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EFFECT OF PROTECTIVE COATINGS ON THE STRESS-CORROSION PROPERTIES O F SUPERSONIC -TRANSPORT SKIN MATERIALS TENTH QUARTERLY STATUS REPORT to NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For the Period Between 1 March, 1965 and 31 May, 1965 Contract No. NASr-117 J. 0. Honeycutt A. C. Willhelm Southern Research Institute Birmingham, Alabama 35205 1 5 J u n e 1965
7325 - 141 7 -XI1
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TABLE O F CONTENTS ” Page INTRODUCTION . . . . . . . . . . . . . . .
WORK PERFORMED . . . . . . . . . . . . .
PROCEDURES . . . .
RESULTS AND DISCUSSION . . . . . . . . . . .
Visual Examination . . . . . . . . . . . .
Bend-Ductility . . . . . . . . . . . . . .
CONCLUSIONS . . . . . . . . . . . . . . .
FUTUREWORK . .
REFERENCES i S O U T H E R N R E S E A R C H I N S T I T U T E LIST OF ILLUSTRATIONS Page Figure
-
1 Construction of the self -stressed specimen.
(All dimensions a r e in inches). . . . . . . .
2 Flow Sheet of Experimental Conditions . . . . .
3 Schematic drawing of clamping members with speci- men in place . . . . . . . . . . . . .
4 AM 350 Bend-Ductility Results after 1,000 hours exposure at 550" F . . . . . . . . . . .
5 Rene' 41 Bend-Ductility Results after 1,000 hours exposure a t 550" F . . . . . . . . . . .
6 Ti-8A1-1Mo- 1 V Bend-Ductility Results after 1,000 hours exposure a t 550" F 15 . . . . . . . . .
7 AM 350 Bend-Ductility Results after 1,000 hours at 9570 Humidity . . . . . . . . . . . .
Rene' 41 Bend-Ductility Results after 1,000 hours exposure at 957" Humidity . . . . . . . . .
Ti-8A1- 1Mo- 1 V Bend-Ductility Results after 1,000 hours at 9570 Humidity . . . . . . . . . .
ii S O U T H E R N R E S E A R C H INSTITUTE LIST O F TABLES Page Table
I Visual Examination o f 1,000-Hr Exposed Specimens . 7
AM 350 Bend-Ductility Data-l,OOO Hours Exposure to
F and Humid 95" F Atmospheres . . . . . 8
Dry 550" I11 Rene' 41 Bend-Ductility Data-1,OOO Hours Exposure
550" F and Humid 95" F Atmospheres . . . . 9
to Dry N Ti-8Al-lMo-lV Bend-Ductility Data-1,OOO Hours Exposure to Dry 550" F and Humid 95" FAtmospheres .
iii S O U T H E R N R E S E A R C H INSTITUTE REPORT ON EFFECT OF PROTECTIVE COATINGS ON THE STRESS-CORROSION PROPERTIES O F SUPERSONIC- TRANSPORT SKIN MATERIALS INTRODUCTION This report summarizes the progress made during the fourth quarter of a project being performed by Southern Research Institute under Contract No.
NASr-117. This quarter consists of the period between 1 March, 1965, and 31 May, 1965.
The purpose of this r e s e a r c h project is to determine whether selected These data w i l l coatings w i l l protect metal substrates from stress-corrosion.
provide needed additional information on the feasibility of using commercially available protective coatings to prevent corrosion of the skins of supersonic- transport aircraft (SST). The coatings and substrates to be evaluated were cho- sen from the results of earlier work on this contract (1, 2 ) l .
Pertinent background information and a detailed description of the spe- cimen preparation and environmental exposures, along with the general evalua- tion procedure, were presented in earlier progress reports and w i l l not be repeated here. Described briefly, the program consists of various s t r e s s - corrosion exposures applied to self-loading type specimens constructed a s shown in Figure 1 . The substrates, coatings, exposure conditions and evaluation methods a r e charted in Figure 2 .
