R. J. AGNEW and J. K. TRUITT Texaco Research Center, The Texas Co., Beacon, N. Y.
W. D. ROBERTSON Hammond Metallurgical Laboratory, Yale University, New Haven, Conn.
Corrosion of Metals in Ethylene Glycol Solutions Data will a i d in interpreting service performance, but not necessarily in predicting complete service behavior. The more important chemical variables glycol concentration, pH, oxygen concentration, chloride ion concentration, and temperature - w e r e investigated, both with and without solution renewal
-
This
investigation represents part of a systematic study of corrosion by aqueous ethylene glycol solutions of the metals and alloys used in internal combustion engines. The more important chemical variables, including glycol concentration, pH oxygen concentration, chloride ion concentration, and temperature, were studied. Two general procedures were used: constant composition tests in which the solutions were continuously replaced, and static tests in which the solutions were not renewed, corresponding to the closed automotive system. The results will not make it possible to predict complete service behavior but will aid in interpreting service performance.
Gesellschaft fur das Studium der Motorbrennstoffe (7). Wormwell and his associates (71) have made an extensive study of sodium benzoate and sodium nitrite as corrosion inhibitors for ethylene glycol, and Caplan and Cohen (2) have shown that corrosion rates of some metals may be accelerated a t low temperatures in certain inhibited glycol systems. Finally, Twiss and Guttenplan (9) have investigated the corrosion of a n aluminum alloy as a function of velocity and impurities in water used to make u p glycol solutions. Aside from general articles on the care of cooling systems and patents
on corrosion inhibitors, these publications appear to represent the extent of available information on corrosion in ethylene glycol solutions.
Experimental Procedure Test Cell. Corrosion rates were determined by two general procedures : In one, the composition of the solution was held constant throughout the test period by continuous passage of unused ethylene glycol solutions, saturated with air, through the cell; in the other, 150 ml. of solution remained in the cell and
SOLUTION
J GAS
IN
1955, approximately 888,000,000 pounds of ethylene glycol (70) was produced in the United States; 75%, or about 70,000,000 gallons, was used as one component of antifreeze in automotive cooling systems. A comprehensive review of automotive antifreezes (7) lists 170 references, of which only one is specifically concerned with corrosion in ethylene glycol solutions. I n 1948 Green, Kratzer, and Emch (3) published experimental data on corrosion rates of metals in a variety of unspecified inhibited ethylene glycol solutions. In 1949 a n extensive review of the physical and chemical properties of various antifreeze solutions, including experimental data on corrosion rates and inhibitors, was published by Schweiz
Test cell incorporating provision for renewal of solution through central capillary and circulation of solution b y means of air lift
ION
VOL. 50, NO. 4
APRIL 1958
649
E 100 lJY
p
80
60 40
20
0 TIME, HOURS
Figure 1 . Time dependence of corrosion of metals in 40% ethylene glycol-water solution a t 160" F. and p f l 7
TIME, HOURS Figure 2. Corrosion of copper and brass under conditions of no solution renewal and solution flow through test cell at 160" F.
was continuously circulated and aerated during the test period. T h e test unit was a n adaptation of one described by Robertson ( 5 ) . Test solution flowed from a 5-gallon glass reservoir through a precision-bore Flowrator tube and into the test cell through the central glass capillary, which also supported the test specimens. The rate of flow, defined by the requirement of constant composition, was maintained a t 5 ml. per minute, which maintained p H within f 0 . 5 unit. Test solution overflowed continuously through a side a r m tube and was collected for future recovery. Gas entered the cell through the circulating air lift connected to the bottom and side of the cell. The air lift keot the solution saturated with pas without direct impingement of gas bub-
Table I.
