WO1999051362A1

WO1999051362A1 - System and method for inhibiting corrosion of metal containers and components

Info

Publication number: WO1999051362A1
Application number: PCT/US1999/007836
Authority: WO
Inventors: Paresh R. Modi
Original assignee: Lockheed Martin Corporation
Priority date: 1998-04-08
Filing date: 1999-04-08
Publication date: 1999-10-14
Also published as: EP1071520A1; JP2003521578A; EP1071520A4; AU3486899A

Abstract

A method and system for inhibiting corrosion of aluminum and other metal-containing components and structures exposed to water are disclosed. In one embodiment, the silicate solution is used as a test fluid medium for structural testing of aluminum-alloy or other metal container structures including propellant tanks (14), in which a structure filled with the medium is then subjected to various structural load testing. In another embodiment, the silicate solution is used as a test medium for proof pressure hydrostatic or load testing of launch vehicle (10) booster tanks. The silicate film (18) protects the underlying base metal surface against corrosion during these tests. The film (18) also protects the base metal surface in normal atmospheric conditions from exposure to humidity and other atmospheric moisture after removal of the test medium from the propellant tank (14) following completion of testing.

Description

SYSTEM AND METHOD FOR INHIBITING CORROSION OF METAL CONTAINERS AND COMPONENTS

RELATED APPLICATION This application claims the benefit of U.S. Provisional Application No. 60/081,094 filed April 8, 1998.

Field of the Invention The present invention relates generally to methods for inhibiting corrosion of metal-containing surfaces that are exposed to aqueous solutions for short or prolonged periods and, in particular, relates to processes for testing tanks, containers, and other structures in which water is used as a test medium that is brought into contact with one or more surfaces of such structures. The present invention relates also to methods for inhibiting corrosion of metal surfaces exposed to normal atmospheric conditions, including environments in which humidity may not be controlled.

Background of the Invention Various structures having aluminum or other metallic surfaces may be subject to testing using water or water-containing solutions or materials. Such testing generally involves filling a structure to be tested with water or an aqueous solution, subjecting the structure to various stresses, and then examining the walls of the structure, and in particular, any seals or joints in the walls, for deformation, leakage of water, or other irregularities. When such testing is conducted using source water or a similar water- containing solution over short periods (e.g., of less than four hours), minor discoloring corrosion of aluminum-containing surfaces and components of the structure may occur. When testing is conducted using source water or a similar water-containing solution over extended periods (e.g., of more than four hours), pitting corrosion and other substantial degradation of metal-containing surfaces and components may occur. This, in turn, can diminish the integrity of the structure being tested.

Space launch vehicles are generally constructed of components comprised of lightweight aluminum alloys. For example, the interior walls of booster tanks used in propulsion, and components within the booster tanks, are often fabricated from aluminum alloy materials. To evaluate the strength and integrity of such booster tanks, it is desirable to conduct structural testing, such as static load or proof pressure hydrostatic testing. This testing generally involves filling the booster tanks with a test medium, such as water, for periods varying from one to seven weeks. However, the use of source water or a similar aqueous solution as a test medium, typically results in substantial corrosion and degradation of the interior aluminum alloy walls and components of the booster tank. Such corrosion may lead to undesirable deposits of materials on the walls and components of the booster tank. Such deposits are incompatible with liquid oxygen and rocket propellant fuels that may fill the booster tank, can be a fire hazard, and may impair the performance of working parts exposed to these propellants such as the rocket propellant engine. The interior walls of such booster tanks may be complex structures, having ridges, such as may be found in isogrid structures, and other test components that extend radially inwardly from the wall, into the interior cavity of the booster tank, making difficult the application of paint or other conversion coatings on such walls to protect them from corrosion.

A further concern for space launch vehicles is that such structures may be produced many months before projected use. Such vehicles may have to be transported over long distances from the point of production to the point of use. During transportation and storage, it may not always be possible to maintain such vehicles in a controlled environment, and they may be exposed to atmospheric moisture that can cause corrosion of aluminum-containing surfaces. It is therefore desirable to protect such aluminum-containing surfaces from corrosion during fluid testing and also later during transportation and storage, prior to use.

Summary of the Invention An object of the present invention is to provide a method for protecting the aluminum alloy and other metal surfaces of tanks, containers, and other structures from corrosion that may occur when such surfaces are exposed to an aqueous media (e.g., during testing, such as performance or validation testing).

