barium by Van Wazer and Campanella ( 1 3 ) t o explain t h e 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 t o deposit on the local cathodic areas of the steel where i t exerts a protective action t h a t resists the usual rather rapid breakdown of the initial oxide film. Consequently, it tends t o 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 t o retard the breakdown of this film t o 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. I t s 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 t o 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 t o be of major significance. Here again the potentials appear t o reflect the condition of t h e oxide film. The initial oxide film on t h e 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 t o 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 t o 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 t o expect t h a t 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 t o 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 t h e best means for the solution of the problem, water treatment appears t o be a preferable practical means.
1780
Preliminary tests have indicated t h a t 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 t o 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 t o be followed readily and is amenable t o 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
Corrosion by Water corrosion tests. Similarlv the accuracy of the mkters Glassy phosphate has been found to b e a very effective ( 1 2 % ) was deemed adequate particularly since the same inhibitor for the corrosion of steel coupled to copper and of meters were used in all tests. zinc coupled to either copper or steel. The inhibition of the They were inserted in the respective circuits only when galvanic attack on steel coupled to copper is chiefly due current measurements were to a marked increase in the cathodic polarization; the taken; at all other times the cells were short-circuited. glassy phosphate film on the copper is primarily responsible Potential measurements of for the inhibition of the galvanic attack on the steel. The the plates were made with a Beckman Model G meter effect of a number of factors, commonly encountered in in conjunction with a Beckpractice, on the inhibition of the galvanic corrosion of steel man saturated calomel halfcell. Adjustment of the curcoupled to copper with the glassy phosphate is quite rent magnitude during the The nomenclature for the similar to their effect on the inhibition of the attack of steel polarization tests was accomsodium phosphate glasses plished by insertion of varialone. The minor differences that exist appear to be due used here is a modification of able resistors in series with that suggested by Partridge to the fact that the film of glassy phosphate, chiefly rethe cell and meter; three vari(6). T h e numerical prefix able resistors, 1000, 5000, and sponsible for the inhibition of the galvanic attack, is that d e s i g n a t e s t h e r a t i o of 50,000 ohms, in series proved NazO:Pz06. T h u s , a 1.1on the copper cathode. best suited in these tests. sodium phosphate glass reThese resistors were shortfers t o a NapO:PpOs ratio circuited a t all times except of 1.1:l. This corresponds during the actual polarizat o the commercial product tion tests. Calgon which was used in all the tests described. T h e use The metal test plates were cleaned thoroughly in a n alkaline cleaner, thoroughly rinsed, and towel dried prior t o test. T h e of other phosphate glasses would yield results which would be in qualitative though not quantitative agreement. copper cleaning also included a brief pickel in 1: 1nitric acid. Coldrolled, mild steel plates were used in tests where steel wasinvolved. The steel-copper couple was chosen for more detailed study; The pickling solutions employed a t the conclusion of the tests, where weight loss was determined, were 1: 1 hydrochloric acid the influence of glassy phosphate concentration, agitation, the inhibited with 0.1% dibutylthiourea for steel, 5% hydrochloric presence of previously formed corrosion products, and the chloacid for copper, and 25% ammonium acetate for zinc. A pickling ride ion concentration were among t h e factors investigated. blank was run in each case and the observed weight losses corrected accordingly. Current flow and polarization data were supplemented by weight loss data.
which might yield some information concerning the nature of the inhibitive action. Preliminary tests were carried out t o determine the effect of 1.1-sodium phosp h a t e g l a s s ( C a l g o n ) on several of t h e more commonly encountered galvanic couples: z i n c - s t e e l , z i n c copper, and steel-copper.
