M A R C D A R R IN
M utual Chemical Company o f Am erica, Baltimore, M d .
W H EN DISSIMILAR metals are in contact in an aqueous medium, the corrosion of o n e 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 con
ditions encountered in practice. The effect of time of exposure, temperature, aeration, submergence of panels, initial p H , 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 p H , and chromate con
sumption. Practical’ applications are described for air condition
ing systems, refrigeration brines, automobile systems, Diesel engines, power rectifiers, and other recirculating and quiescent systems.
T
H E theory and complexity of contact corrosion, also know n as galvanic or bim etallic corrosion, have been described by m any investigators [4-7, 10, 17, 22). A practical exam ple is a brass flange bolted to an iron ta n k containing w ater; th e brass is electronegative to th e iron which corrodes anodically, especially near its contact w ith th e brass. I f th is corrosion is localized so as to form pits, it is called “ anodic p ittin g ” . Loss of m etal from a corroded area is proportional to th e cu rren t; however, electrical m easurem ents are difficult to evaluate, since some of th e current m ay trav el to local cathodic areas on th e anodic m etal (4).W ith passage of tim e, corrosion products appear w hich m ay stim u late th e a tta ck by blocking off regions t h a t become stag n an t, and th is m ay result in concentration cells or th e local depletion of an inhibiting agent. Furtherm ore, corrosion products m ay form discontinuous films which ten d to stim ulate corrosion, or th e corrosion products (or th e inhibitor) m ay produce a protec
tiv e coating. T his form ation of a protective coating is one of th e reasons why th e ra te of corrosion frequently dim inishes w ith time.
1 T h e first p a p e r in t h is series (9) desc rib e d a m e t h o d for t h e e v a lu a t i o n of c h r o m a t e corrosion in h ib ito r s in b i m e t a ll i c s y s t e m s , a n d p r e s e n te d d a t a for s ix - m o n th ex p o s u r e s. T h e p r e s e n t r eport in c lu d e s r e su lt s a f te r fiv e y ears.
If, in th e foregoing example, a little sodium chrom ate is added to th e w ater, corrosion of th e ta n k will stop, and th e iron is said to have been passivated. T his control of corrosion is due to anodic polarization (6), which is probably caused by a very thin film on th e surface of th e anodic m etal. L ate r this protection m ay be im proved by th e deposition of a fairly th ick and adherent coating containing hydrous oxides of iron and chromium (10,16).
T here is no satisfactory su b stitu te for exposure tests. In m ak
ing these tests it is possible, by proper evaluation after a compar
atively sh o rt exposure, to estim ate w hat m ay be expected after several years, provided com parative short-period and long-period d a ta are available for sim ilar systems. Such com parative d ata are presented herew ith for some of th e m ore commonly en
countered bim etallic system s. D etails regarding preparation of te s t panels and m anner of exposure were reported previously (9).
These d a ta are supplem ented by p H records, analyses for chrom ate consum ption, m easurem ents of weight loss, etc. For some bim etallic system s, such as iron-copper, weight loss was found to provide an approxim ation of th e general condition of th e system ; for some alum inum systems, such as aluminum- copper, w eight change was alm ost meaningless (Table VI).
M ore significant for alum inum system s were th e depth of pits and th eir character, as shown in T able IV (footnote c). Variations in th e character of pits, as well as th eir depth, were taken into ac
count in determ ining the corrosion score of any particular system.
F or m any purposes th e au th o r considers th a t corrosion scores pro
vide th e m ost reliable m eans available for direct comparison.