A further object of the present invention is to provide a method for inhibiting corrosion of metal-containing surfaces that may be exposed to an aqueous medium or normal atmospheric conditions such as a humid environment.

The present invention relates to a system and method for inhibiting corrosion of metal-containing surfaces that may occur when such surfaces are exposed to water or an aqueous solution, such as during testing processes. More specifically, the invention is directed to a method for inhibiting corrosion through the use of an alkali metal silicate as an additive to the water or aqueous solution that will be brought into fluid contact with a metal-containing surface.

In one aspect of the invention, the method may include the steps of bringing water combined with an alkali metal silicate into contact with an aluminum-containing surface and maintaining such contact between the mixture of water and silicate and the aluminum-containing surface for at least a first period of time. Contacting the mixture of water and silicate with such a surface results in the formation of a thin silicate film on the surface that protects the aluminum-containing surface from corrosion. The addition of this mixture of silicate and water, in appropriate concentrations, thus functions to inhibit the corrosion that ordinarily results from exposure of such a surface to source water or a solution containing source water. In one embodiment, the alkali metal silicate comprises sodium silicate. The same effect may be achieved where the surface comprises stainless steel, alloy/carbon steel, and other metals.

In another aspect of the present invention, the method includes the steps of forming a test medium comprised of an alkali metal silicate and water, and contacting the silicate and water test medium with an aluminum-containing surface to form a protective film on the surface during the testing process. The step of forming the combination of alkali metal silicate and water may comprise the step of mixing the silicate with water, in appropriate concentrations, to form a silicate solution, preliminary to the step of contacting the solution with the aluminum-containing surface. The step of forming the combination of alkali metal silicate and water may alternatively include the step of contacting the aluminum-containing surface of a structure to be tested with water and then mixing silicate with the water to form a silicate solution test medium that in turn forms a protective film on the aluminum surface. In one embodiment of the invention, the silicate comprises sodium silicate.

For purposes of enhancing the anti-corrosive effect of the invention, the water to be combined with the silicate may comprise deionized water. Deionized water substantially lacks the ionic impurities that can contribute to corrosion of aluminum and other metal surfaces in contact with this water. In one embodiment, the invention includes a preliminary step of deionizing water to be used in the test medium, before the step of forming the test medium.

A further aspect of the method of the present invention is directed to protecting the aluminum-containing inner wall surface of a launch vehicle propellant tank during testing of such tank, where the test medium is comprised of deionized water. In this aspect of the invention, the method generally includes the steps of contacting an aqueous test medium comprising a sodium silicate solution with at least a first section of the inner wall surface of the tank, and then, while maintaining contact between the test medium and the first section of the inner wall surface, applying various stresses to the tank as may be required by the test protocol. The sodium silicate solution of the test medium forms a protective film on the section of the inner wall surface in contact with the test medium that inhibits corrosion, so avoiding the severe damage to the walls and components within the tank that may otherwise occur on exposure to test medium containing source water. In one embodiment of the method of the present invention, the method includes forming the test medium by combining sodium silicate with deionized water. This step may include dissolving sodium silicate into the water. The step of forming the test medium may be performed outside of the internal containment area of the propellant tank to be tested. Alternatively, the step of forming the test medium may comprise the steps of introducing the water into the internal containment area of the tank and adding the silicate (e.g., by injection) to the internal containment area during the step of introducing the water. The method may further include the step of deionizing the water to be used in the test medium prior to combining with the silicate.

The present invention is adaptable to systems or structures in which an aqueous solution flows over, sprays, or immerses components, such as an inner wall of a tank or container, comprises metallic material, such as aluminum. It is adaptable for systems in which water or an aqueous solution, or a solid or semi-solid material containing water or stored in water, is contained or held within a container having walls comprising metallic materials such as aluminum, such as tanks, vats, bins, silos, pipes, vats, or sinks. It is also adaptable for systems in which a metal-containing surface is exposed to atmospheric conditions where the humidity level is not controlled.

Brief Description of the Drawings For a more complete understanding of the present invention, and the further advantages thereof, reference is now made to the following detailed description taken in conjunction with the drawings, in which:

Figure 1 is an external view of a launch vehicle. Figure 2 is a perspective view of liquid oxygen and rocket propellant tanks, in which a structural test using a fluid medium might be performed.

Figure 3 is a perspective view of the inner wall surface of a propellant tank.

