CAUSES OF CORROSION CURRENTS R. B. MEARS AND R. H. BROWN
Localized attack of metal specimens is often the result of electrochemical corrosion, not of simple chemical solution. The appearance of specimens suffering this type of attack has led laymen to attribute it to the presence of impurities in the corroding metal. However, impurities are only one possible cause for this type of attack. Other known or possible causes are: (1) grain boundaries, (2) orientation of grains, (3) differential grain size, (4) differential thermal treatment, (5) surface roughness, (6) local scratches or abrasions, (7) difference in shape, (8) differential strain, (9) differential pre-exposure to air or oxygen,
I
K MANY neutral solutions the corrosion of the common
structural metals appears to be associated with the flow of electric currents between various parts of the metal surface at finite distances from one another. This statement is supported by much qualitative evidence, and in the case of steel (16) and of aluminum (7) the quantities of current flowing during corrosion account for the amount of corrosion which occurs. I n other words, corrosion of the common metals in neutral solutions appears to be electrochemical in nature rather than a simple solution or chemical action. One of the characteristics of such electrochemical corrosion is that often the resulting attack is not uniform but is confined to certain areas of the metal surface while adjacent areas may be virtually free from any sign of attack, The reason for this unlike behavior of the adjacent areas is of considerable importance in order that steps can be taken to prevent attack. Since corrosion in these cases is electrochemical, i t seems probable that the differences in behavior are associated with local potential differences. Several causes of such potential differences are discussed below, and in some cases the magnitudes of these potential differences or of the resulting currents are given. Also, wherever possible, service examples are described which illustrate the cause in question. POTENTIAL DIFFERENCES ASSOCIATED WITH THE METAL
The appearance of specimens suffering electrochemical corrosion immediately suggests that the attacked areas differed in composition from the unattacked areas (Figure 1). Actually none of the commercial metals or alloys are entirely homogeneous so that small particles of other phases (or even variations in composition within a single phase) can usually be detected upon microscopic examination of even the purest commercial metals. I n some alloys two or more phases are desired in order that certain physical or other properties will be obtained. If the various phases can be identified by chemical, spectrographic, or x-ray means, the solution potentials of massive pieces of these phases can be measured in the solu-
Aluminum Company of America, New Kensington, Penna.
(10) differential concentration or composition of the corroding solution, (11) differential aeration, (12) differential heating, (13) differential illumination, (14) differential agitation, (15) contact with dissimilar metals, (16) externally applied potentials, and (17) complex cells. In cases encountered in practice, 4, 6, 10, and 11 appear to determine the local sites of attack more frequently than do any of the others. The magnitudes of potential differences generated by several of the causes listed have been determined experimentally and are given in the paper. In several cases examples illustrating the special attack resulting from these causes are described.
tion or solutions under consideration against some standard reference electrode, as the 0.1 N calomel half cell. Alternatively, by means of a fine tubulus or moistened fiber the solution potential of the separate phases can be measured in situ in the experimental specimen. The solution potentials of several of the constituents present in various metals or alloys were measured by the first method described above and are given in Table I. From the data it would be predicted that selective attack of the aluminum matrix would occur adjacent to particles of CuAl2. As Figure 2 illustrates, such selective attack actually occurs. The specimen from which this picture was made was an aluminum-base alloy containing 10 per cent copper. It was heat-treated a t 500" C. for 48 hours and quenched in cold water. Since only a portion of the copper was soluble in the aluminum, spheroidized particles of primary CuAlzremained. The specimen was given a metallographic polish, and the polished surface was exposed to air-free