1956
INDUSTRIAL A N D 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 8z 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 i n boiler construction was determined at various temperatures in inhibited hydrochloric acid solutions. Twenty-tw-o metals 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 con tents.
October 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
Of the theories proposed ( 7 ) to explain the mechanism of inhibitors, the one generally accepted takes into account the fact that steel surfaces are composed of cathodic and anodic areas; bhe cathodes are the points of the crystal boundaries, the slag inclusions, and the location of other impurities, and the anodes are the exposed iron surfaces. When acid attacks steel, iron is dissolved at the anodes as ferrous ions and hydrogen is formed a t ishe cathodes. The inorganic inhibitors, such as arsenic, form a metallic deposit on the cathodic areas of the steel (3, 4)and the organic inhibitors form films at these areas by adsorption through the nitrogen, sulfur, or oxygen atoms ( 2 , S , I 2 , IS, 16,17)present in the organic material. The metallic deposit or the adsorbed organic film of the inhibitor acts as a blanket that covers the cathodic areas and results in an interfacial resistance to the evolution of hydrogen gas at the interface between the cathodic areas of the steels, the deposited or 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 by equivalent amounts. The net effect is that the acid attack is less than what it would have been in the absence of an inhibitor. Hydrochloric acid solutions containing an inhibitor do not attack all metals to the same extent and various kinds of steel corrode t o different degrees in the same inhibited acid solution. It has been pointed out by Farmer (9) that two steels used in boiler construction may show different corrosion in the same inhibited acid solution. Thus, it is necessary, prior to acid cleaning for the removal of scales, to know the corrodibility of the metals used in the construction of the unit. The object of the work reported herein was to measure, in inhibited hydrochloric acid solutions, the corrodibility of the various metals employed in boiler construction. EXPERIMENTAL PROCEDURE \Then an investigator undertakes a study of corrosion, the probiem of the experimental procedure always arises. The method by which the laboratory tests are carried out should duplicate as nearly as possible the corrosion that takes place in actual practice. Thus the experimental procedure used in this work included the following items: 1. The metals investigated were those actually manufactured (or boiler use. 2. The physical characteristics of the metal surfaces were changed in cleaning no more than was absolutely necessary. 3. The period of the corrosion test was 6 hours, the average length of a boiler acid cleaning treatment. 4. The corrosion tests were made a t various concentrations 3f hydrochloric acid in order t o cover the range necessary to disrolve the various amounts and types of deposits encountered in wid cleaning. 5. The corrosion tests were carried out a t the various temperaCures used in acid cleaning. 6. The inhibitors used were those actually employed in acid cleaning.
METALSINVESTIGATED. The metals used in this investigation, their application in boiler construction, and their physical oharacteristics are listed in Table I. The metals were obtained from four manufacturers and are designated in the table by the letters 9,B, C, and D. CORROSIONTEST. Cleaned specimens of the metal were weighed prior to submerging in the acid solutions which were maintained a t a constant temperature by means of a water bath. The acid solutions were preheated to the desired temperature in the water bath prior to the start of the tests, and the metal specimens were preheated in a similar manner in a beaker of water maintained a t the same tempeiature. The metal specimen was suspended in the center of the acid container b a glass hook that passed through a small hole in the center o f t h e specimen. The acid could contact all sides of the test coupon and the hydrogen gas could escape freely from the metal surface. Fresh acid was used for each test in sufficient quantity t o limit the reduction in strength to not more than 1% of the acid con-
1957
TABLEI. BOILERMETALSUSEDIN CORROSIOK STITDY Code Specification Boiler tubes SA-83A SA-83A SA-83A SA-178A
Manufacturer Source A A
B
A
Drums and headers 98-53 A SA-70 A SA-1O6A A SA-212A A Hand- and manhole plates A SA-7 SA-IO51 SAE-1035
D C
Alloy superheater tubes A SA-209T1 SA-213 SA-213 SA-213 SA-213 SA-213 SA-249
T11 T16 T16 T18 T18 T8
A A
B
A
B A.
