Corrosion
Aluminum Alloy - White to gray powder
.
DEVELOPMENT OF CORROSION
All corrosive attacks begin on the surface of metals. If allowed to progress, corrosion can penetrate into the metal. Because corrosion never begins inside metal parts, there will always be evidence on the surface when corrosion occurs. When corrosion products form, they often precipitate onto the corroding surface as a powdery deposit.
Crevice Corrosion - Crevice corrosion occurs when the electrolyte in a crevice has a different concentration than the area adjacent to the crevice. This type of corrosion is also known as concentration cell corrosion. Electrolyte inside the crevice contains less oxygen and more metal ions than electrolyte just outside the crevice. As a result, the metal surfaces have different activities--even though they may be part of the same metal--and corrosion occurs inside the crevice. This form of corrosion often occurs between faying surfaces or when a surface is covered by a foreign material. (such as dirt) or under gaskets, rubber, or plastic tape. Metals that rely on a tightly adhering passive film, such as corrosion- resistant steels, are prone to crevice corrosion. Three general types of crevice corrosion are: 1. Metal ion concentration cells 2. Oxygen concentration cells 3. Active-passive cells
.
Intergranular Corrosion - Intergranular corrosion is an attack on the grain boundaries of the metal. A highly magnified cross-section of any commercial alloy shows the granular structure of the metal. This consists of quantities of individual grains, each having a clearly defined boundary, that chemically differ from the metal within the grain. The grain boundaries are frequently anodic (i.e., tend to corrode more easily) to the metal within the grain. When in contact with an electrolyte, rapid corrosion occurs at the grain boundaries
.
Levels of preservation for aircraft and SE are defined below. Dehumidification (Level III) is the preferred method of preservation - • Level I: 0-90 days (+/- 3 days) • Level II: 0-1 year • Level III: 0-indefinite
.
Low Alloy Steel - Reddish-brown oxide (rust)
.
METALS AFFECTED BY CORROSION
.
Magnesium - Magnesium alloys are the lightest structural metals used for SE construction. These alloys are highly susceptible to corrosion, which appears as white, powdery mounds or spots when the metal surface is exposed to the environment without a protective finish.
.
Magnesium Alloy - White, powdery, snow-like mounds and white spots on surface
.
Nickel and Chromium - Nickel and chromium are used as protective platings. Chromium plating is also used to provide a smooth, wear resistant surface and to reclaim worn parts
.
FACTORS INFLUENCING CORROSION
a. Type of Metal. The metals most commonly used in SE construction are aluminum, steel, and magnesium. Cadmium, nickel, chromium, and silver are sometimes used as protective platings. Metals have a wide range of corrosion resistance. The most active metals (i.e., those that tend to lose electrons easily)--such as magnesium and aluminum--corrode easily and are listed at the top of Table 2-1. The most noble metals (i.e., those that do not lose electrons easily)--such as gold and silver--do not corrode easily and are listed at the bottom of Table 2-1. b. Dissimilar Metal Coupling (Galvanic Corrosion). When two dissimilar metals make electrical contact in the presence of an electrolyte, the rate at which corrosion occurs depends on the difference in their activity (see Table 2-1). The greater the difference in activity, the faster corrosion occurs. For example, magnesium corrodes very quickly when coupled with gold in a humid atmosphere. But aluminum corrodes very slowly, if at all, when in contact with cadmium. c. Anode and Cathode Surface Area. The rate of galvanic corrosion also depends on the size of the parts in contact. If the surface area of the corroding metal (the anode) is smaller than the surface area of the less active metal (the cathode), corrosion will be rapid and severe. However,if the corroding metal is larger than the less active metal, corrosion will be slow and superficial. For example, an aluminum fastener in contact with a relatively nonreactive monel structure may corrode severely, while a monel bracket secured to a large aluminum member would result in a relatively superficial attack on the aluminum sheet. d. Temperature. High temperature environments tend to produce more rapid corrosion because of accelerated chemical reactions. This is also true in humid environments because of the higher concentration of water vapor in the air. In addition, nightly drops in temperature can cause greater amounts of condensation, which leads to increased corrosion rates. e. Heat Treatment and Grain Direction. When heat-treated, heavy