SHEPPARD T. POWELL, H. E. BACON, AND J. R. LlLL Professional Building, Baltimore 7, Md.
T H E USE of controlled calcium carbonate scale for corrosion prevention in cooling tower systems serving steel equipment has been inveaigated at plants constructed for war production, when copper alloys were scarce. The study revealed factors that markedly affect the success of the treatment, which have hitherto had little recognition. If rising temperatures cause the actual pH of the water to drop at the same rate as the calcium carbonate saturation pH, scale of uniform thickness will be deposited over the entire temperature range. Conditions under which this occurs are described.
H E protection against corrosion of metals in recirculating fresh water cooling systems is a problem of enormous economic importance. The need for such protection has been greatly magnified in the various war industries because of the shortage of suitable corrosion-resisting metals and the serious impact on production of forced outages of critical equipment, imposed by failures resulting from corrosion. Of the many corrective measures tried, some have been partially or wholly successful in circulating systems, while others have been complete failures. A critical investigation of the conditions which have occurred will often reveal the basic reasons for the results obrained by such treatments. In the h a 1 analysis, the protection of any metal from corrosion depends upon the creation and maintenance of a physical dam between the anodic or cathodic part of the metallic surfaoe and the electrolyte (water). Such coatings may contain inorganic or organic substances or both; they may be the product of anodic or cathodic corrosion reactions or may be precipitated nonselectively on all areas from the liquid phase. For complete protection these deposits must be nonporous and continuous; and when this favorable condition is attained, all corrosive attack will be prevented, since there will be no possible contact of the vulnerable metal areas and the corroding medium. It is not the purpose of this paper t o attempt to survey the history of success or failure of all the treatmen'ts which are being offered by proponents of various chemical compounds or to evaluate their relative merits. This paper presents a summary of experiences and conclusions concerning the fundamental requirements for corrosion protection by the deposition of calcium Carbonate scale and the specific control procedures which must be provided to ensure success. Some of the principles involved are equally applicable to other types of film inhibitors. The deliberate production of a calcium carbonate scale to protect water distribution systems is carried out by many municipal water treatment plants and has been applied widely in industry. Of the several control methods available, the authors find it convenient to use the Langelier saturation index ( 2 , s ) as determined by calculation with the aid of the curves shown in Figure 1 or by the marble test'. This method of corrosion prevention is attractive because the chemicals used are cheap, and both anodic
T
and cathodic areas are protected. It requires better control of more variables than is necessary for some other inhibitors. Calcium carbonate has an excellent history of success, although there have been a number of failures. Some of the reasons for unsuccessful results have only recently been fully understood. They may be summarized as follows: 1. The presence of organic contaminants which prevent adherent im ervious crystalline scale formation, 2. Higl sodium alkalinity or other dissolved solids which tend to increase the solubility of calcium carbonate. 3. Concurrent use of phosphates to prevent excessive scale on hotter surfaces. 4. Suspended matter or biological slimes which are mechanically bound up with the scale and make a porouB, heavy deposit. 5. Erroneous selection of the chemical treatment used to obtain a positive saturation index-eg., the use of caustic or soda ash when lime is required, or vice versa. 0. Mass precipitation of finely divided calcium carbonate instead of deposition as scale. 7. A high "index of uniformity of scalin , resulting in no protection of the cold surfaces and objectiona%y heavy scale on the hotter surfaces.
