The Corrosion-Time Relationship of Iron R. F. PASSANO The American Rolling Mill Company, Middletown, Ohio
T
HE amount of iron corrosion which takes place has been measured by various methods. However, the following are those most commonly used: loss of weight, oxygen consumed, depth of pit, and loss of strength. Each of these may be given in many units. In this review no attempt has been made to express the amount of corrosion in terms of a single u n i t , . In f a c t , there is a question in t h e author's mind whether this can be done generally and still retain the information i n t h e original observations. As a matter of record, the author does not advocate t h e g r a p h i c a l 1 1 ) presentation of data as a means of r e c o r d i n g the i n f o r m a t i o n conT I M IN DAYS LOGtained in observations FIGURE 1. CORROSION OF STEEL (6). H o w e v e r , since IMMERSED IN SODIUM CHLORIDE (2) graphical m e t h 0 d s are most convenient t o picture the general nature of a relationship, all data on the corrosion-time relationship have been presented graphically. yl
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THEORY
U. R. Evans (Ba),following Pilling and Bedworth (18), has pointed out that, if an oxide layer is capable of preventing diffusion of oxygen, the rate a t which the layer increases in thickness is inversely proportional to its thickness. Thus, if y represents the thickness of the layer, t represents time, and p is a constant,
If Equation 1 is integrated: y* = 2pt
the integration c o n s t a n t being zero when the time is m e a s u r e d from the commencement of the formation of the film. T h i s is the equation of a parabola. Thus, we might expect the corrosion-time relationship to be parabolic if the rust layers are capable of preventing diffusion of oxygen.
(2)
There may be a question about a hydroxide corresponding to ferroferric oxide, but mixtures of ferrous and ferric hydroxides result in a substance of approximately that composition on drying. I n 1931 the investigators of the Research Laboratory of Applied Chemistry a t Massachusetts Institute of Technology published a paper (10) which dealt with the theoretical composition of rust layers and their ability to prevent diffusion of dissolved oxygen and ions. They concluded that the mixture of hydroxides corresponding to ferroferric oxide was not resistant to diffusion, but that ferric hydroxide was highly so. Apparently t h e n we can expect a t least two 050 s o r t s of corrosion-time c u r v e s : (1) a l i n e a r :: I / relationship if the oxide or hydroxide has properties like those attributed to ferroferric oxide and (2) a parabolic relationship if the oxide or hydroxide has properties ?.-ax like those attributed to ferric hydroxide. T o facilitate the ease ool w i t h w h i c h we c a n recognize the type of relationship, the majority Of t h e f i g u r e s w h i c h FIGURE2. CORROSION OF THREE follow have been drawn STEELS ROTATED IN OXYGENATED on double logarithmic WATER(9) c o o r d i n a t e s because: (1) Any straight line which passes through the origin will be a straight line on log-log coordinates with a slope of one (i. e., a 45" line); (2) any parabola concave downwards which passes through the origin will be a straight line on log-log coordinates with a slope of one-half (i. e., roughly a 30" line); (3) any exponential which passes through the origin is a straight line on log-log coordinates, and the slope on log-log c o o r d i n a t e s i s t h e exponent; and (4)a logarithmic curve is still logarithmic on log-log coordinates.
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UNDERWATER EXPERIMENTS
PROPERTIES OB IRON OXIDES OR HYDROXIDES Three o x i d e s of iron are k n o w n : f e r r o u s (FeO), ferroferric (FesOl, or FeOFezOs), and ferric (Fe203), and there a r e p r o b a b l y corresponding hydroxides.