distilled water at room temperature for 24 hours. Under these conditions, attack was confined to the aluminum matrix immediately adjacent to the CuA12 particles. In liquids of higher conductivity the entire surface of the aluminum matrix between the CuAlz particles was attacked, which left the particles standing u p in relief. Similar studies revealed that selective attack occurred adjacent to particles of other constituents having potentials which were definitely cathodic to that of the aluminum matrix. Particles such as MnAla, which had potentials similar to that of the aluminum matrix, caused no selective attack of the matrix (Figure 3),while particles such as MgsAls, whose potentials were definitely anodic to that of the matrix, were corroded out and left the matrix unattacked. The potential relations between matrix and precipitated particles are greatly affected by the composition of the solution to which the metal is exposed. For example, while the potential of Mg5A18 particles have been found to be 0.17 volt anodic to an aluminum-magnesium solid solution containing 10 per cent magnesium in sodium chloride solution, it is 0.34 1001
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TABLEI. SOLUTION POTENTIAL ;LIEASUREMENTS~ OF SOLID SOLUTIONS OR COKSTITUENTS PRESEKT IN METALSOR ALLOYS Substance Aluminum Silicon CuAln FeA1a AI containing 4 7 C u in solid s o h . A1 containing l & Mg&i in solid s o h . MnAle A1 containin0 4 7 RIg in solid soln. A1 oontainin; 4 % Zn in solid s o h MgZnz nzg6Als Zinc Magnesium Mg oontaining 11% S n i n solid s o h Mg oontaining 447, S n i n solid s o h Mg containing 4% A1 in solid s o h . Copper Cu containing 5% Sn. probably i n solid s o h . Cu containing 10% AI, probably i n solid s o h . C u containing 30% Zn, probably in solid s o h (annealed 70-30 brass)
Potentia!, T‘olts -0.84
-0.26 -0.83 -0.56 -0.69 -0.83
-0.85 -0.87 -1.02
-1.04 -1
nr
-1.10 -1.730 - 1.645 -1.690 1.680 -0.20 -0.0s -0.15
-
-0.25
Trnn
-n
Fccontaining 5 % Cr in solid soln. Fe containing 12% Cr in solid soln. Stainless steel (18% Cr 8% Ni)
-0.50
+
RR
-0.27
-0.15
Some of these d a t a were taken from a paper b y Dix (11);all measurements were made i n solution containing 53 grams NaCl and 3 grams H20n per liter against [t 0.1 N calomel cell. (1
volt cathodic in a solution containing 6 per cent sodium chloride and 1 per cent sodium hydroxide. Hence, in the neutral sodium chloride solution the precipitated particles are selectively attacked, whereas in the alkaline sodium chloride solution the matrix is selectively attacked. While there is no question but that difference in potentials caused by the presence of constituents is responsible for some of the commercially serious corrosion, in general it appears that the importance of this source of potential difference has been greatly exaggerated in the past. Possibly the reason is that the appearance of a section of metal which has suffered localized attack suggests constituents or Ympurities” as the cause of the trouble. Actually, there are several other causes of localized attack which are of far more practical importance than are impurities or constituents.
Grain Boundaries. Practically all metal parts of a size which is commercially important are made up of a multitude of individual grains or crystals. Single crystals of certain of the metals have been produced in substantial size, but they are in the nature of laboratory curiosities. It is clear that the boundary between any two grains is a region which is definitely heterogeneous compared with the body of the grain. Not only does the orientation of atoms in adjacent grains differ but also small particles of separate phases usually precipitate out of solid solution selectively a t the grain boundaries. Recent measurements ( I d ) have indicated that the potentials of grain boundaries of high-purity aluminum (99.986 per cent AI) differ from the potentials of the grain centers in 20 per cent hydrochloric acid when the metal is in either the annealed condition or has been given a solution heat treatment. I n these cases it is not certain whether the small amount of impurities remaining in the metal and possibly precipitated selectively at the grain boundaries contributed to the potential differences or whether they were caused solely by the grain boundary as such with the impurities playing a negligible part. Data of this kind obtained from very pure metals are largely of academic interest. However, measurements have also been made of the more usual case where a second phase precipitates a t the grain boundaries. An average value for the difference in potential between grains and grain boundaries in a case of this kind was obtained by selecting two coarse-grained specimens from one lot of
