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EFFECT OF PROTECTIVE COATINGS ON THE STRESS- CORROSION PROPERTIES O F SUPERSONIC-TRANSPORT SKIN MATERIALS ELEVENTH QUARTERLY STATUS REPORT to NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For the Period Between1 June, 1965, and 31 August, 1965 Contract No. NASr-117 J. 0. Honeycutt A. C. Willhelm
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Southern Research Institute Birmingham, Alabama 35205 24 September, 1965 7475-1417-XIII .I 4 .
ABSTRACT I This report summarizes the work performed under NASA Contract No. NASr-117 during the period between 1 June and 31 August, 1965. The purpose of the project is to determine whether certain commercially available coatings, selected on the basis of earlier work, will retard o r prevent stress corrosion of substrate alloys that are promising candi- dates for use as the outer skins on supersonic-transport aircraft (SST). The program en- tails the exposure, for durations of 1000, 3000, 5000, and 7000 hr, o f self-loaded specimens to various combinations of scratch damage, hot salt at 550" F, and humid salt a t 95" F .
Susceptibility to damage from stress corrosion w a s evaluated by means of a bend test that revealed the residual ductility of the exposed specimens.
This report presents the results from the 3000-hr exposures, and compares them to the results from the previously reported 1000-hr exposures. Tentative conclusions de- rived from these results are: The AM 350 SCT stainless steel substrate w i l l require protection from stress 1.
corrosion in salt-laden humid environments.
The inherent ductility of the solution-treated-and-aged Rene 41 used in these 2.
experiments is inconsistent to the extent that its vulnerability to s t r e s s corro- sion within 3000 h r is obscured.
3 . Duplex annealed Ti-8AI-lMo-lV alloy w i l l require protection from s t r e s s corro- sion when exposed to dry salt at 550" F.
Aluminum-Modified Silicone provides excellent protection for a t least 3000 h r in 4.
either hot-salt environments at 550" F o r in humid-salt environments a t 95" F.
5. Catalytically Cured Silicone provides excellent protection for a t least 3000 h r in humid-salt environments at 95" F but it quickly shredded from each of the sub- strates in the hot-salt environment at 550" F.
Zinc in Silicate Vehicle apparently has a large deleterious effect on the ductility 6.
of Rene 41 and Ti-8-1-1 regardless of the exposure conditions. It provides some protection on AM 350 but is not a s effective as Aluminum-Modified Silicone o r Catalytically Cured Silicone.
Electrophoretic Aluminum, which was evaluated in a blistered condition and on 7.
specimens with exposed edges, did not provide significant protection f o r Ti-8-1-1 in hot salt a t 550" F, which was the only substrat e-environment combination in which it w a s evaluated.
8. Flame-Sprayed Aluminum, which w a s evaluated on specimens with the inside surfaces essentially uncoated, provided some protection (more than Electro- phoretic Aluminum or Zinc in Silicate Vehicle, but considerably less than Aluminum-Modified Silicone) on Ti-8-1-1 in hot salt at 550" F, which w a s the only substrate-environment combination in which it w a s evaluated.
SOUTHERN RESEARCH INSTITUTE .
TABLEOFCONTENTS Page INTRODUCTION . . . . . . . . . . . . . . .
WORKPERFORMED . . . . . . . . . . . . .
PROCEDURES . . . . . . . . . . . . . . .
RESULTS AND DISCUSSION . . . . . . . . . . .
Visual Examination. . . . . . . . . . . . .
Bend-Ductility . . . . . . . . . . . . . .
CONCLUSIONS . . . . . . . . . . . . . . .
FUTUREWORK . . . . . . . . . . . . . . .
I . . . .