sections of metals do not cool uniformly because they tend to vary in chemical composition from one part of the metal to another. This can cause galvanic corrosion if one area is more active than another. Alloys that are fabricated by rolling, extruding, forging, or pressing have properties that depend highly on direction (parallel to grain elongation vs. cross grain). For example, exposed end grain corrodes much more easily than flattened elongated surfaces in sheet stock. This explains why exfoliation occurs at the edge of aircraft skin sections or next to countersunk fasteners. f. Electrolytes. Electrically conducting solutions are easily formed on metallic surfaces when condensation, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic stack gases, and engine exhaust gases can dissolve on wet surfaces- -thereby increasing the electrical conductivity of the electrolyte and the rate of corrosion. g. Oxygen. When some of the electrolyte on a metal surface is partially confined (such as between faying surfaces or in a deep crevice), metal in the confined area corrodes more rapidly than other metal surfaces of the same part outside the area. This type of corrosion is called an oxygen concentration cell or differential aeration cell. Corrosion occurs more rapidly than expected because the reduced oxygen content of the confined electrolyte causes the adjacent metal to become anodic to other metal surfaces on the same part immersed in electrolyte exposed to air. h. Electrolyte Concentration. In the same way that metals can corrode when exposed to different concentrations of oxygen in an electrolyte, corrosion will also occur if the concentration of the electrolyte on the surface varies from one location to another. This corrosive situation is known as a concentration cell. i. Biological Organisms. Slimes, molds, fungi, and other living organisms (some microscopic) can grow on damp surfaces. Once they are well established, the area tends to remain damp--thereby increasing the possibility of corrosion. Their presence can cause the areas they occupy to have different oxygen and electrolyte concentrations. In addition, corrosive wastes are secreted, which causes corrosion. j. Mechanical Stress. Manufacturing processes such as machining, forming, welding, or heat treatment can leave stresses in SE parts. Almost all alloys in SE construction are sensitive to a form of corrosion known as stress corrosion cracking. This residual stress causes corrosion to proceed more rapidly in structurally important regions of the part until failure occurs. k. Time. As time goes on, metals naturally tend to corrode. In some cases, the corrosion process occurs at the same rate--no matter how long the metal has been exposed to the environment. In other cases, corrosion can decrease with time (because of the barrier formed by corrosion products) or increase (if a barrier to corrosion is being broken down).
CAUTION An operating procedure, practice, or condition, etc., that can damage equipment if not carefully observed or followed.
-
Atom - The smallest unit of an element. There are more than 100 elements, including metals (such as aluminum, magnesium, iron, nickel, titanium, cadmium, chromium, copper, and carbon) and non-metals (such as hydrogen, oxygen, sulfur, and chlorine).
.
WARNING An operating procedure, practice, condition, etc., that can result in personal injury or death if not carefully observed or followed.
-
WARNING: Disconnect the negative battery terminal first for preservation. Reconnect the negative terminal last upon depreservation.
-
WARNING: Unless it is a touch-up job, do not apply new primer or topcoat over yellow paint or paint that is suspected of containing lead and /or chromates
-
When 50% or more of the yellow coating system on a piece of SE requires rework because of defects, strip the entire unit to facilitate changeover to the white paint system.
-
Aluminum - Aluminum and aluminum alloys are widely used for SE construction. The corrosion product of aluminum is a white to gray powdery material (aluminum oxide or hydroxide), which can be removed by mechanical polishing or brushing with an abrasive
.
A successful corrosion prevention program is the result of a concentrated effort by all operating, maintenance, and other support personnel involved with SE. An organized and vigilant preventive maintenance effort will: a. Reduce maintenance time spent repairing corrosion damage. b. Reduce the number of maintenance actions. c. Increase SE reliability, availability, and longevity
-
CAUTION: Do not apply Extend rust treatment (A15) to surfaces in direct sunlight. Do not return unused material to the original container or dip brushes into the original container. Never add solvents to Extend rust treatment (A14).
