Handbook Corrosion (1992) WW Part 12 docx

Fig. 2 Profile of shuttle mission. Each shuttle orbiter can fly a minimum of 100 missions and carry as mu ch as 29,500 kg (65,000 lb) of cargo and up to seven crew members into orbit. It can return 14,500 kg (32,000 lb) of cargo to earth. klb = 1000 lb; klbf = 1000 lbf The requirement to achieve a minimum-weight orbiter (68,000 kg, or 150,000 lb, dry weight) has necessitated use of the most efficient structural materials and processes. The requirement for 100-mission reuse has extended advancements in thermal protection materials well beyond the state-of-the-art existing at the inception of the design. Corrosion Control Program: Space Shuttle Orbiter The key to a successful corrosion control program for the space shuttle orbiter was to develop sound technical and management programs. Although the major structural parts of the orbiter, such as the wings, tail, fuselage, and cabin, were manufactured by only a few companies, it was estimated that more than 20,000 suppliers were responsible for providing systems and parts for the vehicle. It was necessary to review and control all orbiter parts to provide the high levels of reliability required. The material and process management program consisted of the following key elements: • A material and process group in engineering • A drawing review system requiring sign-off by a materials engineer • A tracking system for all materials • An orbiter materials and process control specification • A corrosion-control and finishes specification • A stress-corrosion control plan Each material application was reviewed by a qualified material and process engineer who had sign-off authority on drawing, engineering orders, and material rework dispositions. A material tracking system was set up at the inception of the program to prevent 12 material-related hazards from occurring on the orbiter. These include controls for atmospheric corrosion and stress corrosion, fluid and propellent incompatibilities, age life, flammability, toxicity, offgassing, and condensation of volatile condensible matters. Materials and finishes were identified, evaluated, and, when accepted, entered into a computer. Material identification even included those materials used in minute quantities, such as the ink used to stamp part numbers or the nearly invisible cetyl alcohol lubricants on fasteners. A master directory (index) of the behavior of each material in the 12 hazardous categories was maintained and used as a reference. In each hazardous category, a series of encoded, acceptable engineering approaches for each "buyoff" was listed to assist the engineer. For example, a part may be made from a material having a stress-corrosion threshold of 50% of its tensile yield strength, yet be acceptable because: • It is adequately coated • It experiences no significant tensile stresses in the critical stress- corrosion direction (including residual and installation stresses) • It is in a benign environment, such as the cabin In a few cases, the complexity of the part, such as a motor, precluded a separate evaluation of each material, and the entire configuration was qualification tested in its intended-use environment to avoid these hazards. A Material and Process Control Specification (MC999-0096) was placed on all major subcontractors. A similar specification controlled parts that were designed and manufactured in-house. These specifications included controls for fluid systems compatibility, stress corrosion, atmospheric corrosion, and galvanic corrosion. The controls imposed are summarized below. Control of Fluid Systems Compatibility. A fluid systems compatibility analysis is required that covers all fluids and materials used in the system, such as testing, processing, inspection, and operation, along with known or expected trace contaminants. Fluid system compatibility refers to interaction problems involved with materials and the liquid or gaseous subsystems. The problems experienced generally fall into the following categories: • Autoignition: Spontaneous ignition of the material or the fluid • Impact ignition: Ignition brought about by shock or impact within the fluid • Catalytic reaction: Reactions such as the catalytic decomposition of the fluid • Material degradation: This includes such phenomena as chemical attack, corrosion, galvanic corrosion, stress corrosion, hydrogen embrittlement, and crack growth acceleration with metals and includes embrittlement, abnormal swelling, leaching of plasticizers, ultraviolet degradation, and so on, with nonmetallic materials • Fluid degradation: Reactions in which the physical or chemical characteristics of the fluid are altered • Potential ignition: Ignition due to proximity to electrical ignition sources Materials selection was required to minimize the compatibility problems with the fluid systems. Material-fluid combinations that result in autoignition, impact ignition, or another catastrophic