Airplane corrosion

A key aspect to airworthiness for an aircraft is the endurance of its structural components. Corrosion is a problem that has always accompanied metals as structural materials, it is one of the defining aspects of an aircraft structure. Predicting the behaviour of numerous types of corrosion is a challenging task. It is important to stress that in order to increase safety and reduce the cost of designing and manufacturing a new aircraft (for example by using composite materials, even for the metal variety), it is necessary to implement stringent production control.

The main variables connected to corrosion are the aircraft operating time (usually measured in flight hours) and the environmental conditions (temperature and humidity) to which it is exposed. Corrosion is the result of chemical reactions (or electrochemical processes for example with water, oxygen etc ...) and physical phenomena (structural stresses, external electric field, or radiation) that occur on the surface. Furthermore, it is essential to know that corrosion is not only influenced by the chemical-physical characteristics of the material but also that the coupling of two different metals can accelerate it. In addition, aircrafts are in contact with dust, dirt, and prone to bird strikes that could damage the paint initiating corrosion. Finally, we must not forget that the in-service aircrafts on the ground are often exposed to the environment, for example: rural, urban, industrial and marine. In the marine environment, the presence of salts (sodium chloride) intensifies the corrosive processes as they are “effective electrolytes”, harmful to aircraft made of aluminium alloy (aluminium combined with copper, silicon, manganese, magnesium and zinc) which are still the most used materials in the aviation industry today.

Corrosion is a continuous process and cannot be defined in a precise range. The observation of the phenomenon in real time helps us define what type of corrosion and when we can find the damage in a given area of the aircraft at a given time.

The mathematical model proposed at the base of a neural network collects all the data from the past planned inspections of the aircraft. Based on the type of corrosion found and the operational conditions at each instance, it is possible to form a base for maintenance program optimization. Like all machine learning applications, data collection becomes an essential step for a correct evaluation.

It is here that the project manager assumes a key role, as he will coordinate all the stakeholders involved in the process, from project engineering to maintenance engineering.

The diagram in the figure shows a possible result of the analysis performed, in which it is reasonable time to start the inspection for intergranular corrosion is 35,000 FH fort a generic section of the fuselage of a particular aircraft.

Note:

- Surface corrosion, usually localized in areas where stagnation of water or condensation can occur, is a typically electrolytic phenomenon that accelerates its reaction in areas where the geometric shape is singular, for example, rivets, welds, joints; areas of contact between a metal and a non-metal such as composites, glass, rubber; non-removed impurities or corrosion products may form sediments on the surface.

- Intergranular corrosion occurs along the grain of the material when stresses are applied perpendicular to the grain boundaries. High Strength Aluminum are susceptible to this type of corrosion if heat treatment has been applied incorrectly or if they are exposed to corrosive environments.