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The Resistivity of Iron and Its Application to the Chemical Industry By D. M. Strickland THEAMERICAN ROLLINGMILL Co , MIDDLETOWN, ONIO
ROM the early days of alchemy down to the present time, the service life of iron when exposed to chemical corrosion has been a vital factor in the development of the chemical industry. Practically every branch of the chemical industry is, either directly or indirectly, dependent upon iron. Iron is required for plant construction. It is used for roofs, sidings, vats, caldrons, drums, ventilating hoods and ducts, pipes, coils, condensers, blowers, evaporators-in fact, it is the universal metal with multitudinous uses.
F
FIG.1
Economies perfected by the use of corrosion-resistant iron are more quickly apparent in the chemical industry than in other fields of human activity. Although, from a plantmaintenance viewpoint, the rust problem is important to all manufacturers, chemical corrosion of ferrous metals is much more severe than ordinary rusting. The user of chemical equipment finds it advantageous to study his particular requirements carefully and choose the iron which by extended service life yields the best return on the initial investment. The relative resistivities of ordinary steel and iron to chemical action present striking differences. Obviously, the more resistant metal ensures greater economies. Research investigations and scientific studies reveal the fact that comparatively small variances in ferrous-metal purities exert tremendous influences from a corrosion-resistance standpoint. Surprisingly longer service‘ life, for example, is enjoyed by commercially pure iron over the mildest steels procurable-a fact which led to the development of the former specialty and contributes to its increasing use. The rusting away of quickly made steel is not a theory but a sad truth. No less a scientist than Sir Robert Hadfield,l eminent English metallurgist, sounds the warning when he states that the 1921 production of pig iron did not equal the 1921 rust loss. This wanton waste of raw materials demands more than a passing thought. Even though the present generation will not suffer from a lack of iron, what of the future? The human race without iron is a race without refinement-a world of savages, a recurrence of the crudities of former stone ages and cliff dwellers. 1 Engineering Supplement, Manchester Guardian Commerical, October 26, 1922
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Large tonnages of ferrous metals are required by the chemical industries, and every pound which gives a relatively longer service life adds its modicum to the conservation of our natural resources. All comparatively recent steelproducing inventions were designed for tonnage output. A tremendous sacrifice of former quality resulted and buyers of common steel awoke to the fact that their cheaper installations were virtually “penny wise and dollar foolish” purchases. In former times, however, iron was produced in small quantities, and a remarkable degree of purity was often secured. These purer irons satisfactorily resisted corrosive influences and stand to-day as landmarks of service-service which repaid the original investors over and over again. I n this connection, it is interesting to recall that William Murdock (1754-1839) invented gas lighting, and erected his first gasometer near Birmingham, England, in 1792.2 Fig. 1 illustrates this forerunner of the modern gas-holder. Although in partial ruin, it still stands as a testimony to the ingenuity of the inventor, who so satisfactorily designed the original installation that it gave continuous service from 1792 to 1911, nearly 120 years. In a paper read before the 1921-1922 session of the Institute of Civil Engineers, Sir Robert Hadfield refers to this old gasometer and mentions the research investigations instigated for the purpose of studying the nature of the sheet iron used in its construction. Later, analysis revealed the fact that “it is apparently ordinary wrought iron, and copper is practically absent.” Present-day commercially pure iron is even purer than this trusty servant of the chemical industry. Whereas the carbon, sulfur, manganese, and copper impurities in the Murdock iron total 0.175 per cent, commercially pure iron usually carries in the aggregate less than 0.1 per cent of these elements. I n fact, the summation of nine impuritiessulfur, phosphorus, carbon, manganese, copper, silicon, oxygen, hydrogen, nitrogen-must be less than 0.16 per cent to characterize the purest iron manufactured in commercial quantities to-day. Modern commercially pure iron has been developed by the adaptation of scientific purification processes to 20thcentury production methods. The resistivity of such iron to chemical corrosion is of decided importance to the industry.
