mediate mixture and to a change of 16 mol yo (A3) in the concentration of the Group I classification of mixed compounds in the final mixture. Thus, deficiencies in the properties of the final redistribution mixture may be overcome by a relatively small reduction in the concentration of ligands X and Y in the initial reaction mixture. An alternative approach to modifying the properties of the [ (R0)2PSS]rZnzOH redistribution mixture is by simply increasing the size of ligand 2. Results in Table VI show the effect on properties when the size of the ligand was increased from six to 13 carbon atoms without altering the number of mixed compounds theoretically present in the redistribution mixture and when the size of the ligand was increased from six to eight carbon atoms coupled with a reduction in the number of mixed compounds in the final redistribution mixture. The first such change in size had a beneficial effect, whereas the other had not, a t least, a n adverse effect. Thus, in addition to size and number, structure of the 2 ligand also affected the physical properties of the final mixture. Relative to the four isomeric hexyl radicals, the single n-octyl radical improved the viscosity and pour point but not the oil solubility of the redistribution mixture. Conclusions
I n addition to their theoretical interest, compound redistribution mixtures of [ (R0)zPSS I8ZnzOHand [(RO)zPSS]*Zn offered improvements in physical properties which were not possible with the individual compounds. As a consequence, certain proportions of the limiting ligands could be incorporated to reduce the average molecular weight without adversely
affecting viscosity, pour point, oil solubility, and the crystallization tendency. The properties of such mixtures depended on a number of structural features and correlated with the concentration of the limiting mixed compounds calculated from the multinomial theory. Acknowledgment
The author is indebted to Richard L. Postles for helpful discussions and to Stewart H. Wiggins, Lane V. Papannou, and Joseph N. Skwish for help in the numerical calculations. literature Cited
Bacon. W. E.. Bork. J. F.. J . Ora. Chem.., 27., 1484 - f1962). ~ - - . ~ Calingaert, G:, Beaity, H.‘A., “0“rganic Chemistry, An Advanced Treatise,’’ Vol 11, p 1806, Wiley, New York, N.Y., 1943. Elliott, J. S., Jayne, G. J. J., Brazier, A. D. (to Castrol, Ltd.), U S . Patent 3,595,792 (July 27, 1971). Feller, W.. “Probability Theorv and Its Amlications.” _. , D. 33. Wilev. New York. N.Y.. 1950: Higgins,’ W. A., LeSeur, 51. W. (to Lubrizol Corp.), U.S. Patent 3,000,822 (September 19, 1961). Kendall, M. G., “The Advanced Theory of Statistics,” Vol I, 5th ed., p 126, Griffin, London, England, 1952. Kosolapoff, G. M., “Organophosphorus Compounds,” P 236, Wilev. New York. N.Y.. 1950. Lockhiit, J. C., “Redistribution Reactions,” pp 6-30, Academic Press, New York, N.Y., 1970. Lowe, W. (to Chevron Research Co.), U.S. Patent 3,428,563 (February 18, 1969). Moedritzer, K., Advan. Organometal. Chem., 6, 171 (1968). Wiese, H. F. (to Lubrizol Corp.), U.S. Patent 3,446,735 (May ~
27.I 1969).
