FACTORS IN FORMATION OF PROTECTIVE FILMS UPON STEEL BY WATERS TREATED WITH GLASSY PHOSPHATES’ G. B. H A T C H H a l l Laboratories,
O W E N RICE
he., Pittsburgh8 Pa.
Calgon, Inc,, Pittsburgh, Pa.
THE RATE of formation of protective films upon metal surfaces in threshold-treated waters is a function of the rate OF supply of the glassy phosphate to these surfaces and is dependent upon the type and velocity of Row, as illustrated b y the data From continuous flow tests. Two types of small-volume batch tests, designed to provide adequate motion OF the liquid relative to the metal surface, were tested and found to yield results amenable to the seme interpretations as those from continuous flow tests. By means of these batch teats effects of intermittentmotion, temperature, couples, and inhibitor reactions upon inhibitive action of the glassy phosphate were determined. Tests of the inhibitor were made in brines as well as in tap water.
flow conditions in laboratory corrosion tests should simulate, as closely as possible, those which are to be encountertxi in practice. This implies that in the majority of cases the tests should be madu with high rates of motion of the water relative to the steel surface, since most water systems are characterized by intermittent or continuous flow of high velocity, with consequent marked turbulence. These tests of the el’fect of flow velocity upon the corrosion rate in the presence of an inhibitor were conducted with steel wool. T o simulate more cluscly the types of flow condition encountered in practice, tests were made at various flow velocities with steel tubing, both with untreated and Calgon-treated waters. Continuous flow tests generally neeessitate elaborate and cumbersome equipment. They require water in such quantities that it is impractical to employ other than the laboratory tap supply. While the character of this water can be altered somewhat by chemical treatment, it is often desirable to test waters which cannot readily be synthesized in this manner. Butch tests would prove of considwarble utility, if they could be made to yield results comparable to those obtained in continuous flow tests. Two types of batch tests, designed to ensure agitation, have been investigated and found to yield results subject to the same interpretations as those obtained from continuous flow tests. The eflccts of agitation, alternate motion and stagnation, temperature, bimetallic couples, brines, and inhibitor reactions upon the inhibition of corrosion by a phosphate glass have als3 been studied by batch tests. In all of them, weight loss was employed as the means for evaluating corrosion.
NTEREST in corrosion inhibition has been greatly stimulated by the present emergency. Corrosion difficulties can no longer be alleviated simply by replacements with the more resistant but critical metals. It has become essential to utilize black iron pipe which generally necessitates a solution t o the problems of highly discolored “red water” and frequent replacements which often result when such a nonresistant material is used. To decrease the corrosivity of the water itself, which generally implies deaeration, is difficult and often impossible. Thus, the problem usually resolves itself into one of increasing the resistivity to attack of the metal surface. This may be accomplished by treatment of the water with corrosion inhibitors. The importance of the various factors which affect the corrosion rate-agitation, temperature, and composition of solution, particularly with regard t o pH and oxygen concentration, t o enumerate the more critical-have been stressed repeatedly in the literature. Yet surveys (6) have shown that control of many of these variables has been frequently neglected in corrosion tests. Agitation, particularly, often has been overlooked; this factor is of even more importance, if possible, in tests of corrosion inhibitors than in other corrosion tests. Previous tests (S) have shown that the effect of flow velocity upon the corrosion rate of steel is radically altered (7) by the presence of the glassy phosphate inhibitor, Calgon2. The rate of formation of a protective film is dependent upon the rate of supply of this material to the metal surface. Consequently, increased flow velocity, at least during the period of formation of the protective film, often results in a decrease in the rate of corrosion. Speller and Kendall (8),who worked with uninhibited waters, found that the corrosion rate increased as the flow velocity was raised. In view of the marked and opposed effect of flow velocity upon the rate of corrosion with untreated and with inhibited waters (3),
I
CONTINUOUS FLOW TESTS
Tests were conducted to determine the effect of flow velocity upon the corrosion of steel tubing by tap water as well as the effect of several concentrations of Calgon. The flow velocity was varied by two methods. In one series different volumetric rates of water were passed through tubes of essentially the same diameter; in the second, a constant volumetric rate of water was passed through tubes of different diameters. VARIATION IN VOLUMETRIC FLOW RATE. Figure 1A Shows the arrangement of tubes used in this test. A constant flow of Pittsburgh tap water, treated with the desired concentration of inhibitor by means of the constant-head tank arid constant-rate dispenser previously described (a), is passed through this tube assembly. The rate of flow in liters per minute through the individual tubes is indicated by the figures above the tube sections; these values are maintained by adjustment of the screw clamps on the outlet tubes. Five-centimeter sections of glass tubing (4.8-mm. bore) are attached directly before and after each of the steel tubes to minimize disturbance of flow at the inlet and outlet of the test sections. The test sections consist of 16.8-cm. lengths of Shelby cold-drawn seamless steel tubing; 4.57-mm. bore in the tests with 0 and 2 p.p.m. of inhibitor, 4.22-mm. bore
The first paper in this series appeared in 1940 (3). Calgon i6 a sodium phosphate glass containing 137% P~OS.Thia materikl has often been termed a technical-grade sodium hexametaphosphate in accordance with the historical, though dubious, nomenclature of Fleitmann (1). 1
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INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1945
1
A
FLOW RATES-
I
‘JTEST
SCREW CLAMPS /
SECTIONS
I
0
300
FLOW VELOCITY-FVSEC 2 4 6 UNTREATED
-
Arrangement of Steel Tubes for Continuout-Flow Corrosion Tetta at Different Velocitiea
50 100 150 200 FLOW VELOCITY-CWSEC.
