studies on the role of sulfamic acid as a descalant in desalination ...

STUDIES ON THE ROLE OF SULFAMIC ACID AS
A DESCALANT IN DESALINATION PLANTS 1
Anees U Malik, I.N.Andijani, N.A.Siddiqi,
Shahreer Ahmed and A.S. Al-Mobayaed*
(*Chemistry Lab, Al-Jubail Desalination Plant)
ABSTRACT
Sulfamic acid (HSO3NH2) amido sulfuric acid) has long been used as an industrial
cleaning agent due to its remarkable property of solubilising hard scales and most of the
deposits. Furthermore, it can be used on stainless steels with no problem of pitting or
chloride - induced stress corrosion cracking (SCC). Sulfamic acid finds application in
MSF desalination plants for cleaning demisters, heat exchangers and cooling water systerns.Optimization
in the sulfamic acid treatment is desirable in terms of plant efficiency
and economic considerations. However, inspite of its capability as a potential
descalant and an excellent cleaner, the industrial chemical treatment is somewhat
qualitative due to the lack of corrosion and dissolution data regarding sulfamic acid.
The corrosion and dissolution behavior of stainless steels and copper-base alloys in
aqueous sulfamic acid and HCl solutions have been studied. Short and long terms
immersion tests along with electrochemical polarizarion resistance technique have been
used to determine the descaling and corrosion rates. The effect of addition of corrosion
inhibitor to the acid on the corrosion rates of the alloys has also been studied. Some
<strong>studies</strong> have been canied out under plant simulated conditions when scaled/deposited
demisters were treated with sulfamic acid and hydrochloric acid.The results of the <strong>studies</strong>
indicate that an optimum concentration of sulfamic acid or HCl for descaling the
alloys can be detennined depending upon the nature and extent of scaling. The
mechanism of corrosion of materials in presence of sulfamic acid has also been discussed.
INTRODUCTION
Saudi Arabia is the world’s largest desalinated water producer from multi stage flash
evaporation (MSF) seawater desalination plants. These plants use acid or additive
1 Issued as Technical Report No. SWCC RDC)-32 in December, 1993.
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treatment for scale control. Scales are generally formed in the heat input and recovery
sections of MSF units because of high brine concentration, high top temperature,
or high pH. The formation of scales on the demisters is one of the major problems
encountered in the additive treated plants. No demisters fouling was observed in acid
treated unit operated for several years. Demister is arranged at the upper part of the
flash chamber to intercept any such brine droplets and to prevent them from being
carried out into the distillate tray. Therefore, acid cleaning system is very essential to
remove alkaline deposits during operation with additive. Different SWCC’s desalination
plants use different kinds of acids. The Al-Jubail desalination plant uses additive
treatment for scale control, the scaled demisters are cleaned mechanically by dipping
them in a 5% sulfamic acid solution with addition of a corrosion inhibitor (IBIT).
Equal amounts of HCl and H2S04 with corrosion inhibitor (IBIT) are used in Al-
Khobar desalination plant. HCl with a different kind of inhibitor, ARMOHIB 28
has been used in Jeddah desalination plant while Khafji plant uses 5% HCl without
addition of any inhibitor. Sulfamic acid is also used in removing scales from open
circuit cooling water systems and heat exchanger tubes in desalination plants.
Sulfamic acid (HS03NH2 amido sulfuric acid) is the monoamide of sulfuric acid and
is formed in orthorhombic crystals (m.p. 205 o C). The pure crystals are non-volatile,
non-hygroscopic and odorless. It is highly stable and it can be kept for years without
any change in properties. Sulfamic acid behaves as a strong acid in aqueous solution
but the corrosion rates are significantly lower in comparison to other acids [l] (Table-l).
The low corrosion rates can be reduced further by the addition of corrosion
inhibitors [2]. The aqueous solutions of sulfamic acid do not emit corrosive fumes
and solubilize hard water scales and form soluble compounds with most of the industrial
deposits [3]. It can be used for cleaning stainless steels with no problem of chloride
induced stress corrosion cracking. Due to these formidable properties, acid
cleaners based on sulfamic acid are extensively used in a large variety of household
and industrial applications. Recently, suggestions have been made to replace sulfamic
acid by HCl for removal of the scales from demisters due to slower action of
the former and also much higher costs involved in using sulfamic acid in comparison
to HCl. One obvious problem in using HCl is the possibility of pitting and/or SCC
on stainless steels through Cl - induced attack. As the corrosion data for stainless
steels and other construction materials in presence of sulfamic acid are lacking, a
systematic study has been carried out to study the corrosion behavior of sulfamic acid
to qualitatively evaluate the capability of sulfamic acid as a cleaning agent/descalent.
