Use of Wetting Agents in Conjunction with Acid Inhibitors P. H. CAKDVTELLA4xuL. H. EILEKS Dowell Incorporated, Tulsa, Okla.
H
YDROCHLOKIC
Hydrochloric acid solutions containing various thiourea aniourit of iron dissolved and nitrogen-ring compounds have definite rates of attack acid solutions are at the anodes and thus on steel, and the addition of wetting agents lowers this often used to treat the oilretard the acid attack on rate of attack. The wetting agents when used alone in the bearing zones of oil wells the steel. acid solutions are not corrosion inhibitors. The extent to increase the permeability The generally accepted of the formation and thereof the decrease in the amount of corrosion as brought explanation of the mechaabout by the wetting agents seems to depend upon the by increase oil production nism by which organic ininitial degree of wetting of the steel by the aqueous acid( 2 6 ) . Another important hibitors retard the rate of application of hydrochloric organic inhibitor solutions. The lower the degree of acid attack upon steel seems wetting of the steel by the acid organic inhibitor solutions, acid solutions is in the reto be that this type of inthe greater is the effect of a wetting agent in reducing the moval of water-deuosited hibitor forms a film over the amount of rorroqion. scales from industrial equipcathodic areas of the steel. ment such as boilers ( Z i ) , No other theory seems to condensers (27), heat exfit the facts as satisfactorily changers (Z), and water lines (18). Inasmuch as the hydrochloric as the theory of cathodic adsorption of organic inhibitor moleacid solutions are in contact with steel when used for these purcules (3,6, 22, 26). poses, it is essential that an inhibitor be present in the arid in The organic materials that are inhibit’ors in acid solutiona conorder to retard the acid attack on the iron. tain nitrogen, sulfur, or oxygen. The inhibitor molecules when A steel surface is composed of cathodic and anodic areas. The dissolved in an acid solution migrat’e to the cathodic areas !There cathodes are the point of the crystal boundaries and slag incluthey are adsorbed onto the steel surface through the nitrogen, sions and the location of other impurities, whereas the anodes arc. sulfur, or oxygen atoms. The majority, if not all, of the organic the exposed iron surfaces. When acid attacks steel, iron is di+ materials which are acid inhibitors are capable of forming ions solved a t the anodes as ferrous ions and hydrogen is formed at when dissolved in an acid solution. Whether the ions or the the cathodes. There are two methods by which the action of inhibitor molecules migrate to the cathodes (9), an organic film acid on steel can be retarded. The amount of iron dissolved at is formed, which may consist of molecules, ions, or diRharged the anodes may be decreased by the formation at these areas of a ions of the inhibitor. In this paper, the words “adsorbed inprotecting film consisting of an acid-insoluble iron compound, hibitor film” are used to denote this organic film. or the amount of hydrogen formed a t the cat,hodes may be deThis adsorbed film of inhibitor acts as a blanket covering the creased by the deposition at these areas of some material that v d l cathodic areas and results in an interfacial resistance to the evoluretard the formation of hvdrogen. This would dwrt=av thr tion of hydrogen gas at the interface between the cat,hodic.areas
INHIBITOR ANI WETTING &EN 1
25
50
7.5
1
10.0
12 5
HYDROCHLORIC ACID
Figure 1. Effectiveness of Fatty Alcohol Sulfate in Conjunction with Pyridine and Picolines (O.l$%o by Weight) in Preventing Corrosion of Steel by Hydrochloric Acid
Figure 2. Effectiveness of Fatty Alcohol Sulfate in Conjunction with Lutidines and Collidine (0.1% by Weight) 1951
1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
2.5
I
5.0
I I 7.5 10.0 % HYDROCHLORIC ACID
12.5
0.1
I 5.0
7.5
Vol. 40, No. 10
I
1b.o
12.5
150
% HYDROCHLORIC ACID
Figure 3. Effectiveness of Fatty Alcohol Sulfate in Conjunction with Quinoline and Monomethyl Quinolines (0.1% by Weight)
Figure 4. Effectiveness of Fatty Alcoh~l Sulfate in Conjunction with Xsoquinoline, 3-;\.lethylisoquinoline, and Acridine ( O * l % by Weight)
of the steel, the adsorbed inhibitor film, and the acid solution. As a consequence of this interfacial resistance the amount of hydrogen formed and the amount of iron dissolved are decreased in equivalent amounts, with the net effect that the acid attack is reduced from what it would have been if no inhibitor had been present. “Interfacial resistance” as used here refers to the total effect brought about by the presence of the adsorbed film. This resistance is made up of the electrical effects, such as any repulsion the film may have toward the ions in solution, any barrier the presence of film has to the passage of current across the metal-solution interface, and also includes the apparent overvoltage of hydrogen on the film. To be a good inhibitor, a material must be positively adsorbed -that is, it must be a t a somewhat higher concentration a t the metal-liquid interface than Tithin the bulk of the liquid. hlost organic inhibitor materials when dissolved in acid solutions have only slight effect on lowering the surface tensions or causing the acid solutions to form zero contact angles upon steel surfaces. This paper presents results of a study of the effect upon the corrodibility of steel by inhibited hydrochloric acid solutions having low surface tensions and high degrees of wrtting for steel surfaces.
