CORROSION PROBABILITY R. B. MEARS AND R. H.BROWN A l u m i n u m C o m p a n y of America, New Kensington, Pa.
HEN a large area of certain metals (for example, iron) is exposed to ordinary water, corrosion is practically certain to occur somewhere. However, if the exposed area is small, the probability of the development of attack is considerably reduced so that frequently attack does not commence a t all on some of the specimens. The chance of attack occurring on specimens of any given size under a definite set of conditions has been termed the “corrosion probability” (3) and is defined mathematically as P
=
NO x 100 NT
where P = corrosion probability No = number of specimens on which one or more points of attack develop NT = total number of specimens In previous papers (3, 6, 9) the effect of different factors (composition of the liquid, the gas, or the metal, temperature, duration of experiment, etc.) on the corrosion probability for iron and steel has been described. Some work was also done with zinc (6). The present paper comprises a study of the effect of variation in such factors on the corrosion probability and intensity of attack for aluminum. As has been pointed out (Y),measurement of the corrosion probability is even more important in the case of aluminum than for ordinary iron or steel. Under many “natural” exposures, the corrosion probability of unpainted steel is approximately 100 per cent; that is, the entire exposed metal surface is attacked. In the case of aluminum, however, because of the much more continuous and protective oxide film which forms on exposure to air or water, attack is often initiated only a t localized spots on the metal surface. Evidently these spots represent points of special weakness or porosity in the otherwise relatively continuous, protective coating. It is important, therefore, to study the factors which affect the frequency of occurrence of such points, as well as those which influence the velocity of attack once breakdown has commenced. A knowledge of these factors will often make it possible to prevent attack from being initiated a t all. Clearly, if a material had a negligibly small corrosion probability in all natural exposures, it would be as desirable as one which showed a negligibly sniall intensity of attack. In each series of experiments all the factors except one were kept constant, thus isolating the effect of variation in this one factor. The factors so far investigated are (a) area of metal exposed to the liquid, ( b ) influence of the number of points of attack on the conditional velocity of corrosion, ( c ) temperature at which the experiment was conducted, (d) concentration of salt (sodium chloride) in the presence and absence of a heavy metal salt (copper nitrate), and (e) magnitude and direction of applied potential.
The effect of several external variables on corrosion probability and intensity of attack for aluminum specimens has been investigated. An increase in the area of metal increases the probability that attack will develop somewhere on the specimen but decreases the number of breakdowns per unit area. The average depth of attack increases to a maximum value when 4 sq. cm. of metal is exposed. When the number of points of attack on a specimen is increased] the average depth of attack decreases. An increase in the temperature of the experiment increases the probability of attack but decreases the average depth of attack. With increase in the sodium chloride concentration, the probability of attack and the average depth of attack both increase. A t a critical potential, attack of aluminum cathodes immersed in either sodium or aluminum chloride solutions can be prevented. However, at higher or lower applied potentials] attack occurs. The results indicate that the attack of aluminum in chloride solutions is at least partially electrochemical in nature. Statistical analysis of the data indicates that the number of weak places in the film on aluminum increases as their size or intensity decreases. Also, a frequency distribution curve for specimens containing various numbers of points of attack differs from the curve that would be obtained, i f the occurrence of one pit did not influence the occurrence of other pits, in a manner to be expected from corrosion theory.
is so much more continuous than the air-formed film on iron and steel that much larger areas had to be exposed in order to assure a reasonable frequency of attack, even in rather concentrated salt solutions. Thus, for aluminum the exposed area (except in that series of experiments where this factor was the variable under consideration) was 1.5 X 1.5 em. or 2.0 X 2.0 cin. This compares with the 0.3 X 0.3 cm. or 0.1 x 0.1 em. areas generally used for iron or steel. Another difference in the behavior of aluminum, as compared to steel, was noted: In the tests on steel it was found that, if attack commenced at all in any small exposed area, practically
Experimental Method The method employed was of necessity different from that used previously for iron or steel. The oxide film on aluminum I087
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days, the specimens were removed and examined. The number of points of attack on each specimen was counted and the depth of attack was measured by microscopic examination of cross sections of the specimens. The metal employed was commercially pure aluminum (2s-0). This material contained 0.13 er cent silicon and 0.73 per cent iron. The balance was agminum with traces of other elements. It was in the “annealed” temper.