Vol. 33, No. 8
sheet rolled from an alloy containing 96 per cent aluminum and 4 per cent copper. This sheet had been given a solution heat treatment followed by aging a t an elevated temperature in order to cause precipitation of CuAlZ particles, largely at the grain boundaries. The centers of the grains on one specimen were coated with wax, while the grain boundaries were left uncoated. Only the grain boundaries of the other specimen were coated with wax. Then the edges and backs of the two specimens were wax-coated, they were placed face to face in a sodium chloride solution, and the potential difference between the two specimens was measured with a type K potentiometer. Under these conditions the measured potential difference was 0.044 volt, and the grain boundaries were anodic to the grains. Individual values of grain and grain-boundary potentials could be obtained by attaching a fine tubulus to the end of the calomel cell. Contact between the cell and the area of the specimen it was desired to measure could then be made by means of the droplet of solution which adhered to the end of the tubulus. Values obtained in this manner for the same aluminum-copper alloy are shoTvn in Figure 4. Potential differences between grains and grain boundaries have also been measured in the case of 70-30 brass. Samples having a grain size of approximately 0.5 inch (1.25 em.) were furnished through the courtesy of C. S. Smith. This coarsegrained material was prepared by the strain-anneal method of Carpenter and Elam (9). Prior to measurement, the surfaces of the specimens were etched with a solution containing 25 per cent by volume of concentrated nitric acid (specific gravity 1.42) to reveal the grain structure. The method of preparation used was essentially that mentioned above for the aluminum-copper alloy specimens. However, in this case the grain boundaries of one specimen and the grains of the other specimen were coated with Bakelite varnish instead of wax. TF’hen these two specimens were immersed in a solution containing 1 per cent by volume of ammonium hydroxide (28 per cent KH3) and connected together through a milliammeter, a current flowed between the specimens. The direc-
FIGURE1. ALUMINUMSPECIMEN EXPOSEDTO A SOLUTION CONTAINING CHLORIDESAND COPPER SALTS Attack is of t h e pitting type, as illustrated i n the sectional view taken along t h e line marked above in the surface view.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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1003
OF AN ALUMINUM ALLOYCONTAINING OF AN ALUMINUM ALLOYCONTAINING FIGURE3. SURFACE FIGURE2. SURFACE 10 PER CENT COPPERAFTER 24-HoUR EXPOSURE TO AIR12 PnR CENT MANGANESE AFTER 24-HOUR EXPOSURE TO AIR-FREEDISTILLED WATER( X 100, UXETCHED) FREEDISTILLED WATER( X 800, CNETCHED)
The absence of special attack adjacent to MnAle particles is noted.
Special attack is apparent adjacent to CuAlz particles.
conservative since such errors would tend to reduce the experimental values. It seems probable that preferential grain-boundary attack of most metals or alloys is caused or accelerated by electrochemical effects. However, thus far it has not been possible to check the current distribution between the grains and grain boundaries for other materials directly, because of the difficulty in obtaining specimens with sufficiently coarse grains, Nevertheless, indirect evidence has been obtained that electrochemical effects contribute to the intergranular corrosion of 18-8 stainless steels. According to Bain, Aborn, and Rutherford (S),the chromium content of the iron-chromium solid solution a t the grain boundaries, in the case of stainless steel which is susceptible to intergranular corrosion, becomes reduced as a result of chromium carbide precipitation. They considered that this zone, depleted in chromium, then corroded out more rapidly than the surrounding areas which were richer in chroTABLE 11. D~SCRIPTION OF ALUMINUM AND MAGNESIUM ALLOYS mium and thus gave the intergranular type of at% Alloying Elements and Normal Impurities Alloy tack. However, it seemed NO." AI Mg Cu Si Zn Mn Cr . . . . . . . . . . . . . . . . . . . possible that electrochemi2s 89 + 3s Balance ... . . . . . . . . . 1 . 2 .... cal effects were also im179 Balance 0.5 4.0 . . . . . . 0 . 5 .... 245 Balance 1.5 4.5 . . . . . . 0 . 6 .... portant. Therefore, the 52s Balance 2.5 ... ... . . . 0 . 2 5 solution potential of a 53s Balance k.3 0.7 ... ... 0 . 2 5 Balance 56s ... ... 5.2 ... 0 . 1 0 . 1 sample of 18-8 stainless Balance 1.0 61s 0.25 0.6 . . . . . . 0.25