In order to investigate the effect of the wetting agent, the SUIface tensions and the contact angles formed by the hydrogen bubbles on the metal surface in contact with the acid solutions were determined. A steel strip was placed in acid and a bubble of hydrogen was photographed as it was being formed. The diameters of the bubbles photographed were less than 1 mm. and were magnified from 50 to 100 times in the final picture. From the photograph made directly on bromide paper the contact angles were measured by means of a tangentmeter ( 2 ) . The reproducibility of the contact angles v a s la^ The contact angles as measured under this procedure are advancing angles The surface tensions were determined by means of a du Nou? tensiometer, taking into consideration the corrections of Harkins and Jordan ( 5 ) . In this investigation various nitrogen-ring compounds oi the pyridine, quinoline, and acridine series, as well as four thiourea compounds were used as inhibitors for acid solutions. The mateiials tested xiere of best grades available and no attempt wa5 made to purify them further. The two wetting agents used were commercially available materials, a saturated hvdrocarbon suiEonate x-ith an average CIOchain (Du Pont Petrowet R) and B fatty alcohol sulfate nith chains containing from eight to ter, carbons (Du Pont Petrowet WN).
EXPERIMENTAL PROCEDURE DISCUSSION QF RESULTS
ffeighed sandblasted mild steel strips (A.S.T.M. A10-39), 1 x 2.75 X 0.125 inch, ?ere placed .in 150 ml. of various concentrations of hydrochloric acid solutions that contained different organic inhibitors and wetting agents. The acid solutions were preheated to the desired temperature in a water bath prior to the start of the tests and were maintained during the test a t & constant temperature of 79.4 “.C. by,means of the water bath. The metal strips yere preheated in a similar manner in a beaker of lvater maintained a t 79.4’ C. The strips were left in the acid for 6 hours, then rinsed in water and scrubbed lightly with a toothbrush and a mild abrasive, sued as Bon Ami, in order to remove the small amount of carbon and other impurities on the surface 0: the metal. FolloFying this the strips were rinsed in acetone, dried a t atmospheric temperature, and reweighed. AS the weight loss of the strips and the surface areas were known, it was possible to calculate the corrosion rate of the metal expressed as pounds of metal loss per square foot of surface ex osed to the acid per 24 hours of contact time. If the tests had teen run for 24 hours, the corrosion rates might have been somewhat different. All corrosion tests were run a t least in triplicate and the checks varied no more than 1% from the average.
EFFECT O F ADSORBED CROSS-SECTIOYAL AREA. Mann (9, 10, 11) has pointed out that in the case of the aliphatic and aromatic amines the organic materials are adsorbed through the nitroger. onto the metal surface. It was found 1% hen using these amines that the greater the cross-sectional area of the inhibit01 projected onto the metal surface the better the inaterial is as an inhibitor The results reported herein are in general agrcement xith this conclusioii. In Figure 1 are given the results obtained ivith pyridine and the monomethyl substituted compounds of pyridine, the picolines. The broken lines are the results obtained s i t h organic inhibitors dissolved in hydrochloric acid and the solid lines are the corresponding results when a wetting agent is added to t h e acid solution of the organic inhibitor. When the pyridine a n c the picolines are adsorbed through the nitrogen atom, the order of decreasing cross-sectional areas of these materials, ab projected onto the metal surface, will be 2-picoline, 3-picoline, 4-picoline.