Figure 4 is an enlarged partial cross-sectional view of the inner wall of a propellant tank, in which a thin silicate film has formed on the interior surface of such wall;

Figure 5 is a flow chart of one embodiment of the method of the present invention;

Figure 6 is a flow chart of another embodiment of the method of the present invention; and

Figure 7 is a flow chart of a further embodiment of the present invention, as applied to proof pressure hydrostatic testing of a launch vehicle propellant tank.

Detailed Description In the following description, the invention is set forth in the context of structural testing of large space launch vehicle booster tanks, whose interior walls and components are fabricated primarily from aluminum alloys. The structural testing process is intended to evaluate the strength and integrity of the interior walls and components of the tanks. During this process, the tanks are substantially filled with a fluid medium, and the medium is then left in the tanks for an extended period of time. While filled with fluid medium, the tanks may be subjected to various forms of external and internal pressure. A preferred method of testing includes using source water as a fluid medium, primarily because it is inexpensive, easy to handle, and can be disposed without environmental concerns. A problem with using source water, however, is that when it comes into contact with certain metal surfaces, including those containing aluminum, the metal surface can become corroded.

The embodiment described herein is utilized with booster tanks of two varieties, the liquid oxygen (LOX) oxidizer booster tank, 13 feet in diameter and 58 feet in height, with a capacity of approximately 50,000 gallons, and the rocket propellant (RP) fuel booster tank, 13 feet in diameter and 32 feet in height, with a capacity of approximately 26,000 gallons. The inner wall surfaces of both types of tanks are comprised of aluminum alloy barrel panels fabricated in an isogrid pattern, such that the inner wall surfaces have a multiplicity of interconnecting ridges. The inner wall surfaces form an internal containment area which functions to hold the liquid oxygen or rocket propellant fuel material. Each tank has aft and forward domes, also fabricated of aluminum alloys. Because of the complexity of the inner wall surfaces of the tanks, it is not feasible to apply paint or other corrosion inhibiting material to such surfaces to protect the inner surfaces during structural testing. While the present embodiment is set forth in the context of launch vehicle booster tanks, the invention can be used with a wide variety of tanks, components, and other structures having metal-containing surfaces.

In the present embodiment, the inner wall surfaces of the booster tanks are fabricated from several aluminum alloys, such as 2014-T62 (forward and aft domes), 2014-T651 (barrel panels), 2219-T6 (outlet sump), and 7050-T7451 (manhole cover with stainless steel fasteners). The invention is not limited to these alloys, and it is contemplated that the invention may be applied to various metal compositions, with a greater or smaller or no aluminum content. In addition, while the embodiment of the invention described herein is primarily addressed to inhibiting corrosion of aluminum- containing surfaces, that the invention will protect other metal surfaces, including those comprised of stainless steel, alloy/carbon steel, and magnesium alloys, or a dissimilar metals assembly. It should be noted that non-metallic materials used in the present embodiment such as valves, pumps, etc., generally shall be free of plasticizers, particularly for a liquid oxygen system. Also, the method and system of the present invention may be applied to a smaller helium pressurization tank (with diameter of about three feet and length of about six feet), which is used to pressurize the rocket propellant and liquid oxygen to the rocket engine. This tank is fabricated of 6061 -T6 aluminum alloy liner and carbon composite overwrap, and it is subject to hydrostatic pressure testing with an aqueous test medium. Figure 1 is a view of a launch vehicle 10 containing liquid oxygen and rocket propellant fuel tanks. Figure 2 depicts liquid oxygen 12 and rocket propellant 14 tanks in which the method and system of the present invention may be employed. Figure 3 is a perspective view of the inner wall of a barrel panel of a propellant tank, having an inner wall surface 16. In some instances, a propellant tank such as the rocket propellant tank 14 depicted in Figures 1 and 2, may be the subject to structural testing that includes contacting the inner wall surface 16 depicted in Figure 3 with a fluid test medium comprising of water. The system and method of the present invention protects the inner wall surface 16 from corrosion that may occur during testing with an aqueous test medium by depositing a thin silicate film on the inner wall surface 16. Figure 4 is an enlarged partial cross-sectional view of the inner wall surface 16 following contacting the surface with a fluid medium comprising sodium silicate, in which a protective silicate film 18 has formed on the inner wall surface 16. The method of the present invention generally includes the step of contacting a test medium comprising water and an alkali metal silicate with at least one aluminum alloy inner wall surface of a tank, the inner wall surface defining an internal containment area, to form a protective silicate film. While the test medium may comprise any of a variety of alkali metal silicates, a preferred source is sodium silicate. In one embodiment of the present invention, prior to the contacting step, the invention includes a step of removing gross contaminants (e.g., aluminum chips) from at least the inner wall surface, such as by rinsing the inner wall surface with water. Removal of gross contaminants is advantageous, because such contaminants may impair formation of the protective silicate film. In another embodiment of the present invention, the method includes, prior to the step of contacting the test medium, the step of forming the test medium by mixing an alkali metal silicate, such as sodium silicate, with water outside the tank. In another embodiment, the step of forming occurs prior to the step of contacting the test medium by injecting (such as by a pump) the alkali metal silicate into water as the water flows through a conduit connected to the internal containment area defined by the inner wall surface of the tank.

