Corrosion Prelim

Corrosion

is the degradation of materials, usually metals, due to chemical or electrochemical reactions with their environment.

Pure Metals

are not suitable for aircraft construction and are used only in combination with other metals to form alloys.

Climate

The environmental conditions that an aircraft is maintained and operated under greatly affects corrosion characteristics.

Geographical Location

The flight routes and bases of operation expose some airplanes to more corrosive conditions than others.

Micro - organism

Slimes, molds, fungi and other living organisms (some microscopic) can grow on damp surfaces.

Manufacturing Process

such as machining, forming, welding, or heat – treatment, can leave stresses in aircraft parts.

Direct Chemical Attack

or pure chemical corrosion, is an attack resulting from direct exposure of a bare surface to caustic liquid or gaseous agents.

Electrochemical Attack

Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action, converting it back to a metallic compound.

Forms of Corrosion

depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion producing agents present.

Surface Corrosion

(also referred to as uniform etch or uniform attack corrosion) is the most common form of corrosion.

appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products.

Filiform Corrosion

is a special form of oxygen concentration cell that occurs on metal surface having an organic coating system.

It is recognized by its characteristic wormlike trace of corrosion products beneath the paint form. o Polyurethane finishes are especially susceptible to filiform 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.

Dissimilar Metal Corrosion

Extensive pitting damage may result from contact between dissimilar metal parts in the presence of a conductor.

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

The solution may consist of water and ions of the metal that are in contact with water.

A high concentration of metal ions normally exists under faying surfaces where the solution is stagnant and a low concentration of metal ions exist adjacent to the crevice, created by the faying surface.

Oxygen Concentration Cells

The solution in contact with the metal surface normally contains dissolved oxygen.

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.

Intergranular Corrosion

This type of corrosion is nan attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure.

Aluminum alloys and some stainless steels are particularly susceptible to this form of electrochemical attack.

Exfoliation Corrosion

an advanced form of intergranular corrosion and shows itself by lifting up the surface rains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface.

It is visible evidence of intergranular corrosion and is most often seen on extruded sections where grain thickness is usually less than in rolled forms.

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.

Fretting Corrosion

is a particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion.

It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris.

Fatigue Corrosion

involves cyclic stress and a corrosive environment.

Metals may withstand cyclic stress for an infinite number of cycles so long as the stress is below the endurance limit of the metal.

Galvanic Corrosion

occurs when two dissimilar metals make electrical contact in the presence of an electrolyte.

The rate which corrosion occurs depends on the difference in the activities.

Common Corrosive Agents

substances that cause corrosion of metals are called corrosive agents.

Acids

moderately strong acids severely corrode most of the alloys used in airframes.

Alkalis

as a group, alkalis are not as corrosive as acids.

Substances particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust).

Salts

most salt solutions are good electrolytes and can promote corrosive attack.

Some stainless – steel alloys are resistant to attack by salt solutions but aluminum alloy, magnesium alloys, and other steels are extremely vulnerable.

Water

the corrosiveness of water 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 corrosiveness is the conductivity or ability to act as an electrolyte and conduct a current.

Atmosphere

the major atmospheric corrosive agents are oxygen and airborne moisture.

Corrosion often results from the direction action of atmospheric oxygen and moisture on metal, and the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys.