Ind. E n g . C h e m . Res. 1990, 29, 1460-1466
1460
Corrosion Inhibition of Structural Steels in the COz Absorption Process by 1-(Hydroxyethy1idene)-1,l -diphosphonic Acid I s a o Sekine,* T e t s u y a Shimode, and Makoto Y u a s a Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, J a p a n
Koichi T a k a o k a Keiyo Plant Engineering Co., Ltd., 2-8-8 Ichikawa-minami, Ichikawa, Chiba 272, J a p a n
Corrosion inhibition of stainless steel (type 304, (UNS S30400)) and mild steel (SS 41 (UNS K02600)) in the COPabsorption process was investigated by corrosion tests and physicochemical measurements in an aqueous potassium carbonate solution containing 1-(hydroxyethy1idene)-1,l-diphosphonic acid (HEDP). T h e maximum value of the inhibition efficiency (7) of corrosion for SS 41 was ca. 80% in the test solution containing 200 ppm of HEDP under air atmosphere by the corrosion weight loss test and approached those values obtained in the test solutions with Na2Cr04 (>99%) and Vz05 (>99%). The corrosion rate result obtained with the corrosion weight loss test corresponded to that obtained with electrochemical measurements. By scanning tunneling microscopy (STM), the molecules of HEDP were adsorbed on the surface of SS 41 to inhibit its corrosion. In the application test, using a Benfield apparatus as a bench-scale system, the 7 value for type 304 was ca. 80-90% in the test solution containing HEDP a t high pressure and temperature. The HEDP also depressed the formation of scale as well as depressed the result in the water-cooling process. Hot and aqueous potassium carbonate (K2C03)solution has been widely used in the process of removing carbon dioxide ((20,) gas. The solution taking up COPgas has a high corrosive property and tends to be sensitive to the stress corrosion cracking (SCC) of carbon steel (Banks, 1967; Bienstock and Field, 1961; Foroulis, 1987, 1989; Johnson, 1987; Naito et al., 1978). Various inorganic compounds such as potassium bichromate (KZCr20,!, !anadium oxide (V206),and a substrate with nickel (Ni) ion have been developed as corrosion inhibitors in this COP absorption process (Field and Beinstock, 1965; Gancy and Durling, 1978; Eickermeyer, 1969). But the addition of their inhibitors in this solution needs to be avoided to the utmost because their inhibitors are subject to environmental pollution control. It has also been reported that organic inhibitors without pollution control have no effect in this COz absorption process (Ohkubo, 1978). Recently, the steels used as the structural material in the COz absorption process have changed from carbon steel to stainless steel, but the corrosion protection of the stainless steel in this process is imperfect and insufficient. Moreover, no work has been conducted to depress the formation of scale and sludge on the surface of the steel in this COz removing process. Recently, the organic reagent 1-(hydroxyethy1idene)1,l-diphosphonic acid (HEDP) has been industrially used as a corrosion and scale inhibitor in a water-cooling system (Schweitzer, 1969;Ralston, 1972; Liu and Nancollas, 1975; Varsanik, 1975). In this paper, we attempt corrosion and scale inhibition by HEDP in this COz absorption process without pollution of heavy metal and develop an application of the HEDP inhibitor; the effect of corrosion inhibition of mild steel (SS 41 (UNS KO2600 (UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineerings and cosponsored by ASTM)), carbon steel) and stainless steel (type 304 (UNS S30400)) by the HEDP reagent in this process was investigated by corrosion tests, electrochemical measurements, and surface analyses. The inhibition behavior of scale formation by HEDP was also preliminarily evaluated by physicochemical measurements. Experimental Section Materials a n d Preparations. SS 41 and type 304
plates were used as specimens. Their chemical compositions are shown in Table I. The inhibitors and additives were HEDP (Mitsubishi Monsant), monoethanolamine (MEA, Kanto Chemicals), diethanolamine (DEA, Kanto Chemicals),triethanolamine (TEA, Kanto Chemicals), monopropanolamine (MPA, Kanto Chemicals), sodium chromate (Na2Cr04!,Kanto Chemicals), vanadium pentoxide (VzOs,Shudzui s Pure Chemicals), sodium molybdate (Na2MoOr,Kanto Chemicals), zinc sulfate (ZnS04,Kanto Chemicals), and sodium silicate (Na2Si03).These chemicals were reagent grade and were used without purification. A 23% aqueous K2C03 solution was prepared with KzC03 (Kanto Chemicals, reagent grade) and distilled three times with water. This solution was saturated with COBgas a t room temperature and 1 atm to give a test solution (pH 8.30-8.45). A 40% aqueous K&O3 solution was prepared with KZCO3and distilled once with water. Then this solution was saturated with COPgas a t 10 kg/ cm2 to obtain the test solution in the Benfield apparatus (the Benfield process is the COPabsorption process using K,CO, solution with diethanolamine as a catalyst) (Benson and Field, 1954). Corrosion Weight Loss Test. The pretreatment of the steel was carried out according to a previous paper (Sekine and Asazuma, 1983),as follows. The SS 41 plate (20 X 50 X 1.5 mm) was polished with emergy papers up to No. 1500 grade. The plate was washed with distilled water and degreased with methanol and acetone under ultrasonic conditions (125 W). Thus, the SS 41 plate was immersed in the test solution (450 mL) containing each inhibitor. The experimental conditions are shown in Table 11. Electrochemical Measurements. The polarization curve (the polarization curve is a current-potential curve obtained under steady-state conditions) and the rest potential (the rest potential is an open-circuit potential of the cell) in this system were measured as in previous papers (Sekine and Asazuma, 1983; Sekine and Hirakawa, 1986; Sekine et al., 1988), as follows. Each section of SS 41 (2.0-mm-diameter), type 304 (Z.O-mm-diameter),and Ni (2.0-mm-diameter) wires and the Cr (3.0- X 1.7-mm) rod was used as the working electrode, and the other parts were masked by poly(tetrafluoroethylene) (PTFE, Teflon) resin. Before measure-
0888-5885/ 90/ 2629-1460$02.50/0 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1461 Table I. Chemical Composition of SS 41 and Type . - 304 steel C Si Mn SS 41 0.08 trace 0.35 type 304 C0.08 C1.00 c2.00
P
S
Ni
Cr
0.014 C0.045
0.018 C0.03
8.00-10.5
18.00-20.00
Table 11. Experimental Condition for the Weight Loss Test W 0 3
steel SS 41 type 304
concn, wt % 23 40
pressure, immersion O C kg/cm2 time, day 50 f 1 air 7 95 f 2 9.5 f 0.5
30
P J'"'" O f co2
T
temp,
%Jf
condition static flowing
Fe bal bal
c
Pressure gauge
8d
r
y valve
PA ,
Benfield system.
Peephole
Function Genera tor
Potentiostat
Ammeter
I
P
Out let
Figure 2. Schematic diagram of the Benfield system.
again and dried with flon gas, the surface of the steel specimen was observed by the STM apparatus (Unisoku USM-101; resolution of the x and y axes, 1 nm; resolution of the z axis, 0.3 nm; tunneling current, 10 PA-20 nA; bias voltage, 10 mV-10 V) (Sekine et al., 1989). For the SEM measurement, the surface of the specimen was observed by using the SEM apparatus (Hitachi S-430) after immersion in the test solution for 7 days. The ATR spectrum of the same specimen was measured by the FTIR spectrophotometer (JEOL, JIR-100). Weight Loss Test by the Benfield Apparatus. The type 304 specimen (raschig ring; height, 25.0 mm; internal diameter, 23.6 mm; thickness, 0.8 mm) was polished with emery papers up to No. 1200 grade. The specimen was washed with distilled water and degreased with methanol and acetone under ultrasonic conditions (125 W). Thus, the specimen was immersed in 4300 mL of the test solution containing various inhibitors by using a Benfield apparatus (Figure 2) (Benson and Field, 1954). The experimental conditions are shown in Table 11. Preinhibition Test of Scale Formation. For the preinhibition test of scale formation, 100 mL of a 1% aqueous calcium chloride (CaC12)solution was added to 200 mL of a 23% aqueous K2C03solution saturated with C02 containing 0-1000 ppm of HEDP and the other inhibitors. The solutions were kept a t room temperature because the scale compounds were precipitated. Their scale compounds were filtrated and dried for 1 day for analysis by the X-ray diffraction method and IR measurement.
