WDER
use of air conditioning, together with more water conservation regulations, has increased the number of cooling towers and evaporative condensers in use, particularly in urban areas. I n many areas, water supply and atmospheric conditions cause corrosion in cooling water systems unless corrosioninhibiting chemicals are used (70). Chromates have been the most satisfactory inhibitors for this purpose. However, disposal of treated water to sewage systems or streams is sometimes subject to legal restrictions, or objections are raised because the treated water is colored ( 9 ) . Colorless sodium nitrite could avoid these objections.
Laboratory Tests Laboratory tests approximating conditions encountered in evaporative cooling equipment were carried out in pairs of 2-gallon, wide-mouthed bottles. In one of these test specimens were completely submerged suspended from glass S-hooks. Water was pumped from the bottom of one bottle by a circulating pump (Production Specialties, Inc., Model 10) at a rate of about 0.5 gallon per minute, discharging into the air about 4 inches above the water surface of the bottle containing the test coupons. Discharge through a Bunsen burner fishtail attachment permitted thorough aeration. Continuous circulation was maintained by siphoning water from the bottom of this second bottle to the bottom of the first bottle. The bottle with the pump also contained a thermostatically controlled electrical resistance heater with which the circulating water was maintained at 100' to 110' F., a representative temperature range for typical air conditioning cooling systems. Standard comparative tests were carried out on a 28-day cycle, in which four hot rolled steel (SAE No. 1015), one yellow brass, and one copper test coupon were suspended in the test bottle. Test coupons were 1 X 4 X I/g inches with a 0.25-inch-diameter hole about 0.25 inch from each end. Test coupons of aluminum, solder (half lead and half tin), and soldered sections of copper sheet were also used in some tests. All coupons had been marked, cleaned, and weighed prior to the test. One steel test coupon was removed af-
Literature Background Subject Sodium nitrite as an inhibitor Mechanism of inhibition by nitrite
Oxidation of nitrite by microorganisms and its prevention Overcoming effect of high chlorides in reducing nitrite inhibition
Ref. (8,11) (9, 3,5 ,
7 . 11) (5, 6)
I
SIDNEY SUSSMAN,
OSKAR NOWAKOWSKI, and JOHN J. CONSTANTINO
Water Service Laboratories, Inc., New York 27, N. Y.
Experiences with
a
SODIUM NITRITE Unpredictable Corrosion Inhibitor Corrosion inhibition by sodium nitrite in cooling waters is adversely affected by the presence of high sulfate concentrations and by other factors which have not yet been identified ter each 7-day interval until the termination of the 28-day test, at which time the remaining coupons were removed. Coupons were examined, cleaned, and weighed after removal from the test bottle. Corrosion data were calculated as milligrams of metal lost per square decimeter of exposed area per day of exposure
(MDD)
.
Test coupons sheared from 1 X l/g inch SAE 1015 steel strip were freed from mill scale prior to use by immersion in inhibited 15% hydrochloric acid for 15 minutes, rinsing, wire brushing, oven drying, and weighing to the nearest milligram. After exposure they were cleaned by soft wire brushing, oven drying, and weighing to the nearest milligram. Copper and brass test coupons were cleaned prior to use on a cloth buffing wheel using eweler's rouge, washed with water and chloroform, oven dried, and weighed to the nearest milligram. After exposure, brass coupons were cleaned like steel. Copper coupons were placed in a 20% sodium hexametaphosphate solution for 24 hours, rinsed along with fiber brushing, oven dried, and weighed to the nearest milligram. All postexposure weights were corrected for any significant weight loss in the cleaning process as shown by blank experiments.
