Chromate Corrosion Inhibitors MARC DARRLN, Mutual Chemical Company of America, Baltimore, i?ld.
siniultaneous loss of chromate, i t is most economical to employ the lowest concentration of chromate which effectively inhibits corrosion. It is better, however, to employ too much than not enough, since excess chromate not only increases the safe limit of salt concentration but in general reduces the amount of chromate consumed in the protection of met'al surfaces. For this reason it is best to start the chromat,e treatment with a Comparatively high concentration, 500 to 1000 p.p,m., and gradually reduce t,he amount of chromate over a period of months t o the lowest concentration a t which the part,icular installation is effectively protected against corrosion. By way of illustration, let u b assume that the cvaporativc loss from a recirculating cooling system is 1000 gallons per day, that the raw Lmter cont,ains 100 p.p.m. of sodium chloride, arid tk1a.C
OETHELI (8), Speller (9),and others (2-6) have slio\i-ii that, chromates and bichromates effectively inhibit the corrosiori of iron in chloride-containing water. In closed water systems, such as are used for cooling locomotive Diesels ( 7 ) ,there is almost no mechanical loss of water, and it is not important to employ low concentrations of chromate; in fact, i t is more convenient to use a comparatively high concentration (4). With open cooling towers and spray ponds which are employed for air conditioning and many industrial purposes, the situation may be very different. In general, somewhat loTver concentrations of chromate are used, the optimum varying for individual installations, When there is little mechanical loss and the water contains much chloride, periodic drawoffs must be made t o waste in order to kecp the chloride rontent low; sinci: t h i h causes a
1
Atmospheric Water Cooliiig Tower Constructed for a: Large Oil Company (Courtesy, Fluor Corporation)
368
consumed per 1000 square feet of completely submerged ferrous surface for various periods a t room temperature (70' F.), aerated, with the pH maintained a t 7.5 to 9.5. Within this p H range there is no measurable change in corrosion rate. At higher temperatures the consumption of chromate is somewhat increased. With partial submersion, there may be greater corrosion at the water line, but this can be avoided by proper design or heavy painting of water-line areas which are seldom located where painting would hinder heat transfer. Figure 1includes examples of a good water containing 10 p.p.m. of sodium chloride, a fair to poor water containing 100 p.p.m., a very bad water containing 1000 p.p.m. such as might result from Concentration in a n evaporative cooling system, and an extreme condition, 10,000 pap.m. of sodium chloride. From the practical viewpoint these graphs show that in all cases the rate of consumption of chromate is relatively high at first but rapidly falls off until it is stabilized within 1 to 3 months. In general, the consumption of chromate is highest at a concentration of about 50 p.p.m. of chromate and lowekt a t about 500 p.p.m.; for practical purposes there is little difference, after conditions have been maintained for 3 months, with chromate concentrations from 250 to 1000 p.p.m. The general effect of chloride is t o increase the consumption of chromate, especially during the first 10 days while the protective film is forming.
the maximum concentration of chloride permissible in the system is 1000 p.p.m. It would be necessary to replace the evaporation with raw water and to run to waste 100 gallons per day, in order not to exceed a concentration of 1000 p.p.m. of sodium chloride. Since this 100-gallon drawoff contains the maintained concentration of chromate-for example, 250 p.p.m. (0.025%)-it would be necessary to add an equivalent amount of chromate to the makeup water. I n this case: 0.025 X 8.33 = 0.21 pound of chromate per day. Since the volume of the make-up water is ten times that of the drawoff, the required concentration of chromate in the make-up would be 25 p.p.m. in order to maintain 250 p.p.m. of chromate in the circulating system. The same relative proportions apply to other additives, such as caustic soda t o control alkalinity. The general method of calculating drawoff and chromate addition is similar for all manner of open coolers, spray towers, cascade towers, natural draft towers, mechanical draft towers, evaporative condensers, spray ponds, natural ponds, or any other device which operates through atmospheric evaporation (1). ESTIMATION O F CHROMATE CONSUMPTION
I n addition to the chromate which is run to waste, some is consumed in forming a protective film. This may be estimated (7') by means of Figure 1 which shows pounds of sodium chromate T h e chromate-inhibition of monometallic ferrous systems is reported in the presence of chloride, in amounts commonly encountered in water-cooling systems. Data show the rate of consumption of chromate a t different maintained concentrations and changes in the rate with passage of time. The advantage is shown of starting with a comparatively high concentration of chromate and maintaining it until the protective film is stabilized. This procedure is particularly applicable to cooling towers or spray ponds where part of the recirculated water is run to waste to avoid excessive c o n c e n t r a t i o n o f mineral salts. Graphs are included which will be of practical assistance to the engineer. Incidentally, the data may prove helpful to a better understanding of the dtzal nature of passivation and thus lead to further technological improvements.
