Protective Film Formation with Phosphate Glasses G. B. HATCH Ccrlgon, h e . , Piffsburgh, Pa.
The inhibition of the galvanic corrosion of steel coupled to cathodic metals by the glassy phosphates has been found to be chiefly the result of a marked polarization of the cathode. This polarization appears to result from the presence of a glassy phosphate film on the cathode. The film is laid down b y an electrodeposition process, A similar process appears operative with uncoupled steel; film deposition apparently takes place on the local cathodic areas as the result of the local action currents. An additional factor enters into the inhibition in differential aeration cells with steel electrodes. The deposition of the glassy phosphate on the cathode causes its open-circuit potential to become less cathodic. Consequently, the potential difference between the two electrodes is lowered in the presence of the inhibitor. Both this decreased potential difference and the increased cathodic polarization are important factors in the reduction of the current flow in these differential aeration cells by the glassy phosphates.
T
H E glassy phosphates have been employed rather widely for corrosion control during the past 12 years. Pitting and localization of attack has not been a problem where recommendations regarding p H and control of contamination-Le., the system have been either chemical, bacterial, or algal-of followed. Even when glass) phosphate feeds insufficient to obtain adequate protection, have been used, they have not caused pitting. Yet these phosphates have been classified from time to time as anodic or “dangerous” inhibitors (3,9) on the basis of unpublished data (4). There appears to have been a natural tendency t o consider all phosphates together in evaluating their corrosion inhibitive properties. Actually the inhibitive as well as most of the other properties of the glassy phosphates are markedly different from those of the orthophosphates. The inhibitive action of the glassy phosphate6 on the galvanic attack of steel coupled to copper has been found to be chiefly the result of a marked increase in the cathodic polarization. (The term “polarization” is used in the rather broad sense frequently employed in the corrosion field to denote changes in the potential of an electrode from its open circuit value which result from the flow of current. Thus it includes potential drops across poorly conductive surface films as well as counterelectromotive forces.) Only a slight flow of current, hence a low rate of attack on the steel, suffices to equalize the potentials of the two metals in the presence of the inhibitor. This equalization is chiefly the result of a shift in the potential of the copper cathode, and increased cathodic polarization presumably is caused by deposition of a glassy phosphate complex on the copper surface (6). Such behavior certainly would be unexpected if the material was an anodic inhibitor. Consequently, further investigation appeared warranted in order to determine whether the behavior of the phosphate glass with respect to the copper-steel system was specific or generally applicable, particularly t o couples where steel constituted the anodic member. Evidence regarding the protective glassy phosphate film is August 1952
based chiefly on the formation of interference colors together with indications of a more indirect nature (7, 8). More positive evidence of this film appeared desirable, particularly eince many of the conclusions reached in the tests with galvanic couples involved its formation. As a result, phosphate analyses were made of films from metals that had been exposed t o the action of the glassy phosphates. Iron and steel are the primary interest in many practical systems, and in certain of these cases dissimilar metal couples are of little concern. However, differential aeration cells practically always are present in an actual system. Thus investigation of the effect of the glassy phosphate on differential aeration cells with steel electrodes has been included. Experimental