cE Potentiostat
Recorder
Figure 1. Schematic diagrams of electrochemical measurements.
ments, the test electrodes were polished with No. 1500 emery paper, washed with acetone in an ultrasonic bath, and dried. The counter electrode and reference electrode (the potential of the electrochemical parameter was measured with reference to a SCE in this paper) were a platinum plate (30 X 40 X 0.5 mm) and a saturated calomel electrode (SCE), respectively. The working electrode was reduced a t a constant potential of -1.2 V for 10 min before the polarization curve measurement. The polarization curves for the above steel wire and rod in the test solution containing inhibitors were obtained by first polarizing from the rest potential toward the cathodic direction by the potential sweep method (20 or 50 mV/min) and then similarly toward the anodic direction (potentiostat and galvanostat, Hokuto Denko HA-501; function generator, Hokuto Denko HB-104; Figure 1A). The rest potential of each steel was measured in the same test solution for ca. 20 h after cathodic reduction a t a constant potential of -1.2 V (vs SCE) for 10 min (electrometer, Hokuto Denko HE-106; Figure 1B). Surface Analyses. The surface of steel was analyzed by scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and attenuated total refraction (ATR). The STM measurement was conducted as follows: The specimen (20 X 20 X 1.5 mm) of SS 41 was polished with emery papers up to No. 1500 and micropolish alumina suspension (0.05 pm), subsequently defatted in methanol, washed ultrasonically in acetone, and kept in hexane. The specimen of SS 41 was immersed in the test solution a t room temperature for 1-10 min after the specimen was ultrasonically washed in ethanol and water. Then the specimen was taken out of the test solution. After the specimen was ultrasonically washed in ethanol and water
Results and Discussion In this COPabsorption process and test solution system, the corrosion reactions are as follows (Ikeda and Mukai, 1985; Parkins and Foroulis, 1988): KPC03+ C02 + H 2 0 2KHC03 (1)
* K+ + HC03Fez++ HC03- * FeCO, + H+ KHCO,
(2) (3)
Though the pH value of the solution becomes smaller from 12.0 to 8.5 by addition of COz,this solution has a corrosive property. That is, the corrosion reaction proceeds as in
1462 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Na,Cr 0, a n d V,O, conc i wt 'io 0
60
50
u u E
. F
30
,0-3
- T + b ,
102 "
IO-'
-
i
ioo
'
HEDP
NapO, v2oc
C
0
g
20
b u' 0
10
C ( x 1 O-' M
Figure 4. Langmuir adsorption isotherm of HEDP on SS 41 in 23
0
wt % KzC03 solution saturated with C 0 2 at 50
0
200 300 400 500 HEDP conc i p p m
100
OC.
1000
Figure 3. Corrosion rate of SS 41 vs inhibitor concentration in 23 w t % KzC03 solution saturated with COz at 50 "C.
reaction 3 to produce iron carbonate (FeCOJ. The corrosion rates of SS 41 were determined by the corrosion weight loss test in 23 5% aqueous K2C03solution saturated with C02 and containing various inhibitors (HEDP concentration, 0-1000 ppm; Na2Cr0, and V2O5 concentration, 0-1 %), at room temperature (Figure 3). The corrosion rate of SS 41 decreased with increasing concentration of HEDP, gave a minimum value at 200 ppm of HEDP, and increased in concentrations greater than 200 ppm of HEDP. The inhibition efficiency (7) of corrosion was calculated by corrosion weight loss before and after immersion and is determined as follows: (4) 11 = [(W, - w)/W01100 where Wa and W are the corrosion weight losses of the uninhibited and inhibited steels, respectively. The best efficiency in the HEDP system was ca. 80% a t 200 ppm of HEDP. This value was close to those obtained in the Na2Cr04or V205system. Since the HEDP molecules are adsorbed on the surface of SS 41 and form a protective film, it is thought that the corrosion of SS 41 is depressed. The relationship between the corrosion inhibition and the adsorption of HEDP was explained by the adsorption isotherm. It is reported that the type of adsorption, which shows good corrosion inhibition, obeys the Langmuir adsorption isotherm (Saha and Kurmaih, 1986; Sekine and Hirakawa, 1986). Assuming that the inhibitor forms a monomolecular layer on the steel surface at maximum inhibition, the surface coverage (0) at each case is calculated from the results of the corrosion weight loss test. Test is, 8 can be given as follows (Chin and Nobe, 1971): (5) 0 = (WO - w)/(W, - WID) where Wa and Ware the corrosion weight losses of uninhibited and inhibited steels as described in eq 4 and W, is the corrosion weight loss of the given maximum inhibition. Figure 4 shows the plot of C / 0 vs C, where C is the concentration of inhibitor (moles/liter) in solution. The plot gave a straight line with a slope of unity. Consequently, the adsorption of HEDP on the steel was found to be governed by the Langmuir absorption isotherm, i.e., O = K C / ( 1 + K C ) or C/O = 1 / K + C (6) where K is the equilibrium constant of adsorption.