CELL
Experimental The apparatus which was used for investigating the current flow between dissimilar metal couples and the polarization characteristics of the metals is shown in Figure 1. The test specimens consist of metal plates (1.5 X 1.5 inches) which are held parallel to and a t a fixed distance (0.5 inch) from each other by means of a Micarta spacer. Connections to the individual plates are made with 0.5 inch wide metal strips, each of the same composition as the plate to which it is connected. The test specimens were placed on the spacer and the connection strips over these; Micarta caps were then placed on each end, and the assembly was rigidly fastened together b y means of a steel bolt t h a t passed through the spacer and the two caps. The connecting strips were insulated from the top of the spacer t o well above the water line with adhesive plastic tape and were bolted t o a Bakelite cover. Air passed through a diffusion disk located directly beneath the test specimens at a rate of approximately 500 ml. per minute and provided both agitation and maintenance of the dissolved oxygen concentration. A glass tube which extended to within approximately 1 inch from the bottom of the container permitted the insertion of a calomel half-cell for potential measurement of the individual plates during polariza tion determinations; this served to keep the reference electrode a t a relatively remote position from the plates. (The position of this tube is displaced in Figure 1 to prevent obstruction of the view of the test strips; actually it is in a plane midway between and parallel t o the plates). T h e whole assembly was inserted in a glass jar, which contained the test medium (1.8L.) where i t was centered and held in place by means of spring clips attached t o the Bakelite cover. The 'ar in turn was held in a thermostatically controlled water bath 135" f 0.2" C., unless otherwise noted). The current generated by the bimetallic couples was measured by means of a milli- or microammeter connected across the two terminals that lead t o the test strips. Currents of 2 ma. or greater were measured with a milliammeter and external shunt resistance, those between 0.1 and 2 ma. with the shunt resistance disconnected , and a microammeter (0-200 ma.) was used for values below 0.1 ma.; the meter resistances were 0.82, 9, and 360 ohms, respectively. These meter resistances caused some reduction of the current, particularly in the uninhibited systems, as will be considered later in conjunction with the polarization tests. T h e extent of this reduction was not considered significant in view of the duplicability t o be expected in
August 1952
I NSU LAT IO N
TEST PLATES
SPA C E R C ROSS-SEC T IO N
,SPACER
a n a
7Y@
CAPS Figure 1.
Apparatus for Galvanic Corrosion Tests
Pittsburgh t a p water was used as the corrosive medium or the base for its preparation; the analyses of this water a t the times of the various tests are included in Table I. The p H was in the range of 6.5 t o 7 which is in accordance with field recommendations for the use of t h e glassy phosphate for corrosion control. The tests were of the small volume batch variety, hence relatively high concentrations of glassy phosphate were required in comparison with practical applications. The phosphate which can be brought t o the metal surface in a batch test is limited b y
INDUSTRIAL AND ENGINEERING CHEMISTRY
1781
t h a t initially present, Zn.-STEEL whereas fresh inhibitor is c o n t i n u a l l y passed over the surUNTREATED faceinpractice. Rate of supply of this phosphate t o the metal Burface, rather than concentration alone, is the d e t e r m i n a n t factor in protective 100 PPM. film formation (2,3). 0 A series of tests Cu.-Zn. was conducted in order t o check the effect of the glassy phosphate on several of t h e m o r e c o m m o n 1y encountered bimetallic c o u p l e s . T h e e f f e c t of 100 p.p.m. of 1.1-sodium phosphate glass on the current flow bet w e e n copper-steel, 4 CU.STEEL zinc-steel, and copperzinc immersed in Pittsburgh t a p water a t 80 C. was investi2 gated. The data thereby obtained are shown in Figure 2 ; the water analysis iE given in column 1 of Table I. The curves 0 20 40 for t h e u n t r e a t e d TIME- HR. water show considerable variation in indiFigure 2. Influence of Calgon on vidual shape, but all Current Flow between Dissimilar have one common Metal Plates (80' C.) feature: they are in a rather high current range, a