All tests were conducted with a pH between 6.5 and 7 and a calcium: 1.1-sodium phosphate glass ratio of 1:5 or greater. Both these conditions are in accordance with standard recommendations for field applications and have been discussed previously (7, 8). The nomenclature used for the sodium phosphate glasses is a modification of that suggested by Partridge (11). The numerical prefix designates the ratio of NaLO:Pz06. Thus, a 1.1sodium phosphate glass refers to a NazO:Pe06 ratio of 1.1:1. This corresponds to the commercial product Calgon which wa8 used in all of the tests reported here. The use of other phosphate glasses would yield results in qualitative though not quantitative agreement. The tests were of a small volume batch t pe, hence glassy phosphate concentrations considerably higher tzan used in actual practice were employed (8). Agitation was accomplished by aeration unless otherwise noted. Galvanic Couples. Certain metals show no visible interference colors or other evidences of film deposition after exposure to glassy phosphate treated waters. A number of these such as stainless steel, gold, and platinum are cathodic to steel. The action of the glassy phosphates on the galvanic attack of steel coupled to such metals offers a rather severe test of the generality of the hypothesis that the inhibitive action of the phosphate glasses on the galvanic attack of steel is chiefly due to its deposition on the cathode with a subsequent marked elevation of the polarization thereof. Current flow tests were conducted in order to determine the effect of 100 p.p.m. of 1.1-sodium phosphate glass on couples of mild steel with stainless steel (Type 302), gold, and platinum, respectively. The procedure and ap aratus used for these current flow and polarization tests were igntical to those previously described in conjunction with tests with copper-steel couples (5). The test specimens consisted of 11/2 inch by J1/2 inch metal plates which were separated by a distance of ‘/z inch. Cold-rolled steel strip was used for the anodes, and gold (heavily gold-plated copper), platinum, and stainless steel (Type 302), respectively, were used as cathodes. The strips were cleaned in an alkaline detergent prior to use. The stainless steel was subjected to an additional passivation treatment in 30% nitric acid. Pittsburgh tap water was used as the test medium or the base for its preparation. The com osition of this tap water varies somewhat. the analyses of the gatches used in the tests are shown in Table I. The tests were conducted at 35’ & 0.2’. The data obtained are shown in Figure 1. A very low current flow level is obtained in each of the treated systems: the glassy
INDUSTRIAL AND ENGINEERING CHEMISTRY
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~~
Table 1.
Analyses of Tap Water Used
Bicarbonate Chloride Total hardness (as CaCOa) Calcium Magnesium Sulfate Total solids
Water Sample Analysis, P.P.M. DifferGold and ential Stainless steel platinum aeration tests tests tests 14 10 22 21 12 16 27 7
79 22 6
..
166
96
84
97 25 9
76 192
71
whereas the increase in cathodic polarization is very pronounced. The same factor-a marked increase in cathodic polarizationappears responsible for the inhibitive action of the glassy phosphate on the galvanic attack of steel coupled to stainless steel, gold, or platinum as was the case when it was coupled to copper ( 5 ) . Thus the function of the phosphate glass with regards inhibition of the galvanic attack of steel appears to be largely independent of the nature of the cathodic metal to which it is coupled. Film Deposition. The surfaces of stainless steel, gold, and platinum which had been coupled to steel in these tests showed pronounced interference colors after exposure to the water treated with the glassy phosphate. Qualitative tests with a non-metallic cathode (graphite) coupled to steel showed a marked reduction in current flow in the presence of glassy phosphate together with the formation of interference colors on the graphite. These observations imply that the deposition of a film of complex phosphate on an electroconductive surface can be caused, or a t least increased, by making the surface the cathode in a corrosion cell, since graphite alone has shown no indication of adsorption of the glassy phosphate. The film appears to be laid d o m by a process of electrodcposition. A more definite indication that an electrodeposition process is involved in the formation of the corrosion inhibitive film of glassy phosphate appeared desirable to check the hypothesis based on interference colors. Consequently, films from coupled and uncoupled metals were analyzed for phosphate.