T he following designations correspond to corrosion scores:
D e s i g n a t i o n Sco re D e g r e e of Cor rosion
P e r f e c t 1 0 0 N o i n d ic a t io n
E x c e l le n t A b o v e 95 M in o r, b u t v e r y s a tis fa c t o r y G o o d 85 t o 95 D e f in it e , b u t s a tis fa c to r y
F a i r 75 t o 8 5 Q u e s t io n a b l e , b u t p r o b a b l y s a ti s f a c to r y P o o r 65 t o 7 5 P r o b a b l y uns a tis fa c to r y
B a d L e ss t h a n 6 5 S e v e r e
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 37, No. 8 change if th e tests were continued. T h e principal variatio n s from this general behavior occurred w ith th e alum inum system s, which h ad a little higher corrosion score after five years th a n a fter six m onths, for reasons previously rep o rted (9). W ith 'm o st of th e o th e r bim etallic system s p ro tectiv e films were form ed in th e presence of chrom ate alm ost im m ediately. T h e foregoing general behavior does n o t ap p ly to ferrous system s containing very small
Table II. Effect of Temperature on Corrosion Score“ after Exposure in Tap Water, without and with Chromate
tem is g reater if reducing substances are present in th e air.
August, 1945 743
Table IV. Effect of Submergence of Panel on Condition below Water Line after 2-Year Exposure in Tap Water, without and with Chromate at 7 0 F.
( N o n a e r a t e d , q u i e s c e n t , p H 7 . 5 - 8 . 5 a t s ta r t ; s u b m e r g e d a r e a , 12 s q u a r e in o h e s ; v o l u m e o f l iq u id , 8 o u n c e s ; ra tio 01 p a n e l a r e a t o c o n t a c t m e t a l , 6 t o 1; p a n e l m e t a l is s h o w n first i n e a c h s y s t e m .)
C h r o m a t e C o n o n ., P . P . M .
C o n d it i o n W t . L o s s 6, M g. D e p t h of P i t s ' , In. p H a t E n d
C h r o m a t e L e ft, P . P . M .
S y s t e m *A° F u l l “ *A F u ll •A F u l l •A F u l l •A F u ll
F e - C u N o n e Bad B a d 4 . 8 0 4 * 3 . 9 9 3 * 0 . 0 1 4 - 1 0 .0 1 2 * 5 . 8 8 . 0
1 000 G o o d Kxc. 0 . 9 8 0 * N o n e 0 . 0 1 2 “ < 0 . 0 0 1 » 8 . 5 7 . 8 5 0 8 9 7 2
Z n-C u N o n e B a d B a d 3 . 0 9 3 * 3 . 5 9 5 * 0,022<.>.‘ 0.020*.*.» 8 . 9 7 . 8
1000 F a ir G o o d 0 . 0 6 1 * 0 . 0 6 0 * < 0 . 0 0 1 “ N o n e 8 . 8 8 . 5 5 5 6 7 2 8
Z n -F e N o n e B a d B a d 3.325«' 3 . 0 5 9 * 0.022<*>.> 0 . 0 1 0 '. * 8 . 0 7 . 6
1 000 G o o d G o o d 0 . 0 0 4 0 . 0 3 4 < 0 . 0 0 1 “ N o n e 8 . 7 8 . 1 7 0 5 8*1*4
A l-C u N o n e B a d B a d 0 . 3 0 7 * 0 . 2 9 8 * > 0 . 0 3 * . » > 0 .0 3 < .‘.».» 7 . 2 7 . 8
1 000 B a d P o o r 0 . 6 7 5 * 0 . 2 2 8 * 0 . 0 1 5 » . ’ > 0 . 0 3 “ 8 . 9 8 . 9 N o n e 109
A l- F e N o n e F a i r / B a d 1 . 120» 1 .2 7 7 » < 0 . 0 0 1 ‘" < 0 . 0 0 1 » 7 . 1 7 . 3
1000 B a d E x o . 0 . 0 6 7 * N o n e > 0 , 0 3 “ . « . “ N o n e 9 . 0 7 . 6 7 4 8 8 8
F e o n l y N o n e B a d B a d 4 . 0 9 6 4 . 1 8 4 0 . 0 0 7 “ 0 . 0 0 9 * 5 . 2 7 . 8
1000 E x c t . E x o . 1 . 9 2 2 N o n e 0 . 0 1 4 “ N o n e 8 . 1 7 . 2 56 0 9 3 8
Zn o n ly N o n e B a d B a d 3 . 5 6 3 3 . 3 7 2 > 0 . 0 3 s'1 O .O I S W ,« 7 . 1 7 . 2
1000 a S u b m e r g e n c e.