Chemical Analysis of Test Metals (Per cent by weight)
Metal
-
650
bles on the test specimens; it also provided circulating flow of solution past the specimens. The temperature of the test cell was maintained constant, A z o F., by immersion in a suitable constant temperature bath (oil or water). In tests without continuous replacement of solution, a cell nithout overflow side arm was employed and the specimens were suspended from a glass rod. The gas exit was connected to a 2-foot water-cooled condenser to prevent loss of water vapor or glycol oxidation products. Laboratory compressed air used for saturation of the solutions was purified by passing it through a glass wool filter and through 20% caustic solution, followed by a water scrubber. The flow of air was metered by a U-tube manometer and was controlled a t 100 ml. per minute by a Hoke needle valve. For tests in the absence of oxygen, tank nitrogen was purified by passage over reduced copper powder (4) before entering the flowmeter Materials. High purity (dynamite grade) ethylene g1)col was used in all tests. Solutions were prepared with distilled water. Free carbon dioxide was removed by blowing with nitrogen for at least 1 hour. The p H of solutions was adjusted with oxalic acid and sodium hydroxide solutions. T o obtain a p H of 7 or higher, solutions were first taken to p H 4.0 with oxalic acid and then raised to the final p H with sodium hydroxide. Solutions a t p H 4 were first adjusted to p H 9 with sodium h>-droxide and oxalic acid was subsequently added until p H 4 was obtained. Tests involving chloride ion were carried out with solutions made up with water containing either 50 or 200 p.p.m. of chloride ion as sodium chloride. When the solution was not renewed in the test cell, the p H changed in time. The change in p H during the test varied with the initial p H and the metal. With a few exceptions a t extreme values (Table 11), the p H rose from low initial values
INDUSTRIAL AND ENGINEERING CHEMISTRY
Fe
Hot-Rolled Steel
Mg
99.2 0.02
Si
0.11
cu Mn
Pb Sn Zn A1 C Ni Cr
P Ti
0.05
0.46
... ...
99.7
...
0.01 0.01 0.30
Cold- ColdRolled Rolled Copper Brass Solder
...
...
... 99.8 ...
0.01
... ...
... ...
... ... ...
0.12
0.17
...
0.09
... ...
... ...
... ...
... ...
...
...
...
......
...... ...... 67.5 ... ...... ... 73.6 ... 25.3 33.9 ... ...... ...... ...... ...... ...... ......
Cold-Rolled A1 (2s) 0.53 0.031 0.24 ... 0.14
... ...
... ... ... ...
...
99.2-99.8
...
... ... ... ...
... ... ...
... 99.9 ... ... ... ... ...
Cast Cast AI Iron (1936 (1950 Cylinder Engine Head) Block) 0.9
...
92.7
...
...
3.9 8.21 0.4 0.5 2.28 83.9
... ... ...
... ...
...
2.40 0.16 0.80
... ...
...
3.21 0.18 0.24 0.11 0.20
M E T A L C O R R O S I O N IN A N T I F R E E Z E S to 6 to 7 and decreased from higher values to the same range in 48 to 96 hours. Replacement of solutions maintained a constant p H ( f 0 . 5 unit) throughout the test. Preparation of Metal Specimens. Brass, copper, steel, and solder were received in strip form approximately '/z ,inch wide and '/le inch thick; aluminum was cut from l/ls-inch thick sheet. Cast iron specimens were machined from a n engine block and the cast aluminum specimens from a cylinder head. The compositions of these materials are given in Table I. Specimens were cut to 102 X 12.5 X 1.3 mm. and a 4-mm. hole was drilled, 5 mm. from one end. Sharp edges and imperfections in the surface were removed by grinding with a 220 grit emery belt (a separate belt was used for each metal). All test specimens were degreased in hot benzene vapor for 30 minutes. Solder and cast aluminum were then stored in a desiccator until weighed. Other metals were etched in acid solutions as follows: steel and cast iron, 4 N sulfuric acid a t 175'to 185" F.; aluminum, 10% sodium hydroxide a t 140" to 160' F., rinsed in water, and dipped in concentrated nitric acid for 5 seconds; copper and brass, immersed for 15 seconds in 3 to 1 (volume) concentrated nitric acid and glacial acetic acid. After etching, specimens were rinsed in distilled water, wiped with a cloth to remove excess moisture, dried a t approximately 140" F., and cooled in a desiccator. Specimens were generally prepared immediately before test, but, in no case, mare than 24 hours in advance of the test. After completion of the test, corrosion products were removed with a soft brush, then electrolytically cleaned for 3 minutes as a cathode a t 0.16 ampere per sq. cm. in a solution of 5% sulfuric acid containing 2% by volume of pyridine. A small correction for loss of metal in the cleaning procedure was applied to weight loss data. Test Procedure. Two weighed specimens of metal were hung from. the glass hooks on the central capillary or rod of the test cell and the solution was run into the test cell. The level of the solution was adjusted to the bottom of the upper entrance of the circulating side arm tube and the test cell was placed in the constant temperature bath for 10 minutes before air (or nitrogen) was admitted. The reflux condenser was fitted to the side arm and air flow a t 100 ml. per minute started. Time of test was measured from this instant.
