Coating composition and method of forming

A coating composition comprises a slurry consisting essentially of an aluminum neutralized phosphate bonding solution and aluminum powder. The bonding solution advantageously contains a relatively small but essential amount of vanadium pentoxide and, preferably, magnesium. A process for forming the bonding solution component of the coating slurry includes equilibrating an aqueous phosphate solution with a small but controlled and necessary amount of solute aluminum prior to adding aluminum powder to form the slurry. The present invention overcomes the problem of bonding solutions which require environmentally disadvantageous chromates or molybdates.

TECHNICAL FIELD 
This invention relates generally to a corrosion and oxidation resistant 
coating composition and a method of forming said coating, and more 
particularly to a chromium and molybdenum free bonding solution component 
of the coating composition containing a relatively small but essential 
amount of vanadium pentoxide and a method for forming said composition. 
BACKGROUND ART 
Aluminum metal-phosphate coating compositions for protecting metallic 
surfaces from oxidation and corrosion, particularly at high temperatures, 
are well known in the art. For example, U.S. Pat. No. 3,248,251 issued 
Apr. 26, 1966 to Charlotte Allen teaches a coating composition containing 
chromium and/or molybdenum to inhibit the reaction between an aqueous, 
acidic, phosphate component of the composition and a solid particulate 
metallic material, preferably aluminum powder. 
Heretofore, phosphate-aluminum powder coating compositions for protecting 
metallic surfaces from oxidation and corrosion have been based on an 
acid-base reaction to neutralize the bonding solution and contain either 
hexavalent chromium or molybdenum to inhibit the oxidation of metallic 
aluminum. Both hexavalent chromium and molybdenum are considered toxic 
chemicals and are therefore environmentally disadvantageous. In 
particular, hexavalent chromium is rated as a carcinogen. Molybdenum is 
classified as a toxic heavy metal. 
The present invention is directed to overcoming the problems set forth 
above. It is desirable to have an oxidation and corrosion-resistant 
coating for metallic surfaces that does not require either chromates or 
molybdates to stabilize the reaction between the bonding solution and a 
particulate material component, e.g., powdered aluminum. Furthermore, it 
is desirable to have a bonding solution for such coatings that is free of 
both chromium and molybdenum. 
DISCLOSURE OF THE INVENTION 
In accordance with one aspect of the present invention, a coating 
composition comprises a mixture of a bonding solution and aluminum powder, 
in a ratio of about 100 ml of the bonding solution to from about 50 to 
about 150 g of the aluminum powder. The bonding solution consists 
essentially, by weight, of about 47% to about 80% H.sub.2 O, about 15% to 
about 35% H.sub.3 PO.sub.4, less than about 20% of at least one magnesium 
compound, from about 0.1% to 3.0% V.sub.2 O.sub.5, and aluminum in 
solution in an amount sufficient to substantially equilibrate said bonding 
solution with respect to aluminum. 
In another aspect of the present invention, a method of forming a slurry 
coating, comprises mixing together about 500 to about 900 parts by weight 
water and about 150 parts by weight H.sub.3 PO.sub.4 and forming a dilute 
phosphoric acid solution. Aluminum, either in its elemental form or as a 
compound, is added in an amount sufficient to substantially equilibrate 
the dilute phosphoric acid solution with respect to aluminum. Magnesium, 
either as a carbonate or oxide in an amount less than about 120 parts by 
weight, may also be added to the mixture. Vanadium pentoxide is added in 
an amount of from about 1 to about 30 parts by weight. The mixture is 
stirred, or agitated, for a period of time sufficient to permit completion 
of the reaction of the added compounds with phosphoric acid, and form a 
bonding solution that is substantially equilibrated with respect to 
aluminum. The equilibrated bonding solution is then mixed with aluminum 
powder to form a slurry containing from about 50 g to about 150 g of 
aluminum powder for each 100 ml of the neutralized bonding solution.

BEST MODE FOR CARRYING OUT THE INVENTION 
In the preferred embodiment of the present invention, a coating composition 
for ferrous metal surfaces consists essentially of a bonding solution 
neutralized by an oxidation-reduction reaction, and finely divided 
aluminum particles. 
