Par 5-105 Page 5-235
SECTION 8. TAP TESTING
GENERAL. Tap testing is widely used for a quick evaluation of any accessible aircraft surface to detect the presence of delamination or debonding.
The tap testing procedure consists of lightly tapping the surface of the part with a coin, light special hammer with a maximum of 2 ounces (see figure 5-22), or any other suitable object. The acoustic response is compared with that of a known good area.
A “flat” or “dead” response is considered unacceptable. The acoustic response of a good part can vary dramatically with changes in geometry, in which case a standard of some sort is required. The entire area of interest must be tapped. The surface should be dry and free of oil, grease, and dirt. Tap testing is limited to finding relatively shallow defects in skins with a thickness less than .080 inch. In a honeycomb structure, for example, the far side bondline cannot be evaluated, requiring two-side access for a complete inspection. This method is portable, but no records are produced. The accuracy of this test depends on the inspector’s subjective interpretation of the test response; therefore, only qualified personnel should perform this test.
FIGURE 5-22. Sample of special tap hammer.
5-106.5-111. [RESERVED.]
SECTION 9. ACOUSTIC-EMISSION
5-112. GENERAL. Acoustic-Emission is an NDI technique that involves the placing of acoustical-emission sensors at various locations on the aircraft structure and then a pplying a load or stress. The level of stress applied need not reach general yielding, nor does the stress generally need to be of a specific type. Bending stress can be applied to beamed structures, torsional stress can be applied to rotary shafts, thermal stresses can be applied with heat lamps or blankets, and pressure-induced stress can be applied to pressure-containment syst ems such as the aircraft fuselage. The materials emit sound and stress waves that take the form of ultrasonic pulses that can be picked up by sensors. Cracks and areas of corrosion in the stressed airframe structure emit sound waves (different frequencies for different size defects) which are registered by the sensors. These acoustic-emission bursts can be used both to locate flaws and to evaluate their rate of growth as a function of applied stress. Acoustic-emission testing has an advantage over other NDI methods in that it can detect and locate all of the activated flaws in a structure in one test. Acoustic-emission testing does not now provide the capability to size flaws, but it can greatly reduce the area required to be scanned by other NDI methods.
Par 5- Page 5-236
5-113. APPLICATIONS. A wide variety of structures and materials, such as: wood, plastic, fiberglass, and metals can be inspected by the acoustic-emission technique by applying stress on the test material. The emission-producing mechanism in each type of material may differ, but characteristic acoustic-emissions are produced and can be correlated to the integrity of the material. Acoustic-emission technology has been applie d quite successfully in monitoring proof tests of pressure vessels and tests of fiber-reinforced plastic structures of all kinds. There are now ASTM standards and ASME codes applying to its use in testing gas cylinders and both metal and fiber-reinforced plastic vessels, tanks, and piping.
For a welded structure such as a pressure vessel, acoustic-emission testing works well with relatively simple instrumentation. However, slight movement of bolted or riveted joints can also generate acoustic signals. Thus a complex structure may have many acoustic sources besides flaws in its components.
These unwanted emission sources greatly complicate acoustic-emission tests of complex structures. The difficulties are not prohibitive, but they put a premium on the intelligent use of signal processing and interpretation. Therefore, b ecause of the complexity of aircraft structures, application of acoustic-emission testing to aircraft has required a new level of sophistication, both in testing techniques and data interpretation. Research and testing programs are currently in progress to determine the feasibility of acoustic-emission testing on several different types of aircraft.
5-114.5-119. [RESERVED.]
SECTION 10. THERMOGRAPHY
5-120. GENERAL. Thermography is an NDI technique that uses radiant electromagnetic thermal energy to detect flaws. The presence of a flaw is indicated by an abnormal temperature variant when the item is subjected to normal heating and cooling conditions inherent to the in-service life, and/or when artificially heated or cooled. The greater the material’s resistance to heat flow, the more readily the flow can be identified due to temperature differences caused by the flaw.
5-121.5-126. [RESERVED.]
SECTION 11. HOLOGRAPHY
5-127. GENERAL. Holography is an NDI technique that uses visible light waves coupled with photographic equipment to create a three-dimensional image. The process uses two laser beams, one called a reference beam and the other called an object beam. The two laser beams are directed to an object, between beam applications the component is stressed. The beams are then compared and recorded on film, or other electronic recording medium, creating a double image. Indications of applied stresses or defects are shown as virtual images with a system of fringe lines overlaying the part. Holography is most commonly used for rapid assessment of surface flaws in composite structures.
5-128.5-133. [RESERVED.]