mode of failure were not permitted. The use of electrical and electronic components exposed to nondielectric fluid systems was avoided. Buyer approval was required prior to using electronic components in hazardous fluid systems. Metallic materials listed in Appendix I of MC999-0096 are rated for compatibility with gaseous oxygen (GOX), liquid oxygen (LOX), nitrogen tetroxide (N 2 O 4 ), hydrazine (N 2 H 4 ), monomethyl hydrazine (MMH), and low-pressure ( 3.1 MPa, or 450 psi) and high-pressure (>3.1 MPa, or 450 psi) hydrogen. Nonmetallic materials listed in Appendix II of MC999-0096 are rated for compatibility with low- and high-pressure GOX, LOX, N 2 O 4 , N 2 H 4 , MMH, liquid hydrogen, and hydraulic fluid. Materials that are compatible and noncompatible with titanium are listed in MF0004-018. Lubricants for static service with special fluids (application to elastomers, metals, and threads) are: Fluid Lubricant Ammonia Krytox 240 AC; Braycoat 3L-38RP; Braycoat 815Z oil Deionized water Krytox 240 AC; Braycoat 3L-38RP; Braycoat 815Z oil Freon 21 DC F-6-1101 FC 40 DC F-6-1101 Lubricants for dynamic service with special fluids must be resolved on an individual basis. Use of the above lists did not absolve the seller of full responsibility for verifying compatibility under the particular design conditions used by his fluid system. Control of Stress Corrosion. The subcontractor was required to prepare a stress-corrosion plan utilizing MSFC Specification 522A as a guideline for controlling stress corrosion and to take the actions necessary to prevent such failures. Wherever possible, the supplier was required to select materials that are either not susceptible to stress corrosion or have a high resistance to stress corrosion in the anticipated life cycle environment. Where susceptible materials were used, the supplier was required, at the minimum, to take the following actions to reduce stress-corrosion problems to the extent feasible: • Select less susceptible alloys, tempers, or clad products • Reduce sustained stress levels on the part below stress- corrosion threshold levels, especially in the more susceptible short-transverse grain direction • Protect the part from the detrimental enviro nment by hermetically sealing or coating the part or by inhibiting the environment (closed system) • Avoid or reduce residual stresses in parts or assemblies by stress relieving, by avoiding interference fits, or by shimming assemblies • Avoid galvanic couples, which may tend to accelerate stress corrosion • Provide for regular inspection of parts to determine surface flaws and cracking during the life cycle of the part • Improve the surface quality by reducing surface roughness and/or increasing surface compre ssive stresses • Avoid the use of titanium in contact with silver, silver-plated material, or silver- plated fasteners, such as silver-plated A-286 nuts Control of Galvanic Corrosion. Dissimilar metals were not to be used in intimate contact unless they were suitably protected against galvanic corrosion. Because of the seriousness of galvanic corrosion, every effort was made to avoid the use of dissimilar metals, to exclude moisture or other electrolytes from the system, and to protect metal surfaces in the contact area. Metals were considered compatible if they were in the same grouping as specified in MSFC-SPEC-250, Class II, or if the difference in solution potential was 0.25 V or less. Control of Atmospheric Corrosion. All parts, assemblies, and equipment, including spares, were finished to provide protection from corrosion in accordance with the requirements of MSFC-SPEC-250, Class II, as a minimum. All organic finishes and anodized aluminum that contact titanium were limited to surfaces not normally exposed to propellants. A finish specification delineating the finishes used on each specific material in any particular application and corrosion control procedure was prepared by the orbiter contractor. The finish specification and related procedures from the subcontractor were required to provide for in-process corrosion control. Specific requirements were also given for: • Surface preparation for adhesive bonding • Finish systems for interior and exterior surfaces (including those surfaces to which the therm al protection system was to be bonded) • Fastener installations • Joints and faying surface sealing • Protection for parts to be shipped for vehicle final assembly Designing to Control Corrosion of the Space Shuttle Orbiter From a corrosion control standpoint, it is convenient to separate the space shuttle orbiter into four categories; primary structure, fluid systems, mechanical systems, and avionics systems. Each of these areas has its own unique problems. Primary Structure Weight and cost both dictated that the primary structure of the orbiter be made from aluminum. The majority of this structure was made from heat-treated alloys of the 2000 and 7000 series (Fig. 3). However, some 