The bonding solution component of the coating embodying the present 
invention requires that it contain sufficient aluminum in solution so that 
it is substantially equilibrated with respect to aluminum, i.e., that the 
amount of aluminum in solution be substantially at the saturation point 
and the bonding solution therefore essentially inert with respect to any 
subsequent additions of aluminum. Aluminum powder (Al), alumina (Al.sub.2 
O.sub.3) or aluminum hydroxide (Al[OH].sub.3) may be used to provide 
equilibration of the bonding solution. If alumina or aluminum hydroxide 
are selected, it is desirable to heat the mixture to increase the reaction 
rate. Magnesium, while not essential, may desirably be used to at least 
partially neutralize, i.e., reduce the acidity, of the aqueous phosphoric 
acid mixture either before or after equilibration of the mixture with 
aluminum. It has also been found that the addition of a relatively small, 
but essential, amount of vanadium pentoxide (V.sub.2 O.sub.5) to the 
bonding solution significantly enhances the oxidation and corrosion 
resistance properties of the coating following exposure to a high 
temperature environment, such as that found in the compressor section of a 
gas turbine engine during operation of the engine. 
Preferably, the bonding solution includes either magnesium carbonate 
(MgCO.sub.3) or magnesium oxide (MgO), and has a composition consisting 
essentially of, by weight, from about 47% to about 80% H.sub.2 O from 
about 15% to about 35% H.sub.3 PO.sub.4, less than about 20% of either 
MgCO.sub.3 or MgO, 0.1% to about 3.0% V.sub.2 O.sub.5, and from about 0.1% 
to about 3.5% aluminum in solution. If magnesium, either as a carbonate or 
oxide, is not used as an aid to neutralization of the bonding solution, 
aluminum in amounts represented by the upper limits of the above stated 
ranges will be required to sufficiently neutralize the phosphoric acid 
solution and also equilibrate the solution with respect to aluminum. As 
used herein, the formula "H.sub.3 PO.sub.4 " to is used with reference to 
absolute, or 100% pure, full strength phosphoric acid; the term 
"phosphoric acid" means the 75% (industrial) strength phosphoric acid that 
is commonly commercially available; and the term "phosphoric acid 
solution" is used to identify a water-diluted solution of phosphoric 
acid. 
When MgCO.sub.3 is used as a partial neutralizing agent, the bonding 
solution desirably has a composition consisting essentially of, by weight, 
from about 47% to about 67% H.sub.2 O, from about 27% to about 35% H.sub.3 
PO.sub.4, no more than about 20% MgCO.sub.3, from about 0.1% to about 3.0% 
V.sub.2 O.sub.5, and from about 0.1% to about 3.5% aluminum in solution. 
Preferably, the magnesium carbonate neutralized bonding solution consists 
essentially of, by weight, about 62% H.sub.2 O, about 27% H.sub.3 
PO.sub.4, about 9% MgCO.sub.3, about 1.7% V.sub.2 O.sub.5, about 2% 
Al(OH).sub.3 and about 0.2% aluminum powder. 
When, MgO is used as a partial neutralizing agent, the composition 
desirably consists essentially of, by weight, from 52% to about 80% 
H.sub.2 O, from about 15% to about 35% H.sub.3 PO.sub.4, no more than 
about 10% MgO, from about 0.1% to about 3.0% V.sub.2 O.sub.5, and from 
about 0.1 to about 3.5% aluminum in solution. Preferably, the bonding 
solution partially neutralized with MgO consists essentially of, by 
weight, about 65% H.sub.2 O, about 25% H.sub.3 PO.sub.4, about 4% MgO, 
about 1.7% V.sub.2 O.sub.5, about 3.4% Al(OH).sub.3 and about 0.3% 
aluminum powder. 
In accordance with the present invention, the bonding solution component is 
formed by diluting H.sub.3 PO.sub.4 with water, in a ratio in parts by 
weight, of from about 500 to 900 parts water to 147 parts H.sub.3 
PO.sub.4. In the preferred embodiment of the present invention, the above 
proportions are met by mixing 196 parts of 75% industrial grade phosphoric 
acid with 650 parts water. 