Physical Characteristicb Seamless tubing2 inches o.d., 0.15 Seamless tubing 3 inches o.d., 0.14 Seamless tubing 4 inches 0.d. 0.15 Electric welded tubing 2 iAches inah t~hirk ~~ ..~ ...... . . Seamless tubing 6 inches o.d 0.72 Seamless tubing 3 inches a d ' ' 0.38 Seamless tubing 2 inches 0.d:: 0.14 Seamless tubing 5 inches o.d 0.50 Seamless tubing 3 inches 0.d:' 0.33 Seamless tubing 3 inches 0.d.: 0.30
inch thick inch thick inch thick o.d., 0.10 inch inch inch inch inch inch
thick thick thick thick thick thick
Seamless tubing 3 inches o.d., 0.53 inch thick Plate 0.75 inch thick Seamless tubing 2.5 inches o.d., 0.14 inch thick Plate 0.75 inch thick Forged handhole plate, dia. 3.5 X 4 inches, integral shaft Plate 0.875 inch thick Forged handhole plate, dia. 3.5 inches, center hole 1 inch Seamless tubing 2.5 inches 0.d.. 0.25 inch thick Seamless tubing 2 /nches o.d., 0.20 inch thick Seamless tubing 2 i,nches o.d., 0.20 inch thjck Seamless tubing 2 p c h e s o.d., 0.20 inch thick Seamless tubing 2 inches o.d 0.12 inch thick Seamless tubing 2 inches 0.d:' 0.22 inch thick Atomic hydrogen welded jubing 2 inuhes a d . , 0.07 inch thick
tent. The majority of the tests were carried out using 1 liter of acid, but when necessary as much as 5 liters were used per test in order not to spend the acid appreciably. The clean metal coupons were allowed to remain in the acid solutions for 6 hours after which they were removed, rinsed with water, and scrubbed lightly with a toothbrush using a mild abrasive, such as Bon Ami, in order t o remove the small amount of carbon and other impurities on the surface of the metal. The strips were then rinsed in acetone, dried a t atmospheric temperature, and reweighed.
PREPARATION OF TESTSPECIMENS. I n the study of the corrodibility of metals in inhibited hydrochloric acid solutions the size of test specimen and the preparation of the metal surface prior to the corrosion test are of extreme importance. As the metals were purchased in the form of tubing and plates, it was necessary to cut specimens for the corrosion tests. I n sawing these specimens there was a certain amount of cold working of the metal which would have an effect on the experimental results. Because the primary object of this investigation was to ascertain the corrodibility of metals used in boiler construction, it was necessary to eliminate the effect of the sawed edges. This was accomplished by using test coupons of various sizes, that had known ratios between the areas of the sawed surfaces and the sum, for the tubes, of the areas of the inner and outer surfaces; or for the plates, of the front and back sides. The areas of the sawed edges and of the normal (unsawed) surfaces were determined from the measurement of the dimensions of the pieces. Coupons of a t least three different sizes were used for each determination of corrodibility and for many of the metals four different sizes were used. The size of the specimens varied from 1 X 1 inch pieces t o tube sections 6 inches in length. As the ratio between the sawed and normal (unsawed) surfaces, the original total surface area, and the weight loss were known for the various coupons, it was possible to calculate the corrodibilities of the sawed and normal surfaces. The corrodibility was expressed as the corrosion rate in terms of pounds of metal loss per square foot of surface exposed to the acid per 24 hours of contact time. The tests were conducted for only 6 hours; if run for 24 hours the corrosion rates might have been somewhat different. Corrosion rates may be expressed in other terms, such as grams per square decimeter per day or inches penetration per year. To convert pounds per square foot per day t o grams per square decimeter per day multiply by 48.8 and to change pounds per
INDUSTRIAL AND ENGINEERING CHEMISTRY
1958
VoB. 40, No. 10
LEGEND
2
0.4
S A N D B L A S T O--B ACID PICKLE &--A
u)
\
cn' m
-I
0.08
0.04 0.00 0.08
0.04
O/e
Figure 1.
Efrect
HYDROCHLORIC ACID
or Different RPethods
0.00 0
2
Figure 2.