G o o d E x o . 0 . 0 1 4 b I n c lu d e s b o t h m e t a ls.
0 . 0 0 5 0 . 0 0 5 “ N o n e 8 . 1 7 . 8 3 7 0 8 8 8
c M a x im u m d e p t h o f p its o n p a n e l, e x o lu s i v e o f e d g e or c o r n e r co rro sio n; s u p e r s cr ip t n u m b e r s i n d ic a t e pr incip al t y p e s o f c orrosion as l is t e d b e l o w ; w h er e th e r e is m o re t h a n o n e t y p e , t h e m o s t s e v e r e is reco rd ed first:
1 G en er a l co r r o sio n • E d g e co r r o sio n u C o r r o s io n nea r w a t e r lin e
J R o u n d e d p its 7 Corner co r r o sio n B l i s t e r p its
* W id e p its • P e r f o r a t io n of c o a t i n g u E r u p t e d p its 4 N a r r o w p i t s • P e r f o r a t i o n of p a n e l 14 G rain b o u n d a r y p its 5 E l o n g a t e d p i t s 10 C o r r o s io n a t b i m e t a ll i c c o n t a c t
d W e ig h t lo ss a l m o s t e n t ir e ly f r o m p a n e l. • I n c lu d e s w e i g h t lo ss a b o v e w a t e r lin e.
/ C o n d it io n o f iro n v e r y b a d . # W e i g h t Io s bc hiefly f r o m c o n t a c t m e t a l .
Table V . Effect of p H on Corrosion Score of System Iron-Copper ( T w o - m o n th e x p o s u r e o f ir o n p a n e l s in c o n t a c t w it h c o p p e r i n t a p writer, w it h o u t a n d w i t h c h r o m a t e , a t 1 6 0 ° F . , n o n a e r a t e d , q u ie s c e n t , f u ll s u b m ergence)
C h r o m a t e , P . P . M . p H Sco re
N o n e 7 . 5 6 0 (bad)
1000 6 . 5 91 (good)
1000 7 . 0 9 6 (exc.)
1000 8 . 5 9 6 (exc.)
Table I I I compares aerated and nonaerated system s. T he ef
fective inhibition afforded by chrom ate in aerated system s is of practical im portance, since m ost recirculated w ater system s are subject to some aeration. F or example, in an autom obile cooling system th e ra te of corrosion m ay be increased by th e entrance of air through fau lty pum p packing.
Submergence. Corrosion is m ore severe if th e m etal is not completely subm erged, and it is m ost noticeable near or ju st above the waterline. T able IV compares results obtained w ith some bimetallic system s, th ree fourths and fully subm erged. T he fact th a t some three-fourths subm erged panels show indication of cor
rosion in chrom ated m edia, which provide perfect inhibition for fully subm erged panels, cannot be ascribed to a higher concentra
tion of dissolved oxygen, because aeration has practically no ef
fect in th e presence of chrom ate. T he increased corrosion is prob
ably due to m oisture condensed on the m etal above th e w ater level, and th e m etal corrodes beneath this condensed w ater which contains no chrom ate, or there m ay be concentration cell effects ju st below th e w ater level. F or sim ilar reasons, atte m p ts to in
hib it corrosion of steel storage tanks, from which aviation gaso
line is displaced by sea w ater, have proved unsuccessful.
Effect o f pH . A t a concentration of 500 p.p.m . the p H of a sodium chrom ate solution is about 8.0, while th a t of th e bichro
m ate is about 5.5. M ixtures of chrom ate and bichrom ate have interm ediate pH . In this report concentrations refer to p a rts per million chrom ate equivalent (1000 p.p.m . N a 2C r 0 4 = 920 p.p.m . N a2C r207.2H j0 ), irrespective of w hether it is present as chrom ate or bichrom ate, unless otherw ise stated . Figure 1 shows pH v al
ues for practically all concentrations used for com bating corro
sion. D a ta were obtained w ith c.p. chemicals in B altim ore city w ater which had a pH of 7.5 and also in distilled w ater. T he chrom ate graphs for distilled w ater and ta p w ater cross a t about 500 p.p.m ., which is th e concentration m ost commonly used for corrosion control.