I
-1.2
-10
-0.8
-0.6
-C
E vs. SCElV
Figure 5. Polarization curves of SS 41 in 23 wt 9'0 KzC03 solution saturated with COz containing various inhibitors at 50 "C.
The Zn2+ion (ZnSO,) and sodium molybdate (Na2Mo0,) reagents have a synergistic effect with HEDP in some corrosion reactions (Aramaki, 1976). In order to increase the inhibition efficiency of corrosion, the Zn2+ ion and Na2Mo04were added in the test solution with HEDP, but their efficiencies did not increase by a synergistic effect. It seems that no synergistic effect is influenced by oxygen content and pH of the solution. MEA, DEA, TEA, MPA, Na2Mo04,and sodium silicate (Na2Si03)were also used as inhibitors in this system. Their reagents did not show the good corrosion inhibition effects. Figure 5 shows the polarization curves of SS 41 in test solutions containing various inhibitors. Both the cathodic and anodic current densities decreased in comparison with those obtained in the nonadditive system (blank solution). On the other hand, the only anodic current density in the HEDP system was much smaller than that in the nonadditive system. The rest potential vs immersion time curves for SS 41 in test solutions containing various inhibitors are shown in Figure 6. In the Na2Cr04or V,05 system, their rest potentials shifted to the noble potential direction with an increase of immersion time and kept a constant potential at ca. -0.2 V vs SCE after 5 h. The rest potential of SS 41 in the HEDP system was kept constant from the beginning. From this result, both Na2Cr04and V206inhibitors form passive films on SS 41, depressing the cor-
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1463 I
1
0
IO
20
Time / h
Figure 6. Rest potential vs immersion time curves of SS 41 in 23 w t % K&O, solution saturated with C02a t 50 OC.
/ s'
HO CH, Figure 7. Adsorption model of HEDP (A) and the formation of the iron complex of HEDP (B).
rosion reaction, while HEDP molecules adsorb on SS 41 and form the resistance film having a corrosion protective property. The NazCrOaand V205inhibitors have a passive effect, that is, both have the properties of cathodic and anodic inhibitors. The HEDP inhibitor has an adsorption effect, that is, the property of an anodic inhibitor. In particular, the solved HEDP molecule discharges hydrogen ions in the solution and is negatively charged. The HEDP ion and surface metal ion form the six-membered ring complex of metal and inhibitor on the metal surface. The complex is mainly formed at the anodic point of the steel
surface (Figure 7A) (Saha and Kurmaih, 1986). However, when the concentration of HEDP is higher, a little increase of the corrosion rate is observed. Iron dissolved in the solution, and the corrosion rate increased owing to the acceleration of the complex formation between two HEDP ions and one ferrous ion by excess HEDP ion being dissolved in the solution (Figure 7B) (Ueno, 1978; Sekine and Hirakawa, 1986). The SS 41 surfaces in the initial immersion time were observed in situ by STM (Figure 8). In the system of the test solution without HEDP, the SS 41 surface before immersion was microscopically flat and clean (Figure 8A). The surface after immersion became uneven and unclean (Figure 8B). The degree of unevenness was accelerated with increasing immersion time (Figure 8; A, 0 min; B, 1 min; C, 3 min; and D, 5 min). However, in the system of the test solution containing HEDP, the degree of unevenness on surface was lower than that observed in the system of the test solution without HEDP after each immersion time (Figure 9). The surface after immersion was measured in the bias voltage range 0.3-1.0 V to change the STM image. It is reported that the change of the STM image is observed in the STM images of copper oxide and titanium oxide on metal (Kajimura et al., 1987) and steel corroded in glycolic acid solution (Sekine et al., 1989; 1990). The tunneling current is unstable, and the surface is covered with a semiconductor as an oxidized product (work function ($) < 10 eV) or a solid electrolyte (Kajimura et al., 1987; Sekine et al., 1989). It is considered that HEDP molecules are adsorbed on the surface of SS 41 to form adsorption layer (resistance film). Similarly, in the SEM measurement after immersion for 1 day, the degree of unevenness on the steel surface immersed in the test solution containing HEDP was lower than that immersed in the test solution without HEDP. The ATR spectra in the infrared region were measured for the surfaces of the steels immersed in the test solution without and with HEDP. In the former system without
Figure 8. STM images of SS 41 in blank solution. (A) Before immersion, (B) after immersion for 1 min, (C) after immersion for 3 min, and (D)after immersion for 5 min (bias voltage is 0.3 V).