0
FLOW VELOCITY 2 4
-
FTJSEC 6
WREATED
FIGURE 3
with the two higher concentrations. Prior to use the tubes are cleaned in an alkaline cleaner, rinsed, pickled, rinsed, and again immersed in the alkaline cleaner to ensure complete removal of acid; they are then rinsed with water followed by acetone, dried at 100’ C., and weighed. Upon completion of the tests (10 days) the tubes are pickled in 6 N hydrochloric acid, containing 1% thiourea as inhibitor and treated in the same manner as after the initial pickle. The results (Figure 2) me expressed as the average rate of corrosion in milligrams per square decimeter per day over a l o d a y period. The average water temperature during these tests was 28 O C. ; the average tap water analysis is given in Table I. AS is evident in Figure 2, the weight loss with the untreated water is very low under quiescent conditions and rises sharply as the velocity of flow increases; it then levels off, and further increases in the rate of flow exert but a slight effect. In contrast, the curves for the Calgon-treated waters show an initial inflection, the degree and breadth of which decreases ~ E Ithe phosphate concentration increases; a rather sharp decrease in weight loss follows, and then the curves gradually level off. Both under stagnant conditions and at very low velocities, the weight losses for the treated waters differ little from that for the untreated; as the velocity increases, the inhibitive action of the phosphate glass becomes more marked. The shape of the curve for the untreated water appears to result from the effect of flow velocity upon the oxygen concentration in the water in close proximity to the metal surface. With the exception of t,hestagnant tests, the oxygen concentration of the bulk of the water which passes through a given tube is not radically altered by the corrosion which occurs, as indicated by the magnitude of the weight losses. At low rates of flow, the oxygen concentration in the vicinity of the metal surface is, however, rapidly depleted by reaction with the metal, and the corrosion rate is limited by the speed with which oxygen can be supplied to
Table 1. Analyses of Tap Water Uted in Tettt TeataShowninFigureNo. 2 3 6 7 8 10 11 6.3 6 . 8 6 . 7 6.7 6.3 6.4 6.9 Earbonate. p.p.m. 16 16 22 18 14 16 20 Chloride, p.p.m. 21 10 14 18 11 9 13 Tota,hardnePs,p.p.m. Sulfate, p.p.m. Totalsolids, p.p.m. Oxygen. p.p.m.
iii iz gg t! t; i: 226 86 .... .... 113.. 94.. 114.. 6.6 9.5
2 0
8 0
200
A. Volometric rate of l o w proportldnal to velocit~ B . Conitant volumetric rate of l o w
CaCOa
t
I 0
Figure I .
753
J 0
50
Ib3
150
200
FLOW VELO~ITY-CWSEC’.
I
250
Figure 2. , Influence of Flow Velocity on Corrotioa of Steel Tubes of Constant Diameter (Velocity Proportional to Volumetric Flow Rate) and Effect of Calgon Concentration Figure 3. In’fluence of Flow Velocity on Corrosion of Steel Tubet at Constant Volumetric Flow Rate, and Effect of Calgon Concentration
this layer; this, in turn, is dependent upon diffusion and mechanical mixing induced by flow. The latter furnishes a more rapid method for the supply of oxygen (diffusion from any distance as a controlling rate factor would imply a very slow reaction) ; this is c o n b e d by the low weight losses a t zero flow. As the flow velocity increases, the liquid in close proximity to the metal surface is renewed more rapidly; hence, its oxygen concentration remain8 a t a higher level, and the corrosion may proceed faster. As the velocity of flow continues to increase, renewal of the layer in close proximity to the surface becomes so rapid that little change in oxygen concentration results from the corrosion process. Consequently, further increases in the rate of flow will cause only a slight increase in the oxygen concentration near the surface and, 12 13,14 as a result, will increase the corrosion rate by 6.8 7.0 only a slight amount. 14 18 The data for the waters treated with inhibitor 11 16 include the period of formation of the protective film. Consequently, they reflect the initial corrosion 120 169 rate of the unprotected surface as well as the rate of formation of the film and its protective action. The
i!