T able V shows the effect of raising th e pH from 5.5 to 8.5 for th e system iron-copper in w ater containing chromate.
T h e general effect of pH on other bimetallic system s m ay be estim ated from T able I.
In general, this laboratory has found th a t an initial pH of 7.5 to 8.5 is th e optim um range for m ost bimetallic systems, and th a t variations from pH 7.0 to 9.5 are unim portant.
T his m eans th a t for m ost pur
poses a fairly high pH , corre
sponding to th e chromate state, is a little m ore effective th a n th e equivalent am ount of hexavalent chromium in th e bichrom ate sta te (low pH ).
Since th e i n t r o d u c t i o n of sodium bichrom ate plus caustic soda is equivalent to th e use of s o d i u m c h r o m a t e , large users employ bichrom ate plus caustic ra th e r th a n chromate because of the lower cost and commercial availability of the bichrom ate (19).
Changes in pH during Exposure. Some observed pH changes during long exposures are shown in Tables IV and VI. Aeration m ay lower th e p H of an aqueous system , while solution of metal tends to raise th e pH . Furtherm ore, there is a liberation of alkali from chrom ate as it is reduced to th e triv alen t state, b u t the rate of reduction decreases as th e p H is increased. Because of these compensating factors, th e p H of a chrom ated system often tends to become stabilized w ithin a range where pH variations are un
im portant (7.5 to 9.5).
Figure 1. pH of Dilute Chromate and Bichromate Solutions (Beckman Meter)
Table VI. Effect of Silicate on Chromated Systems of 500 p.p.m . chrom ate plus 40 p.p.m . m etasilicate was m ore effec
tiv e th an 1000 p.p.m . chrom ate w ithout m etasilicate.
Chromate Concentration. T he effect of concentration of chro
m ate is show n in T able V II. F o r m ost ferrous system s in ta p
In th is case a v ery protective, fairly thick, dense, greenish coating m ay be form ed (Figure 2, panel 1). Several investigators (10) have reported t h a t th is ty p e of film is a hydrous com bination of iron and chromic oxides (triv alen t). I n these p a rtic u la r tests a well w ater was em ployed which h a d a slight sulfidic odor, and probably some reduction of th e chrom ate was caused by sulfidic
Table VII. Effect of Chromate Concentration on Appearance of Panels
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 745
H 5 B 1 1 7 A 3 2 4 1
Figure 2. Plant Exposure in a Moving Stream C h r o m a te ,
P a n e l N o . S y s t e m P. P . M.
1 1 5 B Fe-brasB N o n e
1 1 7 A F e -b r a ss 72
3 F e o n ly N o n e
2 F e o n ly 72
4 F e o n ly N o n e
1 F e o n ly 72
E x p o su r e T im e , W t. Loss,
M o . M g . C o n d itio n
1 1 . 1 3 6 B a d
1 0 . 0 4 8 G o o d
1 0 . 7 3 4 B a d
1 G a in G o o d
2 1 . 4 5 4 B a d
2 G a in G o o d
compounds. U nder these conditions a concentration of 72 p.p.m . chromate provided excellent protection, even when in contact with brass (Figure 2, panel 117A). I t m ay be th a t th e presence of sulfidic m aterial favors th e form ation of dense protective coat
ings, since low concentrations of chrom ate have been found effec
tive for gas holders, as described late r in this report.
Cathodic Protection. Sometimes it is desirable to reduce the corrosion of a m etal in an aqueous m edium b y placing in contact with it, another m etal which will m ake it cathodic. T his kind of beneficial bim etallic action, which protects by th e sacrificial dis
solution of an anodic m etal, is known as cathodic protection (4) ■ Examples are Alclad (1) alum inum alloys, galvanized ice cans, zinc plates near a ship’s propeller, tinned copper, etc. Although chromate functions chiefly as an anodic inhibitor, it is effective a t low concentrations (less th a n norm ally required for complete inhibition) in assisting a cathodic inhibitor. F o r instance, as Table V III shows, th e corrosion resistance of galvanized iron was improved by th e presence of a very sm all am ount of chromate.