REFERENCES . . . . . .
i SOUTHERN RESEARCH INSTITUTE LIST OF ILLUSTRATIONS L Figure Page
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1 Construction of the self-stressed specimen . . . . . . 2
2 Flow Sheet of Experimental Conditions . . . . . . . . 3
3 Schematic drawing of clamping members with specimen in place Bend-Ductility Results from Undamaged AM 350 after 3000-hr
exposure at 5 5 0 ° F . . . . . . . . . . . . . . 13
5 Bend-Ductility Results from Damaged AM 350 after 3000-hr
exposure a t 550" F . . . . . . . . . . . . . . 14
6 Bend-Ductility Results from Undamaged Rene 41 after 3000-hr
exposure at 550" F . . . . . . . . . . . . . 15
7 Bend-Ductility Results from Damaged Rene 41 after 3000-hr
exposure a t 550" F . . . . . . . . . . . . . . 16
Bend-Ductility Results from Undamaged Ti-8A1- 1Mo-1V after
3000-hr exposure at 550" F . . . . . . . . . . . 17
Bend-Ductility Results from Damaged Ti-8A1- 1Mo- 1V after
3000-hr exposure a t 550" F . . . . . . . . . . . 18
10 Bend-Ductility Results from Undamaged AM 350 after 3000-hr
exposure a t 95% Humidity. . . . . . . . . . . . 20
11 Bend-Ductility Results from Damaged AM 350 after 3000-hr
exposure at 95% Humidity. . . . . . . . . . . . 21
12 Bend-Ductility Results from Undamaged Rene 41 after 3000-hr
exposure a t 95% Humidity. . . . . . . . . . . . 23
13 Bend-Ductility Results from Damaged Rene 41 after 3000-hr
exposure a t 9570Humidity. . . . . . . . . . . . 24
Bend-Ductility Results from Undamaged Ti-8A1- 1Mo- 1V after
3000-hr exposure a t % % y o Humidity . . . . . . . . . 25
ii SOUTHERN RESEARCH INSTITUTE LIST OF TABLES Page Table
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Visual Examination of 3000-hr Exposed Specimens . . . 7
L I AM 350 Bend-Ductility Data-3000-hr Exposure to Dry, II
550°F and Humid, 95"FAtmospheres . . . . . . . 9
1 1 1 Rene 41 Bend-Ductility Data-3000-hr Exposure to Dry,
5 5 0 ° F andHumid, 95" FAtmospheres . . . . . . . 10
Iv Ti-8Al-1Mo-1V Bend-Ductility Data-3000-hr Exposure to
Dry, 550" F and Humid, 95" F Atmospheres . . . . . 11
V iii ~~ SOUTHERN RESEARCH INSTITUTE REPORT ON .
EFFECT OF PROTECTIVE COATINGS ON THE STRESS-CORROSION PROPERTIES OF .
SUPERSONIC- TRANSPORT SKIN MATERIALS INTRODUCTION This report summarizes the progress made during the fifth quarter of a project being performed by Southern Research Institute under Contract No.
This quarter consisted of the period between 1 June, 1965, and NASr-117.
31 August, 1965.
The purpose of this research project is to determine whether selected These data will coatings will protect me tal 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 s u b s t r a t e s to be evaluated w e r e chosen from the results of earlier work on this contract (1, 2)l.
W Pertinent background information and a detailed description of the specimen preparation and environmental exposures, along with the general evaluation procedure, w e r e presented in earlier progress reports and w i l l not be repeated here. Described briefly, the program consists of various stress-corrosion exposures applied to self-loading type specimens construct- ed as shown in Figure 1. The substrates, coatings, exposure conditions and evaluation methods a r e charted in Figure 2.
WORK PERFORMED During this quarter the second group of exposed specimens (3000-hr exposure) w a s removed from the exposure atmospheres, visually examined , rinsed , and subjected to compressive loading for bend-ductility evaluations.
We also re-evaluated the bend ductility of seven 1000-hr-exposure specimens which had fractured prematurely while bending around a radius of the clamp- ing fixture. We found that, with proper clamping, these specimens could be reloaded so that the fracture would occur at the proper location-near the center t of one o r both bowed members. The original ductility data reported for these specimens have been amended in accordance with the new ductility readings , but these corrections caused no significant changes in the comparative status of the coatings and substrates involved.