Par 5-134 Page 5-237 (and 5-56) SECTION 12. SHEAROGRAPHY
5-134. GENERAL. Shearography was developed for strain measurements. The process now provides a fu ll-field video strain gauge, in real time, over large areas. It is an enhanced form of holography, which requires the part to be under stress. A laser is used for illumination of the part while under stress.
The output takes the form of an image processed video display. This technique has been used effectively in locating defects, such as disbonds and delaminations, through multiple bondlines. It is capable of showing the size and shape of subsurface anomalies when the test part is properly stressed. Shearography has been developed into a useful tool for NDI.
It can be used easily in a hangar environment, while meeting all laser safety concerns. Other applications include the testing of honeycomb structures, such as flaps and control surfaces.
Shearography offers a great increase in the speed of inspection by allowing on-aircraft inspections of structures without their removal, as well as inspections of large areas in just seconds.
5-135.5-140. [RESERVED.]
Par 6-1 Page 6-238
CHAPTER 6. CORROSION, INSPECTION & PROTECTION
SECTION 1. GENERAL
GENERAL. The purpose of this chapter is to provide information that will help maintenance personnel to prevent, control, identify, and treat various types of corrosion. (Reference to AC 43-4A, Corrosion Control For Aircraft for a more in -depth study on the detection and treatment of corrosion.)
Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action and converts it back to a metallic compound.
Four conditions must exist before electrochemical corrosion can occur. (See figure 6-1.) They are:
A metal subject to corrosion (Anode);
A dissimilar conductive material (Cathode), which has less tendency to corrode;
Presence of a continuous, conductive liquid path (Electrolyte); and Electrical contact between the anode and the cathode (usually in the form of metal-to-metal contact such as rivets, bolts, and corrosion).
Elimination of any one of these conditions will stop electrochemical corrosion. (See figure 6-2.)
NOTE: Paint coatings can mask the initial stages of corrosion. Since corrosion products occupy more volume than the original metal, painted surfaces should be inspected often for irregularities such as blisters, flakes, chips, and lumps.
FACTORS INFLUENCING CORROSION.
Some factors which influence metal corrosion and the rate of corrosion are the:
Type of metal;
Heat treatment and grain direction;
Presence of a dissimilar, less corrodible metal;
Anode and cathode surface areas ic and cathodic surface areas (in galvanic corrosion);
Temperature;
Presence of electrolytes (hard water, salt water, battery fluids, etc.);
Availability of oxygen;
Presence of different concentrations of the same electrolyte;
Presence of biological organisms;
Mechanical stress on the corroding metal;
and, Time of exposure to a corrosive environment.
Most pure metals are not suitable for aircraft construction and are used only in combination with other metals to form alloys. Most alloys are made up entirely of small crystalline regions, called grains.
Corrosion can occur on surfaces of those regions which are less resistant and also at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most active metals, (those which lose electrons easily), such as magnesium and aluminum,
Par 6- Page 6-239
FIGURE 6-1. Simplified corrosion cell showing conditions which must exist for electrochemical corrosion.
FIGURE 6-2. Elimination of corrosion by application of an organic film to metal surface.
corrode easily. The most noble metals (those which do not lose electrons easily), such as gold and silver, do not corrode easily.
(those which do not lose electrons easily), such as gold and silver, do not corrode easily.
Corrosion is quickened by high -temperature environments that accelerat e chemical reactions and increase the concentration of water vapor in the air.
Electrolytes (electrically -conducting solutions) form on surfaces when condensation, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing the electrical conductivity of the electrolyte, thereby increasing the rate of corrosion.
When some of the electrolyte on a metal surface is partially confined, (such as between faying surfaces or in a deep crevice) the metal around this area corrodes more rapidly than elsewhere on the metal . This type of corrosion is called an oxygen concentration cell.
Corrosion occurs more
Par 6-2 Page 6-240 rapidly, because the reduced oxygen content of the confined electrolyte cause s the adjacent metal to become anodic to other metal surfaces on the same part that are immersed in electrolyte exposed to the or exposed to air.
Slimes, molds, fungi, and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area usually remains damp, increasing the possibility of corrosion.
Manufacturing processes such as machining, forming, welding, or heat treatment can leave stresses in aircraft parts. This residual stressresidual stress in aircraft parts and can cause cracking in a corrosive environment when the threshold for stress corrosion is exceeded.
COMMON CORROSIVE AGENTS. Substances that cause corrosion of metals are called corrosive agents. The most common corrosive agents are acids, alkalies, and salts. The atmosphere and water, the two most common media for these agents, may also act as corrosive agents.
Any acid s will severely corrode most of the alloys used in airframes. The most destructive are sulfuric acid (battery acid), halogen acids ( i.e., hydrochloric, hydrofluoric, and hydrobromic), nitrous oxide compounds, and organic acids found in the wastes of humans and animals and volcanic dust.
