Volume 1 Paper 12
Laboratory Study of Corrosion Effect of Dimethyl-Mercury on Natural Gas Processing Equipment
S. Wongkasemjit and A. Wasantakorn
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JCSE Volume 1, Paper 12
Submitted 25 February 1999, accepted for full publication 13 December 2000
Laboratory Study of Corrosion Effect of Dimethyl-Mercury on Natural Gas
S. Wongkasemjit* and A. Wasantakorn
The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok
e-mail address : mailto2('dsujitra','chula.ac.th')
Dimethylmercury (DMM) has been discovered to be one of the
organomercury compounds in natural gas, besides mercury metal. The effect of
the DMM on the corrosion of carbon steel and aluminium metal was rather
frightening, as compared to that of elemental mercury or mercuric chloride,
which is well-known as a severely corrosive agent. The results showed that DMM
in methanol or petroleum ether as solvent gave similar corrosion patterns to
the elemental Hg solution. It revealed uniform corrosion characteristics on carbon steel
and pitting appearance on aluminium. The higher the DMM concentration or the
higher reaction temperature, the more severe corrosion occurred because of the
higher elemental Hg concentration generated. Trace amounts of hydrogen chloride
or hydrogen sulfide, which are also present in large
quantities in natural gas,
remarkably increased the corrosion potential of DMM on metal. Corrosion rate
in the HCl+DMM solution was approximately 700 times faster than the one containing only DMM and 40
times faster than the one containing only acid.
It is well known that ppm amounts of elemental mercury
in natural gas or condensates result in corrosion of equipment in oil or
natural gas processing plants1-4. This is the main cause of
equipment failure, especially failure of aluminium heat exchangers. The
gas separation parts of the Petroleum Authority of Thailand (PTT) at Map Ta
Put has already encountered this problem, and had to be shut down for a period
of 4-5 months resulting in lost income of
over 10 million US dollars.
§3 The effects of elemental mercury in natural gas on
corrosion was not reported until 1973 when a catastrophic failure of aluminium
heat exchangers occurred at the Skikda liquefied natural gas plant in Algeria5.
It was found that mercury corrosion caused the failure. After this discovery,
a study of the Groningen field in Holland revealed similar corrosion in the
§4 Organic mercury compounds are typical mercury derivatives
formed along with elemental mercury in natural gas8. Most of
them are liquids which are extremely volatile9, and can not
be trapped by mercury absorbants. To remove them, it is essential to decompose
and convert organomercury to elemental mercury
either by heat and/or catalyst10-11.
From the thermochemical data collected12 for mercuric
chloride and dimethylmercury, mercuric chloride, which is a severe corrosive
agent to metals, needs approximately 105 kcal/mol to break the bonds between
mercury and chlorine atoms while the total energy required to break the bonds
between methyl groups and mercury atom is approximately 58 kcal/mole to give
elemental mercury. Therefore, dimethylmercury is believed to be a stronger
§5 Decomposition of dimethylmercury can be only done by either
pyrolyzing or photolyzing13. One example was to decompose
dimethylmercury in the presence of oxygen to give elemental mercury. It was
also found that dialkylmercury could be decomposed by a mineral acid like
hydrochloric acid to give mercuric chloride14, and as a
result might be able to corrode natural gas equipment.
§6 To address the question of whether dimethylmercury will
corrode natural gas equipment, this research was thus focused on
investigating corrosion of metals by organomercury compounds. Dimethylmercury,
which is the most stable organomercury compound, was selected to study its
corrosive behavior toward carbon steel and aluminium specimens.
