1436
INDUSTRIAL AND ENGINEERING CHEMISTRY
the material being studied. In this way the replica will serve as an internal standard of calibration. Figure 12 illustrates the possibilities of this method of determining precise linear dimensions. This procedure of using an internal standard will entail some slight loss of resolution because of the introduction of chromatic aberrations by the replica film, which must be somewhat thicker than the usual thin films. However, for objects which are far above the limit of resolution of the electron microscope, the chromatic aberration can be neglected.
Vol. 34, No. 12
When low magnifications are used, it has sometimes been found difficult to focus the images on the fluorescent screen. A low-power telescope (2.5 diameters) focused on the viewing screen has proved helpful in arriving at the optimum focus. As Figure 13 shows, the telescope has been inserted through one of the observation ports of the electron niicroscope. The optical system of the telescope is such that the fluorescent screen is viewed almost normally. The periscope arranged to view the intermediate image has also been of help in orienting specimens.
Corrosion of Metals and Alloys by Flue Gases J
LOUIS SHNIDMAN AND
J
JESSE S. YEAW Rochester Gas and Electric Corporation, Rochester, N. Y.
F
LUE products, resulting from the combustion of most industrial fuels, exhibit a surprising activity with respect to the corrosion of materials with which they come into contact from the time they leave the burner or fuel bed until they are finally discharged into the outer atmosphere. The presence of the oxides of sulfur has generally been justly blamed for much of the difficulty (1-11). The suggestion has been made that the oxides of nitrogen, which always occur in small quantities in the products of combustion, catalyze, accelerate, or otherwise promote the activity of these vapors (11). A field study (2) conducted recently, in which a total of 783 domestic units in ten cities was inspected, indicated that: 1. The condensation of vapors, which are formed during the combustion of fuel gases containing sulfur, results in the corrosion of combustion chambers and flue pipes. 2. Other constituents of the products of combustion do not appear to be significant from a practical standpoint with respect to corrosion. 3. Sulfur must be reduced to less than 2 grains per 100 cubic feet (4.6 grams per 100 cubic meters) to eliminate corrosion problems in the field (Figure 1).
Section of Coke Conueyor
It has been demonstrated that the elimination of sulfur from fuel gases does not necessarily eliminate corrosion ( I I ) , but the field survey (9)did reveal that corrosion by sulfurfree gases would not create serious field problems. The complete removal of sulfur from industrial fuels is not practicable a t present. The substitution of corrosion-resistant materials for those which are susceptible is one possible method for overcoming this corrosion problem, and the testing of certain metals and alloys in flue gas atmospheres is the subject of the following discussion.
December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1437
The corrosion behavior of metals and alloys was studied when exposed to flue gases. Stainless steel (18-8) showed practically no corrosion. Other metals and alloys, such as aluminum, iron, copper, lead, etc., were corroded more or less rapidly from the time of exposure. No chrome-nickel steel was attacked appreciably during the first year, and only a few samples were attacked within several additional months. After 18 months all but three of twenty-two samples showed severe attack that resulted in perforation of some test strips within a few months of the first signs of corrosion. A sample containing 17 per cent chromium and no nickel was attacked first; increase of chromium content to 27 per cent delayed but did not prevent severe corrosion. The 18-8 chrome-nickel steels have a wide range in susceptibility depending upon their source and previous treatment. The low carbon content of 18-8-S samples apparently had no i d u e n c e on the eventual rate of corrosion. Fractions of titanium and columbium did not impart resistance to 18-8 steels exposed to flue gases two years, whereas molybdenum did. The 25-12 chrome-nickel steel showed low susceptibility; an increase of nickel to 20 increased the rate of the attack. The expected lives of 20-gage metals and alfoys in contact (one side) with flue gas from combustion of manufactured city gas are tabulated.
