IXDUSTRIAL AND ENGINEERING CHEMISTRY
1010
Vol. 23, No. 9
Corrosion Rates of Steel and ComDosition of Corrosion Products Formed in OGygenated Water as Affected by Velocity' B. E. Roetheli and R. H. Brown RESEARCHLABORATORY OF APPLIEDCHEMISTRY. DEPARTMENT OF CHEMICAL ENGINEERING. MASSACHUSETTS INSTITWTE CAMBRIDGE, MASS.
OF
TECHNOLOGY.
Studies were made of the rates of corrosion of steel Since in oxygenated acid obtained in oxygenated water at various speeds of Heyn and B a u e r (8) s o l u t i o n s in which films of rotation. As the rotational velocities increased, the observed that the vecorrosion products do not corrosion rate increased to a maximum, decreased form, the corrosion rate was locity of the corroding meto a very low value, and increased again to a somewhat dium h a d a m a r k e d effect found to be directly proporhigher value at very high velocities. The variations in upon the rate of corrosion of tional to the velocity (Y), i t corrosion rate are due to (1) differences in the type seems highly probable that iron in o x y g e n a t e d water. and uniformity of corrosion-product film formed, the corrosion rate in oxyF r i e n d (6) a n d R u s s e l l , (2) changes in liquid-film thicknesses, and (3) erosion. Chappell, and White (9) obg e n a t e d w a t e r of a metal At low velocities the corrosion product formed is a nont a i n e d r e s u l t s which were covered with a corrosionresistant granular magnetic oxide of iron and corrosion p r o d u c t film which offered very similar to those of Heyn rates increase with increasing velocity because of a and Bauer, but not identical. little or no resistance to oxyreduction in the thickness of the liquid film. At gen diffusion would increase These i n v e s t i g a t o r s found higher velocities, corrosion rates are low because of the with increasing velocity. On that increasing the velocity formation of ferric hydroxide which is extremely rethe other hand, since increasincreased the corrosion rate sistant to the transfer of oxygen. At extremely high ing the velocity of oxygenated until a maximum rate was velocities, corrosion rates increase because of the water promotes the formation r e a c h e d , and that further nonuniformity of the ferric hydroxide film caused of ferric h y d r o x i d e r a t h e r i n c r e a s e of velocity caused by mechanical removal of the corrosion product. than the magnetic oxide (the the rate to decrease to a very A maximum corrosion rate is reached at some velocity former being more resistant low v a l u e . A l t h o u g h the because of the presence of two opposing tendenciesto oxygen diffusion than the above investigations yielded namely, (1) the accelerated transfer of oxygen due to a latter), it would be expected a n a l o g o u s results, the abreduction in liquid-film thickness and (2) the increased that if the r a t e of oxygen solute v a l u e s did not check rate of formation of ferric hydroxide resulting from the diffusion could be held conv e r y closely. Speller and oxidation of ferrous ions. stant in any given type of Kendall (10) under radically atmaratus. the corrosion rate different conditions of experimentation found by the oxygen-drop method that the cor- in oxygenated water would decrease with 'increased velocity. rosion rate of steel pipe with water flowing through it in- These opposing tendencies may be diagrammatically reprecreased with increasing velocity and approached an asymp- sented by the broken lines in a sketch such as shown in Figure 1. totic value which varied depending upon the diameter of pipe On the basis of this reasoning it would therefore be exbeing tested. I n their investigation in acids Friend and Den- pected that, upon increasing the velocity, the corrosion rate in nett ('7) found that the corrosion rate of an iron disk rotated in oxygenated water should a t first increase because of decreased dilute sulfuric acid (0.05 to 1 per cent) was directly propor- water-film thickness and then decrease because of increased tional to the speed of rotation. This investigation differed film resistance. The net effect would be expected to follow from those reported by others (6, 8 , 9, 10) in that no films of some curve such as is indicated by the solid line in the sketch. I n consideration of these hypotheses it