phosphate exerts a marked inhibitive action on the galvanic attack in each of these three cases. The curves for the stainless steel-steel couple differ in shape from those previously obtained for the coppcr-steel system. The initial current is l o x for both the treated and untreated systems, probably a reflection of a low potential difference between the stainless steel and the steel surface covered with the initially present oxide film. The curve for the untreated system rises rather sharply as destruction of the oxide film on the steel anode proceeds, levels off a t a relatively high value, then decreases slightly towards the end of the test. The curve for the treated systems drops almost to zero during the initial stages, then rises slightly and levels off a t a quite low value. Apparently the glassy phosphate offers sufficient protection to the oxide film under the conditions of the test to delay greatly but not entirely to prevent its breakdown; the low rather constant level The coatings were removed from the metal plates by solution in 4.4 N sulfuric acid. The acid was treated with 0.05% during the latter portion of the test seems to represent the di-n-butyl thiourea which served as a pickling inhibitor when general slight galvanic attack of the steel surface coated with the steel was involved. (This particular inhibitor does not interfere glassy phosphate after destruction of the initial oxide film. with subsequent phosphate analyses.) The acid solutions of the The curves for the gold-steel couples are qualitatively similar coatings were then diluted 1 to 10 and the phosphate was reverted-Le., hydrolyzed to the orthophosphate-by a 15-minute t o those previously obtained for the copper-steel system (6). treatment, in an autoclave with steam a t 15 pounds per square The current rises during the early stages of the test in untreated inch. The phosphate contents were then determined by the water as destruction of the oxide film initially present on the steel Truog and hIeyer modification of the DenigAs colorimetric prosurface proceeds. The current drops during the corresponding cedure ( 1 2 ) . stages of the test in treated water as formation of the film of Table I1 shows the influence of coupling several of the more glassy phosphate proceeds. There is a slight lag in the rate of noble metals with steel on the quantities of the glassy phosphate current decrease during the initial stages of the test in the treated deposited on their surfaces when exposed to waters treated with water; its cause has not been determined, although a somewhat 1.1-sodium phosphate glass. (The analyses of the water used in similar lag was present in certain of the tests with the copperthese tests is shown in column 2 of Table I.) Very little of the steel system (6). phosphate was picked up by either the uncoupled gold or platiThe current flow for the untreated platinum-steel couple denum, but slightly greater quantities were deposited on the creases rather sharply during the initial stages of the test, then copper and stainless steel. In each of these tests the quantity of rises slightly and levels off. The usual initial rise attributed to breakdown of the oxide film on the steel anode is not present. Apparently i t is masked by changes in conditions a t the cathode which produce a gradual increase in its polarization. The STAINLESS STEEL S T E E L subsequent rise in current flow appears to reflect GOLD-STEEL the later stages of the oxide film breakdown. Polarization data for these tests with couples of stainless steel-steel, gold-steel, and platinuma stee! are shown in Figure 2; these data were 2 collected after the systems had been coupled for 48 hours. The dotted line a t the intersection of the anodic and cathodic polarization curves repre5 sents the arithmetical mean of the potentials of U the two electrodes with an external resistance of 0.82 ohm. It defines the potential a t the point of intersection of these anodic and cathodic polarization curves rather closely and thus aids in their extrapolation to this point of intersection (5). 0 20 40 0 20 40 0 20 40 TIME -HRS The curves for each of these couples are quite Figure 1. Influence of Calgon on Current Flow between Stainless Steel similar to those which were obtained for the copperand Steel (left), Gold and Steel (Center), and Platinum and Steel steel system ( 5 ) . I n each of the tests the inhibitor (Right) raises the anodic and cathodic polarization, but the increase in anodic polarization is relatively slight Plates in aerated Pittsburgh tap water
$
