La definición de categorías constitucionales

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11.3 TYPES OF CORROSION

There are many forms of corrosion. The form may depend on the metals involved, their function, atmospheric conditions and corrosive agents present.

The following are the more common found on aircraft structures.

Surface Dissimilar Metal

Intergranular Exfoliation

Stress Fretting

Crevice Filiform

Pitting Corrosion Fatigue

Microbiological Hydrogen Embrittlement 11.3.1 SURFACE CORROSION

General roughening, etching or pitting of the metal surface, frequently

accompanied by a powdery deposit of corrosion products, may be caused by direct chemical or electrochemical attack. Corrosion can spread under the surface coating unnoticed, until the paint or plating is lifted off the surface by the corrosion products or forms blisters.

Surface corrosion is a fairly uniform corrosion attack, which slowly reduces the cross-section of the metal. It is, possibly, the least damaging form of corrosion.

A mild attack may result in only general etching of an area, whilst a heavier attack may produce deposits which depend on the type of metal that is being attacked.

‘Pure’ aluminium, stainless steel and copper have more resistance to surface corrosion than aluminium alloy, magnesium alloy and non-stainless steels. This type of corrosion only becomes serious over a period of time and gives a warning of worse corrosion to follow.

11.3.2 DISSIMILAR METAL CORROSION

Galvanic action leads to one of the more common forms of corrosion, which occurs between two dissimilar metals in contact with each other and where there is moisture present. It is caused by the difference in galvanic potential of the two metals where plating or jointing compound has been removed or omitted. This type of corrosion can occur, for example, where steel bolts, nuts, or studs are in contact with magnesium-rich alloys such as aircraft wheels.

This may be taking place out of sight and may result in extensive pitting. It may or may not be accompanied by surface corrosion.

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11.3.3 INTERGRANULAR CORROSION

This corrosion is also known as intercrystalline corrosion, and results from micro-galvanic cells at the grain boundaries in the metal (refer to Fig.34).

Corrosion progresses from the metal surface, in narrow pathways, along grain boundaries, often penetrating quite deeply and having a serious, mechanical weakening effect. The amount of metal corroded is small, relative to the volume of metal affected.

Indications of the damage may NOT be visible to the naked eye. Intergranular corrosion may often be detected by ultrasonic, eddy current or radiographic inspection procedures.

Intergranular Corrosion Fig. 34

JAR 66 CATEGORY B1 intergranular corrosion in which the attack occurs in layers parallel to the surface.

The wedging action, of the corrosion products, occupies a larger volume than the alloy, and will cause lifting of the metal surface, causing it to ‘exfoliate’. This occurs at an early stage, when the corrosion is on, or just below, the surface.

Exfoliation corrosion often attacks 7000 series alloys (those with an appreciable amount of Zinc). When the corrosion occurs well below the surface, extensive damage can occur before the surface deformation is apparent.

Spars, stringers and other high-strength parts, which are extruded or hot rolled, are often (because the grains tend to form in layers) susceptible to this kind of corrosion if they have been poorly heat-treated.

11.3.5 STRESS CORROSION

Stress corrosion cracking is a cracking process, caused by the combined action of a sustained tensile stress and a corrosive environment. Only certain combinations of alloys and environments result in stress corrosion cracking, although this type of failure may occur at stresses well below the yield strength of the alloys. Many of the high-strength structural alloys, used in aircraft, are prone to stress corrosion cracking in a wide range of environments and they are particularly susceptible in marine environments.

In aircraft alloys, the principal stresses, causing this stress corrosion cracking, are not the applied service loads, but the stresses developed within the metal during manufacture and during assembly. For example, internal stresses can arise from quenching after heat-treatment, from ‘force fits’, from badly mating parts, or from welding procedures. Service stresses are only significant when they act in the same direction as internal or assembly stresses.

Stress corrosion cracking has three distinct phases in that there is an initial

‘Incubation’ period, (when a stress corrosion crack starts from pitting or film breakdown). The incubation is followed by a period of ‘Slow Growth’ of the stress concentrations and culminates in a short, ‘Rapid Crack-Growth’ rate.

In highly stressed parts (e.g. landing gear components), cracks may originate from a stress raiser such as a scratch or surface corrosion. This problem is

characteristic of aluminium, copper, stainless steels and high-strength alloy steels and may occur along lines of cold working. Signs of stress corrosion are given by minute cracks radiating from areas of the greatest stress concentration. Likely areas for this type of corrosion are U/C jacks, shock absorbers, bellcranks with pressed-in bushes, or other areas where parts are a force fit, highly stressed or have residual stresses induced during the forming process.