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..
..
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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shapes of the curves for the inhibitor-treated samples in Figure 2 appear to be determined by the relative magnitudes of these three factors. At the low velocities the corrosion rate of the unprotected surface apparently increases faster with increasing flow than does the rate of formation of the protective film: As a result, in this range the weight loss increases with increasing flow. As the velocity is increased still furthcr, the formation*rateof the protective film increases faster than does the corrosion rate of the unprotected surface; hence, the weight loss decreases.
300
UNTREATED
200
.ONSTAN1 TUBE DIAMETER ..ONSTAN1
28:C.
PPM.
BN'*
~
Inn loo
!$
7 0 tJi m 3200 MASS FLOW CONSTANT
3 100
0
2
4
6
a
.
--
IO
REYNOLDS NO.%IO-3 Figure 4. Corrosion of Steel Tubes as a Function of Reynolds Number, and Effect of Calgon Concentration
The flow velocities in the tests of Figure 2 are proportional to the volumetric rates of flow bf the water through the individual tubes. Thus, the quantities of dissolved oxygen and inhibitor which pass through the individual tubes aye proportional to the flow velocity. Explanation of the results in Figure 2 solely upon the basis of quantities of oxygen or inhibitor which pass through the tubes does not, however, appear valid. As noted previously, the weight losses are such that the decrease in oxygen concentration of the water upon passage through the tubes would have been relatively low. The effect of the velocity of flow upon the rate at which dissolved oxygen and inhibitor, respectively, are supplied to the metal gurface appears to offer a more logical explanation of the results; the type of flow, whether laminar or turbulent, is dependent upon the velocity and is obviously an important factor with regard to the supply of disaolved substances to the metal surface. CONSTANT VOLUM~TIUC FLOWRATE. To check the effect of flow velocity alone upon corrosion rate, tests were conducted in which the volumetric rate of flow w y kept constant and the flow velocity was varied. The apparatus, with the exception of the tube arrangement, is identical to that shown in Figure l A , as are the cleaning and pickling procedures. A constant flow of water (1 liter per minute) is passed through a aeries of 10-om. length of Shelby steel tubing of graduated bore (i.e., 1.092,0.791.
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0.457,and 0.295 cm.); the direction of flow is from the tubes of larger diameter to those of smaller (Figure 113). Figure 3 shows the results from a series of lO-day tests a t an average temperature of 16" C. With the untreated water the weight loss first increases rapidly as the flow velocity is raised and then levels off. When inhibitor is present, however, the weight loss decreases'as the flow velocity is raised; this effect becomes more marked as the concentration of inhibitor is increased. At the lowest velocity employed in these tests, the weight losses fbr the three lower concentrations of inhibitor were slightly higher than for the untreated water; duplicate tests a t this velocity did not confirm this but rather indicate somewhat erratic results in this region. The data of Figure 3 are similar to those of Figure 2 obtained from tests in which the velocity waa vpied by control of volumetric flow rate through tubes of essentially constant bore. Thus the velocity of flow exerts a marked effect upon the corrosioa rate, as well as upon the rate of formation of protective films as a result of the presence of inhibitors, which is not necessarily dependent upon the volumetric rate of flow. Apparently the character of flow is an important factor with regard t o corrosion rate. A better characterization of the flow is obtained when these weight loss data are plotted against the Reynolds number, as in Figure 4, rather than against the linear velocity; the upper graph shows the data from Figure 2, the lower, the data from Figure 3. Comparison of the two sets of data shows that the lowest Reynolds number attained in the lower series was such that no pronouncpd hump in the curves for the treated waters would be expected. Extrapolation of the curve for the untreated water in the lower graph to zero flow (Le., Re = 0) indicates a weight loss which is still high in c3mparison with the value from the upper graph. Apparently this difference results from the differences in the length of the diffusion paths between the metal surface and any appreciable supply of oxyge 1 in the two different tube arrangements. The dissolved oxygen in the water contained in the small-bore tube represented by the upper curves in Figure 4 will be rapidly depleted by the corrosion process under conditions of stagnation; subsequently, oxygen must diffuse longitudinally along the tube in order to reach the metal surface. Obviously there is a greater quantity of dissolved oxygen in closer proximity to the metal surface in a tube of larger bore. The respective weight losses in the upper series of curves in Figure 4 are greater than in the lower series; apparently the water was more corrosive, partly at least because of its higher temperature of 28' as compared with 16" C. BATCH TESTS
The contixiuous flow tests established the importance of the rates at which bxygen and inhibitor are supplied to the metal surface, and it became obvious that agitation would be an important factor in batch tests. EFFECT OF AGITATION. The effect of agitation upon the inhibition of corrosion of steel plates by the sodium phosphate glass was investigated with tests in which motion of the liquid relative to the metal surface was obtained either by mechanical agitation 'or by aeration in such a, way as to ensure turbulence. Figure 5 shows chief features of the apparatus used in these tests. In the tests with mechanical agitation (Figure 5A) the test strip is mounted on a glass rod and held rigidly in place by Pyseal cement; the glass rod is attached to a rack which is moved back and forth laterally by a connection attached to a pin on the face of a motor-driven disk. The test strip is mounted on a 45" angle to the plane of motion of the rack. The rack moves back and forth, over a distance of 2 inches, a t a rate of forty cycles per minute; the variation in the velocity of the test strip during the course of the cycle ensures turbulence. Figure 5B illustrates the test equipment for agitation by aeration. Air at the rate of 225 ml. per minute is passed through a diffusion plate and the reault-
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1945
ing air bubbles sweep over the surface of the test, strip'; this provides agitation and ensures saturation of the water with air. The test strips are 1 X 2 inch pieces of 20-gage cold-rolled s&l sheet. They are cleaned in an alkaline cleaner, thoroughly rinsed, and dried prior to use. At the conclusion of the tests they are pickled in inhibited acid, rinsed, dried, and weighed in the manner previously outlined in conjunction with the tube sections.