The numbers in parentheses refer to the references at the end of the report.
SOUTHERN RESEARCH INSTITUTE -2- (a) Machined strip.
(b) S t r i p w i t h e n d s bent.
Spot welds t (d -t 2t) D i s t a n c e (c) Completed specimen.
Construction o f the s e l f - s t r e s s e d F i g u r e 1 .
specimen. (All d i m e n s i o n s are in inches ).
SOUTHERN RESEARCH INSTITUTE -3- Steel Superalloy Titanium Alloy S u b s t r a t e Subs t r a te S u b s t r a t e * ~~ ~~ Coating-Subs trate Com bina tions I I 1 Undain aged
rn Damaged coatings
Salt No Salt Salt No Salt Deposit Deposit Deposit Deposit J J t o Humid D r y D r y D r y Humid Humid Humid D r y 100" F 550" F 100" F 550" F 100" F 550" F 100" F 550" F A
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Exposure T i m e s 1000 H o u r s 3000 H o u r s 5000 H o u r s Bend-Ductility Evaluations
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Metallographic Examination Coating 1 - Aluminum-Modified Silicone Coating 2 - Catalytically Cured Silicone Coating 3 - Zinc in Silicate vehicle
Coating 4 - E l e c t r o p h o r e t i c a l l y Deposited Aluminum
Coating 5 - Flame-Sprayed Aluminum
F i g u r e 2.
Flow Sheet of E x p e r i m e n t a l Conditions SOUTHERN RESEARCH INSTITUTE - 4- A detailed description of the compressive loading fixture used in the bend-ductility evaluations was presented in the previous progress report and w i l l not be repeated here. Described briefly, the fixture consists of two clamp- The con- ing members fitted vertically in a manually operated hydraulic press.
figurations of the clamping members with a specimen in place a r e shown in Figure 3. With the specimen installation shown, some specimens tended to fracture at one o r more of the fixture radii. This w a s corrected by installing specimens so that the tab ends extended approximately 1 /4- in. beyond the fixtures.
A d i a l gage calibrated in 0.001-in. increments is employed to provide a reading of the shortening that occurs in the specimen during compression.
PROCEDURES Following the visual examination of the exposed specimens and the r e - moval of salt from appropriate specimens by water rinsing, w e loaded each
specimen into the clamping members with its (D + 2t) distance (refer to Figure
IC) extending horizontally. The lower movable platen of the press was raised to a position where approximately 1/2-in. of the specimen tab ends would ex- tend into the slots of both clamping members. Previously, the tab ends had been inserted to their f u l l 3/4-in. length as shown in Figure 3, but this tended to cause some specimens to fracture at a fixture radius. By inserting only a portion (1/2-in.) of the specimen tab ends, w e provided freedom f o r the bowed members of ductile specimens to deflect to almost complete compression be- fore bearing against a slot radius.
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 mem- bers. The specimen w a s then locked into position by tightening the set screws against the flat face of the t a b ends.
After positioning and securing the specimen in the fixture, w e placed the dial gage in contact with the lower movable platen and s e t it to the zero po- sition. The specimen w a s then compressed by raising the lower platen with the hydraulic pump. Specimen compression was continued until fracture oc- curred, o r until complete compression (maximum specimen shortening with contact between the tab ends) w a s 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.
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Steel Supporting Block ~~ SOUTHERN RESEARCH INSTITUTE - 6- RESULTS AND DISCUSSION V i s u a l Examination The results of the visual examinations of the 3000-hr-exposed speci- mens are listed in Table I.
This table lists general observations on the appear- ance of each substrate-coating combination after exposure to the dry, 550" F, o r humid, 95" F, atmosphere.
