Petroleum Tank Coatings BRIANMEAD,Humbfe Oil & Refbins Company, Houston, Tex. HE pa.inting of pctroleuni tanks has troubled workers in the petroleum industry and paint and varnish nianufacturers for many years. The problem is unusually difficult in the South, especially in the Gulf Coastal region where, in general, humidity is relatively high and temperature changes widely and suddenly. Moreover, the majority of the large refineriesare on tidewater and their tanks, therefore, are subjected to winds that contain salt, which aggravate corrosion.
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shutdown of the plant, in addition to a large property loss. The extreme ease with which this form of iron sulfide reacts with oxygen is proved by the fact that scale removed from a tank and buried under ground for six months has been known to start glowing immediately after being unearthed.’ There is such an urgcnt need of protection against corrosion by hydrogen sulfide that chemists and engineers throughout the petroleum indnstry for more than five years have been testing any and all promising compounds devised by paint and varnish manufacturers.
TANK EXTERIOItJ
Painting tank exteriors is relatively simple because several rntirely satisfactory coatings have been developed for the purpose. The trend in recent years has been toward aluminum paints, partly because such coatings are more bcautiful than the red or black used previously, and partly because aluminum paint reflects more of the sun’s heat as less of it is absorbed by the tank. In some cases sheet aluminum has heen applied to the outside of tanks, which gives even better reflection, lower tank temperatures, and less evaporation loss. The aluminum in the form of thin foil is attached to themetal of the tank roof with a suitable adhesive; tile foiling operation is simple enough to be liaridled by comparatively unskilled workmen. I n some cases a layer of insulating material has been i n s e r t e d between the aluminum and steel, with even better results as far a s evaporation i s concerned.
TANKINTEUIORS Thedifficult problem h a s h e e n the painting of tank interiors because the so-called sour crudes obtained from most of t h e n e w e r oil fields greatly accelerate corrosion. The sour crudes coutain much sulfur in combination w i t h hydrocarbons and evolve hydrogensulfide during prodnction. handlinn. I. and iransportation. This hydrogen sulfide evolution necessitates closed tanks and the protection of workers by gas masks. It also corrodes iron and steel very rapidly. The product of corrosion by hydrogen sulfide is iron sulfide, which is produced in an extremely reactive form, so that on exposure to air it reacts with oxygen with such vigor as to cause it to glow. This glowing constitutes a real menace to the refiner because a t any time there may he present an explosive mixture of hydrocarbon vapors and oxygen in the tanks in which these crudes are being handled. The danger may be even greater in the working tanks in t,he refinery itsclf, where the consequences of a fire may bo the complete
M m a o n s OF TEBT Some of the earliest tests were made nith paints that tiad stood the test of time for outside paint work. They speedily proved unsuitable for the severe conditions in tanks containing sour crudes. Cases are on record of such coatings failingwithin seven days, thereafter offering no protection whatever. It was therefore necessary to try out many kinds of coatings as quickly and as cheaply as possible. The refiner first had to learn by experience what niay he considered satisfactory test conditions. It was soon found that painted samples of tank steel suspended in the vapor space of a tank gave inconclusive results, such samples invariably lasting longer than the same paint applied on the surface of the tank itself. This discrepancy arises because s u s p e n d e d samples donotreach the same temperatures as t h e t a n k roof itself. Accordingly, in later test work the p a i n t e d test pieces have been inserted b e t w e e n the supporting rafters and the r o o f , where the temperature m o r e closely approximates t h e temperature of the metal in the roof it-
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Attempts to test s u c h c o a t i n g s in the laboratory have met with indifferent success because it is very difficult, if not impssible, to simulate actual tank conditions in the laboratory. One enterprising paint company purchased a banel of sour crude and had i t shipped to their own laboratories in a steel drum, The dissolved hydrogen sulfide attacked the interior metal surface of the drum so rapidly that the oil arrived with no trace of free hydrogen sulfide. It is believed, therefore, that the most satisfactory procedure is to use panels in tanks as above indicated. Paint and varnish manufacturers will find that the oil industry is, in general, willing to cooperate in such tests to the fullest possible extent. 1
Devine. J. M.,Wilhelm. C. J., end Schmidt. L., Bur. Miow. Rcpls.
Inreativoliona 3160 (1932).
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I N D U S T K I A L A N D E N G I N E E I< I N G C H E M I S T K Y
Vol. 24, No. 8
Most of the eaperimenting on interior tank coatings has been d i r e c t e d against corrosion within the vapor space of the tanks. T h e c o r r o s i o n of t a n k bottoms from the inside is largely due to dilute acids andelectrolytesin the water that is invariably present in tankage. A lining of concrete prevents such corrosion satisfactorily in a number of cases, and asphaltic materials provc successful in others.