Wystrach, V. P., Hook, E. D., Christopher, G. L. M., J. Org. Chem., 2 1 , 705 (1956). RECEIVED for review May 3, 1972 ACCEPTEDJuly 29, 1972
High-Temperature Corrosion in Gas Turbines and Steam Boilers by Fuel Impurities I. Measurement of Nickel Alloy Corrosion Rate in Molten Salts by Linear Polarization Technique Walter R. May,l Michael J. Zetlmeisl, and lewis Bsharah Corporate Research Laboratories, Petrolite Corp., St. Louis, X o . 65119
Robert R, Annand Tretolite Division, Petrolite Corp., St. Louis, Mo. 63119
T h e demand for greatly increased amounts of energy has forced utilities and other large-quantity users of fossil fuels to explore low-quality fuels for use in steam boilers and gas turbines. Fuels such as unrefined crude oil and residual oil contain large amounts of impurities which result in corrosive deposits in the equipment. Two of these impurities, sodium and vanadium, form catastrophically corrosive, low melting slags t h a t can destroy a vital part in a matter of hours. To whom correspondence should be addressed. 438
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
Development of techniques which inhibit deposit formation, reduce corrosiveness of the slags that do form, and allow higher temperature (and more efficient) operation has required the procurement and study of many data from actual operations and laboratory equipment. These data are usually difficult to obtain and have sufficient variability to require statistical treatment. There has been a great need for a rapid method of studying the interaction of a potentially corrosive slag with a n alloy and of obtaining a reliable evaluation of this interaction. We have adapted the linear polarization technique for this
The linear polarization technique was adapted to instantaneous measurement of the corrosion rate of high nickel alloys a t 750-900°C in molten salts, corresponding in composition to some slags found in gas turbines and steam boilers. A three-electrode cell was used with test and reference electrodes constructed of the alloy of interest. An evaluation of the Stern-Geary relationship and polarization curves for the system made b y standard techniques indicated that a straightforward rate measurement could b e made with the linear polarization technique. The PetroliteTM corrosion rate meter, a commercially available instrument that employs this technique, was used for further measurements. Comparisons between electrochemical rate measurements and weight loss rates verified the applicability of the method, Data from previous investigations compared favorably with these results.
purpose. This technique involves the electrochemical measurement of the corrosion current a t a fixed potential with a threeelectrode cell. The use of the linear polarization measurements to obtain corrosion rates of metals in aqueous systems has been welldocumented (.innand, 1966; Paul and Shirley, 1969; N a r t i n e t al., 1971; Marsh, 1963). To our knowledge, there has been no published work on extension of the linear polarization method to measurements in molten salt systems, although corrosion currents have been measured for iron and several alloys in molten carbonates (Davis and Kinnibrugh, 1970) and sulfates (Baudo et al., 1970) with standard polarization techniques. The adaptation of the linear polarization method to the measurement of t,l-iecorrosion rate of alloys in molten salts similar in composition to deposits resulting from use of lowquality fuels is described in this paper. Experimental
Equipment. An electrochemical cell was constructed as follows. A Thermolyne Type 2000 furnace was modified by drilling '/.,-in. holes through the top and insulation to match those in the top heating element. Rods of the alloy '/8 in. in diameter and 12 in. in length were inserted through two holes to serve as the test and reference electrodes. Platinum wire (16 gauge) was extended through the third hole, bent, and inserted into the melt to form the auxiliary electrode. The rods and wire were insulated from the heating element with ceramic thermocouple insulators. The rods were coated with a castable ceramic material for a distance of about 3 em beginning about 1 cm from the end of the rod. This coating serves three purposes: i t prevents necking of the rod a t the melt-air interface; it holds the electrode surface area const'ant by coating the region that would be covered b y creeping melt; and i t serves as a support for the insulating tubes. The cell container was a 50-cc porcelain crucible. The inexpensive porcelain crucible used a limited number of times was preferable to more expensive alumina and quartz crucibles which were difficult to clean and deteriorated rapidly. We could not detect any contamination of the melt from the porcelain. An aLnotrolTlfModel 4100 potential controller was used to develop the potential for measurement of the polarization curves. The potential being produced was measured with a Keithley Model 200B voltmeter. A Model 103 PetroliteTM corrosion rate meter from the Petreco Division, Petrolite Corp., was used for the instantaneous rate measurements. The principles of this unit have been described in detail by Annand (1966). Materials. UdimetT'I 500 and 700 rods in. in diameter by 12 in. in length were obtained from Met-Cut Research Associates in Cincinnati, Ohio. These were used as the test and reference electrodes.
