Failure of Steam-Water Heat Exchangers
B.F. Brown, Failure of Steam-Water Heat Exchangers,
Vol 1 (No 1) Feb 1961 p 68–74
Gross wastage and embrittlement were observed in plain carbon steel desuperheaters in five new Naval power plants. The gross wastage could be duplicated in laboratory bomb tests using sodium hydroxide solutions and was concluded to be caused by free caustic concentrated by high heat flux. The embrittlement was shown to be caused by the flow of corrosion-generated hydrogen which converted the cementite to methane which nucleated voids in the steel. A thermodynamic estimate indicated that a small amount of chromium would stabilize the carbides against decomposition by hydrogen in this temperature range, and laboratory tests with 2-14% Cr steel verified this.
Boiler tubes; Corrosion environments; Sodium hydroxide; Warships
(Nonresulfurized carbon steel)
Hydrogen damage and embrittlement
Except to those directly concerned with
the service failure, a report of analysis of such a failure is apt to make
prosaic reading unless the failure was spectacular, or the analysis required
brilliant metallurgical sleuthing, or in the process of analysis new basic
information of general import to the field of metallurgy was uncovered. These
characteristics are rare in the average failure analysis, and they do not
apply to the one reported here.
Even to those not involved directly, however, failure analyses may provide
useful secondary services by pointing out gaps in fundamental metallurgical
information of present practical need. (If those who perform uncommitted research
in metallurgy made an occasional failure analysis, or occasionally read a
report of one, some of these investigators might recognize new and fertile
areas of fundamental importance.) Additionally, a failure analysis report
may be useful to remind management of the breadth of capabilities of research
establishment which may be required to analyze expeditiously a reasonably
simple service failure. Finally, the present report may serve to remind anyone
who may need such reminding that a large proportion of service failures submitted
to metallurgists involve either corrosion or mechanical metallurgy or both
— areas of metallurgy that are emphasized in few academic curricula
and that, somehow, unfortunately fail to arouse enthusiastic pursuit by many
students of metallurgy.
The subject service failures occurred in boiler desuperheaters in five
modern Naval power plants of compact construction and having therefore high
fluxes through heat-transfer surfaces. These desuperheaters consist of steel
tubing submerged in the water of the steam drum. Superheated steam, at 950
F in most of these power plants, enters one end of the tubing and, during
its passage, transfers heat through the tube wall to the water of the steam
drum, which in most of these was at 550 F. The effect of this passage is to
bring the superheated steam down to approximately the temperature of the water
in the steam drum, and if the heat flux through the tube wall is sufficient,
it causes boiling on the water side.
One of the service failures is shown in
. The steam inlet flange is seen to the left and the exit flange
to the right. This particular unit illustrates the two failure aspects observed
in all the units, namely extensive corrosion in a few hundred steaming hours,
and embrittlement to a degree which led to breaking off the inlet flange during
disassembly - a degree of embrittlement uncommon in the low carbon unalloyed
steel of which these desuperheaters are made.
Fig. 1 Failed desuperheater unit (about eight feet in length).
The failure analysis is then divided into searches for (1) the cause
of the rapid corrosion and (2) the cause of the embrittlement.
Cause of Corrosion
The corrosion products on the water-sides of all tubes consisted largely
of loose, poorly-adherent magnetite (as identified by x-ray diffraction) together
with crystals of elemental copper and, in some instances, some Fe2O3
(which probably formed after shut-down). Only thin, tightly-adherent
films of magnetite were observed on the steam-sides.