cfl
75s
sufficient amount of inhibitor to form the protective film and maintain it for the duration of the test. Furthdr, the film should form rapidly in such tests; otherwise, inhibitor will be lost aa a result of adsorption of glassy phosphate upon the rust which forms prior to the full development of the protective film. SOLUTION VOLUME.This discussion would seem to indicate that an increase in the volume of water in the batch tests should result in a decreaae in the conceniration of the phlssphate glass required for satisfactory inhibition of the corrosion. The data in Figure 7 show that, when 20 liters of solution were used in aerated batch tests, only 5 p.p.m. of glassy phosphate sufficed for marked reduction of the attack, in contrast to the 25 p.p.m. required in the sma!l volume test shown in Figure 6 A . The peculiar hump in the curve of Figure 7 was obtained in several duplicate tests. It may result from the coagulating action of a very low concentration of glassy phosphate (4}, a factor which might slightly incFease the protective action of the rust film; a t higher concentrations the phosphate glass would exert a peptizing action
(4).
Test Str iD
Figure 5.
Apparatus for Batch Corrosion Tests
All tests were conducted with 1 liter of solution and for five days a t room temperature unless otherwise noted. Since the temperature w b not more closely controlled, comparison of the numerical values obtained is largely limited to the tests in a single figure; these were all conducted simultaneously to ensure equal temperature conditions. Figure 6 A gives the data obtained from mechanically agitated tests with Pittsburgh tap water at pH 6.7; for comparison a stagnant series is included. The weight losses for the agitated tests drop rapidly as the Calgon concentration increases, level off at a low value, then rise slight'y as the concentration increases further. The weight losses for the st.agnant samples are all low compared to those for the agitated untreated water. There is a slight increase in weight loss as the inhibitor concentration increases, followed by a slight decrease with a subsequent rise at higher concentrations. Figure 6B demonstrates that essentially the same results are obtained whether the agitation is provided by aeration or mechanical means. The weight loss for the strips in the untreated water is greater.in the aerated than, in the agitated tests; this probably indicates that the oxygen concentration in the agitated test is slightly below equilibrium with the atmosphere. Comparison of the quiescent testswith Calgon in Figure 6 leads to the conclusion that stagnant tests are not particularly duplicable; this has been noted before (9) and is not surprising in view of the difficulty of preventing convection currents, particularly where the temperature is not held constant. Howdver, in either of these stagnant tests the effect of Calgon is practically negligible. The results of these batch tests are in qualitative agreement with those obtained from the continuous flow tests. However, the concentration of glassy phosphate required for the formation of a protective film upon the metal surface is considerably greater when the solution volumes are small, as in the case of the batch tests, than it is in continuous flow tests. ?'his should be expected since the actual concentration of inhibitor in solution appears t o be important chiefly in so far as it affects the rate of supply of inhibitor to the metal surface (3). In continuous flow tests the inhibitor is constantly being supplied to the metal surface; in batch tests only a limited quantity of inhibitor is present, and this is all that can be supplied to the surface. Hence, to obtain satisfactory inhibition in a batch test, the solution must contain a :Testa indicate that the same results would be obtained with steel if the air bubbles swept to one side so that they did not oontact the metal surface.