-
CAUTION: Do not use steel wire brushes on battery terminals or live electrical circuits.
-
CAUTION: Paint will crack if applied at thick nesses above the specified wet-film thickness. Apply an additional top coat only if required to hide the primer. Check the thickness using a wet-film thickness (WFT)
-
Category A: SE that has anticipated usage within the next 90 days; should be maintained under current SE/PMS directives.
-
Category B: SE that could possibly be used within the next 180 days; should be placed in a minimum of Level I preservation.
-
Category C: SE not needed for extremely long periods of time (i.e., in excess of 180 days); should be placed in Level II or III preservation depending on the resources at the geographical location.
-
DEPRESERVATION. Remove all barrier materials. Remove corrosion-preventive compounds with cleaning compound in accordance with paragraph 4-10.a.1 (nonoxygen SE). If used, remove the desiccant MIL-D-3464 (A75). Perform preoperational inspection in accordance with MIMs/MRCs. Note any discrepancies on a VIDS/ MAF. Record on an SE preoperational record.
-
Do not apply white paint over yellow paint
-
Eliminating any one of these four conditions will stop corrosion. For example, a paint film on a metal surface will prevent the conducting liquid (electrolyte) from connecting the anode and cathode--thereby stopping the electric current. Another example is when two connected dissimilar metal parts placed in pure water corrode very slowly because of the lack of ions to conduct the electric current.
-
Light Corrosion. At this degree, the protective coating is scarred or etched, and the condition of the metal is characterized by discoloration and pitting to a maximum depth of one mil (0.001 inch). This type of damage can normally be removed by lightly hand-sanding. Light corrosion does not affect equipment functions but should be repaired
-
Moderate Corrosion. Moderate corrosion looks like light corrosion, but there may be some blisters or evidence of scaling and flaking of the coating or paint system. The pit depth may be as deep as 10 mils (0.010 inch). This type of damage is normally removed by extensive hand-sanding or light mechanical sanding. Moderate corrosion is likely to cause severe damage if not treated and needs to be repaired.
-
NOTE An operating procedure, practice, or condition, etc., that is essential to emphasize.
-
NOTE: Always use the mildest cleaning method possible to avoid damaging the equipment being cleaned
-
NOTE: During normal Level II and III preservation, the "clock" stops for MRCs and is started again upon depreservation.
-
NOTE: Ensure that powder coating applications is performed in a dry area free of excessive air movement
-
NOTE: Limit application of Extend rust treatment (A14) to exposed metal only.
-
NOTE: Remove corrosion, scale, and old paint using the least destructive method available.
-
NOTE: When blasting aluminum alloys, do not allow the blast stream to dwell on the same spot longer than 15 seconds. Longer dwell times will cause excessive metal removal.
-
RECOMMENDED COATINGS. The coatings recommended for SE are powder coatings or paint coatings. Both powder and paint coating systems include a primer with a topcoat. The primer promotes adhesion to the substrate and may contain corrosion inhibitors. The topcoat provides weather and chemical resistance. Neither the primer nor the top coat is formulated to provide adequate protection alone. Neither should be applied as a single coat. Always apply both a primer and a topcoat.
-
RECORD KEEPING. Preservation of SE is recorded on the SE Custody and Maintenance History Record, OPNAV Form 4790/51. The OPNAVINST 4790.2 series provides guidance for recording preservation information on this form.
-
Severe Corrosion. The characteristics of severe corrosion are severe intergranular corrosion, blistering, exfoliation, scaling, or flaking. The pitting depths are deeper than 10 mils (0.010 inch). Remove this damage by extensive mechanical sanding or grinding. The function of equipment and safety are impaired, and the unit shall not be used until treated or repaired.