For purposes of further inhibiting corrosion, the test medium used for the step of contacting the test medium comprises sodium silicate and deionized water. In this regard, the method may include, before the step of contacting the test medium with the inner wall surface, the step of deionizing source water to produce deionized water. The method may further include, before the step of contacting, the step of inspecting at least the inner wall surface for gross contaminants, (e.g., aluminum chips), and/or removing gross contaminants by rinsing the inner wall surface with water. Conducting these inspecting and/or removing steps enhances formation of the protective film.

As applied to testing of tanks, the method of the present invention generally includes a further step of applying one or more structural or other stresses to at least one feature of the tank, such as would be done during static load testing, while at least one section of the inner wall surface of the tank is in fluid contact with the test medium. The method also includes the step of examining the tank for irregularities while the test medium is in fluid contact with at least one section of the inner wall surface. While the test process might occur over short periods of time, of a few minutes to a few hours, in one embodiment of invention, the test occurs over about seven weeks and the test medium remains in contact with the aluminum containing surface for that period. It is anticipated that the test process could extend for periods in excess of seven weeks with the medium remaining in contact with the surface for these longer periods.

Tests of surfaces fabricated from the aluminum alloys typically used in launch vehicle booster tanks reveal that, if the test medium consists of source water only, substantial corrosion, including discoloration, pitting, and the formation of a white corrosive deposit on the surfaces and sediment in the test medium, occurs. Corrosion tends to be severe when aluminum alloy surfaces are exposed to a test medium consisting of source water for extended periods. When deionized water is used as the test medium, the degree of corrosion is significantly lessened as opposed to that obtained with source water, although discoloration and formation of deposits and sediment may still occur. Accordingly, while the step of contacting the test medium with the inner wall surface of the tank could employ source water in the test medium, in a preferred embodiment, the test medium comprises deionized water. In the present embodiment, the preferred purity of deionized water is such that it meets the requirements of any of Types I, II, III, and IV described in the publication Standard Specification for Reagent Water (1997), also known as ASTM D1193, by the American Society for Testing and Materials or equivalent specification of deionized water.

In the present invention, the concentration of alkali metal silicate may be adjusted, depending upon the type of metal-containing surface, the degree of a corrosion resistance of the metallic surface used, the degree of purity of the water, and the length of time the test medium is expected to be in contact with metal-containing surface. In a preferred embodiment, which employs sodium silicate, the test medium is comprised of about 30 parts per million of a prime sodium silicate solution in deionized water by volume (of the purity described above), which results in aluminum alloy surfaces even after a period of time up to and including seven weeks substantially free of pitting corrosion, deposits and other severe corrosion. In one embodiment, the prime or first solution silicate solution comprises an aqueous sodium silicate solution, by weight: 20% to 40% sodium silicate, Na of 8.83% to 9.06%, 29% to 29.34% SiO₂, sulfates of less than 200 parts per million, chlorides at less than 350 parts per million, aluminum at less than 550 parts per million,

8 iron at less than 80 parts per million, titanium at less than 100 parts per million, and other metallic constituents at less than 59 parts per million, and having a ratio of SiO₂ to Na₂O of 3.23 to 3.29. Lower concentrations of this prime sodium silicate solution in deionized water by volume, in the range of between about 20 parts per million to about 30 parts per million, may be effective to substantially inhibit corrosion of surfaces containing certain metals that are more corrosion resistant (such as the 6000 series of aluminum alloys), and/or where shorter periods of exposure are contemplated. Concentrations of this prime sodium silicate solution in deionized water by volume, between about 30 parts per million and about 175 parts per million or more, produce strong corrosion-inhibiting effects in a range of aluminum alloys, including substantially no pitting corrosion, deposits or sedimentation, plus no discoloration, at the end of a one-week and up to and including a seven-week test period under the conditions above described. While silicate in various forms may be used, a source of this prime sodium silicate solution suitable for use in the present invention is the commercially available Nalco 2513, a product of Nalco Chemical Company. It should be noted that, in the present invention, the method can be carried out at ambient temperatures, preferably in the temperature range of about 65 degrees Fahrenheit to 75 degrees Fahrenheit. In another embodiment, the test medium is a sodium silicate solution comprising sodium silicate in a concentration of between about 11 parts per million, by weight, and 117 parts per million, by weight, in deionized water, per Table 1 below.