Chemicals : All chemicals were reagent grade and
used as received. Dimethylmercury (Hg(CH3)2, DMM),
mercuric chloride (HgCl2) and elemental mercury (Hg) were purchased
from Fluka Ag., Puriss. Petroleum ether (PE, bp. 80-100 °C), cyclohexane, 1,4 dioxane, dimethylsulfoxide, benzene, and sodium sulfide
(Na2S) were purchased from E. Merck (Germany). Absolute methanol
(CH3OH), antimony trioxide, and diethyl ether were obtained from
J.T.Baker. Chromic acid, trichloroethylene, and isopropyl alcohol were from
AJAX Chemicals Co. Phosphoric (H3PO4), hydrochloric (HCl)/sulfuric
(H2SO4) acids, and stannous chloride were obtained from
§8 Specimen Samples : Specimens, carbon steel
and aluminium, were obtained from commercial carbon steel reinforcement bars
and commercial aluminium bars for architectural use. They were prepared and
cleaned prior to use. The preparation and cleaning was according to
§9 Measurements : The compositions of both
carbon steel and aluminium were analyzed by a Shimadzu QC-6 Vacuum Emission
Spectrometer (Table 1). Corroded surfaces were inspected by a Scanning Electron
Microscope (JEOL, JSM-35 CF).
§10 Table 1 Compositions of commercial aluminium and carbon steel specimens
tested by Vacuum Emission Spectrometer
Commercial aluminium specimen
Commercial carbon steel
comment(11) 1. Specimen Preparation
Commercial carbon steel/aluminium bars were cut to make
short strips with dimensions of 11.2 x 25 x 2.75 mm for carbon steel
and 12.3 x 25 x 3 mm for aluminium. A hole was then drilled in the
strips near one end for mounting. The diameter of the hole was 3.5 mm for
carbon steel and 5.0 mm for aluminium. A few of these strip coupons were
chosen for elemental composition analysis by vacuum emission spectrometer.
§12 Specimens were wetted and rubbed with a No. 120 abrasive
paper until their surface was cleaned and smooth. The sizes of carbon steel
and aluminium specimens became 11.1 x 24 x 2.75 mm and 12.0 x 24 x
2.9 mm, respectively. All of the strip coupons were subsequently stamped with numbers on the upper left hand side near the hole. Coupons were then degreased
by scrubbing with a bleach-free scouring powder, followed by rinsing with
distilled water and a mixture of 1:1 ratio of methanol : diethyl ether, and
finally dried with air. They were then weighed with
an accuracy of 0.0001 g, and kept in a dessicator until experiments were
comment(13) 2. Preparation of Corrosive Solution
The minimum solution volume-to-specimen area ratio was 20
ml/cm2 of specimen surface, as recommended in ASTM A G31-72. The
total solution volume was therefore fixed at 150 ml. The selected solvents
were CH3OH and PE.
§14 a) Hg Solution : Since the solubility of elemental
Hg in cyclohexane is about 3 ppm., 5 mg of Hg was thus weighed and dissolved
in 1 liter of cyclohexane.
§15 b) HgCl2 Solution : The selected solvents
were used to dissolve HgCl2. The HgCl2 concentration used
was 150 ppm.
§16 c) DMM Solution : The same selected solvents were
used to dissolve DMM at concentrations
of 0, 50, 100, 150, 200 and
§17 d). DMM + HCl Solution : DMM concentrations, as set in
the method 2 c), were carried out, and 150 ppm of conc. HCl was added into each
flask containing either methanol or PE. The concentrated HCl was a 37% by wt.
solution, so introduction of HCl could thus not be separated from that of water.
The volumes of conc. HCl and DMM required to give 150 ppm concentration in 150
ml is on the order of 0.05 ml, therefore, the total solution volume did not
§18 e) DMM + H2S Solution : Saturated H2S
solution was prepared by passing H2S gas, which was synthesized
from the reaction of conc. H2SO4 and Na2S
powder, through the solvent for 1 h. at ambient temperature. In case of CH3OH,
1000 mg of H2S is soluble in 943 ml of CH3OH15.
The solubility of H2S in diethyl ether is 6.86 x 10-4
mol/g while that of H2S in PE is 3.70 x 10-4 mol/g 16.