October 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1953
10 LEGEND
UPPER C U M 10% HCL 5
U M E R CUM 2.59'0 HCL
0.045 MOLECULAR
W E M
''
50 %
75 10.0 HYDROCHLORIC ACID
12 5
I50
Figure 5. Relationship of Molecular Weight to Effectiveness of Nitrogen Bases at Nitrogen Content of 0.015Yo by Weight
Figure 6. Effectiveness of Fatty Alcohol Sulfate at 0.1% by Weight in Conjunction with Thiourea Compounds at 0.05% by Weight
and pyridine. When the methyl groups are in the ortho position (2-picoline) the molecule has the greatest cross-sectional area projected onto the metal of any of the monomethylpyridines. The effect of the methyl group on the cross-sectional area decreases as the substitution moves away from the nitrogen atom. Of these materials 2-picoline is the best inhibitor and the other materials are in the same order as their projected cross-sectional areas-Le., the greater the cross-sectional area the better the material is as an inhibitor. The results obtained with 2,4,6-collidine and 2,4- and 2,6lutidines (Figure 2) are in agreement with those of the monomethylpyridine series as to the effect of the size of the molecule on its value as an inhibitor. In comparing these three materials with pyridine and the picolines of Figure I, it will be noted that the two lutidines and the collidine are the better inhibitors, Although the same weight percentage of each material in Figures 1 and 2 was used, there actually is more nitrogen present in the acid solution containing the pyridine than in the case of the picolines, for the lutidines there is a slightly smaller amount of nitrogen, and the 2,4,6-collidine acid solution contains the least amount of nitrogen. A direct comparison of different compounds can be made only a t constant nitrogen content (Figure 5). When the 2,4-lutidine is adsorbed in the inhibitor film, the molecule should have a slightly greater cross-sectional area parallel to the metal surface than the 2-picoline because of the additional methyl group para to the nitrogen. This material is also a better inhibitor than the 2-picoline. In the case of the 2,6lutidine, in which both methyl groups are ortho to the nitrogen, the molecule will occupy a somewhat greater cross-sectional area than the 2,4-lutidine, and the graph shows that the material is a better inhibitor. The 2,4,6-collidine should have an even greater cross-sectional area than the 2,4- or the 2,6-lutidines, and the graph shows that the material is the best inhibitor of the three. The effect of the addition of a phenyl group to a molecule is shown in Figures 3 and 4. The order of increasing cross-sectional area of the materials reported in these figures would be isoquinoline, quinoline, 4-methylquinoline, 3-methylisoquinoline, 2methylquinoline, and acridine. The value of these materials as inhibitors for acid solutions is in the same order. The effects of the cross-sectional areas of various materials upon their value as inhibitors should be compared only under conditions of constant amount of nitrogen present. In Figures
1 to 4 a constant weight (0.1%) of the inhibitor was used regardless of the amount of nitrogen in the compound. In Figure 6 are given the results obtained when sufficient nitrogen base was used to give a constant amount of nitrogen (0.015%) in the acid solution. There seems to be a limit to the extent to which the crosbsectional area will affect the value of a material as an inhibitor Quinoline is approximately twice as effective an inhibitor as pyridine, and this may be expected, inasmuch as quinoline should have about twice the cross-sectional area parallel to the metal surface as does pyridine. The value of acridine as an inhibitor is only slightly better than that of quinoline, yet its cross-swtional area is somewhat greater. In Table I are calculated the increases in value of various compounds as inhibitors resulting from the presence of an o-methy! group. The value of the addition of the ortho group remains fairly constant with increased size of the molecule. If the ortho group is of value only in increasing the cross-sectional area, its effect should decrease with increasing size of the molecule. Thus, other factors in addition to the cross-sectional area must influence the value of the material as an inhibitor, especiallv in the case of quinoline and 2-methylquinoline.