5000- and 6000-series alloys were also used. Fig. 3 Typical materials of construction for the space shuttle orbiter Aluminum Airframe. Prior to alloy selection, two surveys were conducted. The first was to identify steps to be taken to avoid stress-corrosion problems with aluminum alloys, and the second to identify a corrosion protection system that could survive the unique spacecraft environment. The stress-corrosion survey, conducted in the early 1970s, indicated that virtually all stress-corrosion failures in service occurred in the 2000-series alloys in the T3, T4, and T6XX tempers and in the 7000-series alloys in the T6XX tempers and perpendicular to the short-transverse direction. Only alloys 2024, 2124, and 2219 have high SCC resistance in the T6XX tempers. In forgings, stress corrosion occurred in end grain runout along the forging parting lines. In extrusions and plate, failures occurred where parts were severely formed, where interference fits had occurred, or where assembly stresses were high. Theoretically, stress corrosion will not occur until all three of the essential elements are present: a susceptible microstructure, a corrosive environment, and surface tensile stresses. If any one of these was eliminated, stress corrosion should not occur. The approach used on the orbiter, however, was to eliminate or minimize all three of these conditions to the maximum practical extent. First, only aluminum alloys were permitted with a minimum 170-MPa (25-ksi) stress-corrosion threshold (according to the supplier's standard test methods: 30 days in salt spray) in all directions. This eliminated the T3, T4, and T6XX tempers of nearly all the 2000- and 7000-series alloys, whose stress-corrosion thresholds could be as low as 48 MPa (7 ksi) or less in the short-transverse direction. The preferred microstructure of the -T73 and -T76 tempers was chosen for 7000-series alloys, and although some weight penalty was incurred, program reliability was well served. Because most of the aluminum structure is designed for compressive loading (buckling, crippling) or shear, a very small weight penalty actually resulted. For the 2000-series alloys, the T8XX tempers were used predominantly; however, in a few cases, the T6 or T62 tempers were used for alloys 2124 and 2219. Second, every effort was made to reduce residual stress levels. Mill products were ordered in stress-relieved tempers (for example, T651 or T851) wherever possible to reduce machining distortion and susceptibility to stress corrosion. Interference fits were limited to stress levels below 67% of the stress-corrosion thresholds. Tables were prepared to allow the materials and process engineer to ascertain stress levels resulting from interference fit pins and bushings into various size lugs. Residual stresses in assembly were minimized by shimming. Forming by bending, which put the short- transverse direction into tension, was not permitted. Finally, the corrosion-protective paint system (described below) was applied to all aluminum parts. As of 1986, no incidents of aluminum stress-corrosion failures have been reported on the orbiter since its inception in 1972. The second survey conducted in the early 1970s involved the selection of a paint system that would meet the unique requirements of the shuttle. The system had to provide protection to aluminum from corrosion for a minimum of 10 years of seacoast exposure, without touchup, because it must also serve as the base to which the thermal protection system (TPS) tiles of the shuttle are bonded. Unlike commercial aircraft, the external surfaces could not be washed, repainted, or protected from water intrusion and crevice corrosion by using water-displacing chemicals. The paint system had to endure temperatures of 175 °C (350 °F) during entry and landing because heat from reentry soaked back into the structure. It had to be capable of surviving space vacuum and low temperatures (-155 °C, or -250 °F) without degradation. Minimum offgassing was desirable to avoid giving off toxic fumes inside the cabin (crew hazard) or the condensation of volatile material on windows or optical (thermal) control surfaces. The need for exceptional corrosion resistance was further mandated by the floating bilge; that is, the orbiter is stacked and launched in a vertical attitude, operates in zero gravity, and reenters and lands horizontally. It was not possible to ensure that all water drains out of the structure in all attitudes. The paint system chosen was a chromate-inhibited epoxy polyamine primer. This system was tough, abrasion resistant, and durable. Surfaces to be painted were either anodized according to MIL-A-8625 type II, Class 1, or chemically filmed according to MIL-C-5541, class 1A. Each coat of paint was 0.015 to 0.023 mm (0.6 to 0.9 mil) thick. A single coat of the chromated epoxy polyamine primer demonstrated 1500 h of salt spray protection without corrosion, even in areas scratched through