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Time of Immersion,
days,
FIGURE 3. CORROSION OF CASTIRONIN CAMBRIDGE WATER AT 68' F. (20' C.) (8) Water velooity through jars, 4.55 gallons (17.24 liters) per hour
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Figure 1 was p r e p a r e d from data g i v e n b y Bengough and Lee (2) for steel immersed in a q u i e s c e n t solution of 0.5 N sodium chloride under oxygen at 2.5"C. The corrosion products are reported to contain about 10 per cent ferrous iron. Here the relationship b e t w e e n loss and time is linear ( s h o w n by the 4.5" line).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
These investigators have also studied the behavior of iron and steel in potassium chloride solutions of varying concentrations. They report (1) that, when the solution is less than 0.001 normal, the loss-time relationship is an exponential function but is linear in more concentrated solutions. Results reported by Chappell ( 5 ) , representing loss of weight on steel specimens immersed in daB stagnant sea water having access go" zao to air, similarly indicate a linear corrosion-time relationship for the conditions of test. Evans (7), using half-immersed 9 04 s p e c i m e n s of iron a n d s t e e l * 4 TIME 6 0 2 4 e in dilute solutions of potassium IN DAYS chloride a n d sulfate which FIGURE 4. CoRRosIoN OF were s t a g n a n t , a 1s o observed IRON IN WASHINGTON WATER CONTAININGVA- a linear r e l a t i o n s h i p between RIOUS A M o m T s OF DIstime and loss of w e i g h t o v e r OXYGEN('I' a period of a p p r o x i m a t e l y 4 days. Figure 2 was prepared from data reported by Forrest, Roetheli, and Brown (9) on three classes of steel rotated in oxygenated water. The corrosion of plain, 14 per cent chromium, and 188 steels begins in proportion to time. While the attack on 188 practically stopped in 2 minutes and on 14 per cent chromium steel in 10 minutes, the plain steel continued to corrode in proportion to time for the period investigated. These observations apparently show that it is possible for the products of corrosion to become completely impervious under certain conditions and stop the attack altogether. In 1929, Forrest (8) showed loss-time curves for cast iron covering nearly a year. The specimens were in motion and
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exposed to Cambridge, Mass., water which was continually renewed over the period. Figure 3 is a reproduction of his data. Figure 4 is a reproduction of loss-time curves obtained in moving Washington, D. C., water containing various amounts of dissolved oxygen as reported by Groesbeck and Waldron (11). They cover a period of 7 days in a water having a pH of 8 approximately. The curves in Figures 3 and 4 are similar to the extent that they show a decrease in amount of attack shortly after the start and probably fall somewhere between attack proportional to time and complete cessation. Figure 5 was prepared from data obtained in this laboratory (16, 17). Iron specimens were exposed to running water from a well. The water contained a large amount of calcium bicarbonate, some of which was found in the rust layers as calcium carbonate. Quite a few analyses of rust have been made but the results are not sufficiently conclusive to warrant discussion a t this time. Each point represents the average loss on at least two samples of fiye specimens exposed at different times to the water, The losses in successive experiments under the s a m e conditions did not differ by more than s h o u l d be left to chance. The specimens started to lose weight in proportion to time, but after a short period (the duration of which decreased as the velocity was raised) the loss ceased to occur in DroDortion to time. I n STEELSIN ATMOSPHERE, SCOTTt i o n s h i p s there is a DALE, P A . (4) time after the i n i t i a l period when the losstime relation may be parabolic (for example, from 6 to 15 days, inclusive, a t a velocity of one meter per minute), but beyond that time the loss is less than would be expected from the parabolic relationship. As it happens, the losses after the initial period when the loss is proportional to time can be most conveniently represented by a logarithmic curve (Figure 5 , lower part). As a result of the evidence considered so far, a tentative theory is proposed: Iron begins to corrode in proportion to time. After an initial period the rust layers may so retard the diffusion of oxygen and ions that the corrosion-time relationship becomes parabolic. As time goes on, the attack may be completely stopped under certain conditions. This proposition is in keeping with the corrosion-time data reported from underwater investigations. Obviously much depends on the properties of the rust layers which are formed. It is certain that the ability of the rust layers to prevent diffusion of oxygen, etc., will be influenced by the nature of the metal and its environment.