tion of this current was such that the specimen with uncoated grain boundaries was anodic to the specimen with uncoated grains. The magnitude of the current was 190 microamperes for specimens having a total exposed area of 58 sq. cm. The open-circuit potential between these specimens was 0.072 volt. It was found that changes in the surface preparation of the specimens and alterations in the concentration of the ammonium hydroxide solution changed the magnitudes of the short-circuit current and the open-circuit potential between two such specimens, but in all cases studied the grain boundaries were anodic to the grains. I n addition, the potentials of individual grains and grain boundaries were measured with a fine tubulus as described above. I n most instances the individual grain boundaries were anodic to their adjacent grains. A definite but unknown error is associated with both methods of measuring the potentials between grains and grain boundaries, since unavoidably the measured grain boundary is wider than the true boundary zone, which in many cases is probably only of atomic dimensions. However, it is important to note that these measured potential differences are
72s 43 195 214 220 355 356 AM3S AM57S AM240
Balance Balance Balance Balance Balance Balance Balance
... ...
...
... ...
4.0
... ... ... ... .......... ... ...
5.0
1.0
.... .... ....
...
3.8 ... 10.0 ... ... 0.5 1.3 5.0 . . . .... 0.3 7.0 ... Balance 1.5 6.5 Balance ... 0: 7 0.2 Balance 10.0 .... ... 0.1 An alloy number containing the letter S indicates that the alloy is wrou ht a number not containing S indicates the alloy is cast Designa(is 2 s - 0 ) signifies the annealed temper: 1/2H, the half-hard temper; tion and H. the full-hard temoer. W or T 4 (as 53s-W or 220T4) indiratss the solution heat-treated tem'per. T indicates alfoys which-hake been-solution heat-treated and aged either ht room temperature or elevated temperatures, depending on the type of alloy.
8
... ...
...
... ...
... ... ... ...
... ...
.... .... .... ....
steei which had been FIQURE4. VARIATION IN treated so as to precipiPOTSNTIAL OF GRAINSAND tate the chromium as chroGRAINBOUNDARIES mium carbide was measAverage potential of grains -0.484 average potential of ured in comparison with grain boundaries -0.585. the solution potential of a similar sample of 18-8 which had been quenched from the heat-treating temperature so as to retain the chromium in solid solution. It was found that the sample in which precipitation of the chromium had occurred was anodic to the other sample by 0.09 volt in a sodium chloride solution containing hydrogen peroxide. Thus,
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if Bain, -4born, and Rutherford are coirect In a-uining that the matenal adjacent to the gi ain l~oundarios becomes clepleted in chromium hp prwipiration. then it iollo\\s that gdvaiiic action betn eeii suc*h depleted areas and the hody of the grains will occur in Wi,lri .. ~ . certain solutions. In addition, the soluFIGURE5. SEVEREATTACK A D J ~ tion potentials of a series of iron alloys containing various amounts of chromium in solid solution were measured (Table I). Increasing the amount of dissolved chromium in the alloy definitely altered the solution potential in a cathodic direction and thus again indicated that electrochemical effects are important in the intergranular corrosion of 18-8 stainless steels. It should be emphasized here that for some alloys in certain solutions the grain boundaries may be cathodic to the grains. In cases of this type the grain centers will be selectively attacked and the grain boundaries will stand out in relief. An example of this granular type of attack was obtained when 99.986 per cent aluminum, which had been furnace-cooled from a temperature of 630' C., was exposed to hydrochloric acid (10 to 20 per cent HC1 by weight). Current measurements made in 20 per cent hydrochloric acid revealed that in this case the grain centers were anodic t o the grains. As mentioned above, other heat treatments caused this material to suffer intergranular attack. I n such cases the grain boundaries were anodic. I n other words, in those cases where the grain boundaries are anodic to the grain centers, attack will be of an intergranular nature; whereas in cases where the grain boundaries are cathodic t o the grain centers, attack will be of a granular nature.