to bare aluminum. The surface to which the external TPS was bonded had a single coat of the chromated epoxy polyamine paint. It achieved additional corrosion protection from a room-temperature vulcanized (RTV) adhesive layer 0.13 to 0.23 mm (5 to 9 mils) thick used to bond the tiles. In the cargo bay area, the single coat of epoxy polyamine primer was overcoated with one coat of polyurethane (MIL-C-83286 or MIL-C-81773) to achieve the required optical properties absorptivity ( ), emissivity ( ), and the proper / ratio for heat control. The interior of the cabin required the use of nonglare coatings and selected colors. Again, a single undercoat of the chromated epoxy polyamine primer was coated with polyurethane. In this case, the polyurethane not only provided a durable color but also acted as a barrier to unacceptable offgassing products of the primer. Parts were painted as details, drilled and assembled, and then repainted upon assembly to coat the fasteners. Although it was desirable from a corrosion standpoint to install all rivets wet, practical manufacturing considerations did not permit it. Automatic riveting machines, which were used to install nearly 90,000 rivets in the wing, could not use wet rivet installations. The weight reduction demands of the program resulted in the elimination of the use of two coats of paint on interior surfaces. More than 500 kg (1100 lb) of paint were used to cover 8175 m 2 (88,000 ft 2 ) of surface. By substituting an anodize coating for one coat of primer, significant weight savings were realized. Consequently, the finish system for Discovery and Atlantis followed the general scheme: Area Coating Exterior TPS surface Anodize + 1 coat chromated epoxy polyamine primer + 1 coat RTV adhesive Exterior non-TPS surfaces Anodize + 1 coat chromated epoxy polyamine primer Interior surface Anodize + either 1 coat chromated epoxy polyamine primer or 1 coat polyurethane Crew compartment Chemical film or anodize + 1 coat chromated epoxy polyamine primer + 1 coat polyurethane or anodize only The forward fuselage was fabricated as a sheet metal skin-stringer design in aluminum alloy 2024-T6. Suspended inside the forward fuselage was an all-welded aluminum pressurized cabin made from aluminum alloy 2219 using the T6 and T8 tempers. It was approximately conical in shape, about 5 m (17 ft) long and tapering from 5 to 2.4 m (17 to 8 ft) in diameter at its forward end. The mild and aft fuselage structures were machined from aluminum alloy 2124-T851 plate. Major frames were of aluminum alloy 7075 in the T76 or T73 tempers. Aluminum honeycomb sandwich was extensively used in the wings and body flap area. Corrosion protection systems must prevent corrosion of the thin (0.025 to 0.075 mm, or 1 to 3 mil) honeycomb core (usually aluminum alloy 5056- H39) and delamination of the skins. Face sheet skins could not be alclad, because corrosion would proceed in the plane of the sheet, resulting in delamination of the bond line. To prevent corrosion, all aluminum cores were protected with conversion coatings and were nonperforated; the face sheets used corrosion-resistant adhesive primers, and the sandwich assemblies were sealed at the edges to prevent water entry. Structural Joints and Fasteners. The corrosion engineers favored the assembly of structural joints with RTV faying surface sealants; however, the electrical bonding requirements or grounding of each panel eliminated this approach. Electrical bonding requires a maximum dc resistance of 2.5 m across joints requiring lightning protection or radio frequency grounding for electrical or electronic equipment. For a typical faying surface joints, local removal of the paint or anodize on the detail (down to bare metal) is required in the area of the fastener. Bare aluminum surfaces are subsequently coated with a chemical film (MIL-C-5541, class 1A). The joint is then bolted with stainless steel fasteners using stainless steel washers under the bolthead and nut to protect aluminum surfaces during application of torque. Fasteners are installed with the shank portion wetted with chromated epoxy polyamine primer. Joints are subsequently touched up with a chemical film and a coat of chromated epoxy polyamine primer around the washers. Faying surfaces are sealed with a continuous fillet of RTV 577, which is a white, thixotropic silicone rubber material. There are approximately 30 different kinds of electrical grounding joints available to suite various designs on the orbiter; the grounding techniques used include jumpers, spot welds, staples, metallized tape, and the method described above. Even an adhesively bonded edge member or a T-section bonded to a honeycomb face sheet must be provided with a ground to the face sheet itself. Dissimilar-metal joints are permitted on the orbiter without additional galvanic protection if they fall within the range shown in Table 1. Table 1 should be used only a guideline, and such factors as