Figure 3 shows another example of th is type of dual pro
tection for bare iron panels in contact w ith zinc, in ta p w ater
a t 70° F. for tw o years, with and w ithout various am ounts of so
dium chrom ate. W hen no chrom ate was present, the cathodic protection due to th e zinc was insufficient to prevent corrosion (panel 12); b u t, when 62.5 p.p.m . chrom ate was introduced, the inhibition of th e iron panel was alm ost perfect (panel 18). In another series of tests (Figure 4), identical in all respects except for contact w ith zinc, 62.5 p.p.m . chrom ate were insufficient to prevent corrosion of th e iron (panel 14).
Effect of Areas. All d a ta in this report are based on a ra
tio, of subm erged panel area to th a t of th e dissim ilar m etal w ith which it is in contact, of 6 to 1. In general, th e smaller area is th a t of th e cathodic m etal; however, there are some examples where th e anodic m etal is th e smaller. I n th e tabulations th e panel m etal is shown first, followed by th e sm aller contact m etal.
T he subm erged areas of all panels were 12 square inches, and th e volume of th e liquid was 8 ounces.
As would be expected, th e effect of increasing th e relative area of th e cathodic m etal is to increase th e corrosion of th e anodic m etal; for example, increasing th e area of copper in contact with an iron panel will increase th e corrosion of th e iron, while
increas-N o chromate 6 2 .5 p.p.m. chromate 1 8 5 p.p.m. chromate 8 5 0 p.p.m. chromate
Figure 3. Iron Panels In Contact with Zinc after Three-year Exposure in Tap Water at 7 0 ° F.
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 37, No. 8
Table VIII. Progressive Exhaustion, of Chromate and Changes in A ppearance of Liquid in Tap Water Containing Low Concentrations
of Chromate elim inate dissim ilar m etals, b u t th is is often im practical for struc
tu ra l reasons or because of existing installations. A p a rtia l rem required depends on individual factors, chiefly as follows: am ount, n atu re, and location of dissim ilar m etals; ch aracter of w ater, in m aintaining th is concentration, since consum ption of chrom ate in th e chem ical protection of th e m etal surface is lowest a t a fairly high chrom ate concentration. If th ere is considerable overflow or o th er loss of liquid from th e system , th e n th e resulting m e
chanical loss of chrom ate m ay be sufficiently im p o rta n t to m ake it advisable to lower th e chrom ate concentration by small am ounts down to th e m inim um concentration a t w hich it is ef
fective; b u t it is seldom desirable to operate below 200 p.p.m . since th is entails frequent testin g and is less effective.
U sually th e concentration of chrom ate m ay be ju d g ed by
August, 1945 747 often are desirable as a factor of safety to compensate for acci
dental losses, presence of dissim ilar m etals, etc. If m etals such as brass are unavoidably exposed, th e concentration of chrom ate should be th e highest practicable. Sodium chloride brine re
quires about twice the concentration of bichrom ate as calcium chloride brine, to provide satisfactory protection (especially if iron is exposed) ; however, less chrom ate is consumed in th e pro
tection of th e zinc surface.
Although optim um proportions vary for individual conditions, the general recom m endation of th e American Society of Refriger
ating Engineers (2) is to use about 100 pounds of N a2C r20 j.2H 20 per 1000 cubic, feet of calcium chloride brine, or 200 pounds for sodium chloride brine. These proportions correspond for calcium chloride brine to 1600 p.p.m . bichrom ate ( 1750 p.p.m . figured as chrom ate); for sodium chloride brine, 3200 p.p.m . bichrom ate (3500 p.p.m . figured as chrom ate). The alkalinity is adjusted by addition of caustic soda, the precise am ount depending on indi
vidual conditions. Present practice of several eastern ice plants is to use a little higher concentration for calcium chloride brine—
namely, 125 pounds bichrom ate per 1000 cubic feet (about 2000 p.p.m .); for sodium chloride brine, 200 to 225 pounds bichro
m ate. T his treatm en t has been in use for a num ber of years, and has been found to keep th e cans, steel work, and pipe work in the brine system s in alm ost perfect condition.