Area of Metal Exposed to Liquid Experiments with various areas of exposed metal were conducted in two different solutions: ( A ) 10 grams sodium chloride per liter and ( B )0.3 gram sodium chloride plus 1.0 gram copper nitrate per liter. In both cases, as shown in Figures 1 and 2, increase in exposed area greatly increased the “probability” of corrosion, which is equal to the percentage of the specimens which develop one or more breakdowns. This increase in probability with area was also found in the case of iron (9). However, the number of breakdowns per unit area decreased at first as the area was increased, finally reaching a nearly constant value at about 4 sq. cm. for both solutions A and B. This drop in the pits per unit area with increase in exposed area is another striking example of the effect that one corroding point has in inhibiting attack a t adjacent regions (1, 8). The fact that one breakdown has ocALUMINUM COOLING AND SEEDING PANSFOR HANDLING FATTYACIDS
,
,-
the entire area became corroded. I n rontrast, in the case of aluminum the attack initiated a t one tiny point within the corroding area generally remained localized. This allowed not only the probability of corrosion but also the number of individual breakdowns for each specimen to be obtained. The specimens were prepared as follows : The metal sheet, 0.040 inch (1.02 mm.) thick, was degreased by double washing in gasoline, then dipped in dilute hydrofluoric acid solution (25 cc. er liter of water) for 20 seconds, and in concentrated nitric acizcontaining 100 cc. of hydrofluoric acid per liter for 1 minute. The specimens were then washed and boiled in distilled water for 2 hours. This formed a uniform coating of a-A120a.Hz0 on the metal. This method of surface preparation was employed for the following reasons: 1. It was desirable t o obtain a uniform and reproducible surface on the specimens. 2. Grinding the specimens with emery paper (the method employed for steel) was considered not entirely satisfactory; because of the softness of the aluminum, emery particles are likely to remain embedded in the metal. The presence of such particles might influence the results. 3. In many types of service the conditions are such that the film formed (ar-Al203.H20) is similar to that obtained by the method employed. 4. Boiling, after etchin assured the removal of traces of acid which might otherwise ke left on the metal surface if etching, followed only by rinsing in cold water, were employed. 5. Regardless of the method of surface preparation employed, the shape of the curves obtained would probably be the same, although they would be shifted to a different position on the axes. Other methods of surface preparation have actually been employed to some extent, and the results obtained substantiate this belief. In addition, many of the factors studied were found to have a qualitatively similar effect on the corrosion probability to that found for steel, prepared by quite a different method. It is considered that as long as a standard method of surface preparation is adhered to, the effect of the various external variables on the corrosion probability will be similar. That is, the corrosion probability curves will be of the same type, regardless of the surface preparation. The specimens were coated with a wax layer, except for one square bare area of the desired size (usually 4 sq. cm.), They were then stored in a desiccator until used. In conducting an experiment, 50 cc. of the desired liquid were placed in each of twenty half-pint Mason fruit jars. The specimens were then immersed, one specimen to each jar. After the required time, generally 5
AREA EXPOSED-SQ.CM
FIGURE1. EFFECTOF INCREASE IN SIZE OF EXPOSED AREAFOR SODIUM CHLORIDE SOLUTIOKS
l ! l i l l ! I ! ! 1
36
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4 5 6 7 AREA EXPOSED - SO, CM.
8
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S ~ EXPOSEDAREAFOR FIQURE2. EFFECTOF I N C R E A IN SOLUTIONS OF SODIUM CHLORIDE PLUSCOPPER NITRATI
OCTOBER, 1937
curred reduces the chance of another breakdown appearing in the vicinity. The extent of the protected region should depend on the quantity of electric current leaving the attacked region and, therefore, partly on the conductivity of the solution. i
24
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8
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2i 08 2 0
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2
4 6 8 1 0 1 2 1 4 1 6 NUMBER OF PITS PER SPECIMEN
FIGURE3. EFFECTOF INCREASE IN NUMBEROF SITES OF ATTACKo x DEPTHOF PITS
more pits was obtained. Figure 3 shows graphically the relation between the number of pits and the average depth of attack. The average depth of attack decreases rapidly as the number of pits increases from one to four. After this point is reached, little further decrease in depth is attained with increase in the number of pits. For more dilute solutions this relation is not nearly so pronounced. This is probably due to the higher resistivity of these solutions. As pointed out by Evans ( I ) , an increase in the resistivity of the liquid would be expected to decrease the sphere of influence of one corroding area on adjacent areas. Other similar tests using solutions containing copper nitrate as well as sodium chloride gave similar results. However, in this case the decrease in the average depth of pitting with increase in the number of pits per specimen was pronounced even in more dilute solutions. This was probably because of
The intensity of corrosion, as measured by average depth of attack, increases a t first as the area is increased (Figure 2 ) , reaching a maximum a t an area of 4 sq. em. and then falling off slightly for larger areas. This initial increase in the depth of attack with increase in area will also be expected if the corrosion of aluminum is a t least partly an electrochemical phenomenon.