0
Battery of Coke Ovens
Corrosion Apparatus The testing of possible corrosion-resistant substances in the laboratory has been approached in many ways. Generally, the practical value of the results depends t o a considerable degree upon how closely the test procedure coincides with the actual application. For this reason the apparatus shown in Figure 2 was chosen for the testing of various metals and alloys for their susceptibility to attack by flue gas vapors. A pair of corrosion chambers is shown in Figure 2. The combustible gases were passed through pressure regulators and burned on Electrolux refrigerator burners. These burners were equi ped with snap-action valves that shut off the supply of gas i f the flame should be accidently extinguished. The flames burned inside Pyrex tubes, and the excess air was controlled by a variable opening through which the burner heads were thrust. The hot flue products were conducted through a short, water-cooled tube made up of 3/4-inch pipe fittings (stainless steel fittings were substituted in later tests) and into the corrosion chamber
0
0
under a baffle plate as shown. The products were further cooled to the condensing point inside the corrosion chamber by the inverted bottles through which cold water was circulated. Test stri s were suspended on glass hooks from a circular d?sk in the to of the corrosion chamber. The disk was kept in s i w motion by means of a reduction gear and motor. This movement kept the vapors in the box in constant motion, and the fumes were therebv evenlv distributed to all parts of the test stri-s. Excess gases escaped through the hole in t l e roof of the chamber as shown. The rogress of the test was observed through the front of the box wbch WBS closed by ordinary window glass. The de ree of condensation was re ulated by increasing or decreasing &e combustion rates and t8e excess air supply. The adjustment was made so that a foglike mist filled the interior of the corrosion chamber which resulted in a copious precipitation on the test strips. Once started, the test ran continuously day and night for the entire test period with only occasional attention.
Samples Tested Two series of tests were made. The first shows the effect of corrosion upon a variety of metals and alloys by the flue gas vapors from four combustible gases of different composition hnd sulfur content. The second series shows the effect of corrosion upon a group of alloy steels containing chromium and nickel and commonl referred to as stainless steels. The combustible gas used in txe latter series of tests was an ordinary manufactured city gas mixture. Table I summarizes the analyses of the combustible gases.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1438
of the samples in the flue gas atmospheres. But contrary to most of the other metals, the rate of Series A B corrosion was decreased as the sulfur content of Test No. 1 2 3 4 5 the fuel gas increased from 9 to 343 grains per 100 Type of gas h-atural Odorized Purified Foul Purified natural mfd. mid. mfd. cubic feet. The attack appeared to progress by Average analysis, % b y vol. a pitting action, but the rate of loss of metal was CO* 0.0 0.0 2.9 3.1 2.9 Illuminants ... ... 3,5 3.6 3.5 so high that the sample strips were reduced 0 1 0.0 0.0 0.4 0.4 0.2 co ... 10.8 10.3 12.0 to almost paper thinness before actual penetraHZ ... ... 42.9 44.3 46.8 tion occurred. CH4 96.9 96.9 27 . 2 26.3 25.7 C2He 2.7 2.7 ... ... ... Chrome steel reacted quite differently. The CsHa 0.4 0.4 ... ... SZ 0.0 0.0 12.3 12.0 8.9 corrosion losses were small in the low-sulfur gases B. t. u./cu. f t . 1047 1047 540 551 539 but increased sharply in the high-sulfur flue Sp. gr. a t 60' F. 0.575 0.575 0.512 0.519 0.471 Grains per 100 cu. ft. products. Furthermore, pitting action was the Ammonia a s S H J None None 5 100 2 chief factor in the failure of this alloy. Cyanogen a s H C N None Sone None 30-50 None HIS as S h-one Kone None 330 Pione The corrosion of the stainless steel alloy was 0.001 max. 0.4a 9 13 9 Organic sulfur as S negligible in this series of tests. The corrosion a T h e volume of flue gases resulting from t h e combustion of natural gas is about double of this and similar stainless steel alloys is int h a t for the same quantity of manufactured g R 8 ' t o be strictly comparable t h e results f o r the sulfur oontent of these fuels should be oohrected. Thus t h e sulfur 'oontent of t h e cluded in series B. However, the sample inodorized natural gas should be considered a s 0.2 grain/100 0;. ft. t o be comparable with the 9 and 343 grains a s given for t h e manufactured gases. eluded in series A was merely stained after exposure to the flue products from foul gas containing hydrogen cyanide, ammonia, and 343 grains of sulfur per 100 cubic feet after A list of the metals and alloys, together with their respective 13-month exposure. analyses as given by the manufacturer, is presented in Table 11 A~~~~~~~ G ~ ~h~~corrosion ~ rate ~ for the , aluminurns (series A) and in Table I11 (series B). increased rapidly as the sulfur content of the fuel gas inSample strips 1.5 x 7 inches (3.8 x 17.8 cm.) were cut from creased. The corrosion proceeded more or less uniformly the 20-gage (0.032-inch or 0.81-mm.) sheets submitted for this test work. Rough edges and sharp corners were smoothed, and Over the surface, but there lvere midely scattered pits in evidence, and the failure of this group of metals was accelerated by this Pitting action. Alloying Of aluminum with other samples in series A were removed from the corrosion chambers metals decreased its resistance to. corrosion, apparently by every month and the corrosion loss was checked. The samples in series B were allowed to remain for longer intervals. Tenaincreasing the pitting action. cious corrosion products TTere removed by brushing with a stiff coppER G ~ This~ group ~ of metals ~ . shovved a marked wire brush. Possible losses due to this treatment were fully increase in corrosion rate as the sulfur cont,ent of the fuel gas but it seemed to be the most method to Bccomplish the desired end. increased, but t,he action mas found to be relatively uniform; TABLE I. ANALYSESOF COMBUSTIBLE GASES
-
7
. . t
I
~
~
s
~
~
~
~
~
~
.