The last condition is the subject of-the commonest complaint against this method of protection and had led several plants to abandon it. In some cases the trouble actually resulted from mud, organic matter, and slimes, but waa erroneously attributed to an inherent defect in the method of inhibition. The rate of increase in scale thickness with temperature depends on the composition of the water and is subject to control by manipulating the variables in the Langelier saturation index. Finally, a sufficiently thin protective scale has been difficult t o maintain in some cases where terminal temperature differences were small and over-all coefficient of heat transfer (U value) was inherently low, making bare metal surfaces practically imperative. An example is the cooling of hydrocarbon vapors and steam 0'. condensate to 105' F. with cooling water approaching 9 Before a scaling program is undertaken, the system should be thoroughly investigated to make sure that all of the sources of trouble discussed above can be corrected. CONTROL OF SCALING RATE AND THICKNESS
Depending on the water composition, scale can be deposited or redissolved, or may not be affected either way by increases in water temperature. These possibilities occur because the actual pH of water decremes with rising temperature, and the rate of pH decrease depends on the relation between the methyl orange alkalinity and pH (as measured at 77" F.). Figures 2 and 3 show the variation of actual pH with temperatures for water containing 25 and 100 p.p.m. of methyl orange alkalinity, as calculated according to the method of Amorosi and McDermet (1). If the saturation index is calculated aa the dflerence between the saturation pH (pH.) and the actual pH (pH.) corrected for temperature, it may predict quite different behavior a t elevated temperatures from what would be indicated
EDITOR'E NOTE. Thk article wad to have been part of the August issue whioh featured oorrosion inhibitors, but unfortunately data were not prepared in time. The importance of oorrosion oontrol. however, in suoh that the Editm preassnt thin paper now ad part of the reoent concentration of known faota on this subject
1 The marble test is a measurement of the pH change resulting from saturating a sample with oalcium oarbonae (o.P. reagent grade), using a atandard detention time, filtration prooedure, temperature. eto
842
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1945
&13
each case. Comparable conditions have been seen many times in circulating cooling systems. Figure 5B is a graph of the decrease in saturation pH (pH.) against temperature; this is simply an expression of the hhange in constant C of the Langelier index, as shown by the distance between the parallel lines of Figure 1. Figure 5A shows four curves for actual pH (pH.) against temperature, which are essentially parallel with the lower pH. curve. Waters having these pH-alkalinity relations and containing sufficient calcium to have a positive saturation index, will produce scale of about the same thickness at all temperatures because the actual pH decreases at the same rate as the saturation pH. Thus a thin protective film can be maintained on the colder surfaces without plugging the hotter passes of condensers if the correct relation between pH and alkalinity is LLL maintained. The obvious and immediate problem presented to operators is: How can scale of uniform thicknew at all temperatures be maintained by adjusting water composition? If the characteristics of the parallel curves ehown in Figure 5 are plotted as in Figure 6, a curve is obtained which will indicate the necessary pH-alkalinity relations for the desired scaling control treatment. Figure 6 shows the pH for speciiic values of alkalinity at which the deposition of scale would be uniform on heat transfer surfaces at m y temperatures. To control the trqatment 80 as to deposit scale uniformly, it is first necessary to determine the optimum pH for the specific alkalinity from Figure 6. When using Figure 6, it should be remembered that the amount of scale deposited is still dependent on the saturation index value; by complying with the pH and alkalinity requirements of this figure the uniformity, but not necessarily the amount’of scale deposition, is assured. The preceding discussion, relating to calculations of the data shown in Figures 2 to 6, may suggest that the control procedure is unduly complicated for practical application. Actually, a scaling program can be successfully carried out if the water characteristics conform fairly well to the requirements indicated in Figure 6. The change in saturation index with temperature predicts whether scale will be heavier, lighter, or uniform in thickness a8 temperature ip increased, depending on whether its values are positive, negative, or zero. We have called this rate B . I . , the “coefficient of uniformity of scaling”:
I
1
e :
&% s.0
4.0
a.0
3.S
3.0
8.0
L .o
9.S
1.0
*
8
Figure 1.