WORK PERFORMED While the stress-corrosion specimens were undergoing exposures to the d r y 550" F and the humid 95" F atmospheres, we constructed a fixture capa- ble of supporting the exposed specimens for bend-ductility evaluations. After 1,000 hours, the f i r s t group of exposed specimens w a s removed from the ex- posure atmospheres, visually examined, rinsed, and subjected to compressive loading for bend- ductility evaluations.
The fixture consisted of two clamping members fitted vertically in a Both clamping members were mechanically manually operated hydraulic press.
secured to the press, the top clamp to the upper, stationary crosshead of the The numbers in parentheses refer to the bibliography at the end of the report.
SOUTHERN RESEARCH INSTITUTE -2- (a) Machined strip.
(b) Strip with ends bent.
(d + 2t) Distance
(c) Completed specimen.
Figure 1. Construction of the self-stressed specimen. (All dimensions a r e in inches ).
SOUTHERN RESEARCH INSTITUTE - 3 - Steel Superalloy Titanium Alloy Substrate Substrate Substrate
I
Uncoated I [Coating l] iCoating2
oating 3 I l ~ o a t i n g 41 b o a t i w 5
I I I I I I
I Coating-Substrate Combinations
I I
I Undamaged Damaged coatings
u
N o Salt No Salt Deposit Deposit Deposit Deposit I I
I 1 , I 1
Dry Humid Humid Dry 100" F 550°F 100" F 550" F I I 1 - I .
Exposure Times 1000 Hours 3000 Hours 5000 Hours 7000 Hours ,
I Bend-Ductility Evaluations I
I Metallopraphic Examina Coating 1 - Aluminum-Modified Silicone
Coating 2 - Catalytically Cured Silicone
Coating 3 - Zinc in Silicate vehicle
Coating 4 - Electrophoretically Deposited Aluminum
5 - Flame-Sprayed Aluminum
Coating Figure 2. Flow Sheet of Experimental Conditions S O U T H E R N R E S E A R C H INSTITUTE -4- p r e s s by means of a threaded stud, and the bottom clamp to a steel supporting block that was "C" clamped to the lower, movable platen. A dial gage C a l i - brated in 0.001-in. increments w a s mounted on a ring stand adjacent to the press. The height of the dial gage was adjustable on the ring stand and could so that the stem could bear against the movable platen of the be positioned press.
The configuration of the clamping members is shown in Figure 3.
The members consisted of two 3-in. long, 1,25-in. -diameter round steel Both members contained a 1-in. -deep, 0.100-in. -wide slot into which bars.
the specimen tab ends could be inserted. The edges of each slot were rounded to insure against breakage of the specimen at that point where the specimen deflected and bore against an edge of the slots. In order to insure that lateral alignment of the specimen tab ends could be repeated from specimen to speci- men, we inserted two dial-pin locators a c r o s s each slot. Four set screws were used in conjunction with each slot for the purpose of anchoring the speci- mens' tab ends. The s e t screws were arranged in pairs, with each pair ex- tending through the sides on opposite sides of the slot.
PROCEDURES Following the visual examination of the exposed specimens and the r e - moval of salt from appropriate specimens by water rinsing, we loaded each specimen into the clamping members with its (D + 2t) distance ( r e f e r to Figure IC) extending horizontally. The lower movable platen of the p r e s s was raised to a position where the f u l l 3/4-in0 length of the specimen tab ends would extend into the slots of both clamping members.
Once inside the slots, the tab ends were positioned against the dial-pin locator stops and also against a common slot side in both clamping members. The specimen was then locked into position by tightening the s e t screws against the flat face of the tab ends.
After positioning and securing the specimen in the fixture, we placed the d i a l gage in contact with the lower movable platen and set it to the zero position.
The specimen was then compressed by raising the lower platen with the hydraulic pump. Specimen compression w a s continued until fracture occurred , o r until complete compression (maximum specimen shortening with contact between the tab ends) was attained. The dial gage, activated by the upward movement of the lower platen, provided a reading of the bend- ductility o r shortening that occurred in the specimen.
S O U T H E R N R E S E A R C H I N S T I T U T E - 5 - Front View Side View I -l---T I I I I I D I
I I
Upper clamp I I I I I
\ J \ 0 t
\ /' v v 0.100-in. slot 1-in. deep J - - 7 Locator
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Pins I
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Specimen Tab End Figure 3. Schematic drawing of clamping members with specimen in place.