Results Time Dependence of Corrosion. It was important to determine the dependence of corrosion rates of various
Figure 3. Dependence of corrosion of copper on rate of solution renewal a t 160" F. and pH 7
2
4
8
6
IO
SOLUTION RENEWAL RATE, cc. /min.
metals on time at different p H levels. If corrosion rate was constant with time, further studies of other factors would be simplified considerably. Furthermore, the form of the timedeRendence curve is significant with respect to long-time predictions of the corrosion rate, based on limited time laboratory data; constant or declining rates will permit safe extrapolations to longer periods of time, whereas predictions based on accelerating rates cannot be used with safety. The corrosion rates of all seven metals and alloys were determined a t p H 4, 7, and 11 in 40% glycol solution a t 160' F. for 24 to 192 hours. A sufficient num-
ber of points was obtained for each metal to ensure a reliable definition of the corrosion-time relationship. In certain instances tests were run as long as 504 hours to evaluate the relationship over extenqed periods of time. Visual evaluation of the data indicated that in most instances the corrosion losses were linear in time and it was assumed that the curves passed through the origin. Under these conditions the slope of the curve, 6, is the corrosion rate expressed as milligrams per square decimeter per hour. The values for the slope and the standard deviation for the slope, sb, were determined according to the following statistical formulas (72):
tK -% ul 0 K
8 !$ c)
9
NO. OF PASSES UNDERGONE BY CHARGE SOLUTION I
N 0 T E : A L L VALUES BASED ON TESTS OF 96 HOURS DURATION
I 2 3 4 5 NO, OF PASSES UNDERGONE BY CHARGE SOLUTION
Figure 4. Decrease in corrosion rate of copper with number of times corroding solution i s passed through cell VOL. 50,
NO.
4
APRIL 1958
651
Table 11. -
ba
Steel SR NSR
3.99 3.85
pH 7
pH 5.5 S bu
b
Sb
0.144 0.287
... ...
...
Brass SR NSR
PH 9
b
2.81 2.94
...
pH 11
b
Sb
0.052 0.652
Sb
2.74 0.040 2.21 0.154 (PH 9-5)
(PH 7-5)
(PH 4-5) Copper SR NSR
Ethylene Glycol
Corrosion Rate, Mg./Sq. Dm.-Hr.
pH 4 Metal
40%
Corrosion Rates of Metals in
...
...
b
'S
0.015 0.002 0.753 0.488 (PH 11-7.1)
3.12 0.196 0.072 0.019 (pH 4-6.0)
0.452
0.030
0.375 0.024 0.004 0.042 (pH 7-6.3)
3.65 0.218 0.070 0.005 (PH 4-6.4)
0.442
0.033
0.436 0.025 0.060 0.012 (PH 7-6.7)
0.522* 0.092 0.261 0.058 (PH 4-6.5)
0.700 0.085 0.655 0.089 (pH 5.5-7.0)
0.096 0.036 0.030b 0.018 (PH 7-6.7)
0.023 0.004 0.034 0.001 (PH 9-6.5)
9.68 0.559 0.906' 0.155 (pH 11-10.0)
0.056 0.008 0.052 0.015 (PH 7-6.5)
0.157
6.06 0.525 0 . 049' 0.026 (pH 11-10.3)
...
. . I
0.688 0.026 0.033 0.007 (PH 11-6.8) 0.769 0.063 0.036 0.006 (PH 11-7-7)
Solder
SR NSR Aluminum
...
1.17 0.024 0.017 0.012 (PH 4-5.0)
SR NSR
...
0.012
Cast Iron 4.69 0.387 3.23 0.822 6.5jb 0.347 SR Cast aluminum 0.077 0.024 8.82 1.079 1.26 0.126 SR SR. Solution renewal. XSR. No solution renewal. Standard deviation for experimental values about rate; differences in rate may be taken as signifia b. Corrosion rate, mg./sq. dm.-hr. sb. cant if difference exceeds twice standard deviation. 6 Corrosion rate based on linear corrosion time relationship.
=
X
,.. ...
... ...
... ..
lines were drawn in accordance with the slopes derived from the statistical analysis. The corrosion rates and standard deviations are given in Table I1 for each metal a t a number of pH values and for both flowing and static conditions; in the latter case, the p H range is indicated in parentheses. The fit of the experimental data to the calculated straight lines indicated that the assumption of linear corrosion-time relationships held in all cases, within the
time, in hours
Y = corrosion loss, mg. per sq. dm. n - 1 = degrees of freedom T h e relative rates for the different metals a t p H 7 in a continuously renewed solution are indicated by the slopes of the lines in Figure 1. Straight
\
SOLDER
6.0
-
CAST IRON
4.0
-
I
I
\
\ S~EEL
-
I
2.0
DOPP I
\\
_____~
1.0
\
I
1
I
I -
~~
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
PH
Figure 5. Corrosion rates of metals as a function of pH in 40% ethylene glycolwater solutions at 160" F.