8
8 mva
8
8151
ft
WR MIWON
Langelier Saturation Index Chart
by the conventional method; the latter uses the actual pH taken at room temperature. Such discrepancies have led t o alternate methods of calculation, such as Ryznar’s stability index (4). Ryznar describes three water supplies, having saturation indices of +0.54, +0.45, and f0.27 (at 80’ F.), and points out that the ssturation index does not predict the true nature of these waters. The &st is corrosive t o hot water heaters, the second deposits heavy scale in them, and the third is uniformly Batisfactory in both cold and hot services. The above discrepancy between expected performance and actual performance is explained when the effect of temperature on actual pH is taken into consideration and the change m saturation index is plotted over the range 80’ to 160’ F. (Figure 4). These data were taken from the three water supplies cited by Ryznar. It is obvious that the falling index (cam A) predicts increased aggressiveness at high temperatures, the rising index (case B) predicts heavier scale, and the horizontal line (case C) predicts uniform protection. Figure 4 also shows that the conventional saturation index would predict increased scaling in
AS.1. where k
> t1
-
s.1.n
- 8.1.61
The coefficient of uniformity of scaling will be zero if, for any given alkalinity, the pH (at 80’ F.) conforms to the pptimum value shown in Figure 0. At pH values below the curve, for any given alkalinity pH. becomes increasingly higher than pHi as the temperature rises, and the coefficient is positive; the saturation index and male thickness increase. Thus, a protective scale treatment baaed on a high alkalinity, low pH, and a calcium hardness to suit (accordingto the conventional formula) may produce heavy scale at high temperatures. Conversely, if dependence iP placed on a high PET and the alkalinity is too low, the hotter equipment may aotually be aorroded notwithstanding a theoretically correct calcium hardness.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
a44
Figure 2. Actual p H (pH,) vs. Temperature for Water Containing 25 P.P.M. Methyl Orange Alkalinity
Vol. 37, No, 9
Figure 3. Actual p H (pH,) V.I Temperature lor Water Containing 100 P.P.M.Methyl Orange Alkalinity
right of Figure 7. When the optimum pH is established, subtract the actual pH (mewured a t 80’ F.) to,obtain the ApH value. From this APH on the abscissa of Figure 7, project vertically to the alkalinity curve corresponding t o the water supply. The index of uniformity of scaling is now found on the ordinate axis. Since control exactly at the zero point would be very difficult, it is necessary t o consider the latitude in &.I. that is permissible if acceptable results are to be obtained. The practical answer to this question, from observation of plants which are using the treatment, is reassuring. Table I gives the characteristics of cooling water supplies, including ApH and AS.I., for several cooling water systems with notes as to the degree of success experienced. These results are Cooling Water Systems plotted an Figure 7. On this basis a zone of probable good performance is indicated by Marble Optimum Actual Test PH pH .ApH AS.1. the shaded area. If the point n.d. 9.15 8.60 +0.55 +0.13 of intersection between the +0.30 9.10 8.50 +0.60 4-0.15 +0.15 9.30 8.80 +0.50 4-0.11 ApH value and the alkalinity +0.50 9.25 8.50 +0.75 f0.17 +0.50 9.25 8.70 + 0 . 5 5 +0.12 falls within this shaded area, n.d. 9-20 8.62 4-0.58 f0.13 the uniformity of deposin.d. 9.60 8.30 +1.30 f0.25
The significance of the foregoing is shown by plotting (Figure 7) the coefficient of uniformity of scaling against the deviation in pH at SOo F. from the optimum values shown in Figure 6, for several values of methyl orange alkalinity. Obviously, perfect uniformity is predicted a t the intersection of ApH = 0 and AS.1. = 0. Below and to the left of this point, rising temperature will be accompanied by corrosion or resolution of scale; above and to the right, by heavier scale. For uniform scaling control by means of the curves shown in Figure 7, the optimum pH for the particular alkalinity in question should be determineti from Figure 6 or the tabulation at the lower
Table 1.