Thus, in the case of PE, H2S gas was first dissolved into 10 ml of
diethyl ether to saturation, then 140 ml of PE was added. The following corrosive solutions were prepared and studied using
the method of ref. 16:
- CH3OH or PE + saturated H2S
- CH3OH or PE + saturated H2S + 200 ppm
- CH3OH + 300 ppm H2S solution + 200
- CH3OH or PE + 150 ppm HCl
- CH3OH or PE + 150 ppm HCl + 200 ppm DMM
comment(19) 3. Procedure
One pair of specimens was
immersed in an Erlenmyer flask
containing corrosive solution. The flask was closed loosely by a stopper to
release gas produced during the corrosion process and prevent the suppression
of the reaction. Temperatures studied were varied from -10 to 70 °C with an increment of 20 ° C. At low
temperatures, at or below 0 °C, the flask was stored in a
refrigerator, whereas at higher temperatures a temperature controlled bath was
used. Duration of exposure depended on temperature. At low temperatures, the
corrosion process took a long time, 700-1,000 h. while at ambient and higher
temperatures, the reaction took place very rapidly, 74.6 mdd, and needed to be
observed at all time.
comment(20) 4. Specimen Cleaning after the Exposure
The cleaning process was done by following the methods of
ASTM G1-72 (Reapproved 1979). Procedures were varied depending on type of
metal being cleaned, as follows;
§21 a) Aluminium Specimen Cleaning : Coupons were
cleaned as well as possible with a plastic knife. Oily or greasy deposits were
removed by soaking in trichloroethylene followed by the cleaning solution
containing chromic acid, phosphoric acid, and water at 80 °
C for 25 min. The coupons were then rinsed with distilled water, isopropanol,
and benzene. Finally, they were dried between paper towels and placed in a
dessicator for 1 h. before weighing.
§22 b) Carbon Steel Specimen Cleaning : After cleaning
the coupons with plastic knife and soaking in trichloroethylene, remaining
corrosion products needed to be removed by a bristle brush. They were then
immersed in Clarke’s solution at room temperature for 25 min.
The specimens were rinsed with water followed by isopropanol and dried between
paper towels followed by warm air drying.
comment(23) Results and Discussion
The study concentrated on the surface appearances
of the corroded aluminium and carbon steel specimens and the effect of various
factors on corrosion. The surface appearance was identified using scanning
§24 a) Hg Solution : Figures 1 and 2
corresponding to the effect of Hg solution on the surface of carbon steel and
aluminium specimens, respectively, show how Hg could corrode both samples and
how important DMM was to the system containing DMM.
§25 Figure 1 SEM photograph showing
uniform corrosion of carbon steel in cyclohexane containing Hg (click on the image to enlarge it, press Back to return to the
§26 Figure 2 SEM photograph showing
pitting corrosion of aluminium
in cyclohexane containing Hg
§27 The corrosion occurred via the amalgamation of aluminium
(eq.1)7. The amalgam is generally weaker than aluminium itself,
thus easier to be attacked by water or CH3OH, see eq.2, and Hg
generated would circulate to attack the aluminium again.
Hg + Al � �
� � AlHg (1)
2AlHg + 6H2O � �
� 2Al(OH)3 + 2Hg + 3H2 (2)
§28 The results appeared that at ambient temperature, the
corrosion rates were 236.5 and 0.5 mdd (milligram/sq.decimeter/day) for
carbon steel and aluminium, respectively. There were a lot of small pores
distributed uniformly over the whole surface area of carbon steel specimen
while a big black hole which seemed to be in the form of pitting appeared on
the edge of aluminium specimen.
§29 b) HgCl2 Solution : This is
another severely corrosive agent15. Equation 3 shows the
decomposition of HgCl2 to elemental Hg when reacting with aluminium.
2Al + 3HgCl2 � �
� � 2AlCl3
+ 3Hg (3)
§30 It was found that for the reaction in CH3OH
system aluminium corroded very severely whereas in the PE solution, no
reaction took place. The corrosion rates conducted in the CH3OH and
PE solution were 1,179.5 and 0.015 mdd, respectively. This is due to the fact
that HgCl2 is very soluble in either water or alcohol as methanol or
ethanol, but not in non-polar solvents. PE is thus not a good solvent to
dissolve HgCl2. However, the solvent PE used in the experiment
contained 0.02% of water, as a result, very little reaction between aluminium
and HgCl2 occurred in the PE system.