n indication of considerable acceleration of the corrosion of the anodic members of the pairs as a result of the couples. The current decreases quite rapidly for each of the couples in the treated water as the protective film of glassy phosphate forms, then levels off a t a very low value. The phosphate glass greatly decreases the galvanic attack of the anodic member of the couple in each of these cases. Both of the untreated metal pairs which include zinc as a member show an initial decrease in current followed by a rise; the latter is considerably more marked for the copper-zinc couple. This initial drop appears t o reflect the inhibitive action of zinc compounds (1) (in this case corrosion products) on the attack; this action appears insufficient t o reduce the current t o a low level or even t o maintain the initial decrease for a prolonged period. The initial current rise for the untreated copper-steel system reflects the familiar breakdown of the oxide film initially present on the steel surface. The often-mentioned reversal of potentials in the zinc-steel system does not occur, nor should i t be expected at the sulfate and chloride contents of the water involved O
rent-time curves suffice t o shox t h a t only a portion of the weight loss is accounted for by the current flow between the dissimilar metal plates; this portion is considerably higher (50 t o 80%) for the systems treated with glassy phosphate than for the untreated (10 t o 25y0);this accounts for the more pronounced inhibition of weight loss than of current flow. The magnitudes of the current produced by the couples are insufficient t o repress local cell activity completely, consequently they do not represent the total corrosion; rather the current is a function of the acceleration of attack of the anodic member as a result of the couple. Weight losses of the cathodic members of the couples were too low t o detect in these tests (Table I) except for steel coupled with zinc in the untreated water. Here there was insufficient protection exerted by the zinc on the steel t o repress attack of the latter t o any great extent in the untreated system. Glassy Phosphate Concentration. The effect of the 1.1sodium phosphate glass concentration on t h e current flow between steel and copper plates immersed in Pittsburgh tap water a t 35" C. is shown in the left-hand portion of Figure 3. (The water composition a t the time of these tests is given in column 2 of Table I). The curve for the untreated water is quite similar in shape t o t h a t for the corresponding couple in Figure 2, though it is at a considerably lower level, as a result of the lower temperature, and reflects the same factors- Le., the initial breakdown of the oxide film, rtc. With 10 p.p.m. of 1.1-sodium phosphate glass the currcnt rises slightly a t the start of the test, remains essentially constant for a short while, then gradually rises and finally levels off a t a rather high value, though definitely lower than it is for the untreated water. Apparently the build up of the phosphate film during the initial stages is not sufficient t o compensate for the breakdown of the oxide film;
Table 1.
Analyses of Tap Water Used in Tests Water Analysis, P.P.31.
-_.______________-._
Bicarbonate Chloride Total hardness as CaCOa Calcium Magnesium Sulfate Total solids
Tests for effect of glassy phosphate Tebts for effect of ccncn. and glassy phos- influence on phate on corroded various couples system 18 14 14 21 96
60
18
10 12
27
79 22
84
71
7
4 58
Test for effect of agitation
6
166
I
Tests for effect of discontinuance of treatment and influence of chlorides 22 16
97
25 9 76 192
I---------
1.5
0.5i
(4). Visible interference colors on all the coupled plates in the treated systems in these tests indicated the presence of protective films of glassy phosphate. Weight loss data for these 80' C. tests are shown in Table 11. The inhibitive action of the phosphate glass is even more pronounced in the weight loss than in the current flow data. Calculations based on rough estimates of the areas beneath the cur-
1782
d50PPM 0
20 T 1 ME-H
40
0
R
Figure 3. Influence of Calgon Concentration on Current Flow between Copper and Steel Plates (left) and Loss in Weight of Steel (Right)
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 8