Test medium parts per Test medium parts per Test medium parts per million by volume of first million by weight of first million by weight of sodium silicate solution sodium silicate solution sodium silicate (based on (approximate) 40% weight of sodium silicate in first sodium silicate solution)

20 28 11

30 42 17

175 244 97

210 292 117

Analysis of surfaces fabricated from aluminum alloy after contact with a test medium solution of about 30 parts per million of the prime sodium silicate solution in deionized water indicated that a thin protective film of Al-O-Si compound formed on such surfaces, having a thickness of about 25 angstroms when measured by X-ray photoelectron spectrographic analysis. It is believed that this film forms almost instantaneously after contacting the test medium with the surface, such that a period of exposure of a few minutes would be adequate to form the protective film.

Figure 5 is a flow chart describing a generalized process for forming a protective silicate film comprised of Al-O-Si compound on the inner wall surface of a launch vehicle propellant tank to be subjected to structural testing, wherein the tank has an inner wall surface and an external wall surface. As noted in Figure 5, the process includes the step 20 of contacting a test medium comprising a solution of alkali metal silicate in water with at least a first section of the inner wall surface of the tank to form a reaction product on the inner wall surface comprising a thin film of Al-O-Si compound. This film in turn inhibits corrosion to at least the first section of the inner wall surface caused by the aerated water of the test medium. The step of contacting the test medium generally causes the water of the test medium to become aerated. The process further includes the step 40 of applying stress to at least one of the inner wall surface (e.g., proximate the first section) and the external wall surface of the tank while the test medium remains in contact with at least the first section of the inner wall surface. The process further includes the step 60 of examining the tank (e.g., proximate the first section) for deformation, leakage and other irregularities.

Figure 6 is a flow chart of another embodiment of the present method for testing a tank using an aqueous test medium, wherein a protective film formed by the test medium inhibits corrosion to the inner wall surface of the tank caused by aerated water in the test medium. The context for this flow chart is a launch vehicle propellant tank having an external wall surface and an inner wall surface, with the inner wall surface defining an internal containment area capable of holding the aqueous test medium. The method includes a step 100 of deionizing source water to form deionized water for use in forming the test medium. The method further includes a step 102 of mixing the prime sodium silicate solution and the deionized water in a vessel external to the tank to form a sodium silicate solution that will serve as the test medium, such that the desired concentration of the prime sodium silicate solution in deionized water is uniform across the test medium. Alternatively, the method may include a step 108 of injecting an amount of the prime sodium silicate solution into a conduit carrying deionized water into

10 the tank, thereby forming the test medium in the conduit with a desired concentration of sodium silicate in the test medium. Following either of these steps, the method next includes a step 114 of introducing a first portion of test medium into the internal containment area of the tank to bring a first section of the inner wall surface of the tank in contact with the first portion of test medium, such fluid contact resulting in the formation of a silicate film comprised of Al-O-Si compound on the first section of the inner wall surface. A further alternative, shown on Figure 6, includes the step 116 of introducing deionized water into the internal containment area of the tank, so that it contacts at least one section of the inner wall surface of the tank, and then the step 118 of introducing an amount of the prime sodium silicate solution into the internal containment area to form a sodium silicate solution with the deionized water that serves as the test medium, having a desired silicate concentration uniform across the solution in contact with the inner wall surface of the tank. The method includes a step 140 of applying a first amount of stress to at least one surface of the tank, while the test medium remains in contact with the inner wall surface. Alternatively, the method includes the step of substantially filling the internal containment area of the tank, so that test medium contacts substantially all sections of the inner wall surface of the tank, prior to the step 140 of applying stress. While there are various ways of applying stress to one or more surfaces of the tank, depending on the test process used, in the present embodiment, these may include applying a static load to the external wall surface or applying acoustic waves to the tank or applying sinusoidal forces to the test medium for a slosh damping test, or applying internal pressure to the inner wall surface by injecting an inert gas, nitrogen gas or dry air to raise the internal pressure bearing on the inner wall surface above the ambient pressure of the area surrounding the tank. The internal pressure may be raised by a pressurization pump. Finally, the method includes a step 160 of examining the tank (e.g., the external wall surface) for deformation, leakage or other irregularities.