Water used in these tests was New York City's Croton supply. A typical analysis is :
P.P.M. Total hardness Calcium Alkalinity (methyl orange) Alkalinity (phenolphthalein) Carbon dioxide, free Chloride Sulfate Silica Total dissolved solids
PH
49 32 28 0
4 6 16
5
CaIcd. as CaC03 CaC03 CaC03 CaC03
co2 Son
C1
Si02
73 7.1
For experiments which required higher chloride or chloride and sulfate concentrations this water was fortified by an appropriate amount of sodium chloride, sodium sulfate, or both. Inhibitor COIIcentrations were checked daily in the early stages of each test and every few days thereafter. If found to be more than 25 p.p.m. low, additional sodium nitrite was added to re-establish the desired concentration. Initial laboratory experiments carried out with Croton water (above) confirmed published information (Table I ) showing that corrosion of steel could be
Laboratory test equipment consisted of thermostatically controlled electric heater and small pump (left), with spray head and glass-supported test coupons in right jar. Interconnecting siphon joined the two units
(8)
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581
Figure 1. Extensive pitting of cleaned steel test coupons after 28-day laboratory test
Figure 2. test
Specimen at left was in water containing 500 p.p.m. chloride, 1400 p.p.m. sulfate, and 500 p.p.m. sodium nitrite. Remaining specimens were in water containing 150 p.p.m. chloride, 420 p.p.m. sulfate, and (left to right) 350, 500, and 770 p.p.m. sodium nitrite
W a t e r contained 150 p.p.m. chloride, 420 p.p.m. sulfate, and 770 p.p.m. sodium nitrite. Coupons show (left to right) deep pitting of aluminum, etched appearance of copper, dezincified surface layer on brass, and widespread pitting of steel
almost completely inhibited under the experimental conditions used with sodium nitrite concentrations of 200 p.p,m. and higher. Because of published warnings that nitrite concentrations must be raised as chloride concentrations increase, further experiments were carried out using Croton water with the addition of sodium chloride to raise the chloride concentration to 150 p.p.m. These tests (Table 11), showed, as expected, that increased chloride concentration raised the minimum sodium nitrite concentration necessary for good protection from about 200 p.p.m. in Croton water to 250 to 500 p.p.m. at the 150-p.p.m. chloride level. With 6 to 12 p.p.m, chlorides present, almost complete protection was obtained with 200 p.p.m. sodium nitrite and complete protection was obtained with 250 p.p.m. sodium nitrite (Table I). I n contrast, with 150 p.p.m. chlorides there was an appreciable corrosion rate at 200 p.p.m. sodium nitrite, and with 250 p.p.m. sodium nitrite duplicate experiments (Table 11, Nos. 116 and 124) gave results which differed more widely than the usual variation obtained with replicate experiments using the test technique. Thus 250 p.p.m. sodium nitrite appeared to be a borderline concentration, so that the suggested sodium nitrite concentration of 200 p.p.m. above the chloride concentration would appear to be a satisfactory level for this water composition.
On the basis of this laboratory confirmation of published data (@, nitrite treatment of the water in several air washers and cooling towers was started using a control minimum of 500 p.p.m. sodium nitrite. Weighed steel test coupons were submerged in the pans of each unit to check on inhibitor effectiveness. When, as discussed Iater, these test coupons showed that degree of inhibition was not always as great as would be expected on the basis of the sodium nitrite concentration present, available information on nitrite as an inhibitor and on actual water conditions in the equipment under tests was reviewed. In the waters used for systems treated with sodium nitrite, sulfates exceeded chlorides by a factor of about 2.8. As water in the evaporative cooling equipment concentrated, both sulfates and chlorides concentrated. In many cases scrubbing sulfur dioxide and trioxide from the atmosphere caused sulfates to increase faster than chlorides (70). Al-
Table II. Corrosion of Steel at Higher Chloride and Sulfate Concentrationsa More inhibitor was needed as chloride concentration increased NO.
A.
66 115 116 124 139
B.
PiaNOn, P.P.M. 0
Corrosion, MDD 54 31.7 0.7
20 42 100 43 200 38 250 0 39 500 0 Standard 28-day laboratory test, 100110' F. Croton water: C1- = 6-12 p.p.m.; sod-- = 16-33 p.p.m.; pH = 7-8.
582
C.
52.5 19.8 10.3 1.3 0
0 350 500
770
62.6 106.0 97.0 74.2
500 P.P.M. Cl-; 1400 P.P.M. Sod--
153 140 161 a
0 200 250 250 500
1.50 P.P.M. C1-; 420P.P.M. Sod-154 162 163 164
Corrosion was negligible above 200 p.p.m. inhibitor
No.
NaN02, Corrosion. MDD P.P.M. 150 P.P.M. C1-; 16 P.P.M. SO1--
Expt.
Table I. Corrosion of Steel in Low Chloride and Sulfate Water"
Expt.
