessels in existence, a
half of this number being fabricated of quenched-and- tempered (Q & T)
alloy steels (ASTM A517), which represented the upper end of the strength range
examined. At that time about 2000 vessels were examined by vessel manufacturers
for evidence of cracking, and about 400 vessels contained some defects, but the
DOT commented that most of the defects were associated with poor workmanship,
the balance probably being caused by SCC. In the period from 1965 to 1977 the
writer examined eight cargo vessels that were fabricated of ASTM A517 Q & T
steels with a postweld heat treatment and that carried a variety of ammonia
grades, some of them without water inhibition . The observed cracks, in all
in-stances identified by metallographic examination as being caused by SCC,
occurred on the inside surface, mostly in the lower quadrant of the vessel,
that is exposed primarily to liquid ammonia at or within a few inches of
internal welds as well as in areas opposite external welds, especially opposite
those welds that attach load-bearing pads. Cracks were also associated with
repair welds that were not heat treated after the repairs.
Until the mid-1960s the service performance of high strength
quenched-and-tempered steel (ASTM A517) truck-transport cargo vessels was
excellent . However, as SCC started to occur in such vessels, NACE
recommended a set of preventive measures that were adopted in the U.S. as DOT
regulations in 1968 and 1975 . These regulations required that cargo vessels
of Q & T steels in liquefied ammonia service be used only for ammonia
containing at least 0.2 percent water. (An initially permitted ammonia of at
least 99.995 percent purity was later eliminated from the requirements, because
this purity definition considered only water and oil as impurities and did not
include air as a contaminant). Cargo vessels of other than Q & T steels
were permitted for use with any grade of ammonia. The regulations also stated
that all new cargo vessels for ammonia service be postweld heat treated. A wet
fluorescent magnetic particle inspection  of the internal surfaces was also
required of all vessels in conjunction with the hydrostatic retest every 5
years. A major vessel fabricator reported that subsequent to these requirements
SCC damage to Q & T cargo vessels has decreased substantially.
Beneficial and Detrimental Factors
The experience of SCC with liquefied ammonia containers and the resulting
regulatory activity in the U.S. prompted several investigations to be conducted
in laboratories to further determine the factors involved. Radd and Oertle 
suggested that the cracking was due to hydrogen embrittlement, that water
additions were acting to accelerate rather than inhibit cracking, and that the
observed inhibition was due to dissolved oxygen in water rather than
intrinsically to the water. Based on accelerated tests with C02-saturated
ammonia, Naito et al.  proposed that SCC in the test environment was
initiated by pitting and propagated by hydrogen embrittlement. Addition of
about 0.75 percent water to this environment reduced the cracking. Also, the
addition of 0.01 percent NH4
C1 suppressed the SCC. Furukimi et al.  proposed to cover up the SCC
problem by decreasing the hardness of the surface of high-strength Q & T
steel to less than 190 HV by decarburizing the steel surface and controlling
the cooling rate during the Q & T treatment.
At U.S. Steel Research many investigations were conducted by the use of the
slow strain-rate technique . This technique, which is effective in
producing SCC in relatively short time periods, employed dynamic straining to
stress a specimen at a rate that is much slower than that used in a
conventional tension test. Wilde  developed special sampling and gas
chromatography analysis techniques that overcome the problem of selective
evaporation frequently encountered in determining the small concentrations of
gaseous contaminants in liquefied ammonia. With respect to SCC by liquefied
ammonia most of the slow strain rate tests were conducted on the high-strength
ASTM A517 steel. Deegan et al. [14, 15] have demonstrated by the use of anodic
and cathodic polarization, in conjunction with the slow strain method, that SCC
in air-contaminated ammonia is anodically controlled and that cathodic
polarization prevents cracking.