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Corrosion by Water phate film should provide an indication whether the potential difference that caused the current flow was a 0) prerequisite for film formation. 3 Copper-steel couples in oxygen-free W (nitrogen-saturated) water provide 8 -04 such a case; the current flow rapidly 2 falls t o an extremely low value in this 5 system as is shown in Figure 3. (The P analysis of the water used in these 2-02 tests is shown in column 1 of Table -+ w I.) Nitrogen was bubbled through the solution for 24 hours prior t o the 'B introduction of the couple in order t o 0 10 0 05 10 15 0 a5 Lo remove dissolved oxygen. On admisCURRENT- MA sion of air the current rose abruptly then decreased and leveled Figure 2. Influence of Calgon on Polarization of Stainless Steel and Steel (Leff), off at a low value. The gradual deGold and Steel (Cenfer), and Platinum and Steel (Right) crease in current flow after the admisPlates coupled for 2 days in aerated Pittsburgh tap water sion of air is characteristic of the initial periods of tests in air and apparently reflects the progressive formation of the film of glassy phosphate. The results appear t o indiTable 11. Influence of Couples on Film Formation on Metal cate that formation of the protective film commenced only after Surfaces by a Glassy Phosphate air had been admitted t o the system. Phosphate analyses of films from copper and steel plates which (2 days a t 35" C.) had been coupled in water saturated with nitrogen and treated Film on Metal, Mg. Calgon/ Calgon Concn., Sq. Dm. with 1.1-sodium phosphate glass (100 p.p.m.) for 48 hours are Metal P.P.M. Uncoupled Coupled with Steel shown in Table 111. (Column 2 of Table I contains the analysis Stainless steel (302) 50 0.7 4.7 of the tap water used in these tests.) These data show very little Gold 100 0.2 5.2 Platinum 100 0.2 3.4 deposition of the glAssy phosphate on either steel or copper in 5.2 Copper 100 0.5 nitrogen saturated water in contrast to the marked film formation on each of these metals in aerated water. The results Table 111. Influence of Oxygen on Film Formation on of these film analyses confkm the indications obtained from the Coupled Copper and Steel with a Glassy Phosphate current-flow tests that glassy phosphate film formation is very (2 days a t 36" C . ) low in the absence of oxygen when the current flow is extremely Film on Metal, Mg. Calgon/Sq. Dm. low.
7
Water saturated with Air Nitrogen
Copper
Steel
4.3 0.02
5.6 0.1
I
1.0
the glassy phosphate deposited on the metal was markedly increased when it was coupled t o a steel anode. These analytical data confirm the indications of the effect of coupling on the film formation that were obtained from visual observations of interference colors. 'The deposition of the glassy phosphate film is greatly increased when the metal is made the cathode in a corrosion cell. Steel, either alone or in any of the couple combinations considered, shows marked interference colors after exposure t o a glassy phosphate-treated water. Although this might seem t o indicate that a different coating process was operative for steel than for the more cathodic metals, it appears rather unlikely. It seems more probable that the explanation lies in the local cell activity on the steel surface. A greater glassy phosphate film deposition in a single metal system might be expected on steel than on the more noble metals previously considered-i.e., copper, gold, platinum, and stainless steel-as local cell activity is considerably more pronounced with the steel. The local cell activity on the steel also persists in the couples that have been discussed since the galvanic corrosion accounted for only a portion of the weight loss. Glassy phosphate film formation might be expected t o proceed on the cathodic areas on a steel surface in much the same manner as it does on the cathodic member of a bimetallic couple. The effect of the elimination of the current flow between the members of a bimetallic couple-or between the local anodes and cathodes on a single metal-on the deposition of the glassy phos-
August 1952
0
IO
20'
30
40
T I M E - HRS.
Figure 3. Influence of Air (Oxygen) on Current Flow between Copper and Steel Plates Immersed in Calgon-Treated ( 1 00 P.P.M.) Pittsburgh Tap Water
The way in which oxygen is involved in the glassy phosphate film formation is not d e h e d too specifically by the data considered so far. The question remains as t o whether the oxygen or the potential difference with the resultant current flow is the prerequisite for the protective film formation. A bimetallic system that gave appreciable current flow in the absence of oxygen might provide an answer t o this question. Magnesiumsteel is such a system. The phosphate analysis of the film removed from a steel panel, which had been coupled to a magnesium alloy (FS-IA) strip for 24 hours in a solution treated with 50 p.p.m. 1.1-sodium phosphate glass through which nitrogen was
INDUSTRIAL AND ENGINEERING CHEMISTRY
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bubbled, showed a coating equivalent to 2.4 mg. per square dm. of this phosphate. (This test was conducted in a calcium sulfate solution equivalent to 100 p.p.m. CaCOs.) Thus, the potential differences reflected by the current flow appear to be the determinant factor for deposition of the glassy phosphate film, rather than the presence of oxygen. The very slight deposition of glassy phosphate on either metal of the coppersteel couple in the absence of oxygen apparently resulted from the lack of appreciable potential Z K differences in the system. The copper and t h e local cathodes on t h e steel were rapidly polarized by hydrogen with the result that all the metal surfaces quickly attained the same potential. The rapidity of this current decrease was such that very little deposition of phosphate could be expected during the period of appreciable current flow Steel alone is often the major concern in