Cleaned test coupons from 28-day laboratory
0 500 500
59.9 70.0 53.6
Standard 28-day laboratory test, 100'-
110' F. Croton water fortified by NaCl or KaC1 and NalSO4 to indicated Cl- and so4-- concentrations; pH = 7-8.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
though most published information mentioned only chlorides as a possible cause of reduced inhibition by sodium nitrite, one report ( 7 ) suggested a similar effect by sulfates. Therefore additional laboratory tests were carried out to examine the effects of proportionate increases in both sulfates and chlorides on the corrosion inhibiting action of sodium nitrite. These additional experiments (sections B and C, Table 11) showed a surprisingly strong influence of sulfate concentration in reducing the effectiveness of sodium nitrite. In fact, with high sulfate concentrations the addition of sodium nitrite in concentrations very effective at lower dissolved solids levels and at even the same chloride level increased the corrosion rate beyond that observed with the same chloride and sulfate concentrations in the absence of sodium nitrite. In section B, increases in corrosion rates over those from the control experiment in which no sodium nitrite was used were far beyond differences normally encountered in running replicate experiments by the laboratory procedure. Although the chloride and sulfate concentrations selected were higher than those encountered in evaporative cooling equipment in many parts of the country. they are within the ranges encountered by the authors in many units using the soft make-up waters available in the coastal areas of the Middle Atlantic States. At the suggestion of Fox (4)an experiment was run in which sodium nitrite concentration exceeded combined chloride and sulfate concentrations by 200 p.p.m. Surprisingly, the steel corrosion rate (Table 11, No. 164) was still well beyond that of the sodium nitrite-free blank carried out with the same chloride and sulfate concentrations. With 500 p.p.m. sodium nitrite present, a smaller corrosion rate was obtained in a water containing 500 p.p.m. chlorides and 1400 p.p.m. sulfates than in a water containing only 150 p.p.m. chlorides and 420 p.p.m. sulfates (Table 11, Figure 1). Confirming published information,
CORROSION INHIBITOR when pH was reduced to 5.5 to 6.5 with 150 p.p.m. chloride and 250 p.p.m. sodium nitrite present, corrosion rate of steel in a 28-day test was 95.5 MDD in contrast to the 1.3 to 10.3 MDD corrosion rate obtained with p H between 7 and 8 (Table 11. n'os. 116 and 124). Under these same conditions (Table 111), active corrosion of brass, copper. and solder took place. ,4s expected from published data, the corrosion rate of solder was particularly high under these circumstances. LYhen the pH was raised to 7 to 8, solder showed only negligible corrosion (0.6 MDD) with the same sodium nitrite concentration (Table IV). However, when solder was coupled with copper by soldering two strips of copper together to form the test coupon, corrosion rate was considerably higher, with most of the attack taking place on the solder. Here, too, there was a surprising observation in that with corresponding concentrations of sodium nitrite corrosion rate was higher in distilled water (Nos. 67 and 68) than in Croton water fortified with 150 p.p.m. chlorides (Sos. 66 and 69). In the case of distilled water runs, the higher concentration of sodium nitrite appears to have accelerated corrosion rate. Copper was not greatly affected under these experimental conditions (Table V: Figure 2) but both brass and aluminum corroded at rates in excess of those found in the absence of any inhibitor despite the presence of sodium nitrite. In one experiment ( S o . 164), sodium nitrite exceeded the combined chlorides and sulfates by at least 200 p.p.m. In the case of aluminum, with low or moderate sulfates, sodium nitrite gave complete protection at least up to 150 p.p.m. chloride (Nos. 49, 124). However, when sulfate content was increased along with chloride content, the addition of sodium nitrite actually accelerated corrosion rate over that observed with the same chloride and sulfate concentration in the absence of any inhibitor.
Typical test coupon assembly for field use (before exposure) Individual coupons were insuloted from mounting bolts b y rubber grommets and separated by rubber washers
tive condensers in which sodium nitrite was being used for corrosion control. Most of these units were air washers used for humidification or dehumidification. These field test results have frequently been unpredictable (Table VI). For example, section A presents results of tests carried out under average conditions
Table IV. Expt. NO. 63* 61 1216
6P 68" 66d 69d
Corrosion of Metals"
Table V.
C1-, P.P.M.