After 3000 h r of exposure to the dry, 550" F atmosphere, the visual ap- pearance of the bare Rene 41 substrate remained unchanged whereas the bare substrates of AM 350 and titanium were discolored. The humid, 95" F exposure caused no visual changes in the bare substrates of titanium and Rene 41, but did, however, cause significant rusting of the bare A M 350 specimens which had been deposited.
The 550" F exposure had no visual effect on any of the substrates coated with Aluminum-Modified Silicone.
However, in the 95" F exposure of the Aluminum- Modified-Silicone-coated specimens, slight rusting occurred on salt laden AM 350 specimens and pin-point corrosion appeared on salt laden and damaged Rene 41 specimens.
The Catalytically-Cured-Silicone-coated specimens showed no visual However, in the dry, 550" F expo- change during the humid, 95" F exposure.
s u r e , this coating shredded and spalled completely from all specimens of each substrate.
Specimens from each substrate material coated with Zinc in Silicate Vehicle contained spotted a r e a s of a brownish-gray discoloration after exposure to the humid, 95" F atmosphere. A grayish-white oxide, appearing in combina- tion with these brownish-gray discolorations, developed on the salt-deposited specimens. The grayish-white oxide w a s especially severe on the salt deposited Rene 41 specimens. The dry, 550" F atmosphere caused no visual changes in any of the zinc coated specimens in the absence of salt. However, in the salt- deposited specimens a grayish-white oxide formed and appeared only a t the im- mediate areas where the salt w a s in contact with the coating.
N o visual changes were observed in the Electrophoretic-Aluminum and Flame-Sprayed-Aluminum-coated specimens, but these were exposed only to the 550" F atmosphere.
In general, the visual appearance of the 3000-hr-exposed specimens showed no marked differences in comparison with the 1000-hr-exposed speci- mens. There w e r e slight differences noted, however, in the specimens with -~ ~ SOUTHERN RESEARCH INSTITUTE -7- Table I Visual Examination of 3000-Hr Exposed Specimens Substrate Exposure Visual Observations a f t e r Exposure Coating AM 350 550" F Dark brownish-bronze color Over e n t i r e surface.
B a r e 95" F Rust spots on specimens with salt.
550" F No change.
Aluminum- Modified Silicone AM 350 95" F Slight r u s t and s a l t stains on s a l t deposited specimens- no change in unsalted specimens.
550" F Coating spalled and shredded wer entire surface within Catalytically Cured Silicone AM 350 48 hr.
95" F No change.
No 550" F Grayish-white oxide only on specimens with salt.
Zinc in Silicate Vehicle AM 350 change in specimens without salt.
95" F Spotted areas of brownish-gray discolorations on all specimens exposed. Grayish-white oxide noted only on specimens with salt.
550" F Yellowish gold color Over entire surface.
B a r e Titanium 95" F N o specimen exposed.
Titanium 550" F No change.
Aluminum- Modified Silicone No unsalted specimen 95" F Salt stains-otherwise no change.
exposed.
5%" F & shredded Over entire surface within 48 hr.
Catalytically Cured Silicone Titanium Coating spalled 95" F No change. Only salt-deposited specimens exposed.
Zinc in Silicate Vehicle Titanium 550" F Grayish-white oxide only on specimens with salt. No change in specimens without salt.
95" F Only s a l t deposited specimens exposed-spotted areas of grayish-white oxide.
Electrophoretic Aluminum Titanium 550" F No change.
95" F No specimens exposed.
550" F Only s a l t deposited specimens exposed. No change.
Flame-Sprayed Aluminum Titanium N o specimens exposed.
95" F B a r e Rene 4 1 550" F Slight s a l t stains-otherwise no change.
95" F No change.
550" F No change.
Aluminum- Modified Silicone Rene 41 95" F No change i n unsalted specimens. On salt-deposited speci- mens there were slight s a l t stains on the undamaged and heavy s a l t stains with s e v e r a l areas of pin-point corrosion on the damaged specimens.