cathode-to-anode area ratios, corrosive environments, and other detrimental factors must be evaluated. Table 1 Metals and alloys compatible in dissimilar-metal couples The fasteners chosen for the spacecraft are all basically corrosion resistant, but care must be taken with dissimilar-metal combinations. Bolts are typically made of alloys A-286 (965 to 1380 MPa, or 140 to 200 ksi), Inconel alloy 718 (1240 MPa, or 180 ksi), and MP35N (1655 MPa, or 240 ksi). For applications up to 870 °C (1600 °F), Udimet 500 is used (1035 MPa, or 150 ksi). Of these materials, only A-286 has shown any corrosion in service. Thecorrosion is only superficial and of on real concern. It is removed only for cosmetic reasons. None of these alloys is susceptible to hydrogen embrittlement in orbiter vehicle service. Nuts are made from A-286 and Inconel alloy 718 and are lubricated with a thin (0.005 to 0.01 mm, or 0.2 to 0.4 mil) silver plate. Bolts and nuts are always installed with washers. The aluminum, therefore, never contacts the silver plate. Because the hole, as previously mentioned, is coated with wet chromated epoxy polyamine primer, no moisture can penetrate between the stainless or nickel shank and the aluminum hole. Stainless steel washers are separated from the aluminum surface with a dry coat of primer (where electrical grounding is not required) or a chemical film coating plus a touchup of the primmer around the washer (where electrical grounding is required). No problems with galvanic corrosion are experienced in these installations. Inserts, made from A-286, are silver plated and must also be installed into aluminum wetted on their exterior with the chromated epoxy polyamine primer. Where nuts are used in contact with titanium, only molybdenum disulfide-type dry film lubricants are used. Experience has shown that silver in contact with titanium at approximately 265 °C (500 °F) or above can bring about stress-corrosion cracking (SCC) of titanium (Ref 2, 3, 4). Titanium contact with silver is also prohibited by MIL-S-5002. Titanium pin and collar fasteners are used for shear applications. To save weight, the orbiter uses aluminum alloy 2024 collars rather than A-286. Again, the holes are wet coated with the chromated epoxy polyamine primer by applying primer to the fastener shank away from the threads. Both ends of the fastener are subsequently touched up with the primer. Where such fasteners are used on the graphite cargo bay doors, it is necessary to use RTV rubber as a corrosion barrier, thus completely encapsulating the aluminum collar to prevent corrosion. Rivets used on the spacecraft are made from aluminum alloy 2219-T62. These rivets provide good shear strength while avoiding the need to "ice box" rivets after solution treating, as is required with aluminum alloy 2024 rivets. Further, aluminum alloy 2024 rivets would undergo aging at entry temperatures of the orbiter (175 °C, or 350 °F), resulting in an increased susceptibility to corrosion as grain-boundary precipitation initiated. Aluminum alloy 2219-T81 rivets, although also commercially available, lack sufficient ductility to prevent cracking of driven heads. (The widely used aerospace aluminum alloy 7050-T73 rivets were unavailable when the shuttle orbiter was being built.) Other Structural Alloys. No corrosion protection (except for passivation treatments after fabrication) is considered necessary for stainless steels. Stainless steels, particularly the precipitation-hardenable grades, will often display light surface corrosion products after extensive exposure. A light abrasion will remove the corrosion. No effort is made to passivate stainless parts as installed, because chemical spillage is considered more detrimental to the structure than any enhanced corrosion protection gained from passivation. Nickel alloys such as Inconel alloy 718 and Inconel alloy 625 are used for elevated-temperature service with no corrosion protection. Inconel alloy 718 brazed honeycomb panels are used for the conical seals on the vertical stabilizer and for outboard elevon rub panels and flipper door panels. Inconel alloy 625, made as a resistance-welded sandwich, is used to temperatures of 870 °C (1600 °F). The surface of the sandwich is coated with a wear-resistant high-emittance chromium oxide coating. Titanium also requires no further corrosion protection. Titanium, principally as Ti-6Al-4V, is widely used as forging, bar, and plate products throughout the spacecraft. Many other high-strength titanium alloys are also used. The major structural members of the aft thrust structure are made of Ti-6Al-4V. These transmit the thrust of the liquid rocket engines to the orbiter structure. Titanium honeycomb sandwiches, made by the liquid interface diffusion (LID) bonding process, are used as inboard elevon and flipper door panels. Although the honeycomb has a perforated core, no corrosion is experienced. Because of prior experience in which processing and