EffectiFrness of \. arious Inhibitors
4 6 8 Vo HYDROCHLORC ACID
10
12
of Cleaning
Test Specimens
square foot per day to inches p.er year iajsuming uiiiform penetration) multiply by 8.96. Because the metals used for the tests xvere coated with iron oxide and paint' or varnish, it was necessary to clean the metal before it ivas subjected to the corrosion tests. It was important to remove all foreign material from the surface, but it was imperative to remove only a change the physical charac I n order to investigate the effect' of the preparation of the test specimen: a preliminary study IT-as undertaken in Ivhicli three methods were used for cleaning i,hcmetal coupons-sandblasting, acid pickling, and electro-acid cleaning. In the sandblasting operation the strips m-ew placed in the path of an air-borne stream of fine silica dust, to remove any protective paint or varnish coating and mill scale that, might be present. The strips that were cleaned b acid pielcling Tr-erc first washed in acetone t o remove the protec ve coating of paint or varnish, then immersed in 30yc hydrochloric acid solution a t room tcinperature until all the oxide scale had been removed. The length of time required for the acid t o clean the test coupons varied from a few seconds t o 30 minutes, depending upon the amount of oxide present. The acid solution used for this purpose did not contain an inhibitor. Folloving the pickling operation the strips were washed with mater and dried with acetone. I n the electro-acid cleaning process the protective paint or varnish coating was removed by soaking in acetone. The test specimen \vas suspended in a n aqueous solution containing 2%l, sodium hydroxide, ly0 sodium carbonate, and 17; trisodium phosphate, maintained a t a temperature of 180 F. electrical potential of 2.5 volts direct current Tvas applied with the test specimen as the cathode and a steel container as the anode. A current density of approximately 1 ampere per square inch \Tas allowed to flow for 1 minute. Following this the strip mas rinsed in xater and then placed in a 1095 sulfuric acid solution at 60" to
80 ~ ' F cvith ~ the test specimen as the cathode as before and a carbon anode. 'The potcntial in t,his case !vas G volts direct current a t a current density of 0.02 ampere per square inch, The strip was left in contact with the acid until the oxide was removed, nhich required only a few minutes, then rinsed again in water arid dried nith acetone. The specimens cleaned by these three procedures Ivere st,ored over anhydrous calcium sulfate in desiccators until ready for we. IKHIBITORS TESTED.The corrodibilitv of a metal in an inhibited hydrochloric acid solution will depend not only upon tho procedure used for cleaning the metal, but also upon the inhibitor used in the acid solution. In order to investigate the efficiency of different materials, a preliminary s h d y was undertaken using coininercially available materials representing four different types of inhibitors-inorganic, organometallic, aliphatic organic, and aromatic organic. The inorganic inhibitor (Donell Inc., A9) contained arsenic as the active ingredient. The organometallic inliibitor was a solution of a nitrogen base containing considerable copper (American Chemica,l Paint Co., Xurodine 101). One of the organic inhibitors used was a coal-tar aromatic material (Dowel1 Ine., -%25),the exact composition of which is unknown, but containing both nitrogen and sulfur. The other organic inhibitor was dibutylthiourea (Sharpies Chemicals, Inc., 1143) dissolved 2670 by weight in a solution of nitrogen bases. The nitrogen bases, which were not eificient acid inhibitors, n-ere used as a solvent t o expedite the dissolving of the dibutylthiourea. Each inhibitor was used in the various acid solutions at a conceritration of 0.4YG by volume. A saturated hydrocarbon sulfonate wetting agent having an average ten-carbon chain v a s used in connection with all the inhibitors except the organometallic mat,erial. It has been found ( 1 ) that surface active agents improve the value of many materials as acid inhibitors. The wetting agent was used in a concentration of 0.1 % by volume of the commercial product, Du Pont Petrowet It.
October 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1959
metals used in boiler construction, the only results reported herein, except those in Table 11, are for the normal unsawed surfaces of the metals. The results given in Figure 1 were obtained in the preliminary investigation of the effect of cleaning the metal coupons by three different procedures. The metals used for this investigation confoimrd to the -4.8. M.E. specifications SA-83 type A, 58-209 Grade TI, SA-210, and SA-213 Grade T16, and were chosen because they represent steels of the various chemical compositions used in this work. The SA-83A is a low carbon and low silicon steel; the SA-209 T1 is a carbon-molybdenum alloy steel of medium carbon and high silicon content; the SA-21.0 is a medium carbon steel with low silicon content: and the SA-213 T16 is an alloy steel containing chromium, molybdenum, and titanium. The inhibitor used was the nitrogen-sulfur coal-tar material.