Influence of Number of Points of Attack on Conditional Velocity of Corrosion As a further extension of the electrochemical theory of corrosion, it follows that the occurrence of a point of attack a t one spot on a small specimen should not only reduce the probability of attack a t adjacent areas but should also reduce the velocity of attackwhich occurs on these adjacent areas if corrosion is initiated. In an attempt to test this point directly, eighty “identical” specimens, with 2 X 2 cm. exposed areas, were exposed to a concentrated sodium chloride solution (100 grams per liter). After exposure, the specimens in each group were divided into sets depending on the number of pits which had developed. Thus, all of the specimens which had been exposed to the solution with a concentration of 100 grams per liter and had developed one pit only were placed in one set. Those which were exposed under the same conditions but had developed two pits were placed in another set, and so on. The depth of each pit was measured, and thus the average depth of attack for specimens containing one, two, three, or
TEMPERATURE -“C.
FIGURE4.
EFFECTOF INCREASE IN TEMPERATURE OF EXPERIMENT
the fact that in the presence of copper salts higher potential differences are formed between the anodes and cathodes composing local cells on the surface of the aluminum.’ Consequently, even in solutions of low conductivity the protective action of one corroding point will be pronounced.
Temperature of the Experiment In this case also, a solution of 0.3 gram of sodium chloride plus’1.0 gram of copper nitrate per liter was employed. The results shown in Figure 4 indicate that the probability and pits per specimen both increase as the temperature increases. Here again, the effect of variation in temperature on the probability is similar to that found for steel (9). However, the average depth of attack decreases with increase in temperature. This may be due to the small number of pits developing a t low temperature. The fact that a smaller number of pits is initiated a t the lower temperature would tend to increase the velocity a t which they corrode.
Concentration of Salt The effect of increase in concentration of s o d i u m c h l o r i d e in the absence of heavy m e t a l s a l t s is shown in Figure 5, and in
LARGEST ALL-ALUMINUM TANKS IN THE WORLDFOR STORING CHEMICALS WHICHMUSTBE KEPTWATER-WHITE
1 When copper nitrate is present in the solution, copper precipitates on the cathode area with the result that minute cells of aluminum/solution/oopper 81‘0 produced. The potential between the aluminum and copper will be greater than the potential between the anodes and cathodes, developed when the copper nitrate was not present in the sodium chloride solution.
the presence of copper nitrate (1 gram per liter) is shown in Figure 6. The corrosion probability, pits per specimen, and average depth of attack (Figure 6) all rise markedly as the salt concentration is increased. Although the heavy metal salts per se do not appreciably increase the probability of attack, with increasing additions of chlorides the probability increases much more rapidly than when copper nitrate is absent. Increase in the corrosion probability with increase in salt (potassium chloride) concentration was also found for iron (9).
z
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1.6
m
80
8
1.28 60
r = 08340
w
a
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aluminum chloride solution, which cannot yield an alkali hydroxide in the vicinity of the cathode, an applied potential greater than 0.5 volt causes attack of the cathode. It is possible that depletion of hydrogen ions in the liquid immediately adjacent to the cathode results in high hydroxyl-ion concentration in this vicinity, thus causing attack on the aluminum.. This is a point which warrants further study.