After mixing, the diluted phosphoric acid solution is heated to its boiling 
point, and aluminum then added in an amount sufficient to substantially 
equilibrate the solution with respect to aluminum. Only a relatively small 
amount of elemental aluminum such as powdered aluminum or aluminum strips, 
for example from about 10 to about 70 parts by weight, is required for 
this purpose. If an aluminum compound, such as alumina or aluminum 
hydroxide is selected to provide solute aluminum in the mixture, amounts 
of such compounds should be added to provide from about 0.1% to about 3.5% 
aluminum in solution. 
In the preferred embodiment, about 10-40 parts by weight, preferably about 
20 parts by weight, of aluminum hydroxide is slowly added to the boiling 
phosphoric acid solution, boiled together for a period of time, typically 
3 to 10 minutes, sufficient to dissolve the Al(OH).sub.3, and then removed 
from the heat source. 
After the addition of aluminum, either magnesium carbonate, magnesium 
oxide, or mixture thereof is desirably added, in addition to an essential 
amount of vanadium pentoxide, to the aluminum-equilibrated mixture. If 
magnesium carbonate is selected, less than 120 parts, by weight, and 
preferably about 50 parts, are added to the mixture. If magnesium oxide 
(magnesia) is selected, less than 50 parts, by weight, and preferably 
about 24 parts, are added. In the present invention, vanadium pentoxide in 
a amount of from about 1 to about 30 parts, by weight, and preferably 
about 10 parts, is added to the mixture. 
The optional magnesium compounds and the required vanadium pentoxide 
additions are preferably mixed into the aluminum equilibrated solution in 
the following order. A portion of the magnesium (for example about 40%) is 
slowly added while stirring to the hot, but not boiling, equilibrated 
solution. This mixture is stirred until the added magnesium compound is 
completely dissolved, for example about 10 minutes. The solution is then 
again heated to boiling and the vanadium pentoxide is slowly added while 
stirring. Heating and stirring are continued, typically for at least 10 
minutes, until the solution is free of undissolved solids. The solution is 
then again removed from the heat source and the remaining MgO is slowly 
added while continuously stirring. When the solids are completely 
dissolved, or the solution shows only trace amounts of solids, the 
solution should again be heated to boiling for 1 minute and then removed 
from the heat source. 
It is desirable to add a small amount, for example about 2 parts by weight, 
of aluminum to the hot solution to assure complete equilibration of the 
mixture with respect to aluminum. A fine aluminum powder, such as Alcoa 
Type 201, is suitable for this purpose, and should be stirred into the 
solution for about one minute. 
The solution should then be cooled to about 60.degree. F.-80.degree. F. 
(15.degree. C.-17.degree. C.) and measurements made of pH and density. 
Preferably, pH should be between 2.8 and 3.4 and density should be between 
1.28 and 1.32 g/ml. Control of pH will maximize the shelf life of the 
subsequently formed slurry and minimize reaction of the coating with 
carbon steel substrates. If the pH is lower than 2.8, additional magnesium 
(either oxide or carbonate) should be added to reduce the acidity of the 
mixture and bring it into the desired range. Generally this will require 
the addition of less than 3 g of MgO. The solution should be stirred for a 
least 5 minutes after the last addition. 
Ideally, the solution is initially made to the desired density. However, 
due to the boiling required in the introduction of the aluminum hydroxide 
and the vanadium pentoxide, significant water is lost in solution 
preparation. Also, the type and size of mixing container and the heat 
source will effect the amount of water remaining in the solution. 
Therefore, it may be necessary to adjust the density of the bonding 
solution if it is outside the desired range of 1.28 to 1.32 g/ml. 
Preferably, a higher or lower density modifying solution is prepared by 
adding less or more water in the initial preparation stage. The modifying 
solution is then added to adjust the density of the bonding solution to 
the desired range. Alternatively, a low density solution can be boiled to 
drive off water and achieve the desired density. However, a high density 
solution must be diluted by the addition of a low density modifying 
solution because direct addition of water to the bonding solution will 
cause precipitation. 
After assurance that the pH and density of the solution are within the 
desired limits, the solution should be allowed to settle for 24 hours and 
filtered before using. 
