THE EFFECT OF HEAVY METAL IONS ON THE LOCALIZED CORROSION BEHAVIOR OF STEELS 1 Anees U. Malik, Mohammad Mobin, Ismail Andijani, Fahd Al-Muaili and Mohammad Al-HajriSaline Water Desalination Research Institute Saline Water Conversion Corporation (SWCC) P.O.Box 8328, Al-Jubail 31951, Saudi Arabia. E-mail: [email protected]SUMMARY The localized corrosion is the most serious problem encountered in a processing plantsuch as desalination and power plants. The carryover of metal ions by the liquid, aerosol and vapors appears to be the primary cause of corrosion in evaporators, distillate system, boiler tubes, turbines, distillate pipelines and other systems ofdesalination and power plants. The deposition of carryover heavy metals/oxides on steel components in MSF desalination or power plants is a common problem andreported by many authors. The deposits may initiate localized attack in the form ofpitting or crevice corrosion. The pits act as initiators of stress corrosion in the form ofstress corrosion cracking, corrosion fatigue or intergranular corrosion and result in the failure of compone nts. However, this asp ect has been given little attention and is least understood yet has great relevance to seawater desalination and power plants. Keeping in view the above facts, a research project entitled, “The effect of heavy metal ion on the localized corrosion behavior of steels” was formulated by the Corrosion Department of R&D Center Al-Jubail. The project contains the results of an investigation concerning with the effect of heavy metal ions, e.g., Cu, Ni and Zn on the localized corrosion behavior of carbon steel and316L under different exp erimental conditions. The work of the project was divided into five major tasks, namely, literature survey; material and equipment acquisition; immersion test under static condition; immersion test under dynamic condition; andelectrochemical studies. Immersion tests of 1, 6 and 12 months duration were carried out to determine the effect of metal ions on the corrosion rate of steels. The effect of Si a light non-metallic 1 Issued as Technical Report: TR. 3804/APP 96010 in March, 2005.
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THE EFFECT OF HEAVY METAL IONS ON THE LOCALIZED CORROSION
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8/7/2019 THE EFFECT OF HEAVY METAL IONS ON THE LOCALIZED CORROSION
The localized corrosion is the most serious problem encountered in a processing plant
such as desalination and power plants. The carryover of metal ions by the liquid,
aerosol and vapors appears to be the primary cause of corrosion in evaporators,
distillate system, boiler tubes, turbines, distillate pipelines and other systems of
desalination and power plants. The deposition of carryover heavy metals/oxides on
steel components in MSF desalination or power plants is a common problem and
reported by many authors. The deposits may initiate localized attack in the form of
pitting or crevice corrosion. The pits act as initiators of stress corrosion in the form of
stress corrosion cracking, corrosion fatigue or intergranular corrosion and result in
the failure of components. However, this aspect has been given little attention and is
least understood yet has great relevance to seawater desalination and power plants.
Keeping in view the above facts, a research project entitled, “The effect of heavy metal
ion on the localized corrosion behavior of steels” was formulated by the Corrosion
Department of R&D Center Al-Jubail.
The project contains the results of an investigation concerning with the effect of heavy
metal ions, e.g., Cu, Ni and Zn on the localized corrosion behavior of carbon steel and 316L under different experimental conditions. The work of the project was divided into
five major tasks, namely, literature survey; material and equipment acquisition;
immersion test under static condition; immersion test under dynamic condition; and
electrochemical studies.
Immersion tests of 1, 6 and 12 months duration were carried out to determine the
effect of metal ions on the corrosion rate of steels. The effect of Si a light non-metallic
1Issued as Technical Report: TR. 3804/APP 96010 in March, 2005.