This report incorporates the results of a systematic study carried out to investigate
the corrosion behavior of carbon steel, SS 316L, 70-30 and 90-10 Cupro-nickel in
presence of sulfamic acid and hydrochloric acid with or without the addition of an
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inhibitor. The role of acid concentration on the corrosion or descaling rates has specifically
been investigated with special reference to the performance of sulfamic acid
in a desalination plant. The results of an investigation concerning with the scaled
demisters have also been included.
EXPERIMENTS
Materials
Carbon steel, 316L stainless steel and 70-30, 90-10 cupronickel were obtained commercially
in sheet and/or rod forms. The flat, circular flat or cylindrical specimens
were used during the experiments without any further heat treatment. The chemical
composition of the alloys is given in Table-2.
Sulfamic acid in powder form and corrosion inhibitor (IBIT) in liquid form were
obtained from Al-Jubail desalination plant. The inhibitor (ARMOHIB 28) used with
hydrochloric acid was obtained from Jeddah desalination plant.
Sulfamic acid and hydrochloric acid solutions of varying concentration (range 2% to
10%) were prepared with and without addition of a corrosion inhibitor. 450 ml of
10% acid solution with 10% inhibitor was prepared by dissolving 4.5 grams of acid
powder plus 4.5 ml of inhibitor solution in distilled water. Similarly, 450 ml of 5%
acid solution with 5% inhibitor was prepared by dissolving 22.5 grams of acid powder
plus 1.125 ml of inhibitor solution in distilled water.
Techniques & Procedure
Electrochemical Polarization Studies:
Electrochemical polarization resistance experiments were carried out using an
EG&G model 342-2 soft Corr measurement system. The system is consisted of
Model 273 Potentiostat/Galvanostat, Model 342 soft Corr software and Model 30
IBM PS/2. All the experiments were carried out using a corrosion cell with saturated
calomel as reference and graphite as counter electrodes. Circular, flat or cylindrical
test specimens were used. The exposed area of the test specimens which was screwed
in the sample holder was 1 cm 2 .
The polarization resistance measurements were conducted at a scane rate of 0.1
mV/s with starting and final potentials corresponding to -20 mV to + 20 mV vs OCP,
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espectively. Before starting the experiments, the specimens (W.E.) were left in the
acid solution for about 1 hour to attain a steady state which was shown by a constant
potential.
Immersion Tests
Immersion tests were carried on 316L stainless steel, carbon steel, 70-30 and 90-10
cupronickel alloys in 5% sulfamic acid solution and 3% HCl solution with and without
addition of different concentrations of corrosion inhibitor at room temperature
under dynamic conditions. Coupons of about 60x40x2 mm dimensions were cut from
the sheet and abraded with 180 grit SiC paper. The abraded coupons were cleaned
ultrasonically and dried. The dried coupons were weighed.
Two coupons from each alloy were immersed in test solution under dynamic conditions.
Four test solution conditions were used: 5% sulfamic acid 3% HCl with 0%,
2%, 5% and 10% inhibitor.
Descaling Experiments
A used mist-separator (demister) containing scale layers on the knitted SS 316L wire
mesh pad’was obtained from the Al-Jubail desalination plant (Fig.1) for studying the
descaling performance of sulfamic acid and hydrochloric acid and the effect of corrosion
inhibitor in dissolution process. Samples from demister were immersed in different
concentrations of sulfamic acid and hydrochloric acid with and without
inhibitor under static and dynamic conditions. All the experiments were carried out
at room temperature. The deposits/scales on wire mesh were analyzed chemically
and it was found that deposits are the carbonates and hydroxides of magnesium,
(30%) and calcium (2%).