Figure 3.
Higher corrosion rates are obtained N hen the test specimens are cleaned by sandblasting than by the acid pickling or electroacid cleaning procedures. One reason for these higher corrosion rates may be the rough surface resulting from sandblasting, which is not taken into consideration in calculating the corrosion rates, and this method of cleaning may result in cold working of the metal, which also could cause an increase in corrodibility (19). The electro-acid cleaning procedure gave somewhat lower corrosion rates than the acid pickling. This was most noticeable inthose cases in which the uninhibitedacid tended to cause pitting. With theSA-210, the concentrateduninhibitedacid used in the acid pickling caused pitting during the cleaning operation and then when the cleaned specimens were subjected to the actual corrosion test abnormally high corrosion rates were obtained. The pitting in the case of this metal could be prevented during the pickling operation by the use of weak acid, but this procedure would have caused trouble in case of the SA-209 T1 which pitted in uninhibited weak acid solutions. Thus, the most satisfactory uninhibited arid solution for use in pickling would be one that
Corrodibility of SA-834 Boiler Tube Metals
The metals were analyzed chemically ioi ctlrbon, silicon, and sulfur (20). The carbon was determined by combustion, the silicon by the perchloric acid dehydration method, and the sulfur by the hydrogen sulfide evolution volumetric procedure. -4s the work progressed it was found desirable t o anneal a slumber of the metal specimens. This was accomplished by heating them for an-hour to a temperature of 1575" F. in a neutral aalt bath, followed by slow cooling to room temperature. ' DlSCUSSlON OF RESULTS
E m w x OF SPECIMEN PREPARATION. I t was found by using test coupons of various sizes and calculating the corrosion rates of the sawed edges and of the normal unsawed surfaces, that the edges corroded a t a different rate than did the surfaces. A few typical results are given in Table 11. Because the purpose of -bhis work was to obtain information as to the corrodibility of
v)
\O'oe v)
m
-I
TABLE11. EFFECTOF SAWED EDGES "
Metal ,SA-178A sample
SA-210 sample
Condition of Test 10%
ttatio of NealeotNormal Fig t o Sawed effect of Surface sawing
HCI, 17.5' F.
1.88:l 5.54:l 84.1:l 10% HC1, 160° F. 2.65:l 5.25:l 26.4:l 10%
HCI, 176' F.
Corrosion Rate Lb./Sq. Ft./Daf.
0.668:l 1.46:l 1.82:l
4.12:l
0.041 0.031 0,022
0.014 0.010 0.007 0.119 0.096 0,090 0.075
Normal surface
0.079 0.078 0.077 0.032 0.031 0.031
0,022 0.021 0.021
0.166
Eii6
o/o
s
__
I _
Sawed edge
0.159 0.157 0.154
0.06
SA-192 SAMPLE B 0.17 O/o C O/o Si
0.04 -
1 15OOF.
,
0.02
0,006 0.006
0.006 0.056 0.055 0.055 0.055
0
2
4 6 8 IO 12 H Y D R O C H L O R I C ACID Corrodibility of SA-178A and SA-192 Boiler Tube Metals "/o
Figure 4.
1960
INDUSTRIAL AND ENGINEERING CHEMISTRY
O.l(
Vol. 40, No. 10
SA-210 SAMPLE AQ")
g:?:
O/O
0.01 0.035 0.o
o/o gi O/o
S
__-_
I
> 0.01
2\ I-
LL
0.0
0: v) \
0.0
..Y
I
-
Figure 5.
HYDROCHLORIC ACID Corrodibility of SA-210 Boiler Tube Metals
brings about a minimum of pitting. Such a solution v, ould be a weak acid for the metals that pit in concentrated acid and a concentrated acid solution for the metals that pit in veal