Hennecken  also found that SCC of carbon steel in liquid ammonia is anodic
in nature, and expanded further from polarization tests that oxidizers in
ammonia increase the danger of SCC. The anodic nature of SCC in ammonia was
also verified by Wilde and Kim  who demonstrated by the slow strain-rate
test that aluminum, which is anodic to steel in air-contaminated ammonia ,
when coupled to the straining steel specimen can prevent SCC, apparently by
cathodic protection. However, the relatively high electrical resistivity of
liquefied ammonia and the resulting poor throwing power militate against a
successful use of cathodic protection by sacrificial aluminum anodes in
combating SCC in industrial applications. This reservation is supported by
observations that SCC occurred in ammonia truck-transport cargo vessels, which
were equipped with aluminum baffles electrically coupled to the steel vessels
The studies of the ASTM A517 steel also showed that uncontaminated
metallurgical grade ammonia does not cause SCC , but that an 0.38 psig
over-pressure of air can result in SCC . The latter authors also showed
that separate additions of either 9 ppm oxygen or 3 ppm nitrogen to the ammonia
did not cause SCC in A517 steel, but that SCC occurred when both of these
contaminants were added together; a smaller addition of 0.9 ppm oxygen together
with 3 ppm nitrogen also resulted in SCC . Later, Wilde  showed that in
the absence of nitrogen greater amounts of oxygen (200 ppm) can cause slight
SCC in plastically deformed steel. He also determined that 0.02 ppm oxygen
(added as air to ammonia containing 18 ppm water) did not cause SCC in A517
steel slow strain-rate specimens. As the air contamination level was increased,
marked SCC occurred between 1 and 200 ppm oxygen, and as the air contamination
was increased further the SCC severity became less pronounced until at an
oxygen concentration of 920 ppm and above SCC was not observed. The role of
oxygen was also confirmed by Lyle  who concluded from slow strain-rate
tests that oxygen contamination is the primary cause of ammonia SCC of steel,
as well as by Farrow et al.  who concluded that oxygen contamination is
necessary for SCC of mild steel at a level of more than 5 ppm in commercial
grade ammonia, but as little as 1 ppm may be sufficient when water is absent.
The early conclusion by Loginow and Phelps  that a small addition of water
inhibits SCC of steel in air-contaminated ammonia was later confirmed in slow
strain rate tests on A517 steel [14, 15, 21]. Deegan and Wilde  also showed
that for the degree of air contamination used (approximately 2 atmospheres
partial pressure of air at 25°C) a limit for the effectiveness of water as an
inhibitor was observed at 0.08 percent, which is in very close agreement with
the lowest amount of 0.1 percent reported earlier . Hennecken  states
that high oxygen contents in the ammonia require increased water contents to
provide inhibition against SCC in carbon steel, and conversely, that low oxygen
contents could be counteracted by low water contents. He also concludes from
polarization measurements that the presence of oxidizers in liquid ammonia
impede the inhibiting properties of water. Examination of some failures of
agricultural and industrial ammonia vessels, that were discussed earlier,
suggested that the inhibition with water was not practiced continuously and
thus resulted in SCC. In 1975, Poulson and Arup  showed experimentally by
the use of the slow strain-rate test on mild steel that when water-inhibited
ammonia is replaced with a water-free air contaminated ammonia, cracking
started without significant delay, thereby confirming the suspicion that an
intermittent exposure to uninhibited ammonia can result in cracking.
Although cracks in truck-transport cargo vessels and storage spheres appear
mostly in the lower part that is continuously exposed to liquid ammonia [2, 6,
16], various reports from Europe were concerned with SCC in the vapor space of
storage spheres (mostly fabricated of low-strength carbon steel without
postweld heat treatment of the entire sphere) that contained water-inhibited
ammonia. These observations suggested that the inhibitive properties of water
in liquid ammonia are not transferred to the vapor phase. Studies by Ludwigsen
and Arup  showed that ammonia which condenses on a cool steel surface at
nonequilibrium conditions, such as on a cool night, is capable of causing SCC.
They concluded that such a condensed ammonia contains much less inhibiting
water and much more detrimental oxygen than the liquid ammonia from which it
evaporated. Later, Brown confirmed by calculation the paucity of water in the
vapor over liquid ammonia , and Wilde confirmed the enrichment of air in
the vapor by experimentally determining the partition coefficients between
liquid and vapor phases of ammonia to be 587 for oxygen and 603 for nitrogen
. Ludwigsen and Arup  suggest that the liquid ammonia be maintained at
a temperature that is lower than that of the steel in contact with the ammonia
vapor to prevent condensation and consequent SCC. This approach apparently does
not avoid SCC in the meniscus between vapor and liquid. Lunde  reports that
SCC occurred in the vapor phase when the steel was at ambient temperature,
whereas cracking was observed only in the meniscus when the top of the
container was warmer by 5°C than the water-inhibited liquid ammonia and
condensation did not occur.
The environmental effects, that are summarized above, together with
electrochemical studies led to the formulation of a mechanism of SCC Gf
constructional steels in liquefied ammonia [15, 21].
"In oxygen contaminated ammonia an adsorbed film exists on the steel surface
which maintains the corrosion potential at very noble values. During straining
after yield, breaks in the film expose film free metal at slip steps, and this
metal dissolves rapidly under the noble potential imposed by direct galvanic
coupling to the filmed metal surface.