Weld
i~~~~
Orientation of Grains. Grains oriented in different directions would be expected to have different solution potentials. The potentials for the individual grains shown in Figure 4 differ from one another, probably in part a t least because of their difference in orientation (although there are other possible explanations in this case). Several workers have studied this point, but their published results are often indefinite or conflicting ( I , 9, 15, 19, 32). More work along this line is desirable. Differential Grain Size. A fine-grained specimen of a given metal contains a higher internal energy than does a similar coarse-grained specimen. For this reason it would theoretically be expected to exhibit a different solution potential. However, because of the experimental difficulties associated with the preparation of fine- and coarse-grained specimens of a metal without factors other than grain size being different, apparently there is no experimental evidence on this point. Differential Thermal or Metallurgical Treatment. If one portion of a metal surface has been subjected to a different thermal treatment from that on other parts of the surface, differences in potential between these two regions may occur. Under service conditions the heat of welding is likely t o cause such inhomogeneities and there are numerous references in the liteiature (6, f8, 27, 31) to special attack which is caused by the presence of welds. I n this case it is not generally the weld bead which is anodic (if the weld wire used was of the same alloy as the material being
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TO
WELDSOF STAINLESS STEELEXPOSED TO STRAUSS TEST
welded) but usually a zone on either side of the weld where the metal was heated and cooled a t some optimum rate which would throw cathodic impurities out of solid solution. To illustrate, the solution potential of a welded 53s-T aluminum alloy specimen (Table 111) was measured at the weld and also a t various distances from the weld. A band of metal about 1.5 inches (3.8 em.) from the weld had the most anodic potential. When welded 53s-T specimens of this type were exposed to a corrosive salt solution, attack was largely confined to this anodic band. TABLE111. POTENTIALS
O F ALLOYSAT \'ARIOTJS FROM W E L D
DISTANCES
Diatance from Weld, In. (Cm.) Potential=, Volt 1 (2.5) -0.840 4 (10.2) -0.847 -0.830 7 (17.8) 9 (22.9) -0.828 After annealing l(2.5) -0.842 4 (10.2) -0.842 7 (17.8) -0.840 9 (22.9) -0.839 Measured in solution containing 53 grams NaCl and 3 grams HzOz per liter against 0.1 N calomel cell. Condition of 53s-T Alloy As welded
If the welded 53s-T plate was either annealed or given a solution heat treatment and subsequently aged at an elevated temperature, the differences in potential between various portions of the plate were greatly reduced. Similarly, if 18-8 stainless steel which had been quenched from the heat-treating temperature was reheated a t 725" C., the solution potential of the reheated sample differed from that of the unreheated sample by about 0.1 volt in sodium chloride-hydrogen peroxide solution. Potentials of a similar origin have been caused by welding stainless steels, and unless the welded article is subsequently given a suitable heat treatment, special corrosion associated with the weld may occur. A typical example of special attack adjacent to a weld is illustrated in Figure 5 . I n this case the specimen is of 18-8 stainless steel which was welded and then exposed in acidified copper sulfate solution. Effects of the type described above are not confined to the two alloys cited as examples but are general. Whenever local heating results either in changing the nature of the phases present or their compositions, differences in potential will probably occur. POTENTIAL DIFFERENCES ASSOCIATED WITH THE METAL SURFACE
I n addition to those potential differences which are associated with the metal, other potential differences are caused by its surface condition. They are probably somewhat less commonly recognized than are those which have already been discussed.
