I
CORNELIUS GROOT and R. E. WILSON' Hanford Atomic Products Operation, General Electric Co., Richland, Wash.
Intergranular Corrosion of Aluminum in Superheated Steam Behavior of alloys under high temperature-pressure stresses imposed by nuclear reactors opens a broad new field for investigation. For aluminum, resistance may relate to iron content and fine shades in alloy content
Use
OF ALUMINUM in high temperature, water-cooled reactors requires knowledge of its temperature and pressure limitations. Guillet and Ballay ( 5 ) showed that aluminum is subject to disastrous attack a t 300" to 350' C. in superheated steam; 99.8% pure aluminum was destroyed, but an aluminum alloy containing 13% silicon gained only 4 mg. per square cm. in 300 hours. Draley and Ruther (3) have shown that aluminum alloy 1100 is usable u p to 200" C. in demineralized water. They also developed an alloy of 1% nickel in aluminum alloy 1100 (Aluminum Co. of America's M-388), which is
Present address, University of Washington, Seattle, Wash.
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I
Corrosion experiments at high pres. sures were performed in these autoclaves
resistant to high purity water up to 350" C. (4). Dillon, Wilson, and Troutner (2) surveyed most commercial alloys, and found that highly alloyed materials, such as 2018 and 4032, were least attacked a t temperatures up to 350" C. in pure water. Huddle and Wilkins (6) showed that 0.50/0 iron inhibited intergranular attack on aluminum up to 200' 6. in demineralized water. This work was undertaken to determine conditions that would cause intergranular corrosion in water or its vapor at 400 pounds per square inch pressure and below, to measure effects of pressure and temperature on rate or probability of intergranular corrosion, and to discover how intergranular corrosion can be minimized.
Experimental For experiments at low steam pressure, laboratory steam was used; this permitted a constant flow through the system and swept out the hydrogen that was generated. The system consisted of 3 feet of 1-inch schedule 40 stainless steel pipe. A tube furnace on one end superheated the steam. Six inches of magnesia insulation reduced heat loss in the test section. A '/e-inch stainless steel valve allowed a slow stream of steam (and hydrogen, if any) to escape. Temperatures were recorded with thermocouples and a Micromax recorder. Despite the thick insulation, there was a thermal gradient in the tube and presumably in the sample. For experiments a t high pressure, no suitable steam source was available, so the experiments were done in static systems. The autoclaves are rated a t 10,000 pounds per square inch at 650" F. All metal in contact with the vapor is 316 stainless steel, and the nuts are 416 stainless steel. At operating temperatures of 400" to 500' C. the threads tended to gall, and several autoclaves were made useless because of this. The closing technique which minimized galling was: wiping the female threads clean, brushing the male threads with a
short bristle brush to remove carbon or coke deposits, painting only the male threads with a paste of graphite and oil as thinly as possible without leaving holidays, and then assembling and tightening. Appropriate pressure was developed by placing a calculated (7) amount of distilled water in the autoclave--c.g., 5.9 ml. of water in a 100-ml. autoclave to attain 2500 pounds per square inch at 475" C., and 6.4 ml. of water to attain 2500 pounds per square inch a t 450" C . Actually, vapor densities were obtained, but for engineering convenience, results are reported as pressures. The closed autoclaves were then placed in a furnace held a t the desired temperature by an indicating controller (Figure 1). After exposure for a predetermined time, they were removed and air-cooled.
Results Ideally, a corrosion experiment should give quantitative results, such as a penetration rate in milliinches per year; because Pesults can then be compared to determine effect of each variable and
0
05
Figure 1. specimen VOL. 49,
10
1.5 20 TIME IN HOURS
Heating curve of corrosion
NO. 8
AUGUST 1957
125 1
Figure 2. Top.
This sample was not attacked The whole ‘/z Bottom.
X 1 inch sample
Cross section
its interactions. This does not always occur, however, and frequently the sample is completely destroyed; which indicates only that the material is unsatisfactory under the conditions of the test. Corrosion experiments also may result i n no measurable attack on the sample; this is satisfactory qualitatively but not quantitatively, These nonquantitative results have a way of upsetting the best experimental designs. A further difficulty arises from nonuniform corrosion, and in these experiments, corrosion was spectacularly nonuniform. Small holes were eaten through some otherwise undamaged samples. TVere quantitative measures of corrosion obtainable, interpretation would be difficult. In the closed systems, corrosion conditions were not constant and not even known after the start of the reaction. Steam was used up and replaced bv hydrogen as the reaction proceeded. Only in the lowest steam-pressure flowing system were the conditions uniform and even here a temperature gradient existed. Attempts were made to measure the amount of corrosion by using bend, tensile, and bursting tests. These tests were not satisfactory because high temperatures annealed the samples and produced large blank corrections. Bend and tensile tests could measure only corrosion that took place in the test section. Bursting tests were the most satisfactory,
1 252
Figure 3.
These samples were attacked
Top. Somple after exposure at 400’ C. and 431 pounds per sq. inch absolute; 1, after 7 hours; 2, after 2 3 hours; 3, after 4 8 hours; 4, after 72 hours. Bottom, Cross section after 280 hours at 370’ C., 4 0 pounds per sq. inch
because a weak spot anywhere in the bursting specimen would show up. However: the specimens used in this test, cylinders 1.44 inches in diameter and 8.3 inches long, were too large to fit in the autoclaves, and this promising line of investigation was not followed up. Electrical resistance methods were considered, but were discarded because of insensitivity to small holes in a large sheet. This lack of integrity, a small hole in a large sheet, was the criterion for failure in service. As quantitative measures were not applicable to many experiments, and also not sufficiently developed, these experimental results were classified into three categories-no attack, attacked, and destroyed. The samples classified as no attack (Figure 2) generally had a uniform gray film. Those classifed as attacked (Figure 3) generally had raised mounds of a gray powder, consisting of clear crystals of oxide with metallic aluminum specks inside. The destroyed samples had lost their metallic strength and appearance, and some were completely reduced to powder (Figure 4). hutoclaves containing such specimens had a high pressure of hydrogen.
measurable.