Corrosion by Water insufficient phosphate is available a t this low concentration to continue to keep pace with oxide film breakdown. The current drops rapidly t o a low value with 25 p.p.m. of the glassy phosphate then continues to decrease a t a more gradual rate for the duration of the test. The 50-p.p.m. curve falls a t a slightly higher level than the 25, whereas that for 100 p.p.m. is progressively higher. The weight loss values for the steel plates in these tests a t various concentrations of glassy phosphate are shown in the right-hand portion of Figure 3. The upper cdrve represents the observed weight loss (total), and the lower curve is calculated from the areas under the current-time curves and represents the weight loss due to the current produced by coupling with copperi.e., the galvanic corrosion. The current between the dissimilar metals accounts for a much greater portion of the total corrosion in the effectively inhibited cases. (This also was noted in consideration of the data in Table 11.) Some apparent discrepancy exists between the 4 weight loss and current flow re? sa5 sults at 25 and 50 p.p.m.; the w a weight 1084 indicates minimum (r 3 total attaek a t 50 p.p.m., V w h e r e a s t h e c u r r e n t shows minimumacceleration of attack 0 IO 20 by contact with the copper a t TIME -HRS 25 p.p.m. H o w e v e r , b o t h Figure 4. Influence of types of data indicate very Calgon on Current Flow good inhibition a t either conbetween Copper and Steel centrat ion. Plates in Quiescent W a t e r I
Table II. Influence of Calgon (100 P.P.M.) on Weight Loss of Coupled Metals in Pittsburgh Tap W a t e r a t 80' C. Weight Loss, Mg./Sq. Dm./Day Metal Copper-steel couple Steel Copper Zinc-steel couple Zinc Steel Copper-sinc couple Zinc copper
Untreated
Treated
842 0
7.2
391 551
6.1 0
80 1
5.8 0
0
0
Effect of Agitation. The influence of the glassy phosphate on the current Aow between copper and steel in quizscent tap water is shown in Figure 4. Comparable data under conditions of agitation (by aeration) are shown in the initial portion of Figure 5. (The analysis of the water used in these tests is shown in column 3 of Table I.) After a rapid initial rise the current from the couple in the untreated quiescent water remains constant for a while, then gradually drops down and levels off. The initial current rise reflects the familiar breakdown of the oxide film. This breakdown apparently is rapidly masked by oxygen depletion in the vicinity of the electrodes which tends to reduce the current. The final essentially steady state seems t o reflect a condition where oxygen solution a t the water-air interface and its subsequent diffusion to and depolarization of the cathode is the factor that controls the rate of the galvanic attack. The value of the current a t which this quiescent untreated system levels off is much lower than the corresponding value approached in the agitated system (initial portion of Figure 5); this reflects the relatively slow rate of supply of dissolved oxygen under stagnant conditions. The current for the system treated with glassy phosphate shows a rapid initial inflection similar to that in the untreated system but then declines steadily as the protective
August 1952
film slowly builds up. Although the rate of development of the protective film is much slower than in the agitated treated system (initial portion of Figure 5), the glassy phosphate appears to have a much greater effect on the bimetallic system under quiescent conditions than it does on steel alone under similar corfditions (3). (This rather anomalous behavior, t o a conhd'erable extent, appears to be due to the geometrical arrangement of the electrodes in the bimetallic cell.) The data in Figure 5 show the effect of alternate agitation and quiescence on the inhibition of the galvanic corrosion in .the copper-steel system by the glassy phosphate, Discontinuance of agitation in the untreated system results in a rapid drop in the current, a reflection of the oxygen depletion. It then levels off a t a value about the same as that for the untreated bystem which was quiescent from the start (Figure 4). Subsequent agitation of the untreated system results in an abrupt rise t o a quite high level, after which the current decreases somewhat and levels off. The current value attained by this abrupt rise is considerably higher than that approached during the initial cycle of agitation. This seems t o indicate & loss in the rather slight protective quality of the rust coating, apparently the result of reductiofi of the oxide during t h e quiescent 2.c period when the oxygen supply was restricted. The