Surface Roughness. Highly polished metal surfaces may exhibit different solution potentials from those of rough abraded surfaces. Probably one reason is that any film which forms on the rough surface will be much less continuous than ,a f ilm formed on a smooth surface. Table IV gives the potentials between rough and smooth surfaces for several materials. As would be anticipated, the initial potentials for specimens of any given metal differ more from each other than do the potentials after the specimens have remained in the solutions for some time. TABLE IV. EFFECTOF SURFACE ROUGHNESS ON SOLUTION‘ POTENTIAL Metal
Surface Condition
Aluminum (28-!/1H)
Metallographic polish Electrolytic polish 000 emery paper Microtome cut No. 120 -4loxite Metallographic polish 000 emery paper No. 120 Aloxite Metallographic polish 000 emery paper No. 120 Aloxite
Copper Steel a
Potential against 0.1 N Calomel Cell, Volt Steady Initial reading -0
855
-0,854 -0,849 -0.990
-0.817 -0.307 -0,338 -0.378
-0.438 -0.587 -0.627
-0.853
-0.852
-0.853 -0.848 -0.850 -0.341 -0.369 -0.a74 -0.739 -0.775
-0.779
10% sodium chloride a t 26’ C.
Local Scratches. Potential differences caused by local scratches or abrasions are related to those just discussed. Such potential differences are among the most important causes which determine the sites of local attack. I n several previous papers (6, 7 , 8) the methods of measuring potentials of this type were discussed and the magnitudes of such potentials given. Several of these values are given in Table V. As might be expected, the effect of scratches in determining sites of attack is most pronounced in environments where the metal in question forms adherent and protective films of corrosion product. Obviously, if the entire metal surface is attacked and the corrosion products which are formed are soluble, scratches may not be points of special weakness. Therefore, this phenomenon is most in evidence when the metal in question resists attack by forming a protective layer. In corrosive salt solutions attack of large flat surfaces of aluminum alloys is often confined largely or entirely to acci-
F I G U R6.~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1941
.
TABLE V. POTENTIAL BETWEEN SCRATCHED” AND U N S C R A T C ~ D SURFACES AT OPEN CIRCUIT Metal or Alloy Commercially pure A1 (25) 18-8 stainless steel
A
+
0.1 N KCl 0.3% Ha09 0.1 N K 2 0 0 r i- 0.3% HiOa 16% NaCl
E%2%; $ Et%R85 +
Magnesium 0.001 M NaC1 0.238 IIf N a p 0.001 iM KCI -t- 0.0075 IM KzCOs Steel a Scratched area anodic in all cases,
Potential Difference, Volts 0.22 0.33 0.23 0.21 0.20 1.4 0.32
dental scratches. Figure 6 illustrates this type of attack for an aluminum alloy article which was employed in service.
Cut Edges. These are also sites of special weakness under many exposure conditions. Several factors contribute. I n the first place, because of geometrical considerations it is generally more difficult for a continuous protective film to build up over the edges. Often the edges are rough and uneven, and there are ragged and torn metal fragments which have a large ratio of surface area to volume. In addition, if the material was sheared, the edges have been subjected to severe cold working which may also cause them to be more readily attacked in certain environments. It is important to emphasize, however, that cold working does not always produce special susceptibility to attack; in fact, coldworked materials may actually prove more resistant than annealed materials in certain cases. TABLE VI. POTENTIAL BETWEEN
THE
EDGESAND CENTEROF ROOMTEMPERATURE“
SHEARED-SHEET METAL SPECIMENS AT
Metal or Alloy Solution 528-1/&3 A1 alloy 10% NaCl 10% NazCOs 529-1/1H A1 alloy 10% NaCl Copper The edges were anodio to the centers in all cases,
Volt 0.02
0.03 0.01
It is possible to measure the potential between the edges and the center of a metal specimen by a technique similar to that described under the section on grain boundaries. Thus two “identical” sheet mecimens are selected. The edges of one, and alfexcept the edges of the other, are coated with wax. These specimens are then immersed in the desired solution and the potential between them is measured with a potentiometer. Table VI shows the results for pairs of specimens of several materials. While it might be expected that the edges of the specimens would be anodic to the center and so tend to corrode selectively, this is not always the case. For example, Figure 7 is a cross section through an aluminum alloy (5251/2H) specimen which was exposed for one week in 10 per cent sodium carbonate solution at 31 O C. and illustrates a case in which the edges are especially resistant to attack. Thus, it is not possible to predict with certainty whether or not the edges or center of the specimen will be most susceptible. However, it is a general rule that the edges will probably behave differently from the center.