QUIESC ENT
20
0' 1
J
O
Figure 6. influence of Agitation on Corrosion of Steel Plates, and kffect of Calgon Concentration
At the conclusion of the tests of Figure 7 the solutions were filtered. The residual phosphate concentrations were determined by the modified Denigbs colorimetric method of Truog and Meyer (IO)after reversion of the molecularly dehydrated phosphate by refluxing for 4 hours in a solution 0.45 N with sulfuric acid. The decrease in concentration plotted against the initial concentration (Figure 8 ) first rises sharply to a maximum aa the initial concentration increases and then falls off; at the two lower concentrations (0.6 and 1.0 p.p.m.) about 90% of the initial quantity is absent from the solution at the end of the test. Apparently considerable of the glassy phosphate is adsorbed on the rust; when
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the quantity of rust which forms is greatly reduced, as at the higher initial concentrations, the amount adsxbed decreases. This is in agreement with the conclusion previously drawn from continuous flow tests in which the retardation of the formation of the protective film as a result of the presence of previously formed rust was attributed to adsorption of the phosphate glass (3).
Vol. 37, No. 8
more rapidly as the concentration of inhibitor increases than does that for the strips agitated 8 hours per day. Moreover, the rate of decrease in weight loss for the strips agitated 16 hours per day is slower than is generally the caae with continuously agil ated tests. This indicates that if the film is not completely formed before the periods of quiescence, its further rate of development
1 FIGURE I I
FIGURE 7
'I
50
L
0
J
0
.
.
.
.
.
.
.
8
2 4 6 CPLGON C0NC.-PPM.
.
FIGURE B
14001
100 1% CALGON CONC.. WM.
50
; K)
Figure 7. Inhibition of Steel Corrosion in Lar e-Volume (20-Liter) Aerated Batch ?est, as a Function of Calgon Concentration Figure 8. Decrease in Calgon Concentration during Large-Volume (20-Liter) Aerated eCorrosion Tests of Steel Plates (Figure 7) Figure 9. Influence of Calgon on Corrosion of Steel under Conditions of Intermittent Agitation Figure 10. Corrosion of Steel as a Fanction of Temperature, and Effect of Calgon Concentration Figure 11. Influence of Calgon on Corrosion of Steel Strips Coupled with Copper and Brass
INTERMITTENT AQITATION, The flow in numerous systems is intermittent in character; periods of rather high flow alternate with periods in which the system is quiescent. The condition in most dead ends of water distribution systems is probably much better characterized by intermittent flow than by rt continuous, very low flow velocity; a similar situation exists in homes and office buildings. While motion has been found essential for the formation of a protective film, it does not follow that continuous rapid motion is necessary to maintain the film. Continuous flow tests show that the effect of the protective film formed as a result of treatment with the glassy phosphate persists for an appreciable time after the treatment has been discontinued (3). Consequently, moderate interruptions in agitation would not be expected t o interfere seriously with inhibition, once the protective f%n had been formed; Figure 9 shows that this is the case. The weight loss for thb strips subjected t o agitation 16 hours per day decreases
will be slower than if the agitation had been continued. This may be caused by retardation in film development as a result of the presence of rust (S),a small amount of which would form during the period of quiescence. TXIMPERATURE. Figure 10 shows the results from a series of aerated batch tests conducted to determine the effectof temperature on the inhibitive action of the phosphate glaw In these tests the temperature was kept constant within +0.Zo C. T o avoid excessive reversion of the glassy phosphate, the tests were run for 6 hours at 99" C., 24 hours at 60"and 80" C., 48 hours at 40" C., and 120 hours at 10' and 26" C.; the maximum reversion was approximately 40% and occurred in the 99" C. test. The pH of the water used was 6.9. The curve for the untreated water is similar to those obtained by earlier investigators (2, 6). As the temperature rises, the weight loss increases at an approximately linear rate to 60" C.; above this temperature the decreasing solubility of oxygen begins
Auguat, 1945
I N D U S T R I A L A N d E N 0 I N E E R I N 0 C H E M I S TRY
757
loases obtained in &nagitated batch test. As the inhibitor concentration increases, the weight loss initially drops off rapidly, and then rises, finally to such an extent that the attack upon the metal is greater than in the untreated water; at the very high conoentrations there is another drop in the c w e , such as is often found in highly concentrated solutions of salts. It has been postulated that this differencein the behavior of the dilute and concentrated solutions results from a difference in the state of combination of tQ glassy phosphate; in the dilute solutions it is probably all present in the form of the calcium complex, while in the concentrated there is an excess of the free sodium salt (3). Solutions of the free sodium salt tend to attack steel as well as numerous other metals, presumably as a result of the ease with which the glassy phosphate forms complexes with them. One would expect that addition of calcium to a concentrated solution of the glassy phosphate would prevent such attack. Figure 14 shows that this is the case. The solutions in these agitated batch tests (pH 6.9) were prepared with distilled water and contained 1% of glassy phosphate; the calcium waa added as the chloride. At very low concentrations of phosphate glass the absence of calcium will not lead to much actual attack by the sodium salt, since little metal will be required to satisfy the complex. Yet the presence of calcium is decidedly beneficial with regard to the inhibitive action, even at these low phosphate glass concentrations (Figure 15). Solutions in these tests (pH 6.8) were prepared with distilled water and contained 100 