practical water systems. Wide expanses of steel generally are-present which are 80 located that they are immune t o the influence of dissimilar metal couples. Thus, the growth of protective films of glassy phosphate on steel alone is of considerable interest from a practical point of view. Figure 4 shows the rate of formation of this coating OB steel immersed in water treated with 50 p.p.m. 1.1sodium phosphate glass. (These data were obtained from agitated batch tests of the type previously described (8). The analysis of the tap water used in these tests is shown in column 3 of Table I.) The coating deposits quite rapidly during the first day after which its rate of growth becomes much slower. The growth of the protective film on steel roughly parallels the decrease in current flow in the bimetallic systems treated with glassy phosphate which have been considered. This might be expected in view of the indications that the coating on steel alone is deposited on the local cathodes in the eame manner as on the cathodic member of a bimetallic couple. Films scraped from steel which had been exposed to Pittsburgh tap water treated with 50 p.p.m. of 1.1-sodium phosphate glass (for 5 days a t 35" C.) were subjected to x-ray and spectroscopic examination. The x-ray diffraction patterns confirmed the amorphous nature of these f3ms which had been postulated previously. The spectroscopic examination of these films indicated the presence of calcium. This lends support to the earlier suggestion that the inhibitive action of the glassy phosphates in natural waters involves the calcium salts or complexes of these phosphates (8). (A number of other multivalent metal ions appear able to function in a manner analogous to calcium. However, the marked affinity of the glassy phosphates for calcium and its wide occurrence in waters render i t of major practical significance.) Differential Aeration Cells. Further investigation of systems in which steel functions as both the anode and cathode appeared desirable in view of the prevalence of this condition in the field. Differential aeration is one of the more important sources of local potential differences on steel surfaces which are encountered in actual practice. The causes for the differential aeration may be inherent in the equipment design or may be set up during operation as the result of the accumulation of porous deposits, corrosion products, organic growths, and other miscellaneous debris on the 1778
metal surface. Mansa and Saybalski (10) recently have described the effect of 1.1-sodium phosphate glass on differentid aeration cells with steel electrodes. Their primary interest lay in the potential differences in the system which they followed both with an open circuit and with a 1OOO-ohm external resistance load; the current flow also was determined under the later condition We wished to follow the current flow under conditions of a low external resistance such as commonly encountered in prartice. We also wished to determine the polarization characteristics after the protective film had become well established. The experimental cell used in this investigation with steel electrodes was very similar to that used in the bimetallic couple tests; differential aeration was accomplished by wrapping one electrode with Whatman No. 120 paper. The backs of the electrodes were covered with an adhesive plastic tape in order that a more uniform current density might be obtained on the exposed surfaces of the plates. It was hoped that this might, tend t o repress local cell activity on the bare steel electrodc Anti thus cause a greater portion of the total corrosion current t o pass through the external meter circuit. (Electrodes, 2 X Z1/, inches, were used so that the metal surface-solution volume relation was the same as in the teats with dissimilar metal plates.) The influence of the glassy phosphate on the current flow between the steel electrodes in this differential aeration cell is illustrated by the data in Figure 5. (Analysis of the water used in these tests is shown in column 3 of Table I.) The curves for both the treated and untreated water show a very rapid initial development of current-a reflection of depletion of the oxygen in the water adjacent to the covered electrode (the anode). The curve for the u n t r e a t e d syateni then drops off; this IO appears to be due both to destruction of the oxide film Q initially present on I the uncovered steel 5W 05 electrode and to the L Y d e v e l o pm e n t o f iT 2 areas on this surfaw u where the oxygen supply is somewhat ,oo restricted as the re0 40 80 suit of rust accumuTIME- HR Figure 5. Influence of Calgon on lations. The curCurrentFlowfromDifferential Aerarent from the tion Cells (Steel Electrodes) in untreated settles down somePittsburgh Tap Water what after about the first 20 hours, although it shows a slow upward drift for the duration of the test. The current drops much more rapidly in the treated system as formation of the glassy phosphate film proceeds. The low current value at which this curve levels off shows the protective action of this film. The effect of the glassy phosphate on the current flow in these differential aeration cells is very similar t o its effect in the bimetallic systema previously considered. Polarization data for these differential aeration tests were taken twice during the course of the run-after 24 and 120 hours, respectively. These data are shown in Figure 6. The glassy phosphate causes a slight increase in anodic polarization and a marked increase in cathodic polarization in both cases. The chief cause for the current reduction during the early portions of the run-i.e., about the first 24 hours-appears to be this increased cathodic polarization. An additional factor, a decreased potential difference between the bare and covered electrodes, becomes an important contributor to the current reduction during the later stages of the test in the treated water, as illustrated by the data taken after 120 hours. ThiP smaller potential differenrc