ATMOSPHERIC EXPERIMENTS
05
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2
5
10
eo
50
100
Time o f Continuous Exposure. days,log scale
FIGURE 5. RELATIONOF AVERAGE Loss TO TIMEOF EXPOSURE (17) Above: log-log oo6rdinates; below: semilog co8rdinates
Figure 6 was prepared from loss of weight data on sheet specimens exposed to the atmosphere a t Scottdale, Pa., by Buck (4). The appearance of the curves would lead one to believe that the above tentative proposition does not cover the case. The 0.012 per cent copper steel did not lose weight quite in proportion to time, and the losses on the 0.050 and 0.242 per cent copper steels are not quite parabolic. Within the last two years some loss of weight data have been obtained from atmospheric tests a t Middletown, Ohio.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The materials were exposed on two occasions and two samples of each material have been removed so far, the one representing a period of 6 months and the other of one year. Samples 1 and 13 represent the same material exposed a t different times, and 12 and 16 represent a second material exposed on the same two occasions as 1 and 13. The average loss on the duplicate samples removed after 6 months a g r e e d c l o s e l y , but the loss of weight on the duplicate samples removed after one year was quite different. The nature of industrial operations in the vicinity of the test racks had not changed in the interval, so an attempt was made to locate the cause of the difference from the weatther r e c o r d s . The rainfall for the two 6-month periods 23.85 and 23.29 inches (60.58 3 sb 1 ,A ,Io d was and 59.16 cm.), but for the two INW annual periods was 42.73 and 63.90 FIGURE7. CORROSIONi n c h e s (108.53 a n d 162.31 cm.). OF SHEET STEELSIN ATMOSPHERE, MIDDLE- The samples exposed for the year in which 63.90 inches of rain fell TOWN, OHIO (13 and 16) lost more weight than those exposed for the year in which 42:73 inches fell ( l a n d 12). This observation is qualitatively in keeping with Hippensteel’s results on zinc coatings where the weight loss increased when the rainfall was increased artificially ( l a ) . Therefore it seemed possible that rainfall might be a more suitable independent variable than time for these conditions of exposure. Figure 7 shows the losses on the materials exposed a t Middletown, Ohio, against rainfall, and the relationship is apparently consistent with the hypothesis. Although the reference to Buck’s work does not include a statement of the time when the sheet specimens were exposed, the present author believes they were exposed on May 11, 1917 (3). Scottdak, is not far from Pittsburgh and accordingly the rainfall would be expected to be similar. Figure 8 is a revision of Figure 6, using rainfall a t Pittsburgh from May, 1917, as the independent variable in place of time. T h e corrosion-rainfall relationship is consistent with the tentative proposition stated above; the 0.012 per cent c o p p e r steel initially lost weight in p r o p o r t i o n to time Y ) I C x 4 503 RAINFAILL IN CM Lot -. a n d finally t h e loss FIGURE8. CORROSION-RAINFALL was controlled by diffuRELATION (DATAOF FIGURE6) sion. The o t h e r two steels lost weight from diffusion over the greater part of the period investigated. Perhaps if further losses were available, it might have been found that the attack practically stopped on the 0.050 and 0.242 per cent copper steels. This would presume, of course, that the rust layers become more and more impervious with time. Actually the rust layers on sheets in the atmosphere probably never increase in thickness or resistance to diffusion beyond a certain point owing to vibration, lack of adhesion, etc. It is t o be hoped that thesestatements can be read as uncontroversial observations. The sheets perforate in time; hence, the rust layers certainly do not prevent deterioration entirely. The fact that the life of 16-gage sheets is more than double that of 22-gage sheets on test racks qualitatively confirms some flattening of the corrosionRAWALL
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time curves. It is likely that the exact increase in life which is accomplished by increase of thickness is dependent upon the composition of the metal and the test location.