5 % " F Coating spalled & shredded Over entire s u r f a c e within 48 hr.
Catalytically Cured Silicone Rene 4 1 95" F N o change.
No change Zinc in Silicate Vehicle Rene 4 1 550" F Grayish-white oxide only o n specimens with salt.
in specimens without salt.
95" F Spotted arras of brownish-gray discoloration on all speci- Severe grayish-white oxide coating on specimens mens.
with salt.
SOUTHERN RESEARCH INSTITUTE -8- .
coatings of Zinc in Silicate Vehicle and Aluminum-Modified Silicone. These dif- ferences w e r e : Under humid, 95" F conditions the grayish-white oxide on the 1.
3000-hr zinc-coated and salt-deposited specimens w a s much heavier than the oxide layer on similar specimens from the 1000-hr exposure. This oxide layer w a s more prominent on the Rene 41 substrate than on the other substrates.
2. Under humid, 95" F conditions the Aluminum-Modified-Silicone coating on AM 350 and Rene 41 had greater amounts of rusting This corro- and pin-point corrosion after the 3000-hr exposure.
sion was prominent only on the salt deposited and damaged speci- mens.
Bend -Duc tility The complete results of the bend-ductility evaluations on the 3000-hr- exposed specimens are listed in Tables I I , 1 1 1 and IV. The data from these tables, in combination with similar data from the 1000-hr-exposure specimens, are graphically illustrated in Figures 4, 5, 6, 7, 8,and 9 (550" F exposure) and in Figures 10, 11, 12, 13, and 1 4 (95" F exposure). In each figure the bend-ductility (shortening) data is presented in bar- chart form with each bar representing an average shortening value f o r two o r more replicate specimens of a particular substrate-coating combination.
Within each temperature exposure (95" F and 550" I ? ) > each suhstrate- coating com bination is subjected to four exposure conditions. These conditions are: undamaged, no salt; damaged, no salt; undamaged, with salt; and damaged, with salt. At each temperature, two figures are allotted to each sub- strate, with each figure containing only two of the exposure conditions.
F o r each substrate, the data from the undamaged specimens a r e pre- sented in one figure, and the other f i g u r e contains the data from the damaged specimens. Grouped within each of the exposure conditions are separate bars that represent the average bend-ductility results from each substrate-coating combination for the 1000-hr and 3000-hr exposure intervals. Sufficient space has been reserved for future additions of the 5000-hr and 7000-hr data.
The bars for each substrate-coating combination are made with a dif- ferent pattern s o that particular combinations can be easily followed from one figure to the next. The dashed line at 2.0-in. shortening in each figure w a s established on the basis of specimen and fixture geometry, and represents the transition from f u l l ductility to an embrittled condition. Ductility values below this line were considered to indicate significant embrittlement, whereas values at o r above the line indicated f u l l ductility. All specimens with shortening SOUTHERN RESEARCH INSTITUTE -9- o w 0 0 uaQ) m a 00, 0 0 lnln In* w w & & &c; & & O O D W 88 O O D C Q 8 8 Inla -In Y4".
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d d d .9 . 5 5 v v v N N N SOUTHERN RESEARCH INSTITUTE -12- ' values less than 2.0-in. fractured in one o r both bowed members. Some speci- mens with shortening values between 2.0 and 2.5 in. fractured also, but these fractures were considered to be insignificant because the shortening values w e r e within the maximum ductility range.
Because of the type of exposures involved, w e assumed that any signifi- cant reduction in shortening was a result of s t r e s s corrosion unless explainable by other causes. In our analysis of the results, ductility reductions of 0.2 in. o r more w e r e considered to be significant. The apparent improved ductility of the fully ductile 3000-hr specimens can be attributed to the previously explained change in the technique used for installing specimens in the fixture. This instal- lation change provided a greater original distance between the clamping members of the fixture and thereby allowed fully ductile specimens to attain a greater shortening value.