TABLE I. EFFECTOF ORTHO-SUBSTITUTED METHYLGROITPS IN PREVENTING ACIDCORROSION BY NITROGEN BASES (Tests made a t 79.4' C., using sufficient base t o give 0.015% nitrogen In solution) LT, Reductinn i;l Corrosion 2.5% 10% Materials Compared HC1 HCI Pyridine 2-Picoline 22 21 2-Picoline 2,6-Lutidine 36 34 4-Picoline 2 4-Lutidine 32 24 2,4-Lutidine 2:4,6-Collidine 23 28 Quinoline Quina 1din e 31 21 Isoquinoline 3-Methylisoquinoline 23 20
Numerous examples in the study of adsorption indicate that the position of the substituted group has a great effect on the rate and amount of a material adsorbed. The ortho compounds arc usually adsorbed much more strongly than the meta and para compounds (7, I S , 19). This implies that the nature and the arrangement of the atoms or groups within a molecule have cmn-
1954
I N D U S T R I A L AND. E N G I N E E R I N G C H E M I S T R Y
Vol. 40, No. 10
3, 4, and 6 give the corrosion rates of mild stoel in hydrochloric acid solutions containing a rvetting agent and an organic inhibitor. The .rvct,tingagent used is a fatty alcohol sulfate containing from eight t o ten carbon chains. I n Figures 7 and 8 are given the xsults obtained with a saturated liydroearboii sulfonate wetting agent, of an average carbon chain of ten atoms. The results with the two n-ctting a g w t s , in gcncral, are about thc same. The \vetting agents n h m used alone are not inhibitors, as is shoivn in Tablt, rI, but w h t m used in conjunctiou viith the nitrogen bases or t h r thiourea compounds there is a reduction in the amount of vorrosion of the steel 0 ~ ~ that 1 ' given by the acid containing only thi. orgaiiic inhibitor. In general, tlie presence of thc wetting a,gcnt does riot change thr order of the effectiveness of the nitrogen bases or the different thiourras, but the graphs indicate that the wetting agents usually have much more effect in the wralrer arid solution3 tlivri in t h r more concentrated solution.
. Corrosion _ _ _ _ Rates, _ _ _ Lb.: ~ 5y. P t. /U)Y
Figure 7 . Effectiveness of Saturated Hydrocarbon Sulfonate in Conjunction with Pyridine and \'arions Methyl Substituted Pyridines (Q.lY0 by Weight)
.
sidrrable effect on the 6tretiytIi oi the' adsorptiori boiid. 'I'ht. ortho-substituted nitrogen compounds may be brt,ter inhibitors, in part, because of the increased ngth of the adsorption bond, i w s e d by the presence of an ortho-substituted group. Ch'iao and Mann (4)have pointed out that the adsorbed niolecules of the inhibitor may be tilted upon the ~iietalsurface and the angle of inclination may vary for the same corrosion inhi.bitor, depending on the concentration of the inhibitor in the acid solution. That the ort,ho-substituted methyl group brings aboui such a phenomenon may explain in part the results given i i i Table I, but it is difficult to understand from the idea of a tilting nolecule n-hy an o-methyl group should cause approsin1 the same percentage rduction i n coim?icin r ~ i ~ g a ~ ~ofd lt hi i~ ~ e of the molccule. .-Inother factor, n-hich miiy tiavc the olipu~iteefft,ct, is 1 he clozt.rie6s of the packing of t,he illhibitor nmlr~culcs\\it hin the wthoriicd film. The film irhich has the IK~OI(:CUICTthe clo ng else is the hame, gin. bc,tt(:i, protection of l h c acid, because such a Khn xi11 rontain Les~liolethrough Tyhich the hydrogen ion can niigratt: to pick up electron5 irom the metal surface and be rvolved ad a gas. Thus, thr mort. jgmnietrical the moleculr arid lcss the n u r n h of substitut rci groups present, the closer would he the packing of the molecules. From the standpoint of closeness of packing, pyi,idine and qiiinoh e should give the tightest packed film. Thus, \Then equilibrium i s established a t the. metal-liquid i l l terface bet,ween the rate of adsorption of the inhibitor film and $he discharge of the inhibitor into t h e acid solution, a number of ,factors will affect the value of the material as an inhibit,or. 111 addition to the cross-sectional area of the molecule, strength oi' &headsorption bond het\Teen the nitrogen and the metal and the closeness of packing of the moirrulrs into t>headsorbd film must be taken into consideration. Figure 6 gives t,he results obtained \vith tliiourca and the th~,r.isymmetrically substituted diethyl-, diliutyl-, arid diphenylchioureas. Here, as in the case of the nitrogen bases, other farcors must be taken into considcration in addit,ion to the pro,iec:t,ed cross-sect,ional area of tlie molecule onto the metal surface. For example, diethylt,hiourea is better inhibitor than is: dibutylLhiourea for solutions containing more than 10% hydrochloric. acid (the dot-dash lines), but its cross-sect,ional area is less than :hat of dibutylthiourea. . 't'hia . d i t 1 lilies in P'igures 1, 2, EFFECTOF WETTIXC:r \ c