testing solutions had resulted in SCC of titanium alloys, a control specification, MF0004-018, has been imposed. This specification defines a list of fluids that are suitable for titanium and the specific conditions under which their contact is appropriate. Beryllium alloy S-65 (99% Be min) is used structurally for the external tank door and for windshield retainers. Beams providing structural support in the windshield area use either S-65 or CIP HIP-1 (Ref 5), which is also nonstructurally for the navigational base and the star tracker boom, as well as for heat sinks. Beryllium must be protected in service. The beryllium is anodized according to a Rockwell internal specification and is painted with one coat of chromated epoxy polyamine paint or chemically filmed in a manner similar to that used for aluminum (MIL-C-5541). Two coats of the chromated epoxy polyamine paint are then applied. The anodized coating (0.05 mm, or 0.2 mil, minimum) reveals no corrosion when tested in 168 h of salt spray according to ASTM B 117. Steels must be protected in service from the seacoast environment. Steels are often painted with chromated epoxy polyamine paint or plated with nickel or chromium, depending on the service. Cadmium plating is not used except under rare circumstances, because it can easily sublime in space and redeposit on cooler adjacent surfaces. To avoid problems with SCC and hydrogen embrittlement, steel alloys are restricted to 1380 MPa (200 ksi) or less in tensile applications, and precipitation-hardenable steels are restricted to the H1000 or higher-temperature tempers. Steels with tensile strengths as high as 2070 MPa (300 ksi) can be used for applications that involve bearing, compressive, or shear loads; such applications include ball or roller bearings,valve seats, and springs. A more complete description of the corrosion protection of steel alloy parts that are in moving contact with each other can be found in the discussion "Mechanical Systems" in this article. Niobium is used for low-stress applications in the orbiter airframe structure. Tubes and nozzle parts fabricated from niobium alloy C-103 (Nb-10Hf-1Ti-0.5Zr) are used in the reinforced carbon-carbon nose cap for the shuttle entry air data system (SEADS) program involving measurements of aerodynamics pressures. These parts have a VH109 silicide coating to prevent high-temperature oxidation. The coating was chosen because of its performance at the design temperatures (1455 °C, or 2650 °F) (see the discussion "Case Histories" in this section). Niobium alloy C-103 parts are used as closeout members in elevon seals to shield the hot plasma from the interior structure and mechanisms. These parts are coated with an R512E silicide coating. They were designed for service at maximum temperatures of 1370 °C (2500 °F). Silicide coating systems are ceramic in nature and may be chipped by impact on edges or surfaces. Tests were conducted to verify that parts with coating damage down to bare metal could still function for a limited number of flights (see the discussion "Case Histories" in this section). Composite materials are widely used on the orbiter and present no corrosion problems except for graphite epoxy structures. Although graphite is compatible with titanium, corrosion-resistant steels (A-286 and 300-series stainless steels), nickel, and cobalt-base alloys, the galvanic potential between graphite and aluminum or graphite and steel requires special design considerations. Suitable galvanic isolation is accomplished by using a layer of titanium foil, Tedlar, Kapton, or type 120 glass fabric with suitable resin plus two coats of chromate epoxy polyamine primer. All edges of the joints between the graphite and the aluminum or steel are sealed with RTV silicone to preclude moisture intrusion. More than 300 boron/aluminum composite tubes with diffusion-bonded Ti-6Al-4V clevises are used on the orbiter, principally in the mid fuselage to stabilize frames or as pressure vessel supports. The aluminum portion is painted with chromated epoxy polyamine primer. These present no special corrosion design problems. Also, there are no corrosion problems with boron epoxy-bonded reinforcements on titanium thrust structure tubes in the aft thrust structure. Fluid Systems The space shuttle orbiter fluid systems must provide for the storage, transfer, and regulation of 17 different fluids, as shown in Table 2. The fluid systems can be grouped into major functional areas: • Environmental control and life support system (ECLSS) • Electrical power system (EPS) • Reaction control system (RCS) • Orbital maneuvering system (OMS) • Main propulsion system (MPS) • Auxiliary power units (APU) • Hydraulic system (HYD) Table 2 Fluids used on the space shuttle orbiter See also Fig. 4. Index number Fluid/gas Location System Quantity Explosive limit Threshold limit value, ppm Remarks 1 Ammonia (NH 3 ) Aft fuselage ECLSS 44.27 kg (97.6 lb) 16% 25 Two tanks 2 Breathing oxygen (GOX) Mid fuselage ECLSS 32.21 kg (71 lb) NA (a) (b) One tank 3 Freon-21 Mid and aft fuselage ECLSS 272.16 kg (600 lb) NA 1000 System 4 Freon-1301 Crew compartment ECLSS (fire extinguisher) 5.17 kg (11.4 lb) NA 1000 Three tanks [...]