I I I
' I Steel Supporting Block
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S O U T H E R N RESEARCH I N S T I T U T E - 6- RESULTS AND DISCUSSION Visual Examination The results of the visual examinations of the exposed specimens a r e listed in Table I. This table lists general observations on the appearance of each substrate-coating combination after exposure to the dry 500" F o r humid 95" F atmosphere.
The dry 550" F atmosphere caused the bare substrate of AM 350 and titanium to discolor but caused no discoloration of the bare Rene 41 substrate.
The exposure to the humid 95" F atmosphere caused no visual changes in any of the bare substrates with the exception of a few rust spots on the bare, salt- deposited AM 350 alloy. Except for the occurrence of a few salt stains on salt-deposited specimens in the humid 95" F atmosphere, the appearance of the specimens coated with Aluminum-Modified Silicone w a s not changed by either exposure. The Catalytically-Cured-Silicone-coated specimens showed no visual change during the humid 95" F exposure. However, in the d r y 550°F exposure, the Catalytically-Cured-Silicone coating shredded and spalled of each substrate. Each specimen coated with completely from all specimens Zinc-in-Silicate-Vehicle contained areas of grayish-white discoloration after exposure to the humid 95" F atmosphere. The dry 550" F atmosphere caused grayish-white discolorations to appear only on those zinc-coated specimens that had been subjected to salt deposits. These discolorations appeared only at the immediate a r e a s where the salt was in contact with the coating.
Bend-Duc tility The complete results of the bend-ductility evaluations on the 1,000- hour exposed specimens a r e listed in Tables 11, 111, and IV. The data from these tables a r e graphically illustrated in Figures 4, 5, and 6 (550" F ex- In each figure the bend- posure) and in Figures ?, 8, and 9 (95" F exposure).
is presented in bar-chart form with each bar r e - ductility o r shortening data presenting an average shortening value for two o r more replicate specimens of a particular substrate- coating combination.
Four exposure conditions a r e indicated in each figure. These condi- tions a r e : undamaged, no salt; damaged, no salt; undamaged, with salt; and Grouped within each of the exposure conditions a r e damaged, with salt.
separate bars that represent the average results from each substrate-coating combination. The bar for each substrate-coating combination is made with a different pattern s o that particular combinations can be easily followed from one figure to the next. The dashed line in each figure represents the inherent shortening ductility of the substrate involved. The inherent ductility of a S O U T H E R N RESEARCH INSTITUTE rn co * .si Q ld c a l .r( Y c I , g a P P I , .zI P . . 2 .
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during compression of Because of the specimen and bending-fixture geometries, maximum shortening (maximum ductility) ranged between 2 . 0 and 2 . 2 in. All specimens with shortening values less than 2 . 0 in., including those used to measure in- herent ductility, fractured in one o r both bowed members. Some specimens with shortening values between 2 . 0 and 2.2 in. fractured also, but these frac- tures were considered to be insignificant because the shortening values were within the maximum ductility range.
Whether o r not embrittlement has occurred is indicated by comparing the difference between the substrate's inherent ductility and its ductility after exposure.
Because of the type of exposures involved, w e assumed that any significant reduction in shortening was a result of s t r e s s corrosion. In our analysis of the results, ductility reductions of 0 . 2 in. o r more were considered to be significant.
Figure 4, which presents the results from the AM 350 bare and coated specimens exposed to the dry 550" F environment, shows that there was no significant loss in ductility during the f i r s t 1,000 hours, eL+.l?-r in salted o r un- salted specimens.
Therefore, it is indicated that AM 350 alloy is insensitive to hot-salt within 1,000 hours of exposure. If this insensitivity prevails through- out the long-duration exposures, protective coatings would not be necessary f o r A M 350 in this environment, but they would not be harmful i f needed f o r protec- tion in other environments.
The ductility results from Rene' 41 specimens exposed to the dry 550" F atmosphere a r e presented in Figure 5. These results are quite e r r a t i c f o r both bare and coated specimens. F o r example, the bare, damaged specimens ex- hibited g r e a t e r ductility than the bare undamaged specimens, regardless of whether exposed to salt or not. In fact, the bare, undamaged, and unsalted specimens had the poorest ductility of all Rene' 41 specimens subjected to the d r y 550" F environment.