§31 c) DMM Solution : In the flasks containing
blank PE and MeOH solutions, after being immersed for 960 h. at ambient or 70
°C, specimens showed little corrosion, about 2 mg
only, which is most likely due to scratch caused during the cleaning process,
since the corrosion measurement after conducting the blank cleaning also
resulted in almost the same weight loss, 2 mg. However, those immersed in the
solutions containing DMM (see Figures 3 and 4 ) showed similar corrosion patterns
to those immersed in the solution of
Hg in cyclohexane, in that carbon steel and aluminium gave
uniform and pitting corrosion, respectively. The higher the DMM concentration,
the more pores or pits were observed
on the specimens.
§32 Figure 3 SEM photograph showing
uniform corrosion of carbon steel in CH3OH
§33 Figure 4 SEM photograph showing
pitting corrosion of Aluminium in CH3OH
Interestingly, DMM in MeOH seemed to be more corrosive than DMM in PE. The discoveries of Skinner12 and
Bass13 could explain this phenomenon. The C-Hg bond of DMM
is weak and can be easily broken by either photolysis or thermal dissociation
in the presence of a hydrogen donor, such as alcohol, as can be seen from eq.
4. The product is Hg metal which is a very corrosive species towards metals.
PE is a non-polar solvent, and obviously not a hydrogen donor. Hg metal could
thus not be formed. Corrosion on the specimen surface, however, could come
from the presence of trace water in PE.
CH3-Hg-CH3 + MeOH �
� � �
2 CH4 + Hg (4)
§34 The relationships of the corrosion rates versus temperature
are illustrated in Figures 5 (for carbon steel) and 6 (for
aluminium) for both MeOH and PE systems with and without DMM. The overall
corrosion rates of carbon steel specimens were higher than those of aluminium
specimens. Corrosion rates in solutions containing DMM increased significantly
at temperatures above 30° and 50 °C for carbon steel and aluminium specimens, respectively, meaning that
corrosion was a thermally activated process, as discovered by Skinner and
§35 It can also be seen in the case of the PE system, which contains trace
amounts of water. Water could then serve as a proton donor to DMM, as shown in
Especially, at higher temperature, water molecules generate proton easier and
donate proton to DMM faster. As a result, the decomposition of DMM to mercury
metal, which is the cause of specimen corrosion, became easier.
§36 Figure 5 Effect of
temperature on the corrosion rate of carbon steel
§37 Figure 6 Effect of
temperature on the corrosion rate of aluminium
§38 As described previously, organomercury could react
with mineral acids to form mercury salts which are also corrosive compounds14,
for example, forming mercuric chloride from the reaction of organomercury with
§39 Since natural gas also contains CO2, which could
convert to carbonic acid in the presence of water, and H2S, the
following studies were therefore undertaken to determine the effect of acid
mixed in DMM solution on the corrosion rate.
§40 As for the time dependence of the corrosion process, the
experiment was carried out for aluminium in the CH3OH + 150 ppm HCl system, with
and without DMM. The results are shown in Figure 7. It is obvious that DMM did
not play the most important role as the exposure time increased.
§41 Figure 7 Weight loss of aluminium specimen in HCl
solution with and without DMM
§42 d) DMM + HCl Solution : After adding 150 ppm
HCl into the flasks containing various concentrations of DMM (0 to 250 ppm),
the corrosion results were rather frightening, especially with the systems
having CH3OH as solvent, which produced a much more corrosive
reaction than those with PE as the solvent. The reason is that HCl can not be
easily dissolved in PE as compared to CH3OH. However, for both
carbon steel (Figure 8) and aluminium (Figure 9), the corrosion rate
increased with increasing DMM concentration. Moreover, the corrosion
appearance of aluminium specimens seemed to be more uniform than pitting since
it was now distributed over the whole surface area whereas carbon steel still
showed uniform appearance.