... completely through the tube wall in Fig 12( c) The microstructure was then etched with 3% nital to bring out the carbon steel (Fig 12d) Figures 12( e) and 12( f) show x-ray dot maps of nickel and chromium concentrations, respectively, through the inclusion shown in Fig 12( d) Figures 12( b) to (d) show that the carbon steel is anodic to the stainless steel in water and that corrosion was accelerated by this galvanic... with corrosion products Filiform corrosion can be recognized by the following characteristics: • • • Relatively shallow corrosion of the metal surface Meandering, filamentary pathways Occurrence below a paint or other protective film To prevent filiform corrosion, coatings with lower permeability to water and oxygen are required The following is an example of failure of a pressure vessel by filiform corrosion. .. galvanic corrosion depends on the flow of current; therefore, corrosion rates tend to be accelerated when larger differences in potential exist between the two metals Corrosion damage to the anode becomes more severe as the cathode-to-anode ratio increases Corrosion rates also increase as solution conductivities increase To prevent galvanic corrosion, parts can be isolated from each other electrically, can... doublers are added to restore strength Filiform Corrosion Filiform corrosion is a special form of corrosion that occurs underneath a protective film It is a moving oxygen concentration cell Corrosion takes the form of threadlike or filamentary trails and proceeds along the metal surface rather than penetrating through the thickness Significant filiform corrosion can occur in a matter of hours or days... (d) Cross section through fretting corrosion 175× Stress -Corrosion Cracking Stress corrosion requires the simultaneous occurrence of three conditions: a susceptible material or microstructure, a corrosive environment, and surface tensile stresses Control of stress corrosion is achieved by avoiding any one of these conditions (see the discussion "Control of Stress Corrosion" in this section) Each metal... humidities as low as 60% A key condition for the development of filiform corrosion is that the film is semipermeable, permitting oxygen as well as humidity to pass through it Filiform corrosion, therefore, is essentially a form of crevice corrosion in which one member forming the crevice (the protective film) is semipermeable In filiform corrosion, an anodic head, typically 0.08 to 0.13 mm (3 to 5 mils)... cylinder, 2024-T851 aluminum piston 3 12. 0 1.7 250 20.7 3000 41.4 6000 4.0 20.7 3000 41.4 6000 82.7 12, 000 1.5 2.2 320 2.4 350 3.3 480 26 Factor of safety Liquid hydrogen (LH2) Liquid oxygen (LOX) Monomethyl hydrazine (MMH) EPS Aluminum 2219-T6 2-4 635 × 25 × 86 3.4 (accumulator) 635 × 25 × 86 3.4 (accumulator) 121 4 47.8 EPS Inconel 718(b) 2-4 973 38.3 1.5 7.1 1035 8.6 124 0 109 1575 Forward RCS Aft RCS... that the corrosion zones on both sides of the welds were brought about by a preferential anodic phase in the weld HAZs or a preferential anodic zone caused by differences in permeability and thickness of the protective oxide coating If moisture had condensed in the tank, corrosion would have taken place at the tank bottom Because the corrosion path encircled the weld, it was suspected that corrosion. .. revealed that the corroded areas were entirely free of corrosion products (Fig 10) This indicated that the corrosion had occurred at an earlier stage of manufacture and was passivated and removed by chemical cleaning Although searches of manufacturing records could not verify it, the most probable cause of corrosion was believed to be filiform corrosion occurring under a tape used to attach a protective... accounted for the lack of corrosion products, the shallowness of attack (0.1 mm, or 0.4 mil), and the filamentary network observed Galvanic Corrosion Galvanic corrosion results from or is accelerated by dissimilar-metal contact The more electronegative metal becomes the anode and is corroded The more electropositive metal becomes the cathode and is not attacked The severity of galvanic corrosion depends on . controlled parts that were designed and manufactured in-house. These specifications included controls for fluid systems compatibility, stress corrosion, atmospheric corrosion, and galvanic corrosion. . inception of the program to prevent 12 material-related hazards from occurring on the orbiter. These include controls for atmospheric corrosion and stress corrosion, fluid and propellent incompatibilities,. fluid • Material degradation: This includes such phenomena as chemical attack, corrosion, galvanic corrosion, stress corrosion, hydrogen embrittlement, and crack growth acceleration with metals