The Aluminum-Modified Silicone and Catalytically-Cured Silicone speci- mens showed similar e r r a t i c results in that neither salt nor prior mechanical damage affected the ductility results in any consistent pattern. The specimens coated with Zinc in Silicate Vehicle were the only ones that produced consistent This reduced ductility in the results, and all of these had reduced ductility.
undamaged and unsalted specimens decreased slightly but steadily f r o m the least severe condition to the most severe condition. Since the inherent ductility of the Rene' 41 substrate was apparently inconsistent, the effect of the coatings is detrimental was obscured, with the possible exception that the zinc coating both in the presence and absence of deposited salt.
SOUTHERN RESEARCH INSTITUTE - 12-
a B a r e Catalytically-Cured Silicone
Aluminum-Modified Silicone Zinc in Silicate Vehicle 2.50 2.00 1.50 d ...I .!i M
4 1.00
9) J A Lc s v1 0.5
H H
H
H
Undamaged Damaged Damaged Undamaged No Salt N o Salt with Salt with Salt Figure 4. AM 350 Bend-Ductility Results a f t e r 1,000 H o u r s Exposure a t 550" E S O U T H E R N R E S E A R C H I N S T I T U T E -13- B a r e Catalytic.ally-Cured Silicone Aluminum-Modified Silicone a Zinc in Silicate Vekiirlt-.
2.50 2 2.00 .!j 1.00 0.5
U
t + H H
Damaged Undamaged Damaged Undamaged No. Salt No Salt with Salt with Salt * One specimen only.
Figure 5 . Rene' 41 Bend-Ductility Results after 1,000 Hours Exposure a t 550" F.
S O U T H E R N R E S E A R C H I N S T I T U T E - 14- Figure 6 contains the ductility results from bare and coated Ti-8Al-lMo-lV specimens after exposure to the dry 550" F atmosphere for 1,000 hours. The ductility of the bare specimens without salt was not significantly affected but, a s expected, some decrease was caused by the mechanical damage. The ductility of the bare specimens with salt, however, was greatly decreased in the undamaged condition and further decreased in the damaged condition. Therefore, it is evi- dent that coatings or some other form of protection w i l l be needed f o r this s u b - strate if it is to be subjected to this type of exposure in service.
Specimens coated with Aluminum-Modified Silicone and Catalytically- Cured Silicone apparently provided the needed protection for 1,000 hours because the undamaged specimens exposed to salt had no significant decrease in ductility.
Although the damaged specimens exhibited significant losses in ductility, the losses were no greater than those in the damaged but unsalted specimens, indi- cating that the losses were due to the previously inflicted damage. The apparent protection provided by Catalytically-Cured Silicone might be misleading because this coating shredded and spalled from the substrate very shortly after exposure to 550" F. It is probable that the salt deposit was removed along with the coating and allowed these specimens to retain the same ductilities as unsalted specimens.
The specimens coated with Zinc in Silicate Vehicle had much lower ductility than the inherent ductility of the substrate and the decreases were uni- form regardless of the differences in exposure conditions. Since these ductili- ties were uniform under all conditions of exposure, the reductions in ductility must be attributed to the coating itself. The reasons for this behavior of the zinc coating have not yet been investigated, The Elphal-coated specimens had low ductilities similar to those of the However, these specimens were from a different lot of zinc-coated specimens.
material that had been retained from the previous program and that had under- gone several rolling and heating treatments during experimental applications of the coating. The losses in ductility might be due to these treatments rather than to shortcomings in the coating. The characteristics of the coating itself should become more apparent when additional specimens from the longer exposures be- come available.
The Flame-Sprayed Aluminum coating could not be evaluated a f t e r 1,000 hours because the limited number of specimens available were assigned to the longer-duration exposures.
