Kinetics of the Corrosion Process in Condensate Gas Wells NORIIAY HACKERMAN ~ N D 'HAROLD R. SCHVTDTI Irnicersity of Texas, Austin, Ter.
R a t e curves for the corrosion of SAE 1020 steel in a number of natural gas-condensate wells were obtained for relatively long periods of time. The coupons' surfaces were examined microscopically and the composition of the corrosion products was determined by x-ray and diffraction methods. The results substantiated the corrosion classification proposed earlier-namely, corrosive, mildly corrosive due to less aggressive corrodent, noncorrosive, even though the highly active corrodent is present. Equations €or the linear portions of the curves
are given. A n inverse relationship between the thickness of the surface layers and the steady reaction rate is shown to be obtained for the first two of the three corrosive types. In the third type, a change in the nature of the film is evident, which is ascribed to the presence of inhibitors, natural or injected, in sufficient concentration for the conditions of gas production. I t is postulated that diffusion of corroding agent to the metal is the steadyrate controlling factor and that this in turn depends on the nature of the siirface layers.
THE
was not controlled. Following the electrolytic action the Ramples were scrubbed thoroughly with a stiff bristle brush and an abrasive soap, dried, and weighed. The yeight loss of five clean, unexposed coupons due t o the above procedure averaged 4 mg. for t'he 40.3 sq. cm. area available. In viea- of the weight loss incurred by the exposed coupons the correction was conkdered to be ncgligible and was not applied in obtaining the rate curves. In a few cases the composit'ion of the surface layers was studied without removing them from t'hc cxposcd coupons. Small steel plugs were exposed t o the well stream by screwing them into 0.3 X 2.5 X 7.5 cm. steel holders which were inserted into the coupon containers already available (3). The plugs, which were 2 mm. thick, were cut from a 4-nim. diameter ShE 1020 st>eel bar. These plugs were exposed for relatively short periods, 1 to 14 days. By means of a special adapter the plugs wcrc placed on the electron diffraction attachment of an RCA Type EMU-1 electron microscope. The elect,ron beam was allowed to strike the exposed plug surfacc a t a low angle of incidence and the pattern was obtained by reflection. The wells in which the exposures were made are listed below, along with the fields in which they are situated:
problem of corrosion iii condensate gas wells appearh to be yielding to the concerted efforts being devoted t o it. In many cases some alleviat,ion has been obtained by chemical treatment,, either by increasing the p H of the water slightly viit,h alkaline reagents or by treating the metal, in place, with inhibitors. Furthermore, certain low nickel or chromium steel alloys have shown considerable promise in resisting the corrosive action. The nature of the corrosive process has been elucidated to some extent. The corrosive agents have been known for some t,ime to be the lower fatty acids and carbon dioxide in the presence of water. The relative effects of each of these have been studied and it has been shown that t,he corrosive iict,ion of carbon dioxide becomes marked a t a partial p r r s s u i ~of an atmosphere or more ( 4 ) . I n a n earlier paper (3)in this series it, was shown that gas condensate wells could be classified into three types with respect to corrosion characteristics. Since then, this classification has become more firmly establishcd on the basis of a large number of individual coupon exposures in many wells. I n the work reported here quantitative determination of the reaction rates were made for SBE 1020 steel coupons exposed to the fluids of seven wells. Exposures were made in six of the wells which were untreated, in t'wo of the same wells while naphthenic acid was being injected, and in the seventh well while sodium chromate was being used. I n addition, the appearance of the surface was observed microscopically and the composition of the corrosion products adhering to the metal surface was investigated. EXPERIMENTAL METHODS