-
COATINGS FOR SUPPORT EQUIPMENT (SE) - The coatings presented in this section are to be used for corrosion-preventive touch-up and total rework. The primary objective of any paint is to protect exposed surfaces against corrosion and other forms of deterioration. Additional reasons for using particular paints include: a. Reduction of glare (by using lusterless or nonspecular coatings) b. Camouflage and other detection counter measures c. High visibility requirements d. Identification markings.
.
Chromium (plate) - No visible corrosion products;blistering of plating due to rusting and lifting.
.
Copper and Copper Alloys - - Copper and copper alloys are quite corrosion-resistant, with corrosion usually limited to staining and tarnish. - Copper corrosion is evidenced by the accumulation of blue or blue-green corrosion products
.
Copper-base Alloy, Brass, Bronze - Blue or blue green powdery deposits
.
Corrosion -is the electrochemical deterioration of a metal because of its chemical reaction with the surrounding environment. This reaction occurs because of the tendency of metals to return to their naturally occurring states, usually oxide or sulfide ores. For example, iron in the presence of moisture and air will return to its natural state--iron oxide or rust. Aluminum and magnesium form corrosion products that are white oxides or hydroxides. When corrosion occurs, water is usually present in some form (e.g., humidity, moisture, condensation, rain, salt spray, etc.), acting as an electrolyte and reacting chemically with metal surfaces. This is known as electrochemical corrosion.
.
Corrosion Fatigue - Corrosion fatigue is the cracking of metals caused by the combined effects of cyclic stress and corrosion and is very similar to stress corrosion cracking. If it is in a corrosive environment, no metal is immune to some reduction in resistance to cyclic stressing. Corrosion fatigue failure occurs in two stages. During the first stage, the combined action of corrosion and cyclic stress damages the metal by pitting and forming cracks in the pitted area. The second stage is the continuation of crack propagation, in which the rate of cracking is controlled by: 1. Stress concentration in the main cross-section. 2. Physical properties of the metal. Fracture of a metal part due to fatigue corrosion occurs at a stress level far below the fatigue limit, even though the amount of corrosion is small. For this reason, protection of all parts subject to alternating stress is particularly important, even in environments that are only mildly corrosive. Preventive measures are the same as those given for stress corrosion cracking.
.
DEGREES OF CORROSION Light Corrosion Moderate Corrosion Severe Corrosion
.
Exfoliation Corrosion - Exfoliation corrosion is an advanced form of intergranular corrosion and occurs when the surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the grain boundaries. The lifting up or swelling is visible evidence of exfoliation corrosion. Exfoliation occurs on extruded, rolled, wrought, and forged highstrength aluminum and magnesium parts.
.
Filiform Corrosion - Filiform corrosion is a special form of oxygen concentration cell corrosion (or crevice corrosion) that occurs on metal surfaces having an organic coating system. It is recognizable by its characteristic worm-like trace of corrosion products beneath the paint film
.
Fretting Corrosion - Fretting corrosion is a special form of concentration cell corrosion that occurs in combination with surface wear. The corrosion products increase the wear of the surface, and the wear exposes more bare metal surface to be corroded. The overall effect is greater than the single effects of corrosion and wear added together. It has the general appearance of galling, in which chunks of metal are torn from the surface with corrosion at the torn areas or ragged pits. This type of corrosion occurs on faying surfaces of close tolerance and on parts under high pressure in a corrosive environment when there is slight relative movement of parts (such as that caused by vibration). Fretting corrosion is normally encountered in heavily loaded static joints that are subject to vibration and that are not sealed to prevent moisture entry.
.
Galvanic Corrosion - Galvanic corrosion occurs when different metals are in contact with each other and an electrolyte (such as salt water). It is usually recognizable by a buildup of corrosion at the joint between the metals.
.