Discussion If it is considered that attack is initiated only a t weak spots in the oxide coating on the aluminum surface, it might be expected that only the weakest points would break down in mildly corrosive solutions. As the corrosiveness is increased, stronger and stronger points would break down. By applying the Poisson expressionZfor the special case where n = 0 (that is, where the chance of obtaining no breakdowns in a drop becomes equal to e-E, E being the average number of points of breakdown assuming mutual independence of events), the relative frequency of occurrence of spots of any degree of “weakness” can be calculated. The equation can be expressed as :
20 zlco a
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IO
20 x) 40 50 60 70 80 NaCl CONCENTRATION - GRAMS PER LITER
__
W
Inn __
6. EFFECTOF INCREAS~ IN SODIUMCHLORIDE
CONCENTRATION
I n the presence of the copper salt, copper deposition often occurred on specimenswhichwerenot noticeably attacked anywhere. It appears possible that copper can be deposited in situ in a pore in an aluminum coating, effectively blocking it and preventing further attack. In cases where a point of definite breakdown did occur in the aluminum surface, copper deposition was generally much more pronounced in the vicinity of the pit than elsewhere on the surface. Copper was sometimes observed inside a pit, especially in the larger, deeper pits. However, none was ever found a t the bottom of a n attacked region. It seems possible that a cathodic area can pursue an anodic point a t the bottom of an advancing pit until finally even the walls of the pit itself can serve as cathodes to the more shielded bottom of the pit.
p,
OF APPLIEDPOTENTIAL ON FREQUENCY TABLE I. EFFECT DBPTH OF ATTACK ON 25-0 Applied Depth of PitsPotential -Probability-Av. No. of Pits-Av. Volt Anode Cathode Anode Cathode Anode Cathode % % Mm. Mm. Solution, 1 N NaCl 0.097 0.097 1.6 1.6 83 83 0.0 0.092 0.0 0.0 0.0 1.7 0.1 80 0.0 0.0 0.078 2.0 0.2 80 0.0 0.137 0.0 2.5 0.0 83 0.0 0.4 0.0 0.0 0.150 3.3 100 0 0 0.5 0.269 0.203 5.3 11.7 100 0.6 100 Solution. 1 N AlCls 0.0 0.0 0.111 0.0 292 0.5 100 0.193 0 0356 Over 485 100 100 0.6 2000
AND
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34
r----
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100
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t
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80 60 40
20
tions. Too high an applied potential is as dangerous for an aluminum cathode as for an aluminum anode. Under the conditions of the experiment, the optimum state in the sodium chloride solution occurs when the metal is a cathode at an applied potential of 0.4 volt. In aluminum chloride this potential is about 0.5 volt. It is rather surprising that even in a n
1
where p > is the chance of obtaining one or more breakdowns in a given area (that is, the corrosion probability). Since po = e - ” a n d p o + p > o = l , t h e n ~ > ~l -=p O = l-e-E. Figure 7 shows the relation between E and the sodium chloride concentration. This curve may be considered as indicating the relative frequency of “weak” points of various sizes or inten-
Applied Potential Table I shows the effect of applied potential on the corrosion of aluminum in sodium chloride and aluminum chloride solu-
=
0
0
oi
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I 1 I I I I 04 0.6 0.8 10. 1.2 IA GRAMS PER LITER NaCl ADDED TO I GRAM PER LITER
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1.610.0
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FIGURE 6. EFFECT OF INCREASE IN SODIUMCHLORIDE CONCENTRATION IN PRESENCE OF COPPERNITRATE
sities in the oxide film on aluminum. The number of such weak points increases as their size (or intensity) decreases: this conclusion is similar to that reached by similar calculations for oxide films on steel (9) and, by a totally different line of reasoning, for silver iodide films (2). As has been pointed out previously, the frequency of occurrence of specimens with 1,2,3, etc., points of attack cannot be exDected to be similar to that which would be calculated from
* Fisher (4) states: “It may be shown theoretically that if the probability of an event is exceedingly small, but if a sufficiently large number of independent cases are taken t o obtain a number of occurrences, then this number will be distributed in the Poisson series.” The Poisson relationship is strictly applicable only when the aeries of events under obaervation are mutually independent. This relationship oan be expressed re: PO = e - E ;
= Ee-E;
pl
-$
e - E ; PI
--
E’ 31 % - E . . .