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
Corrosion by Water in the treated systems (as compared to that in the untreated) results from a less cathodic potential of the bare steel electrode. The results obtained in these differential aeration tests agree with those of Mansa and Szybalski (10) regarding the inhibition of the current flow by the glassy phosphate. The author found as did they, that the primary effect of this inhibitor is at the cathode. However, he did not obtain the reversal in the direotion of current flow in the treated system which they observed; the cause for this difference may lie in the previously discussed differences in external resistance. The results obtained in these differential aeration tests also are in agreement with Cohen’s potential data for continuous flow tests with steel tubes ( 2 ) . He determined the potential differences between tubes through which water flowed and those in which the water was stagnant; such conditions are analogous to differential aeration cells. The presence of Calgon resulted in a decrease in these potential differences.
culated from the average diameter of the encrusted filter sand with the assumption that the particles were spherical as was indicated by visual and microscopic examination. The drops in both alkalinity and glassy phosphate content were roughly parallel; both gradually decreased and reached zero a t the same time.
5
-0.751
3 -0.65 -0.75
120 HR.
---50PPM.
W
Discussion of Results The data considered here indicate that the current flow between the two members of a bimetallic system causes the deposition of the glassy phosphate on the cathodic member of the couple. The presence of this phosphate film on the cathodic metal markedly increases its polarization. This increased cathodic polarization causes a marked reduction in the current flow between the anodic and cathodic members and consequently in the galvanic attack of the anodic metal. The action of the glassy phosphates on the steel by itself appears analogous t o its action on the bimetallic systems. The airrent flow between the local anodes and cathodes causes the deposition of these phosphates on the cathodic areas. This results in a marked increase in the cathodic polarization, with a consequent reduction in the current flow between these local elements. Thus, the formation of the protective glassy phosphate film on the metal surface is brought about by the electrochemical nature of the corrosion process. The growth of glassy phosphate films on metal surfaces tends t o be self-limiting. The foamation of the protective film decreases the factor-i.e., the corrosion current-that causes its deposition. The corrosion current decreases a8 the film becomes more protective; as a result, the subsequent rate of phosphate deposition decreases. As the corrosion rate approaches a very low or negligible value, the rate of protective film deposition tends to become negligible. The mechanism of the deposition of the glassy phosphate films discussed differs in some respects from that postulated previously ( 7 , 8). The protective glassy phosphate film that is involved in conosion inhibition appears to be laid down by an electrodeposition process rather than by adsoiption. Adsorption probably occurs, a t least on the initially present oxide film [numerous cases of adsorption of the glassy phosphate upon metal salts and oxides have been observed where no corrosion currents were involved (S)], but it appears to account for only a minor portion of the protective film. The magnitudes of the corrosion inhibitive films, as indicated by the analyses considered, for example, Figure 4, are considerably greater than normally are encountered with adsorbed films. They are quite a bit heavier than the glassy phosphate films which are required to coat a calcite surface sufficientlyto render it inactive for the relief of calcium carbonate supersaturation. For example, 0.097 mg. of 1.1-sodium phosphate glass per square dm. was required t o render the calcite surface inactive-roughly 1/50 of that required to attain adequate corrosion protection upon steel (Figure 4). This value was obtained from tests with filter sand encrusted with calcium carbonate similar to those previously described (6). The drops in alkalinity and gl$ssy phosphate content of a water supersaturated with respect to calcium carbonate on passage through a column of encarusted sand were detcwnined The calcite surface was calAugust
1952
-0.65
a
0.2
04
CURRENT- MA.