These desuperheaters were operating in boiler water
to contain enough free caustic (plus some phosphate)
to bring the pH to 11; rapid corrosion to form magnetite would hardly be expected
in such solutions. Furthermore the generating tubes of these same boilers,
exposed to the same water at the same temperature, did not suffer the same
attack. A clue to the cause is the general pattern of the attack in the desuperheater
, where the corrosion
is most severe at the inlet end, with little attack in the central zone where
the circulation of water in the steam drum is high, but severe again near
the downstream end of the first pass (far right-hand side). The attack is
also more severe near the tube support plates than on either side. Judging
from appearance of several desuperheaters in various stages of deterioration,
the advance of the corrosion attack was toward the steam exit end. It thus
appears that the more severe attack occurs (a) where the steam is hotter,
and (b) where circulation is poorer.
One might speculate that boiling on the desuperheater surface might
damage by thermal shock the magnetite film depended upon for protection against
corrosion, but this can hardly have been the case: the generating tubes of
the same boiler certainly experienced thermal shocks certainly no less severe
without exhibiting the same rapid corrosion. A second effect of boiling is
concentration of the boiler-water chemicals. For reasons not well understood,
the magnetite produced in highly caustic solutions tends to be nonprotective.
Whether this involves the dissolution of the iron as the soluble FeOOH-
complex, which later transforms to magnetite, or whether simply
the magnetite which forms in caustic solutions is physically defective, is
not known. But in either case the primary causative of the abnormally high
corrosion rate is postulated to be free caustic.
Cause of Embrittlement.
We now turn to the search for the cause of the embrittlement. Tensile
ductility and Charpy impact data were not taken on the failure metal for the
simple reason that the qualitative evidence for embrittlement was convincing
enough. The fracture of
is suggestive. In a more severely deteriorated unit from another boiler with
1100 steaming hours (less than 2 months), the inlet zone was so badly embrittled
that pieces of the remaining metallic material could be broken out by hand.
The analysis of the metal from a unattacked area of a typical desuperheater
is shown in
Table 1 Analysis of Tube from an Unattacked Area of a Desuperheater
of such steel in the unattacked areas is the typical ferrite plus patches
of fine, rather spherical carbides which one commonly sees in low carbon steels.
The microstructure of the remaining metal under the severely attacked areas,
however, is quite different. A typical example is shown in
Fig. 2 Microstructure of center of attacked tube wall. 250×
Most of the carbides have disappeared, and there is extensive void formation.
Most surprising of all, when one recalls that the unattacked steel shows 1
or 2 ppm hydrogen content, the steel under the severely attacked areas in
most desuperheaters was found to contain more than 100 ppm of hydrogen, and
in one case, 268 ppm. The clue to the disappearance of the carbides was provided
by a mass spectrographic analysis of gas released by (1) drilling the embrittled
metal under mercury, and (2) dissolving the embrittled metal in aqueous cupric
chloride. The analysis showed that about one-third of the gas was methane,
the remainder mostly hydrogen. The hydrogen released by corrosion had decomposed
The mechanical properties of low carbon steel are not drastically deteriorated
by hydrogen in solution, and the embrittlement noted here is attributed largely
to the extensive internal “notching” provided by the crack-like
voids, which may be considered similar in their effect on ductility to the
graphite flakes in gray cast iron. The cause of the embrittlement, then, is
not so much the absorption of hydrogen as the formation of the crack-like
voids. Evidence will be given later to support the postulate that these voids
are formed because of the formation of methane, not because of the large amount
The methane is formed by the reaction
is sometimes referred to as the methanation reaction. The heat treater who
has used methane to carburize steel may be surprised at first to see the reaction
going in the other direction. The reason for the reversal is the temperature
effect; estimating from the free-energy data selected by Kubaschewski and
Evans, the temperature of reversal is about 875° for equilibrium conditions.