.
TABLE I. COMPARISON OF TESTPANELS Max.
Panel No.
1 3 5 7 9
NaCI, P.P.M.
10
100
13
a
b C
0
25 50
11
15 17 19 21 23 25 27
100
29 31 33 35 37 39 41
1,000
43 45 47 49 51 53 55
10,000
Depth of Pitsa, In.
Chromate, P.P.M.
.
0,004 0,008 0.003 0.004
250 600 1000
None None None
0 25
None 0 015
0.009
60
None None None None
100 250 500 1000
None
0 25 50 100 250 500 1000
>5 >5 >5
5 4 1
0 0 >5 >5 >5
5 4 2
0
0,009
>5
0.017 0.002
>5 >5 3 5 >5
None
0.004 0.005
Exclusive of pits on or near edges. Inclusive of edge pits. Calculated from weight loss.
369
No. of Pitsb
Penetrationc, In. per Year
'
Score
Fe Equiv. Panel of ChroCondimate tion Consumedd
23 '72 75 85 90 97 100
Bad Poor Fair Good Good Exc.
0,00444 0.00124 0.00082 0.00009 , 0.00008 0.00005 0.00004
62 53 79 80 85 90
Bad Bad Bad Fair Fair Good Good
0.00711 0,00087 0.00090 0.00044 0.00022 0.00014 0.00012
67 63 58 70 78 78 83
Bad Bad Bad Poor Fair Fair Fair
0.01346 0,00020 0.00027 0.00005 0,00001 0.00000 0.00000
60
Exc.
Grams Fe Oxidized by Air
None
5 21 0 08
0.02
0 09 0 01 0 00
0.02
0.01 0.00 0.00 0.00 None
0.04 0.04 0.01
0.01 0.00 0.01
None
0.05 0.06 0.03 0.02 0.02 0.01
d 324.0 grams NazCr01 consumed (reduced t o trivalent state) is equivalent t o the oxidation of 111.7 grams F e to FepOs.
0.00 0 00
1 72 0 55 0.34 0.03 0 02 0 02 0 01
2.75 0.37 0.37 0.15 0.06
0.03 0.03
INDUSTRIAL AND ENGINEERING CHEMISTRY
370
From Figure 1 and the daily dranoff, the most economical chromate concentration may be estimated-namely, the concentration a t which the sum of the chromate drawn off and that consurncd has a minimum value. It is not advisable ordinarily to maintain chromate concentrations below 100 p.p.m. in the circulating system. Furthermore, it is best to start the tieatment with a comparatively high concentration of chromate (500 to 1000 p p m . ) and later loner the concentration az the protective film becomes stabilized. If conditions encounteied in practice U I e ’ more corrosive than those on ~ l i i c l i the charts are based (due to higher tcmpcrature, bimetallic contacts, old lust, ttc.) the figures obtainpd from the curves may be multiplied by a suitable bafety factor Excess chromate is desirable also bccause there may be a small additional consumption of chromate by rcducing substances which may be present in the raw water or i n the air with nhich it come9 into contact. KO correction is required for leakage and similar mechanical losses of cliromatc (provided they do not exceed the calculated drawoff) since these losses ale compcn~atetl in practice by a reduced drax OB.
-
Vol. 38, No. 4
------
--c
I
-
--
DAYS
EXPERIMENTAL PROCEDUll E
PAXELS.Test specimens having a snrface area of 12 square inches were c u t from a mild steel plate known as type A rank (about 0.1% carbon). All were cut from the same sheet and were uniformly polished, cleaned, and inspected for defects prior to testing. At the end of the exposure and before cleaning, photogiaphs were taken (Figure 2) and corrosion scorcs obtained : Designation Perfect Excelleiit
Score 100
Above 95
Good
85 t o 95
E air Poor
75 t o 84 65 to 74
Bad
Less than 65
_ _
. .