Figure 6. Influence of Calgon on Polarization of Steel Electrodes of Differential Aeration Cells after 24 and 120 Hours’ Immersion in Pittsburgh Tap Water Upper arms of curves show potentials of covered electrodes and lower arms those of bare electrodes
The formation of the protective glassy phosphate coating by an electrodeposition process helps t o clarify the results obtained with copper-steel couples in stagnant solutions (6). The glassy phosphate gave considerably greater inhibition in quiescent tests with the couple than had been obtained in earlier tests with steel alone. It was suggested that the more pronounced action was due to a considerable extent t o the geometrical configuration of the bimetal cell which was employed. The glassy phosphate in the water between the two electrodes in the bimetal cell is readily available for film formation; it is carried to and deposited on the cathode by the galvanic current. The flow of current between the local anodes and cathodes on the steel strip is concentrated close to the surface; the volume of water through which it passes and consequently the amount of the glassy phosphate subject t o its influence are very small-Le., much smaller than in the case of the bimetal cell. Little effect in stagnant waters would be expected with a couple in which the two metals were joined directly; the major portion of the current would be concentrated close t o the joint and consequently would pass through a relatively slight volume of solution. The motion of the liquid with respect t o the metal surface (flow or agitation) is an important factor in the inhibition of attack in bimetallic systems. just as it was with steel alone (7, 8); it determines the rate of supply of the glassy phosphate to the metal surface and consequently the rate of protective film formation. The migration of the glassy phosphate t o the cathode in calcium-bearing waters-i.e., where it is used for corrosion controlis quite different from its behavior in solutions of its sodium salts in distilled water. In the latter instance, the phosphate migrates to the anode as has been indicated by transference studies in these laboratories ( 1 ) . Apparently this difference in behavior is the result of the presence of the glassy phosphate a s a calcium salt or complex in the corrosion control applications The migration of the glassy phosphate t o the cathode in cal$umbearing waters might suggest that the calcium glassy phosphate complex involved is a cation. An alternative explanation appears more plausible; the calcium salt or complex may be present as a positively charged colloidal particle. The particles may consist of numerous phosphate chains linked one to another by calcium-Le., in the same manner as has been suggested for
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barium by Van Wazer and Campanella ( 1 3 ) to explain the extremely high molecular weight values calculated from the results of their polarographic investigation of the barium complex of a phosphate glass. The positive charge may result from adsorption of calcium ions by the colloidal calcium glassy phosphate. The open-circuit potential of the steel anode in a bimetallic system generally tends to be more cathodic in the presence of the glassy phosphate. Apparently the phosphate glass tends to deposit on the local cathodic areas of the steel where i t exerts a protective action that resists the usual rather rapid breakdown of the initial oxide film. Consequently, it tends to retard the usual rather rapid shift of the open-circuit potential of the steel in the anodic direction. Occasionally, the glassy phosphate does not appear to retard the breakdown of this film to the usual extent, perhaps because of unusual weakness of this initial oxide film. At such times the difference between the open-cirouit potentials of the steel anode in the treated and untreated systems is absent or disappears during the course of the tests. Its disappearance during the early stages of a test is often accompanied by transient discontinuities in the current flow-time curve. These variations in the open-circuit potential of the steel do not affect the galvanic attack on the steel to any appreciable extent. Steel is the anode in these couples and the inhibition of the galvanic attack is largely the result of the increased polarization of the cathodic member. The open-circuit potential of the steel anode of the differential aeration cells is slightly more negative in the presence of the glassy phosphate than in its absence; this difference does not appear sufficient to be of major