Figure 7 describes another embodiment of the method of the present invention, which includes use of an aqueous test medium in proof pressure hydrostatic testing of a launch vehicle propellant tank. The booster tank has top and bottom ends, and has an external wall surface and an inner wall surface. The inner wall surface is comprised of aluminum alloy, and defines an internal containment area capable of holding an aqueous test medium. The bottom end of the tank has a port through which test medium can enter and be drained from the internal containment area of the tank. This embodiment

11 contemplates that the test medium is formed in a conduit attached to the port and then is transported through the conduit into the port and from there into the internal containment area. As noted in Figure 6, the method involves the step 200 of first verticalizing the tank. The method then includes the steps of 208 introducing deionized water (formed, for example, by deionization of source water) into the conduit and 210 injecting prime sodium silicate solution into the conduit in the flow of the deionized water to form a test medium comprised of a sodium silicate solution. The turbulence of the test medium in the tank due to flow pressure further homogenizes the test medium. The next step 214 includes contacting the test medium with the inner wall surface of the tank by introducing the test medium into the internal containment area of the tank, from the conduit and through the port located in the bottom end of the tank. In a preferred embodiment, the test medium is introduced at the bottom end of the tank to reduce the likelihood that air pockets will form on the inner wall surface. Such air pockets may adversely affect the testing process and formation of the protective silicate film. Note that a vent hole in the manhole cover helps to vent air while tank is being filled with medium. In another preferred embodiment, the test medium is introduced at a high pressure by a pressure pump to facilitate filling of the tank with test medium in an expedited manner (e.g., when introduced from a storage vessel as described below.) The method then includes the step 222 of substantially filling the internal containment area of the tank so bringing test the medium in contact with virtually all sections of the aluminum alloy inner wall surface of the tank, and leaving a small air space at the top of the internal containment area. The method includes the alternative step 224 of maintaining the test medium in the tank for a first period of time, preferably of about three days. While the tank is still filled with test medium, the method includes the step 230 of pressurizing the tank, to increase the internal pressure bearing on the inner wall surface. This is preferably done by injecting nitrogen gas or dry air, to raise the internal pressure of the tank (e.g., for launch vehicle tanks of this particular capacity, to a pressure in the range of about 80 pounds per square inch for about five minutes). Other methods of pressurization using other gases may be utilized. After the step 230 of pressurizing the tank, the method next includes the step 232 of depressurizing the tank, to return the internal pressure to atmospheric pressure. The method includes an alternative step 234, following said depressurizing step 232, of maintaining the test medium in the tank for a second period of time, preferably of about three days. In another embodiment of the invention, the internal containment area of the

12 tank may be completely (100%) filled with test medium, up to the manhole cover opening (without any air gap) and then the tank can be pressurized with a pressure pump. In the present embodiment, the next step includes a step 260 of examining the tank for deformation, leakage and other irregularities. The description above, concerning composition of the test medium, forming the test medium, and additional uses and applications are all also applicable to static load testing of launch vehicle propellant tanks.

An advantage of the present invention in the field of space launch vehicles is that the silicate used is not felt to produce a fire hazard and is consistent with applicable government requirements for liquid oxygen and other systems. A further advantage of the invention is that, following testing and removal of the test medium, the protective Al- O-Si film remains on the aluminum alloy inner wall surfaces of the propellant tanks, providing further protection from atmospheric moisture during storage and transportation prior to use on the launch pad. In addition, after use, the test medium containing silicate may be disposed as waste water, at silicate concentrations of 210 parts per million or less of prime sodium silicate solution, and after bringing the pH to a range of about pH 6.5 to pH 8.0 without environmental concerns (this requirement may vary from jurisdiction to jurisdiction).

In another aspect of the invention, the sodium silicate test medium may be recycled after initial use. The solution may be prepared and stored for use later, as for example, to meet production requirements. More specifically, in instances where a number of tanks will be processed, the step of injecting 210 may be followed by the step 211 of storing the test medium in a storage vessel as illustrated in Fig. 7. Storage of more than six weeks or recycling after an extended period of use in testing may require a further step of adding an anti-bacterial treatment to the solution.