The procedure for pretreating, exposing, handling, and exainining the 0.16 X 2.5 X 7.5 em. coupons has already been described ( S ) . The solid corrosion products from all coupons exposed were removed by scraping the surface carefully with the edge of a glass microscope slide. X-ray patterns of these materials, pulverized t o a fine powder, were obtained using a General Electric x-ray diffraction unit (G-E XRD-1 Unit). The K lines of copper were filtered through nickel oxide t,o give monochromatic radiation (CuKa). Identification of t,he resulting patterns was made by consulting the A.S.T.M. card file of x-ray patterns (1). The coupons were electrolytically descalcd for 3 minutes in a 5y0 sulfuric acid solution containing 2 ml. of a pickling inhibitor (Rodine 67, American Paint Company) per liter of solution. A current density of 0.4 ampere per sq. em. of surface area was used. A carbon rod was used as the anode; and the coupon, totally immersed in the solution, acted as the cathode. The temperature 1
H u n t Stewart well No. 3 Cotton \'alley Field, La. G r a n t well No. 1 Cotton Valley Field, La. Chicago well I i o , l a Carthage Field, Tex. Jones h-0. 1 Chapel Hill Field, Tex. Opelika Field, Tex. Tullos No. 1 Well 33-5 Erath Field, La. T h c gas can be groduoed froin either t h e Upiier or L o w x Pettit f o r i i i a tion. RESULTS OF FIELD STUDIES
Ht-NT STEWART WELL KO.3. This well was known by field experience to be highly corrosive. Two series of tests were made; the first under normal conditions and the second while naphthenic acid (1 gallon for appr~ximat~ely 6,000,000 cubic feet of gas produced) was bcing injected into the stream t'o rctard tmhecorrosion rate. Laboratory tests had previously showri that naphthenic acid functions as an effective inhibitor in anaerobic systems ( 5 ) . The dat,a obtained from the two exposures are plotted in Figure 1. The equat,ions for the linear portion of these rate curves are: As-removed ?/ = 0.252 0.346 t (1)
+
Descaled
y = 0.390
+
0.352 t
(2)
where 1 is the time in weeks and y is the weight lost in grams per 100 sq. cm. of surface area. The slope gives the steady, or continuing, rate of the reaction. This value may be converted to the more usual corrosion rate units of milligram per square decimet,er per day by multiplying by 1000/7. At the beginning of the exposure a surface layer formed which
Present address, General Electrical Company, Riohland, W s h .
1712
August 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
i
/A
3.0
I
1
2
3
TI
4
5
6
7
0
ME I N WEEKS
Figure 1.
Corrosion R a t e Curves f r o m Coupon Exposures in H u n t Stewart No. 3 Well First exposure: 0, as removed; 8, deacaled
Second exposure:
a,as-removed;
CB,
desoaled
was subsequently partially removed by flaking and possibly by dissolution in the acidic medium. Following this removal a secondary surface layer formed. With either the primary or secondary layer, the metal surface showed pits which were first observed after 2 weeks of exposure. The pits were comparatively small and did not appear t o increase markedly in size. Apparently the surface layers were removed from the metal before the pits penetrated very deeply. A portion of the surfaces of all the coupons which had been exposed for more than 5 weeks was covered with a copper colored layer. The color was due t o interference fringes which indicated that this layer was very thin. The color changed t o blue on being exposed to the atmosphere, indicating that it had thickened through oxidation. An attempt was made to obtain a diffraction pattern of these surface products but only a small amount of the material could be scraped off and no pattern was found. The curves of the as-removed data and the descaled data are almost parallel and show no change in slope after the fist week. This indicates f i s t , that the surface layers are of constant thickness, and second, that the surface layers do not appreciably impede the rate of attack. The x-ray and electron diffraction patterns showed the presence of ferrous oxide and ferrous carbonate in the layers. It is apparent from the curves for the second series of coupons, exposed while the well was being treated with naphthenic acid, that the rate did not change appreciably from that found with the first series, although the inhibitor did cause a change in the nature of the attack. During the first 5 weeks the surface layers increased steadily in thickness. The rate equations for this period are :