1464 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
L------.
475
I//
475
, L
O
---7
'
-0
nm
-7 -----------O
475
nm
Figure 9. STM images of SS 41 in a 200 ppm HEDP solution. (A) After immersion for 1 min, (B)after immersion for 3 min, (C) after immersion for 5 min (bias voltage is 0.3 V), and (D) after immersion 5 min (bias voltage is 1.0 V). Table 111. Weight Loss Test of Type 304 in Benfield System at 95 OC inhibiting agent wt loss, corrosion inhibition and its amt mg rate, mdd efficiency, % blank 46.6 3.97 100 ppm HEDP 8.7 0.74 81.4 500 ppm HEDP 10.0 0.84 18.8 0.1 w t % Na2Cr0, 0.6 0.05 98.7 0.1 w t % v*05 1.8 0.15 96.2 3 % DEA' 12.5 1.06 73.3 100 ppm HEDP + 4.0 0.34 91.4 3% DEA" "0 f 4
OC.
HEDP, the peak based on HEDP was not observed in the ATR spectrum, but in the latter system with HEDP, the peaks based on HEDP were observed. Anyhow, the HEDP was adsorbed on the surface of SS 41 to form the adsorption layer (resistance film) and inhibited the corrosion reaction. As for the application of the corrosion weight loss test, the corrosion rates of SS 41 and type 304 were measured with the bench-scale process, which uses the Benfield apparatus in the flowing system under the experimental conditions of high temperature and high pressure to determine the inhibition efficiency of corrosion as shown in Table 111. 7 for SS 41 in various test solutions could not obviously be determined because the data of the corrosion weight loss varied widely. q for type 304 in the test solution containing 100 ppm of HEDP was ca. 80% and approached those values for type 304 in the test solutions containing NazCr04 (concentration, 0.1%; 7 = 98.7%) or V205(concentration, 0.1%; 7 = 96.2%). From this result, the type 304 passive film was broken by the fluid (1.68 m/s) in the pipe, but the NazCr04or Vz05passive film and the adsorption film of HEDP protected the steel from this corrosion reaction. tl in the test solution containing DEA (3.0%) obtained a more effective value compared with that in the blank so-
lution but fairly lower value than those in the test solution containing NazCrOl (0.1%) or V205 (0.1%). In the simultaneous addition with HEDP and DEA, 7 was higher than that in the test solution containing HEDP and was consistent with those in the test solutions containing NazCrO, (0.1%) or V205(0.1%). The inhibition behavior of HEDP was analyzed by the polarization curve and NEP measurements. From this result, it is considered that the formation of the passive film on the type 304 surface was depressed by the addition of inhibitors, but the Cr and Ni of the component in type 304 and inhibitors formed a stable film to sufficiently inhibit this corrosion reaction. In the C 0 2 absorption process, scale or sludge is easily formed by the experimental or operating conditions of pH, Ca2+concentration, temperature, and so on. The black product was obtained as a scale in the solution without HEDP in the practical equipment. From the analysis results for scale, the scale was composed of atoms of Fe, Cr, Ni, K, and Ca. In the solution containing HEDP, scale was scarcely formed and the formation of the scale was preferably inhibited by HEDP. To clarify the mechanism of scale inhibition by HEDP preliminarily, it is assumed that CaC03,which is a typical carbonate salt, is a quasiscale product in this system. The formation process of this scale (CaCO,) and the interaction between this scale (CaCO,) and various inhibitors were analyzed by X-ray and IR methods in the laboratory test to evaluate this process. When various inhibitors (DEA, NazCr04,and VzOJ were added to the test solutions containing Ca2+ion, a white product precipitated. Figure 10 shows the analysis results of the white product by the X-ray diffraction method. Their white crystals were mainly CaCO, crystals of calcite structure (Smith et al., 1968). The numbers and intensity of peaks by the X-ray diffraction method were not changed with the addition of the inhibitors above. The calcite structur is stable. From this result, it is suggested that the formation of CaC03 (scale) is not inhibited by DEA, Na2Cr04,and V205. Amounts of 0-1000 ppm of HEDP
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1465
4000
3000
2000
1600
1200
800
400 7 5 0
Wavenumber I cm-'
30
50
40
60
70
80
28 I degree
Figure 10. X-ray diffraction patterns for CaC03 by adding various inhibitors.