was deemed adviscorrosion products developed, while in the other investigations cited the solution was essentially neutral and hence films or able to determine the corrosion rates of steel in oxygenated water in an apparatus in which the velocity of metal relative to corrosion products could be formed. A number of previous investigations (1, 2, 3, 4) have de- that of the water would be the only variable condition. monstrated not only the presence of oxide or hydrous oxide Experimental Procedure and Results on metals, but also the effect of their presence on changing The experimental procedure used was essentially that decorrosion rates of metals. These papers do not, however, discuss the effect of external factors, such as distribution of scribed in a previous article in which turbulence could be conions and dissolved oxygen in the corroding medium and the sidered aa a function of speed of rotation alone. Rotating effect of these factors upon the type of film formed. A recent cylinders of low-carbon annealed steel were rendered oxidepaper (6) has indicated the importance of the turbulence of free by treatment with 10 per cent hydrochloric acid. The the oxygenated water upon the type of film formed in the cor- adhering acid was removed by washing with a stream of oxyrosion of steel, and hence upon the corrosion rate. If the gen-free water until the effluent was neutral. The specimen water was stirred rapidly, the film consisted of red gelatinous was then subjected to the action of oxygenated water and the ferric hydroxide, whereas if the film was formed in an unstirred corrosion measured by determining the decrease in oxygen consolution, the corrosion product was a loose granular pre- centration during the test. The only departure from the cipitate of magnetic oxide of iron It was also found that the previous procedure was to equip the synchronous motor with magnetic oxide, under the conditions of its formation, afforded sets of interchangeable gears. By this means the speed of little resistance to the diffusion of oxygen but that the ferric rotation could be conveniently varied. The results of the corrosion rate determinations a t different hydroxide did, thereby reducing the corrosion to a relatively speeds of revolution are represented graphically in Figure 2. low value. The corrosion rates are plotted as cubic centimeters of 1 Received May 1, 1931.
A
S E A R L Y a s 1910
A
I
ISDUSTRIAL A N D ENGIh'EERISG CHEMISTRY
September, 1931
oxygen (0" C., 760 nun. of mercury) consumed per minute per square decimeter of metal, corrected for differences in oxygen concentration. The correction was made by dividing by the arithmetic-average oxygen concentration (cubic centimeter per liter) existing in the corrosion cell during the oxygen-drop determination. At the end of each determination, visual examinations of the films formed during the corrosion-rate determinations showed the conditions recorded in Table I. The effect upon the corrosion rate of the speed of rotation a t which films were allowed t o build up on the specimens is given in Table 11. Table I-Conditions
of F i l m s Formed on Iron at Different Speeds of Rotation ~~~
RCN
1 2
SPEEDOF ROTATION R.p . m. 6.34 57.0
3
85.5
4 5
114.0 171.0
6
228.0
7
278.0 342.0
8
APPEARANCE OF FILM
Large areas of loose black granular film Small areas of loose red gelatinous film Similar to run 1 Similar t o runs 1 and 2, except for some gelatinous red film on black substance Similar to run 3 Similar to runs 3 and 4, except for considerable amount of red gelatinous film Uniform thin layer of red gelatinous film, metal surface visible through this layer Similar t o run 6 Similar to runs 6 and 7 , except for striations in film in direction of rotation.
of Liquid F i l m s a n d Corrosion-Product F i l m s on Corrosion R a t e POINT OF DBTERMITION OF CORROAPPEARANCE NATION OF CORRO- CORROSION R U N SION PRODUCT OF FILMS SIOR RATE RATP R.9. m. R.p . m. cc. 1 0 Granular, black 0 0.0014 2 0 Granular, black 228 0.0098 3 228 Gelatinous, red 228 0.0010 4 278 Gelatinous, red 278 0.00065 5 278 Gelatinous, red 22s 0.00065 6 342 Gelatinous, redb 342 0.0053 7 342 Gelatinous, redb 228 0.0053 aOxygen consumed/sq. dm./minute/av. cc. oxygen/liter. bAt this speed of rotation there were striations on the film due to erosion.
