August, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
(16) Eldredge and Mears, Aluminum Research Lab., unpub. data.
(17) Electrochemische Werke Mtinchen A.-G., German Patent 570,468 (July 5,1929). (18) Evans, J . Chem. SOC.,1930,480. (19) Evans, Trans. Eleotrochem. SOC.,69,213,230(1936). (20) Geller, 2. Metallkunde, 28,359 (1936). (21) Jablezyneski and Pierschdski, Z . anorg. allgem. Chem., 217,298 (1934). (22) Jenckel and Woltmann, Ibid., 233,236-56 (1939). (23) Ibid., 233, 239 (1937). (24) Kempf and Daugherty, Automotive I d . , 81, 156-9, 161 (1939). (25) LamDrev. U. S. Patent 2,153,961(Am. 11, 1939). (26j Lauer, thesis, Univ. of Minnesota, 1931. (27) Lichtenberg, Aluminium, 19,505 (1937). (28) Ibid., 20,264 (1938). (29) Lichtenberg, Mdallwirtshaft, 19,1021 (1940). (30) Lichtenberg and Geier, Aluminium, 20,784 (1938). (31) MacDermott, U. S. patent 1,927,842(Sept. 26, 1933). (32) McNutt, Ibid., 2,095.611 (Oct. 12,1937). (33) Mann, Trans. Electrochem. Soc., 69, 115 (1936). (34) Mears and Benson, Aluminum Research Lab., unpub. data.
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141
(35) Mears and Benson, IND.ENG. CAEY.,32, 1343 (1940). (36) Mears and Eldredge, Trans. Electrochem. SOC.,83, 4 (1942); Proc. Ann. Water Canf. E w . SOC.Western Penna., 3, 1-15 (1942). (37) Reichert, U. S. Patent 2,008,726(July 23, 1935). (38) Reschke and Neunzig, Aluminium, 23,358 (1941). 25, 1336 (1933). (39) Rhodes and Berner, IND.ENO.CHEM., (40) Roch and Rohrig, Aluminium, 21,31(1939). (41) Rohrig, Zbid., 17, 140 (1934). (42) Ibid., 17,559 (1936). (43) Rohrig, Chem.-Ztg., 47,528 (1923). (44) Rohrig, Korrosion u. Metallschutz, 5 , 41 (1929). (45) Rohrig and Geier, Aluminium, 19,448(1937). 34,32 (1942). (46) Schwartz and Munter, IND.ENO.CHEM., (47) Seligman, Proc. World’sDairy Congress, 2, 1202 (1923). (48) Seligman and Williams, J . Inst. Metals, 23, 171 (1920). (49) Ibid., 28, 297 (1922). (60) Seligman and Williams, J . SOC.Chem. Ind., 35, 88 (1916); 36, 409 (1917); 37,159(1918). (51) Sievertz and Lueg, 2. anorg. allgem. Chem., 126, 194 (1923-34). (52) Wolf and Tuxhorn, Aluminium, 22, 186 (1940).
U +!&&
T E C H N O L O G Y UNDER CONDITIONS ENCOUNTERED IN PRACTICE‘ MARC DARRIN Mutual Chemlcal Company
WHEN DISSIMILAR metals are in contact in an aqueous medium, the corrosion of one of the metals is accelerated. The purpose of this report is to describe the technology, rather than the theory, of inhibiting this kind of corrosion b y means of chromate, under conditions encountered in practice. The effect of time of exposure, temperature, aeration, submergenee of panels, initial pH, and chromate concentration are shown as they influence the corrosion of various bimetallic systems. Data include corrosion scores, weight loss, depth and type of pits, changes in pH, and chromate consumption. PracticaP applications are described for air conditioning systems, refrigeration brines, automobile systems, Diesel engines, power rectifiers, and other recirculating and quiescent systems.
