INDUSTRIAL
AND
ENGINEERING CHEMISTRY
A N A L Y T I C A L EDITION PUBLISHED
BY
THE
AMERICAN
CHEMICAL
SOCIETY
0
HARRISON
E.
HOWE,
EDITOR
Chromate Corrosion Inhibitors in Bimetallic Systems * Compared by a Scoring Method Based on Visual Observations RIAHC DARRIN, Jlutual Chemical Company of .imerica, Baltimore, Rld.
Increasing amounts of water being used for air conditioning, washing, cooling, and condensing raise a problem in many localities in respect to water supply and sewage disposal. Present practice is tending toward recirculating systems to save the water and the use of corrosion inhibitors to save the systems. It is well known that the corrosion of many metals is greatly reduced by the presence of sodium chromate or sodium bichromate (3, a), but there are almost no data in the literature showing the relative efficiency of these chemicals for the retardation of bimetallic corrosion in ordinary recirculating water systems. Comparisons are presented in this report for iron, galvanized iron, tinned iron, copper, brass, and aluminum, both alone and in contact with each other. The corrosion of these metals in various water systems was greatly retarded or completely inhibited by sodium chromate at the concentrations indicated. For some combinations sodium bichromate was equally satisfactory, but in general it was not so efficient as the chromate. In addition to prolonging the life of equipment, the formation of slimes may be avoided.
M
systems and the practical value of chromate inhibitors. Table I gives some weight-change data illustrating how difficult they are t o evaluate even for very simple systems in the presence of chromates. Without visual inspection it is not apparent from Table I that the chromate completely inhibited corrosion. With bimetallic systems weight changes have been found to be even more complex and almost impossible to interpret without visual data. On the other hand it has been noted that the impression formed on visual examination corresponds in most instances to practical behavior. This does not apply to corrosion which produces concealed defects such as the intercrystalline corrosion of aluminum, the graphitizing of cast iron, or the caustic embrittlement of steel. Concealed corrosion can be evaluated best by physical measurements, such as loss of tensile strength, or by microscopic examination. Depth of penetration methods are also of practical value. Various electrochemical methods that have been proposed provide fundamental assistance in studying the mechanism of corrosion, but require great skill and theoretical knowledge of the factors influencing corrosion. They are difficult because of innumerable local potential differences between different areas of the same metal, and between corrosion products. With dissimilar metals in contact in aqueous solutions-which is common from the practical viewpoint-the situation is particularly complex, resulting in electrochemical processes which may either intensify or retard corrosion.
ANY corrosion processes may be followed by weight
changes, provided precautions are taken to ensure that corrosion products either remain entirely on the specimens or are completely removed, without removal of uncorroded metal. Such data, however, may require interpretation by corrosion specialists. For instance, iron may increase in weight because of formation of rust scales. Aluminum increases in weight, but the oxide film may be protective. In the presence of an oxidizing passifier such as sodium chromate, Hoar and Evans (1, 6) have noted the formation of a dense inert protective film containing chromic oxide, Cr208. Others have noted the formation of protective films, such as zinc chromate on galvanized surfaces. Wilson and Groesbeck (6) have used salt-spray methods for testing corrosion inhibitors in air-conditioningequi ment, and report that chromates and bichromates are the most egcient.