Cdimet'TM 500 and 700 are high-temperature, high-nickel content alloys produced by Special Metals Co. of L-tica, K.Y. These two materials were chosen because they represent extremes in corrosion resistance for high-nickel alloys. These are well-known materials of gas turbine construction, and their corrosion characteristics have been well-documented in the field (Bieber and Slikalisin, 1970). All materials used to prepare the melts mere reagent grade chemicals obtained from Fisher Scientific Co. The castable ceramic materials used to coat the rods was Ceramacast 505 obtained from Aremco Products, Inc., of Briarcliff Xanor, X.Y. 10810. Results
For a convenient measurement of corrosion rate, the SternGeary relationship should be followed in the region of the polarization curve where the polarized potential is close to the corrosion potential (Stern and Geary, 195'7; St'ern, 1968). This relationship is defined by:
AE
PllPC - -AI 2.3 I c o m ( P a
+
Pc)
where
AE/ A I
=
polarization resistance
Pa
=
Tafel slope of anodic polarization curve
13, = Tafel slope of cathodic polarization curve
I,,,,
=
corrosion current
The PetroliteT" corrosion rate meter uses the more convenient inverse of the polarization resistance, A I / A E , known as the
1
/
1201
20
O
II
0
I/
'bC/2,
io
sb
I00
I20
I40
160
180
2b0
A I (rna/crn2)
Figure 1. Linear polarization curves for UdimetTJI 500 and 700 in V2Oj at 750°C Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
4, 1972 439
100,
I
600500'00-
300-
-2 --.E
200100-
0-
(Y
0
-100-
-200-
-300-400-
-500-600-
-7007 01
U d i m e t T M 5 0 0in VzOo
- 750' - -- 900' 40
Gravimetric ( x ) (rngcm-'hr"l
0
,
do0
00
-
Figure 3. Weight measurement correlations for UdimetTM
- d o 0
500
Figure 2. Polarization cuwes for UdimetTM 500 at several temperatures
polarization admittance. A linear plot of current density vs. potential for UdimetTM500 and 700 is given in Figure 1. The data follow a straight line through about 40 mV. This indicates that electrochemical processes are predominant and that measurements can be made a t 10 mV as called for by the technique used. Polarization curves for UdimetT" 500 in VzOj a t three temperatures are given in Figure 2. The linear regions approximate one decade, a length convenient for extrapolation of the corrosion current from the polarization curve. The Tafel slopes are 270, 320, and 340 mV/decade on the cathodic side and 470, 630, and 560 mV/decade on the anodic side a t 750°, 800°, and 900°, respectively. Similar results were obtained for UdimetTM700. Comparison of values determined from a polarization curve with those from the Corrosion rate meter was good. The Model 103 PetroliteTMcorrosionrate meter is capable of providing a potential of u p t o 100 mV and is calibrated to read directly in mils per year the corrosion rate of an iron surface with an area of 9 cm2. The meter reading was converted to indicate correctly the corrosion current for I'dimet 500 and 700 by the following calculations. An equivalent Tveight was estimated for both alloys by using published elemental analyses and by considering that Co, N o , Si, and T i oxidize to the + 2 state, and d l , Cr, and Fe oaidize to the +3 state. I n this manner, 26.991 and 27.372 were calculated for UdimetThr 500 and 700, respectively. The meter reading was converted to corrosion current for the alloy by the following calculation:
9.0 em2 iiiA/cm2 X 0.0055 Lilloysurface area NPY ~
Factor (includes all constants) X Meter reading(1lPY) Alloy surface area
=
mA/cm2 The factor for VdimetThr500 is 0.04384 and for 700,0.04446. If the corrosion rate meter is not measuring extraneous 440 Ind. Eng. Chem. Prod.