Corrosion, MDD
8-12 8-12 8-12 0
1.2 0.6 0.6 19.3 29.4 6.5 4.4
0 150 160
* Croton
water.
C
Distilled
Brass and aluminum corrosion was greater than in absence of inhibitor NO.
48 154 162 163 164 152 161
Nonferrous
Corrosion, MDD
Metal Brass 250 150 10.0 Copper 250 150 4.3 Solder 250 150 36.8 Conditions given in Table 11, except pH = 5.5-6.5.
0 250 I000 500 2000 0 2000
c1-, P.P.M.
Corrosion of Nonferrous Metals with High Chlorides and High Sulfates"
Expt.
Corrosion was active a t the lower p H
NaN02, P.P.M.
NaN02, P.P.M.
Metal Solder Solder Solder Soldered copper Soldered copper Soldered copper Soldered copper
a Standard 28-day laboratory test: 100-llOo F.; p H = 7-8. water. NaC1-fortified Croton water.
During the past 3 years test coupon data have been obtained from operating air washers, cooling towers, and evapora-
Ill.
Corrosion of Soldera
Corrosion was surprisingly greater in distilled water
Field Tests
Table
which, according to both published recommendations and the above laboratory tests, should have provided excellent corrosion control. In two cases this was so, but the other two installations showed corrosion rates well above those which would have been anticipated. In fact, the first experiment listed illustrates a
a
c1-, P.P.M.
Sod--, P.P.M.
Corrosion, MDD
612 150 150 150 150 500 500
16-33 420 420 420 420 1400 1400
0 0.7 2.6 0.8 0.8 6.2 2.0
16-33 16 16 420 420 420 420 1400 1400
0 1.2
350 500 770 0 500
6-12 150 150 150 150 150 150 500 500
500 250 0 350 500 770 0 500
6-12 150 150 150 150 150 500 500
16-33 16 420 420 420 420 1400 1400
0 0 2.6 5.2 3.7 4.6 3.3 5.9
iYaNO2,
Metal Brass
P.P.M. 500 0 350 500 770 0 500
48 66 124 154 162 163 164 152 161
Copper
49 124 154 162 163 164 152 161
Aluminum
500 0 250
0
1.3 0 0 0 0 1.9 0
Conditions given in Table 11.
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583
case in which the corrosion rate found in the presence of a more than adequate concentration of sodium nitrite was of the same order of magnitude as that which would be expected in the absence of any corrosion inhibitor for equipment of this type in the community concerned. The results in section B, Table VI, were obtained with somewhat higher chloride and sulfate levels. In each case, nitrite concentration was far more than that necessary to exceed chloride plus sulfate concentrations by more than 200 p.p.m. Yet, results varied from complete protection to rather substantial corrosion rates which represented serious attack upon this type of equipment. Section C, Table VI, shows results of tests in which sodium nitrite concentrations were inadequate according to recommended standards. I n two of these cases protection was inadequate, but in the other case, with the lowest average sodium nitrite concentration, corrosion rate was acceptably low. Similar unpredictable results were obtained with nonferrous test coupons placed in the same operating equipment. Field test results were obtained with aluminum test coupons placed in two different air washers because these units had aluminum elements in their construction (Section D, Table VI). The first two experiments in this section show data obtained in the same air washer during two different testing periods. During both of these exposures sodium nitrite concentration and pH were apparently adequate for satisfactory corrosion control according to published data. However, there was reasonably good protection obtained (4.7 MDD) in one exposure, but poor protection during the other, despite the fact
that sodium nitrite concentration was considerably higher during this latter exposure period. I n contrast, the third experiment shows results from an installation in which there was complete inhibition of corrosion despite a borderline sodium nitrite concentration and pH actually below the recommended minimum. Discussion
