Protective coating to retard crack growth in aluminum alloy

A protective coating to retard crack growth in aluminum and aluminum alloys has particular utility when applied to the surfaces of structural parts of aircraft providing a crack growth inhibitor to a crack surface when and as it develops in such parts. This inhibitor is a compound having a vapor pressure high enough to volatilize the inhibitor and emit molecules which will react with the surface of the metal to form a protective film. It is blended within a selected primer and is thereby available for interaction with the surface and tip of any crack which may initiate beneath the primer. A topcoat of low-permeability polyurethane covers the inhibitor-reinforced primer to prevent escape of the inhibitor from the primer.

TECHNICAL FIELD 
This invention relates to protective coatings for aluminum alloys and 
particularly to protective coatings to retard crack growth in such alloys. 
A major limitation in the life of modern aircraft is the development and 
growth of fatigue cracks in the airframe structure. Aircraft are designed 
so that cracks in structural parts will not grow to a critical length 
during the life of the aircraft. These designs are based on data obtained 
by fatigue testing precracked specimens under controlled laboratory 
conditions. Typically, this would be at room temperature and at a relative 
humidity of about fifty percent (50%). Humid environments accelerate the 
rate of fatigue crack propagation. 
Aircraft which operate in coastal regions have a shortened life span unless 
the environment effects are neutralized. This is accomplished in part, by 
protecting all structural parts with paint systems which contain corrosion 
inhibitors and which are relatively impermeable to the diffusion of 
moisture. Such protective paints are of no effect, however, once a crack 
is initiated and the base metal becomes exposed. 
BACKGROUND ART 
Combinations of inhibitors have been heretofore proposed which are 
effective in slowing the rate of crack propagation in high strength steel. 
"Inhibition of Crack Propagation of High Strength Steels through Single 
and Mulifunctional Inhibitors" AFML-TR-76-120, August 1976 describes how 
crack growth in high strength, low alloy steels may be retarded when 
maintained in a aqueous solution by the presence of oxidizing inhibitors 
such as chromate or by the presence of nitrite ions. It also demonstrates 
that a combination of borax and nitrite are more effective than single 
inhibitors in controlling crack propagation in high strength steels 
provided these are maintained in liquid. Experimental results showed this 
system to be equally effective in the presence of sodium chloride and in 
both sustained load stress-corrosion and cyclic load corrosion-fatigue 
conditions. 
The literature also contains data on the use of inhibitors to slow crack 
growth in aluminum alloys. Again, an aqueous solution is essential to this 
operation. "Mechanisms of Corrosion Fatigue Crack Propagation in Al-Zn-Mg 
Alloys" MIT, Department of Metallurgy and Materials Science, February 1972 
discloses that the addition of nitrite ions drastically reduces the 
corrosion fatigue crack growth rates of peak aged 7075-T6 aluminum tested 
in sodium chloride solution. The nitrite ions displace the chloride ions 
from the crack tip and change the fracture morphology from brittle to 
ductile. In "Material Evaluation: Part II-Development of Corrosion 
Inhibitors", AFML-TR-79-4127, September 1979, retardation of crack growth 
in high strength aluminum alloys exposed to distilled water and to 0.1 
Molar sodium chloride solutions is effected through the use of 
borax-nitrite and piperazine inhibitors. 
In summary, the prior art disclosures on the use of inhibitors to control 
crack growth rate are predicated upon a continuous exposure of the crack 
path to solutions which contain the inhibitors. In the airplane 
application there is no liquid to carry the inhibitor to the crack tip and 
surfaces. At best, therefore, the prior art practices have been to 
periodically wash or rinse the aircraft with solutions containing the 
various inhibitors in an effort primarily to remove all salt residue which 
are known to accelerate crack growth. 
DISCLOSURE OF INVENTION 
The present invention appreciates the above short-comings in the state of 
the art and proposes to overcome these by the provision of a protective 
coating especially designed and adapted to retard crack growth in aluminum 
and aluminum alloy. In so doing it is recognized at the outset to more 
closely approximate the normal aircraft environmental exposure, namely air 
with varying degrees of relative humidity; but most important there is no 
liquid to convey the inhibitor to the crack surfaces and tip. 
In contrast to the prior art, therefore, the method of inhibition 
envisioned by the present invention is absorption of volatile crack growth 
inhibitors on the freshly generated fracture surface and at the tip of the 
advancing crack. To this end it is proposed herein to employ such a crack 
growth inhibitor incorporated into a selected primer for stress-critical 
areas where the inhibitor volatilizes and is available for reaction with 
the fresh surface of any crack which might form. This volatile crack 
growth inhibitor reinforced primer may be and preferably is covered with a 
low-permeability organic film, as a topcoat to prevent the loss of the 
inhibitor to the atmosphere. 
More specifically, the proposed volatile crack growth inhibitor is a 
compound having a high vapor pressure which will react with the surface of 
the metal to form a protective film. It is blended with the primer, which 
is an organic paint, i.e., a polymeric organic material mixed with a 
solvent to facilitate its application in the form of a thin film over a 
substrate or metal. This primer is one which is compatible with the 
selected inhibitor with respect to the curing properties thereof and the 
physical properties of the ultimate coating. Preferably either MIL-P-23377 
epoxy-polyamide or MIL-P-87112 polysulfide are to be used in this 
application. The epoxy-polyamide material is obtainable from Andrew Brown 
Division of Koppers Company, Inc., 656 Owenby Drive, Marietta, Georgia 
30060 or from DeSota, Inc., 1700 S. Mound Prospect Road, DesPlaines, 
Illinois 60018. The polysulfide material is obtainable from Products 
Research and Chemical Corporation, 2919 Empire Avenue, Burbank, California 
91504 under the company designation PR-1432GP.