Res. Develop., Vol. 1 1 , No. 4 , 1972
0
Gravimetric (a1 ( r n g ~ m - ~ h r - ' l
Figure 4. Weight measurement correlations for UdimetTM
700
electrochemical processes, there is a n implied correlation between the electrochemical measurements and rates of weight loss obtained by directly weighing the electrode. Corrosion rate meter measurements were taken on a n electrode a t a fixed temperature every 5 mill for 15-60 mill (depending on the rate of weight loss), and a weighted average of the readings was taken to calculate the weight loss. I n the case of the two highly inhibited slags, the rates were averaged over 19.75 hr for the Sa2S@4-9j~IgS04-4Vz05 slag and 67 h r for the 0.2l';a2S04-94.8hIgS@4-5~~zO~ slag. This method of averaging the readings was used to minimize changes in corrosion rate, errors owing to changing melt characteristics, and minor temperature variations. At the end of the run, the rods were weighed after cleaning. We experimented with cleaning techniques and found that a combination of dipping in molten equal weight mixture of caustic soda-soda ash with cathodic polarization followed by light abrasion with KO.400 Emery clot'h to remove remaining slag, ceramic coating, and oxide gave the best results. The shape of the rods suited them ideally for placing in a n electric drill and turning while abrading them with the Emery cloth. After the weight loss was determined by weighing, it was divided by the time and surface area. The average corrosion
Table 1. Comparison of Corrosion Rates Measured Electrochemically a n d Gravimetrically Slag composition-mole ratio
vzos
vzo5
Na20-Vz05
Temp,
O C
Gravimetric wt loss rate, mg cmJ hr-1
Electrochemical wt loss rate, mg cm-2 hr-1
Ratio of gravimetric to electrochemical
750 800 800 900 900 800 900 900 900 800 800 900 800 900 800 900 800 900
UdimetTM500 27.5' 51. la 18.1 82. 3' 64 28.1 137 0.837 0.057 12. 1' 4.02 25.8 3.56 5.4 7.0 34.8 9.7 20.2
28.7 64.5 36.4 138 90.4 84.7 82.0 0.752 0.043 6.71 10.0 10.3 5.17 6.62 24.2 60.8 8.32 11.5
0.96 0.79 0.50 0.60 0.71 0.33 1,67 1.11 1,33 1.80 0.40 2.51 0.69 0.82 0.29 0.57 1.17 1.75
750 800 850 900 950 750 750 800 850 950 950 750 750 800 800 850 850 900 950 1000 750 850 950 950 800 900 1000 800 850 900 800 900
UdimetTM700 21.8 45.4 51.6 94.5 150 19.6 21.0 61.4 132 206 344 6.34 6.67 9.34 7.43 7.7 17.7 9.1 12.1 28.2 29.9 50.5 51.0 65.2 30.5 17.1 20.4 14.6 20.9 18.3 14 29.5
12.11 58.7 80.7 104 113 37.6 45.5 70.2 94.9 73.9 94.2 9.16 7.39 8.08 9.75 7.33 6.48 14.5 15.1 15.5 32.1 36.5 36.1 39.2 8.94 12.1 15.8 16.9 14.5 19.8 27.8 57.2
1.80 0.77 0.64 0.91 1.32 0.52 0.46 0.87 1.39 2.79 3.65 0.69 0.90 1.16 0.76 1.05 2.73 0.63 0.80 1.82 0.93 1.38 1.41
1.66 3.41 1.42 1.29 0.87 1.44 0.93 0.50 0.52
a Cleaned in molten caustic-sodium carbonate with cathodic protection followed by minimal abrasion. All others were cleaned totally by abrasion.
rate was converted t o weight loss by use of the procedure described above. The data are presented in Table I, and the correlations are presented 111 Figures 3 and 4. Corrosion rate measurements were made with L-dimetTA' 500 electrodes a t several temperatures on two slags with known corrosion characteriitics. These data are compared with the measurement on V20s in Table 11.
