Special Corrosion Problems in Oil Refining By Robert E. Wilson and W. H. Bahlke STANDARD OIL C o .
( I N D I A N A ) , &’HITINO,
IKD
This article describes specialized corrosion problems encountered in the petroleum industry. In “fire and steam” distillation of crude oils most of the corrosion is caused by the hydrolysis of magnesium chloride in the salt which is almost always present. This causes rapid corrosion in all parts which come in contact with the condensed water containing hydrochloric acid and apparently some other corrosive compounds. Methods of preventing or minimizing this corrosion are discussed. Nickel-chromium or high-chromium steels and some special bronzes stand up rather well, while ordinary steels are very poor. In coking distillations of crude, or in cracking stills, the most severe corrosion occurs in the parts of the apparatus above 315.6” C. (600” F.) and is due to hydrogen sulfide and probably other corrosive sulfur compounds. Under these conditions the chromium (“stainless”) steels, aluminium, and calorized iron stand up remarkably well, while copper and some bronzes are much worse than ordinary steels. Monel has no advantage over steel under these conditions. The addition of lime to pressure steel charging stock markedly reduces but does not stop corrosion. Some experiments on the rates of corrosion of still by pure hydrogen sulfide at high temperatures brought out important points as to the characteristics of the corrosion reaction and the protective coating formed.
HE corrosion of petroleum-refining equipment, with its attendant losses, constitutes one of the major problems confronting the designer and operator of such equipment, and much must be done to reduce or eliminate such losses. The most popular palliative for corrosion is to estimate the rate and allow so much extra thickness to the metal used. While this may be the most economical solution of the problem in a few cases, it is scarcely a satisfactory general remedy. This paper is directed to a consideration of special corrosion problems in the petroleum industry and hence does not consider the well-known corrosive conditions in water lines and sewers, acid-handling and recovery systems, and refrigerating systems, which it shares with many other industries. A comparatively small number of compounds and conditidns are responsible for most of the corrosion losses peculiar to the petroleum industry. Crude petroleum gives two of thesenamely, active sulfur-containing compounds and hydrochloric acid arising from the hydrolysis of magnesium chloride in emulsified salt water. Treatment for the removal of the objectionable sulfur and other compounds is likely to cause the introduction of a third corrosive agent, sulfuric acid, and its reduction product, sulfurous acid. These three, in addition to the increased activity of sulfur-bearing components a t high temperatures in cracking processes, constitute the major sources of corrosion peculiar to oil refining. Two methods of prevention will naturally be suggestednamely, chemical neutralization of the corrosive substances, and the use of corrosion-resisting equipment. The former method is easier to apply to existing equipment, but involves a continuing cost which is likely to be large on account of the tremendous quantities of oil handled by a given still, and the reaction products frequently cause other trouble,
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ISD UST R I A L AND E NGI iVE E RI S G C H E MI ST R Y
April, 1923
as detailed later. The latter method means new and generally rather expensive equipment, though the cost per barrel of oil handled is generally very small. Most of this paper is accordingly devoted to tests of corrosion-resisting materials, though attempts in other directions are also described. To be of value in the present state of the art, it was considered essential that the tests be made under actual operating conditions, and extend over a period of several months a t least. This paper summarizes the results of tests conducted during the past two years. Corrosion of Crude Distilling Equipment
I n coking distillations without steam the most serious corrosion is in the hotter part of the equipment, such as the still, air-cooled towers, if any, and vapor lines, and is mainly due to the action of corrosive sulfur compounds, particularly hydrogen sulfide, most of which is formed during the cracking which takes place near the end of the run. This same action is observed in a more aggravated way in pressure still corrosion. There is also some corrosion due to sulfur compounds in the run-down lines, although in general sulfur corrosion in the absence of water drops off quite rapidly with temperature. T a b l e I-Tests
in S t o v e Distillate R u n - D o w n L i n e -Loss
METAL Calite casinga Blork ..- _ tin - ._
in mg./sq. cm /day1st 2nd 3rd period period period 53 days 32 days 25 days 0.015 0.064 0.069 ... 0.086
... ...
... ... ... ...
