CORROSION (AME M-7)

CORROSION

Many aircraft structures are made of metal, and the

most insidious form of damage to those structures is

corrosion. From the moment the metal is manufactured,

it must be protected from the deleterious effects

of the environment that surrounds it.

Water or water vapor containing salt combines with

oxygen in the atmosphere to produce the main source

of corrosion in aircraft. Aircraft operating in a marine

environment, or in areas where the atmosphere contains

industrial fumes that are corrosive, are particularly

susceptible to corrosive attacks

The appearance of corrosion varies with

the metal. On the surface of aluminum alloys and

magnesium, it appears as pitting and etching,

often combined with a gray or white powdery deposit.

On copper and copper alloys, the corrosion forms a

greenish film; on steel, a reddish corrosion byproduct

commonly referred to as rust.

Types of Corrosion

Direct chemical  attack

Electrochemical attack

Direct Chemical Attack

Direct chemical attack, or pure chemical corrosion,

is an attack resulting from a direct exposure of a bare

surface to caustic liquid or gaseous agents.

The most common

agents causing direct chemical attack on aircraft

are:

(1) spilled battery acid or fumes from batteries;

(2) residual flux deposits resulting from inadequately

cleaned, welded, brazed, or soldered joints; and

(3) entrapped caustic cleaning solutions.

Electrochemical Attack

An electrochemical attack may be likened chemically

to the electrolytic reaction that takes place in electroplating,

anodizing, or in a dry cell battery. The reaction

in this corrosive attack requires a medium, usually

water, which is capable of conducting a tiny current

of electricity. When a metal comes in contact with a

corrosive agent and is also connected by a liquid or

gaseous path through which electrons may flow, corrosion

begins as the metal decays by oxidation.

Forms of Corrosion

Surface Corrosion -

Surface corrosion appears as a general roughening,

etching, or pitting of the surface of a metal, frequently

accompanied by a powdery deposit of corrosion products.

Closer inspection will

reveal the paint or plating is lifted off the surface in

small blisters which result from the pressure

Dissimilar Metal Corrosion -

Extensive pitting damage may result from contact

between dissimilar metal parts in the presence of a

conductor. While surface corrosion may or may not

be taking place, a galvanic action, not unlike electroplating,

occurs at the points or areas of contact where

the insulation between the surfaces has broken down

or been omitted. This electrochemical attack can be

very serious because in many instances the action is

taking place out of sight, and the only way to detect

it prior to structural failure is by disassembly and

inspection

Intergranular Corrosion

This type of corrosion is an attack along the grain

boundaries of an alloy and commonly results from a

lack of uniformity in the alloy structure.

Intergranular

corrosion may exist without visible surface evidence.

Very severe intergranular corrosion may sometimes

cause the surface of a metal to “exfoliate.”

Stress Corrosion

Stress corrosion occurs as the result of the combined

effect of sustained tensile stresses and a corrosive

environment. Stress corrosion cracking is found in

most metal systems; however, it is particularly characteristic

of aluminum, copper, certain stainless steels,

and high strength alloy steels (over 240,000 psi).

Fretting Corrosion

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