HE theory and complexity of contact corrosion, also known as galvanic or bimetallic corrosion, have been described by many investigators (4-7, 10, 17, 88). A practical example is a brass flange bolted t o an iron tank containing water; the brms is electronegative to the iron which corrodes anodically, especially near its contact with the brass. If this corrosion is localized so as to form pits, it is called “anodic pitting”. Loss of metal from a corroded area is proportional to the current; however, electrical measurements are difficult to evaluate, since some of the current may travel t o local cathodic areas on the anodic metal (4). With passage of time, corrosion products appear which may stimulate the attack by blocking off regions that become stagnant, end this may result in concentration cells or the local depletion of an inhibiting agent. Furthermore, Corrosion products may form discontinuous films which tend to stimulate corrosion, or the corrosion products (or the inhibitor) may produce a protective coating. This formation of a protective coating is one of the reasons why the rate of corrosion frequently diminishes with time.
T
1
The drat paper in this series (8) described a method for the evaluation
of chromate corrosion inhibitors in bimetallic systems, and presented data for six-month exposures. T h e preaent report includes resulta after five years.
of America,
Bdtimore,
Md.
If, in the foregoing example, a little sodium chromate is added to the water, corrosion of the tank will stop, and the iron is said to have been passivated. This control of corrosion is due t o anodic polarization (6),which is probably caused by a very thin film on the surface of the anodic metal. Later this protection may be improved by the deposition of a fairly thick and adherent coating containing hydrous oxides of iron and chromium (10, IS). There is no satisfactory substitute for exposure tests. In making these tests it is possible, by proper evaluation after a comparatively short exposure, to estimate what may be expected after several years, provided comparative short-period and long-period data are available for similar systems. Such comparative data are presented herewith for some of the more commonly encountered bimetallic systems. Details regarding preparation of teat panels and manner of exposure were reported previously ( 9 ) . These data are supplemented by p H records, ar,alyses for chromate consumption, measurements of weight loss, etc. For some bimetallic systems, such as iron-copper, weight loss was found to provide an approximation .of the general condition of the system; for some aluminum systems, such as aluminumcopper, weight change wtw almost meaningless (Table VI), More significant for aluminum systems were the depth of pits and their character, as shown in Table IV (footnote c). Variations in the character of pits, as well as their depth, were taken into account in determining the corrosion score of any particular system. For many purposes the author considers that corrosion scores provide the most reliable means available for direct comparison. The following designations correspond to corrosion scores: Designation Perfect Esrcellent Good
g$zr Bad
Score 100 Above 95 85 t o 95 75 to 85 65 to 76 Less than 65
Degree of Corrosion No indication Minor, but very satisfactory Definite but satisfactory Cpestiorhble, but probably satisfactory robably unsatisfactory Severe
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Vol. 37, No. 8
Table 1. Effect of Time of Exposure on Condition of Bimetallic Systems in Tap Water after Various Periods of Exposure at 70" F.
Table II.
(Nonaerated, quiescent, P / r submergence for 3 years followed b y full submergence for 2 years; p H unadjusted, with and without chromatejnhibitor; concentration of inhibitor equivalent t o 1000 p.p.m. NazCrOd; Baltimore city water. about pH 7.5 a t s t a r t ; bichromate nystems about p H 5.5; chromate systems a b o u t p H 8.0) 3 Yr. 5 Yr. 18 Mo. Type of Panel 6 Mo. Iron, in contact with brass Bad Bnd Bad Bad I n t a water Good Exc.~ Good Plus &chromate Good Good Exc. Exr. Exc. Plus chromate
(Nonaerated. quiescent, a/, suhmergence. p H 7.5-8.5 a t start) 1000 P.P.M. No Chromate Chromate Contact 70° F.. 160" F.. 70° F.. 160' F , T y p e o f Panel .Metal 6 mo. 6 wk. 6 ma. 6 wk. Iron Copper 44 67 96 100 Galvanized Bare iron 58 58 93 90 Galvanized Copper 48 85 Aluminum Iron 54 60 "Aluminum Copper 53 62 69 90
Iron, contact copper I n t a p water Plus bichromate Plys chromate Iron, contact tin In ta water Plus gichromate Plus chromate Galvanized, contact hare iron I n t a p water Plus bichromate Plus chromate Galvanized, contact hrass In t a p water Plus bichromate Plus chromate (:alvaniaed, contact copper I n t a water Plus b c h r o m a t e Plus chromate Galvanized, contact tin In t a water Plus Eichromate Plus chromate Aluminum, contact iron I n ta water Plus tichromate Plus chromate Aluminum, contact brass In t a p water Plus bichromate Plus chromate Aluminum, contact coppei I n ta water Plus gichromate Plus chromate Aluminum, contact tin In t a water Plus &chromate Flus chromate
Bad Good Exc.