Visual Scoring This method is based on simple observations and the weighted scoring of characteristic corrosion criteria, which may be expressed numerically, and can be duplicated by different observers with a degree of precision satisfactory for practical purposes. The method may be varied to simulate
TABLE I. WEIGHTCHANGEAFTER 6 MOXTHSAT 70’ F T a p water 500 p. p. m. KazCrO4 600 p. p. m. XarCrzOi
Corrosion tests conducted at this laboratory have revealed little relationship between weight changes in bimetallic 755
Aluminum
Iron
+0.123 4-0.021 +0.004
t0.007
-0.849 1-0.012
Galvanized Iron -0.564 1-0.017 -0.015
Tinned Iron -0.161 f0.008 -0.020
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 13, N o . 1 1
The nature of the scoring method is such, however, that ordinary changes in weightings seldom result in important n the relative corrosion order. Of course the same changes i weightings must be used in comparing corrosion scores. A general idea of the protection efficiency may be obtained from the sum of the liquid and metal corrosion scores, but for many purposes it is best to consider them separately. In a bimetallic system, 100 represents perfect inhibition; with single metals, 60 indicates complete protection. A limitation of this method is that it does not afford a numerical comparison between monometallic systems and bimetallic systems, except under special conditions. There are technical difficulties in making comparisons between these systems due to differencesin the nature of the initial corrosion centers or pits. The corrosion trend in monometallic systems is slower in developing. This was particularly evident with aluminum and to a lesser extent with zinc. For aluminum in tap water, the corrosion scores were better after 18 months than after 6 months. These changes appear to be due to the initial formation of many corrosion centers which are heavily weighted, because in most systems even small pits are an indication of the start of severe corrosion. It happens, however, that these initial pits, instead of deepening, either spread until they join to form a fairly uniform and resistive surface
$1 +FE
AlrSn
A l r B r a ~ AI*CY
FIGuaE 1. CORROSION TESTS vLlr,uus p s r u o s l C"LIUII,"LL>. OG",lL,& may us rawalgllrau to obtain comparisons for different applications. Illustrative of a way in which corrosion criteria. mav be weighted for an air-conditioning system, cloudiness ma? be allowed 4 points out of a possible 100. Low scoring for oloudiness is indicated became it may not be harmful. If a precipitate settles, it may result in some harm and should be iven a greater weight. If the preeipitste is such that it may Jog the system, a still greater weight is indicated. Ifthe general appearance score is zero, the precipitate and cloudiness scores also will be close to zero. Other corrosion criteria mav be selected and weighted in a similar manner. The accompanying data further illustrate the scoring method. Forty points were allowed for the condition of each of the metals in contact; 20 points for the corrosion products in the liquid. The corrosion of each metal was scored on the basis of five types of observation: discoloration, roughening, pitting, depth of pits, and general corrosion. By general corrosion are meant the nature and distribution of corrosion, the geueral impression of the observer, and unclassified factors. Evidence of corrosion in the liquid was scored in a simiiar manner based on cloudiness, precipitation, and general appearance. Under general appearance of the liquid are included unclassified variables, the nature of the precipitate, and its probable harmful effect in a recirculating system. The weightings employed in this report, shown in Table 11,are arbitrary but logical and illustrate a practical application. Different weightings, however, are used for different applications. For instance, different weightings are indicated for an automobile cooling system than for an air washer. If discoloration or slimes are unimportant they may be disre garded. For some purposes the entire liquid score is disre garded. The scoring procedure in this laboratory is for the observers to check all of the descriptive terms on a report form. These data are assembled and weightinp assigned according to the particular purpose of the corrosion project.
Cloudiness precipitate
Geneial appearance
. ...
For Condition of Liquid None (4). slight 3). moderate (2). considerable (O\ None (5). slight ( 5 ) . moderate (2). oonsiderable (0) G w d (8).fair (6).p001(14). bad (2). very bad (0)
"
..
. .
. . ...
or change in hght reflection so as to become invisible except under special illumination and low magnification. The procedure employed in this report is limited to corrosion criteria which can be seen by unaided visual inspection. It has also been found, when inhibitors are absent, that oxygen is so rapidly consumed as to cause corrosion to d e pend on the rate a t which additional oxygen is obtained from the air, thus making numerical comparisons between some uninhibited systems meaningless-beyond the general observation that it is very severe as compared to inhibited systems. Irrespective of the explanation, a study of about a thousand corrosion record sheets indicates that technical difficulties in making numerical comparisons are very much less when inhibitors are present. The corrosion scores of all inhibited systems are given in Table 111. Scores are also given for uninhibited bimetallic systems for purposesof general oomparison, although they may be only approximately correct because of variations in oxygen deficiency. Corrosion scores have not been assigned to monometallic systems in tap water, but it can be seen from Figures 1, 2, and 3 that corrosion was severe in comparison with the inhibited systems. If numerical comparisons are desired between these systems a different procedure is indicated, involving aeration; but this is beyond the purpose of the present investigation which is to evaluate the protective efficiency of chromate inhibitors.
November 15. 1941
ANALYTICAL EDITION
1.51
iron plates. Tin refers to heavily tinned iron plates, Both galvanizing and tinning were done by doubledipping after cutting and drilling. Brass refers to a commercial grade, analyzing about 60 per cent copper, 40 per cent aine. Copper and aluminum refer t o the commercially pure metals, analyzing about 99.9 per cent copper and 99.0 per cent aluminum. The iron panels were cut from a mild steel sheet known as type A tank (about 0.1 per cent carbon). All panels were uniformly polished, cleaned, and inspected for defects prior t o testing. The snels tested at 160” F. were placed in thermostatically controlEd ovens with uniform heat distribution. No light was admitted t o the ovens. The room-temperature test panels were shielded from direct sunlight and were inspected at frequent intervals up t o 18 months. The principal data. on which this report is based are the comDasite observations of four men for the same D U L ~ Sshown in the
details of scoring b> different observers, hut the total corrosion score of each observer for any particular Condition was essentially the same, as was the composite score. The corrosion scores of the monometallic systems did not prove to be a measure of the relative merit of the inhibitors because either sodium chromate or sodium bichromate gave such perfect inhibition that comparison of their scores was almost meaningless.