Figures 4 and 5, which present the results from the AM 350 bare and coated specimens exposed to the dry, 550" F environment, show that, after 3000-hr exposures, damaged and undamaged specimens sustained no significant losses in ductility, either in salted o r unsalted conditions. It is indicated that the AM 350 alloy remains insensitive to hot-salt after 3000 h r of exposure.
Should this insensitivity continue to prevail throughout the longer-duration expo- s u r e s , it would indicate that protection from s t r e s s corrosion would not be neces- s a r y for AM 350 in this environment, but that protective coatings would not be harmful if needed for other reasons.
The ductility results from the Rene 41 specimens exposed to the dry, 550" F atmosphere are presented in Figures 6 and 7. The results have con- the 3000-hr tinued to he erratic f n r hOth h2re 2Ed cnated spEfcirr?eI1s thrwgh exposure. F o r example, the bare, undamaged specimens with salt exhibited greater ductility than those undamaged without salt, and the bare, damaged spe- cimens with salt w e r e more ductile than those without salt. The coated Rene 41 specimens exhibited similarly erratic and unexplainable ductility patterns.
Therefore, it is indicated that the effects of the coatings have been obscured by the erratic ductility of the substrate.
Figures 8 and 9 contain the ductility results from bare and coated Ti-8A1-1Mo-1V specimens after exposure to the dry, 550" F atmosphere.
After 3000 h r the bare specimens without salt continued to retain good ductility in the undamaged condition, but a considerable ductility decrease occurred in those with previously inflicted mechanical damage. However, the bare speci- salt exhibited large ductility losses in both the 1000-hr and 3000-hr mens with exposures. The similar ductilities produced by both exposure times indicate that the 550" F-dry-salt condition causes its maximum ductility damage within 1000 hr. Therefore, it remains evident that coatings o r some other form of protection will be needed f o r this substrate if it is subjected to this type of exposure in service.
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I I I I SOUTHERN RESEARCH INSTITUTE The results from the Aluminum-Modified-Silicone and Catalytically- Cured-Silicone coatings indicate that they continue to provide protection after 3000 hr. The undamaged specimens exposed to salt showed no losses in ductility, although some anomalous loss w a s exhibited by the undamaged specimens not exposed to salt. The ductility losses in the damaged speci- mens, in both the salted and unsalted conditions, w e r e equivalent to the losses produced by the mechanical damage alone, showing that the two coatings pro- vided protection from the hot-salt exposure. The apparent protection provided by the Catalytically-Cured-Silicone coating is misleading because it spalled from the substrate within 48 h r after the s t a r t of the exposure. The resulting bare specimens probably retained their ductility because the salt deposit w a s removed by the spalled coating.
The ductilities of the specimens coated with Zinc in Silicate Vehicle continued to remain uniformly low after 3000 hr of exposure. Since the ducti- lities of these zinc coated specimens in both the 1000-hr and 3000-hr exposures have been most uniform under all conditions of exposure, the low ductilities must be attributed to the coating itself. The reasons for this behaviour of the zinc coating are still undetermined.
I The Electrophoretic-Aluminum- coated specimens were within the f u l l ductility range in both the undamaged and damaged conditions when no salt w a s involved. However, in the presence of salt, both the undamaged and damaged specimens exhibited low ductilities. The ductilities of these salted specimens w e r e in the same range as those ductilities exhibited by the bare, salt-deposited specimens. These ductility losses might have occurred because the edges of the specimens w e r e not coated, o r because the coating w a s in poor condition from the blisters that developed when the coating was applied by an experimental technique.
The only Flame-Sprayed-Aluminum-coated specimens exposed at 550" F were those exposed to s a l t in the undamaged and damaged conditions. The ducti- lity values of both the undamaged and da'maged specimens were well below the maximum ductility range. However, no unsalted specimens were involved in this exposure interval. Therefore, the cause of the reduced ductility will be in doubt until additional specimens from the longer-duration exposures become available.