C hrom ate has been found effective also for com bating the corrosion of some alum inum system s in strong calcium chloride brine and for preventing th e dezincification of brass' in ammonia- containing brines {18). This laboratory also has obtained inter
esting results w ith sparingly soluble complex zinc chromâtes (24) for inhibiting th e corrosion of galvanized system s in calcium chloride brine.
As in air conditioning, chrom ate is used to prevent corrosion on the w ater side of amm onia condensers, when recirculated cooling water is employed. T his elim inates th e form ation of thick rust layers and slime deposits, which increase the condensing pressure and cause decreased p lan t efficiency and lowered ice capacity.
To x i c i t y o p Ch r o m a t e. B ichrom ate or chrom ate m ay have an irrita n t action if exposure is prolonged. Covering of existing skin lesions and simple washing, precautions tak en w ith alm ost all d ry chemicals, suffice to elim inate difficulty from this source.
T he dilute solutions employed for corrosion control present no appreciable hazard, although it is self-evident th a t they should not be swallowed or allowed to remain in contact with the body for long periods of tim e. C arry-over into th e air from the spray cham ber of an air conditioning u nit is prevented by built-in baffles. T he m axim um permissible carry-over is the equivalent of 0.1 mg. CrOs per cubic m eter of air (14), which value is not attain ed in practice. If it is desired to detect th e presence of chrom ate, th is m ay be done by noting any yellow color im parted to a piece of bleached m uslin hunfe over th e air-discharge grill;
the precise am ount m ay be found by using an im pinger tu b e (13), followed by colorim etric determ ination w ith diphenyl carbazide.
Refrigerating Brines ( 8 ) . Ice cans are usually m ade of gal
vanized steel; tan k s an d pipe work m ay be galvanized or made of b;ire iron or steel. On galvanized work some of the underlying ferrous-zinc alloy m ay be exposed. Bronze or other dissimilar m etals m ay be present , b u t in general th e brine system is chiefly zinc-iron. T he principal brines are 20 to 25% solutions of calcium chloride or sodium chloride. A t these concentrations these salt solutions are som ew hat less corrosive th a n a t lower concentra
tions. However, a t all concentrations th eir a tta c k on m etal is severe, and th e cost of replacing ice cans is a serious item in the upkeep of many, ice plants, while th a t of repairs and replacem ent of brine piping and pum ps is often large in cooling and freezing plants. T he severity of corrosion depends on th e m etals in con
tact, th e n atu re of the brine, its oxygen contení, tem perature, and pH, and th e presence of inhibitors. W hen chrom ate is used, the presence of dissolved oxygen has no effect.
For an all-galvanized system , w ith no dissim ilar m etal in con
tact, 500 p.p.m . bichrom ate affords satisfactory protection in cal
cium chloride brine under ordinary conditions, provided the pH and chrom ate concentration are adjusted from tim e to tim e to replace th a t consumed in th e chemical protection of the metal surface or th a t m echanically lost. U nder p lan t conditions th e
half-N o chromalc 6 2 . 5 p . p . m . chromate 1 2 5 p .p .m . chromate 2 5 0 p .p .m . chromate
Figure 4. Iron Panels with N o Other Metal after Three-Year Exposure in Tap Water at 7 0 ° F.
life of th e chrom ate content of a brine is about fpurteen m onths, which m eans th a t about 800 p.p.m . bichrom ate should be the initial concentration in order not to drop below 500 p.p.m . w ithin six m onths. A nother m ethod of m aintaining the chrom ate con
centration is to add bichrom ate whenever it is necessary to add calcium chloride to m ake up for th e dilution caused by condensa
centration is to add bichrom ate whenever it is necessary to add calcium chloride to m ake up for th e dilution caused by condensa