p.p.m. of glassy phosphate; the calcium was added aa the chloride. A weight ratio of calcium to Calgon of 0.2 gives maximum protection against attack with the 1% Calgon solution (Figure 14); the slight inflection in the curve above this ratio may result from excessive precipitation of Calgon by calcium at these high ratios. In the more dilute solutions of the phosphate glass (Figure 15) the inhibition is quite marked at a calcium to Calgon ratio of 0.2 but continues to increase aa this ratio is further raise& The presence of 1270 p.p.m. sodium chloride appears to result in a slight increase in the amount of calcium required for maximum inhibition. This increase in the amount of calcium required is considerably more pronounced at very high sodium chloride concentrations (5and 15%) aa is evident from the data in Figure 16; solutions in these tests (pH 6.8) were prepared with distilled water and contained 200 p.p.m. Calgon. The calcium was added as the chloride. Numerous metals other than calcium can prevent the attack upon steel by the sodium phosphate glasses; apparently the tendency toward complex formation of the material should be satisfied sufficiently so that attack of the metal will not be re-
to play a determinant role, and the weight loss levels off, then drops rather sharply. The curves for the treated waters fall progressively below that for the untreated, in order of inhibitor concentration. At 10 p.p.m. the weight loss is very low and almost constant up to about 20" C.; but aa the temperature rises further, the curve parallels that for the untreated water. When the concentration is raised to 25 p.p.m., the weight loss remains very low up t o about 40"C., rises to a maximum at 60" C., then drops off. At 50 and 100 p.p.m. the maxima also occur at 60" C. but are much smaller in magnitude. The shape of the curves for the treated waters appears t o be a reflection of that for the untreated, As the corrosion rate for the untreated water increases, a more rapid rate of supply of inhibitor (in these batch tests, a higher concentration) is required (3)t o keep the weight loss at a low value. The shift in the maxima of the weight loss curves from 80' to 60' C, at the higher inhibitor concentrations appears to indicate the influence of another factor. Apparently the protective film which results from the presence of the phosphate glass forms more rapidly at the higher temperature; visual observation of the rates of formation of interference colors on steel strips in the treated waters at different temperatures tends to confirm this concIusion. COUPLES. Figure 11 shows the results of agitated batch tests in which steel strips were coupled respectively with strips of c o p per and brass of equal size. These data show that the couples do not appreciably alter the inhibitive action of the phosphate g l w . However, it would be expected, from the earlier discussion, that a somewhat more rapid supply of phosphate glass to the metal surface would be required in the presence of these couples than in their absence-since they would increase the corrosion rate in the untreated water-if the formation of the protective film were to proceed at the same rate. BRINES.Figure 12 shows the effect of glassy phosphate concentration on the corrosion of steel by 5 and 15% brines of sodium and calcium chlorides; these tests were of the agitated batch type (pH 7.0). The Rodium chloride brines were treated with 500 p.p.m. calcium to improve the inhibitive action of the phosphate glass (the reason will be discussed later). These data indicate that the inhibitor functions in much the same manner in sodium and calcium chloride brines BSJ in tap water. I n the case of the 15% calcium chloride brine a more rapid rate of ~ ~ p p ofl yphoe phate glass (Le., a higher concentration) is required to attain minimum weight loss than with the 6% brine. All of these brines appear to require a somewhat more rapid rate of supply of inhibitor in order to attain maximum protection than is generally the case with tap water. INHIBITOR REACTIONS
The chemical reactions of an inhibitor are important factors in both testing and application. To mention a few of the more obvious examples, chromates Could not be expected to maintain their efficiency in the presence of easily oxidizable material, nor could an easily oxidizable inhibitor, such as some ot the organic materials, be employed in case8 where it was necessary to maintain a chlorine residual. Consequently, some discussion of the special properties of the glaasy phosphates seems desirable a t this point. The apparent discrepancy in the behavior of dilute and concentrated solutions of glassy phosphates with regard to the corrosion of steel merits first Consideration. Figure 13 shows the effect of a wide range of Calnon concentrations in tap water upon the weight
A. SODIUM CHLORIDE (TECHNICAL QRADE)
r-5% NaCl
9
40
15%NaCl
20
0
50
IO0 I50 CALGON CONC.-RPM I
i
I '0
50
103 I50 CALGON C0NC.-RRM.
Rsure 1% Influence of G l g o n on Corrosion of Steel by Brines
i
758
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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
quired to satisfy this tendency. I n this respect the action of calcium is of major interest, as far as practical applications are concerned, since calcium forms a tightly bound complex with the phosphate glass and is practically a universal constituent of natural waters. The calcium to Calgon ratio required for
voluses of very soft waters, since relatively high inhibitor concentrations are required in such tests. Another problem which involves the same properties is provided by the feed solution employed for the addition of the sodium phosphate glass t o the water to be treated; the solutions
5
8
CALCIUM :CALGON -WT RATIO 0.5 1.0 1.5 29 i FIGURE I6
v)
4
40.
s 20. 0
0 0
Figure 15. Influence of Calcium Concentration on Corrosion of Steel in Presence of 100 P.P.M. Calgon Figure 16. Influence of Calcium Concentration on Corrosion of Steel by Sodium Chloride Brines in Presence of PO0 P.P.M. Calgon
200 300 400 CALCIUM C0NC.- BBM.