AI UMINUM SHEET EXPOSED FOR 2 CORROSIVE SALTSOLUTION d
SURFACE O F SCRATCHED
MONTHSIN
Solution
Attack is confined t o scratches; in other environments scratches may not he particularly susceptible t o attaok.
Shape of Specimen. The shape of the specimen may sometimes be of controlling importance, as might be inferred from the preced-
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may be set up between the film-free and the film-coated-regions which will result in special attack. The potentials beTABLE v-11.
POTENTIALs" BETWEEP; FILM-FREE .1ND FILM-
COATED AREAS Material Anodically oxidized commercially pure A1 ( 2 s - 0 )
Condition of Surface Continuous coating Local imperfections Roughened by numbers stenoiled vrior t o oxidation Scratched after oxidation Duralumin-type alluy (243-T) Abraded, exposed t o air 2 min. Abraded, measured a t once Commeicially pure A1 ( 2 s - 0 ) With "normal" air-formed film Film removed by oontinuous scratching with glass stylus Measured in NaCl solution against 0.1 N calomel half cell.
Volts -0.442 -0.511 -0.797
- 0.851
-
0.673 -0.682 -0.83 >--3.0
Q
tween film-free and film-coated areas of some aluminum alloy specimens are shown in Table VII. The effect of film formation on the potential of a commercially pure (2s-0)aluminum specimen is shown graphically in Figure 8. When a freshly etched aluminum specimen was exposed to a sodium chloride solution (53 grams per liter) through which air was being bubbled, the potential fell rapidly at first probably because of film formation. After 2 minutes the rate of change in potential decreased but was still appreciable even after 30 minutes. The potential altered less rapidly when hydrogen was bubbled through the solution, qrobably because of less rapid film formation under these conditions. FIGURE 7. SHEARED-SHEET SPECIMEN OF 52SJ/zH AFTER FOR A WEEK EXPOSURE IN 10 PERCENTSODIUM CARBONATE Surface shown above and cross section below, taken along t h e lines marked on t h e surface.
ing paragraph. Convex surfaces generally show lovier hydrogen overvoltages than concave surfaces (19)and so may develop different solution potentials. Slthough no service cases where corrosion currents set up by this cause have been called to our attention, it is reasonable to suppose that such special attack might develop under suitable conditions. This is especially probable since previous work established that wires of small diameter corrode more rapidly than wires of larger diameter (18). I n the section on cut edges it was mentioned that strain hardening may contribute to special edge attack. Such behavior may not be confined to edges. Any portion of a specimen subjected to plastic deformation may have a different solution potential from a similar specimen which has not been deformed. Several cases of special corrosion resulting from this phenomenon have been reported (11,17, 18,34,56), but none have come directly to our attention.
Differential Strain.
Differential Pre-exposure. If a portion of a metal surface is exposed to some environment which differs from that to which adjacent areas of the surface are exposed and then subsequently the entire surface is exposed to a uniform environment, corrosion currents may flow between these areas. Suppose that a drop of hydrochloric acid falls on a metal surface. At the local area in contact with the acid the natural protective layer may be broken down. When the entire specimen is subsequently exposed to a salt solution, attack may be confined solely to the small area which had previously contacted the acid. Conversely, a portion of the metal surface may be exposed to air, oxygen, boiting water, or some other environment which causes the formation of a protective film. On subsequent exposure to salt solution, a substantial potential difference
POTENTIAL DIFFERENCES ASSOCIATED WITH THE LIQUID
I n addition to corrosion currents set up by heterogeneities in the metal itself or on its surface, heterogeneities in the corroding environment may a130 be important Heterogeneities which have probably caused the greatest difficulty under service conditions are those T5-hich result from differences in concentration in different portions of the corroding liquid (21, 29). i.62 1.14