1011
differences in the composition of the corrosion products. In the previous investigation it was shown that the presence of two products of corrosion had a major bearing upon the ultimate corrosion rates of steel in oxygenated water-namely, (1) a granular magnetic oxide of iron and ( 2 ) a gelatinous ferric hydroxide. The former afforded but little resistance to the passage of oxygen and various ions so that a high corrosion rate resulted; the latter, being of a gelatinous character, was found to be highly resistant and when present reduced the corrosion rate to a large degree. The factors determining which of these two types of films would be produced were found to be the solubilities of the ferrous and ferric hydroxides. The former required much higher hydroxyl-ion Concentration for its precipitation than the latter a t the same metallic-ion con-
UWN CORROSION RATE O I $TELL IN OIICLNATED WATER
Table 11-Effect
POINT OF FORMA-
Discussion of Results
An inspection of Table I shows that marked differences are to be noted in the character of the films formed upon corroding oxide-free steel surfaces in oxygenated water a t different speeds of rotation. Figure 2 shows that corrosion rates also vary markedly, after the initial period, when the rotational velocity is changed. The variation of corrosion rate with speed of
Figure 1
rotation is more strikingly brought out in Figure 3 in which the ordinates represent the corrosion rates a t the end of 900 minutes (obtained from Figure 2 ) , and the abscissas, the speeds of rotation. These phenomena may be explained by means of the data in Table I1 and the conclusions reached in a previous investigation ( 5 ) which attributed differences in corrosion rates to
00 MINUTES TlUE
Figure 2
centration and, under conditions of inadequate agitation, the concentration of hydroxyl ions in the liquid film became such that precipitation of ferrous hydroxide occurred. After the precipitation the ferrous hydroxide reacted with the ferric which was formed a t a lower pH, and the loose granular magnetic oxide resulted. However, when the agitation of the liquid was violent, the hydroxyl-ion concentration in the liquid film was kept below the value necessary for the precipitation of ferrous hydroxide, and the concentration of oxygen in the regions where ferrous ions were formed was kept high so that the formation of ferric ions was accelerated and the precipitation of ferric hydroxide would occur without the formation of any of the ferrous hydroxide. In this manner a very protective film was formed. In runs 1 and 2 of Table I1 it is shown that films formed a t low speeds of rotation are poorly protective and that the rate of corrosion depends primarily upon the rate a t which oxygen can pass through the liquid film-i. e., upon the speed of rotation. From runs 3 to 7 of the same table it may be seen that the film formed a t high speeds of rotation controls the corrosion rate despite the presence of a high oxygen concentration a t the corrosion products-liquid-film interface. In consideration of the facts presented in the previous investigation (6) and in Table 11,the effect of rotation upon the corrosion rates of steel in oxygenated water indicated in Figure 3 was to be expected. At low velocities the corrosion product consisted largely of the porous granular black magnetic oxide of iron. Increasing the velocity increased the rate of corrosion by decreasing the thickness of the water film in contact with the corrosion products and permitted a more rapid transfer of oxygen. At high velocities (above 228 r. p. m.) the corrosion product, consisting primarily of ferric hydroxide, was extremely protective and the corrosion rate was independent of the velocity until velocities were attained a t which (as evidenced by the presence of striations in the film) erosion of the film began to take place. At intermediate 7-elocitiesa balance was obtained between two tendencies: ( I ) the increased proportion of ferric
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1012
hydroxide formed tending to reduce corrosion rates, and (2) the increased rate at which oxygen moved through the liquid film which tended to increase the transfer of oxygen to the metal surface. From the results of this investigation, which are qualitatively in agreement with those of Heyn and Bauer (8), Friend (6), and Russell, Chappell, and White (9) and obtained in similar forms of apparatus, and in disagreement with those of Spel-
OOi8
0 012
0.008
0.004
&OOOC 0
I
f
LO
'
I
00
1
'
'
1
8
I
120 IW 200 REVOLUTIONS El MlNU'E WCLD or ROTATION
"
240
1
200
'
"
320
'
330
Figure 3
ler and Kendall (10) whose results were obtained in pipes, the authors are led to believe that the consideration of velocity alone is not sufficient for the correlation of the existing data in the literature. It is believed that turbulence which, as has been found in hydrodynamic researches, is affected by the velocity, the shape of the vessel, the viscosity of the liquid, and the density of the liquid, has a marked effect upon the types of corrosion product produced and hence upon the corrosion rate obtained under different conditions of investigation.