Zinc
I ZirFe , Z n c J n ;Zn+8rasi:ZntCu FIGURE2. CORROSION TESTS
Experimental Data
The tabulated data for iron and copper in contact illustrate the scoring method. The data for the aluminum series, zinc series, and iron series present the corrosion scores for the more important metal combinations. The plates and washers may be considered separately or in combination, depending on the purpose of comparison.
HOT WATER TESTS. Corrosion inhibition in hot water roughly pwalleled that at room temperature. As would be ex‘pected, corrosion is more rapid. A t the end of 6 weeks at 160‘ F. the corrosion scores of most of the bimetallic test panels dropped t o figures comparable to 6 months at 70’ F. With combinations
Testing conditions which are not specified were identical for all specimens. “Tap water” refers to Baltimore city water. “Chromate” and “bichromate” refer to solutions of sodium chromate, Na&r04, and sodium bichromate, Nar Cr20r.2Hz0, at concentrations equivalent to 1000 parts of NalCr04 per million. AU regular tests were run in closed %ounce screw-cap glass jars, without agitation or aeration. Some aerated tests also were run. The aerated tests indicated that corrosion in the presence of sodium chromate is essentially the same in closed as in aerated systems. If no chromate is present the corrosion is very much more rapid when aerated. Since there are varying degrees of aeration in practical applications, and since aeration is often the rate-making factor when chromates are absent, the most conservative way of evaluating chromate corrosion inhibitors is without aeration. I n other words, the conditions of test used in this investigation represent the minimum relative retardation of corrosion d o r d e d by the chromate. There are many conditions of aeration where the merit of the chromate i s considerably greater than that indicated. There was some access of air when the test samples were inspected, but all were handled in an identical manner, and the admission of air was less than under any ordinary condition of practical use. The black appearance of the corrosion products on the iron test panels in tap water (Figure 3) indicates insufficient oxygen. The photographs (2),Figures 1, 2 and 3, show the general appearance of the corrosion tests after 6 months at room temperature. The top rows contain sodium bichromate, the middle rows sodium chromate, and the bottom rows tap water. The bolts and washers show the effect of contact with a dissimilar metal. Markines on the nhotoeranh show the date metal first. followed by the washer m h l . -In^each test the halts and nuts are the same metal as the washers. Zinc refers t o heavily galvanized
IKON
Fc-an
i-erbrasa
Fe+Cu
FIGURE 3. CORROSION TESTS
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 13, No. 11
T A B L111. ~ SCORING METHOD T a p Water
Chromate
Bichromate
Iron and Copper in Contact 6 Months a t 70" F. Iron Plates Discoloration Considerable None Roughening Considerable None Pitting Moderate None Depth of pits Deep None General corrosion Very bad Slight
Slight None None None Slight
Co per Washera L o o l o r a t ion Roughening Pitting Depth of pits General corrosion
Slight None None None Slight
Slight None None None None
Slight None None None None
Liquid Cloudiness Precinitate Gene& appearance Plate score (40 -perfect) Washer score (40 -perfect) Liquid score (20 -perfect) Protection efficiency (100 perfect)
Considerable considerable Bad 6 36 2
None None Good 37 39 20
Slight Slikht Fair 36 39 14
-
44
96
89
Aluminum Systems", 6 Month8 a t 70' F. Aluminum Plates Alone Plate score Liquid score I n Contact with Iron Washers Plate score Washer score Liquid wore ~~
X
X
40 20
40 20
T a p Water
Chromate
Bichromste
Zinc Systems", 6 Months a t 70" F. Galvanized Iron Plates Alone Plate score X 40 Liquid score X 20
39 20
I n Contact with Plain Iron Washers Plate score Washer score Liquid score
20 34 4
39 37 17
35 37 14
I n Contact with Copper Washers Plate score Washer score Liquid score
35 4
9
36 39 14
28 22c 14
20 39
40
8
20
29 260
11 14 7
40 39 20
I n Contact with Brass Washers Plate score Washer score Liquid score I n Contact with Tinned Washers Plate score Washer score Liquid score
40
16
39 39 17
Iron Systemso, 6 Months a t 70' F.