Saturation Index Data for Ten
Methyl Orange Calcium AlksHardCalcd. Plant Protection Uniformity linity neas S.I. A5 Good Satisfactory 95 112 4-1.2 B” Exoellent Excellent 90 119 $0:70 C5 Good Good 125 224 0 46 Do Good Satisfactory 136 204 0.70 E” Good Excellent 135 160 f1.00 F5 Excellent Excellent 107 105 +0.69 Gb Good Uneatisfactory 188 110 +0.69 Hb Fair Unsatisfactory 210 170 +1.30 n.d. 9.70 8.70 +1.00 $0.19 Unsatisfactory 200 250 +1.10 f0.10 9.70 8.30 +1.40 f0.25 Poor 87 90 f0.83 +0.26 8.70 9.00 +0.30 -0.04 Plants A t o F obtain protection of both hot and cold surfacee with slightly heavier scale on hot surfaces at those marked “eatisfaotory”. b Plants G, HI and J obtain very slight corrosion of cold equipment, and light to heavy scale in hot equipment; a hi her saturation index and lower coefficient of uniformity are indicated. C%l’lsntK experiences scale on colder surfacea but corrosion of hot surfaces; a higher alkalinity to raise the optimum pH would bring the coe5cient of uniformity within the positive range.
;Eo,:
is likely to be satisfactory. For systems in which the water temperature differential is not greater than about 70” F., the additional chemicals and effort to control the treatment to achieve a zero
September, 1945
IN D U S TR I A L A N D E N 0 IN E E R IN 0 C H E M I S T R Y
ApH and A8.I. value is not always justified. Cams where the temperature differentials are extremely high may require maintenance of a ApH value which will result in a AS.1. of 0.1 or even less. THIN WALE AT LOW' TEMPERATWES An inlet temHEAVY SCALE AT HlQH TEMPERATURES perature of 80 O F. and an outlet temperature of 150' were used as the basis for calculation of Figure 7, as t h i s covers most coolingwater systems. It makes no difference if this figure THIN SCALE AT BOTH HlQH AMD is used for a sysLOW TEMPERATURES tem with an inlet temperature other than SOo F. or an outlet temperature other than 150' F., aa long as the actual pH, used is measured at about 80' F. TEMPERATURE,*f. To adjust the c o m p o s i t i o n of Figure 4. Conventional and Corrected cooling water for Saturation Index vs. Temperature for Three production of uniWater Supplies of Different Chemlca form scale, it may Chrracteristio be necessary to -SIturatlon Index, EO* to 1SOo F., calculated hem PHmearund at 80' F. vary the hardness - . - Ltun(1on Index cowrded lor doeraace In actual with lime or repH between 80Dand I SOo F. duce the alkalinity with acid. Frequently the use of caustic soda or soda rtsh to raise the saturation index gives unsatisfactory results and requires the substitdtion of lime for a sodium alkali to produce a dense, thin, and uniform scale. Figuw 8 shows the variation of saturation index with temperature for cooling waters at two plants listed in Table I where excellent results are being obtained. One plant produces lowboiling hydrocarbons, the other a copolymer. Both have steel
I
045
THIN SCALE AT LOW TEMPl!RATURES CORROSION AT HfGH TEMPERATURES
I
~
TIMPERATUREI.F.
Figure 5. Temperature VI. Saturation H Decrease and V.I Actual p H for Four Alkalinity Conditions Ghich M a k e There Ch8ng.r Equal as Temperrture i s Increased
tubular heat exchangers and other water-jacketed equipment using water from a cooling tower. Both report good control of corrosion by a hard, dense scale which is slightly heavier in hotter units, but without significant loss of heat transfer. EXAMPLES
The following example iS given to illustrate the use of Figures l', 2, and 3 in determining the conventional saturation index and also the corrected saturation index: Assume a water of the following composition: P.p.m. as CaCOa Calcium hardness Methyl ?range alkalinity Total solids pH. at 80' F.
200 100 600
8.a
It is desired to know the 8.1.at 80" F. and also at 160" F. From Figure 1: 80'
PHE H at 80' F.
i
onventional S.I. p H at 160° F.
(~ie. a)
Corrected 8.1. at
x
160'
1. NITMIL OIUNOL ALMALINRI-REM. AS C&Os
Figure 6.
Optimum pH-Alkalinity Relations for Uniform Scale Deporition at All Temperatures
F.
F.
2.70 2.70 2.12 7.62
8.30
f0.78
... t . .
