Type of Corrosion.

Type of Corrosion

There are two general clasifications of corrosion which cover most of the specific forms. These are : Direct chemical attack and Electrochemical attack. In both types of corrosion the metal is converted into a metallic compound such as an oxide, hydroxide or sulfate. The corrosion process always involves two simultaneous

Forms of Corrosion

Type of Corrosion

Corrosion Control

Corrosion Removal

Preventive Maint.

changes : The metal that is attacked or oxidized suffers what may be called anodic change, and may be considered as undergoing cathodic change. Direct Chemical Attack. Direct chemical attack, or pure chemical corrosion, is an attack resulting from a direct exposure of a bare surface to coustic liquid or gasous agents. Unlike electrochemical attack where the anodic and cathodic changes may be taking place a measurable distance apart, the changes in direct chemical attack are occurring simultaneously at the same point. The most common agents causing direct chemical attack on aircraft are : Spilled battery acid or fumes from batteries, residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints, and entrapped caustic cleaning solutions. Spilled battery acid is becoming less of a problem with the advent of aircraft using nickel-cadmium batteries which are usually closed units. The used of these closed units lessens the hazards of acid spillage and battery fumes. Many type of fluxes used in brazing, soldering, and welding are corrosive and they chemically attack the metals or alloys with which they are used. Therefore, it is important that residual flux be removed from the metal surface immediately after the joining operation. Flux residues are hygroscopic in nature, that is, they are capable of absorbing moisture, and unless carefully removed, tend to cause severe pitting. Caustic cleaning solutions in concentrated form should be kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrotion removal are, in themselves, potentially corrosive agents, and particular attention should be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occure, a noncorrosive cleaning agent should be used even though it is less efficient. Electrochemical Attack. An electrochemical attack may be likened chemically to the electrolytic reaction which takes place in electroplating, anodizing, or in a drycell battery. The reaction in this corrosive attack requires a medium, usually water, which is capable of conducting a tiny current of eletricity. When a metal comes in contact with a corrosive agent and is also connected by a liqiud or gaseous path through which electron may flow, corrosion begins as the metal decays be oxidation. During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal ( become neutralized ). Different areas of the same metal surface have verying levels of electrical potential and if connected by a conductor, such as salt water, will set up a series of corrosion cells and corrosion will commence. All metals and alloys are electrically active and have a specific electrical potential in a given chemical environment. The constituents in an alloy also have specific electrical potentials which are generally different from its other. Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to become anodic and the less active metal to become cathodic, thereby establishing condition for corrosion. These are called local cells. The greater the differnce in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed to develop. As can be seen, the conditions for these corrosion reactions are conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur this is the basis for effective corrosion control. The electrochemical attack is responsible for most forms of corrosion o aircraft structure and component parts.

Common Corrosive

List of Agent

Bugs