~
29 20 5
39 38 20
X X
40
14
Iron Plates Alone Plate score Liquid score
20
37 20
6 36 2
37 39 20
36 39 14
37 25
I n Contact with Copper Washers Plate score Washer score Liquid score
10 34 9
19 39 11
14 40 7
I n Contact with Copper Washers Plate score Washer score Liquid score
I n Contact with Brass Washers Plate score Washer score Liquid score
13 34 9
30 39 17
17 33 b 9
I n Contact with Brass Washers Plate score Washer score Liquid score
18 39 2
37 39 20
36 366 17
I n Contact with Tin Washers Plate score Washer score Liquid score
23 15 9
40 40
40 39 20
I n Contact with Tinned Washers Plate score Washer score Liquid score
21 31 2
37 39 17
37 35 17
20
(i X d m n t e a t h s t corrosion was severe in comDarison with inhibited svstern;.- a-s-csn-be-seen-in photographs, but method employed does not give directly com arable figures for monometallic systems in t a water. b Slightly rower score of brass .washers in resence of so&um bichromate is due chiefly t o discoloration, whlch may be $isregarded for many purposes. C Low score of cop er washer in contact with zinc in presence of sodium bichromate ia due t o formation of a hard, rough, tightly-adhering layer of
of aluminum-copper and iron-copper, sodium chromate was more beneficial in hot water than in cold, whereas with aluminum-iron, zinc-iron, and zinc-copper combinations, sodium chromate was a little more effective at room temperature. CHROMATE: CONCENTRATION TESTS.Owing to acceIerated corrosion a t 160' F., a series of tests was run at this temperature, with iron plates in contact with copper washers, using triple proportions: 3000 parts per million. There was no corrosion after 8 weeks at 160' F. in the presence of sodium chromate, and only sli ht corrosion in the presence of sodium bichromate. Corrosion ako was negligible with sodium chromate a t a concentration of 1000 p. p. m. In other words, the protection afforded by sodium chromate was about the same a t both concentrations. Lower concentrations were not included in this series, but there are ample records a t this laboratory indicating that under most conditions concentrations of 200 to 500 p. p. m. provide good protection. Concentrations below 100 p. p. m. protect aluminum and zinc, but may cause stimulation of corrosion with iron systems. REGULATED pH TESTS. A set of corrosion tests was run with iron plates in contact with copper washers, with the alkalinity of the sodium chromate adjusted to H 7.0 by addition of sodium bichromate, the total parts per m8lion of chromium being kept the same. Tests were for 8 weeks at 160' F. using a concentration equivalent to 1000 p. p. m. Another set was run with 3000 p. p. m. For further comparison, a third set was run a t the same time wjthout adjusting the alkalinity. These tests indicated that adlustment to pH 7.0 does not result in an im-
crystalline zinc compound over surface of copper, causing i t to be re orted Scraping off this deposit a8er 18 months revealed that copper ha qot actually been attacked. The scoring method is undouhtpdly too severe In this instance, b u t shows that an undesirable condition cxists when sodium bichromate is used in this system rather than sodium chromate. Somewhat similar conditions exist with brass washers. as considerably roughened and fitted.
pressive difference. If anything, sodium chromate without adjusting alkalinity gave slightly better protection. AERATEDTESTS. A set of aerated corrosion tests was run with the iron systems at 70" F. The air was drawn from outside the building by means of a water aspirator. It was well filtered, water-washed, and sup lied a t a low steady pressure. The air was bubbled through alytest samples continuously at a slow uniform rate. All samples were tested at the same time and aerated in an identical manner. For comparison a closed set also was run at the same time without aeration but with other conditions identical. The corrosion of all the aerated test panels in tap water was very much faster and more severe. The behavior of the aerated samples in the presence of sodium chromate was the same as the corresponding nonaerated test panels. EIGHTEEN-MONTH TESTS. The bimetallic systems retained essentially the same corrosion order, which was evident a few days after starting the tests, up to the time the photographs were taken 6 months later. Rescoring after 18 months showed essentially the same order as at 6 months. There were no signs that the sodium chromate had lost its effectiveness. In some cases the protection was better after 18 months than after 6 months. PRACTICAL TRIALS.In trials with several small York airconditioning units (34 X 37 inch washers), one Carrier (26 X 30 inch washtar), and one large Sturtevant air-washer (60 X 60 X 84 inch spray chamber), weekly observations over a period of 18 months indicated that the use of sodium chromate not only prevented corrosion but also eliminated the formation of organic slimes. The nature of the microorganisms destroyed was not
ANALYTICAL EDITION
November 15, 1941
investigated. Two new units without chromate accumulated a large amount of slime during the same period. One old unit which had accumulated slime and rust prior to the test completely cleared during the 18-month chromate treatment. Because of fluctuations in overflow the concentration of sodium chromate varied between 100 and 500 p. Each wTeek it was adjusted t o 500 p. p. m., on the basis o&%ow color which was found satisfactory for practical purposes. Although chromate replacement is frequently due to mechanical loss through the water overflow, some consumption is due to the oxidation of organic material removed from the air. Many bacteria, molds, and accompanying odors are destroyed. Sodium bichromate is a little more reactive than sodium chromate, and consequently has a shorter life if much organic material is present. The foregoing results have been substantiated by other practical observations extending over 4 years, including several large air-conditioning units.