L A s shown by the results from the humid 95" F exposure on AM 350 specimens, Figure 7, the bare specimens without salt retained good ductility i n both the damaged and undamaged conditions. The salted specimens, how- ever, were rapidly attacked. Both damaged specimens fractured prematurely before the completion of the 1,000 hours, a s did one of the undamaged speci- mens. The other undamaged specimen retained f u l l ductility. Each of the S O U T H E R N RESEARCH INSTITUTE -15- Bare Ic[B Zinc in Silicate Vehicle Aluminum-Modified Silicone Electrophoretically Deposited Aluminum @ Catalytically-Cured Silicone Flame-Sprayed Aluminum 2.50 2.00 1.50 1.00 0.5 Damaged Undamaged Damaged Undamaged with Salt with Salt No Salt No Salt * No specimens available for this exposure.
Figure 6. Ti-8Al-,lMo-lV Bend-Ductility Results after 1,000 Hours Exposure at 550" F.
S O U T H E R N R E S E A R C H INSTITUTE - 16- B a r e Catalytically-Cured Silicone @ Aluminum-Modified Silicone Zinc in Silicate Vehicle 2.00 1.50 1.00 0.5
H H H
t-----l
Damaged Undamaged Damaged Undamaged No Salt No Salt with Salt with Salt a Average between one specimen that had no significant change in ductility (2.171-in. shortening) and one specimen that fractured during exposure (zero shortening).
* Specimens fractured during exposure ( z e r o shortening).
Figure 7. AM 350 Bend-Ductility Results a f t e r 1,000 Hours a t 95 7 ' Humidity, ~~ S O U T H E R N R E S E A R C H INSTITUTE -17- premature failures occurred near the end of the bowed specimens rather than at the center where the maximum s t r e s s and the scratches were located. Al- though one specimen retained f u l l ductility for some unknown reason, these r e - sults indicate that salt and a humid atmosphere cause rapid s t r e s s corrosion to occur in bare A M 350, and that the attack may be most severe a t a certain critical intermediate stress.
The ductility of all the coated AM 350 specimens with the exception of those coated with Zinc in Silicate Vehicle was essentially unchanged after the humid exposure, indicating that Aluminum-Modified Silicone and Catalytically- Cured Silicone provided adequate stress-corrosion protection. The zinc coat- ing, on the other hand, apparently caused some ductility losses that bore no consistent relationship with the presence of salt o r mechanical damage. The apparent lack of effects from salt on the zinc coated AM 350 is exemplified by the relatively high amount of ductility shown in undamaged specimens with salt a s compared to the lower ductility in those undamaged and without salt.
The ductility results from bare and coated Rene' 41 specimens after exposure to the humid, 95" F atmosphere a r e presented in Figure 8. Contrary to the e r r a t i c ductilities after 550" F exposure (Figure 5), the bare substrate showed a steady ductility drop from the least severe to the most severe exposure condition, indicating that salt-laden humid environments might cause s t r e s s corrosion to occur in Rene' 41 material.
However, specimens coated with Aluminum- Modified Silicone and Catalytically- Cured Silicone showed ductilities that were quite e r r a t i c in relation to the severity of the exposure conditions.
F o r example, the Aluminum-Modified Silicone specimens were more ductile when damaged than when undamaged, either with o r without the presence of salt, and the undamaged, unsalted specimens had l e s s ductility than the others.
On the other hand, the Catalytically-Cured Silicone specimens under the most severe condition (damaged, with salt) were more ductile than those under the l e s s severe conditions. The specimens coated with Zinc in Silicate Vehicle exhibited low ductilities under all four exposure conditions.
The ductilities of titanium specimens exposed to the humid, 95" F atmosphere are presented in Figure 9. Because of a shortage of specimens, no bare specimens and only a few coated specimens (undamaged, with salt) The results from these few specimens were exposed in the 1,000-hour group.
show that there was no ductility loss in the specimens coated with Aluminum- Modified Silicone and Catalytically-Cured Silicone. There was, however, a drop in the ductility of those specimens coated with Zinc in Silicate Vehicle.
S O U T H E R N R E S E A R C H I N S T I T U T E -18- B a r e Catalytically- Cured Silicone Aluminum- Modified Silicone $iB Zinc in Silicate Vehicle 2.50
G
1.00
B
c n 0.5
H H H w
Damaged Unda mage d Dam aged Undamaged No Salt No Salt with Salt with Salt Figure 8. Rene' 41 Bend-Ductility Results a f t e r 1,000 Hours Exposure a t 95% Humidity.