§43 The temperature dependence of corrosion rates of solutions
containing only HCl and HCl with DMM were different, as seen in Figures 10 and
11 for carbon steel and aluminium, respectively. In the case of
corrosion by the solutions without DMM and having CH3OH as solvent,
the rates were obviously much lower than the ones with DMM after the
temperatures went up beyond 30 (for carbon steel) and 50°C
(for aluminium). This is simply because the DMM + HCl solutions contained not
only corrosive HCl, mercury metal, but also mercuric chloride produced by the
reaction of DMM/CH3OH and DMM/HCl as discovered by Skinner/Bass12-13
and Dessy14, respectively. For the case of PE as the
solvent, the rates were much lower, but still higher than the ones without HCl.
This phenomenon could be explained by the poor solubility of HCl in PE in
comparison to CH3OH. However, when temperature was elevated, the
solubility was increased from 1.53 x 10-3 to 9.83 x 10-3 mole/l.
§44 Figure 8 SEM photograph showing
uniform corrosion of carbon steel in CH3OH
+ DMM + HCl
§45 Figure 9 SEM photograph showing
uniform corrosion of aluminium in CH3OH +
DMM + HCl
§46 Figure 10 Comparison of
corrosion rate of carbon steel in HCl solutions with and without DMM
§47 Figure 11 Comparison of
corrosion rate of aluminium in HCl solutions with and without DMM
§48 e). DMM+ H2S Solution : Since H2S
is a component mixed together with elemental Hg and Hg derivatives in natural
gas, it was substituted in place of HCl in this study. Only aluminium
specimens were studied and the results are listed in Table 2.
§49 Table 2 : Comparison of
corrosion rates of aluminium in CH3OH/PE + H2S/HCl + DMM
solutions at ambient temperature for 130 h.
CH3OH + saturated H2S
CH3OH + 150 ppm HCl
CH3OH + 150 ppm HCl + 200 ppm DMM
CH3OH + saturated H2S + 200 ppm
CH3OH + 300 ppm H2S + 200 ppm
PE + saturated H2S
PE + 150 ppm HCl
PE + 150 ppm HCl + 200 ppm DMM
PE + saturated ppm H2S + 200 ppm DMM
Note : mpy is mils (0.001 inch) per year
§50 As can be seen, the corrosion in
the solutions containing
both acid and DMM increased drastically (Figure 12). Both acids have the same
potential to cause corrosion. The rates were extremely high when DMM was present
together with the acids. For example, CH3OH + 150 ppm HCl + 200 ppm
DMM corroded the specimens approximately 2675.9 mdd, which was 40 times more
than that corroded by the solution without DMM. The CH3OH +
saturated H2S + 200 ppm DMM solution was severe as well. The rate
was almost 100 times faster than the one without DMM.
§51 Figure 12 : SEM photograph
showing aluminium corroded by the CH3OH +
saturated H2S + 200 ppm DMM solution at ambient temperature for 130
§52 As expected, when using PE instead of CH3OH, the
corrosion rates were much lower. The system of PE + 300 ppm H2S +
200 ppm DMM also gave a lower rate than that of PE + 150 ppm HCl + 200 ppm DMM.
This is due to the content of water in HCl since HCl used in the system was
37% of HCl in water while H2S used was in the form of gas purged
into the PE solvent directly. The reactivity between H2S
or HCl and DMM was then low. However, in gas phase as in natural gas, the gas
molecules move around easier and faster, resulting in higher chance for those
molecules to collide with each other. The reactivity between H2S
and DMM should thus be better and the corrosion rate would be higher.
Based on the results of this study we find that
organomercury compounds found in natural gas catalyze the corrosion of
aluminium and steel components of gas pipelines and equipment in the presence
of trace amounts of water. Decomposition of DMM leads to the formation of
elemental mercury, which catalyzes corrosion by water. Furthermore, in the
presence of small amounts of HCl or H2S the corrosive action of DMM
is increased by several orders of magnitude and can lead to catastrophic
failure in a relatively short time. These observations suggest that for
natural gasses that contain mercury, steps should be taken to determine the
presence or absence of organomercury compounds and eliminate them if possible.
This study was supported by the Royal Thai Government
Research Fund. Deep appreciation goes to Professor Erdogan Gulari, Faculty of
Engineering, University of Michigan, Ann Arbor, for his helpful suggestions,
discussions and manuscript proof-reading.
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