5
2
100
Figure 13. Influence of Wide Range of Calgon Concentration on Corrosion of Steel Figure 14. Influence of Calcium Concentration on Corrosion of Steel in 0 Calgon Presence of '17
-
CALCIUM: CALGON W% RATIO 0:5 1.0 L5
loo
I\
FIGURE IS
FIQURE I7
60:\
?l
4 44 d I
p270 W?M. NaCl
Figure 17. Influence of H on Corrosion of Steel by a 1 0 4 Solution of Calgon
0
50 100 150 CALCIUM CONC. RRM.
maximum inhibition varies somewhat with the composition of the solution, as mentioned previously. Further the ratio is higher than is 'required for formation of a soluble calcium complex. This may be because both calcium and iron compete for the phosphate glass; the equilibrium thus set up is displaced in the direction of formation of the calcium complex by excess calcium. With few exceptions the calcium concentrations encountered in practice are sufficiently high to ensure maximum efficiency of the inhibitor, although condensate and a few zeolite effluents of almost zero hardness may require the addition of traces of calcium if maximum inhibition is to be obtained with the phosphate glass. Even in the case of sodium chloride brines, where higher values of this ratio are required, generally sufficient calcium is present as an impurity so that further treatment is unnecessary. Thevalue of this ratio can prove an important factor in batch tests with small
2 INITIAL pH
employed for this purpose are generally quite concentrated (525%). Corrosion of the proportioning equipment by these highly concentrated solutions of the sodium phosphate glasses may be minimized by the addition of calcium as mentioned previously or by adjustment of the pH of these solutions. Figure 17 shows the effect of pH upon the weight loss of steel strips in 10% Calgon solutions; these data are the results of agitated strip tests. The weight loss of the strips which were not pretreated drops from a high value a t pH 7.0 to almost zero at pH 7.5; duplicate tests showed that the pH at which the attack became negligible varied from 7.5 to 9.0. When attack occurred at pH 7.5 or above, several days might elapse before the corrosion became visible, although the weight loss for the 5-day test period was high. This appears to imply that, although attack of the oxide film initially present upon the steel surface is greatly re-
Augd, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
duced when the pH is raised to 7.5,attack upon the base metal still proceeds at a rather high rate. To check this hypothesis, a series of tests was conducted with strips which had been pickled to remove the oxide film prior to immersion in the gl-y phosphate solutions. The data in Figure 17 obtained with these pickled strips show that a considerably higher pH (9.5) is required to prevent attack of the steel strips when the oxide film haa been removed by pickling. Since damage t o the oxide film may occur in the proportioning equipment, particularly in the case of moving parts, it is advisable t o maintain a pH above 9.6 if steel equipment is used to handle concentrated solutions of the sodium phosphate glass. DISCUSSION
The results indicate that corrosion data from batch teste are amenable to the same general interpretation aa those from continuous flow tests, provided adequate motion of the liquid relative to the metal surface is maintained in the batch tests. The advantages of batch tests as compared with those of the continuous flow type have been mentioned earlier-namely, they are more convenient, require less elaborate equipment, and readily permit tests in a given laboratory of waters of widely different compositions. However, batch procedures are not applicable to all types of waters, which constitutes the chief limitation of tests of this type. The ease with which the solution composition varies, under certain conditions; constitutes a serious disadvantage of tests of the batch type; such changes may result from absorption or loss of carbon dioxide or by the accumulation of corrosion products. Changes in the carbon dioxide content have not proved particularly troublesome with tBe surface supply employed in the majority of the tests described here, since from the start this water is practically a t equilibrium with the carbon dioxide in the atmosphere. If waters high in carbonate, bicarbonate, or carbon dioxide are employed, an atmosphere of controlled composition would prove useful, particularly for the aerated tests, to prevent loss or absorption of carbon dioxide. Similarily, if oxygen concentrations other than the value at equilibrium with air are desired, they may be obtained by controlled composition of the test atmosphere, Changes of the solution composition as a result of the accumulation of corrosion products are more serious as they tare often not amenable to correction. Considerable trouble will be encountered in tests of the batch type i.f measurements of the corrosion of steel in low pH waters (i.e., less than about pH 6)are attempted; the 'pH of the solutions will increase as corrosion proceeds and will generally bvel off around pH 7.0-7.5. If buffers are used to maintain the desired pH level, they are generally needed in rather high concentration; the data obtdnod will then be a measure of the corrosion in the buffer solution rather than in the water in question. Consequently, continuous flow tests are preferable, if not essential, for waters of low pH. The relatively high concentfations of inhibitor required in the small-volume batch tests for satisfactory reduction of the corrosion rate may lead to unfavorable inhibitor reactions with certain waters. 