A silicate solution with a concentration of silicate in deionized water in the range described above may be used to form a protective film on components with aluminum- containing, stainless steel, carbon/alloy steel, or other metallic surfaces. The solution also may be used to protect the interior walls of pipelines, tubes, fittings, pumps, and other equipment during testing or other processes where an aqueous medium is employed. The invention is adaptable for systems or structures in which an aqueous solution flows over, sprays, or immerses components whose outer walls consist of aluminum-containing or other metallic surfaces. The Al-O-Si film protects the metal

13 surface underlying the film against normal atmospheric conditions (such as humidity). It is also adaptable for storage systems in which water or a mild aqueous solution, or a solid or semi-solid material containing water or stored in water, is held in a container having walls comprised of aluminum or other metallic materials, such as tanks, vats, bins, silos, pipes, vats or sinks.

While various embodiments of the present invention have been described in detail, it is apparent that further modifications and adaptations of the invention will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.

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Claims

What is claimed is:

1. A method for testing of a structure having an external wall surface and an inner wall surface, wherein the inner wall surface defines an internal containment area capable of holding fluid, the inner wall surface comprising a metal, said method comprising the steps of: contacting a first amount of a test medium with a first section of the inner wall surface, and wherein said test medium forms a corrosion-inhibiting film on the first section of the inner wall surface; and while said test medium is in contact with said first section of the inner wall surface, applying a first amount of stress to at least one of the inner wall surface and external wall surface of said tank.

2. The method of claim 1 , wherein the test medium comprises an alkali metal silicate and water.

3. The method of claim 2, wherein the water in the test medium is deionized water.

4. The method of claim 3, wherein the deionized water meets the requirements of at least one of Types I, II, III and IV of ASTM Dl 193.

5. The method of claim 1 , wherein said test medium comprises a first sodium silicate solution dissolved in deionized water.

6. The method of claim 5, wherein said test medium has a concentration of at least about 20 parts per million of said first sodium silicate solution in deionized water.

7. The method of claim 5, wherein said test medium has a concentration of between about 30 parts per million and about 175 parts per million of said first sodium silicate solution in deionized water.

8. The method of claim 1 , wherein said test medium comprises a sodium silicate solution having a sodium silicate concentration of between about 11 parts per million, but weight, and about 117 parts per million, by weight, in deionized water.

9. The method of claim 1 , wherein said test medium comprises a first sodium silicate solution comprising by weight: 20% to 40% sodium silicate, Na₂O of 8.83% to 9.06%, SiO₂ of 29% to 29.34%, sulfates of less than 200 parts per million, chlorides of less than 350 parts per million, aluminum of less than 550 parts per million, iron of less than 80 parts per million, titanium of less than 100 parts per million, and other metallic

15 constituents of less than 59 parts per million, and having a ratio of Si0₂ to Na₂O of 3.23 to 3.29.

10. The method of claim 1 , wherein said first section of the inner wall surface comprises an aluminum alloy.

11. The method of claim 1 , wherein said first section of the inner wall surface comprises an aluminum alloy selected from the group consisting of the 2000 series, 6000 series and 7000 series of aluminum alloys.

12. The method of claim 1 , wherein said first section of the inner wall surface comprises one of stainless steel, carbon/alloy steel and magnesium alloy.

13. The method of claim 1, further comprising, prior to said contacting step, the step of storing at least the first amount of the test medium in a storage vessel.

14. The method of claim 1, wherein said step of applying stress includes applying at least a first amount of stress to at least one external wall surface of the tank.

15. The method of claim 1, wherein said step of applying stress includes introducing a first quantity of a first gas into the tank to raise the internal pressure against the inner wall surface of the tank above ambient pressure.

16. The method of claim 15, wherein the first gas introduced into the tank comprises one of an inert gas, nitrogen gas and dry air.

17. The method of claim 15, further comprising the steps of: depressurizing the tank to ambient pressure; and while said test medium remains in the tank, examining the tank for at least deformation and leakage.

18. The method of claim 1 , further comprising, prior to said contacting step, the step of forming said test medium.

19. The method of claim 18, wherein said step of forming said test medium includes the step of dissolving a first portion of a first sodium silicate solution into deionized water to form a sodium silicate solution of at least 20 parts per million of said first sodium silicate solution in deionized water.

20. The method of claim 1, further comprising the step of: while said test medium remains in the tank, examining the external wall surface of the tank for deformation and leakage.

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21. The method of claim 1 , further comprising prior to said contacting step, the step of removing contaminants from said inner wall surface.