‘
As-removed
y = 0.000
Descaled
y = 0.080
+ +
by corrosion or by erosion. At this point, the second break in the rate curves, the initial rate of attack of the first five-week period was resumed. The slope of the curves from the seventh week on are: as-removed, 0.360 and descaled, 0.353. The surface layers on the coupons exposed from 5 to 7 weeks afforded a protection analogous to that found in the Grant well. The tabling effect had been observed previously ( 3 )in a series of experiments made in the Tullos No. 1 well. The corrosion product found in the surface layers formed during the exposure to the inhibited stream proved t o be mainly ferrous carbonate as determined by x-ray data. An important difference between the first and the second exposure is that in the latter pitting was not observed, whereas in the former, the incidence of pits mas high. GRANTWELL No. 1. Two series of tests were made on this well, one under natural conditions and one while the well was being treated with naphthenic acid in the same dosage as described just above. The data obtained are plotted in Figure 2. The equations of the curves from the first exposure are: As-removed
y = 0.030
Descaled
y = 0.200
(3)
0.322 t
(4)
During the fist 2 weeks blisters formed on the surface which subsequently broke and were swept away. This is believed t o be due to a gas-forming reaction. The ridges remaining from the blisters were gradually covered over until the fifth week, corresponding to the beginning of the flat portion of the rate curve. The surface layer then appeared to be continuous-Le., no pits or blisters were observed. As can be seen a t the first break in the curves of Figure 1, these thickened layers were obtained by the fifth week and remained through the sixth week. Some blisters again appeared a t the sixth week indicating that the surface penetration was occurring again. Between the sixth and seventh week the layers apparently were undermined and sufficiently loosened by the blisters so that they were removed either
+ +
0.105 t
(5)
0.136 t
(6)
The slopes of the two curves are slightly different, indicating a slow thickening of the surface layers a t the expense of the basis metal. Micrographic examination of the surfaces showed them to be covered with a corrosion product which was very adherent in spots but which would flake off in others with subsequent formation of a second layer. These coupons were difficult t o descale but pitting was not observed. X-ray and electron diffraction of the corrosion product showed the presence of ferrous oxide and ferrous carbonate. Examination of the curves for the second series of tests shows that there was an initial build-up of corrosion product although the corroding constituents penetrated the layers allowing the reaction t o proceed a t a constant rate. The equations for the curves are: As-removed
y
=
-0.130
Descaled
1/ =
+O.lOO
+ 0.060 t + 0.097 t
(7) (8)
The surfaces appeared t o be the same as those found in the previous test, although some blister formation was observed. The blisters broke with subsequent flaking, leaving an apparently clean surface. The surface remaining after flaking appeared hazy under the microscope indicating that a thin film remainpd 01
f
I
I
-
U
X ‘u
0.281 t
1713
P0
09
a 0.0 n
w
n
2
3
4
5
6
7
8
TIME IN W E E K S Figure 2. Lower met
Corrosion R a t e Curves from Coupon Exposures i n Grant No. 1 Well from first exposure and upper setfromseoond. 0, asremoved;
0, descaled
INDUSTRIAL AND ENGINEERING CHEMISTRY
1714
I
was rapidly formed after the layer broke away. The corrosion product was found t o contain ferrous carbonate. CHICAGO WELLYo. 1. Two series of exposures were made in this well-the first while the gas mas being taken from the Upper Pettit formation and the second from the Lower Pettit forma,tion. The data obtained are plotted in Figure 3. The stream from the L-pper Pettit formation is of intermediate corrosive character, although this is not. obvious from the graphs. The equations for the lines are: As-removed
q = 0.302
-
0.020 t
(9)
Descaled
q = 0.400
+
0.014 t
(10)