_-_
-
-
Degi eo of Coil osion
S o indication hIinor,but vel>
satisfactory Definite, hut satisfactoi y
Questionable Probably unsatisfactory
I
-
+ + -
Seveic
The SCOICS are the composite obaeivations of foul men. There wexe n o significant differenccs between the scores reported by different men or between duplicate panels. After the exposed panels were cleaned with a soft bristle brush, theii weight loss and condition were evaluated, additional photographs were taken (Figures 3, 4, and s),and depth of pits were ineawred optically. MEDIA. Twenty-eight media 1% crc employed (Figure 2 ) . Large amount, of these media were prepaied in advance, the pH was adjusted to 8.0-9.0, rind cliromate analyses were run by precise clwtrometric titration. The fiist sales (10 p.p.m. sodium chloride) 11aa prepared from Baltimore city watei which was
10
20
30
40
80
60
70
80
80
100
110
120
130
b40
I50
DAYS
Figure 1. The Cuniu1aLiv.eConsumption o f Chromate per l O O D Square Feet b y Waters Containing Various Aniounts of NaCI
60
170
IBO
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
April, 1946
-
371
found to contain this amount of chloride a t the time the solutions were prepared. The other three series were prepared by addition of calculated amounts of C.P. sodium chloride to Baltimore city water. EXPOSURE.Duplicate specimens were exposed in each medium a t room temperature (70" F.). Individual jars were employed for each panel, which was completely submerged and aerated with a n excess of water-washed air. Other details were the same as previously described (4,6). ANALYSES.During the period of exposure, a t frequent intervals so as to avoid important changes in chromate concentr& tion, the panels were transferred to other jars containing fresh media, and the old media analyzed. The difference bctwecn the total chromate in each jar, before and after eaeh transfer, was recorded as the amount consumed. At first the chromate consumption was so rapid that daily analyses were required. Analyses for chloride and pH determinations showed no ap.preciable change, and these data are omitted. The fact that chloride analyses were constant precludes the possibility of any appreciable loss ~f chromate other than on thc ferrous surface. EFFECT OF TIME
10
80
30
40
50
60
70
BO
SO
DAYS
100
110
180
130
140
150
160
170
180
Figure 1 shows the cumulative consump tion of chromate for the odd-numbered panels by daily increments up to 182 days (6 months). Similar data, obtained for the duplicate even-numbered panels, are not shown. As a result of minor differences during the first few days the curve level of most of the duplicate panels was either a little above or below those shown in Figure I, but their relative positions and shapes were the same (Figure 6). During the first 2 weeks a great many more points were obtained than are indicated on the graphs, They are omitted for clarity, but each is included as an increment of the cumulative curve. For instance, the solid triangular point for 10 days and 50 p.p.m. chromate (Figure 1, graph for 10 p.p.m. chloride) is the sum of the consumption during each of the previous days. Although all points are satisfactorily close to the smoothed-out curves, few points are precisely on the curves. There appears to be a periodic trend for points to fall above the curve and then below, the deviation becoming less with lapse of time. During the first few days these deviations were so great that increments for a n upper curve were sometimes less than for a lower curve. For this reason misleading conclusions might be drawn by comparing rates based on analyses a few days apart. Instead, conclusions should be based on the slope of the smoothed-out curves. Inspection of the points on these graphs indicates that it is impossible t o establish the average shape of the curves without data extending over 30 to 90 days. Data beyond 90 days are chiefly confirmative for
INDUSTRIAL AND ENGINEERING CHEMISTRY
312
c
0
25
NaKrOd Concentration, P.P.M. 50 100 250
500
Vol. 38, No. 4
the systems studied. Other data indi. cate that there would be no important change in the corrosion behavior of these systems after 6 months, up to at least 5 years ( 4 ) .
1000
NaCI, P.P.M.