22. The method of claim 1, wherein said contacting step comprises the step of: maintaining contact between the first section of the inner wall surface and the first amount of test medium for a period of between about one week and about seven weeks.

23. A method for inhibiting corrosion of a structure, wherein said structure has an inner wall surface defining an internal containment area capable of holding fluid and an external wall surface, the inner wall surface comprising an aluminum alloy, and wherein the internal containment area has a top end portion and a bottom end portion, comprising the steps of: introducing a first portion of water into the internal containment area, wherein at least a first section of the inner wall surface is in fluid contact with the water; and introducing a first amount of sodium silicate solution in the first portion of the water to form a sodium silicate medium, said sodium silicate medium being in fluid contact with the first section of the inner wall surface, and wherein a corrosion-inhibiting film is depositable from said sodium silicate medium on the first section of the inner wall surface.

24. The method of claim 23, further comprising, prior to said introducing step, the step of deionizing the first portion of water.

25. The method of claim 23 , further comprising prior to said introducing step, the step of removing contaminants from at least the first portion of the inner wall surface.

26. The method of claim 23, wherein the concentration of sodium silicate solution in water is at least about 30 parts per million.

27. The method of claim 23, wherein the concentration of sodium silicate solution in water is less than about 210 parts per million.

28. The method of claim 23, wherein said sodium silicate solution comprises by weight: 20% to 40% sodium silicate, NajO of 8.83% to 9.06%, SiO₂ of 29% to

29.34%), sulfates of less than 200 parts per million, chlorides of less than 350 parts per million, aluminum of less than 550 parts per million, iron of less than 80 parts per

17 million, titanium of less than 100 parts per million, and other metallic constituents of less than 59 parts per million.

29. The method of claim 23, wherein said first section of the inner wall surface comprises an aluminum alloy.

30. The method of claim 23, further comprising the steps of: while said sodium silicate medium remains in contact with said first section of the inner wall surface, applying a first amount of stress to at least one of the inner wall surface and the external wall surface; and examining the structure for deformation and leakage.

31. A tank prepared according to the process of: introducing a first amount of solution into an internal containment area defined by an inner wall surface of the tank, said solution comprising a first amount of sodium silicate and a first amount of deionized water; and maintaining fluid contact between the first amount of solution and at least a first portion of the inner wall surface for a first period of time to form a silicate film on said first portion of the inner wall surface.

32. The tank prepared according to the process of claim 31, wherein the process further comprises the step of: prior to said step of introducing the first amount of solution, deionizing said water.

33. The tank prepared according to the process of claim 31 , wherein said step of introducing the first amount of solution comprises the steps of introducing the first amount of deionized water and injecting the first amount of sodium silicate into said deionized water during said step of introducing said first amount of deionized water to form the first amount of solution.

34. The tank prepared according to the process of claim 31 , further comprising the steps of: while the solution remains in contact with said first portion of the inner wall surface, applying a first amount of stress to at least one of the inner wall surface and the external wall surface; and examining the tank for deformation, leakage and other irregularities.

35. A method for proof pressure hydrostatic testing of a tank, wherein said tank has an internal wall surface defining an internal containment area capable of holding

18 liquid, the internal wall surface comprised of aluminum alloy, with a fluid conduit attached to the internal containment area at a bottom end portion of said tank, said method comprising the steps of: verticalizing said tank; introducing a first amount of deionized water into said fluid conduit; injecting a first amount of sodium silicate solution into said conduit during said introducing the first amount of deionized water step to form a test medium; introducing a first portion of said test medium into the bottom end portion of said tank; pressurizing the internal containment area of said tank; depressurizing the internal containment area of said tank; and examining said tank.

36. The method of claim 35, further comprising, after said introducing the first portion of said test medium step, the step of maintaining said test medium in contact with said internal wall surface for a first period of time.

37. The method of claim 35 , further comprising, after said depressurizing step, maintaining said test medium in contact with said internal wall surface for a second period of time.

38. The method of claim 35, wherein said sodium silicate solution comprises by weight: 20% to 40% sodium silicate, Na₂0 of 8.83% to 9.06%, SiO₂ of 29% to 29.34%, sulfates of less than 200 parts per million, chlorides of less than 350 parts per million, aluminum of less than 550 parts per million, iron of less than 80 parts per million, titanium of less than 100 parts per million, and other metallic constituents of less than 59 parts per million, and having a ratio of Si0₂ to Na₂O of 3.23 to 3.29.

39. The method of claim 38, wherein sodium silicate solution has a concentration of at least about 30 parts per million in deionized water.

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