Samples were cut from d single piece of 1100 aluminum that had been impact-extruded and annealed. Experiments at 40 pounds per square inch were in a temperature qradient. Intergranular corrosion started at the hot end, and worked toward the cold. The temperatures given here are those for the walls of thr apparatus at the point where intergranular corrosion of the sample was visible aqd at the point where it was complete Tt’here duplicate values agreed, only one entry appears in the tabulations. TVhere several experiments were done a t the same temperature and pressure, but for different time intervals and all showed no attack, onlv the one for the longest time is listed.
Effect of Temperature and Pressure
The first series of tests was run to define the region of temperatures and pressures where intergranular corrosion was
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4.
Sample destroyed
H I O H -STR E S S ALUMINUM C 0 R R 0S I ON Table I. Temp.,
c.
370 430
500
Intergranular Corrosion of 1100 Aluminum Press., Lb./Sq. In. Abs.
Time,
Hr. Low Pressure 40 40 40 40 40
Results5 N A
260 260 260 260 66
A
D N
Medium Pressure 325 350
375 400
380 395 395 395 395 410 410 430 430 430 400 430 430 430
65 24 48 64 74 24 90 2 3.5
7 r.
16 23 48 72
N A A A A A A N A A A A A A
High Pressure 300 325 350
375 400
500
1250 1750 2250 2250 2250 2550 2750 3700
20 64 2 3.5 20 2 2 2
N A A A D A
a N = no attack; A = destroyed.
Attack of M-388 Aluminum Alloy b y Superheated Steam
D
D attack; D
=
Table I may be interpreted in terms of induction and attack time. Induction time is that for visible attack; attack time, that from initial attack to destruction. However, exact induction time is not known. The bombs were opened a t arbitrary times, and the sample was observed. If the sample was not at-
Table 11. Intergranular Corrosion of M - 3 8 8 in Superheated Steam Temp., O
c.
Press., Lb./Sq. In. Time, Abs. Hr. Medium Pressure
Resultsa
475
400
64
N
500
400 400 400
2 16 48
N A D
tacked. induction time was greater than " the sample exposure; if the samples were attacked, induction time was less than the sample exposure, and the sum of induction plus attack time is greater than the sample exposure. Finally, if the sample was destroyed, exposure time must be greater than the sum of the induction time plus attack time. So considered, induction time decreases rapidly with increasing pressure, or more likely, with increasing concentration of the corroding substance. Increase from 40 to 2550 pounds per square inch steam pressure dropped induction time from 260 hours to less than 2 hours a t 370' to 375' C. Induction time is likewise reduced by increasing temperature. At 400 pounds steam pressure, induction time is more than 65 hours a t 325' C., and less than 24 hours a t 350' C. The attack time correspondingly decreases with increasing pressure. At 400' C. and 430 pounds steam pressure, attack time is more than 72 hours, but less than 2 hours at 400' C. and 2700 pounds steam pressure. Attack time also decreases with temperature ( 7 ) .
High Pressure
Since Draley (3,#), had shown M-388 so resistant to intergranular attack by water, its resistance to superheated steam also seemed probable. Table I1 verifies this. I n addition to aluminum, M-388 contains 0.9 to 1.3% nickel, 0.45 to 0.7Oyo of iron, and in maximum percentages, silicon, 0.17; copper, 0.15; lithium, 0.008; cadmium, 0.003; boron, 0.001. Other impurities of 0.05% each total 0.15%. Induction time for M-388 at 400 pounds per square inch steam pressure was greater than 64 hours at 475' C., and between 2 and 16 hours at 500' C. At about 3000 pounds, it was greater than 162 hours at 400' C., approximately 2 hours at 450" C., and less than 2 hours at 500' C. Attack time varied, but at 400 pounds it was less than 48 hours, at 500' C. At about 3000 pounds it ranged from several hours a t 425 ' C. to less than 2 hours at 500' C. This lot of M-388 is about 100" C. better than the 1100 alloy of Table I-i.e., it shows the same induction time a t a temperature 100' C. higher.
400
2750
162
42 5
3000 3000 3000
2
N N
7
D
23
Aluminum lot, Surface Preparation, and Metallurgical Condition
The preceding experiments left no doubt that the sample of M-388 tested was better than that of 1100 tested under these conditions. Thus, the more general problem was: Are all lots of M-388 better under all conditions than 1100? The next series of tests, intended to show effects over longer time intervals,
450
2 500
2
A A
460
3350 3350
2 4.5
A D
475
2500 2500
500
2 16 2
3700
N = no attack; A destroyed.
=
N
D D
attack; D =
Table 111.
Temp., 'C.
Intergranular Corrosion of 1 100 Aluminum (Second lot) Press., Lb./Sq. Time, Inch Hr.
325 425 450
Results
No attack
720 66 66
400 400 400
No attack
No attack
Table IV.
Intergranular Corrosion"
Element
Table I Susceptible
Ag A1 B Bi Cr rcu Fe Ga Mg Mn Ni Pb Si Sn Ti
...
Table I11 Resistant T
S
S
