CHARACTERISTICS The appearance of corrosion will vary with the metal involved. The following discussion includes brief descriptions of typical corrosion product characteristics. These descriptions are only for the most common materials used in gas turbine propulsion and support equipment.

Iron and Steel Possibly the best known and most easily recognized of all forms of metal corrosion is the familiar reddish-colored iron rust. When iron and its alloys corrode, dark iron oxide coatings usually form first. These coatings, such as heat scale on steel sheet stock and the magnetite layer that forms on the inside of boiler tubes, protect iron surfaces rather efficiently. However, if sufficient oxygen and moisture are present, the iron oxide is soon converted to hydrated ferric oxide, which is conventional red rust. Hydrated ferric oxide, red rust, does not protect surfaces. It destroys surfaces.

Aluminum Aluminum and its alloys exhibit a wide range of corrosive attacks, varying from general etching of surfaces to penetrating attacks along the internal grain boundaries of the metal. The corrosion products of aluminum are seen as white-gray powdery deposits.

Copper and Copper Alloys Copper and its alloys are generally corrosion resistant, although the products of corrosive attack on copper are commonly known. Sometimes copper or copper alloy surfaces will tarnish to a gray-green color, while the surface will remain relatively smooth. This discoloration is the result of the formation of a fine-grained, airtight copper oxide crust, called a patina.

Patina offers good protection for the underlying metal in ordinary situations. However, exposure of copper alloys to moisture or salt spray will cause the formation of blue or green salts called verdigris. The presence of verdigris indicates active corrosion.

Cadmium and Zinc Cadmium is used as a coating to protect the area to which it is applied and to provide a compatible surface when the part is in contact with other metals. The cadmium plate supplies sacrificial protection to the underlying metal because of its great activity. During the time it is protecting the base metal, the cadmium is intentionally being consumed. Zinc coatings are used for the same purpose, although to a lesser extent. Attack is evident by white-to-brown-to-black mottling of the surfaces. These indications do NOT indicate deterioration of the base metal. Until the characteristic colors peculiar to corrosion of the base metal appear, the coating is still performing its protective function.

Nickel and Chromium Alloys Nickel and chromium alloys are also used as protective agents. They are used as electroplated coatings and as alloying constituents with iron in stainless steels and with other metals such as copper. Nickel and chromium plate provide protection by the formation of an actual physical noncorrosive barrier over the steel. Electroplated coatings, particularly chromium on steel, are somewhat porous. Eventually, corrosion starts at these pores unless a supplementary coating is applied and maintained.

TYPES OF CORROSION As stated previously, corrosion may occur in several forms, depending upon the metal involved, its size and shape, its specific function, the atmospheric conditions, and the corrosion-producing agents present. Those corrosion types described in this section are the most common forms found on gas turbine engines and machinery structures.

Direct Surface Attack The surface effect produced by reaction of the metal surface to oxygen in the air is a uniform etching of the metal. The rusting of steel, tarnishing of copper alloys, and the general dulling of aluminum surfaces are common examples of direct surface attacks. If such corrosion is allowed to continue unabated, the surface becomes rough, and in the case of aluminum, frosty in appearance. Direct surface attack is sometimes referred to as uniform etch corrosion.

Galvanic Corrosion Galvanic corrosion is the term applied to the accelerated corrosion of metal caused by dissimilar metals being in contact in a corrosive medium.

Dissimilar metal corrosion is usually a result of faulty design or improper maintenance practices which result in dissimilar metals coming in contact with each other. This is usually seen as a buildup of corrosion at the joint between the metals. For example, when aluminum pieces are attached with steel bolts and moisture or contamination are present, galvanic corrosion occurs around the fasteners.

Pitting The most common effect of corrosion on aluminum alloys is pitting. It is caused primarily by variations in the grain structure between adjacent areas on the metal surfaces that are in contact with a corrosive environment. Pitting is first noticeable as a white or gray powdery deposit, similar to dust, that blotches the surface. When the superficial deposit is cleaned away, tiny pits or holes can be seen in the surface. These pits may appear either as relatively shallow indentations or as deeper cavities of small diameters. Pitting may occur in any metal, but it is particularly characteristic of aluminum and aluminum alloys.

Intergranular Corrosion Intergranular corrosion is an attack on the grain boundaries of some alloys under specific renditions. During heat treatment, these alloys are heated to a temperature that dissolves the alloying elements. As the metal cools, these elements combine to form other compounds. If the cooling rate is slow, they form predominantly at the grain boundaries. These compounds differ electrochemically from the metal adjacent to the grain boundaries. These altered compounds can be either anodic or cathodic to the adjoining areas, depending on their composition. The presence of an electrolyte will result in an attack on the anodic area. This attack will generally be quite rapid and can exist without visible evidence.

As the corrosion advances, it reveals itself by lifting up the surface grain of the metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. This advanced attack is referred to as EXFOLIATION. Recognition and necessary corrective action to immediately correct such serious instances of corrosion are vital. This type of attack can seriously weaken structural members before the volume of corrosion products accumulate on the surface and the damage becomes apparent.

Fretting Fretting is a limited but highly damaging type of corrosion caused by a slight vibration, friction, or slippage between two contacting surfaces that are under stress and heavily loaded. Fretting is usually associated with machined parts such as the contact area of bearing surfaces, two mating surfaces, and bolted assemblies. At least one of the surfaces must be metal.

In fretting, the slipping movement at the interface of the contacting surface destroys the continuity of the protective films that may be present on the surfaces. This action removes fine particles of the basic metal. The particles oxidize and form abrasive materials that further accumulate and agitate within a confined area to produce deep pits. Such pits are usually located where they can increase the fatigue potential of the metal.

Fretting is evidenced at an early stage by surface discoloration and by the presence of corrosion products in any lubrication. Lubricating and securing the parts so that they are rigid are the most effective measures for the prevention of this type of corrosion.

Stress Stress, evidenced by cracking, is caused by the simultaneous effects of tensile stress and corrosion. Stress may be internal or applied.

Internal stresses are produced by nonuniform deformation during cold working conditions, by unequal cooling from high temperatures during heat treatment, and by internal-structural rearrangement involving volume changes. Stresses set up when a piece is deformed. Examples of internal stresses include those induced by press-and-shrink fits and those in rivets and bolts.

Concealed stress is a more dangerous condition than design stress. Concealed stress corrosion is difficult to recognize before it has overcome the design safety factor. The magnitude of the stress varies from point-to-point within the metal. Stresses in the neighborhood of the yield strength are generally necessary to promote stress corrosion cracking, but failures may occur at lower stresses.

Fatigue Fatigue is a special type of stress corrosion. It is caused by the combined effects of corrosion and stresses applied in cycles. An example of cyclic stress fatigue is the alternating loads to which the connecting rod of a double-acting piston in an air compressor is subjected. During the extension (up) stroke a compression load is applied, and during the retraction (down) stroke a tensile or stretching load is applied. Fatigue damage is greater than the combined damage of corrosion and stresses. Fracture of a metal part due to fatigue corrosion generally occurs at a stress far below the fatigue limit in a laboratory environment, even though the amount of corrosion is very small. For this reason, protection of all parts subject to alternating stress is particularly important wherever practical, even in environments that are only mildly corrosive.