652
INDUSTRIAL AND ENGINEERING CHEMISTRY
e . .
...
limits of significance, except for cast iron and solder a t 4 p H ; a significantly better representation of the data was obtained for these metals by use of a parabolic equation of the following form: Cast iron Solder
j = 9 . 0 6 ~- 0 . 0 1 6 7 ~ ~ y = 1 . 2 0 4 ~- 0 . 0 0 4 4 9 ~ ~
However, because of the poor reproducibility of the data for cast iron and solder, the corrosion rates based on a straight line are believed to be a satisfactory first approximation for comparison purposes. Therefore, the corrosion rates given in Table I1 and used throughout the remainder of this paper are based on this assumption. I n general, the results obtained in the absence of replenishment of the test solution were much more erratic than those obtained with solution replacement. I n many instances the pH changes rapidly in time and, when the corrosion rate is markedly dependent on pH, the values are only approximations. Of all metals investigated, only steel maintained approximately linear rates over the entire range of 4 to 11 pH. With brass, copper, and aluminum, rapid initial corrosion took place in the unreplenished solutions but the rate soon leveled off to a negligible value, except for aluminum a t p H 11. The data for solder resembled closely those obtained with solution replacement, except a t 11 pH, where extremely erratic results were obtained. To interpret the large differences in rate observed for copper under replace-
METAL C O R R O S I O N IN ANTIFREEZES ment and nonreplacement conditions, tests were made in which the specimens were first subjected to corrosion under nonreplacement conditions for 144 hours, after which they were transferred to conditions of solution replacement without intermediate treatment of the specimen surfaces (Figure 2). Figure 3 shows the effect of rate of flow of solution through the cell a t p H 7 on the corrosion rate of copper. These experiments indicate that the corrosion rate is determined by the concentration of copper in solution. Further substantiating evidence for this conclusion was obtained by repeatedly passing the same solution through the cell a number of times at 1 ml. per minute to determine whether the rate would approach that of nonredacement conditions as the became saturated with corrosion products (Figure The decreasing corrosion rate is well represented by an-expression of the form: * s)4
2 ;j
8
'p
ci
w'
2 $
E TEMPERATURE, O F .
Figure 6. Corrosion rate of steel in 40% ethylene glycol-water function of temperature in an open system
solution as a
Logy = -Kx + A
where
y = corrosion rate, mg. per sq. dm. per hour x = number of times solution was used A, K = constants
The solutions from tests which varied in length from 24 to 192 hours under the nonreplacement conditions were analyzed for total soluble copper content. The copper content rose very rapidly during the first 48 hours of test and then remained approximately constant a t about 18 p.p.m. However, on re-use of the solution, under the condition of solution replacement, the final solutions varied in copper content from 3 to 4 p.p.m. at the end of one pass to 7 to 8 p.p.m. for five passes. This indicated that although the corrosion rate approached that oibtained with no solution renewal, as the number of solution passes increased, the solutions were not saturated with copper ion; extrapolation of the preceding relationships (insert, Figure 4) indicates that the rate approaches the limit of that for no solution renewal at about six passes. The fact that the flowing solutions were not saturated with copper ion is not surprising, when it is considered that 5760 ml. of solution passed through the cell during each 96-hour test. Tests made with fresh glycol solution to which 18 p.p.m. of cupric ion was added as cupric chloride, and under the condition of solution replacement, prevented corrosion of the copper specimens during 96 hours. Under comparable conditions, the corrosion rate in a solution containing 9 p.p.m. of cupric ion was 0.29 mg. per sq. dm. per hour; in the absence of cupric ion the rate was 0.375 mg. per sq. dm. per hour. All these data definitely indicate that the difference in corrosion rate of
0
TEMPERATURE , O F .
Figure 7. Corrosion rate of copper in 40% ethylene glycol-water solution as a function of temperature in an open system
TEMPERATURE,
OF:
TEMPERATURE, OF.