In the absence of any other impurity, oxygen has the tendency to refilm the
bare metal and to some degree inhibits the severity of SCC depending on the
concentration and the strain rate. Nitrogen dissolved in the ammonia inhibits
the filming of the bare slip steps by a process of 'competitive adsorption'
with the oxygen and consequently increases the severity of SCC through a
process of adsorption rather than through a process of electrochemical origin.
Water inhibits SCC by acting as an additional film former which hastens
passivation and healing of film breaks at slip steps".
In this proposed model no mention is made about the contribution of carbon
dioxide to the SCC process. Wilde  has shown that, contrary to the early
conclusion that carbon dioxide is essential to the cracking process , this
normal constituent of air when added to ammonia resulted in severe general
corrosion, but not SCC; for SCC to occur, air must also be present. In this
study Wilde  also examined the effects of various other contaminants and
concluded that CO, CO+O2, CH4, SO2, S02+02,
NH4C03, (NH4)2CO3 NH4N03,
NaN03, N0, N02, N20, NaC1, NH4C1,
NaN02, and NaNH2+air did not cause SCC when added to an
air-free test ammonia. Some of the test additives (S02, S02+02,
NH4N03, and NH4
C1) caused general corrosion to varying degrees.
Although it has been shown convincingly that additions of water on the order of
0.1 percent and more can effectively inhibit SCC in air contaminated ammonia,
there are end uses of ammonia in which even such small amounts of water cannot
be tolerated, such as in metal heat treating or refrigeration applications.
Therefore, some studies were made with additives that would be compatible with
the end use of ammonia, primarily amine compounds . The tests showed that
some inorganic amines and ethylenediamide inhibited cracking in
air-contaminated ammonia. Because the molecular constitution of hydrazine (N2H4)
is similar to that of ammonia (NH3), this compound was subjected to
further tests at concentrations ranging from 0.002 to 1.0 percent. The
inhibitive properties of hydrazine in air-contaminated ammonia were observed at
concentrations of 0.025 percent N2;H4
and above. Apparently hydrazine acts as an oxygen scavenger inasmuch as in a
hydrazine-treated ammonia, that was initially contaminated with air and which
subsequent to the hydrazine addition did not cause SCC, the oxygen content had
decreased to zero. The inhibitory effects of hydrazine and water were also
reported in relation to mild steel as well as those of ammonium carbonate,
ammonium bicarbonate, and carbamate . Lyle  reported that water,
methane, and nitrogen exhibit inhibiting properties, the latter being at
variance with the findings discussed earlier [15, 21].
The preceding discussion of practical experience and the many investigative
studies of SCC of steels in liquefied ammonia service suggest several measures
that could prevent SCC or reduce the probability of cracking. However, it
should be recognized that each measure has limitations when considered
separately. For maximum benefit as many procedures as possible should be
employed in the attempt to combat SCC.
Air contamination (even a few ppm) is considered the primary cause of SCC of
steels in liquefied ammonia. It is therefore good engineering practice to purge
new vessels of air and to eliminate any conceivable possibility of air ingress
into the ammonia handling system, such as at detachable ammonia transfer
points. Because air tends to concentrate in the vapor space above a liquid
ammonia pool (there is about 600 times more air in the vapor phase than in the
liquid in equilibrium with it) it is advisable in the case of large storage
spheres to continuously condense the ammonia vapor with a simultaneous
discharge of the noncondensables, which is mostly air, and to return the
condensed liquid to the vessel. For relatively small units it was demonstrated
that a bleeding or boil-off method can effectively remove air from ammonia to
avoid SCC [2, 13]. Good results in reducing SCC damage to storage spheres have
been achieved by a careful control of the oxygen content in ammonia having a
low water content .
Similar to the detrimental effect of oxygen, the inhibitory effect of water
content in liquid ammonia has been well documented, the optimum dose apparently
being 0.2 percent water in those applications where this amount of water can be
tolerated although it was suggested that a lower water concentration may be
adequate when the oxygen content is low [16, 20]. Note that for effective
inhibition of SCC the required water content must constantly be present in
ammonia. Should the water content drop below the critical concentration even
for a relatively short time, as by influx of water-free ammonia into a vessel,
the irreversible-SCC damage can occur. Thus it is important to monitor the
water content on a frequent and regular basis and to replenish the water if it
starts to fall below the critical concentration. In metal heat treating and
refrigeration applications where this low concentration of water in ammonia is
undesirable, laboratory tests have shown that 0.025 percent hydrazine addition
is adequate in inhibiting SCC. Although industrial utilization of hydrazine in
ammonia is still forthcoming, such use of hydrazine should verify the
The inhibitory effect of water may not be realized in the vapor space of liquid
ammonia storage tanks, and calculations and laboratory studies indicate that
measures, which prevent condensation of ammonia on the steel surface, would
tend to avoid SCC. Simultaneous reduction of the oxygen content of the ammonia
vapor should also be beneficial.