>litrerial 'I'rited
alone .'.cid and O . l ~ cfatty a l c o h o l b u l f a t e .Acid and 0.1% .silitirar?d hydrooar. hon siilfonntr .ICld
5c.:
15%
0.966
1.56 LA1
10% HCl 2.79 2.98
O.Qil?J
1.60
%.\I4
:3,94
2.5%
HCI
HC1
0.Y51
HCl
3.89 3.99
merits of contact, angle and surface given in T I. In expressing the contact, angla the coiivention has been observed of reporting the value as that oiitaiiied when measured through. the water phase. Photographs of hydrogen bubbles on steel in contact v-ith inhibitcd hydroi%hloricacid solutions are shown in Figure 9. TThcn the fatty alcohol sulfate n-etting agent was used in the 2.5% h drochloric wit1 solutions in addition to the organic inhibitors 1; I l l , the contact angles n'ere alw-ays zero regardless of the organic inhiliitor and the surface tensions varied from 29.3 to 29.7 dynes pc>r c'iii. In the case of the saturated hydrocarbon sulfonate, the 1-ontact angles ere also zero for. 2.57, hydrochloric acid solutions c,ontaining the organic inhibitors given in Table IT1. Thcl surface, ten4ons varird from 28.3 to 28.8 dynw pc.1. cm. ti~iirioiiare
Figure 8. Effectiveness of Saturated Hydrocarbon Sulfonate (0.1qo hy Weight) in Conjunction with Nitrogen Bases (Oslq, by Weight) and Thiourea Compounds (0.05q0 b) Weight)
October 1948
195s
INDUSTRIAL AND ENGINEERING CHEMISTRY
the decrease in corrosion rate is about 50%. When the contact angles are fairly low, as in the case of the picolines, the improvement is likewise smaller. During the corrosion tests it was noted that, when no wetting agent was used, bubbles of hydrogen would form on the metal surface and occasionally one of the bubbles would gron until it was fairly large. Such a bubble of hydrogen was considerably larger than the average bubble leaving the Figure 9. Photographs of Hydrogen Bubble on Steel metal surface. Eventually In 2.5 70hydrochloric acid solution containing 0.05 9% sym-diethyl thiourea the bubble would leave the Left. X 100. Contact angle 48' measured through a ueous phase metal, and whenever it Right. X400. Contact angle O 0 measured through aqueous phase. %ohtion also contained 0.1% of fatty alcohol sulfate did, another was usually formed in the same mot (as tal an could be determined by visual observation) and When a contact angle is formed by a liquid on a solid, the equaseemrd without fail to grow also to be a large bubble. After tion expressing the relationship between the different interfacial tt series of such bubbles had been formed on the surface, it tensions and th? contact angle (E))is: was noted that a pit was being formed. If a wetting agent was present along with the inhibitor in the acid solution the bubbles of hydrogen never grew very large; in fact, the majority of the bubbles were much smaller than the where A l j is the adhesion tension, SIis the surface tension of the bubbles when no wetting agent was used. Before the bubbles solid, SI3is the interfacial tension between the solid and aqueous grew to be of appreciable size, in the presence of a wetting agent, solution, Sa is the surface tension of the aqueous solution, and they left the metal surface. The wetting agents did not 013 is the contact angle. entirely prevent pitting, but they did greatly reduce the amount In Figure 10 are given the result,s obtained by plotting adhesion and size of the pits. Inhibitors such as the substituted thioureas, tensions against the percentage decrease, as brought about by the quinoline, and its derivatives, which tended to cause pitting withpresence of the wetting agent', in the amount of corrosion of st'eel out a wetting agent, gave only slight amounts of pitting when by inhibited hydrochloric acid solutions. The adhesion tensions used with a wetting agent. are a measure of the degrees of wet,ting of the steel by the inhibThis decrease in the amount of pitting may be attiibuted to the ited hydrochloric acid solutions and are calculated from t'he surfact that the contact angles were zero for the acid solutions conface tension and contact angle measurements. The surface tensions for this work were determined a t the liquid-air interfa,ce. whereas the contact angles were measured a t the steel-liquid-hy6 drogen interface. The assumption that the surface tensions at the liquid-air and a t the liquid-hydrogen interfaces are the same may not be entirely correct, but for these calculations it should A 1 I I I z not introduce,an appreciable error. 8 5 The greatest reductions in corrosion as brought about by the wetting agent are in the cases of the lowest degrees of wetting of THIOURE the steel by the inhibited hydrochloric acid solutions. With the 0" thiourea and substituted thiourea compounds where the contact 84 angles are fairly high, 32" t,o 54'-i.e., low dcgrees of wetting-