UNDERGROUND EXPERIMENTS There are eighteen soils from the Bureau of Standards’ investigation on which sixteen deepest pit measurements are available on the sixteen wrought specimens a t five inspection periods. These data offer a means of investigating the nature of the corrosion-time relationship for a third type of exposure. Curves for three of the eighteen soils are shown in Figure 9. The depth of pit in soil 23 (13-16) increases in proportion to time for approximately 4 years after which the increase in depth of pit seems to be controlled by diffusion. The depth of pit in soil 43 appears to be ‘ l s ay controlled by diffusion over SML Na 23. the whole p e r i o d c o v e r e d by the investigation. T h e 3 501LNQ42. data from soil 22 would in&S-o,, SOL Na22. dicate that the depth of pit is governed by diffusion for a 8 wh:lle, but after a certain time O 5 U o (approximately 6 years) the a‘ nt4 IN rcuum.M. OF pits cease to increase further FIGUIIE 9, in depth. WROUGHTPIPE NIPPLESIN At least two o t h e r s o i l s THREE TYPESOF SOIL show the same type of curve as soil 43 (soils 29 and 40); these are probably all relatively wet soils. The most frequently occurring type of curve is that shown by soil 22 where the depth of pit apparently approaches a limiting value. Necessarily the limiting depth which is approached is not the same for all soils showing this type of curve. For example, the limiting value for soil 42 is in the neighborhood of 0.12 inch (0.305 em.), soil 46 about 0.09 inch (0.229 cm.), soil 19 approximately 0.06 inch (0.152 cm.), etc. These soils are clays, loams, etc., and the rainfall in the loeations is intermittent. If these observations can be generalized safely a t some time, a pipe with a wall thickness greater than 2 3a should last indefinitely, where 2 is the average of the deepest pits a t the limiting value and u is their standard deviation. Thus, it may conceivably be profitable to know the nature of the time-corrosion relationship for various soils or groups of them, whether it continues or flatten8 out, and if so, a t what value, if we are to use metals and coatings economically. Apparently the depth of pit data from ferrous metals buried underground are consistent with the above tentative proposition. Admittedly, the theory is the result of inductive reasoning. It was stated after a survey of the underwater experiments because the author’s experience would indicate that the variables of underwater experiments are more thoroughly understood than are those of atmospheric and underground service.
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CONCLUSIONS
It would be difficult to write a single mathematical equation which, with the substitution of constants, would cover the relationships observed, considering that: 1. There are real grounds at times for doubting whether the essential experimental conditions were maintained throughout the time that the observations were made. 2. The observations in papers are not often presented in such a way that the essential information which they contain is available. 3. If provisions 1 and 2 do exist, the number of observations available is not sufficiently large to make the experimental average coincide with the objective average.
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4. Any curve (or series of curves) which satisfies a tenable hypothesis and which does not miss an experimental value by more than its error is ordinarily a satisfactory representation of the experimental facts.
Apparently the tentative proposition stated in the text covers the state of affairs. The author is of opinion that the use of calculated rates of corrosion is likely to be misleading except when the corrosion-time relationship is linear or when one has a knowledge of the nature of the corrosion-time relationship existing under the particular conditions, whatever they may be. This opinion is contrary to the practice of certain outstanding investigators. The fact that calculated rates of corrosion will change regularly or continually when the corrosion-time relationship is other than linear makes it difficult to visualize the significance of a single rate figure. This discussion has a close bearing on the meaning of the phrase “protective rust layer.”’ If the visible products of corrosion completely stop attack on the metal, there is no doubt that the rust layer is protective. If the corrosion-time relationship is linear, the rust layer is certainly not protective. However, there are many occasions when the attack lies between a linear relationship and complete cessation. In one sense of the word, the rust layers which are capable of 1 Note the distinction between a “protective rust layer” and a “protective film.” A protective film is a n oxide, etc., which is thin enough t o be invisible when in contact with the metal: it influences the initial distribution of attack and can immunize the metal from attack, as the film on BOcalled stainless materials does under many conditions.