Figure 8. Corrosion rate of solder and aluminum in solution as a function of temperature
40%
ethylene glycol-water
VOL. 50, NO. 4
APRIL 1958
653
Table 111. Corrosion Rates of Metals in 40% Glycol Solutions at - 1 0" F. under Static Conditions
Metal Steel Copper Brass Solder Aluminum Cast iron Cast aluminum
Av. Corrosion Rate, Mg./Sq. Dm.-Hr. Distilled 200 water p.p.m. C10.008
+O.OOl -I-0.003 0.008 $0.001 0.043 0.002
0.031 $0.001 +0.001 0.026 0.004 0.050 0.012
copper (and brass) between conditions of solution renewal and no renewal is largely due to the copper ion concentration. Dependence of Corrosion Rate on pH. The relationship between p H and corrosion rate under conditions of solution replacement in which p H is constant throughout a test is shown in Figure 5 and Table 11; data for steel in water, under the same conditions, are included for purposes of comparison. I t is apparent from Figure 5 that the p H range of 6 to 8 is optimum for over-all low corrosion losses of the metals involved, and that tests conducted a t high and low p H levels are meaningless unless the p H is carefully controlled. In the absence of solution renewal, those solutions initially a t p H 4 increased with time to a slightly higher p H (5.0 to 6.5), dependent to some extent upon the metal present (Table 11). Temperature Dependence of Corrosion Rate. The effect of temperature upon corrosion rate was studied only under conditions of solution renewal. The conditions were limited to the following: solution flow rate 5 ml. per minute, 40% ethylene glycol solution, zero chloride ion content, and 100 ml. of air per minute. Results of these experiments are shown in Figures 6 to 8. T h e general effect of increase in the temperature was to increase the corrosion rate, as would be expected from the usual effect of temperature upon reaction rates. This increase was most marked in the cases of steel a t p H 4 to 9. In certain instances the data a t a given temperature were limited to a single test time and consequently the precision is questionable, especially for copper and solder a t 220' F.; because of the marked dependence of corrosion rate on p H for both solder and aluminum above p H 10, the temperature dependence of rate in this range of p H could not be determined with confidence. In view of the low temperature accelerated corrosion of some metals in boraxinhibited glycol solutions reported by Caplan and Cohen (2), it was of interest to determine whether appreciable corrosion of automotive cooling system metals would occur in uninhibited glycol
654
solutions under low temperature conditions simulating the standing of a vehicle during the winter months. Two specimens of a given metal were suspended in a 40y0 glycol solution a t p H 7 contained in a stoppered bottle and stored a t -10' F. Separate tests were run for 30 and 90 days' duration on solutions prepared with distilled and 200 p.p.m. chloride ion water. The average corrosion rates of the individual metals are shown in Table 111. Although steel, cast iron, solder, and cast aluminum exhibited small corrosion rates, no accelerating effect of low temperature was detected. Additional tests were made a t 0' F. in the regular test cell without solution renewal on steel, brass, solder, and aluminum (2s) in 40% distilled water solution of glycol a t p H 7 and with a n air rate of 5 ml. per minute. The corrosion rates, based on 168-hour tests, were (mg. per sq. dm. per hour) : Steel Brass Solder Aluminum
0.011 +0.009 0 005 S0.015
These results are comparable to those obtained under static conditions and show that low temperature has no accelerating effect on these metals in uninhibited glycol solutions. Oxygen Dependence of Corrosion Rate. T o determine the effect of oxygen on corrosion rate, comparable tests were made with purified nitrogen as the saturating gas. Tests were conducted with 4070 glycol, either renewed a t the rate of 5 ml. per minute or without solution replacement, at 160' F. with 100 ml. of saturating gas per minute. The solutions were blown with nitrogen for 18 hours or longer before beginning a run and the reservoirs of solu-
Table IV.
tion were kept blanketed with nitrogen throughout the test. Before the test was initiated, all lines were blown out with nitrogen, the solution was charged to the cell, and nitrogen admitted for 10 minutes before immersion of the metal specimens. I n Table IV, the average results for runs in nitrogen are compared with the corresponding results obtained in air. The data show that oxygen has a pronounced effect on the corrosion rate of the metals under study, with constant renewal of solution a t the different p H levels, except for steel a t p H 4 and aluminum a t all pH levels. T o determine whether the oxide film on aluminum could be maintained in the presence of chloride ion, without oxygen in the solution, tests were made a t p H 4 and 7 with a chloride ion concentration as high as 20,000 p.p.m. Even under these conditions, corrosion rates of the same low order were observed. I t was concluded that sufficient oxygen was liberated by electrolysis of the water to maintain the protective oxide film in repair. Chloride Ion Dependence of Corrosion Rate. The effect of chloride ion upon corrosion rate was studied both with and without solution renewal. The standard test conditions were 160" F., 40y0solution, 5 ml. of air per minute, and 4. 7, and 11 pH. A few runs were made in isolated instances at p H 5.5 and 9, where an appreciable difference had been found betlveen p H 4 and 7 or 7 and 11. All initial tests were made with solutions prepared from water containing 200 p.p.m. of chloride ion. In instances where appreciable differences existed in tests made with solutions containing 0 and 200 p.p.m. of chloride ion, additional tests were made with solutions made up with water containing 50 p p.m. of chloride ion.