The first two samples were covered with a loosely adherent, reddish corrosion product which looked like rust but proved to be ferrous carbonate. Thereafter, the surface layers were black and appeared to be much like t,hose from the Grant well. The layers flaked off in rat'her large pieces, but no pitting was observed. On removing the corrosion product of the 2-week sample for x-ray analysis, a white waxlike substance was observed below the black layers. Enough of this subst'aiice was obtained for an x-ray sample and this yielded oiily the ferrous carbonat'e pattern. Between the second and third week the production rate was decreased from 5,000,000 to 1,000,000 cubic feet per day causing the as-removed curve to change its direction as shown on the graph. The descaled weight loss thereafter changed very litt'le, although penetration of the layers did riot stop completely. After the fifth week the production rate was increased to 3,000,000 cubic feet per day. The Lower Pettit formation is only mildly corrosive. The equations for the curves in Figure 3 are: As-removed
g = 0.115
Descaled
y = 0.185
+ +
0.013 t
(11)
0.027 t
(12)
The surface layers t,hrough the first Tr-eek gave gold colored interference fringes indicating very thin films. They also appeared very thin under t,he microscope. The surface after 2 xveeks looked as it, had previously but there were large breaks in the film. The plugs designed for surface reflection electron diffraction study were exposed to this stream for 1- and 2-week periods. The patterns were very clear but only ferrous oxide and ferrosoferric oxide could be identified. TGLLOS WELLXo. 1. The coupons from this well were not attacked t o any great degree as is apparent from the curves in Figure 4. The rate equat>ionsof the straight portion of the curves are : As-removed
y = 0.000
Descaled
y = 0.090
+ +
0.018 1
(13)
0.039 t
(14)
The surface was shiny aft,er 6 weeks' exposure and still showed some of the original polish scratches. Photomicrographic examination of t.he coupons showed extensive blister formation on the surface. It appeared as t.hough there had been a limited amount of penetration through the film wit'h the liberation of a gas which caused blister formation. Comparison of this surface with that due t o a 1-month exposure in a neighboring well, the T U ~ ~KO, OS 2, showed t,hemt o be similar. The Tullos No. 2 had previously been shown t o be noncorrosive (3). The noncorrosive character mas attributed to the presence of a natural inhibitor in the stream. Originally the Tullos No. 1 well was corrosive, much like the Hunt Stewart well. Since t h a t time the well has been reworked and in addition naphthenic acid was added t o the stream. The well stream became noncorrosive and the naphthenic acid injections were stopped to see how long i t vould take t.he well t o return t o its corrosive character. The well continued noncorrosive for 10 months; consequently, it was assumed that the change in character was due t o the reworking. It is improbable that naphthenic acid would
Vol. 41, No. 8
8 .
-
L
1
I
0,5c
Figure 3.
TIME I N WEEKS Corrosion R a t e Curves from Coupon Exposures in Chicago No. 1 Well
Upper curves for gas production from Upper Pettit and lower curves for production from Lower Pettit. 0, as-removed; 8, deacaled
be effective for t h a t length of time. The present experiment was started 8 months after reworking. The small amount of corrosion product obtained from the surfaces proved t o contain ferrous carbonate by x-ray analysis. The film from the TLIIIOS No. 2 also contained ferrous carbonate. Plugs for electron diffraction study were exposed t o the Tullos No. 1 well. Two tests were made; in the first, a 3- and 5-day exposure was obtained and at a later date another 3-day exposure was made. The patterns obtained were not very clear; however, ferrosoferric oxide and ferrous carbonate were found to be present. JONES WELL No. 1. A 19-meek exposure was made in this well. The data obtained are plotted in Figure 5 . The well appeared to be of the intermediate type until the fifth week a t which time the weight loss curves broke sharply t o a lower rate. The attack after this break indicates t h a t a protective film had formed and the penetration of the surface layers was markedly decreased. The rate equations for the straight portions of the curves are: As-removed
y = 0.165
Descaled
y = 0.385
+
+
0.009 t
(15)
0.013 t
(16)
Loosely adherent surface layers *wereformed and no pitting was observed.