RESULTS AND DISCUSSION
Polarization Resistance Tests
Table 3 lists the corrosion rate values of alloys in three sulfamic acid concentrations,
e.g., 2%, 5% and 10% at room temperature under static condition, the values are
computed from the linear polarization resistance plots. These values are based on
the cathodic and anodic tafel values which were obtained from previously carried out
tafel plot runs. The results show that without inhibitor, the corrosion rates of carbon
steel are highest (< 60 MPY) and that of SS 316L are lowest (> 0.5 MPY), cupronickel
alloys occupying middle position (2-6 MPY). It appears that the effect of vary-
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ing sulfamic acid concentration (2,5 and 10%) on the corrosion rates is not very
significant as indicated by the similar corrosion rates of the alloys. However, the
addition of inhibitor to sulfamic acid lowers down the corrosion rates considerably
especially that of carbon steel and cupronickel alloys. Moreover, the effect of variation
in inhibitor concentration on corrosion rates is quite significant. For example,
with 10% acid and 5% inhibitor, the corrosion rates are : carbon steel: < 9 MPY, SS
316L: < 0.4 MPY and cupronickel: < 0.8, and addition of 10% of inhibitor reduces the
corrosion rate still further e.g., carbon steel: < 4.7 MPY, SS 316L : < 0.18 MPY and
cupronickel : < 0.07 MPY. Table 4 lists the corrosion rates of alloys at 5% sulfamic
acid without inhibitor and with additions of 2%, 5% and 10% inhibitor under
dynamic conditions. The results indicate that the corrosion rates of alloys under
dynamic conditions are generally higher than that under static conditions. Figures 2
to 4 show trends in the variation of corrosion rate of alloys with increasing inhibitor
concentration under static and dynamic conditions.
Tables 5&6 lists the corrosion rates of three alloys by polarization resistance technique
in HCl with and without addition of inhibitor (ARMOHIB 28) under static and
dynamic conditions. A comparison of corrosion rate values of three alloys in sulfamic
acid and HCl indicates that the latter has rates l-2 order of magnitude higher than
the former.
An analysis of the corrosion rates of alloys obtained from polarization resistance
techniques indicates that under static or dynamic conditions, there is a steep fall in
corrosion rates on increasing the inhibitor concentration up to 5% followed by a slow
decrease or virtually no lowering in corrosion rates in the inhibitor concentration
range of 5-10%.
Immersion Tests
The results of immersion test of alloys in 5% sulfamic acid solution in presence of
inhibitor (concentration : 0, 2, 5 and 10%) at room temperature and under dynamic
condition are summarized in Table 7. For SS 316L no perceptible weight change was
recorded for the coupons immersed under all test solution conditions for a duration
of 70 days. For carbon steel, highest corrosion rate was 139 MPY without inhibitor
followed by 81 MPY on addition of 2% inhibitor, the corrosion rates lowered down
to about 47 at 5% and 10% inhibitor concentrations, respectively.
Cupronickel alloys have much lower corrosion rates than carbon steels, the effect of
inhibitor addition is very pronounced at 5% and higher concentrations of inhibitor
when corrosion rates are lowered down from 0.06 to about 0 MPY. Fig. 5 shows
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trends in the variation of corrosion rates of alloys with increasing inhibitor concentration
under dynamic conditions. Effect of addition of inhibitor is clearly seen on the
coupons. Fig. 6 shows uniform dealloying on the coupon when there was no addition
of inhibitor. While Fig. 7 shows selective dealloying when the amount of inhibitor
was 2%. Figs 8, 9, 10 show typical microstructures representing dealloying at different
inhibitor concentrations, Corrosion rates of 70 Cu-30 Ni alloys are significantly
lower than 90 Cu-10 Ni alloys.
Table 8 lists the result of immersion tests of alloys in 3% hydrochloric acid with 0%,
3%, 5% and 10% inhibitor under dynamic conditions at room temperature. All the
three alloys show very high corrosion rates when there is no addition of inhibitor viz
SS 316L (75 MPY), 70 Cu-30Ni (630 MPY) and 90 Cu-10 Ni (605 MPY). Corrosion
rates lower down drastically after the addition of inhibitor except in the case of SS
316L where the corrosion rate is 48 MPY with 3% inhibitor. Fig.11 shows trends in
the variation of corrosion rates of the three alloys with increasing inhibitor concentration
under dynamic condition. Comparing the corrosion rate values determined
from electrochemical polarization and immersion tests, two important points are
emerged: (i) the trend in lowering of corrosion rate on increasing inhibitor’s concentration
is similar in both the methods i.e. the minimum corrosion rates are at about
510% inhibitor concentration (ii) under dynamic condition, the corrosion rates for
carbon steel and cupronickels determined from immersion tests are higher than that
determined by polarization resistance technique. The higher corrosion rates during
immersion tests could be attributed to the process of breaking and reformation of the
protective oxide film till a thick oxide deposit is formed on the metal which does not
allow further corrosion. The rates determined by electrochemical polarization
resistance technique are lower because of the range of potential traverse is very small
within +/- 20mV from 0.C.P and obviously, the potentials applied within such a narrow
range would not likely to induce any change in the surface characteristics of the
alloy.