Oxygen Dependence of Corrosion Rate in 40% Ethylene Glycol at
160" F. Solution Renewal Corr. Rate, __Mg./Sq.
~
Urn.-Hr.
Metal
pH 4
Air
Nitrogen
11 4 7 11
3.99 2.81 0.015 3.12 0.38 0.69
4.3 0.58 0.03 0.01a
Brass
4 7 11
3.65 0.44 0.77
0.03 0.00 0.03a
Solder
4 7 11 4
0.52 0.10 9.68
0.24 0.21 0.24 1.04 0.08 9.2
Steel
7 Copper
Aluminum
INDUSTRIAL AND ENGINEERING CHEMISTRY
7 11 a
Weight gain.
1.17 0.06 6.06
0.02Q
0.02
pH 4-5
7-5 11-7
No Solution Renewal Air Nitrogen Corr. ratc, Corr. rate, mg./aq. mg./sq. dm.-hr. PH dm.-hr. 4-5.9 3.85 0.15 2.94 0.15 6.8-7.2 11-10.8 0.75 0.02
4-6.0 7-6.3 11-6.8
0.07 0.04
4-6.4 7-6.7 11-7.7 4-6.5 7-6.7 11-10.0 4-5.0 7-6.5 11-10.3
'
0.00
0.03
4.0-4.4 6.8-6.1 11.0-10.8
0.07 0.06 0.04
4.0-4.3 6.8-6.0 11.0-10.9
0.02a 0.04" 0.02a
0.26 0.03 0.91
4.0-4.2 6.8-5.9 1 1.0-10.8
0.02
4.0-5.0 6.8-7.2 11.0-10.6
0.06 0.07 0.08 0.01a 0.OP 0.20
0.05 0.05
0.02a
0.01a
METAL C O R R O S I O N IN ANTIFREEZES Table V.
Chloride Ion Dependence
of Corrosion Rate in 40% Ethylene Glycol a t 160" F. C1- Concn., P.P.M.', No Solution Renewal
C1- Concn., P.P.M.n, Solution Renewal Metal
PH
Steel
4 7 11
Copper
4 5.5 7 11
Brass
4
5.5 Solder
7 11 4 5.5 7 11
0.69 3.65 0.44 0.44 0.77 0.52
...
0.10 9.68
Aluminum
... ... ... ... ... ... 3.1 ... 0.69 11.6
... ... ...
4 1.17 7 0.06 9 0.16 11 6.06 7.7 Chloride ion content of water used to prepare 40% solutions.
The results of these tests, summarized in Table V, indicate little effect from the chloride ion, except in the isolated cases of steel and copper a t p H 4 and with solder. I n the case of solder a t p H 4, the corrosion rate was a linear function of time over the period tested and showed no decrease in time as experienced in the absence of chloride ion. Because of limited tests of aluminum a t p H 11 and the poor reproducibility normally experienced with this metal a t the high pH, no significance was attached to the differences in the latter results. Without solution replacement, increases in corrosion rates in the presence of chloride ion were found for all metals a t p H 4. An appreciable increase was also noted in the case of solder a t p H 7 and the corrosion rate of aluminum a t p H 7 and 11 increased three- or fourfold in the presence of chloride ion; but the higher rates were not such as to cause serious concern by the present standards of utility. Glycol Concentration Dependence of Corrosion Rate. The effect of glycol concentration was studied a t concentration levels of 20, 40, and 60 volume % ethylene glycol in distilled water a t 160' F. in the presence of air and with solution flow only (Table VI). The most marked effect of concentration was observed with steel (Figure 9); in this case, the concentration range was extended to zero glycol content. No consistent change in corrosion rates with glycol concentration for metals other than steel were observed. Insufficient runs were made a t 20 and 60'%0g1ycol concentration to allow a statistical analysis of the data to be made; consequently,
Corr. rate, mg./sq. dm.-hr.
PH 4.0-5.2 7.2-6.5 11 * 0-10.2
4-6.0
4.0-5.5 5.5-6.5
0.77 0.17
0.31 0.66 2.94 0.35 0.23 0.71
7-6.3 11-6.8
0.04 0.03
11.0-7.5
0.07
4-6.4
...