T I M E IN WEEKS
CL
0
T l V E IN W E E K S 1
I
I
1
1
I
2
3
4
TIME
5
IN MONTHS
6
7
8
0
Figure 4 (Top). Corrosion Rate Curves from Coupon Exposure in Tullos No. 1 Well Figure 5 (Center). Corrosion Rate Curves from Coupon Exposure in Jones No. 1 Well Figure 6 (Bottom). Corrosion Rate Curves f r o m Coupon Exposure in Well 33-5 0, as-removed; 8,dencaled
August 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
Very clear electron diffraction patterns of the corrosion product of this well were obtained. Two types were found: one (taken from a sample exposed during the first 8 weeks) which could not be identified; and a second (from a coupon exposed 19 weeks) which contained the same unidentifiable pattern and superimposed on it the pattern of sodium chloride. This well was expected t o produce hydrogen sulfide but no evidence of sulfide products could be found in the diffraction patterns. WELL 33-5. This well was initially a very corrosive well, but injection of sodium chromate in a n amount t o maintain 500 t o 1000 p.p.m. of chromate in the effluent water apparently gave rather complete protection from corrosion ( 7 ) . The chromate, in aqueous solution, was added continuously to the stream, and from the returns it was evident that only a small amount was used to maintain the protection. A 9-month exposure was made in this well. The data are plotted in Figure 6. The rate equations for the straight portions of the curves are:
'
As-removed
y = -0.018
- 0.0005 t
(17)
Descaled
y = +0.062
+ 0.0009 t
(18)
The slope as calculated was divided by 30/7 t o give t in weeks. The negative slope of the as-removed equation shows that the film thickened gradually. The layers were penetrated slightly according t o the descaled rate equation. The surfaces exhibited by the coupons were unique in appearance. After a month, the surfaces showed the original scratch marks plus a thin film which apparently was broken here and there, then eroded for a short time, and finally healed by the inhibitor. This was typical of the surfaces throughout the run except that a t the end of the run the surface appeared to have more corrosion product and fewer of the original scratch marks. Identification of the surface layers was rather difficult because of the small amount of corrosion product and the weak patterns obtained. The x-ray diffraction patterns showed the presence of ferrous carbonate, ferrous oxide, and iron. The sample from the coupon exposed for 3 months also yielded the two strong lines of ferrosoferric oxide. A plug for electron diffraction surface reflection study was exposed in this well. The pattern was not clear but the lines identified were judged t o be due t o ferrosoferric oxide. The protective film formed on iron by a chromate has never been completely identified (6). The substances found in this study are logical components of the slow oxidation of iron ( 8 ) regardless of whether they offer protection or not. Several of the steps in the over-all mechanism can be postulated. The acid attacks the iron surface and the ferrous ion liberated is either converted t o ferrous carbonate by the carbonate ion, or t o ferrous oxide by the alkaline chromate. B y a disproportionation reaction, the ferrous oxide is converted into finely divided iron and ferrosoferric oxide. The finely divided iron can then re-enter the process by being attacked by the acid components. T o show that ferrosoferric oxide forms, a laboratory experiment was made in which a clean iron coupon was placed in a system containing acetic acid, potassium dichromate, water, and hydrocarbons. The solutions were deaerated and introduced under anaerobic conditions into a reaction tube and sealed. The tube was rotated in a water bath operated at a constant temperature of 60" C. After 4 hours the tube was opened and the coupon removed. An x-ray pattern of the surface layers showed the presence of only ferrosoferric oxide. DISCUSSION
The micrographic examinations of the surface layers formed on steel coupons exposed t o various wells provided further evidence for the three types of surface layers previously postulated (3). Some of the observations found in this study are collected in
1715
Table I. The wells classified as corrosive wells were reported as giving rise t o pits and to surface layers which were porous and relatively adherent. According to the present study, the Hunt Stewart well iVo. 3, operating under natural conditions, fitted into this category. Pit formation was evident and the rate of attack was very high. The surface layers were found to be relatively thin, having a thickness of 4 . 2 , ~after 4 weeks' exposure. The second type of surface layers identified in the work cited above were those formed from wells of intermediate corrosive character. The layers were reported t o be thicker than those of the more corrosive wells. The present work showed that the layers formed on the coupons exposed to the Grant well No. 1, operating under natural conditions, were representative of this type of layer. thick. These After 4 weeks the layers were found, to be 9.9,~ layers flaked off, usually in rather large pieces. This is to be expected of a thick, rigid layer which in most cases has appreciable internal stresses and strains and thus is sensitive to temperature changes and to deformities in the surface of the basis metal.
TABLE I. RESULTSO F LfICROGRAPHIC EXAhIINATIONS SURFACE LAYERSFORUED ON STEEL COUPONS EXPOSED TO VARIOVS WELLS
OF
Well H u n t Stewart H u n t Stewarta Grant Grant" Carthage Upper Pettit Carthage Lower Pettit Tullos No. 1 Jones No. 1 33-5a a Inhibited.