TYPES O F CORROSION
Figures 3, 4, and 5 show the appcarance of the odd-numbered panels after 6-month exposure and cleaning. The reverse sides of the panels are essentially the same. The appearance of the cvennumbered duplicates was also thc same. The original identification marks XI hich w r e lightly scratched are still vi4hle on most of the panels which were exposed in chromate-containing media. The control panels exposed without chromate were more severely corroded than Figure 3 shows, and the original markings were completely obliterated. Weight losses were exceedingly high, about ten to one hundred times greater than iyith the lowest concentration of chromate (25 p.p,m.). Without rhromate the most severe corrosion occurred with the lowest chloridc concentration (10 p.p.m.), the opposite of what happened when chromate was present. Types of corrosion are indicated by the following numbers for each panel (Figures 3, 4,and 5 ) :
10
100
1,000
0 = no corrosion 1 = general corrosion 10,000
2 = rounded pits 3 = wide pits .
4 = narrow pits
5 = elongated pits 6 = edge corrosion 7 = corner corrosion
Figure 2.
Condition of Iron Panels after.6-Month Exposure at 70' F., Total Submergence, Aerated, pII 7.5-9.5
Figure 3.
Control Panels (No Chromate)
The order of the numbers designates the order of prcdominance of various types of corrosion. Table I lists tho number and depth of pits, penetration per year, corrosion score, and panel condition. It also shows the iron equivalent of the total chromate reduced (consumed) during the 6-month period. The difference bet'ween t,his figure and the weight loss represents the amount of iron oxidized by the air used for aeration; it indicates that t,here is a small amount of atmospheric oxidation even in the presence of chromate, although t,he amount so oxidized is almost negligible compared to the amount of atmospheric oxidation (corrosion) when no chromate is present. For instance, with panel 1 (Table I) in Baltimore city n-ater containing no chromate, the amount, of iron oxidized by atmospheric oxygen in 6 months was 5.214 grams as compared to 0.079 gram (panel 3) when a concentration of 25 p.p.m. of sodium chromate was maintained.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1946
EFFECT 0F.CHLORIDE
Figure 7 shows the effect of chloride concentration on the consumption of chromate per 1000 square feet of ferrous surface, for Various concentrations of chromate after &month exposure. The four curves are given similar inflections so as to pass through as many experimental points as possible; they pass through all points except the one indicated by a n arrow. A duplicate exposure, shown below the arrow by a solid triangle, is on the curve. "he curves show a rapid drop in the amount of chromate consumed as the concentration of chromate was increased from 50 t o 100 p.p.m., a tendency to flatten between 100 and 250 p.p.m.,
373
and a rapid drop t o a minimum at about 500 p.p.m. except for the highest chloride concentration which tended t o continue dropping with increase in chromate concentration. For less than 50 p.p.m. the consumption of chromate dropped, but corrosion became quite noticeable; however, the amount of corrosion was much less than without chromate. Figure 8 shows the effect of time of exposure on the consumption of chromate, for various concentrations of chromate in the presence of 100 p.p.m. of sodium chloride. These curves follow the same form as those of Figure 7. After 10 days under the conditions specified, the consumption of chromate was almost stabilized for concentrations of 500 to 1000 p.p.m.; after 30 days i t was fully stabilized for 500 p.p.m. and about 90 days were required for lower concentrations of chromate. DU4L SATURE OF PASSIVATION
25
P.P.M. NarOOl
66-3
Panel N o
consumea, Weiaht 1()ss, mg. Corrosion types Score Panel condition
mg.
=I
95 2-6-1
72 Poor " h i
-- _-
66-1 7
fifi-R 1
?!E ILi
iOo0128
589 2-5-3-1-6 53
56 1 3-5-1-2 58
145 416 3-5-6-1 63
Bad
Bad
Bad
fifi-AS
10,000
r
-*l
24Ch
A'
b ?
1
*
Panel So. NaCI, p.p.m NazCrO: consumed, mg. Weight loss, rng. Corrosion types Score Panel condition
66-5 10
(36-19 100
66-33
71
123
382 5-3-2-6-1 60
177 439 3-5-1-6 58
66-47 10,000 174 433 3-5-1-2-6 58
Bad
Bad
Bad
66-21
66-49 10,000 77 * 176 3-1 70 Poor
106 2-1-1-5 75
Fair
-6
1000
Panel No.