0.07
4.0-6.4
7-6.7 11-7.7
0.06 0.04
7.2-6.8 11.0-7.2
0.22 0.07 0.03 0.05
2.8
4-6.5
0.26
0.70 0.67 0.12 3.3
PH 4-5 7-5 11-7
...
...
200
Corr. rate, mg./Sq. dm.-hr. 3.85 2.94 0.75 0.07
0 60 200 Corr. Rate, Mg./Sq. Dm.-Hr. 3.99 4.8 6.4 2.81 3.2 3.2 0.02 0.2 4.4 3.12 3.3
... 0.38
'
0
...
...
... 0.69
12.5 1.44 0.11 0.14 9.7
...
...
...
...
4.52 2.54 0.34
. 5.5-6.7
7-6.7 11-10.0
0.03 0.91
4.0-6.5 5.6-6.9 6.8-7.3 11.0-9.8
4-5.0 7-6.5
0.02 0.05
4.0-5.5 6.8-6.5
1.25 0.15
11-10.3
0.05
11.0-10.3
0.20
... ...
the precision of some of the data, particularly for solder and aluminum a t p H 11, is not high. However, because the rates a t these extreme values of p H are far beyond the useful range, no additional work was done to define the values more closely. Discussion of Experimental Data. .For all metals and chemical conditions studied, the corrosion rate was linear with time, or decreased with time. Accordingly, predictions may be made with considerable assurance. Furthermore, the fact that none of the corrosion rates increased in time, especially in the
GLYCOL CONCENTRATION
9 . .
...
...
...
absence of solution replacement, indicates that detrimental (corrosionaccelerating) products of glycol oxidation do not accumulate in the test solutions. Also, the magnitude of corrosion rates, under conditions of nonreplacement of solution, were either equal to or lower than those obtained under the condition of solution renewal. The finding that in the absence of solution renewal, solutions initially a t p H 4 increase slightly rather than decrease in p H with time shows that oxidation of glycol under these controlled conditions does not result in highly acidic products. These results indicate that
, PER CENT
(VOL.)
Figure 9. Corrosion of steel in ethylene glycol-water solutions of varying glycol concentration at 160" F. Freezing point curve indicated for comparative purposes
VOL. 50,
NO. 4
APRIL 1958
655
Steel, solder, and aluminum are less deTable VI.
Glycol Concentration Dependence of Corrosion Rate at 160’ Glycol Concn., Vol. % 20 40
0
Metal Steel
7 9 11 Copper
Brass
4 7 11 4
7 Solder
Aluminum
11 4 7 11 4
7 11
10.93 7.78 8.06 . , I
...
9 . .
... ... ...
... ... . a .
.
I
.
...
... . a .
possibly many reported cases of a large decrease in pH, accompanied by excessive corrosion, in extended service use of commercial glycol antifreeze may be attributed to extraneous factors like leakage of exhaust gases into the cooling system rather than degradation of glycol. Data in Table I1 are significant with respect to the design of laboratory test procedures and the chemical interpretation of corrosion rates. From the relative standard deviations, it is apparent that reproducibility is considerably better in tests in which the solution is continuously replaced. The change in p H is frequently large in a static test; accordingly, an interpretation of results in terms of chemical variables is almost impossible, in the range where corrosion rate varies significantly with pH. Comparison of corrosion rates in the two testing conditions is complicated by the change in p H ; however, the metals seem to fall into two distinct classes in the p H range of most interest. Steel and solder are not greatly affected by solution replacement and the corrosion rates of copper or brass are very greatly increased by solution replacement. In the latter case, experiment has demonstrated that, in the presence of oxygen, the corrosion rate of copper is largely determined by the concentration of copper in the solution. Steel, solder, and aluminum corrosion products are relatively insoluble in the presence of oxygen and, except a t the highest p H level, the concentration of metal ions in solution is negligible. The large increase in corrosion rate with pH, below 6 or above 9, clearly defines the optimum range of operation. The data also emphasize the need for information obtained under well controlled conditions, as the corrosion rate of steel rapidly diminishes but the corrosion rates of aluminum and particularly of solder rapidly increase above a p H of 10. Thus, the simple expedient of
656
60
Corr. Rate, Mg./Sq. Dm.-Hr.
PH
4
F.