S O U T H E R N R E S E A R C H INSTITUTE -19-
R B a r e
Zinc in Silicate Vehicle
&iil Aluminum- Modified Silicone
a Electrophoretically Deposited Aluminum
Catalytically-Cured Silicone Flame-Sprayed Aluminum 2.50 2.00 rn Q)
4 1.50
.E3 .fj
. ! !
d 0 )
z 1.00
m C 0.5
H H H b - 4
Undamaged Damaged Undamaged Damaged N o Salt No S a l t with Salt with Salt * No specimens available for exposure.
Figure 9. Ti-8AI-lMo-lV Bend-Ductility Results after 1,000 Hours a t 9 5 % Humidity.
S O U T H E R N R E S E A R C H INSTITUTE -20- CONCLUSIONS On the basis of the results from the 1000-hour exposures, we d r a w the following conclusions: 1. The AM 350 SCT stainless steel substrate will require protection f r o m s t r e s s corrosion in salt-laden humid environments.
The inherent ductility of solution-treated and aged Rene' 41 is incon- 2.
sistent to the point of obscuring its vulnerability to s t r e s s corrosion within 1,000 hours.
Duplex annealed Ti-8-1-1 alloy will require protection from s t r e s s 3.
corrosion when exposed to dry salt at 550" F. Its vulnerability to s t r e s s corrosion in salt-laden humid environments has not yet been determined in this program.
4. Aluminum-Modified Silicone on the A M 350 and Ti-8-1-1 substrates provides excellent protection against s t r e s s corrosion f o r 1,000 hours, either under dry 550" F conditions o r humid 95" F conditions. It prob- ably provides protection for Rene' 41 superalloy also, but its effects were obscured because of apparent inconsistencies in the inherent ductility of this substrate.
5 . Catalytically-Cured Silicone provides excellent protection on all three substrates in the humid 95" F environment. Its protective qualities at 550°F a r e in doubt because it quickly shredded and peeled from all three substrates when exposed to the elevated temperature.
all 6. Zinc in Silicate Vehicle has a deleterious effect on the ductility of three substrates after exposure to the humid 95" F environment. In the dry 550" F environment it has no deleterious effects on AM 350 SCT stainless steel, but is possibly deleterious on Rene' 41 and is definitely deleterious on Ti-8-1-1 alloy.
7. Insufficient specimens were available in the 1000-hour exposures f o r , ' indicating the effects of Electrophoretically Deposited Aluminum and Flame-Sprayed Aluminum on the Ti-8-1-1 alloy, which was the only substrate to which these coatings were applied.
SOUTHERN RESEARCH INSTITUTE -21- FUTURE WORK 1 % During the next quarter, bend-ductility evaluations w i l l be performed on specimens from the 3,000-hour exposure interval which ends on 22 June, When necessary for additional clarification of results, we shall make me tal- l, 000-hour and lographic examinations of selected specimens f r o m both the 3,000-hour exposures.
Submitted by: jbm+ J. 0. Honey utt, Jr.
V A s s is tant Metallurgist A. Clyde Willhelm Research Metallurgist Approved: /J. R. Kattus, Director Metallurgy Res ear ch Birmingham, Alabama 15 June 1965
73 2 5 - 141 7 -XI1
(45: 15) c bf SOUTHERN RESEARCH INSTITUTE -22- REFERENCES Holder, S . G., Jr., and Willhelm, A. C., "Protective Coatings for 1.
Sheet Metals in Supersonic Transport Aircraft, 'I final summary re- port from Southern Research Institute to NASA on contract NASr-117, 15 June 1963.
Honeycutt, J. O . , Jr., and Willhelm, A. C., I 1 Evaluation of Protec- 2.
tive Coatings for Skin Materials on Supersonic Transport Aircraft, 'I final summary report from Southern Research Institute to NASA on Contract NASr-117, 24 June 1964.
S O U T H E R N R E S E A R C H I N S T I T U T E