Q
8
5
TABLE111. SURFACE TEKSIOKS AND CONTACT ANGLES h f E A S U R E M E N T AT
25 C.
(Surface tensions determined a t liquid-air interface. Contact angles formed by hydrogen bubbles on steel surface in acid solutions) Surface Tension, Material Added DynesICm. Contact Angle,' to 2.5% HCI 15 67.3 0 , 1 % pyridine 0.1'70 2-picoline 65.6 19 0 . 1 % 3-picoline 66.6 16 12 0 . 1yo 4-picoline 68.4 35 0 , 1 % 2,4-lutidine 68.6 0 . 1 % 2,G-lutidine 32 68.8 36 67.3 0 . 1 % 2,4,6-collidine 64.1 22 0 . lYOquinoline 24 65.6 0 . 1 % quinaldine 24 64.1 0 . 1 Yo lepidine O.lYO acridine 67.7 39 64.2 32 0 . 0 5 % thiourea 58,6 48 0 .OS% diethylthiourea 54 0 .05To dibutylthioyrea 57.0 62,l 43 0 . 0 0 % diphenylthiourea
I 3
B
B
I I - -
z -PICOLINE--*'
3 PlCOLl NE-\
8! 21
PgRIDINE
-0
4 PlCOLlNE- 4
L
o
I1
Figure 10. Relationship of Degree of Wetting (Adhesion Tension) of 2.59" Hydrochloric Acid Solutions of Inhibitors and Percentage Reduction in Amount of Corrosion of Steel by Inhibited Hydrochloric Acid Solutions In presence of fatty alcohol sulfate wetting agent. Nitrogen hasea and wetting agent at 0.1% and thioureas at e.@;% all by weight
1956
INDUSTRIAL AND ENGINEERING CHEMISTRY
iainiiig the inhibitors and n ilttiiig agcliit. This irittans that the acid solutions containing the 1% etting agents completely wet the metal and in so doing prevent the bubbles of hydrogen from having any more than a very slight contact with the metal surface. Because the acid wets the metal it will prevent the bubble* of hydrogen from becoming ~ e r ylarge before they escape from ihe metal and as a result all the hvdiogen is liberated in the form of extremely small bubbles. In the forming of small bubblcs *hereis less tendency for the acid t o cause pitting (8). Although the lower corrosion rates are due in part to decreased pitting, other factors influeliced by the piesence of the m t t i n g agent may also have an effect. I t is doubtful that the amount of corrosion can be decreased as niuch as 50% by the lone factor of reducing the amount of pitting. Thus, other factors in addition i o the decrease of pitting must be t a l r ~ nin consideration to explain the value of wetting agent%
m n c u SIONS The presence of wetring agent3 in acid solutions coiitainirig Irganic inhibitors reduces the amouni of corrosion of a metal, to an extent that seems to depend upon the initial degree of wetting of the metal by the aqucou.. acid-organic inhibitor solution, The lower the degree of wetting of the metal by the acid organic inhibitor solutions, the greatpr is the effect of wetting agents in reducing the amount of corrosion. partial explanation mal tit. that the higher degree of wetting resulting from the addition of wetting agents brings about a reduction in the size of the hydiogeri bubbles, and that thi- cause? a decreasp in the pitting type 01 vorrosion. The Tvork reported herein 13 111 agrcwnent ~ i t the h theory that rtic c r o s ~ e c t i o n a area l of a n adborbed organic molecule as projected onto a metal surface is one of the factors thal determine rhe value of the material as a corrosion inhibitor. The crosswctional area of the adsorbed molecule as projected onto a metal surface determines the area protected from the hydrogen ions In general, the value of a material &s a coirosion inhibitor inweases through the series. pyridine, the picolines, the lutidines,
Vol. 40. No. 10
rollidine, quinoline, isoquinoline, lepidine, quinalidinc, arid ac.1I dine. Thiouiea and the syrnmctrical substituted derivative\, diethyl, dibutyl, arid diphenyl, do not in all cases show increaqed value as inhibitors xith increased cross-sectional area. Iri the case of the o-methyl substituted nitrogen-ring compounds, the increase in value as an inhibitor as brought about by the presence of an o-methyl group was fairly constant regardless of the size of the molecule. A possible explanation is that the ortho-substitutpd group ma); affect the strength of the adsorption bonds LITERATURE CITED
(1) Alquist,, F. Tu’., Groom, C. H., and Williams, G. F., Z’runs. Am. Soc. Mech. Engrs., 65, 719 (1943). ( 2 ) Bartell, F. E., and Cardwell, P. H., J . Am. Chem. Soc., 64, 494
(1942).