150' F. 2.70 2.70
1.50 6.90
8.ao
-I-1.40
7.87
+0.87
This would mean that from 80" to 150' F. the S.I. would increase +0.19 which would mean appreciably heavier scaling at 150" F
I N D U S T R I A L A N D EN 0 I N EE R I N 0 C H E M I S T R Y
846
Vol. 37, No. 9
and an actual pH of 9.0, using Figure 7 alone, aa follows: ApH = optimumpH - 9.0 = 8.65 9.0 = -0.85
-
Figure 7 shows that this is in the descaling region, and the condition can be corrected by increasing the alkalinity t o nearer 100 p.p.m. CONTROL PROCEDURES
p~
s
OPTIMUM p~
- pnamo*n
Figure 7. Index of Uniformit of Scaling vs. Devietionr in pH at 80' F. from Optimum Value Required for bniform Scele Thickness between 80' and 150' F.
In order to maintain a protective scale OD water-cooled surfaces, it is necessary t o make simple routine analyses regularly. In addition to determining the components of the calculated Langelier saturation index, it is helpful to run the marble test to determine the true saturation index. This value reflects compensation for organic compounds or other contaminants which inhibit crystalline scale formation, and thus .require the maintenance of a higher calculated index than would be necessary in the absence of inhibiting substances. The calculated index may be +1.0, for instance; if inhibitors are present, the true index may be as low as +0.2. Such inhibition probably contributes to the range of tolerance, in Figure 7, on the side of heavier scaling.
(Good results arc obtained in the area bounded by shaded liner.)
than at 80°F. Figure 6 shows that the optimum pH for an alkalinity of 100 p.p.m. is 9.20. If the optimum pH minus 8.30 = the actual pH at 80" F. is taken on Figure 7 (9.20 +0.90),the point of intersection of this value (0.9)and the 100 p.p.m. alkalinity curve is at a AS.1. slightly above 0.2 in the increased scaling region. This hypothetical condition could be corrected either by decreasing the methyl orange alkalinity value, by increasing the pH value, or by changing both conditions. It would be necessary, at the same time, to make some adjustment of the calcium hardness content of the water to keep about the same scaling rate. If the alkalinity were decreased to 25 p.p.m., the S.I. difference at 80" and 150' F. would be zero and a uniform scale deposition would be expected. The composition of the water would now be:
-
Calcium hardness Methyl orange allc&3ty;p.p.m, Total rolids, p.p.m. PH
320 25 500 8.3
From Figure 1:
/
posited on heat ex-
tions require shutting down equip ment and forced outages, this isoften impractical and other control inspection techniques must be provided. Some operators have installed eo 100 I20 140 miniature singlet u b e h e a t exTEMPERATURE, changers, fabriFigure 8. Conventional and Corrected Saturation Index VI. Temperature for cated Of pip Two Cooling Water Systems Producing and assembled with Satisfactory Uniform Protective Scele unions. The exposed inner tube is coupled in sections, and one piece is removed every week or two and tagged with a record of the water conditions that produced the results it illustrates. Specimen heat exchangers are operated at several temperatures covering the entire plant range. OF.
80' F. 2.50 3.30 2.12 7.92 8.30 4-0.38
€50'
F.
2.50 3.30 1.60 7.30
7.68
4-0.88
It is often impractical to change only one of the controlling factors and in such cases both pH and alkalinity should be adjusted. Figure 7 shows that actud descaling a t increased temperatures is possible. Whenever the actual pH of the circulating water is greater than the optimum pH values given in Figure 6, an increase in temperature will result in a decrease in the scaling rate. Without going through each step of the calculation, this behavior can be shown for a condition of 50 p.p.m. alkalinity
LITERATURE CITED
(1) Amorosi, A.
M.,and McDermet, J. R.,Proc. Am. Soe. Tmtino
Materials, 39, 1204 (1939). (2) Hoover, C.P.,J . Am. Water Woilca Assoc., 30,1802 (1938). (3) Langelier, W.F.,Ibid., 28,1600 (1936). (4) Ryznar, J. W.,Ibid., 36. 472 (1944).