treated system shows a slight gradual de4 E crease in current after tl d i s c o n t i n u a n c e of Z w agitation and only a a a 1.c very slight rise a t the 3 0 start of the second agitated cycle. The data show that the quiescent period has no deleterious action on the protective film of glassy phosphate. In fact, the film ap0 20 40 60 pears to continue t o TIME-HR grow d u r i n g t h e Figure 5. Influence of Calgon on period of quiescence. Current Flow between Copper and (Here again the geoSteel Plates under Conditions of metrical arrangement intermittent Agitation of t h e e l e c t r o d e s seems t o be an important factor as was mentioned in conjunction with the data in Figure 4.) Hysteresis, The effect of discontinuance of the treatment after the protective film of glassy phosphate has become well developed is illustrated by the data in Figure 6 for the coppersteel system. (The analysis of the water used in these tests is given in column 4 of Table I.) The current remains essentially constant for about 8 hours after the treatment is discontinued. It then begins a gradual increase which continues for the duration of the test. This gradual increase in current is an indication of the progressive breakdown of the protective film. This film dehitely persists for some time after the treatment is diacontinued in these galvanic couple tests, but the effect is not nearly as pronounced as with steel alone ($',a). Effect on a Corroded System. Figure 7 shows the effect of the glassy phosphate on r~ copper-steel couple which previously had been exposed t o aerated untreated water for 25 hours. D a t a for a continuously untreated system as well as for one which had been treated from the start are included for purposes of com-
E
INDUSTRIAL AND ENGINEERING CHEMISTRY
1783
Table 111. Influence of Chloride Concentration on Weight Loss of Steel Coupled to Copper and Influence Thereon of Calgon Chloride Concn., P.P.M. 16 516 1016
(%day test a t 3 5 O C.) Weiglit Loss, Mg./Sq. Dm./Day Untreated 50 p.p.m. Calgon 100 p.p.m. Calgon 778 18 23 914 18 35 820 44 33
I
I
0
05
I 0
I
25
50 TIME-HR.
75
100
Figure 6. Effect of Discontinuation of Calgon Treatment on Current Flow between Copper and Steel Plates
parison. (The analysis of the water used in these tests is shown in column 2 of Table I . ) The current flow in the initially untreated system falls off quite rapidly on the addition of the glassy phosphate and levels off at a low value. However, the rate of current decrease in this system is somewhat slower than in the system which was treated from the start. This retardation of the protective film formation as a result of the presence of previously formed corrosion products is similar t o t h a t observed with steel alone (8, S), although i t appears less pronounced. I t s cause apparently is the same as for steel alone; the previously formed rust adsorbs considerable of the glassy phosphate. The curves for the untreated systems show some divergence, but this appears t o be within t h e range of duplicability generally expected in corrosion tests. Influence of Chloride. The inhibitive action of the glassy phosphates on steel has shown little sensitivity t o the presence of chlorides, in contrast t o the behavior of numerous other inhibitors. Tests were conducted in order to determine whether this insensitivity persisted in the galvanic attack of steel coupled t o copper. Figure 8 shows the influence of the glassy phosphate on the current flow between copper and steel plates a t chloride levels of 516 and 1016 p.p.m. (These chloride concentrations were obtained by treatment of $he water shown in column 4 of Table I with sodium chloride.) Comparable data a t a third chloride level (16 p.p.m.) are contained in Figure 3. The current flow curves for the untreated water a t t h e two higher chloride concentrations fall a t a considerably higher level than does t h a t for the lower chloride content, an indication of increased galvanic attack at these higher chloride levels. T h e initial rate of increase of current rises with t h e chloride concentration, apparently a reflection of the more rapid destruction of the oxide film which was present initially. The maxima in t h e current flow curves during the intermediate stages of the tests become more marked at the higher chloride concentrations. There is also a more pronounced decrease in current during t h e latter stages of the tests as the chloride concentration increases.