66-7 10
100
~ % $ ; p c ~ s u m e d , mg. Weight loss, mg. Corrosion type8 Score Panel condition
24 19 6-2 85
39 36 6 79
66-35 1000 52 65 3-2-1 77
Fair
Fair
Good
Figure 4.
Effect of Low Concentrations of Chromate
These data provide evidence that the mechanism of chromate inhibition has a dual character. The initial, almost instantaneous passivation or anodic polarization of the metal surface is folloncd by the slow formation of-a more protective layer which becomes stabilized within 30 to 90 days under the conditions described. For the most part this film is invisible, but in time it may impart a color t o the surface. This evidence tends to reconcile the two general theories regarding the mechanism of passivation: (a) induced changes in the nature of the metal surface and ( b ) oxide film theory as anticipated by Faraday and later observed by Evans and co-workers (2, 6, 9). Both mechanisms may be essentially correct and may occur in the order named. Whatever the nature of the initial induced changes which cause a barrier in or on the metal surface, this barrier (be i t force or molecular) is slowly reinforced by deposition of hydrous ferric-rhromic oxides. During the time this outer layer is forming, there is a comparatively rapid consumption of chromate (reduction from hexavalent to trivalent). It is during this period that localized corrosion may occur if the chromate concentration is low and the chloride high. Afterward there is no corrosion of practical importance with a maintained chromate concentration above 100 p.p.m., nor is there much harmful corrosion with a maintained chromate concentration as low as 25 p.p.m. The foregoing is based on behavior a t 70" F. As would be expected, some preliminary tests showed that the protective layers formed more rapidly a t higher temperatures.
INDUSTRIAL AND ENGINEERING CHEMISTRY
374
Apparently, with concentrations of sodium chromate betwvucn 100 and 500 p.p.m. the induced changes on the metal surfaccz: are uniform, but’ during the inceptive period there may be sonic unevenness in the deposition of the outer film. Presumably, after building to a certain extent, it ia ruptured and a little iron dissolves from or tlirougli thc inner barrier (which is probably anodic t o the outer layer); this dissolved iron, together \Tit11 simultaneously reduced hydrous chromic oxide, is deposited to rebuild the outer layer. This mechanism repeats itself in 5 cyclic manner with decreasing amplitude and frequency with lapse of time until conditions are substant,ially constant. During the inceptive period it is to consider that the iron d directly from the surface of thc metal than through a special kind of oxide film, from which it follo~vs that the initial passivation probably resides in some induced changes in the nature of the metal surface rather than in a highly resistant oxidc Iayw. The oxide layer demonstrated 1)s Evans appears to be a product of the second stage which starts very quickly since the first stage is completed almost instantly. As the concentration of chiomate was increased from 50 t o 500 p.p.m., Panel T o . there was a rapid decrease in thc NaCl p.p.m. N a z C h consumed, i n g . rate of ieduction of chiomntc and Weight loss, mg. Corrosion types the rate of oxidation of iron. This Score means that the induced changes .on Panel conditioii or in the metal surface are more pelfect Kith increase in chromate concentration. When the chromate concentlation was low (25 to 50 p.p.m.), the behavior fluctuated widely during thc inceptive period, apparently as a rcsult of imperfect or uneven primari passivation of the metal sui facc; thiz condition became stabilized n ith time but more slowly than TI ith liighei concentrations of chromate. T h r total amount of iron oxidized \\as of thc Bame order for chromate cnncentraPanel No. NaC1, p.p.ni. tions of 25 and 50 p.p m., although NapCrOl coiiwrned. inz, the reduction of chromate n a s usuWeight loss, mg. Corrosion types ally a little less, and the proportion Score Panel condition of iron oxidized direetly by air a little greater, for 25 than for 60 p.p m. of chromate, There is some additional but less definite evidence that, when the chromate concentration is high (above 500 p.p.m.), the opposite effects occur. With a chromate concentration of 1000 p.p,m. thcre n a s almost no fluctuation from the start, although the total amount of iioii oxidized was usually slightly le-5 than n i t h 500 p.p.m. of chromatc, the reduction of chromnte \I a < wmc what greater. SUMMARY
Experimental data presented here are consistent with a dual theory for the mechanisnl of passivation. The nature of the time functions a p p e a ~ st o
Pmiel Xu. TiaCl, p.p.ni. NanCrOI consumcii, Weight loss, nig. Corrosion types Score Panel conditioii
4
preclude 5 simpler mechanism, hut tkic possibility or a more cornplex mechanism is not excluded. Fundamental data (Figure I) show ihat, wvlien chromatc is used to inhibit corrosion in watcr systems under the conditions specified: (a) Rat,e of consumption of chromate is relatively high a t first, but rapidly falls otf until i t is stabilized within 1-3 months. ( b ) Consumption oE chromate is highest a t n concentration of about 50 p.p.m. chinmatt: and lowest a t aboul 600 p.p.m. (c) There is little difference, al‘tcr conditions haw: been maintained for 3 months, with chromate concentrations from 260 arid 1000 p.p.m. (tl) the chloridc content of t h o
GO-11
to 1 1
(3-25 100 11 18
G
Ci ’37
8.5
EXC.