These experiences with sodium nitrite as an inhibitor in laboratory and field tests indicate that knowledge of the factors affecting its inhibitor action is still incomplete. Although numerous comments have been made about the effects of chlorides in reducing the inhibitory powers of sodium nitrite, both laboratory and field results given here, as well as the report of Akal’zin and Glushenko (7), suggest rather strongly that sulfates also have a very considerable effect in reducing the inhibiting action of sodium nitrite. Since these experiments were completed, Fox (4)has obtained preliminary results in static corrosion tests which tend to confirm the fact that sulfates significantly reduce the inhibiting action of sodium nitrite. Such an effect is not noticed when sodium chromate is used under the same laboratory or similar field conditions. For example, in laboratory tests using a circulating water containing 500 p.p.m. of chloride and 1400 p,p.m. of sulfate at pH 7 to 8, the corrosion rates for mild steel were 59.9 MDD in the absence of any inhibitor, 54 to 70 MDD when 500 p.p.m. of sodium nitrite was also present, but only 3 MDD when 500 p.p.m. of sodium chromate was present. Similar
comparative data have been obtained under a variety of experimental conditions, and in all cases in which complete inhibition was not obtained, corrosion rate obtained with sodium nitrite as the inhibitor was much higher than that obtained with the same, or in many cases a smaller, concentration of sodium chromate as the inhibitor. With regard to the results obtained with nonferrous metals, there have been numerous published warnings that sodium nitrite does not effectively protect many of these metals under conditions satisfactory for the protection of steel. Recommendations have been made for supplementing the nitrite by borax: sodium benzoate, and other inhibitors. In the case of cooling waters, addition of the necessary concentrations of such supplementary inhibitors frequently makes the cost of treatment prohibitively high. Where corrosion rates for nonferrous metals are not excessively high, it may be satisfactory to accept these corrosion rates and to protect the steel in the system by addition of sodium nitrite to the circulating water. Conclusions
Sulfates adversely influence the action of sodium nitrite as a corrosion inhibitor, although it is not known whether this is the only such factor, in addition to chlorides and low pH. Further study of the factors influencing the action of sodium nitrite as a corrosion inhibitor is needed to make this compound a reliable tool in the hands of the corrosion engineer and water treatment chemist. Additional tests are under way in this and at least one other laboratory. Literature Cited
Table VI.
Unit” AW
Field Test Coupon Results Show Unexplained Variations in Corrosion Rates Corrosion Rate Average Circulating Water Analyses NaNOz, c1-, so4--, Days pH p.p.m. p.p.m. p.p.m. exposure MDD A. Steel 7.7 7.1 7.5 7.4
1231 480 1077 1620
6 21 4 5
B. AW
7.3 6.9 7.8 9.6 9.0 9.5
850 586 2312 4951 1130 3985
EC CT EC
8.4 8.2 7.9
140 654 64
AW
7.8 7.8 6.7
2320 700 268
CT AW
a
43.0 12.2 0.0
128 220 341 319 173 385
70 30 57 86 58 30
30.0 26.1 32.6 7.7 4.0 0.0
240 378 95
62 62 43
28.5 26.0 5.9
675 67 28
57 13 13
31.7 4.7 0
Aluminum 163 22 7
AW = air washer, CT = cooling tower, EC = evaporative condenser.
584
0.0
Steel 89 243 38
D.
56 42 29 74
Steel 21 44 113 114 32 73
C.
33 85 21 23
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Akal’zin, P. A., Glushenko, V. V., Izvest. Vsesoyuz. Teplotekh. Inst. 21, No. 9, 21-4 (1952). (2) Cohen, M., J . Electrochem. Soc. 93, 26-39 (1948). (3) Cohen, M., Pyke, R., Marier, P., Ibld., 96, 254-61 (1949). (4) Fox, W. C., Solvay Process Div., Allied Chemical & Dye Corp., Syracuse, N. Y., private communication, Jan. 11, 1957. (5) Hoar, T. P., Corrosion 14, 103t4t (1958). ( G ) Holdworth, J. (to Imperial Chemical Industries), Brit. Patent 686,836 (Feb. 4, 1953). (7) Pyke, R., Cohen, M., J. Electrochem. SOC. 93, 63-78 (1948). (8) Solvay Process Div., Allied Chemical & Dye Corp., New York, N. Y., “Sodium Nitrite for Rust and Corrosion Prevention.” (9) . , Sussman., S.., Corrosion 13, 701t-10t (1957). (10) Sussman, S., IND. ENG. CHEM.44, 1740-4 (1952). (11) Wachter, A., Zbid., 37, 749-51 (1945). RECEIVED for review April 18, 1958 ACCEPTED October 7, 1958 Meeting-in-Miniature, New York Section, ACS, New York, March 1958.