1784
As a result, the average current for the untreated system is higher a t 516 p.p.m. than at 1016 p.p.m. chloride. The decrease in current during the latter stages of the tests appears a t least partially explicable on the basis of mechanical obstruction of the steel surface with rust. The current flow-time curves for the systems treated with the glassy phosphate a t the different chloride levels are quite similar. The current decreases at a somewhat slower rate as the chloride concentration increases, but a very low level is attained in each case. The inhibitor causes a marked reduction of the galvanic attack throughout the range of chloride concentrations investigated. Tests were also run with 100 p.p.m. of 1.1-sodium phosphate glass a t the different chloride levels; the resultant values fell sufficiently close t o those a t 50 p.p.m. t h a t they were omitted from Figure 8 in the interests of clarity. The curve for 100 p.p.m. of 1.1-sodium phosphate glass fell slightly above ithat for 50 p.p.m. a t 516 p.p.m. of chloride, and their relative positions were reversed a t 1016 p.p.m. of chloride. The weight loss data for the steel in these galvanic couple tests of the influence of chloride are included in Table 111. The weight losses of the steel anodes in the untreated systems vary with the chloride concentration in t h e same general manner as did the current flows, although t o a lesser extent. As might be expected, the galvanic attack accounts for a considerably greater portion of the total attack in the higher conductance waters-i.e., a t the higher chloride concentrations. The weight loss data for the steel anodes in the treated systems show that the glassy phosphate causes a marked reduction in t h e attack a t all three chloride levels. Fifty p.p.m. of 1.1-sodium phosphate glass provides slightly greater inhibition a t the two lower chloride concentrations than does 100 p.p.ni., whereas the reverse is true a t 1016 p.p.m. of chloride. Thus the data for both weight loss and current flow indicate t h a t the inhibition of the attack of steel coupled to copper by the glassy phosphate is quite insensitive t o the chloride concentration, a t least in the range tested. This is the same conclusion as was reached previously with respect t o the inhibition of the attack on uncoupled steel with the glassy phosphate (3).
0
25
50
75
100
Figure 7. Influence of Previously Formed Corrosion Products on Inhibition of Current Flow between Copper and Steel Plates with Calgon
Polarization Tests. The polarization characteristics of the individual electrodes of the copper-steel couple were investigated in order t o gain some insight into t h e manner in which the glassy phosphate functioned in the inhibition of galvanic attack. The term polarization is used in the rather broad sense frequently employed in the corrosion field t o denote changes in the potential of a n electrode from its open-circuit value which result from the
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
Corrosion by Water I
I
1
516 PPM CHLORIDE
I
1016 PPM CHLORIDE
4'1 2.o
50 PPM 0
on
-LpoTr
0
20
40
ILO
0 TIME- HRS
20
40
Figure 8. Influence of Calgon on Current Flow between Copper and Steel Plates at High Chloride Concentrations
0
flow of current. Thus it includes potential drops across poorly conductive surface films as well as counterelectromotive forces.
05
1.0 C U R R E N T - MA
Figure 9. Influence of Calgon Concentration on Polarization of Copper and Steel Plates Polarization data after 48 hours exposure to aerated water
-
The polarization measurements were made after the tective film of glassy phosphate had become well establiziod, generally near the conclusion of the current flow tests. The potentials of the individual electrodes of the bimetallic couple were measured as functions of the current. Adjustment of the current was accomplished by means of variable resistors in series with the bimetallic cell and the meter. The measurements were carried out as rapidly as possible (generally within a period of 10 to !5 minutes) in order to avoid undue interference with the corrosion process. At times the potential values tended to drift; measurements were taken after the values had steadied down and remained constant for 1 minute. T h e arithmetical mean of the potentials of the two dissimilar metals with 0.82 ohm external resistance was taken as the short-circuited potential of the couple; the value thus obtained was indicated by the dotted line at the intersection of the polarization curves for the anode and cathode. (There was no measurable difference between these potentials in the treated systems; the maximum difference for the untreated systems was 4 mv.) Influence of Glassy Phosphate Concentration. Figure 9 shows the results of polarization measurements with copper-steel couples after exposure t o various concentrations of 1.1-sodium phosphate glass for 48 hours at 35' C.; these data are from the same series of tests as the current flow and weight loss data shown in Figure 3. The upper arms of each of these curves represent the potential of the steel anodes as