Average Thiokness of Surface Type Layers, p Corrosive 4 2 Corrosive 6.7 Intermediate 7.6 Intermediate 9.9
4
Slope of R a t e Curve 4 . neremoved &ied 0.346 0.362 0.281 0.322 0 105 0.136 0.060 0.097
Exposure Time, Weeks 4 4
4
--I
Not classified
6.2
4
-0
020
0.014
Noncorrosive Noncorrosjve Noncorrosive Noncorrosive
3.3 4.6 7.0 3.0
4
0.013 0.018 0.009 -0.0006
0.027 0.039 0.013
4
10 22
0,0009
Ferrous carbonate was found to be present in the layers formed in each of these wells. Because this material constituted the largest proportion of the surface layers formed in all but one of the wells studied, it follows that this substance is the most insoluble of the compounds which can be formed under well conditions. This compound may not precipitate a t the low p H normally found for the water phase in the wells. I t s formation, however, could be explained on the basis of the fact that in the region of the metal surface the pH increased because of the reaction of the metal with the acid waters. If this Were so, ferrous carbonate would precipitate on the metal surface and act as a more or less efficient mechanical barrier, depending on the conStinuity of the layer. The carbonate scale is somewhat soluble in the acidified stream and dissolves from the exterior surface of the layer. This was brought out clearly by consideration of the effects due to exposure to the stream from the Upper Pettit formation in the Chicago well. During the run the flow rate was reduced, less of the corrosive stream passed over the scale, and the rate of dissolution of the ferrous carbonate was decreased. At the same time penetration of the surface layer by hydrogen ion, and possibly b y carbonate ion would allow solid ferrous carbonate to be regenerated next to the metal. Thus the thickness of the surface layers would increase. The formation of the ferrous carbonate could also be realized by the diffusion of the ferrous ion outward through the solid surface layer a t a rate in excess of that of the carbonate ion inward through the pores of the scale. Under such conditions precipitation would occur on the outer portion of the surface layer. Where ferrous oxide was also present it had probably formed by the reaction of the ferrous ions in a region of high pH. Undoubtedly this was then partially converted to ferrous carbonate. A recent report (4)on the effect of the partial pressure of carbon dioxide on the type of surface layers formed on steel showed t h a t
INDUSTRIAL AND ENGINEERING CHEMISTRY
1716
\Then the partial pressure of carbon dio3ide Tvas 1 atmosphere or more the surface had a marked tendency t,o pit. Addition of a small amount of acetic acid to such a 81-stem had little effect. n l t h acetic acid alone as the corrosive agent, the steel showed an even acid etch. Exposure of coupons to acetic acid and to carbon dioxide, ranging from 0.2 to 0.5 atmosphere resulted i n rapid initial attack with subsequent formation of a thick surface layer of ferrous carbonate, similar to that formed on the coupons from the intermediate type n-ell. A correlation bet!?-een the partial pressure of the carbon dioxide present in the wells studied here with the type of layers formed Tms made. The agreement is good for the first tn-o types of n-ells. Mlth the third type the metal was scarcelj- attaclced, even ivhen the partial pressure of carbori dioxide was as great p s that in highly corrosive Jvells, The third type of layer observed was previously described as being orgaiiic in nature ( 5 ) . The appearance of the films from the Tullos ~ve11s and the Carthage ~ v c l l (Lower Pettit) n-we transparent initially arid thinner t h a n those formed in the intermediate n-ells. The values for the thichcss of the sui~l'acelayers after Iweekn' exposure are given in T a h l e I. 1 riietallogmphic examination of a cross section of the Tullos well S o . 2 surface layer on a coupoil exposed for 1 moiitii as made. It i ~ a fso u n d t o be about 5 , thick, n-liich is i n good agreemeill v i t h the value found for the noncorrosive Tullos ~vellS o . 1 iri this stud\-. The partial pressure of carbon dioxide in the noncorrosive Tullos !vel1 S o . 