Good
GG-13
68-27
100 I ~ P .
Vol. 38, No.
A00 0 .0
20.1
0
7-6
100 Perfect
00
Figure 5 .
13.5
Good
(io-8‘3 1000
2’3 37 fi- 7 85 (;ooil
lOOC! 33.2 9 . !J o;7 P r) Good
(Xi 3 10,000
.3--3--ii3”
t?
i8
Fair
10.000
:3-7-(?
Effect of High Concert trations of (:hrornate
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1946
315
Figure 6. Duplicability of Results f
Figure 7. Effect of Chloride after 6-Month Exposure t
9
I \Ill
I
DAYS
tems such as are commonly employed for air conditioning and o t h e r cooling p u r poses.
!
Figure 8. Effect of Time in the Presence of 100 P.P.M. Sodium Chloride
ACKNOW LEDGR.1ENT
ys 182
ao
The author is indebted to 0. F. Tarr for many helpful suggestions in the planning and interpretation of results; and to J. W. Fankhanel, of the Research & Development Department, for the preparation of panels, analyses of media, and other details of the 6-month exposures. Corrosion scores were checked by W. H. Hartford and R. L. Costa. The author wishes t o take this opportunity of thanking F. N. Speller and S. T. Powell for reviewing this report.
80
IO
LITERATURE CITED 100
PO0
300
400
500
600
700
800
N a p r 0, p ~ m .
water is increased, the consumption of chromate increases, especially during the first10 days while the protective film is forming. ( e ) Rate of consumption of chromate fluctuates in a cyclic manner, but the amplitude and frequency decrease with lapse of time until conditions are substantially constant. These data have important practical applications. They show that it is most advantageous t o start treatment of a water system with a comparatively high concentration of chromate and t o maintain such a concentration until the protective film is formed. This concentration may be lowered later. An example was given to illustrate how the optimum concentration of chromate may be estimated for inhibiting corrosion in recirculating water sys-
800
1000
(1) Am. Soo. of Heating and Ventilating Engrs.,
Heating, Ventilating, Air Conditioning Guide, 1945,23rd ed. (2) Bauer, O., “Korrosion metallischer Werkstoffe”, Vol. I, Ann Arbor, Mich., Edwards Bros., 1943. (3) Darrin, Marc, Am. Sot. Refrigerating Engrs., Corrosion Rept., pp. 21-7 (1944). (4) . ~ ~M ~ ~ ~ ~ i ~~ cHEM.,’37, ,, 741-9 (1945). (5) Darrjn, Marc, IND.ENO.CHEM.,ANAL.ED., 13, 785-9 (1941): Proc. Ann. Water Conf., Eng. Sac. Western Penna., 2, 59-69 (1941). ( 6 ) Evans, U. R., “Metallic Corrosion, Passivity and Protection”, London, Edward Arnuld & Co., 1938. (7) Mutual Chemical Co. of Am., “Chromate Corrosion Inhibitors
for Internal Combustion Engines” (in press). B.E., and COX,G. L., IND. ENG.CHEM.,23, 1084-90
(8) Roetheli,
(1931). (9) speller, F. N., ll~orrosion-causes and Prevcntiol19,,2116 New York, McGraw-Hill Rook Co., 1935.