TENTATIVE Smcincmioxs For the purposes of defining the requirements of a protective coating for the vapor space of tanks, and of stimulating discussion of the problem, the following tentative specifications and comments are suggest& (1) The c o n s i s t e n c y must pennit application by spray. S p r a y e d jobs for such interior tank work are charier a n d i n v a r i a b 1v m o r e s a t i s f a c t o r y than hand-brushed jobs because corirers inaccessible to the brush can be adequately protected. (2) Poisonous pigments, vehicles, and solvents must be absent. This requirement is introduced as much for the benefit of the paint manufacturer as for the user. Although painters can be safeguarded against ahnost any kind of solvent, elaborate safety precautions arouse prejudice against the product. (3) Drying must take place rapidly. Tan& are expensive and must be kept to a bare minimum; consequently, tanka taken out of service for cleaning and repainting must go back into service as quickly as possible. (4) Heat treatment to harden the coating must not be necessary. Some of the more satisfactory synthetic resins are plastic before hardening and attain their full value only after a heat treatment. It is impossible to apply such heat treatment to tank coatings, except in so far as the heat of the sun may be utilized. ( 5 ) The coating must bond well to steel. The refiner is generally williiig to sand-blast the surface to he protected, thus maranteeing removal of scale. It is important that this sand-blasting be done carefully for tank scale invariably contains gas and compounds capable of liberating gas, which will ruin tho best paint job in a short time. (0) The coating must be cheap. IIowever promising the coating nray be, it will not be tried if its initial cost is too high. (7) The color should be light in order to facilitate ohservation of the progess of the paint job. The interiors of tanks are dark and frequently cannot be satisfactorily illuminated. If it is to be applied in two coats, they should differ in color. (8) The coating must be unaffected by water and water vapor. I n all hut the driest powible climates the underside of the tank roof will be exposed to wat.er vapor during the day and have liquid water condensed on it after sundown. Almost all tanks contain a foot or two of water below the crude oil or distillate, and this water distils up through the oil into the vapor space. I'rotjably more coatinp fail on account of this
that is either attacked by liydrogeii sulfide or that can act as a carrier for it. 111 one case a coating was devised in which some rubber was used in order to give greater impermeability to water. Unfortunately the rubber carried the hydrocarbons and the hydrogen suEde through the protective coating to the metal surface that was to be protected. (12) The coating must resist excessive temperature changes. During the heat of the suirimer, especially in tlie Gulf Coastal region, the metal of the tank roof varies in temperature from about 180" F. to as low as 60' or TO" f. I n the case of sudden thunderstorms this change may take place in the course of a few minutes and must not affect the structure of the coating. Many of the earlier types of coat,ings failed bemuse they lacked suscient elasticity. (13) The coating must be unaffected by exposure to air. h5ost crude tankage, and probably 5 majority of refinery distillate tankage, is open to the air and breathes in or out according to temperatiire changes. It is, of course, this breathing that involves the greatest hazard if the air so breathed in comes in contact with iron sulfide scale and starts it glowing. It must be appreciated that a satisfactory coating should be capable of withstanding not only each individual factor that could contribute to its failure, but also any possible combination of these factors. That the problem is not easy of solntion is to be seen from the fact that i t has bcen continuously attacked for more than fiye years and that at the present time there is no one coating that has been accepted as entirely satisfactory.
COATIN~S TESTED The different methods of protection that have been tried are listed as follows, together with a brief discussion of the applicability of each type: Conventional paint with linseed oil or similar veliicle. In general. where such oostings have failed. the vehicle has been
August, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
softened by continued exposure t o hydrocarbons and water, either in liquid or vapor form. Varnishes and resins. The earlier coatings failed mainly from inadequate flexibility to withstand the temperature changes and the consequent expansion and contraction of the metal. If sufficiently flexible for this purpose, they were softened too readily by hydrocarbon vapor. Synthetic resins and condensation products. These also failed in some instances on account of their lack of elasticity, which permitted the development of pinholes. Metal was attacked at these points of failure, and the products of corrosion were built up and gradually destroyed the whole coating.
It is not meant that the above should convey the impression that all types of coatings have failed equally. Some of the later types of synthetic resins appear promising and seem to avoid the shortcomings of the earlier coatings. The question may well be asked, "What should be the minimum life of a satisfactory coating?" The answer is difficult, since naturally there are differences of opinion among refiners. It is believed, however, that no coating can be considered satisfactory that gives less than two to three years in crude tankage or one year in sour distillate service.
