What is corrosion? Corrosion can be defined as the deterioration of a material due to a reaction with the surrounding environment. Metals corrode because we use them in chemically unstable environments. Very few metals are found in nature in a metallic state like copper, gold and silver. All other metals are transformed from minerals or ores into metals that are inherently unstable in their environment. These unstable metals tend to revert to their more stable mineral forms. Some metals form protective ceramic films (passive films) on their surfaces which prevent or slow down their corrosion process. We can prevent corrosion by using metals that form naturally protective passive films, but these alloys are generally expensive, so we have developed other means of corrosion control. These elements are discussed later in this article. Types of Corrosion There are two common classifications of corrosion: direct chemical attack and electrochemical attack. In both cases, the metal is converted to a compound such as a sulfate, oxide, or hydroxide. Corrosion always involves two simultaneous changes. The oxidized metal undergoes what is called an anodic change, and the corrosive agent is reduced and undergoes a cathodic change. Direct chemical attack is corrosion resulting from direct contact of a bare surface with a caustic liquid or gaseous substance. Unlike electrochemical etching in which anode and cathode changes can occur remotely, changes in direct chemical etching occur at the same time and point. The most common substances causing direct chemical attack on an aircraft are: residual flux deposits resulting from insufficiently cleaned, welded, brazed or brazed joints spilled into the middle of the paper......luminum. There are many types of stainless steels. These are generally impervious to corrosion, but they can undergo crevice corrosion and undergo stress corrosion cracking in some cases. Titanium is a common metal found in nature, but its limited use means that small-scale production comes with a high price. However, it is widely used in the aerospace industry. There are two general types of titanium alloys: aerospace alloys and corrosion-resistant alloys. Engineering designers must consider the amount of metal needed to resist the expected load for a given application. Since they can make mistakes or the structure can be misused, they are required to over-design the plane by a safety factor. Once the necessary mechanical load safety factor has been taken into account, it becomes necessary to determine whether corrosion tolerance is necessary to ensure aircraft safety..