...pa
5
E d
6-B
Where PO, PI,pz, pa, and p s are the chances of obtaining, respectively, 0, 1. 2, 3,or D occurrences of the event in question, E is the mean of the series of 2)(z 3 ) . .I. observntiom and z/ is the factorial of z or z(z - 1)( 5
-
-
. ...
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OCTOBER, 1937
1091
the application of the Poisson equation. This equation applies only in cases where the occurrence of events is mutually independent. Theory and experiment have shown that this is not true for the corrosion phenomenon. Since the occurrence of one site of attack has been found to reduce the chance of attack a t adjacent areas, it is to be expected that the frequency of occurrence of specimens with a small number of pits 225 Lu
g
200
w m
G hL50
5 125
4 ’ 3 E la0 E
N U M B i R OF PITS
9 a75 e
FIGURE8. FREQUENCY DIBTBIBUTION
CURVESFOR SPECIMENS WITH VARIOUS NUMBERS OF PITS
050 025
One curve is obtained from the experimental data, and the other from the Poisson equation.
w
O
o
a05 010 015
020 0.25 a3o a35 040 a45 a50 055 CONCENTRATMN OF NaCl-GRCUWS PER LITER REPRESENTING INTENSITY OF WEAK POINTS
FIGURE 7. RELATIONBETWEEN INTENSITY OF WEAKPOINTS AND’:THEIR RELATIVE FREQUENCY would be higher than that predicted from the Poisson equation whereas the frequency of occurrence of specimens with slightly more pits would be lower than that predicted. That is, the mode (the most frequently occurring value, IO) of the curve of frequency distribution vs. number of pits would occur a t a lower number of pits than would the curve obtained from the Poisson relationship. It has also been found that attack is very likely to occur on areas which are screened from oxygen, as by corrosion p r o d u c t8s The effect of special attack under corrosion products should be most pronounced for specimens developing a relatively large number of pits, since the average depth of attack does not fall off appreciably after some definite number of pits for any given specimen is passed. (This limiting number of pits is six in the case of the specimens from which the data in Figure 8 were taken.) It is to be expected, then, that a larger number of specimens would be encountered which have a relatively large number of pits than would be anticipated from application of the Poisson e x p r e s s i o n . Figure 8 presents the relation between the number of pits and their f r e q u e n c y of distribution. The data for this curve were obtained by grouping the results of the series of experiments mentioned under “Influence of Number of Points of Attack on Conditional Velocity of Corrosion.” The relationship, as calculated from the Poisson ex-
.
pression, is also given. Deviations between the two are qualitatively in agreement with those that would be predicted from a consideration of the theory, as outlined in the foregoing.
Acknowledgment The authors wish to express their appreciation to I?. C . Frary, E. H. Dix, Jr., and W. L. Fink for their advice and encouragement. They also wish to thank other members of the Metallurgical Division of this laboratory, especially C. W. Cline, for assistance in the microscopic examination of the specimens.
LIQHTBUOYAT ENTRANCE TO CHARLESTON, S. C., HARBOR The 25-foot superstructure is all-aluminum construction.
Literature Cited (1) Evans, U. R., Trans. Am. Electrochem. SOC.,57, 415 (1930). (2) Evans, U. R.,and Bannister, L. C., Proc. Roy. Soc., A125,370(1929). (3) Evans, U. R.,and Mears, R. B., Ibid., A146, 164 (1934).
R. A., “Statistical Methods for Research Workers,” p. 55, Oliver and Boyd, 1932. (5) Mears, R. B., Iron Steel Inst. (London), Carnegie Schol. Mem., (4) Fisher,
1935, 69.
R. B., Iron Steel Inst. (London), 3rd Rept. Corrosdon
(6) Mears,
C m m . , 1935, 113. (7) Mears, R. B., J . Iron Steel Inst.
(London), 131, 284 (1935).
(8) Mears, R. B., and Evans, U. R., Trans. Faraday Soc., 30, 421 (1934). (9) Ibid., 31, 627 (1935). (10) Shewhart, W. A., “ E c o n o m i c
Control of Quality of Manufactured Product,” p. 63, New York, D. Van Nostrand C o . , 1931.
RECEIVED May 20,1937. Presented before the Division of Industrial and Engineering Chemistry a t the 94th Meeting of the American Chemical Society, Rochester, N. Y., September 6 to 10, 1937.