.
~
~
~
~
~
~
~
~
e
~
b
Results for Series A The samples in series A (8, 9) were removed once each month, and the loss by corrosion was determined. With the exception of the stainless steel, the corrosion of the metals and alloys began upon exposure t o the flue gas vapors and proceeded rather uniformly throughout the entire test period. The averaged result^ in terms of loss by weight and by volume (depth of corrosion) are included in Table IV. In general, the texture of the corrosion products was finer and adhered more closely to the strips as the sulfur content of the combustible gas decreased. This did not necessarily indicate that the depth of the corrosion would be correspondingly less, as shown by the results for iron and lead, although the corrosion rat'e of most of the samples increased greatly as the sulfur in the fuel gas was increased. FERROUS GROUP. As might be expected, black iron showed by far the most immediate and severe corrosion of any
&' Grains of
(ru/fur per /OO
TABLE11. ANALYSESOF METALSAND ALLOYS(SERIES A), Type of Metal Ferrous group Black iron Chrome steel Chrome-nickel steel Aluminum group Aluminum A1-Mg alloy A1-Cu alloy' Copper group Commercial copper Silicon bronze b Si-Mn bronze 85-15 brass Others Commercial lead Commercial zinc a Aluminum coated.
Cr 16:OS
19.17
...
0.25 .
.
I
... ..... . ...
...
... b
Ki
A1
Cu
0:267
... ... ...
... ... ...
.. , ... ...
99 97.25 95.0
4.0
... ... ... ...
... .. , ... ... ... ...
9.05
... ..,
... ... ...
96 96 84-87
... ...
Zn
Fe
.. ..
Rkst
*.
1 0
.*
0 . .
0.5
0:ZS.C
... ... .3. .
2.75-3.75
0.7a'-i'.25
... ..
... ...
..
..
..
1
15 (ipprox.) 0.10
..
.. C
...
0.37 0:35
..
.*
hIn
0:2?3 0.310
Rest
..
Contained also 0.5'% of other constituents.
Si
Maximum.
...
..* ...
... ... ... ...
IN
Cu ff of @as
PERCEKT Mg
Pb
....*.. .. .. ..
. . . . . .. ..
Sn
....* .
2.5 0.5
.. .. .. .. .. ~.... .0:ioc .. .. .. .. .. .. .,
P 0
;bio
0.027
...
... ...
S
C
0:6i4 o.OiG 0.018 0.10
... ... ...
... ... ... ...
... ... ...
... ...
...
. t .
...
... .... .. ... ... ..... . ... ...
~
~
December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE111. ANALYSISOF STAINLESS STEELS(SERIES B) Sample
No.“ 1 3 2
Manu- Standard faaturer T y p e N o . A 430 B 446 C 446
Approx. Compn., % Cr 17 27 27
Ni
....
C C B A B A
3 02 304 302 304 304 302
18 18 18 18 18 18
17 I9 18
B C A
321 321 321
18 19 18
21 22 20
C B
347 347 347
18 18 18
A
16
B
14 l5
A
C
316 316 316
18 18 18
11 10
B
309 309
25 25
C
12 A 310 13 C 310 a All samples were supplied b Analyses also inolude: Si C After 24-month exposure.
Analysisb, %
I
-
-
-
8 8 8 13 13 13
-
9 10
12 12
...
17.06 23-30 25-30
... ... ... ...
17-19 17-19 17-19 18.89 17-19 18.89
7-9.5 7-9.5 7-9.5 8.44 7-9.5 9.11
omparison to the other metals and alloys included.