Alkalies, as a group, are not as corrosive as acids.
Aluminum and magnesium alloys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Substances particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust).
The major atmospheric corrosive agents are oxygen and airborne moisture , both of wh ich are in abundant supply. Corrosion often results from the direct action of atmospheric oxygen and moisture on metal , and the presence of additional moisture often accelerates corrosive attack , particularly on ferrous alloys. The atmosphere may also contain other corrosive gases and contaminants, particularly industrial and marine salt spray, which are very corrosive.
The corrosiveness of water , rain, or snow will depend depends on the type and quantity of dissolved mineral and organic impurities and dissolved gasses (particularly oxygen) in the water. One characteristic of water that determines its corrosivity is the conductivity or its ability to act as an electr olyte and conduct a currentmakes it corrosive is its conductivity . Physical factors, such as water temperature and velocity , also have a direct bearing on theits corrosiveness.
MICRO-ORGANISMS.
Bacteria may be either aerobic or anaerobic. Aerobic bacteria require oxygen to live. They accelerate corrosion by oxidizing sulfur to produce sulfuric acid.
Bacteria living adjacent to metals may promote corrosion by depleting the oxygen supply or by releasing metabolic products. Anaerobic bacteria, on the other hand, can survive only when free oxygen is not present. The metabolism of these bacteria requires them to obtain part of their sustenance by oxidizing inorganic compounds, such as iron, sulfur, hydrogen, and carbon monoxide. The resu ltant chemical reactions cause corrosion.
Fungi are the growths of micro-organisms that feed on organic materials. While low humidity does not kill microbes, it slows their growth and may prevent corrosion damage. Ideal growth conditions for most micro-organisms are temperatures between 68 and 104 °F (20 and 40 °C) and relative humidity between 85 and 100 percent.
Damage resulting from microbial growth can occur when any of three basic m echanisms, or a combination of these, is brought into play. First, fungi have a tendency to hold moisture, which contributes to other
Par 6-11 Page 6-241 forms of corrosion. Second, because fungi are living organisms, they need food to survive. This food is obtained from the material on which the fungi are growing. Third, these micro-organisms secrete corrosive fluids that attack many materials, including some that are not fungi nutrient.
Par 6-11 Page 6-242 Microbial growth must be removed completely to avoid corrosion. Microbial growth should be removed by hand with a firm non-metallic bristle brush and water. Removal of microbial growth is easier if the growth is kept wet with water.
Microbial growth may also be removed with steam at 100 psi. Protective clothing must be used when using steam for removing microbial growth.
6-5.6-10. [RESERVED.]
Par 6-11 Page 6-243 SECTION 2. TYPES OF CORROSION
GENERAL. All corrosive attacks begin on the surface of the metal making the classification of corrosion by physical appearance a convenient means of identifying the many manifestationsication. (See figure 6-3.)
FIGURE 6-3. Corrosion attack.
GENERAL SURFACE CORROSION. General surface corrosion (also referred to as Uniform Etch or Uniform Attack Corrosion) is the most common form of corrosion and results from a direct chemical attack on a metal surface and involves only the metal surface. (See figure 6-4.) General surface corrosion usually occurs over a wide area and is more or less equal in dispersion. On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if allowed to continue, the surface becomes rough and possibly frosted in appearance. The discoloration or general dulling of metal created by exposure to elevated temperatures is not to be considered general surface corrosion.
FIGURE 6-4. General surface corrosion.
PITTING CORROSION. Pitting corrosion is one of the most destructive and intense forms of corrosion. It can occur in any metal but is most common on metals that form protective oxide films, such as aluminum and magnesium alloys. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny holes or pits can be seen in the surface. (See figures 6-5(a) and 6-5(b).)
These small surface openings may penetrate deeply into structural members and cause damage completely out of proportion to its surface appearance.
FIGURE 6-5(a). Pitting corrosion (external view).
FIGURE 6-5(b). Pitting corrosion (magnified cross section).
Par 6-11 Page 6-244 CONCENTRATION CELL CORROSION.
Concentration cell corrosion, (also known as Crevice Corrosion) is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though the joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material. Metal ion concentration cells and oxygen concentration cells are the two general types of concentration cell corrosion. (See figure 6-6.)
Metal Ion Concentration Cells . The solution may consist of water and ions of the metal which is in contact with water. A high concentration of the metal ions will normally exist under faying surfaces where the solution is stagnant, and a low concentration of metal ions will exist adjacent to th e crevice which is created by the faying surface. An electrical potential will exist between the two points; the ar ea of the metal in contact with the low concentration of metal ions will be anodic and corrode, and the area in contact with the high metal ion concentration will be cathodic and not show signs of corrosion.