Vol. 23, N o . 9
Conclusions From the results of this investigation it may be concluded that: (1) Variations in velocity during the corrosion of steel in oxygenated water alter the corrosion rate by influencing the thickness of the liquid film and the type of corrosion product formed. ( 2 ) The corrosion product formed on steel at low rotational speeds is, for oxygen concentrations normally encountered, a granular nonresistant magnetic oxide of iron. (3) The product formed a t high speeds is a protective gelatinous ferric hydroxide. (4) When the corrosion product is formed at low velocities, the corrosion rate is governed by the resistance offered by the liquid film to the transfer of oxygen, whereas, when formed a t high speeds, the resistance of the corrosion product controls the rate of corrosion. (5) A maximum rate of corrosion is obtained as velocity is increased because of the combined effect of two opposing tendencies: (a) the decreased resistance to diffusion of oxygen due to a reduction of liquid-film thickness and (b) the increased resistance of the corrosion product at the speed at which ferric hydroxide commences to form. Literature Cited (1) Brown, Roetheli, and Forrest, IND.ENG.CHBM.,98, 350 (1931). (2) Evans. J . Chcm. Soc., 1997, 1020. (3) Evans, Zbid., 1919, 2651. (4) Forrest, Roetheli, and Brown, IND. E N Q . C H E M . , 99, 1197 (1930). (5) Forrest, Roetheli, and Brown, Zbid., 38,650-3 (1931) (6) Friend, "Corrosion of Iron," Carnegie Scholarship Memoirs, Vd. XI,p. 129,Iron and Steel Institute, London, 1922. (7) Friend and Dennett, J. Chcm. SOC.,191,41 (1922). (8) Heyn and Bauer, Mill. kgl. MafcriuZpri4fungsom1, 98, 62-137 (1910). (9) Russell, Chappell, and White, IND. ENG.CHEM.,19, 65-73 (1927). (10) Speller and Kendall, Zbid., 16, 134-139 (1923).
Effect of Oxygen Concentration on Corrosion Rates of Steel and Composition of Corrosion Products Formed in Oxygenated Water' G. L. Cox and B. E. Roetheli RESEARCH LABORATORY OB APPLIEDCHEMISTRY,
DEPARTMENT~OFCHEMICAL ENGINEERING,
MASSACHUSETTS INSTITUTE
OF
TECHNOLOGY, CAMBRIDGE, MASS.
Short-time studies of the corrosion rates of steel in water ide. The presence of the resistant ferric hydroxide a t containing varying amounts of dissolved oxygen (3 t o 18 cc. high oxygen concentrations is attributed to the rapid rate per liter) show the rates t o be approximately proportional of oxidation of ferrous ions to the ferric state which prevents the precipitation of ferrous hydroxide and t h e subto t h e oxygen concentration at concentrations below 5.5 cc. per liter. A t higher concentrations, marked deviations sequent formation of the granular magnetic oxide of iron from the linear relationship occur. The deviations be- present on this metal a t low oxygen concentrations. Income more pronounced a s the oxygen concentrations in- creasing the velocities serves to decrease the thickness of crease, indicating t h a t t h e resistance t o t h e transfer of t h e liquid film and t o increase t h e rate of transfer of oxygen t o the metal surface increases with increasing oxygen which a t low concentrations is present in amounts oxygen concentration because of changes in the nature insufficient to oxidize the ferrous ions completely so t h a t the corrosion process is accelerated. A t high oxygen conof the corrosion products. A t low oxygen concentrations the corrosion product centrations the rate of oxidation of ferrous ions is acformed is composed primarily of a porous black magnetic celerated by increased velocities and the resistant ferric oxide of iron, while a t high concentrations the corrosion hydroxide film is built up more rapidly. product is predominantly a red gelatinous ferric hydrox. . .. . . . . .. .. ..
R
APID developments in recent years in the methods of
corrosion testing in laboratories and on service installations have resulted in the need for accurate information regarding the effects of various factors which IReceived
May 4,19311
are likely to influence the validity of the tests. The theory of aqueous corrosion has been frequently stated and t.ested by various investigators, notably, Whitney (18), Walker (14,15), McKay (9, IO), Evans (2, 8, 4, 6, 6), Wilson (Ig), Whitman (18, I7),Speller (II), and others. However, much contra-