Life Expectancy
T b pitting factors, derived from the corrosion data showing the extent and nature of the attack, do not include the time factor and do not, therefore, indicate the relative life of the metals in contact with flue gas vapors. If it be assumed that the pitting action proceeded in proportion to the time elapsed, then the maximum life of the samples, in continuous contact with flue gas vapors as in the experimental tests, may be estimated from the following : where L T Y D
= = = =
L= T Y / D max. life of metal, years thickness of metal, inches time exposed t o corrosive action,, years depth of deepest pit in Y years, inch
That this assumption is fairly consistent with the facts (at least for 20-gage metal) is shown by the data of Figure 4.
TABLEIX. DEPTHOF CORROSION AND PITTING FACTORS IN SERIES B AFTER 24 MONTHS Sample No.
Av. Depth of Corrosion, In.
1
3 2 5
I
I
/o
/5
20
0.00095 0.00029 0.00039
25
Table IX shows that the pitting action was an important factor in the failure of this series of alloy steels. Another situation is also revealed by these data-namely, the extreme increase in the rate of the attack by the flue gas vapors after the initial start was made. Thus, in the case of the plain 18-8 chromium-nickel steels (samples 4 to 9) the loss of metal ranged from 0.00003 to 0.00095 inch, and the corresponding pit factors ranged from 250 down to 18. This shows that the depth of the pit first formed a t the time of the initial loss of metal was 250 times as deep as the average loss suffered by the alloy. As the corrosion became more widespread, the
17 19 18 21 22 20 16 15 14 11 10
12 13 a
0.00015 0.00012 0.00003 0,00192 0.00161 0.00023 0.00210 0.00153 0.00047 0.00000 0.00000 0.00000
0 0 0 0
00016 00010 00190 00124
Strips penetrated duiing exposure.
D e p t h of Deepest P i t , In. 0.0180 0.0200 0.0162 0.0175 0.0150 0.0140 0.0095 0.0100 0.0075 O.0390a 0.0320" 0.0140 0 . 0360n 0 , 0370a 0.0140 0.0002 0.0002 0.0002 0 0140 0 0070 0 0470a 0 0190
Pitting Factor 6 15 18 18 40 50 60 80 250 20 20 60 17 24 30
..
.. .. 90 70 25 15
..
1444
These curves represent measurements of the deepest pits found after 19- and 24-month exposure. The steep slopes indicate the rapidity of the attack. I n some cases the experimental results were too minute to be entirely indicative, but in several other cases actual penetration of the samples occurred during exposure, and the lives of these samples were definitely measurable. The dotted lines indicate the extrapolated data for the metals of series A. The data of Table X were calculated from the results of the corrosion tests after 19 and 24 months of exposure, respectively. These results show that the life expectancies of these alloys can be estimated with a fair general agreement even from corrosion data which are quite minute (see also Table V). TABLEx. MAXIMEMLIFE O F 20-GAGE STAINLESS CONDENSIXG FLUEGASVAPORS Sample No.
A 4
17 19
-
Max. Life, Years ,-
*
19-mo. test 24-mo. test
4 4
2 2 4
A 4
2”
-
Max. Life, Years Sample No. 19-mo. test 24-mo. test
16
2
14
b b b
b b b
12
2
25
15
TABLE XI.
hfAXIhXUM
Metal or Alloy Iron Copper Zinc Aluminum Lead Steels Chrome Chrome Stainless Stainless Stainless Stainless Stainless Stainless
LIFE OF ‘2o-G.4GE
h’lETAL IN
hpprox. Compn. Commercial steel And a few of its alloys Commercial S n d some of its alloys Commercial
17 27 18-8 18-8-Ti 18-8-Cb 18-8- M 0 26-12 25-20
FLUEGASES” Life. Yeara l b
7 8 8 15
3 3-4 4-8 2-5 b 2-5 b c
5-9
2-3
b
The combustible gas wa6 manufactured city gas. b Strips were penetrated during exposure tests. 0 Corrosion very light. no calculation possible with available d a t a .
a
STEEL I N
13 3 3 5 Test strip penetrated during exposure. b Corrosion very light, no calculation possible with d a t a available..