Below this temperature, the lower the temperature, the higher will be the
driving force for the reaction to go to the right, and it should be higher
at room temperature than at boiler temperatures. But a mild steel tube, periodically
refilled with HCl plus a trace of phosphorus until it had corroded one-third
through the tube wall at room temperature, presumably with a fairly high hydrogen
flux through the wall toward the abraded outer surface, showed no signs of
void formation. A clue to the reason for this is suggested in
, which shows the microstructure of a severely
corroded steam inlet flange from a desuperheater. This is cast steel having,
originally, a microstructure of pearlite plus grain-boundary ferrite and coarse
Widmanstatten ferrite. Voids have formed in the grain-boundary ferrite, and
the adjacent pearlite is beginning to decompose, with the carbon diffusing
to the void to form methane. It is noted in all these embrittled specimens
that there are very many fewer voids than there were originally carbide particles.
Carbon must therefore diffuse a considerable distance, and the positive temperature
coefficient of diffusion of carbon would thus be expected to tend to offset
the negative temperature coefficient of the energetics of the methanation,
so that at a sufficiently low temperature this reaction will not take place
to any important extent.
Fig. 3 Section of attacked cast steel flange showing voids surrounded by
The postulated mechanisms for the corrosion and embrittlement of the
present service failures are summarized schematically in
. Rapid boiling concentrates the caustic at the
surface of the tubes causing accelerated corrosion of the iron, which forms
nonprotective magnetite deposits; the cathodic partial-reaction corresponding
to the anodic oxidation of iron is the reduction of hydrogen, which dissolved
in the steel; rapid corrosion means high hydrogen levels in the steel, and
high temperatures permit high diffusivity of carbon to a void where, with
the ubiquitous hydrogen, it can react to generate methane. Under the energetics
of the methanation reaction, the voids grow to cracks readily visible under
the microscope, and these serve as embrittling internal stress-raisers.
Fig. 4 Schematic summary of mechanisms of deterioration.
Several tests were performed in the laboratory to serve as a guide check
on some of the above. Ideally, these tests should have involved boiling, but
because of the complexity of apparatus required to perform such tests at high
temperatures and, therefore, high pressures, a simplified bomb system was
used instead. Rods 2 in. long and
in. diam were drilled to provide a longitudinal cavity
of about 0.3 in. diam, leaving tube walls about 0.1 in. thick. These rods
were of annealed plain carbon steel. Concentrated solutions were prepared
in such proportion that at the same dilution, all would show room-temperature
pH of about 11. These solutions included a solution with a high ratio of NaOH
, a high ratio of Na3
to NaOH, and, in a separate series, 40% NaOH of reagent grade.
The solutions were introduced into the bombs, the end were closed with
pressure-fit plugs, and the closures were welded shut. The bombs were then
inserted into a furnace at 950 F**
hr, removed, sectioned, and examined metallographically. The essential findings
are shown by
Fig. 5 (a) Section through wall of plain carbon steel bomb after 24-hr exposure
at 950 F to concentrated high free-caustic solution. Corroding surface, at
left, shows absence of carbides and presence of voids. (b) Same as (a) except
solution was high in phosphate and low in caustic. 100×
a shows complete
decomposition of the pearlite and cracking toward the corroding surface in
the solution containing a high concentration of free caustic.
b shows the absence of this attack in the high-phosphate
solution. This was a failure analysis, not a research and development project,
so the corrosion rates were not measured, but subjectively there appeared
to be a much thicker layer of oxide in the specimen of
a than that of
A test of the concept that it is the methane, not the hydrogen, that
caused the embrittlement was made indirectly by selecting a steel that should
have carbides thermodynamically stable to hydrogen.
