Warnick_J_Safety335_Module_9_Questions with Answers.docx

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SFTY 335 - Module 9 1. What are the main types of corrosion concerning aircraft? Filiform corrosion is a unique form of oxygen-concentration cell corrosion or crevice corrosion, which occurs on metal surfaces having an organic coating system. It is identified by its characteristic worm-like trace of corrosion products beneath any painted film. Filiform occurs when the relative humidity of the air is 78-90% and the surface is slightly acidic. Corrosion starts at breaches in the coating system and fastener heads and extends along the surface of the painted sheet underneath the coating. If filiform corrosion is not removed, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams. Stress corrosion is intergranular cracking of a metal that is caused by a combination of stress and corrosion. Stress may be caused by internal or external loading. Internal stresses are produced by non-uniform deformation during cold working, by unequal cooling from high temperatures and by internal structural re-arrangement involving volume changes. This is especially true when working aircraft skins and parts. Pitting Corrosion The most common type of corrosion on aluminum is pitting. Pitting first appears as a white, powdery deposit. It starts on the surface of a material and then extends vertically into the material. This type of corrosion is a dangerous form of the vertical extension, which decreases the material strength. You do not get a large indication of this type of corrosion on the surface. This corrosion can also be the starting point for intergranular corrosion. Intergranular corrosion is along the grain boundaries of a metal and there is little or no indication shown on the surface. Each grain has a clearly defined boundary, which differs from the metal within the grain center. The grain boundary and the grain center react with each other as anode and cathode when it comes into contact with an electrolyte.
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  • Summer '14
  • faying surface, main types of corrosion

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