A s shown by the results plotted in Figures 10 and 11, which pertain to the humid, 95" F exposure on AM 350 specimens, the bare specimens without salt retained good ductility in both the undamaged and damaged conditions.
I I Most of the bare specimens with salt deposits, however, were rapidly attacked and fractured long before the 1000-hr exposure had been completed. On the l other hand, two undamaged specimens, one within the 1000-hr group and one within the 3000-hr group, did not fracture during the exposure and retained f u l l ductility in the bend test.
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0 0 f) SOUTHERN RESEARCH INSTITUTE -22- In each damaged and salt-exposed specimen, all of which failed prema- turely, the fracture occurred in both bowed members. In each of these speci- mens, one bowed member fractured near its center at the location of maximum s t r e s s (and scratch damage) whereas the other bowed member failed near its The tab end. We were unable to discern which bowed member fractured first.
premature failure in the undamaged specimen occurred near the tab end of only one bowed member.
Although two of the undamaged specimens retained f u l l ductility, the re- s u l t s from the majority of specimens indicate that salt and a humid atmosphere The anomalous ductile can cause rapid s t r e s s corrosion in the AM 350 material.
behavior of two of the undamaged specimens indicate that the attack might be dependent upon obscure critical conditions that w e r e not controlled in these experiments.
The f u l l ductility shown by the specimens of Aluminum-Modified-Silicone and Catalytically-Cured-Silicone coatings on AM 350 remained unchanged after 3000 h r of humid exposure, indicating that these coatings provided adequate stress-corrosion protection. The specimens coated with Zinc in Silicate Vehicle and exposed for 3000 h r appeared to regain some of the ductility losses exhibited by the 1000-hr specimens. Whereas the 1000-hr specimens w e r e fully ductile only in the undamaged and salt-exposed condition (an anomaly in itself), the 3000-hr specimens w e r e ductile in all conditions except the damaged and salt-exposed condition (an apparently reasonable result). Although the 3000-hr results indicate that Zinc in Silicate Vehicle provides a significant amount of protection in humid and salt-laden environments, the overall results indicat e that this coating has an erratic effect on the ductility of the substrate.
The ductility results from bare and coated Rene 41 specimens after ex- posure to the humid, 95" F atmosphere are presented in Figures 12 and 13.
Although the 1000-hr results from bare specimens had a reasonable pattern of reduced ductilities as related to the exposure conditions, the 3000-hr results reflected the erratic ductility that is apparently characteristic of the Rene 41 substrate. Similarly erratic patterns w e r e developed with the 1000-hr and 3000-hr coated specimens. Therefore, the protective qualities of the coat- ings could not be accurately evaluated.
The ductilities of the titanium specimens exposed to the humid, 95" F Because of a shortage of specimens, atmosphere are presented in Figure 14.
no bare specimens and only a f e w coated specimens w e r e exposed with the 1000-hr and 3000-hr groups. These few specimens represented only one condition (undamaged, with salt). The specimens coated with Aluminum- Modified Silicone and Catalytically Cured Silicone, after 3000-hr exposure, attained f u l l ductility in contrast to the deteriorated ductility that r e s u l t e d from 1000-hr exposure. The greater ductility drop in the 1000-hr specimens coated with Zinc in Silicate Vehicle was apparently diminished by the 3000-hr Further interpretation of exposure, but not to the level of complete ductility.
these results might be possible when additional specimens from the longer ex- posure be come available.
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b CONCLUSIONS On the basis of the results from the 1000-hr and 3000-hr exposures, we draw the following conclusions: The AM 350 SCT stainless steel substrate w i l l require protection 1.
from s t r e s s corrosion in salt-laden humid environments.
2. The inherent ductility of the solution-treated-and-aged Rene 41 used in these experiments is inconsistent to the extent that its vulnerability to s t r e s s corrosion within 3000 h r is obscured.