Fro. 2-(a)
(a) (b) COMMERCIALLY PUREIRON, ( b ) WROUGHT IRON
Although each service condition is an individual study and all inclusive deductions cannot be drawn from any single test, yet, this iron has been investigated under so many 2
Presidential Address, Society of British Gas Industries, April 18, 1918.
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different conditions both at home and abroad that, to say the least, the results secured are indicative of the service to be expected when commercially pure iron is used for industrial installations subjected to those corrosive conditions simulated by the tests in question. Typifying the results of such investigations, the following tests are described. Each investigation has been scientifically conducted, all test specimens being uniform in size, gage, and surface treatment; uniform temperature conditions prevailed and due consideration was given to equalities of strength and volume of testing solutions, molecular c~ncentration,~ and all other important variables. ACID CONDITIOKS Representing acid corrosion, an acetic acid test is submitted. A 3 per cent solution was chosen and properly prepared samples were immersed therein for 45 days. The test was conducted in duplicate at a temperature approximately 100” F. The results tabulated are averages of closely agreeing weight losses and show the greater resistivity of commercially pure iron. MATERIAL Commercially pure iron Steel
.
S P 0 015 0 006 0 048 0 077
C 0 010 0 140
hln 0 012 0 288
Loss Cu Oz./Sq Ft. 0 018 1 94 0 372 3 80
FIG.4
bolts “are practically in the same condition as when put in use,” whereas “the wrought-iron bolts are hopelessly corroded.” MATERIAL Commercially pure iron Wrought iron
S 0.036
ANALYSIS C Mn O.qO9 0.025 0.034
....
0.026 0.032
0.010 0.221
0.028 0.028
0.025 0,054
0.18
0.034
0 . 2 6 5 , 0.025
0.050
0.12
P
Si
....
ALKALINE COXDITIONS The action of ammonia has been determined by an accelerated test of 8 days’ duration for which hot, concentrated ammonia was chosen. All specimens were immersed in the same container, fresh ammonia being added daily. MATERIAL Commercially pure iron Mild steel Metallic copper
Loss
0 z . j S q . Ft.
0.04 0.14 2.06
The following tabulation indicates the comparative resistivities of ferrous metals when exposed to sodium hydroxide (caustic soda or lye). A 25 per cent solution was used and the test run in duplicate for 100 days at room temperature. FIG.3
An interesting report has recently been completed by Tatlock and Thompson14independent analysts of Glasgow, Scotland. The following excerpt has a bearing on the problem of acid corrosion:
MATERIAL Commercially pure iron Mild steel
Loss
O z / S q Ft. 0 017
0.079
The samples, which were. pieces of plate, each measuring 2 by F / 4 in., were tested with regard to the loss in weight caused by immersion in 5 oz. of dilute acid, for 50 hrs. Each of the samples was tested in a separate vessel, the strength of the acid being 9 parts of water, 3 parts of sulfuric acid, and 1 part of hydrochloric acid. The following results give the percentage loss : MATERIAL^
Loss in Weight Per cent 1.6 25.7
Commercially pure iron Puddled iron Charcoal iron 40.6 Mild steel 51.1 a Trade names changed by the author t o the classification noted.
Fig. 2 illustrates an interesting investigation conducted by a British steel and coal company. For three years and three months bolts of commercially pure iron and wrought iron were exposed to the action of acidified pit water, the illustration clearly showing that the commercially pure iron
* Strickland, Chem. Met. Eng., 26 (1922), 1165. City Analyst’s and Gas Examiner’s Laboratory, 156 Bath St., Clasgow, Scotland, March 21, 1922. 4
FIG 5
OTHERCHEMICAL CONDITIONS The action of a 3 per cent solution of ammonium chloride was established by an investigation conducted a t room temperatures for 420 days.
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Loss Mn Cu Oz./Sq.Ft. 0 . 0 2 5 0.078 3.87 0.280 0,200 4.38
smooth enamel surface free from any such defects. This indicates a freedom from gas or gas-forming constituents which is not true of the other materials.
A similar test at room temperatures has been completed for which 5 per cent magnesium chloride was employed. Weight losses were determined at the end of 420 days’ immersion.