significance. Here again the potentials appear to reflect the condition of the oxide film. The initial oxide film on the covered steel electrodes disappears quite rapidly, regardless of whether or not the inhibitor is present. This might be expected since access of the glassy phosphate to this covered surface would be quite slow. The presence of the glassy phosphate results in a less cathadic potential of the bare steel plates which are the cathodes in the differential aeration cells. This difference probably would be even more pronounced, were it not for the development of rust covered (anodic) areas on the bare steel plate in the untreated system. The less cathodic potential of the bare steel in the presence of the inhibitor appears to reflect the reducing conditions a t the cathode. Interference of oxygen access to the metal surface b y the glassy phosphate film may intensify this effect-
i.e., in a manner similar to that suggested by Mansa and Szybalski (10). The general retardation or failure of this effect t o show up on the local cathodes of the steel in the bimetallic couples with a more noble metal may reflect lower current densities on these cathodic areas as a result of diversion of a large portion of the current from the local anodes t o the dissimilar metal cathode. I n retrospect, the inhibitive efficacy of the glassy phosphates in actual practice might have led one to expect that their primary action would be on the cathodic areas. Had the action been primarily on the anodes, the efficacy should have been expected to be rather low. Flow or agitation long has been recognized as a primary requisite for adequate film formation by the low concentrations of the glassy phosphate used in threshold treatment. Protective film formation has been found to occur most rapidly where the flow and consequently the rate of supply of the glassy phosphate to the metal surface was the highest. Flow also governs the rate of dissolved oxygen supply to the surface, and this in turn governs its potential. Thus, the areas where the protective film formation has been highest were those which should have been the most cathodic. Acknowledgment
The author wishes to acknowledge the assistance of Mary Joan Pavlich with much of the experimental work described in this paper. literature Cited (1) Albrecht, K., private communication. (2) Cohen, M., Trans.Electrochem. SOC.,89, 105 (1946). (3) Evans, U. R., “hletallic Corrosion, Passivity and Protection,” 2nd ed., p. 327, London, Edward Arnold and Co., 1946. (4) Hamer, P., and Powell, L., Ibid., p. 327. (5) Hatch, G. B., 1x11. ENG.CHEM.,44, 1780 (1952). ENG.CHEM.,31,51 (1939). (6) Hatch, G. B., and Rice, O., IND. (7) Ibid., 32, 1572 (1940). (8) Ibid., 37, 710 (1945). (9) Kahler, H. L., and George C., Corrosion, 6,331 (1950). (10) Mansa, J. L., and Snybalski, W., Acta Chem. Scand., 4, 1275 (1950). (11) Partridge. E. P.. Chem. Eno. News. 27. 214 (1949).
RECEIVED for review February 18, 1952.
ACCEPTED June 20, 195%.
Inhibition of Galvanic Attac Steel with Phos G.B. HATCH Calgon, Inc., Piffsburgh, Pa.
ATER systems in which only a single metal is involved are almost as rarely encountered in practice as they are desirable from a corrosion standpoint. Consequently, galvanic attack in the proximity of dissimilar metal contacts is a common problem. Electrical isolation of the offending metal pair may prove a possible remedy in a few cases; all too often the system will be grounded in too many different places t o render this practical. “Waster sections” offer another method for alleviation which also is of rather limited applicability. Although elimination of the offending dissimilarity of metals may be the best means for the solution of the problem, water treatment appears to be a preferable practical means.
1780
Preliminary tests have indicated that the molecularly dehydrated phosphates are effective inhibitors for the reduction of galvanic attack in bimetallic systems (S), but the effect of various factors commonly encountered in water systems on such inhibition has not been described. The current flow between dissimilar metals immersed in water appeared t o be a promising means for investigating the effect of various factors on the inhibitive action of the glassy phosphates in relation to galvanic attack. The magnitude of this current is a measure of the influence of the couple on the attack of the anodic member of the pair-Le., the galvanic attack. The method permits the growth of protective films to be followed readily and is amenable to polarization tests,
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 8