BEST MODE FOR CARRYING OUT THE INVENTION 
To the primer, either the epoxy-polyamide or the polysulfide material, add 
approximately 5%, i.e. between 4% and 6% by weight of the volatile crack 
growth inhibitor. The most effective inhibitors have been found to be (1) 
a fatty acid amine such as that known commercially as CORTEC VCI 560 
compound and obtainable from ACR Electronics, Inc., 3901 North 29th 
Avenue, Hollywood, Florida 33020, and (2) an cyclic amine compound, 
preferably one which is the reaction product of hexafluoroisopropanol and 
cyclohexylamine mixed in a 1 to 1 molar ratio. The cyclic amine is 
preferred over the fatty acid amine because when compounded as herein 
proposed it is effective not only in retarding the cracking growth of 
aluminum in a high humidity environment but also has no detrimental effect 
in a moisture free situation. All other known volatile crack growth 
inhibitors including CORTEC VCI 560 compound are significantly effective 
only in a humid environment and in the absence of moisture, accelerate to 
some degree the rate of crack growth which can be highly objectionable in 
some aircraft operational areas. 
The preferred method of mixing the inhibitor with the primer is to 
calculate the amount of inhibitor needed to make approximately a 5% by 
weight formulation. The correct weight of inhibitor is then dissolved in 
just enough methyl ethyl ketone solvent to dissolve it completely. All 
mixing operations are done at room temperature, i.e., about 80.degree. F. 
(23.3 Celsius). 
Both of the above named primers are two part systems, i.e., composed of a 
base material and a catalyst. The catalyst is added to the base material 
and thoroughly mixed. The methyl ethyl ketone containing the dissolved 
corrosion inhibitor is then mixed with the primer. Where a one part system 
primer is employed the corrosion inhibitor dissolved in the methyl ethyl 
ketone is blended directly into the primer. 
The ultimate coating may be applied to the aircraft or aircraft part by the 
conventional methods of spraying or brushing. Inasmuch as the inhibitors 
are volatile and tend to evaporate from the primer, it is recommended that 
the inhibitor reinforced primer be overcoated as stated above with a low 
permeability topcoat. An acrylic, epoxy resin or polyurethane may be 
employed for this purpose, the polyurethane paint often used on aircraft 
being the preferred. If a crack should initiate beneath the topcoat, the 
inhibitor vapor reacts with the fresh surface and slows down any further 
growth of the crack. 
Comparative Tests 
In order to demonstrate the effectivity of the protective coating of this 
invention the following test is offered: 
a strip of 7075-T6 aluminum 24 inches (60.96 cm) long, 4 inches (10.16 cm) 
wide and 0.1 inches (0.254 cm) thick was prepared with three (3) machined 
slots at spaced intervals. 
the strip was subjected to loads fluctuating between 2,500 and 10,000 psi 
(1795.5 and 7190 kilograms per square centimeter) at a rate of 6 cycles 
per minute until an initial crack developed in the strip adjacent each 
slot. 
the back side of the top slot was painted with about a 0.001 inch (0.00254 
cm) thick film of standard epoxy polyamide primer (MIL-P-23377 which 
includes about 8% chromate corrosion inhibitor). 
the back side of the middle slot was painted with about a 0.001 inch 
(0.00254 cm) thick film of the same primer formulated with 5% by weight of 
an cyclic amine vapor phase inhibitor 
(hexafluoroisopropanolcyclohexylamine). 
the bottom slot was left untreated as the control. 
a Petri dish in which a water saturated swab of cotton was placed was 
sealed around each slot to assure a 100% relative humidity atmosphere. 
the strip was cycled as before until the most rapidly growing crack reached 
the length of 1 inch (2.54 cm). While cycling the three crack lengths were 
monitored at regular intervals of about 2 hours with the aid of a 20 power 
microscope and a tabulation was made of stress cycles versus crack length 
for each of the cracks. 
Following are the results after 25,000 fatigue cycles: 
Bottom Slot (Control):0.24 inches (0.609 cm) 
Top Slot:0.19 inches (0.483 cm) 
Middle Slot:0.06 inches (0.152 cm) 
The above shows that the crack exposed to the protective coating of this 
invention (middle slot) grew at about 25% of that of the control (bottom 
slot) while the crack exposed to the standard primer with corrosion 
inhibitor grew at about 76% of that of the control. Stated differently, 
the crack exposed to the protective coating herein proposed grew at about 
one-third the rate of the crack exposed to the standard primer with 
corrosion inhibitor. 
In order to establish the superiority of a cyclic amine volatile inhibitor, 
over other amine volatile inhibitors in a low humidity (dry) environment 
tests were conducted in a similar manner as above except that no primers 
were used and the test inhibitors were deposited in the Petri dish without 
actual contact with the associated crack and one (1) gram of silica gel 
dessicant was substituted for the water saturated swab. The same cycling 
and monitoring procedures were followed and the results are tabulated 
below: 
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Inhibitor Crack Length at 40,000 Cycles 
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None (Control) 0.12 inches (0.305 cm) 
Cyclic Amine 0.11 inches (0.279 cm) 
(Hexafluoroisopropanol- 
Cyclohexylamine) 
Non-Cyclic Amine (Thiourea) 
0.40 inches (1.016 cm) 
(Guanine) 0.15 inches (0.381 cm) 
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The above shows that the non-cyclic amines actually increase the rate of 
crack growth compared with the control while the cyclic amine produced a 
slight decrease in crack growth rate. The two non-cyclic amines selected 
above show the relatively wide range of crack growth with the particular 
amine. In no case tested, however, did any non-cyclic amine perform as 
well as the control specimen when exposed to a dry environment. Of the 
cyclic amines tested only hexafluoroisopropanol-cyclohexylamine 
outperformed the control specimen in a dry environment.