Hydrogen Embrittlement - Hydrogen embrittlement is the weakening of materials such as high-strength steel (typically 180 Ksi and above), some high-strength aluminum, and some stainless steels when they are exposed to acid paint removers, plating solutions, and other acidic and more alkaline materials. This occurs when a cathodic reaction on the steel surface produces hydrogen, which diffuses into the bulk metal, accumulates at grain boundaries, and weakens the structure. If the part is under load or contains residual manufacturing stresses, sudden catastrophic failure occurs when the part can no longer sustain the internal and/or applied stresses. Hydrogen embrittlement has been known to occur in parts stressed to only 15% of nominal tensile strength.
.
PRESERVATION AND DEPRESERVATION OF SUPPORT EQUIPMENT (SE) - PURPOSE. Prevention of deterioration is one of the more important features of OPNAVINST 4790.2, Naval Aviation Maintenance Program (NAMP). The readiness of in-use assets and the safeguarding of reserve stocks of SE can be seriously impaired by corrosion damage and deterioration. Deterioration is greatest when SE is dirty, inactive for an extended period of time, or being shipped. Therefore, a form of protection must be afforded to the equipment under these circumstances. If the work of repairing deterioration damage is added to the normal workload, maintenance becomes very difficult, time-consuming, and costly. SCOPE. This section describes initial preservation treatment, preservation maintenance procedures, and necessary depreservation steps. Activities may use the following categories to determine the required level of preservation—as well as applicable MIMs or directives. The maintenance officer is responsible for determining and ensuring that the appropriate pieces of SE are placed in preservation
.
PREVENTIVE MAINTENANCE - The two most important factors in preventing corrosion and the only ones that field personnel can control are the removal of electrolytes and the application of protective coatings. Corrosion can be minimized by frequent washing because the extent of corrosion depends on the length of time electrolytes are in contact with metals. If noncorrosive cleaners are used, frequently cleaning a surface in a corrosive environment will reduce the likelihood of corrosion. In addition, keeping chemical treatments and paint finishes in good condition will minimize corrosion. The degradation of nonmetallic materials can be minimized by avoiding the use of unauthorized maintenance chemicals and procedures. When repairing or replacing nonmetallic materials, use only approved materials. Dedication to proper preventive maintenance practices maximizes equipment reliability.
.
Pitting Corrosion - The most common corrosion on aluminum and magnesium alloys is called pitting corrosion. It is first noticeable as a white or gray powdery deposit, similar to dust, that blotches the surface. When the deposit is cleaned away, tiny pits or holes can be seen in the surface. The combination of small active anodes to large passive cathodes causes severe pitting.
.
Preferred and Alternate Coatings. The preferred coatings for SE are as follows: - 1. Primer. MIL-P-53022 Type II (A62) polyamide epoxy applied at manufacturer's recommended dry film thickness. 2. Topcoat. MIL-C-85285 Type II (A65) polyurethane applied at a dry film thickness of 2 mils. Alternate coatings include the following: 3. Primer. MIL-P-53030 (A63) waterborne epoxy applied at manufacturer's recommended dryfilm thickness. 4. Topcoat. MIL-C-22750 (A67) (lead free), polyamide epoxy applied at a dry-film thickness of 2-3 mils
.
Stainless Steel - Basically, stainless steels (or corrosion-resistant steels as they are more properly described) are alloys of iron with chromium. Many other elements—such as nickel, sulfur, molybdenum, vanadium, cobalt, columbium, titanium, and aluminum—are added in various amounts and combinations to develop special characteristics. Stainless steels are much more resistant to common rusting, chemical actions, and high-temperature oxidation than ordinary steels because of the formation of an invisible oxide film or passive layer on the surface of these alloys. Corrosion- and heat-resistance are the major factors in selecting stainless steels for a specific application. However, it should be emphasized that stainless steels are not a cure for all corrosion problems because of service conditions that can destroy the oxide film on their surfaces. Stainless steels are highly susceptible to crevice corrosion and stress corrosion cracking in moist, salt-laden environments. In addition, if proper techniques of sealing and protective coating are ignored, they can cause galvanic corrosion of almost any other metal with which they are in contact. Stainless steels may be magnetic or nonmagnetic
.