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making ferrous metals corrode a t in-between rates, as it were, are also protective. Thus, the author believes that constructive discussion on the significance of the phrase “protective rust layer” will be a valuable contribution for further discussion. LITERATURE CITED Bengough, Lee, and Wormwell, Proc. Roy. SOC. (London), 134A, 308 (1931). Bengough and Lee, J . Iron Steel Inst., 135, 285 (1932). Buck, D. M., Proc. Am. SOC.Testinn Materials, 1 9 , I I . 224 (1919). Buck, D. M., Trans. Am. Electrochem. SOC.,39, 109 (1921). Chappell, J.Iron Steel Inst., 85, 270 (1912). Dodge et al., Proc. Am. SOC.Testing Materials, 33, I (1933). Evans, “Corrosion of Metals,” pp. 12-14, Arnold, 1924. Evans and Hoar, Proc. Roy. SOC.(London), 137A, 343 (1932). Forrest, Proc. Am. SOC.Testing Materials, 29, 11, 128 (1929). Forrest, Roetheli, and Brown, IXD. EKQ.CHEX, 22, 1197 (1930). Ibid., 23, 650 (1931). Groesbeck and Waldron, Proc. Am. SOC.Testing Materials 3 1 , 11, 279 (1931). Hippensteel, Borgman, and Farnsworth, Ibid., 3 0 , I I , 456 (1930). Logan, Ewing, and Yeomans, Bur. Standards, Tech. Papers, 1928, KO.368, 447. Logan and Grodsky, Bur. Standards J . Research, 7, 1 (1931). Logan and Taylor, Ibid., to be published, 1933. Passano, Proc. Am. SOC.Testing Materials, 32, 11, 468 (1932). Passano and Nagley, Ibid., 33, I1 (1933). Pilling and Bedworth, J. Inst. Metals, 29, 529 (1923). ’
RECEIVEDJune 9, 1933. Presented a t the Third Corrosion Conference Bureau of Standards, Washington, D. C., March 30 and 31, 1933.
Holocellulose, Tota1 Carbohy dr ate Fraction of Extractive-Free Maple Wood Its Isolation and Properties GEO. J. RITTERAND E. F. KURTH, Forest Products Laboratory, Madison, Wis.
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E S E A R C H workers The total carbohydrate portion of extractivedetermination a n d i s t h e r e free here termed (~hOIOCellUloSe,~~ fore objectionable for routine h a v e long had t h e work. Accordingly, the Forest desire to develop rapid is in “lid form by a rapid method Products L a b o r a t o r y undermethods for isolating, in a solid f r a c t i o n , the e n t i r e c a r b o deaebed at the Forest Products Laboratory for took t h e d e v e l o p m e n t of a hydrates in wood which has been routine work. rapid m e t h o d . It was found extracted with alcohol-benzene The total acetyl groups, the total carbon di- that repeated alternate treats o l u t i o n and with hot water. oxide-forming and a part of the mentsof theextractive-free wood with chlorine and alcohol-pyriSuch a wood fraction methoxyl groups Of the wood are in the hobdine solution removed in about afford a c o n v e n i e n t means -_ celluLose* 10 hours all except a small perf o r s t u d y i n g the nature and centage of the lignin; the rethe relationship of the c a r b o hydrate components and their substituent groups, acetyl, maining small percentage was removed in 30 minutes with a carboxyl, and methoxyl. Some of these components and solution of calcium hypochlorite. The residue remaining substituent groups are partially or wholly lost in the isolation from the foregoing procedures is the carbohydrate fraction. I n one phase of this study the chemical characteristics of Cross and Bevan cellulose which constitutes only about of the material are compared with those of Skelettsubstanzen. 80 per cent of the carbohydrates. Chlorine dioxide in a solution of pyridine in water has In stating those comparisons a short appropriate name for been used by Schmidt (12) in isolating a carbohydrate frac- the material is desirable. Since the material is composed of tion designated Skelettsubstanzen which constitutes practically hemicelluloses and cellulose, it has been termed “holocellulose, the total carbohydrates of extractive-free wood. Schmidt’s meaning whole or entire cellulosic material. procedure requires approximately one month for making the MATERIAL 1 The author8 Dropose the word “holocellulose” as preferable t o Skeletb ~
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subatanrcn, maintaining that the latter term does not describe the material correctly either from the physical or chemical point of view. The word “holocellulose” ae yet has not been formaUy or generally accepted by workere in this field.
i \ / ~ ~ ~ l ~ (60-80 mesh) that was extracted with and with hot Water was used’ alcohol-benzene The alcohol-pyridine solution was prepared by diluting