Discussion
The data presented in Table I indicate reasonable agreemerit betlyeen the electrochemical and gravimetric measurements. ii linear regression analysls of the data \vas made. The best fit line Ivith 9570 confidence limit (cl) for UdlmetThT500 is presented in Figure 3. The expected correlation with a slope of unity fell within the 95% cl. The fraction of variance reInd. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 4, 1972
441
Table II. Corrosion Rates (mg Naps04
Mole ratio MgSOa
0 14.3 1.23
0 0 93.21
hr-l) for UdimetTM 500 in Some Slags of Interest
Wt ratio Mg/V Na/V
VnOs
0 0
100 85.7 5.56
4/1
0 1/6 O.l/l
movedwas 0.557, indicating 99% confidence in the data fitting a straight line. The best fit line with the 95% cl for UdimetTR' 700 is presented in Figure 4. The expected correlation with a slope of unity fell'within the 95% cl only after omitting the data t,aken above 900°C. The fraction of variance removed was 0.759, indicating 99% confidence in the data fitting a straight line. This information indicates that the test correlation decreases above 900°C wibh UdimetThT500 and 700 electrodes. One possible cause of this is intermittent loss of electrical contact because of bubbling and calcining melts. This would increase the resistance effect on the measurements and make the corrosion rate measurements a t high temperatures low relative to weight loss. This is the observed effect. Oxidative instability of the alloys is also a possible cause of poorer correlation a t higher temperature. There are few data in the literature on corrosion measurements of high-nickel alloys in a crucible test that can be compared directly with t'his work. A 42 mg em-2 hr-1 corrosion rate for XmonicTM80-1 in Tr20j-Na20 a t 800°C was reported by Macfarlane (1963). This compares favorably with 35.4 and 40.2 mg hr-l a t 750" and 800°C for L-dimetTM 700 measured in this work. Ot'her workers (Annand, 1966; Stern, 1955) dealing with aqueous systems have gott'en better agreement than we have demonstrated between t'he gravimetric and electrochemical data. However, they examined a small temperature range, no variations in corrosion medium, and systems with much lower corrosion rates. A discussion of several of t'he factors that can influence the correlation between electrochemical and gravimet'ric data follows. Temperature. The Tafel slopes of the polarization curve are expected to vary directly with temperature according to the equation dA@
-
2.3 RT
o=dlogi-aF where
p
=
Tafel slope
A@ = polarization potential
i T LY
= = =
I; =
current temperature reaction order Faraday const'ant
Variations of Tafel slopes with temperature were calculated by use of the proportionality indicated in the above equation. The calculated and experimental values are compared below for L-dimetT" 500 in J i ~ O s : Temp
7 50 800 900
7 50 800 900 442
Found, mV/decade
Cathodic 270 320 340 Anodic 420 630 560
Expected
... 283 309 ...
492 538
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 4, 1972
Temp, OC 700
750
800
850
900
950
.. 55.2 ..,
28.7 67.9
64.5 73.1 0.0789
...
90.4 120 0.247
138 0.444
,
...
106 0.158
...
On the cathodic side the slope for 800°C was 12y0 and a t 9OO"C, 9% greater than expected from the variation with temperature. On the anodic side the 800°C slope was 23%, and the 900°C slope 4% greater than calculated. A large portion of the change in slope can be attributed to that expected from the temperature coefficient of the Tafel relationship. The change in slope beyond that expected from temperature variation could be owing to a change in mechanism. However, we believe that it can also be accounted for by the fact that the corrosion rate is extremelyrapid and the electrodQsurface area changes during the measurement of the polarization curve. As the surface area decreases, the mA/cm2 term tends to become low because the surface area value is larger than the true value. Therefore, the true Tafel region would be extended further out, and the slope would be less. I n consideration of the overall accuracy of the data, we felt that a temperature correction was not justified. Slag Composition. ri wide range of slag compositions was examined. There is little reason to doubt that the cathodic reactions in a slag of pure V20j would be quite different from those in a slag containing Na2S04, JIgSOd, and V Z O ~These . slags containing carbonates and sulfates were also susceptible to calcination which occurred a t higher temperatures. Calcining slags gave erratic measurements, probably owing to intermittent loss of electrical contact in the slag as mentioned above. Cleaning Procedures. Cleaning of the electrodes was the most subjective step in the measurements and was a large source of error. We first experimented wit'h removal of the slag wit'h abrasion. This worked fairly well since the slag was much softer than the alloy, and the rods could be burned with a n electric drill to facilit,ate the abrasion. This technique had the disadvant'age of either not removing all the slag or too much metal. We tested excessive abrasion and found that' by doubling the normal abrasion time, a n additional 5% weight loss was found. We compared this technique with dipping in molten caustic soda-soda ash with cathodic polarization. This resulted in removal of the slag, but the electrode was covered with an oxide layer which was easily removed wit'h abrasion. The latter method seemed preferable since it necessitated less abrasion. The measurements involving cleaning by this method are nobed in Table I. Surface Irregularities and Surface ilreas Measurement. Every possible precaution was taken to avoid irregular surfaces. However, after the electrodes had been used a few times, the surface tended to become slightly pitted and irregular. This added error into the surface area measurement. The exposed surface was about 1 cm2. With this small a measurement (even though considerable care was taken in measuring the rods), a small error in measurement would result in a large error in the surface area. Another surface area problem is associated with the fast corrosion rate. The surface area changed significant,ly within a few minutes, particularly a t the higher temperatures. Jlechanism Changes with Temperature. The alloy becomes unstable a t high temperatures owing to air oxidation. This becomes the dominant source of weight loss above 900°C.