104
'lo
J 202
30
40
Blank
018 116
50 60 2 8 /degree
70
80
Figure 11. X-ray diffraction patterns for CaC03 by adding HEDP.
were added to the test solution containing Ca2+ion. The production of white precipitate decreased with increasing content of HEDP. The white precipitate in each test solution was analyzed by the X-ray diffraction method (Figure 11). In a 10 ppm HEDP solution, the peaks were similar to those obtained in the blank solution. The white precipitate was mainly CaC0, crystals of calcite structure (Smith e t al., 1968). It is suggested that HEDP does not influence the structure of CaCO,. The peaks obtained in a 70 ppm HEDP solution differed from those obtained in the blank solution. The structure of CaC0, changed from a calcite structure to a vaterite one (Smith et al., 1968). In the solution containing more than 200 ppm HEDP, the white precipitate was not already formed. The structure of CaC0, is believed to be amorphous because the peaks of the X-ray diffraction were not seen. To confirm the behavior of HEDP on scale, the white precipitates were analyzed by their IR spectra as shown
Figure 12. IR spectra of CaCO, scales.
in Figure 12. The IR spectrum of the scale produced in the blank solution did not show the peaks based on phosphoryl (P=O), phosphono ((HO),PO), hydroxy (OH), and P-C groups of HEDP. With the concentration of HEDP, the IR spectra show remarkable remarks at 670 (up-c), 960 and 1080 (up+), and 3400 which are based on HEDP. It suggests that the complete crystal structure of CaC0, is not formed by the addition of HEDP and by the interaction between HEDP and CaC03. Anyhow, the addition of HEDP would lead to the inhibition of scale formation based on CaC03. It is reported by Reddy and Nancollas (1973) that HEDP demonstrates a greatly reduced rate of growth of calcite seed crystals and the mechanism involves selective adsorption of the phosphonate onto crystal growth sites, thereby stopping further growth and precipitation. In this system, it is thought that the HEDP disturbs the crystallization and its growth of CaC03 (scale). Anyhow, as with the water-coolingprocess, HEDP would be used as a corrosion and scale inhibitor in the COPabsorption process without pollution.