The value of degasified commercially pure iron for vitreousenamel purposes cannot be overestimated. A homogeneous metal not only safeguards the enameler and reduces all culls and seconds to a minimum, but also givestthe chemical: industry a source of supply for enameled equipment which is free from blisters or surface irregularities and satisfactory for service. COMMERCIAL INSTALLATIONS
MATERIAL S Commercially pure iron, . 0.020 Steel . . . . . . . . . . . . . . . . . . . 0.025
P C 0 , 0 0 7 0.010 0,004 0.050
Loss MATERIAL S P C Mn Cu Oz./Sq.Ft. Commercially pure iron. . 0 . 0 2 0 0 , 0 0 6 0.010 0.018 0 . 0 2 0 0.675 Steel . . . . . . . . . . . . . . . . . . . 0 . 0 3 5 0,013 0.080 0.400 0.281 0.850
Fro R
Striking differences are noted when ferrous metal, are exposed to the action of 20 per cent aluminium sulfate. In 12 days the following results were secured, room-temperature conditions prevailing during the investigation MATERIAL Commercially pure iron Steel
S 0 015 0 048
P 0 006 0 077
C 0 010 0 140
Mn
0 012 0 288
Loss Cu O z / S q Ft. 0 018 0 07 0 372 5 84
The action of ammonium nitrate follows, 3 per cent solution being used for a test conducted at room temperature for 191 days. MATCRIAL Commercially pure iron Steel.. . . . . . . . . . . . . . .
S P C 0 , 0 3 6 0 , 0 0 6 0.010 0 . 0 4 3 0.077 0.100
The chemical investigations described above have definite. applications from a commercial standpoint. Valuable as such tests are, howexTer, actual service records are equally, if not more, important, because in the final analysis commercial economy with conservation of ferrous metal products is the universal desire. Distinctive scientifio experiments must go hand in hand with an outstanding service record, otherwise the applicability of any ferrous metal under consideration is not fully established. Fig. 36 typifies economies resulting from longer ferrousmetal service. The commercially pure iron beaterhood is still in good condition after eight .years of service. The steel previously used failed in two years. This is an Ohia paper-mill installation, the hood being exposed to moist paper pulp, dyes, alum, and other chemicals. Galvanized commercially pure iron has given satisfactory service since 1908 at a Kentucky cement plant. The usual cement-plant conditions prevail, the cominercial value of the iron being demonstrated by the long service life. Another interesting example is furnished by a Califoriiia sulfur company. Although subjected to salt air and sulfur fumes, the commercially pure iron equipment is in excellent condition after ten years of service. The iron has outlasted any ferrous metal used prior to the present satisfactory installation. A twelve-year-old service life is illustrated by Fig. 4. The sheets are on the buildings of an Indiana chemical company. The commercially pure iron i s uniformly resisting the action of locomotive fumes, corrosive cheniical dust, sulfuric and phosphoric acids.
Loss Mn Cu O z / S q Ft 0 , 0 2 1 0.040 8.73 0 . 2 8 0 0 . 2 4 8 16 09
YITREOUSENAMELED COATING Nead and Kenyon6 have published a comprehensive report describing various enameling, welding, and fusion tests. This research was primarily undertaken to determine “the suitability of different materials both for welding rods and for material to be welded,” and the experiments establish the fact that “commercially pure iron manufactured in the basic open-hearth furnace is practically free from gases or gas-forming constituents.” The first method tried consisted in coating 15-in. wide 16gage strips of various materials with vitreous enamel. The enamel was baked on the strips by the passage of an electric current through them. This heated the strips quickly and uniformly t o the proper temperature. T h e materials tested consisted of commercially pure iron made in a basic open-hearth furnace, copper-bearing open-hearth iron, basic open-hearth mild steel, charcoal iron, Bessemer steel, and silicon steel. In all cases except the open-hearth irons, t h e enamel contained bubbles a n d blisters. The iron samples produced a perfectly 5
J A m . Welding SOC.,October, 1922, p. 9.
FIG.7
The commercially pure iron roof pictured in Fig. 5 already has seven years of satisfactory service to its credit. It is in excellent condition, whereas the steel pre\