Weighted average per day
0.015
0.064 0.069 Cyclops metal (rough) 0.086 Cyclops metal (polished) Stainless steel (Firth-Sterling) 0.112 0.119 0.152 hardenedb 0.096 0.153 0.182 0.116 Bearing metal 0.184 0.213 0.157 Gun metal 0.251 0.19 0.324 Manganese bronze 0.252 0.232 0.276 Hills-McCanna No. 45 0.265 0,282 0.246 Hills-McCanna No. 50 0.271 0.29 0,247 Commercial bronze 0.285 0.37 0.189 Nickel bronze 0:279 0.296 0.27 0.248 Rolled nickel 0.333 0.310 0.137 0.600 Titanium-aluminium bronze ... 0.314 0.186 0.475 McGill metal 0.354 0.403 0.294 Muntz metal tubing ... 0.374 0,435 0.296 Ambrac tubing 0.378 0,481 0.154 Cold-rolled brass 0:421 0.396 0.475 0.425 Monel 0.333 0.444 1.09 0.116 Aterite 0.336 0.471 0.194 0.506 Aluminium 0,582 0.513 0,765 0.314 Ascoloy (bar) b ... 0.554 0.82 0.343 Vanadium bronze ... 0.76 Ascoloy (sheet) b 0.54 1.03 0.776 ... Copper 0:?76 0.814 Calorized iron ... 0.814 1.03 Everdur metal 1.03 ... ... 1.28 Cast iron 0.506 0.97 1.89 1.17 Copper-bearing steel 1.34 1.98 2.01 1.71 Byers wrought iron 1.47 2.01 ... 1.81 0.95 1.60 Wrought iron 2.42 1.85 0.78 1.95 Vismera steel 2.45 2.09 1.71 2.26 Climax alloy steel 2.23 2.26 Still-bottom steel 1.65 2.34 2.58 a Iron-nickel-chromium-aluminium allov which is no longer made. Present- Calite A a n d %-are iron-nickel-chromium alloys: b Further experiments are under way to determine the cause for the difference between Firth-Sterling Stainless Steel and Ascoloy. The latter pits badly above the water line under these conditions and this accelerates the rates of corrosion in successive runs. Samples of varying hardness showed very similiar results: the effect of variations In carbon and chromium content is now being studied.
...
... ... ... ...
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I n running crude with steam the temperatures are lower and there is little cracking to produce corrosive sulfur compounds, and therefore much less corrosion in the hotter portions of the equipment. However, the magnesium chloride present in the salt water, which almost invariably accompanies crude oil, hydrolyzes a t high temperatures in the presence of steam and gives off considerable quantities of hydrochloric acid. This is not active a t high temperatures, but as soon as any steam is condensed the acid dissolves and causes very rapid corrosion in heat exchangers, condensers, and run-down lines. This type of corrosionis also observed to a considerable less extent in ordinary coking distillations, because whatever water may be present in the oil does come off with a certain amount of hydrochloric acid. Indeed, the concentration of hydrochloric acid is about the same in this water
INDUSTRIAL AND ENGINEERING CHEMISTRY
356
as in the water from the steam distillation, but the total volume of acid and water is very much smaller and there is none coming off during most of the run, so that this form of corrosion is generally much less except in places where the water may be trapped and remain in some part of the system throughout most of the run. When such conditions occur, as, for example, around valves in horizontal lines, the water absorbs not only hydrochloric acid but hydrogen sulfide from the latter part of the run, and this acid solution, together with corrosive compounds in the oil, causes much trouble, and it seems to be almost impossible to obtain very high corrosion resistance from any practicable metal. I n order to simulate this latter rather severe condition and thus also to secure some information as to what would stand up well in stills running crude with steam, three series of tests have been made in the stove distillate run-down line from an ordinary crude tower still, the samples of metal being placed in an enlargement in the line in such a way that approximately the lower half thereof was in the water which was trapped in this depression. Between each test the samples were carefully cleaned and weighed. The results of the three series of tests are shown in Table I. The test includes a wide variety of metals, many of which were selected as being possibly available for valve trim, etc., and others more as a matter of scientific interest. Approximate figures as to the composition of the less familiar alloys are given in Table IV. It will be noted that the agreement between the different periods of the test was not particularly good, probably because this test combines two kinds of corrosion and the amount of water trapped off in the line undoubtedly varies from time to time. The salt water content of the crude may also have varied. I n general, those metals which stand up well against ordinary acid corrosion do not stand up as well against sulfur corrosion, and vice versa, and changes in the amount of water would therefore change the relative position of the various materials. In general, however, it may be said that Cyclops metal, hardened stainless steel, nickel chromium steel, and block tin stood up very well, with some of the bronzes nearly as good, while various kinds of wrought iron and steel were
Vol. 17, No. 4
it appears that the use of heavy cast-iron condensers, lines, and valves, the latter trimmed with some of the abovementioned more resistant metals, is the best solution of this rather difficult problem, especially if care is taken to keep water from collecting in any part of the condenser or rundown system. Table 11-Tests
of Samples Hung in First Tower of Crude Still (Total time of test, 75 days)
METAL Natural Ascoloy (chromium steel)" Polished Ascoloy (chromium steel)" McGill metal (forged bronze) Aluminium Stainless steel (Firth-Sterling) hardened Muntz metal tubing Brass Manganese bronze Vanadium bronze Aluminium bronze (Hills-McCanna) Wrought iron"
LOSS
Mg./sq. cm./day None
0.004 0.019 0.0216
0.0254 0.049 0.0515 0.0945 0.11 0.147 0.183 0.222 0.24 0.312
Monel ......