a function of the current and the lower arms those of the copper cathodes. The intersections of the respective anodic and cathodic potential curves represent the potentials of the short-circuited couples. The currents that correspond t o these intersectidns are those generated by the short-circuited couples; comparison of these values with the respective maximum experimental currents noted in the curves provides an idea of the magnitudes of the current decrease due to the resistance of the meters. Ten p.p.m. of the phosphate causes little change in the slope of the polarization curve for the anode (Figure 9), nor in t h a t for the cathode in the vicinity of its intersection with the anodic curve. The rather slight inhibition of current that occurs appears t o be due t o the slightly lower potential level of the anode. At 25 p.p.m. of 1.1-sodium phosphate glass, the potential of the anode drops considerably, and the slopes of the anodic and cathodic polarization curves become steeper; the change in the anodic slope is relatively slight in comparison with that of the cathodic. Thus the marked inhibition of current flow a t this concentration appears t o have been due chiefly t o a marked increase in the polarization of the cathode. The level of the curve for the anode at 50 p.p.m. is much higher than at 25 p.p.m. Otherwise the curves are quite similar in shape and again the chief cause for the August 1952
low current flow is the very high cathodic polarization. The anode potentials a t 100 p.p.m. are slightly above those a t 25 p.p.m., and the curve shows the same characteristics as were noted at 25 and 50 p.p.m. I n each instance where the supply of the glassy phosphate is sufficient t o cause a marked decrease in current flow, the inhibition of the galvanic attack is chiefly the result of a marked increase in the cathodic polarization. The open-circuit potentials and the general levels of the polarization curves for the steel anodes vary in a rather peculiar manner with the concentration of glassy phosphate. Moreover, this variation is rather erratic. For example, the 48-hour polarization tests of a duplicate series of t h a t in Figure 9 at 50 p.p.m. gave a considerably less anodic level of the polarization curve for the steel. Similar variations have been observed throughout the concentration range. These variations do not ap8ear t o affect either the nature or degree of the inhibition of the galvanic attack; consequently, they do not appear t o be of major significance. The general tendency appears t o favor less negative open-circuit potentials of the steel anodes in the presence of the glassy phosphate, although exceptions t o this generality are rather common. The polarization data for the other tests with the copper-steel system which have been discussed are not to be considered in detail since they provide little additional information. I n all cases the presence of the glassy phosphate caused a marked increase in the cathodic polarization.
Discussion The data reported here indicate t h a t the glassy phosphate exerts a marked inhibitive action on both the galvanic and the total attack on steel coupled t o copper. The inhibitive action is primarily due t o a marked increase in the polarization of the copper cathode. This implies t h a t the film of glassy phosphate, which is chiefly respdnsible for the inhibition of the current flout is located on the surface of the copper cathode. The concept of the glassy phosphate film on the copper cathode as the factor t h a t controls the rate of the galvanic attack of the steel anode clarifies some of the differences between the inhibition of the attack of steel coupled t o copper and the inhibition of the attack on steel alone. One such difference is the less pronounced retardation of the development of the protective film as a result of the presence of previously formed corrosion products in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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copper-steel system than for steel alone. The copper surface is relatively free from corrosion products after the preliminary exposure t o the untreated water as compared t o that of the steel which is coated heavily with rust. Access of the glassy phosphate t o the surface of the copper is not obstructed to nearly the extent that it is to the surface of the steel. Consequently, the protective film forms faster on the copper surfaces (and thereby reduces the galvanic attack of the steel t o 1% hich it is coupled more rapidly) than it does on the steel. Another difference is the less pronounced hysteresis of the inhibitive action on the galvanic attackof steel coupled to copper than on the attack of steel alone. Apparently the protective film on copper does not persist as long after discontinuance of the treatment with glassy phosphate as does that on steel. The failure of variations in the open-circuit potential of the steel to affect the inhibition of the galvanic attack of this metal in the copper-steel couple also is clarified. These changes a t the anode have little effect on the