Oxygen Concentration Cells. The solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under gaskets, wood, rubber, and other materials in contact with the metal surface. Corrosion will occur at the area of low oxygen concentration (anode). Alloys, such as stainless steel, which owe their corrosion resistance to surface passivity, are particularly susceptible to this type of crevice corrosion.
ACTIVE-PASSIVE CELLS. Metals which depend on a tightly adhering passive film, usually an oxide, for corrosion protection are prone to rapid corrosive attack by active-passive cells.
Active-passive cells are often referred to as a type of concentration cell corrosion. However, the active-passive cell is in actualitactually two forms of corrosion working in conjunction.
The corrosive action usually starts as an oxygen concentration cell. As an example, salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell. The passive film will be broken beneath the dirt particlesalt crystals. Once the passive film is broken, the active metal beneath the film will be exposed to corrosive attack.
(See figure 6-7.) Rapid pitting of the active metal will result. This reaction can become locally intense due to several factors. First the reaction is augmented by the affected area, in thatsince the proportion of the exposed base metal is small compared to the surrounding nonreactive metal. This effectively concentrates the focal point of the reaction, often resulting in deep pits in a short time and a greater rate of corrosion.
FIGURE 6-6. Concentration cell corrosion.
Par 6-16 Page 6-245 FILIFORM CORROSION. Filiform corrosion is a special form of oxygen concentration cell which occurs on metal surfaces having an organic coating system. It is recognized by its characteristic worm-like trace of corrosion products beneath the paint film. (See figure 6-8.) Polyurethane finishes are especially susceptible to filiform corrosion.
Filiform occurs when the relative humidity of the air is between 78 and 90 percent and the surface is slightly acidic. This corrosion usually attacks steel and aluminum surfaces.
The traces never cross on steel, but they will cross under one another on aluminum which makes the damage deeper and more severe for aluminum. If the corrosion is not removed, the area treated, and a protective finish applied, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams. Filiform corrosion can be removed using glass bead blasting material with portable abrasive blasting equipment or sanding. Filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent, using coating systems having a low rate of diffusion for oxygen and water vapors, and by washing the aircraft to remove acidic contaminants from the surface.
INTERGRANULAR CORROSION. Intergranular corrosion is an attack on the grain boundaries of a metal. A highly -magnified cross section of any commercial alloy shows the granular structure of the metal. It consists of quantities of individual grains, and each of these tiny grains has a clearly -
FIGURE 6-8. Filiform corrosion.
defined boundary which chemically differs from the metal within the grain. The grain boundary and the grain center can react with each other as anode and cathode when in contact with an electrolyte. (See figure 6-9.) Rapid selective corrosion of the grain boundaries can occur. High-strength aluminum alloys such as 2014 and 7075 are more susceptible to intergranular corrosion if they have been improperly heat-treated and then exposed to a corrosive environment.
EXFOLIATION CORROSION. Exfoliation corrosion is an advanced form of intergranular corrosion and shows itself by lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. (See figure 6-10.) It is visible evidence of intergranular corrosion and is most often seen on extruded sections where grain thickness are usually less than in rolled forms.
FIGURE 6-7. Active-passive cell.
Par 6-16 Page 6-246
FIGURE 6-9. Intergranular Corrosion of 7075-T6 aluminum adjacent to steel fastener.
FIGURE 6-10. Exfoliation corrosion.
GALVANIC CORROSION. Galvanic corrosion occurs when two dissimilar metals make contact in the presence of an electrolyte. (See figure 6-11.) It is usually recognizable by the presence of a build-up of corrosion at the joint between the metals.
STRESS CORROSION CRACKING. This form of corrosion involves a constant or cyclic stress, acting in conjunction with a damaging chemical environment. The stress may be caused by internal or external loading.
Internal stress may be trapped in a part of structure during manufacturing processes such as cold working or by unequal cooling from high temperatures. Most manufacturers follow up these processes with a stress relief operation , . Even so, sometimes stress remains trapped. The stress may be externally introduced in a part or structure by methods such as by riveting, welding, bolting, clamping, press fit, etc. If a slight mismatch occurs, or a fastener is over -torqued, internal stress will be present.
Internal stress is more important than design stress, because stress corrosion is di fficult to recognize before it has overcome the design safety factor. The level of stress varies from point to point within the metal.
Stresses near the yield stre ngth are generally necessary to promote stress corrosion cracking. (See figure 6-12.)
However, failures may occur at lower stresses. Specific environments have been identified which cause stress corrosion cracking of certain alloys.