Cast iron Bearing metal Vismera steel Climax alloy steel Copper-bearing steel Still-bottom steel Byers wrought iron Aterite" Nickel Ambrac tubing Gun metal Commercial bronze Nickel bronze Phosphor bronze" a Samples in tower 203 days.
0.376
0.409 0.415 0.48
0.52 0.78 1.154 1.67 1.96
2.49 4.41 4.86
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Effect of Lime on
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very poor. Brass, monel, soft stainless steel, aluminium, calorized iron, and cast iron occupied an intermediate position in these tests. The variation between the best and poorest metals was by no means so great as in the case of sulfur corrosion a t high temperatures mentioned later. In general,
/
Fiqure 1 A
-
Effect of Lime on Pressure S t i l l Corroslon
-
From this table it will be noted that the stainless and other chromium steels and aluminium stand up remarkably well, with some of the bronzes next. Various kinds of iron and steel are grouped rather closely with markedly greater corI
Chcm.
Me;.Eng., 99, 491 (1920); 13, 1122 (1920); 96, 1119 (1922).
April, 1925
IiVDUSTRIAL AiVD ESGILVEERISG CHEJfIISTRI'
rosion, and metals containing large amounts of copper or nickel show very high rates of corrosion. These same differences are brought out even more strikingly in the tests in the pressure stills. Corrosion of Cracking Equipment
The corrosion of pressure stills is recognized as being one of the principal sources of expense and danger in pressure still
357
facts should make possible much better protection against corrosion of most pressure still equipment. Some experiments have also been carried out on the prevention of sulfur corrosion in Dressure stills bv addine some alkali to combine with the suliur and preveniits liberation as hydrogen sulfide. It was found possible to cut the corrosion to less than half by adding two or three equivalents of lime per equivalent of dulfur in-the pressure still charging stock,
Figure 2
operation. Practically all of this corrosion can be traced to but it did not by any means stop corrosion, and the accumuthe action of sulfur compounds, of which the most reactive is lation of the solid matter and resulting danger from hot tubes probably hydrogen sulfide formed by the thermal decomposi- or hot bottoms made the method of protection seem of little tion of sulfur compounds in the oil. This corrosion is es- value. Caustic soda was also tried, with generally similar pecially rapid a t temperatures above 371.1" C. (700" F.), results. Some comparative corrosion tests on different dropping off to a negligible amount a t 260" C. (500" F.). metals on a small pressure still run with and without lime are T o illustrate the severity of this corrosion condition, a case shown in Figures 1 and ld. is known where a rather poorly designed fractionating tower had the entire inside eaten out in about two years. Table 111-Tests in Pressure Stills Up to the present the general method of caring for this corLoss N o . of rosion has been to build the stills thick enough to stand several METAL Mg./sq. cm./run runs years of corrosion before reaching the danger point, it being Aluminium 60 None Calite casting 60 None generally considered that there was nothing else available a t a Calorized plate 50 None Duralumin 10 None reasonable price which could be safely used for these large 33 Stainless steel (Firth-Sterling) 0.03 units operated a t high pressures and temperatures. While Ascoloy 94 0.03 Aluminium bronze 0.11 38 this may still be true for the main large drums or shells of Galvanized iron 0.55 50 Zinc 0 . 6 0 10 pressure distillation equipment, it does not appear to be true Cast iron 30 2.73 in the case of smaller parts, such as tubes, heat exchangers, Vismera steel 10 2.96 Special Climax alloy steel 33 3.32 fractionating towers, valves, etc., because, as Table I11 Still-bottom steel 30 3.44 Copper-bearing steel 10 3.49 shows, there are a number of metals which have a surprisingly 20 t o w chromium alloy steel 3.68 high resistance to this form of corrosion. Wrought iron 3.69 10 Monel 10 3.85 The following tests were made by supporting strips in the Bronze 10 19.25 Brass 10 21.5 upper part of a commercial pressure still operating a t an averCopper 40.6 10 age temperature of about 390" C. (735" F.) for about 30 hours during each run. The sulfur content of the charging These plots are drawn to a log scale on account of the wide stock averaged between 0.25 and 0.80 per cent. The strips were so arranged above a pan that the lower part thereof variation between the different metals. The parallelism bedipped into circulating reflux liquid from the runback, but tween the two curves with and without lime indicates that the in general the rate of corrosion on the two parts of the strips percentage reduction in corrosion is nearly the same on all was not greatly different. metals. I n the cases of the brass and aluminium the results These results are shown in Table 111. It will be noted that are a little uncertain, as