1 is as great as that found in the corrosive nells. From the appearance mid thiclaiess of the surface layers and t>he low rate of attack on the basis nieta.1 of the coupons cxposcd to this stream, it is reasonable t o conclude that a natural inhibitor is present, in the n-ell stream which forms a relatively impc~rvioui, coherent film. This is further borne out by the effect of organic substances added to the \Yell streanis as shon-n in the second exmade on the Hunt Stenart !vel1 and Grant well. The addition of naphthenic acid l o the Grant, \\-ell caused the coupons to increase in Tveight initially and the surface layers to become thicker. In the case of the Hunt Stewnrt v d l the film became thicker and the pitting n-hich had previously been observed ivas no longer found. d t one time during the exposure the rate curves assumed a slope approximating that' usually associated with a noncorrosive well. It, appears that the inhibitor decreased the rate of diffusion of t,he corroding const,itii-
Vol. 41, No. 8
ents. In other words, it seems to have plugged, to some extent a portion of the pores normally present in the surface layer. I t is believed that in every case the controlling step after the initial attack on the metal surface is the diffusion of t,he const'ituents through t8hefilm. This is shonm t o be t,he case with t h e first four vc-ells list,ed in Table I. For these the corrosion ratei.e.>the slope of the descaled x-eight loss curve thickness of the surface layers decreases. Coupons in the last four wells of Table iirrturally or by chromate, had thin, impervi the metal \%-asnot attacked to any degree. The one .ified i v d l listed iii the table apparently is of the noncorrosive rype according t o the data. ~
lCKYOWLEDGhlENT
The authors t,ake pleasure in acknoxledging the fiiiaiicial support of the Corrosion Research Project.; Committep of thc Natural Gasoline Association of .Imerica. They also wish t o express their apprec ion to the many field n-orlcers IT-ho helyed in making the enupoil exposures. AIost of the n-orli referred t o may be found in the minutes of the Corrosion Research Project Comiiiittce ( S n t ~ u l a lGasoline Liesociation of -inicrica, 422 Kciinedy Building, Tulsa, Okla.). The information is du.e to m m y field vl.or.lters and inveAgat,nrs assnciated u-ith the committee iind much of the work is as yet unpublished. LITERATURE CITED
(1) Ani. 9 o c . Testing Materials, X-Ray Diffraction. Data C R ICIS,
Philadelphia, Pa. L-.Ii., "Metallic Corrosion. Paasivit;-. arid Protection,"
( 2 ) Evan,,
2nd ed., p . S I , London. EdwardA%rnoldand Co., 1946. ( 3 ) Hackelman, N . , and Shock, D. A , , IND.ENG. CHEM., 39, SCiS
(1947).
(4) Shock, D. A , , N.G..k.l. Corrosion Research Projects Committee, M i n i i t e s , 3 , 373 (1947). (5) Shock, D . A , , and Hackerman, N., IKD.ENG.Cxmr., 39, 1283 (1947). (G) Uhlig, H. H., Chem. E?LQ.X e u s , 24, 3154 (1948). (7) Yale, IT. D.. Corrosion, 2, S5 (1946). RZCEIVED -4pril 8. 1918. Based in part o n a thepis submitted t o t h e faculty of he University of Texas i n partial fulfillment of t h e requirenicntb for t h e degree of doctor of Dliilosophy.
0 USING MEASUREMENTS OF FILM THICKNESS AND RIASS C. ICESNETH EIEERTS U . S . Bureau of Mines, Bartlesuille, Okla.
T
HE data published by Haclierman and Schniidt ( 1 ) suggest
that the products of the corrosion they n-ere studying niay be partially identified by the density of the surface layers formed on the tcst c,oupons. The microscope procedure used by the authors ( 3 ) in determining the average thiclrness of the surface layers provided a nicasure of the volume of the corrosion product, in an area of 100 sq. cm. Det,ermining the n-eight loss of the coupons on exposure both before and after they were descaled provided a measure of the mass of the corrosion product jn the same area. Densities computed from the mass and the volume
of the corrosion product agree for the niost par0 xvith the densities giver1 in the literature for the compounds identified by x-ray analysis. Each set, of equat,ions given by Hackerman and Schmidt has been solved using the value of t (Jxeks of exposure) given in their Table I. The measured values of surface layer thickness, also given in Table I, and the difference in the coniput,ed values of the pairs of the equat,ioris have bccn used in the following equation to estimate the density of the corrosion products: 100 (10-%)d = y' - y (1)