Postweld heat treatments are effective in substantially reducing the
probability of SCC of steel in contact with air-contaminated ammonia. Although
postweld heat treatments are frequently considered only from the standpoint of
reducing the residual stresses, it should be recognized that such thermal
treatments result in two consequences, both of which are beneficial in reducing
SCC. First, properly conceived and executed thermal treatments can
substantially lower the residual stresses that result from fabrication (welding
and cold forming), erection, and restraint of the steel structure, such as
storage sphere. Secondly, an appropriate postweld heat treatment can
significantly reduce the peak hardness in the heat affected zone (HAZ) of the
weldment . Thus, a hardness peak of 443 HV in the HAZ of an ammonia tank of
ASTM A202 Grade B hot rolled steel was reduced to 280 HV by a postweld heat
treatment, and similarly a 447 HV peak hardness was reduced to 290 HV in an
ASTM A517 Q & T steel weldment . Such reduction in HAZ hardness benefits
the resistance to SCC, especially in conjunction with other measures. In this
context note also that guidelines promulgated by the Compressed Gas Association
for repairs of vessels permit minor welding repairs to be performed without a
subsequent heat treatment . However, the size of the area repaired by
welding or the extent of welding do not affect the hardness of the HAZ and its
susceptibility to SCC. Therefore from the standpoint of avoiding SCC, it is
good engineering practice, that steel equipment for liquefied ammonia service
be heat treated after any welding operation regardless of size, which is a
requirement in the United Kingdom . Although it is generally considered to
be difficult and costly, a successful full-scale heat treatment of a large
fabricated and erected storage sphere was reported from Japan . Shot
peening has been proposed to reduce residual stresses in areas of weld joints
or even to induce compressive surface stresses. However, experience has shown
that this approach only reduced temporarily the incidence of ammonia SCC, but
did not prevent it [16, 27, 31].
It is recognized that, in general, the probability of SCC of steel in liquefied
ammonia service is the greater, the higher the strength of the steel. The
formulation of a lower strength limit, below which SCC would not occur, is not
practical because it depends on the level and nature of contamination, the
degree and nature of inhibition, the magnitude of residual and operating
stresses and the operating temperature. Nevertheless, a lower-strength steel
appears to provide a higher degree of resistance to SCC than a higher-strength
variety. With respect to the stress that exists in a steel component other
aspects pertain; the upper limit is governed by applicable statutory
regulations, and the lower end, which is desirable from the standpoint of
combating SCC, is governed by the operating pressure, the technologically
achievable reduction of residual fabricating stresses, and economic and safety
considerations. It is good engineering practice to avoid stress concentrating
features. Note also that if SCC has initiated, the effective stress at the tip
of the crack can be very high indeed. Novel design aspects, such as external
reinforcements that would place the interior surface of a tank in compression,
could reduce the operating stress level and benefit in decreasing the
likelihood of SCC.
Many catastrophic failures, with resultant personal injuries and property
damage that have their origin in SCC, could be avoided by periodic and frequent
inspections of the steel surfaces that are in contact with ammonia, and by
making repairs when needed. Cracknell  has proposed an inspection schedule
for as-welded carbon steel vessels that is based on oxygen and water contents.
Guild  recommends inspections to be conducted both before and after
pressure testing of storage spheres, because fine cracks can open up during the
pressure test and become visible afterward. In the writer's experience, the
most sensitive inspection method for detecting the frequently very fine
stress-corrosion cracks is the wet fluorescent magnetic particle method with an
alternating current yoke 16, 7, 31]. An acoustic emission technique , which
detects actively propagating cracks by sensors applied to the outside of an
operating vessel, could furnish a continuous report on the soundness of the
vessel if equipped with appropriate monitoring, processing, and warning
*Reprinted with permission from Materials Performance, Vol. 25, No.
12, December 1986, pp 18-22. ©1986, National Association of Corrosion Engineers
Houston, TX. Reprinted text has been edited for clarification and space
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Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.