Bad Good Exc.
Bad EX^.^ Exc.
Bad Exc. Exc.
Bad Good Good
Bad Exc. Good
Bad Exc. Good
Bad Good Good
Bad Goodb Goodb
Bad Good Exr.
Bad Enc. Exc.
Bad Exc. Exc.
Poor Poorb Exc.
Bad Fair Exc.
Bad Fair Gooda
Bad Fair Exc.
Bad Bad b Goodb
Bad Fair Exr.
Bad Fair Good
Bad Fair Good
Bad Exc. Exc.
Bad Exc. Exc.
Bad Exc. Exc.
Bad Exc.
Ba,d Fair Exc.
Bad Fair Exc.
Bad GoodC Good
Bad Good Good
Bad Bad Good
Bad Poor" Exc.C
Bad Poor Exc.
Bad Poor Good
Bad Bad Po01
nad Bad Faira
Bad Bad Fair
Bad Ba,d Fair
Bad Exc. Exc.
Bad Goot4.C EXO.
Bad Exc. Exc.
Bad Erc. Exc.
Bad Exc. Exc.
Bad Exc. Exc.
Rad GoodC Exc.
Bad Exc. Exc.
EFFECT
-
ffs, ".
a
Z?
100 = perfect.
Table 111. Effect of Aeration on Condition after Exposure in Tap Water Containing Various Amounts of Chrornrte at 70" F. (Full submergence, p H 7.5-8.5 at s t a r t ; nonaerated systems were quiescent excees air used for aerated systems; panel area 12 square inches, volume oi liquid 8 ounces) Panel and Chromate Concn. Nonaerat ed Aerated Iron, n o other metal No c b o m a t e 2 mo fair; 6 mo., bad 1 mo., bad 1 wk.', n o chromate; 6 24 hr., nochromate; 1 6 2 . 5 p.p.m. mo., bad mo., bad 250 p.p.m. 1 yr., exc.; 3 yr., exc. 1 yr.. exc.; 3 yr.O Galvanized, n o other metal No chromate 2 mw.'fair; 6 mo.. bad 1 mo.. poor; 6 mo.. badb 62.5 p.p.m. 3 yr., exc. 100 p.p.m. 3 yr., exc.c 3 yr., exc. 125 p.p.m. LI 3 yr. exc.4 200 p.p.m. 3 yr., exc. 250 p.p.m. 3 yr., 500 p.p.m. 3 .yr., exc. 1000 p.p.m. Iron, contact brass 1 mo. fair. 6 mo.. bad No chromate mo'' bad 1 wk:, no' chromate; 6 2 . 5 p.p.m.
.
I
EXC.