8/7/2019 THE EFFECT OF HEAVY METAL IONS ON THE LOCALIZED CORROSION
element has been specifically studied. The important experimental conditions, which
include the nature of aqueous medium, metal ion concentration, temperature, pH and
flow condition, have been taken into account. In general, there is a negligible effect of
metal ions on the corrosion rate of 316L in presence of 50 ppb, 1 ppm and 100 ppm
concentration in either seawater or distillate water under different experimental
conditions. The effect of metal ions on the corrosion rate of carbon steel is quite
pronounced and follows interesting trends. The results from immersion tests show a
decrease in the corrosion rates of carbon steel in distillate water containing higher
concentration of Cu and Ni under both static and dynamic conditions. In seawater, a
higher concentration of Cu is detrimental under dynamic condition. The presence of Zn
in the aqueous medium influences the corrosion behavior of steel differently than that
of Cu and Ni. A higher concentration of Si in seawater, under dynamic condition,
effectively decreases the corrosion rate.
Electrochemical techniques like free corrosion potential, potentiodynamic polarization
and polarization resistance measurements and AC impedance have been used to
investigate the role of heavy metal ions on the corrosion behavior of carbon steel and
SS 316L. The instantaneous corrosion parameters as obtained by electrochemical
techniques show an increase in corrosion rates with increasing metal ion
concentrations. Under controlled laboratory conditions, there is no evidence of
localized attack in presence of different concentrations of metal ions.
1. INTRODUCTION
Corrosion is the major cause of component or material failure in desalination and
power plants. Though one or more forms of corrosion are involved during corrosionfailure in desalination plants general and pitting corrosion are more common modes of
failures [1]. In seawater processing systems, if dissolved oxygen and pH are under
control, general corrosion is the predominant mode of attack on conventional
construction materials such as carbon steel. This form of corrosion is easily
controllable and is desirable in the sense that it permits predictive estimates of service
life. Pitting, the most detrimental form of attack, is often responsible for the corrosion
failures of components in desalination and power plants. In process plants, it accounts
at least 90% of metal damage by corrosion [2]. Though there are several causes of
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The presence of nickel in both distillate water and seawater has almost similar effect to
that of copper on the corrosion rate of carbon steel. In distillate water, the highest
corrosion rate of 33 mpy is observed in presence of 50 ppb of nickel under dynamic
condition, this is followed by a corrosion rate of 21 mpy in presence of 1 ppm of
nickel. Both under static and dynamic conditions, a lowering in corrosion rate is
observed only in presence of 100 ppm of nickel. In seawater, under static condition
though there is a slight increase in corrosion rate with increasing metal ion
concentration, there is no significant effect under dynamic condition.
(3) Effect of Zinc
In distillate water, under static condition, except for 100 ppm of zinc ion concentrationwhich indicated an increase in the corrosion rate, there is no appreciable effect of zinc
on the corrosion rate of carbon steel. Under dynamic condition, corrosion rate is
almost unaffected in presence of all the three concentration of zinc. In seawater, under
static condition, the presence of zinc has no significant effect on the corrosion rate of
carbon steel. However, under dynamic condition there is a decrease in the corrosion
rate in presence of zinc at all the three selected concentrations.
(4) Effect of Silicon
The effect of silicon, a light non-metallic element, on the corrosion rate of carbon steel
was specifically studied. The silicon solution in distillate water and seawater were
specially made and kept in plastic containers. The immersion tests were also carried
out in plastic vessels. In both distillate and seawater, under static condition, presence
of silicon appears to have no significant effect on the corrosion rate of carbon steel.
However, under dynamic condition, the corrosion rates appear to decrease with
increasing Si concentration.
3.1.2 SS 316L
Tables 7-10 show the effect of metal ions on the corrosion rate of SS 316L in both
distilled water and filtered raw seawater at different pH at 25 oC. The results are
obtained under static condition. SS 316L is unaffected in distilled water (pH 6.5) and
seawater (pH 8.5) for an immersion period of 1, 6 and 12 months duration. The effect
of different metal ions on the corrosion behavior of 316L is summarized as follows:
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In general, there is negligible effect of metal ions on the corrosion rate of 316L in
presence of 50 ppb, 1 ppm or 100 ppm concentration at pH 4, 6.5 or 8.5 for an
immersion period of 1, 6 or 12 months duration in both distilled water and seawater.