Rahmel (1969) reported on attempts to measure corrosion currents of several metals and alloys with polarization techniques. H e had little success and attributed his difficulties to the high conductivity of v2os. Manakov and coworkers (1962) measured the conductivity of V205and found 0.25 and 1.83 ohm-’ cm-l a t 600’ and 1000°C, respectively. T h e work presented here indicates that the high conductivity of V2O5 has little or no effect on the ability to measure polarization curves or corrosion currents. There has been considerable development of the mechanism of vanadic corrosion (Pantony and Vasu, 1968a,b,c;Greenert, 1972a,b; Brasunas and Grant, 1952; Halstead, 1970; Monkman and Grant, 1953). Although the factors controlling the kinetics are rather complex (ion mobility, melt conductance, and formation of oxygen radicals, among others), the basic mechanism which results in metal loss is a simple oxidation reaction involving the metal and oxygen. The data presented here support this mechanism and demonstrate that the corrosion can be followed with a straightforward electrochemical measurement. Nonelectrochemical processes such as dissolution of the metal in the elemental state are eliminated as major factors. Magnesium and sodium removals are well-known techniques for controlling high-temperature corrosion (Foster, 1970). The melt compositions were chosen with varying amounts of sodium and magnesium to resemble deposits that might be found in a gas turbine or steam boiler. Several observations were made on these data that indicate t h a t they are consistent with known corrosion characteristics of various slags. Pure VzOS and Na2SO4-MgSO4-V~O5gave comparable corrosion rates in the 750-950’C range. T h e h’az0-hIg0-Vz05 composition gave rates almost a n order of magnitude lower than the Vz05 and Na2SO4-hIgS04-VZ05 compositions, a n observation that agrees with many other data. The carbonate compositions were in the same range as the oxides, probably owing to calcination of the carbonates to the oxides at the testing temperatures. The NazO-VsOs composition, with the exception of the measurement at 750”C, gave values lower than pure V20s. This observation is consistent with the data of Niles and Sanders (1962) who found the Na20-Vz05 to be slightly lower than VZOS,whereas materials such as h’’az0-3VzOj and Na20-6V205 were several times more corrosive than V20S. Since we are interested in applying this method to inhibited systems t h a t corrode a t rates several orders of magnitude lower than uninhibited ones, we have verified the method over a wide range of corrosion rates. I n Table I1 corrosion rates of three slags of well-known corrosion characteristics are compared at several temperatures. Cunningham and Brasunas (1956) and Buckland e t al. (1952) have reported that mixtures of Na2S04and VZO5with the Na2S04 in the 15-2074 range are more corrosive than either pure V205 or Xa2S04. This observation was verified in the data presented in Table 11. LTp to 900°C, the Ka2S04-6V206was more corrosive than V205, but they became approximately equivalent at 900°C and above. For a highly inhibited slag, we chose one with a 4/1 wt ratio of magnesium to vanadium and a O . l / l wt ratio of sodium to vanadium. The data in Table I1 demonstrate that the corrosion rate in this slag is a factor of over l o 3less than in pure VzOS, as expected from the earlier burner test, crucible test, and field data.