$
100 p.p.m. 125 p.p.m. 250 p.p.m.
a Improved corrosion score for long exposu,res is due in p a r t t o initial formation of a number of corros?on centers which are heavily weighted, because small pits a r e usually an indrcation of the s t a r t of severe corrosion. In some systems these small initial pits, instead of deepening, either spread until they join t o form a uniform and resistive surface, or change in light reflection so a s to conceal their presence. b Low corrosion score of some galvanized systems is due to formation of a rough crystalline layer of zinc chromate on t h e met,al,surface, causing i t t o appear roughened or pitted. Removal of this de osit seldom reveals any important corrosion of t h e underlying metal. endoubtedly the scoring method is too severe when this condition exists. Long exposure tends to reduce this difficult.):. Apparent variation due to border-line scores.
OF EXPOSURE CONDITIONS
For most purposes six month8 are sufficient to predict the behavior of a bimetallic system in water a t normal temperature. Shorter periods make it difficult to evaluate the extent of corrosion and may fail to reveal its true nature. Table I shows that after six months there was no important change in the corrosion behavior of the described bimetallic systems up to at least five years; since the rate of corrosion decreased rapidly with lapse of time, it is probable that there would be little further change if the tests were continued. The principal variations from this general behavior occurred with the aluminum systems, which had a little higher corrosion score after five years than after six months, for reasons previously reported (9). WithBmost of the other bimetallic systems protective films were formed in the presence of chromate almost immediately. The foregoing general behavior does not apply to ferrous systems containing very small Time of Exposure.
EffFct of Temperature on Corrosion Scorea after Exposure in Tap Water, without and with Chromate
500 p.p.m. 1000 p.p.m. a
1 mo., no chromate; bad 1 mo., no chromate, f fair; 6 mo., elongated pits, 0.003 in. deep 6 mo., exc.; 1 yr., I mo.. 100 p.p.m. chromate; 1 yr. no goodo; 3 y r . l chromate, fair; 3 yr., bad 01 1 yr., exc.h; 3 yr., exc. 3 yr., exc. 3 yr.. exc.
No data for precisely same conditions; protection probably excellent:
b After one year weight loss 0.521 g. C After one year; weight loss 0.004 g . ;
94 p.p.m. chromate. After one year, weight loss 0.050 6.; 126 p.p.m. chromate. After one year, weight loss 0.005g.; 490 p.p.m. rhromate. / Protection probably unsatisfactory. 9 After one year, 148 p.p.m. chromate. h After one year, 471 p.p.m. chromate. d e
amounts of chromate which may be depleted during the period of exposure; more will be said about these systems later. Temperature. Exposure should be under about the same ten]perature conditions as will be met in use. Accelerated tests at higher temperatures are not suggested, except as a quick means ot estimating probable trends. Although the total corrosion score+ at different temperatures may provide a fair comparison, the appearances of the panels and liquid may fail to give a true picture of the character of the corrosion. As Table I1 shows, the general trend for several bimetallic systems for a six-week expo>ure at 160" F. was not very different from six months a t 70' F., except for the system aluminum-copper. For this system accelerated tests at 160' F. indicated good protection, whereas six months at 70' F. showed the protection to be poor, with some. improvement after one year and three years. When there is a fluctuating temperature, it is safe to figure on the concentratioii required to inhibit at the highest temperature. Aeration. Although aeration greatly stimulates most corrosion, it has practically no effect when sufficient chromate is present to inhibit. The concentration of chromate required for an aerated system is essentially the same as for a c l o s d system; however, the pate of consumption of chromate in an aerated system is greater if reducing substances arc present in the air.
August,
1945
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
743
Table V shows the effect of raising the p H from 5.5 to 8.5 for the system iron-copper in water containing chromate. 3 The general effect of pH on Chromate c&g.: Condition Wt. Lorn*, Mg. Depth of P i t a o , In. pH at End Left, P.P.M. other bimetallic systems may System P.P.M. 8/45 Full* :/4 Full :/4 Full 1/4 Full */4 Full be estimated from Table I. Fe-Cu None Bad Bad 4.8043 3.993d 0.01411 0.012* 6 . 8 8.0 I n general, this laboratory has 1000 Good ESC. O.98Od None 0.01211