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11.3.6 FRETTING CORROSION

Fretting corrosion is the result of rubbing movement between two heavily loaded surfaces, one, or both, of which are metallic. The rubbing action destroys any natural protective film and also removes particles of metal from the surface. In its early stages, the debris of this corrosion forms a black powder. These particles form an abrasive compound, which aggravates the effect of the rubbing action and the surface is continually removed to expose fresh metal to the corrosive attack. This form of attack can eventually cause cracking and fatigue failure.

The most likely areas affected are gears, screw jacks, loose panels, splined hydraulic pump drives and rivets (when they become loose). , It may be serious enough to cause cracking and fatigue failure.

11.3.7 CREVICE CORROSION

Crevices are liable to preferential attack, usually by a differential aeration form of corrosion, intensified by the high ratio of cathode to anode area involved. The attack is more severe where crevices collect dust and moisture (Fig. 35).

Low Oxygen Concentration (becomes anodic)

High Oxygen Concentration (becomes cathodic)

Crevice

Crevice Corrosion Fig. 35

Severe localised corrosion occurs at narrow openings or gaps between metal components, often due to flexing. Corrosive agents are able to penetrate into the joint.

11.3.8 FILIFORM CORROSION

Filiform corrosion occurs beneath thin, protective coatings, on aluminium and steel alloys, with the paint or coating often bulging or blistering. On aircraft structures, the attack often starts at fasteners and extends as thread-like lines of corrosion under the paint. It may not be readily visible until it has become quite severe. The damage tends to be very shallow and is not, usually, structurally dangerous.

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11.3.9 PITTING CORROSION

Pitting corrosion can occur on aircraft materials when the protective film, whether applied or natural, breaks down locally and this may also lead to intergranular corrosion. The corrosion often stems from the screening effect of silt, scale or corrosion deposits that reduce the oxygen concentration at local points on the metal surface, which establishes differential concentration cells.

Local rough spots, inclusions, contaminations and lack of homogeneity in the alloy or metal are also possible causes of pitting. In size and depth, the pits are widely variable and a large number of pits can give a surface a ‘blotchy’

appearance.

Aluminium and magnesium alloys, chromium-plated and stainless steels (including nitrided surfaces), are all particularly susceptible to this form of corrosion. Pitting corrosion of an aluminium alloy component can be detected by the appearance of white powder on the surface of the metal (refer to Fig. 36).

Pitting Corrosion of an Aluminium Alloy Component Fig. 36

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This is similar to stress corrosion cracking, except that the applied loads are cyclic instead of static. Crack propagation is aided by the corrosion that occurs, at the root of the crack, during the tensile part of the loading cycle.

11.3.11 MICROBIOLOGICAL CONTAMINATION

This is caused, directly or indirectly (and in one or more ways), by micro-organisms which are not only able to produce corrosive substances (such as hydrogen sulphide, ammonia and inorganic acids), but can also act as depolarisers or catalysts in corrosion reactions. Local depletion of oxygen and water, held in contact with a metal surface, by matted fungi and micro-organisms, all contribute towards establishing corrosive environments.

The commonest form of microbiological corrosion in aircraft, is that, which is caused by contamination of fuel tanks (unless the fuel has an additive to protect against it). The growth of the fungi depends on several conditions, but a high ambient temperature can drastically increase the rate of growth, and especially so when the temperature is above 30C with a high relative humidity. This microbiological growth is sometimes called Cladosporium Resinæ.

Where fungal growth has formed, there is a probability that corrosion of the tank will occur. The organisms, resembling a mucous, can cause problems with filters and with the fuel contents gauge units. The roots of the fungus, penetrating the internal sealing and protective coatings of fuel tanks can cause further problems.

In well-developed contaminations, a dense mat of fungus forms on the floor of the tank, retaining water and preventing free flow to the water drain-valve. In integral fuel tanks, this can result in serious corrosion of the aircraft structure such that penetration of the bottom wing skin has been known to occur.

Spillage, of organic materials, from around galley and toilet areas, provides a further source of microbial contamination.

There is evidence that such spillage can be more corrosive than its chemical composition (acidity and chloride content) possibly due to fermentation by yeast and bacteria.

11.3.12 HYDROGEN EMBRITTLEMENT OF STEELS

Many of the standard surface protection treatments, including cleaning and electroplating, are liable to introduce hydrogen into steel. To avoid embrittlement, the steels must be ‘baked’, at a temperature of around 200C, following the treatments. The duration of the baking is dependent on the strength of the steel.

High-tensile steels are much more susceptible to hydrogen embrittlement than are other metals.

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Hydrogen embrittlement shows itself in slow strain-rate mechanical tests and not by fast rate tests such as in impact testing. These steels can show a sudden failure after many weeks of loading at well below their normal yield strength.