Summary Visual scoring is adapted to the evaluation of chromate corrosion inhibitors. Results may be duplicated with an accuracy sufficient to be of practical value. Sodium chromate is effective in retarding, or completely inhibiting, the corrosion of many bimetallic systems in water. Sodium chromate and bichromate are effective in retarding corrosion in most monometallic systems. Sodium bichromate also reduces corrosion in many bimetallic systems, but it is not so generally effective as sodium chromate. Aluminum and copper, aluminum and brass, and zinc and copper combinations are difficult to inhibit. With these combinations, sodium chromate is very helpful but not completely inhibitive. The relative retardation of corrosion by sodium chromate is essentially the same in hot water as in cold. Neutralizing the alkalinity of sodium chromate to pH 7.0 does not result in any impressive differences. If anything, sodium chromate is more effective a t its natural alkalinity of about pH 8.5. Although the rate of corrosion of most systems is greatly accelerated by aeration, there is no important difference be-
759
tween aerated and closed systems when sodium chromate is present. Increasing the concentration of sodium chromate slightly improves the protection, but this is not indicated except as a factor of safety when recirculating water is subject to dilution. No harm results from excess sodium chromate. Amounts less than about 100 p. p. m. should not be used for iron systems. Sodium chromate retains its inhibitive properties until lost from the system, either mechanically or by chemical reduction. For many practical purposes its depletion may be judged by loss of color.
Acknowledgment The writer wishes to take this opportunity to acknowledge the assistance and helpful suggestions of the Baltimore staff of the Mutual Chemical Company of America, particularly 0. F. Tarr, W.H. Hartford, and D. F. Altimier, and to express appreciation to G. H. Young, E. Ward Tillotson, W. A. Hamor, and others associated with Nellon Institute for many thoughts and constructive criticisms.
Literature Cited (1) Hoar, T. P., and Evans, U. R., J . Chem. SOC.,1932, Part 11, 2456-81.
(2) Perkins, E., illustrations from unretouched photographs taken in
natural colors, Mutual Chemical Co. of America, 270 Madison Ave., New York, N. Y. (Lantern slides are available in natural colors and will be loaned on request, sizes 3.25 X 4 and 2 X 2 inches.) (3) Roetheli, B. E., and Cox, G . L., IND. EKG.CHEM.,23, 1084-90 (1931). (4) Speller, F. N. “Corrosion Causes and Prevention”, 2nd ed., pp. 372 et seq., New York, McGraw-Hill Book Co., 1935. (5) Van Brunt, Charles, and Remscheid, E. J., Gen. Elec. Rev., 39, 128-30 (1936). (6) Wilson, S. H., and Groesbeck, E. C., Natl. Bur. Standards, Research Paper 1305 (June, 1940).
Controlling Apparatus to Eliminate Waste of Water in Using the Ordinary Filter Pump FREDERIC E. HOLMES Children’s Hospital Research Foundation and Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, Ohio
T
HE cost of operating the ordinary aspirator or suction pump is usually concealed in a single water bill which
includes more legitimate uses as solvent, diluent, etc. The cost for a single aspirator may be on the order of thirty dollars per year, depending upon several obvious factors. For filtration, the removal of a supernatant fluid from a precipitate, the removal of used samples and reagents from such apparatus as the Van Slyke blood gas pipet, and many distillations under reduced pressure where rather large variations in pressure are not detrimental, a closed suction system with automa“u ~ conc trol of the supply of water to the aspirator has proved satisfactory. The controlling apparatus shown in the accompanying figure has been found suitable for the purpose. It is highly reliable, having been in operation night and day for months during the past 3 years without attention or interruption. P indicates the filter pump, connected t o the vacuum line through a check valve (Bunsen valve), W is a whistle valve, and
V is a rubber tube connecting flask B with the vacuum line. In operation, mercury in flask A depresses lever LA, opening the whistle valve and starting the as irator. Increasing vacuum in the system “lifts” mercury from task A t o flask B until the shift