(3) Chappell, E. L., Roetheli, B. E., and McCarthy, B. Y . , IND.
EXG.CHEM., 20, 582 (1928). (4) Ch’iao, Shih-Jen. and Mann, C. 8..Ibid., 39, 910 (1947). ( 5 ) Harkins, W.D., and Jordan. H. F., J. A m , Chem. Soc., 52, 1751 (1930). (6) Jimeno, E., Grifoll, I., and Morral, F. R., Trans. Electrochem. Soc., 69, 105 (1936). (7) Linner, E. R., and Gortner, R. A., J . Phus. Chem., 39, 35 (1935). (8) McCulloch, L., J. Am. Chem. Soc., 47, 1940 (1925). (9) Mann, C. A , , Tvans.Electrochen. Soe.. 69, 115 (1936). (10) Mann, C. A., Lauer, B. E., and Hultin, C. T., IKD. E m . CHEM., 28, 159 (1936). (11) Ibid., 28, 1048 (1936). 112) Mnnner. H. P.. T m n s . Electrochem. Soc.. 69. 85 IlR.76) (13) Phelps, H. J., J . Chem. Soc., 1929, 1724. (14) Pon-ell, S. T., Trans. Am. SOC.Mech. Engrs., 68, 905 (1946). (15) Putnam, S. W., and Fry, W. A, IND.ENG.CHEM.,26,921 (1934). (16) Rhodes, F. H., and Kuhn, 1%’. E., Ibid., 21, 1066 (1929). (17) Rush, J. S., and Jennings, ITr*S.,Petroleum Refiner, 23, 07 11944). (18) Speller,‘F. N., Chappel, E. L., and Russell, R. P., Trans. Am. Inst. Chem. Dngrs., 19, 165 (1927), (19) Weiser, H. B., “Colloid Chemistry,” p, 77, New York, ,John Wiley & Sons, 1939. (20) Young, T., Trans. Roy. Sac. (London),95, 65 (1805) \-
HEcnrveD July 17, 1947. Presented before the 15th Midwest Regional Meeting of the A 4 ~ m ~ I CHEAIICAL C A ~ SOCIETY.Kansas City, Mo.
Corrosion of Boiler
J
Inhibited Hydrochloric J
P. H. CARDWELL AND S. J. MARTINEZ Dowell Incorporated, Tulsa, Okla.
CID solutions have beexi used for many years as pickling agents in the iron and steel industry to remove oxide deposits from metallic surfaces. Inhibitors are used in the acid solutions to retard the attack on the iron by the acid without appreciably affecting the dissolving of the scale. Only during the past few years has inhibited acid been used extensively and commercially for removing deposits and sludge encrustations from industrial equipment such as boilers, heat exchangers, condensers, water lines, and other miscellaneous units. The use of hydrochloric acid solutions for dissolving encrusta;ions from industrial equipment has been an outgrowth of the development of efficient inhibitors. During the removal of the deposits the acid comes into contact with steel and other metals; thus, inhibitors must be used to protect the metal from appreciable amounts of corrosion. Of the many materials which are inhibitors for hydrochloric acid solutions, Some give very little
protection to the metal whereas others give nearly complete protection. Therefore in acid cleaning extreme care must be taken in selecting inhibitors in order to prevent serious damage (16‘) to the equipment being cleaned. 7
1he ,
corrodibility of metals used in boiler construction was determined at various temperatures in inhibited hydrochloric acid solutions. Twenty-tw-ometals representing fourteen different A.S.M.E. code specifications as well as forty boiler handhole plates were used in this investigation. A study was made of the effectiveness of four acid inhibitors and of the method of preparing the test coupons. Considerable variation in the corrodibility of the metals was attributed in part to their carbon and silicon contents.