Stainless Steels (300-400 series) - Rough surface; sometimes a red, brown, or black uniform stain
.
Steel - Ferrous (iron) alloys are used to manufacture many components and assemblies in ground SE (e.g., missile gantries, silo crib structures, frames and bodies of trailers and vans, and lesser structural parts such as brackets, racks, and panels). If unprotected, ferrous alloy surfaces (with the exception of stainless steels) are easily corroded in the presence of moisture. Ferrous alloy surfaces are normally painted or plated to prevent corrosion. Corrosion of steel is easily recognized because the corrosion product is red rust. When ferrous alloys corrode, a dark corrosion product usually forms first. When moisture is present, this coating is converted to red rust, which will promote further attack by absorbing moisture from the air.
.
Stress Corrosion Cracking - Stress corrosion cracking is the intergranular cracking of a metal caused by the combined effects of constant tensile stress (internal or applied) and corrosion. Internal or residual stresses are produced by cold working, forming, and heat treatment operations during manufacture of a part and remain concealed in the part unless stress relief operations are used. Other hidden stresses are induced in parts when press or shrink fits are used and when slightly mismatched parts are clamped together with rivets and bolts. All these stresses add to those caused by applying normal loads to parts in operation. Metals have threshold stresses below which stress corrosion cracking will not occur. This threshold stress varies from metal to metal and depends on the characteristics of the stress that is applied. 1. Associated Hazards. Stress corrosion cracking can be extremely dangerous because it occurs at stress levels far below the rated strength of a metal, starting from what is thought to be a very minor corrosion pit. Parts can completely sever in a split second or they can crack slowly. The rate of cracking is very unpredictable. 2. Causes. Specific environments that cause stress corrosion cracking of certain alloys have been identified. Salt solutions, sea water, and moist salt-laden air may cause stress corrosion cracking of heat-treatable aluminum alloys and stainless steel. Magnesium alloys may stress corrode in moist air. Stress corrosion can be prevented by placing an insulating barrier between the metal and the corrosive environment and by applying protective coatings and water displacing corrosion-preventive compounds.
.
TYPES OF CORROSION
.
Uniform Surface Corrosion - Uniform surface corrosion results from a direct chemical attack on a metal surface and involves only the metal surface. On a polished surface, this type of corrosion is first seen as a general dulling or etching of the surface, and, if the attack continues, the surface becomes rough and possibly frosted in appearance. This type of corrosion appears uniform because the anodes and cathodes are very small and constantly shift from one area of the surface to another. An example is the etching of metals by acids. The discoloration or general dulling of metal created by exposure to elevated temperatures is not considered to be uniform surface corrosion.
.
Electrolyte - A liquid solution (usually water) containing ions. Salt water is an electrolyte.
...
Electron - A negatively charged particle much smaller than an atom. An electrical current occurs when electrons are forced to move through metal conductors. Electrons also flow through water solutions—but only in the presence of ions.
...
Ions - Atoms or groups of atoms bound together that are either positively or negatively charged. An electrical current occurs when ions are forced to move through water solutions. Ions cannot move through metal conductors.
...
THEORY OF CORROSION - When a metal corrodes, the metal atoms lose electrons and become metal ions in an electrolyte solution. The positively charged metal ions can combine with negatively charged ions to form corrosion products, such as metallic chlorides, oxides, hydroxides, and sulfides. Four conditions must exist before this type of corrosion can occur. a. A metal must be present that has a tendency to corrode. The corroding metal is known as the anode. b. A dissimilar conductive material (the cathode) that has less of a tendency to corrode than the anode must be present. Examples include a different metal, a protected part of the same metal, or conductive plastics. c. A conductive liquid (electrolyte) must connect the anode and cathode so that ions can carry electrical current between them. d. Electrical contact between the anode and cathode (usually in the form of metal-to-metal contact) must exist so that electrons can move from the anode--where they are released--to the cathode.
...