7.04 4.37 4.80 0.06
3.99 2.81 2.74 0.02
4.96 0.22 0.12 0.00
3.29 0.38 0.23
3.12 0.38 0.69
2.20 0.24 0.74
3.49 0.41 0.12
3.65 0.44. 0.77
3.44 0.33 0.59
1.86 0.52 17
0.52 0.10 9.68
1.55 0.65 12
1.00 0.11 12
1.17 0.06 6.06
0.76 0.02
17
maintaining a high p H in a cooling system containing soldered radiator joints could well cause rapid failure of the system. The observed temperature dependence of corrosion rate (Figures 6 to 8) is not completely understood a t present. The experiments were conducted in a system open to the atmosphere and, accordingly, the oxygen solubility was defined by the temperature. Under these conditions, it would be anticipated that the corrosion rate should go through a maximum with temperature due to the increasing rate of the chemical reaction with temperature and the decreasing oxygen concentration. .4 maximum in the rate is observed in water and salt solutions for iron (8) and copper (6); when the corrosion is dependent on the reduction of oxygen, the rate goes to zero a t boiling point of water for copper and to a small value for steel, which is capable of reducing hydrogen ions in the absence of dissolved oxygen. In a 40% ethylene glycol-water system the rates do not appear to follow any simple pattern; in particular, they do not fall off significantly as the temperatures approach the boiling point of the solution (220” F.). The solubility of oxygen in the system as a function of temperature is not known and, of course, must be different than in water; also, the glycol may undergo a chemical change to yield a reducible compound a t the higher temperatures. In any event, however, oxygen or some other reducible species is required in amounts equivalent to the metal oxidized by the corrosion reaction. For metals like iron which are more active than hydrogen, the reducible species can be the hydrogen ion; for copper, the identity of the necessary oxidizing agent is not certain, but sufficient oxygen is probably available in these solutions at 220’ F. For copper and brass, the absolute dependence of the corrosion rate on oxygen is demonstrated by the data in Table IV.
INDUSTRIAL AND ENGINEERING CHEMISTRY
_.pendent on oxygen concentration, though the difference in rates for steel depends on the relative concentration of oxygen and hydrogen ions and, consequently, on the pH. The effect of chloride ion is particularly important in connection with the water that may be used to dilute the glycol in practice. It appears that chloride generally increases the rate as anticipated, but the magnitude of the increase is relatively small, except for aluminum a t all p H levels and solder in the higher p H range. Subsequent work has shown that the principal effect of chloride ion is observed in galvanic corrosion, in which the rate of reaction a t the anode of two coupled metals is considerably increased by the presence of chloride ions in the solution, The most significant effect of glycol concentration was observed with steel. The corrosion rate decreases almost linearly with increasing glycol concentration in the intermediate range of pH. An understanding of the over-all corrosion phenomena in actual automotive cooling systems depends on knowledge not only of factors affecting corrosion of individual metals but also galvanic corrosion of coupled metals and the accompanying electrochemical phenomena. With regard to the use of these results, the experiments were designed to provide data necessary for the interpretation of service experience and the improvement of antifreeze formulations, The documented conclusions pertain to the stated conditions and are not a substitute for service tests involving other factors. literature Cited
(1) Brooks, D. B., Streets, R. E., Natl. Bur. Standards, Circ. 474 (November 1948). (2) Caplan, D., Cohen, M., Corrosion 9, 284 (1953). (3) Green, D. H., Kratzer, J. C., Emch, P. I., ASTM Bull. No. 154, 57 (October 1948). ( 4 ) Meyer, F. R., Rouge, G., Angew. Chem. 5 2 , 6 3 7 (1939). (5) Robertson, W.D., Trans. Electrochem. SOC.98, 94 (1951). (6) Robertson, W. D., Nole, V. F., Davenport, W., Talboom, F., submitted to Electrochem. SOC. (7) Schlapfer, P., Bukowiecki, A., Schweiz. Gesellschaft fur das Studium der Motorbrennstoffe, Bern, Berichte, No. 15, 1949. Speller, F. N., “Corrosion, Causes and Prevention,” p. 153, McGrawHill, New York, 1935. Twiss, S. B., Guttenplan, J. D., Corrosion 12, 263, 311 (1956). U. S. Tariff Commission, “Synthetic Organic Chemicals, United States Production and Sales, 1955,” Rept. 198,2nd Series, 1956. Wormwell, F., Mercer, A. D., J . A$jI. Chem. 3, 22 (1953). Youden, W. J., “Statistical Methods for Chemists,” Wiley, New York, 1951 RECEIVED for review February 27, 1957 ACCEPTED .4ugust 10, 1957 I