The slurry coating embodying the present invention is formed by mixing the 
above described equilibrated bonding solution with very fine atomized 
aluminum powder, desirably having a nominal particle size of no more than 
about 45 .mu.m and preferably a mean particle size of about 3 .mu.m to 8 
.mu.m. The aluminum powder is added to the bonding solution in amounts to 
provide a ratio of from about 50 g to about 150 g of the aluminum powder 
for each 100 ml of the bonding solution. Preferably, the slurry contains 
about 100 g of atomized aluminum powder, such as Reynolds 400 aluminum 
powder, for each 100 ml of above described bonding solution. The aluminum 
powder should be sifted through one or more sieves to assure that there 
are no agglomerates prior to mixing into the bonding solution. If the 
coating is to be used in applications that require a very smooth surface, 
such as on gas turbine compressor blades, the atomized aluminum powder 
should be screened through a 400 mesh sieve. 
The slurry formed as described above is typically applied to a metallic 
surface prone to oxidation or corrosion, by dipping or spraying, with 
spraying being the preferred process. A protective coating for the 
metallic surface is preferably formed by applying the slurry in two coats, 
each about 0.001 inch (0.025 mm) in thickness, dried at 140.degree. 
F.-180.degree. F. (60.degree. C.-82.degree. C.) for 30 to 60 minutes and 
cured at about 650.degree. F. (343.degree. C.) for 30 to 60 minutes 
between each coat. The coatings, as cured, are not electrically conductive 
and therefore cannot provide galvanic protection against corrosion of the 
underlying substrate material. However, the coatings can be made 
electrically conductive by bead peening or by heating as specified in 
MIL-C-8175B specification. Thus, the coatings can, by mechanical or 
thermal processes, be made electrically conductive and thereby provide 
galvanic as well as barrier protection of the underlying ferrous alloy 
metal substrate. 
Desirably, after the second coating is applied, dried, cured and processed 
to make it electrically conductive, the surface of the coating is sealed. 
It has been found that the above described bonding solution is useful, by 
itself, when applied as a seal coat to seal the surface of the dried and 
cured slurry coats. The bonding solution seal coat seals any undesirable 
porosity in the earlier applied coating and provides additional oxidation 
and corrosion protection of the substrate. Further, the bonding solution 
provides a smooth surface that enhances the flow characteristics of 
aerodynamic components onto which it is applied. Also, the rate of 
consumption of aluminum in the coating during service is decreased. The 
seal coats are dried and cured at the same time and temperature as the 
above described slurry coatings. 
The bonding solution, as described above, may also be used to form a slurry 
mixture with other particulate materials, such as insoluble solid metal 
oxides, nitrides or carbides, solid lubricants or abrasive particles. 
Examples of such materials, which should desirably be essentially inert, 
or nonreactive, with the bonding solution, include iron oxide, aluminum 
oxide, boron nitride, silicon nitride and graphite. 
Industrial Applicability 
The chrome and molybdenum-free coatings embodying the present invention are 
particularly useful for providing corrosion and oxidation protection to 
ferrous metal alloy surfaces operating at elevated service temperatures, 
such as the compressor blades, stators, and casings of gas turbine 
engines. 
A number of test specimens were prepared to compare the coating embodying 
the present invention with current commercially available and accepted 
coatings, all of which undesirably contain chromium or molybdenum. The 
test specimens were inspected for surface appearance and roughness, and 
tested for conductivity, mechanical properties including resistance to 
spalling and bond strength, thermal stability, resistance to corrosion, 
and resistance to corrosion at elevated temperatures. The coatings 
embodying the present invention were found to be at least equal to, and in 
many instances superior to, the coatings containing chromates or 
molybdates. In particular, the coatings containing vanadium pentoxide in 
the bonding solution were stable at high temperature and exhibited 
significantly greater resistance to oxidation and corrosion after exposure 
to elevated temperatures than coatings without the vanadium pentoxide 
addition and prior art coatings containing chromium or molybdenum. Also, 
it was observed that the test specimens coated with the slurry composition 
embodying the present invention were very smooth, having a surface 
roughness better than, or at least equal to, the best coatings containing 
chromium or molybdenum. 
While the invention has been described with reference to certain preferred 
embodiments, those skilled in the art recognize that certain common 
substitutions may be made without altering the essential properties of the 
claimed coating composition. For example, while water is specified in the 
above examples illustrating the preferred embodiment, other common 
solvents such as alcohol, acetone, or similar liquids, may be substituted 
for a portion of the water in the claimed mixtures without departing from 
the spirit of the invention. 
Other aspects, objects and advantages of this invention can be obtained 
from a study of the disclosure and the appended claims.