Conclusions
Considering all the factors that can influence the correlation, we feel that the agreement we have obtained between the gravimetric and electrochemical measurement is acceptable. Reduction of corrosion rate by several orders of magnitude is expected when efficient inhibitors are employed. We believe that this technique has been adequately demonstrated for screening of materials with a large range of corrosion inhibitor characteristics. The method compares favorably with techniques such as the crucible test and the various types of laboratory burner test rigs. T h e rate data obtained with this technique confirmed earlier data comparing the corrosive characteristics of CdimetTM 500 and 700 as well as the relative corrosiveness of several slag compositions. The corrosion rates for UdimetTM 500 were lower than UdimetTM700 a t comparable temperatures in the same slags. This was expected from previous data on these alloys which indicated the superior corrosion characteristics of UdimetTM 500. The technique appears likely to be applicable to other highstrength, high-temperature alloys of interest. Work in progress in this laboratory with high-cobalt alloys indicates this to be true. Acknowledgment
T h e authors gratefully acknowledge the guidance, encouragement, and help by R. W. Greenlee and E. C. French of the Tretolite Division and R. N . Lucas of the Petreco Division of Petrolite Corp. Literature Cited
Annand, R. R., Corrosiun, 22, 215 (1966). Baudo, G., Tamba, A., Bombara, G., ibid., 26, 193 (1970). Bieber, C. G., Mikalisin, J. R., International Nickel Co., Inc., Suffren, N.Y., Tech. Paper 695-T-OP, November 11, 1970. Bornstein, N. S., DeCrescente, M. A., Corrosion, 26,209 (1970). Brasunas, A. des., Grant, N. J., Trans. A S M E , 44, 1117 (1952). Buckland, B. O., Gardiner, C. M., Sanders, D. G., ASME Paper NO. 52-A-161, 1952. Cunningham, G. W., Brasunas, A. des., Corrosion, 12,3894 (1956). Davis, H. J., Kinnibrugh, D. R., J . Electrochem. SOC.,117, 392 (I970). Foster, A. D., General Electric Gas Turbine State of the Art Engineering Seminar, Saratoga Springs, N.Y., September 1970. Greenert, W. J., Corrosion, 18, 57t (1962a). Greenert, W. J., ibid., 91t (1962b). Halstead, W. D., J . Inst. Fuel, 43, 234 (1970). Macfarlane, J. J., “The Mechanism of Corrosion by Fuel Impurities,” H. R. Johnson, D. J. Littler, Eds., p 261, Butterworths, London, England, 1963. Manakov, A. T., Esin, 0. A., Lepinskikh, B. M., Dokl. Phys. Chem., 142, 171 (1962). Marsh, G. A., Int. Congr. Metal. Corros., Proc. bnd, p 936, New York, N.Y., March 11-15, 1963. Martin, R. L., Annand, R. R., Wilson, D., Abrahamas, W. E., Mater. Prot. Performance, 10 (12), 33 (1971). Monkman, F. C., Grant, N. J., Corrosion, 9, 460 (1953). Niles, W. D., Sanders, H. R., J . Eng. Power, 84, 178 (1962). Pantony, D. A., Vasu, K. I., J . Inorg. ~ V u c l .Chem., 30, 423 (1968a). Pantony, D. A., Vasu, K. I., ibid., 433 (196813). Pantony, D. A,, Vasu, K. I., ibid., 755 (1968~). Paul, R., Shirley, W. L., Mater. Prot., 8 (I), 25 (1969). Rahmel, A., “Mechanism of Corrosion by Fused Salts,” European Federation of Corrosion, Dusseldorf, Germany, April 10, 1969. Stern, J., J . Electrochem. SOC.,102, 663 (1955). Stern, M., Corrosion, 14, 60 (1968). Stern, I f . , Geary, A. L., J . Electrochem. SOC.,104, 56 (1957). RECEIVED for review hIay 5, 1972 ACCEPTED August 16, 1972
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
443