3.2 Electrochemical Studies
3.2.1 Free Corrosion Potential (E corr ) Measurements
Figures 1 to 9 show time vs free corrosion potential plots for carbon steel and SS 316L
immersed in distilled water (pH 6.5) and filtered raw seawater (pH 8.5) in presence and
absence of heavy metal ions under static condition. The results for carbon steel and SS
316L are summarized separately.
3.2.1.1 Carbon Steel
The free corrosion potential of carbon steel in distilled water, in absence of metal ions,
for the first 2 hrs is about -550 mV vs SCE; this is followed by an increase in potential
till a stable potential range of -650 to -750 mV is reached. However, in seawater
without metal ions, a potential of -650 mV is reached during the first 2 hrs, this is
followed by an increase in potential till a stable potential range of -700 to -800 mV is
reached. The results of the effect of heavy metal ions on the free corrosion potential of
carbon steel in both distilled water and seawater are summarized as follows:
(1) Effect of Copper
Figures 1 and 2 show potential vs time plots for carbon steel in distilled water and
seawater containing 0, 1 and 100 ppm copper. In distillate water containing 1 ppm Cu,
a positive shift in Ecorr is noticed for the first 24 hours only whereas in presence of 100ppm Cu, a pronounced positive shift in Ecorr is noticed for a time period extending 17
days. In seawater, though, there is no significant effect of the presence of 1 ppm Cu
on the Ecorr of carbon steel, the presence of 100 ppm Cu slightly shifted the E corr in
noble direction and a potential below-650 mV is maintained for the time period
extending 17 days.
(2) Effect of Nickel
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In distillate water a significant ennoblement in Ecorr is observed in presence of both 1
and 100 ppm nickel (Figure 3). An initial decrease in negative potential was observed
in presence of both 1 and 100 ppm nickel, this is followed by a constant noble
potential. However, in presence of 1 ppm Ni after a time period extending 16 to 17
days, an increase in negative potential is observed till a potential equal to the potential
of carbon steel in absence of nickel is reached. In seawater, the effect of nickel on the
potential of steel is not much pronounced. In presence of both 1 and 100 ppm nickel
the potential is reached to above - 600 mV on the first day of immersion (Figure 4).
(3) Effect of Zinc
The Ecorr vs time plots for carbon steel in distilled water and seawater containing 0, 1
and 100 ppm zinc is shown in figures 5 and 6. In distillate water, though the potential
of carbon steel in presence of 100 ppm zinc is slightly below the potentials of carbon
steel, there is no significant effect of zinc on the Ecorr of carbon steel in both distilled
water and seawater.
(4) Effect of Silicon
Figure 7 shows the potential vs time plots for carbon steel in seawater containing 0, 1
and 100 ppm silicon. Though, there is no effect of presence of 1 ppm silicon on theEcorr of carbon steel, a major positive shift in Ecorr is observed in presence of 100 ppm
of silicon.
3.2.1.2 SS 316L
(1) Effect of Copper
Figure 8 shows the potential vs time plots for SS 316L in distillate water containing 1
and 100 ppm copper. An increase in copper ion concentration shifts the Ecorr of 316L in
more noble direction.
(2) Effect of Nickel
Figure 9 shows the potential vs time plots for SS 316L in distillate water containing 1
and 100 ppm nickel. After an initial negative shift in Ecorr , the increasing nickel ion
concentration appears to shifts the Ecorr of 316L in more noble direction.