Descaling Tests on Fouled Demister
Table 9 shows amount of scales removed from fouled demister (316L) samples after
dipping in sulfamic acid solutions for 19 hours under static conditions. Aqueous sulfamic
acid solutions of concentration 3%, 5% and 10% with addition of 0 %, 2%, 5%
or 10% inhibitor were used during descaling tests. The results indicate that under
static conditions, sulfamic acid only partially removes scales from the demister and
the addition of inhibitor has only marginal effect on descaling. The data further
shows that the descaling is resulted in presence of 3%, 5% and 10% sulfamic acid
solution under dynamic conditions. The rate of descaling is very fast and maximum
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amount of scales were removed from samples within 1 hour. The results indicate that
the addition of inhibitor did not effect the descaling process. As expected the descaling
efficiency in dynamic condition is much better than under static condition.
Descaling test by hydrochloric acid was carried out at 3%, 5% and 10% concentrations
under dynamic conditions Table 9. Results show that within hours, the scales
are totally removed. Comparing these results with that of sulfamic acid it is obvious
that hydrochloric acid is a much more powerful descalant than sulfamic acid. The
plots of acid (sulfamic acid or HCl) versus amount of scales removed Figure 12 can
provide a method to determine the quantity of acid required to remove the scale
completely from the demister or vice-versa under dynamic conditions.
GENERAL CONCLUSIONS
1. Under static conditions, the effect of variation in sulfamic acid concentration on
corrosion rates of steel, cupronickel and AISI 316L is not significant but addition
of inhibitor to sulfamic acid brings down the corrosion rate by l-2 order of
magnitude.
2. The corrosion rates under dynamic immersion conditions for the alloys are
higher than that determined by polarization technique. The lower rates by electrochemical
polarization technique are attributed to the narrow range of applied
potential which would not induce any significant change in the surface
characteristics of the alloy.
3. Under static conditions, sulfamic acid only partially removes scales from the
demister even after submerging it for a period of more than 18 hrs, however,
under dynamic conditions, 3% or 5% sulfamic acid solutions completely remove
the scales within 1 hr. In both the condition, addition of inhibitor to the acid
has only nominal effect on descaling.
4. Besides higher corrosion rates, hydrochloric acid exhibits much higher descaling
characteristics than sulfamic acid.
5. Astandard plot between acid (Sulfamic acid or HCl) versus amount of scale
removed can provide a method to determine the quantity of acid required to
remove the scale from the demister or vice-versa under dynamic conditions.
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CONCLUDING REMARKS
The results of the <strong>studies</strong> carried out on descaling behavior of sulfamic acid indicate
that for descaling of demisters (material: AISI 316L) in desalination plants, sulfamic
acid is an efficient cleaning agent. Since addition of an inhibitor does not have a
significant effect on corrosion rates. Therefore, addition of inhibitor to sulfamic acid
is not required for cleaning purposes. In Cupro-nickel (70-30 or 90/10) heat
exchangers in MSF plants, sulfamic acid is a good descaling agent provided sufficient
amount of corrosion inhibitor is present. HCl in presence of inhibitor is a more efficient
and cost effective descaling agent but due to its high corrosion rate and its tendency
to induce stress corrosion cracking the long term effects of its application are
to be monitored.
REFERENCES
1. Encyclopedia of chemical technology, Vol. 21. Sulfamic Acid, pp 951-57, Wiley
Interscience Publication (1983).
2. Process Industries, NACE Publication (1986), pp 267-268.
3. Metal Hand Book, Vol. 13, Corrosion, Specific Industries and Environments,
American Society of Metals, 1987, pp 1140-1141.
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Figure l(a).
Photograph of a used mist-separator (demister) showing scale layers
on the knitted SS 316L wire mesh.
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Figure 1(b). Close up view of the scaled wires of SS 316L.
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Figure 6. Photograph of 70/30 Cu/Ni coupon after exposing to 5% sulfamic acid
without inhibitor showing uniform dealloying.
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--_-_ -.-.
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Figure 7. Photograph of 70/30 Cu/Ni coupon after exposing to 5% sulfamic acid
with 2% inhibitor showing patches of dealloying.
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Figure 8. Microstructure of 70/30 after exposing to 5% sulfamic acid with 2%
inhibitor showing unaffected zone. 200 x.
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Figure 9.
Same as above but on the dealloyed zone
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Figure 10. Microstructure of 70/30 after exposing to 5% sulfamic acid without
the addition of inhibitor, showing uniform dealloysing. 200 X.
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