inhibition because the film of glassy phosphate on the copper cathode is the factor t h a t controls the rate of the galvanic attack on the steel. The rather erratic behavior of the open-circuit potential of the anode of the copper-steel couple in the presence of the glassy phosphate seems to preclude any specific action of the inhibitor on this potential. It appears more likely t h a t the glassy phosphate exerts a secondary influence on the potential determinant factor. This determinant factor appears t o be the condition of the oxide film on the steel surface. The influence of the glassy phosphate on the open-circuit potential of the anode of the copper-steel couple appears t o be attributable t o its effect on the oxide film on the steel surface. The glassy phosphate apparently deposits on the oxide film present initially on the steel surface and thereby tends to retard
its breakdown. Consequently, it tends to retard the rather rapid shift of t h e potential of the steel in the anodic direction which is characteristic of the untreated system and indicative of the rapid breakdown of the initial oxide film. Occasionally the film of glassy phosphate appears inmfficient t o retard the breakdown of this oxide film to the usual extent (perhaps due to some inherent weakness in this init,ial oxide film); consequently, the shift of the open-circuit potential of the steel in the more anodic direction is not retarded to the usual extent. The inhibition of the galvanic attack of steel coupled to copper by the film of glassy phosphate on the copper cathode raises a number of points for speculation. It appears quite likely that the inhibition of the galvanic attack of steel coupled to other cathodic metals proceeds in a similar manner. Perhaps even the inhibition of the attack on steel alone occurs in a similar fashioni.e., as the result of an increased polarization of the local cathodic areas brought about by the glassy phosphate film. Such speculation chiefly serves t o emphasize the desirability for further experinient,al data. Acknowledgment
The author wishes t,o acknowledge the assistance of Mary Joan Pavlich with much of the experimental work described in this paper. Literature Cited (1) Evans, U. R., J . SOC.Chem. Ind.(London),43, 321T (1924). (2) Hatch, G. B., and Rice, 0.. IXD. EXC.CHEM.,32, 1572 (1940). (3) Hatch, G. B., a n d Rice, O., Ibid.,37,752 (1945). (4) Hoxeng, R. B., a n d Prutton. C. F., Corrosion, 5, 330 (1949). ( 5 ) Partridge, E. P., Chern. Eng. News, 27, 214 (1949). R E C B I V Efor D review February 18, 1952.
A C C E P T E DJ a n e 20, 1952.
Cathodic Protection of Steel in Contact with Water LEON
P. SUDRABIN
Nectro Rust-Proofing Corp.
(N.J . ) ,
Beffeville,
HENRY C. MARKS
N. J.
The protective current requirements for the cathodic protection of steel submerged in water are controlled b y the same environmental factors that influence the rate of corrosion. Higher dissolved oxygen concentrations, temperatures, or velocities that increase the rate of oxygen diffusion to the cathodic areas tend to increase the protective current requirements. Cathodically deposited calcium carbonate coatings, which impede oxygen diffusion, and resistive coatings decrease the protective current needed. Calcium carbonate i s cathodically deposited most readily from water having more than 40 to 50 p.p.m. of both calcium and bicarbonate alkalinity. In the combination of metal protective paints with cathodic protection, the best over-all results are obtained b y controlling the potential at the paint surface so that corrosion activity at the coating flaws ceases, and no excess current i s used to damage the coatings b y ampere-hour effects such as hydrogen evolution, production of alkali, and electroendosmosis.
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Wallace & Tiernon Co., Belleville,
N. J.
ATHODIC protection has a distinct practical position among corrosion control methods in that it is often supplied to steel structures already buriFd in soil or submerged in water without requiring exposure of the structure or treatment of the corrosive environment, During the past 20 years a growing reliance has been placed on the use of cathodic protection for the maintenance of the vast network of underground gas and oil transmission and distribution pipelines. At the same time this method of protection has been developed for steel structures submerged in water, such as dock piling, water storage tanks, clarifier mechanisms, sludge blanket type softeners, deep well pumps, pipes, flumes, traveling screens, sewage digestors, ship hulls, and open top refinery condensers (1, 10,16, 80,89). Although the fundamentals of cathodic protection for soil and water environments are alike, the influence of the Auid character of water requires some special attention. The intent of this paper is to consider separately, in a corrosion system of iron in water, the effect of dissolved oxygen concentration, temperature, velocity, mineral composition, surface films, and coatings on protective current requirements.
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