Selection for Specific Environment
Estimates were made (from the data tabulated by Kubaschewski and Evans)
of the standard free energy of the methanation reactions for Fe3
and for Cr23
, and from these a “rule-of-mixtures”
estimate indicated that at 500 K a carbide containing roughly 60% of chromium
might be stable to hydrogen. Using the distribution ratio of Hultgren and
co-workers for chromium in carbide: chromium in ferrite of 28:1, one is led
to conclude that slightly more than 2% Cr (in a low-carbon steel) could reasonably
be expected to stabilize the carbides against methanation. Conveniently, there
is available a commercial low-carbon steel containing
Bombs similar to the ones just described were made of the
% Cr steel and were tested with the high free-caustic
solutions. Again corrosion rates were not measured, but subjectively the specimen
had a heavy scale
on the inside comparable with that on the specimen of
a, except that the carbides were intact throughout,
but thinner scale than on the specimen in
Fig. 6 Same as
Fig. 5a except
steel containing about
% Cr. 100×
Although the relatively low chromium content might reasonably be expected
to stabilize against methanation, it is far too low to be expected to protect
against normal corrosion. Also, in high pH solutions chromium is taken up
into a soluble complex, so that it would not be expected to prevent corrosion
due to concentrated caustic. Its role thus is not to prevent corrosion but
to prevent the catastrophic embrittlement of the uncorroded steel.
The arguments above suggest an even simpler solution to the cracking
problem where strength is not an important consideration: use a “steel”
with no carbon in it. To test this, a bomb was made of Armco ingot iron and
the experiment for the specimen shown in
was duplicated, with the finding that no cracking was observed.
Nickel and nickel alloys are often used to contain NaOH solutions at
elevated temperatures, but the data available indicated that for conferring
protection to steel against attack by free caustic, the nickel content would
probably have to be so high that the steel would be austenitic and therefore
susceptible to unrelated stress-corrosion cracking. This implies that the
alloy approach is a less desirable one than the water chemistry approach suggested
by the phosphate tests, if adequate control can be maintained under the less-than-ideal
conditions under which these power plants are required to operate.
The analysis, then, is concluded. The cause of the rapid corrosion,
high free-caustic; the cause of the embrittlement, formation of microcracks
by methane generated by hydrogen reacting with carbides in the steel. These
are not proven with the rigor one expects in a research project, as distinguished
from a failure analysis, but they appear to provide simple and adequate explanations
of all known facts. They also carry a clear-cut implied solution: avoid high
free-caustic as by the balanced phosphate method, which should obviate both
the gross corrosion and the embrittlement caused by the corrosion-generated
It is not suggested that high phosphate treatments will guarantee freedom
from all ills in high heat-flux systems. A review of the literature reveals
that we know too little of the complex phosphate chemistry of iron at high
temperatures to be sure that we will not some day encounter some ugly surprises
in such systems.
To avoid ending this report on such a negative note, it is suggested
that we can apply some of the arguments above to explain (a) why caustic-embrittlement
cracking occurs in boilers, and (b) why we can get away with cathodic protection
of mild steel in sea water and wet soil for years without causing embrittlement.
The actual caustic cracking as illustrated by micrographs in the literature
looks very much indeed like the incipient stages of the sort of cracking observed
in this present study, that is, cracking by methane formation, which could
be prevented by balanced phosphate treatment (as is practiced in some quarters)
or by the use of a low chromium steel, as suggested by the present experiments.
In the case of cathodic protection, hydrogen is being reduced on the protected
steel surface at a rate that may be comparable to the rates experienced in
the present failures. The diffusivity of the hydrogen is certainly adequate
to move it throughout the steel; but at ambient temperature the carbon cannot
move to the inclusion, high-angle grain boundaries, or whatever special site
is required to permit reaction with the hydrogen and therewith the nucleation
and growth of the damaging voids. Even if the carbon could diffuse, the energy
available to activate the reaction is so limited at room temperature that
this reaction, like the room-temperature reaction between charcoal and hydrogen,
would be expected to be very slow.
The author wishes to acknowledge
with appreciation the metallographic analyses of the desuperheaters by Mr.
A. J. Edwards, the conduct and metallographic analyses of the “bomb”
tests by Mr. R. Newbegin, the chemical analyses by Mr. D. Walter, and helpful
discussions with Drs. M. C. Bloom and A. J. Pollard. The work was supported
by Code 651 of the Bureau of Ships.
Copyright © 2004 ASM International®. All Rights