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OTHER METHODS USED Of coatings other than paints and varnishes that have been tried, there may be cited: Processes for spraying such a metal as aluminum onto steel. With these, some degree of success has been attained, although it is difficult to obtain complete covering of the metal surface. The use of a thin sheet of metal foil glued to the steel by a suitable adhesive. The greatest obstacle t o this has been the finding of such an adhesive, and to date little progress has been made. The use of galvanized iron as material for the tank roof. With this, some degree of success has been obtained. The substitution of aluminum sheet for tank steel in fabrication of the tank roof. As far as the sheet aluminum is concerned, this appears to be entirely successful, although mechanical problems arise on account of the smaller tensile strength of aluminum. Where steel girders are used inside the tank to support an aluminum roof, the problem of formation of iron sulfide and its come uent hazard, of course, remains. On small tanks this problem %as been overcome by the use of external girders. RECEIVED April 7,1932. Presented before the Division of Paint and Varnish Chemistry a t the 83rd Meeting of t h e American Chemical Society, New Orleans, La.,March 28 to April 1, 1932.
Corrosion of Mild Steel and Alloys by Hydrogen Sulfide at 500" C. and Atmospheric Pressure A. WHITEAND L. F. MAREK Research Laboratory of Applied Chemistry, Massachusetts Institute of Technology, Cambridge, Mass.
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VAILABLE data on the corrosion of metals and
Mild steel is rapidly attacked by hydrogen s u ~ &at 5000 c. and atmospheric pressure. Under the same conditions, aluminum is very slightly ai!hckt?d even after exposure for 3 weeks* High-chromium steels (12 to 20 per cent chromium) resist the corrosion, and the resistance increasesujith increasein chromium content. The presence of high proportions of nickel are detrimental to the corrosion resistance imparted to the alloys - by - chromium.
hydrogen sulfide a t elevatedby temperatures are Data on the inrather limited. crease in weight attained over limited time intervals by metal specimens in hydrogen sulfide atmospheres have been obtained by Scholl (5) a t 900" F. (482.2" at 12000 F. (6490 by Gruber (,$), a n d a t 1470°F. (799" C.) by Sayles (8); these have recently been assembled and reviewed by Pilling and Worthington (6). The effect of oxygen on the low-temperature corrosion of pipe steel by hydrogen sulfide present in moist gases a t low concentrations has been investigated by Devine, Wilhelm, and Schmidt ( 2 ) ,who have also reviewed the literature on this phase of the corrosion problem. Corrosion a t high temperatures, as measured by loss in weight of specimen after removal of scale or by surface penetration, loss in tensile strength of specimen after exposure, or destruction of grain boundaries have not been reported for the case of high-temperature hydrogen sulfide corrosion. Hydrogen sulfide directly attacks the surface of some of the commonly used structural metals, reacting quite rapidly a t high temperatures to form metal sulfides, and it thus causes a reduction in strength by decreasing the effective wall thickness of equipment. It is important that a knowledge of its effect be available in the design of equipment exposed to its action a t elevated temperature. This paper presents the results of an investigation conducted for the purpose of determining the corrosion
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r e s i s t a n c e of various steels (particularly alloy steels) and aluminum when subjected to the action of and dry hydrogen sulfide a t 500" C. (932" F.) and atmospheric pressure. The methods actually employed, while not c o m p a r a b l e w i t h c o n d i t i o n s m e t i n practice, a simple a n d r a p i d method for studying the action of h y d r o g e n sulfide and permittid comparisons to be made between the different metals and alloys. EXPERIMENTAL PROCEDURE
The hydrogen sulfide employed in these experiments was generated as required from ferrous sulfide and dilute sulfuric acid. The metals used were mild steel containing 0.23 per cent carbon, cold-rolled aluminum, and a series of alloys, the compositions of which are shown in Table I. All the samples except those of aluminum were cut from bars of the hobrolled material parallel to the direction of rolling, and were used for the tests as received. Aluminum test strips were cut from a polished sheet of the metal. Samples were cut 4 inches (10.2 cm.) long, 0.5 inch (1.3 cm.) wide, and 0.1 cm. thick. The average area of surface exposed was 28.8 sq. cm. The equipment consisted of a Kipp generator in which the hydrogen sulfide was generated from ferrous sulfide and 10 per cent sulfuric acid; a bubble bottle containing water
