There are two general classifications of corrosion that cover most of the specific forms: 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 involves two simultaneous changes: the metal that is attacked or oxidized suffers what is called anodic change, and the corrosive agent is reduced and is considered as undergoing cathodic change.
RELATED POSTS
Direct Chemical Attack
Direct chemical attack, or pure chemical corrosion, is an attack resulting from direct exposure of a bare surface to caustic liquid or gaseous agents. Unlike electrochemical attack where anodic and cathodic changes take place a measurable distance apart, the changes in direct chemical attack occur 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. [Figure 1]
 |
| Figure 1. Direct chemical attack in a battery compartment |
With the introduction of sealed lead-acid batteries and the use Electrochemical Attack of nickel-cadmium batteries, spilled battery acid is becoming less of a problem. The use of these closed units lessens the hazards of acid spillage and battery fumes.
Many types of fluxes used in brazing, soldering, and welding are corrosive, chemically attacking the metals or alloys that they are used with. Therefore, it is important to remove residual flux from the metal surface immediately after the joining operation. Flux residues are hygroscopic in nature, absorbing moisture, and unless carefully removed, tend to cause severe pitting.
Caustic cleaning solutions in concentrated form are kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrosion removal are, in themselves, potentially corrosive agents. Therefore, particular attention must be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occur, use a noncorrosive cleaning agent, even though it is less efficient.
Electrochemical Attack
Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action, converting it back to a metallic compound. The following four conditions must exist before electrochemical corrosion can occur. [Figure 2]
- A metal subject to corrosion (anode)
- A dissimilar conductive material (cathode) that has less tendency to corrode
- Presence of a continuous, conductive liquid path (electrolyte)
- Electrical contact between the anode and the cathode (usually in the form of metal to metal contact, such as rivets, bolts, and corrosion)
Elimination of any one of these conditions stops electrochemical corrosion.
NOTE: Paint can mask the initial stages of corrosion. Since corrosion products occupy more volume than the original metal, painted surfaces must be inspected often for irregularities, such as blisters, flakes, chips, and lumps.
 |
| Figure 2. 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, that 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 that electrons flow through, corrosion begins as the metal decays by oxidation. [Figure 2] During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal becoming neutralized. Different areas of the same metal surface have varying levels of electrical potential and, if connected by a conductor such as salt water, sets 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. This potential is commonly referred to as the metal’s “nobility.” [Figure 3] The less noble a metal is, the more easily it can be corroded. The metals chosen for use in aircraft structures are a studied compromise with strength, weight, corrosion resistance, workability, and cost balanced against the structure’s needs.
 |
| Figure 3. The galvanic series of metals and alloys |
The constituents in an alloy also have specific electrical potentials that are generally different from each 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 conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater the severity of a corrosive attack if the proper conditions are allowed to develop.
The conditions for these corrosion reactions are the presence of a 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 on aircraft structure and component parts.