3.2.2 Potentiodynamic Polarization Measurements
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Since during metallic corrosion, the total rate of oxidation equals to total rates of
reduction, the dissolution rate of steel equals to the reduction rates of O2 and metal ions
and hence the initial corrosion rate of carbon steel in presence of metal ions is expected
to increase. After reduction of the metal ions the resulting metal get deposited over the
steel surface and form a barrier which is likely to protect the steel from further
corrosion. The extent of protection offered to the steel shall depend upon the ability of
the deposited metal to form a stable protective barrier and nature of protection offered
to steel surface. The deposited Cu and Ni protect the steel cathodically. In aqueous
medium containing high concentration of Cu and Ni a stable protective barrier is more
likely to be formed and hence a lowering in corrosion rate is observed. In presence of
lower concentration of metal ions the observed increase in corrosion rate is due to
absence of a stable protective barrier. In seawater the observed increase in corrosion
rate in presence of 100 ppm Cu under dynamic condition is due to erosive action of
fine suspended solid particles on the barrier film.
In presence of Zn, the corrosion rate of carbon steel in distillate water under both static
and dynamic conditions is unaffected except at higher concentration of metal. Since
Zn is anodic to steel, under the given experimental conditions, it is unlikely to get
reduced and deposit over steel surface and thus decrease the subsequent rate of
corrosion. However, it may remain in the solution in the ionic form affecting the
conductivity of the test solution. The observed increase in corrosion rate in presence of
100 ppm of Zn may be accounted due to the increase in the conductivity of the
solution. In seawater, with increasing metal ion concentration a lowering in corrosion
rate under dynamic condition is probably due to limited transfer of dissolved O2 from
bulk solution to the surface. The presence of higher concentration of Si in the aqueous
medium appears to be effective in lowering down in the corrosion rate of carbon steelunder dynamic condition. The effectiveness of Na2SiO3, under dynamic condition, in
lowering down the corrosion rate of carbon steel has already been established [18].
The results of immersion tests find support from free corrosion potential
measurements. The Ecorr of carbon steel in distillate water and seawater is in the range
of -650 to -750 mV and -700 to -800 mV vs SCE, respectively. In distillate water
containing 100 ppm of Cu and Ni a positive shift is E corr is noticed which is maintained
for a considerable period of time. A positive shift in Ecorr is indicative of the protection
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offered to steel surface. In seawater containing 100 ppm of Cu and Ni the shift is not
much significant. The presence of Zn in the aqueous medium does not affect the Ecorr
significantly whereas the presence of 100 ppm Si in seawater shifts the Ecorr
considerably in noble direction. For 316L also an increasing concentration of metal
ions in the aqueous medium appears to shift the Ecorr in noble direction.
The corrosion parameters obtained from potentiodynamic and linear polarization
measurements show an increase in Icorr and lowering in R p values, respectively with
increasing Cu and Ni ion concentrations indicating an increase in corrosion rate of
carbon steel with increasing metal ion concentration in the aqueous medium. The
parameters measured by the above techniques are instantaneous and the increase in
instantaneous corrosion rate with increasing metal ions concentration is expected and
can be explained on the basis of occurrence of two cathodic reactions (Equ. 2 and 3).
5. CONCLUSIONS
(1) Under controlled laboratory conditions the results from immersion and
electrochemical tests show no evidence of localized attack on carbon steel and
316L in presence of different concentration of heavy metal ions.
(2) The results from immersion tests in the pH range 4 to 8.5 at room temperature
show no effect of metal ions on the corrosion rate of 316L.
(3) The effect of metal ions on the corrosion rate of carbon steel in the pH range of
6.5 to 8.5 at room temperature is quite pronounced and follows interesting
trends.
(4) The presence of Cu and Ni in the aqueous medium produces almost similar
effect on the corrosion behavior of carbon steel.
(5) The corrosion rate of carbon steel in distilled water containing higher concentration of Cu and Ni decreases under both static and dynamic conditions.
In seawater, however, a higher concentration of Cu is detrimental under
dynamic condition.
(6) The presence of Zn in the aqueous medium influences the corrosion behavior of
steel differently than that of Cu and Ni.
(7) A higher concentration of Si in seawater, under dynamic condition, effectively
decreases the corrosion rate of carbon steel.
8/7/2019 THE EFFECT OF HEAVY METAL IONS ON THE LOCALIZED CORROSION
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