Conclusions The corrosion inhibition of structural steels, stainless steel (type 304) and mild steel (SS 41), in the C02 absorption process was investigated by various corrosion tests and physicochemical measurements in aqueous K2C03 solution containing HEDP. The conclusions drawn from these results are as follows: 1. The inhibition efficiency (7)of corrosion for SS 41 in the test solution containing 200 ppm of HEDP under the atmospheric conditions was ca. 80% by the corrosion weight loss test, and it was close to those in the test solutions containing Na2Cr04or V205(concentration, 10-"1 w t 70;7 > 99%). 2. The results of the corrosion weight loss and adsorption isotherm corresponded to those obtained by the polarization curve and rest potential measurements. 3. The HEDP molecule adsorbed on the surface of SS 41 obeyed the Langmuir adsorption isotherm obtained by corrosion weight loss. Then by STM and SEM, the adsorption of HEDP molecules on the surface of SS 41 depressed the corrosion. 4. For the application of the bench-scale system, the Benfield apparatus, the 7 value for type 304 in the test solution containing HEDP under high pressure and high temperature was ca. 80-90% and it was close to those in
Znd. Eng. Chem. Res. 1990,29, 1466-1470
1466
the test solutions containing Na2Cr04(98.7%) or V,O, (96.2 Yo j . 5 . For the results of the water-cooling process, HEDP also inhibited the formation of CaCO,, i.e., the production of scale in this process. 6. The use of HEDP results in corrosion and scale inhibitors close in efficiency to those obtained by employing typical passivating inhibitors such as Na2Cr0, or V205in the C02 absorption process. HEDP is adsorbed on the steel surface to form a protective film and depressed the corrosion of steel and the formation of scale without heavy metal pollution. As for the water-coolingprocess, HEDP would be used as corrosion and scale inhibitors in the COz absorption process without pollution. Registry No. HEDP, 2809-21-4; SS 41, 12732-02-4; SUS 304, 11109-50-5; K2C03,584-08-7; COz, 124-38-9: NazCrO,, 7775-11-3; V205,
1314-62-1.
Literature Cited Aramaki, K. Boshoku Gijutsu 1976, 25, 693. Banks, W. P. Corrosion in Hot Carbonate System. Mater. Prot. 1967, 9, 37. Benson, D.; Field, J. H. Chem. Eng. Prog. 1954, 50, 356 and references therein. Bienstock, D.; Field, J. F. Corrosion Inhibitions for Hot-Carbonate Systems. Corrosion 1961, 17, 9. Chin, R. J.; Nobe, K. Electrochemical Characteristics of Iron in H2SO4Containing Benzotriazole. J . Electrochem. SOC.1971, 118, 545. Eickermeyer, G. A. Japan Patent 9806, 1969. Field, J. F.; Beinstock, D. US.Patent 3181929, 1965. Foroulis, Z. A. Stress Corrosion Cracking of Carbon Steel in Hot Potassium Carbonate/Bicarbonate Solution. Boshoku Gijutsu 1987, 36, 689. Foroulis, Z. A. Stress Corrosion Cracking of Carbon Steel in Aqueous Diethanolamine and Monoethanolamine. Boshoku Gijutsu 1989, 38, 9. Gancy, A. B.; Durling, E. U.S. Patent 4116629, 1978. Ikeda, A.; Mukai, S. COP Corrosion Behavior of Carbon and Cr Steels. Sumitomo Search 1985, 31, 9. Johnson, J. J. Corrosion of a Hot Potassium Carbonate COPRemoval Plant Proc Int. Corrosion Forum 1987, 189
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Received for review September 6, 1989 Revised manuscript received February 7, 1990 Accepted February 21, 1990
Study of Eastern and Western Oil Shale Mineral Activity for Hydrodesulfurization Reactions Abolghasem Shamsi US.D e p a r t m e n t of Energy, Morgantown Energy Technology Center,‘ P.O. Box 880, Morgantown, W e s t Virginia 26507-0880 Studies of the hydrodesulfurization (HDS) reactions of sulfur model compounds over eastern and western oil shale minerals have been conducted to provide a fundamental understanding of the heterogeneous catalytic behavior of minerals in oil shale pyrolysis. The products obtained from the reactions of thiophene over eastern low-temperature ashed (LTA) shales were C1-CI hydrocarbons, whereas with western LTA shales higher hydrocarbons, such as unsaturated cyclic compounds of C5and C6, plus a wide variety of alkylbenzenes were detected. The HDS reaction of thiophene over dolomite, siderite, illite, calcite, and a mixture of these minerals was also studied. The HDS activity of combusted spent shales will be discussed. Introduction oneof the goals of the oil shale program is to develop mechanistic models that provide a reliable predictive capability for the efficient and environmentally acceptable METC.
conversion of oil shale to useful fuels. The development of a predictive model requires a fundamental but detailed understanding of ‘‘Primary” and “secondary” chemical reactions, including the role of minerals during oil shale pyrolysis. During the retorting process, shale oil vapors pass over and contact the mineral surfaces, especially on hot-solid recycle processes. Minerals are known to catalyze
This article not subject to U S . Copyright. Published 1990 by the American Chemical Society