'For example, small-volume batch tests with the glamy sodium phosphates will not reveal maximum inhibitive action with very soft waters, since the higher inhibitor concentrations required in such tests will result in ratios of calcium to sodium phosphate glass too low for maximum efficacy. The dependence of the corrosion rate upon the supply of oxygen and inhibitor to the mejal surface has been stressed. Thus it would appear desirable to plot the corrosion rate against the rate of supply of inhibitor per unit area of metal surface rather than against the concentration of inhibitor in the water; unfortunately the rate of supply of a dissolved material to a solid surface is too complex, a phenomenon to be readily calculable. This factor is a disadvantage of all laboratory techniques for the investigation of corrosion and inhibition in that it does not appear possible to
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calculate, from the results of laboratory tests, the exact concentration of inhibitor which will be required in a specific practical application for the attainment of a given degree of protection. However, a fair approximation of the inhibitor requirement of a specific application can be obtained by choice of a test method in which the flowapproximates that encountered in practice with regard to type and rate. The value thus obtained is likely to be somewhat high, presumably because much larger quantities of water and, consequently, of inhibitor are brought to the metal surface in most field installations than in any equipment practical in the laboratory. The minimum inhibitor concentration required in practice is best found by trial. This may be done by determining the corrosion rates-for example, by measurements of the oxygen drops throughout the system-& various inhibitor dosages; however, in many systems the rate of flow is such that serious corrosion can occur even though the oxygen drop is too small for precise evaluation. An indirect but more generally applicable method, which has proved useful in the case of the glassy phosphates, consists of the determination of the decrease in inhibitor concentration throughout the system. A pronounced drop in inhibitor concentration a t the extremities of the system indicates an insufficient feed of phosphate g l w , since relatively little inhibitor will be adsorbed from solution once the protective film is formed. A continued high loss of inhibitor indicates continued formation of icon oxide (i.e., corrosion), Naturally, both of the above methods presuppose that the treatment at'a given inhibitor dosage is continued long enough to ensure attainment of equilibrium conditions. Experience with systems in the field allows a generally satisfactory estimate of the inhibitor requirement of a given system. In general, systems which are extensive in relation t o the quantiiy of water used require relatively high inhibitor concentrations, particularly if the rate of corrosion in the untreated system is high. For example, a hot water system with poor circulation might require 10-20 p,p.m. of the phosphate glass in order to attain the desired inhibition of corrosion. A relatively extensive distribution system in a small town might require 4 1p.p.m. of the phosphate glass, whereas the average municipality-would obtain a similar degree of inhibition at a feed of 2 p.p.m. Satisfactory protection of the distribution system af a large city is generally attained with 1 p.p.m. The inhibitor requirement estimates based upon field experience indicate the same geueral conciysiou that has been reached in the laboratory tests-namely, the rate of supply of inhibitor to the metal surface is the important factor with regard to the formation and maintenance of protective films rather than solely the concentration of inhibitor in solution. ACKNOWLEDQMENT
The authors wish to acknowledge the assistance of C. S. Bailey Freese with the experimental work. LITERATURE CITED
(1) Fleitmann, T., Pogg. Ann. 78, 233,338(1849). I v o n Steelfnst., Carnegis Schol. M e m , 11, 1 (1922). (2) Friend, J. N., 32,1572 (1940). (3) Hatch, G.B.,and Rice, 0..IND. ENQ.CHEM., (4) Hazel, F.,J . Phys. Chem., 46,516 (1942). (5) Heyn, E., and Bauer, O., Mitt.kgl. Materialprafungsarnt, 28, 62 (1910). (6) McKay, ,R. J., and LaQue, F. L., A.S.T.M. Symposium on Corrosion Testing Procedures, p. 87 (1937). (7) Rice, O., and Hatch, G. B., U. 5. Patent 2,337,856(1943). (8) Speller, F. N.,and Kendall, V. V., IND. ENG.CHEW.,15, 134 (1923). (9) Thompson, J. F., and McKay, R. J., Zbid., 15, 1114 (1923). (10) Truog, E., and Meyer, A. H . , IND.ENG.CHEM.,ANAL.ED,,1, 136 (1929). PnssaNmn before the Division of Water, Sowage, and Sanitation Chemistry CHBMICAL SOCIETY, Pittsburgh, Pa.
at the 108th Meeting of the A M m I C A N