indicated by the dotted lines, bealuminium, calorized iron, stainless steel, and Ascoloy stand cause it was difficult to remove all the scale from these soft up amazingly well under pressure still condition, the poorest metals without abrading some of the metal. The order of corroding less than 1per cent as fast as ordinary steel. There the different metals is about the same as that indicated in is little difference between the steels, all of them corroding Table 111, though differences in the charging stock, the rapidly, while bronze, brass, and especially copper corrode frequency of cleaning, and the individual samples used many times more rapidly than iron, these metals being es- caused considerable variation in quantitative results. There pecially vulnerable to sulfur corrosion. Monel has no advan- is little difference between the results in the vapor and those tage over steel under these conditions. A knowledge of these in the liquid phase, whether with or without lime, though
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
358
the per cent reduction in corrosion on iron in the vapor phase is about 60 as compared with 75 in the liquid phase. T a b l e IV-Analyses of Alloys METAL Calite casting Iron (obsolete formula) Nickel Chromium Aluminium Carbon, etc. ALLOY
Cyclops metal (nickel-chromium stainless steel)
McGill metal
Aterite
Nickel Chromium Iron
P E R CENT
56.6 31.89 4.34 6.93 0.24
18 8 Remainder
Tin Lead Copper Iron Nickel
Trace Trace 88
Nickel Iron Copper Zinc Lead
35
4.5
6.94
Vol. 17, No. 4
then cleaned with acid, washed, and dried, and the results at 393.3' C. (740' F.) obtained (Figure 4). It will be noted that the initial rate of corrosion of the clean metal was very high, but that it drops off rapidly during the first hour and more slowly during the next 2 hours. The runs were then repeated several times to determine whether or not a steady rate was obtained, no cleaning being done between runs. The iron tubes were evacuated between the runs to prevent the formation of hydrogen during the heating and cooling periods. Considering the results as shown in Figure 4, it appears that (a) the coating of ferrous sulfide slows down the rate of corrosion to a small fraction of that on clean metal, but never entirely stops the corrosion; (b) heating and cooling between runs loosens or scales off some of the protective layer and gives a higher initial rate of corrosion, though not so high as for
15
40 1.5 5
Ascoloy
Chromium Iron
Hills-McCanna No. 45
Copper Aluminium Iron
88 10.5
Stainless steel (Firth-Sterling)
Carbon Chromium Iron
0.30 13:O Remainder
14 Remainder except for accidental impurities
1.5
Measurement of Rate of Corrosion of I r o n by Hydrogen Sulfide
.
A few experiments were carried out to throw some light on the main factors affecting the rate of corrosion by hydrogen sulfide a t elevated temperatures. A measured quantity of this gas was passed through iron tubes held in a lead bath a t a predetermined and regulated temperature. The exit gas was then passed through sodium hydroxide solution and the residual gas measured in a gas buret. This, corrected for the alkali-insoluble impurities in the original hydrogen sulfide, gave the volume of hydrogen liberated by the reaction of hydrogen sulfide and iron. I n the later runs these impurities averaged about 0.25 per cent. This gas was made by the absorption of an impure gas in magnesium oxide suspended in water. After considerable gas had been absorbed the solution was boiled and the first few liters of gas discarded, after which the gas reservoirs were filled. A diagram of the apparatus is shown in Figure 2.
clean metal; the rate varies from run to run, probably on account of variation in the scaling off between runs; (c) the rate a t 393.3' C. (740' F.) is several times as fast as a t 315.6' C. (600' F.), thus confirming observations on pressure still equipment; (d) the apparently steady rate of 1.3 cc. of hydrogen per minute corresponds to a corrosion rate of 15.7 mg. of iron per square centimeter per run, about four and'a half times that observed on the pressure still strips where the partial pressure of hydrogen sulfide was, of course, much less. Acknowledgment
The writers take this opportunity to acknowledge the work of E. P. Brown and C. C. Miller on the effect of lime on the rate of corrosion in pressure stills, and of H. G. Schnetzler on the rate of corrosion of iron by hydrogen sulfide.
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Runs a t 68.9' C. (200' F.) and 204.4' C. (400' F.) showed no appreciable reaction. The curve in Figure 3 shows the results a t 315.6' C. (600' F.), plotting both the total cubic centimeters of hydrogen as observed, and the rate of hydrogen formation determined from this smooth curve. The tube was
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Detailed information in regard to the terms of these various fellowships may be obtained from the Bureau of Mines, Washington, D. C., or from the different institutions named.