1s
Vol. 34, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
2a
The increase of nickel in similar alloys from 12 to 20 resulted in a marked and serious increase in the rate of attack. The actual corrosion of the alloy steels was preceded by staining. This was followed by localized pitting action, which continued t o be so severe during the subsequent spreading of the general corrosion that actual penetration of some of the strips occurred during the test exposure. As a result of series A and B, it was estimated that 20-gage (0.032-inch or 0.81-mm.) metal would fail by corrosion and eventual perforation due t o pitting, when exposed on one side only in an atmosphere of condensing flue gas vapors resulting from the combustion of manufactured city gas, within the approximate time limits given in Table XI.
Literature Cited
Summary As a result of the series A tests, in which a number of different metals and alloys were exposed to the action of flue gas vapors, it was found that corrosion began upon exposure and continued more or less rapidly throughout the test period, depending upon the resistance of the individual metal or alloy. Practically no corrosion occurred during these tests in the case of a sample strip of 18-8 stainless steel. As a result of the series B tests, however, in which a number of the so-called stainless steels were continuously exposed to a similar flue gas atmosphere for 2 years, serious attack did occur, which revealed several developments. It was found that none of the chrome-nickel steels were appreciably attacked for a year. Then only a few were affected in several months more of continued exposure. But after about a year and a half, all but three of the twentytwo original samples had succumbed to the effects of the vapors. The attack was so suddenly severe that, within a few months from the time of the first signs, perforation of some of the strips had occurred. As might have been expected from series A, the sample containing 17 per cent chromium and no nickel was attacked first. The increase in the chromium content to 27 per cent delayed the initial attack, but did not prevent subsequent and severe corrosion. The initial attack upon the ordinary 18-8 chrome-nickel steels indicated a wide range in the susceptibility of these alloys depending upon their source and previous treatment. The low carbon content of the special 18-8-S samples did not appear to have influenced the eventual rate of corrosion once it had begun. Of the 18-8 steels made up with fractions of titanium, columbium, and molybdenum, only those containing molybdenum withstood the attack of the flue gases after two years. The samples made up with a 25-12 chrome-nickel content showed relatively low susceptibility to the effects of flue gas.
(1) Barkley, J. F., U. S.Bur. Mines, Tech. Paper 436 (1928). (2) Bosbyshell, J. H., and Yeaw, J. S., Am. Gas Assoo. Proc., 1939, 518-42. (3) Johnstone, H.F., Univ. Ill. Eng. Expt. Sta., Bull. 228 (1931). (4) Maoonaohie, J. E., “Deterioration of Domestio Chimneys”, Toronto, Consumers’ Gas Co., 1932. ( 5 ) Mueller, F. P., Am. Gas Assoc. MonthZg, 18,35 (1936). (6) Murphy, E. J., Am. Gas Assoc. Proc., 1939,553-6. (7) Shnidman, Louis, Ibid., 1935,706-25. (8) Shnidman, Louis, and Yeaw, J. S., Ibid., 1937,697-715. (9) Ibid., 1939,542-52. (IO) Speller, F. N., “Corrosion Causes and Prevention”, 2nd ed., p. 214, New York, MoGraw-Hill Book Co., 1935. (11) Wood, J. W., Parrish, E., and others, “Corrosion from Products of Combustion of Gas”, Parts I, 11,111,and IV, Repts. 33, 34, 36, and 38 of Joint Researoh Comm. of Inst. of Gas Engrs. and Leeds Univ., 1933, 1934, 1935, 1936. PREEENTED before t h e Division of Gas and Fuel Chemistry a t the 104th Meeting of the AMERICAN CHEMICAL SOCIETY.Buffalo, N. Y.
Sulfur Dioxide-Correction On page 1019 of my article in the September issue, the ammonium sulfite-bisulfite process used for the recovery of sulfur dioxide at the Trail smelter was erroneously described as being developed by the American Smelting and Refining Company. The process used was developed by the Consolidated iMining and Smelting Company of Canada, Ltd. [Lee, Lepsoe, and Chapman (to Consolidated Mining and Smelting Company of Canada, Ltd.), U. S. Patent 2,021,558 (Nov. 19, 1935)]. On page 1020 it is stated that the cyclic process has supplanted the one in which sulfur dioxide was recovered as an intermediate step in the production of ammonium sulfate. I am informed by Robert Lepsoe that the ammonium bisulfite acidification with sulfuric acid is still the main process used there for the production of 100 per cent sulfur dioxide. H. F. JOHNSTOND