Duplex annealed Ti-8A1-1Mo-1V alloy w i l l require protection 3.
from s t r e s s corrosion when exposed to dry salt at 550" F.
Its vulnerability to s t r e s s corrosion in salt-laden humid environ- ments has not yet been determined in this program.
4. Aluminum-Modified Silicone on AM 350 and Ti-8-1-1 substrates provides excellent protection against s t r e s s corrosion for 3000 hr, 550" F conditions o r humid, 95" F conditions. It either under dry, probably provides protection for Rene 41 superalloy also, but its effects w e r e obscured because of inconsistencies in the inherent ductility of this substrate.
5 . Catalytically Cured Silicone provides excellent protection on AM 350 and Ti-8-1-1 substrates in the humid, 95" F environment.
Its apparently excellent protective qualities at 550" F a r e mislead- ing because it quickly shredded from all three substrates when exposed to the elevated temperature. Its effects on Rene 41 super- alloy were obscured by the inconsistencies in the inherent ductility of this substrate.
6. Zinc in Silicate Vehicle apparently has a large deleterious effect on the ductility of the Rene 41 and Ti-8-1-1 substrates regardless of the exposure conditions. It provides some protection on A M 350 in the humid, 95" F environment but is not as effective as Aluminum- Modified Silicone and Catalytically Cured Silicone, especially in the presence of scratch damage. In the dry, 550" F environment the zinc coating has no deleterious effects on A M 350, but its protective qualities a r e obscure because this environment was not harmful to the bare substrate.
SOUTHERN RESEARCH INSTITUTE .
- 27- e 7. The Electrophoretic -Aluminum coating did not provide significant protection for Ti-8-1-1 in the 550" F hot-salt environment. How- ever, the failure to protect might have been caused by the uncoated edges o r the blistered condition of the coating. This coating was not evaluated on other substrates o r under other exposure conditions.
The Flame-Sprayed Aluminum coating provided some protection for 8.
Ti-8-1-1 in the 550" F hot-salt environment.
It was more effective in this regard than Electrophoretic Aluminum o r Zinc in Silicate Vehicle but much less effective than Aluminum-Modified Silicone o r Catalytically Cured Silicone. However, its effectiveness might have been reduced because the inside surfaces of the specimens w e r e essentially uncoated. This coating w a s not evaluated on the other substrates or under other exposure conditions.
FUTURE WORK During the next quarter, bend-ductility evaluations will be performed on specimens from the 5000-hr exposure interval which ended on 14 September.
When necessary for additional clarification of results, w e shall make metallo- graphic examinations of selected specimens from the looo-hr, 3000-hr, and 5000- h r exposures. In addition, we shall collect samples and perform a preliminary evaluation of new coatings that have been brought to our attention. A s soon as the 5000-hr specimens have been evaluated, we shall arrange to visit approxi- mately three of the leading aircraft manufacturers to d i s c u s s all aspects of the coatings problem with them.
Submitted by: J. 0. Honeycutt, Jr.
Assistant Metallurgist * Approved: A. CGde Willhelm Research Metallurgist I J. R. Kattus, Director Metallurgy Research Birmingham, Alabama 24 September, 1965 7475- 1417-XIzI (45 : 15)cbf SOUTHERN RESEARCH INSTITUTE v --28- REFERENCES 1. Holder, S . G. , Jr. , and Willhelm, A. C. , 11 Protective Coatings for Sheet Metals in Supersonic Transport Aircraft, 'I final summary re- port from Southern Research Institute to NASA on contract NASr-117, 15 June, 1963.
I 2. Honeycutt, J. 0. , Jr. , and Willhelm, A. C., 11 Evaluation of Protec- tive Coatings for Skin Materials on Supersonic Transport Aircraft, " final summary report from Southern Research Institute to NASA on Contract NASr-117, 24 June, 1964.
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