- The heating of some protective coatings may cause gassing that can be corrosive to metal components.
- Some commercial coating strippers contain acids that attack PCB laminates, and discolor or corrode copper.
- Certain silicone sealants contain acetic acid that is highly corrosive to metal components in avionics circuits.
- Some potting compounds revert to a liquid form under certain conditions. This reversion process reduces moisture protection in electrical connectors.
- Degradation of polyvinyl chloride gives off acetic fumes which are corrosive to most metals used in avionics equipment.
- Shrinkable elastomers (heat shrink) tubing, although not a problem directly, can cause damage to adjacent circuitry when heat guns are applied to shrink the tubing.
- . Some dry film lubricants contain graphite, which is an excellent lubricant, but graphite is also corrosive. Graphite, in the presence of moisture, promotes galvanic corrosion. Other dry film lubricants contain molybdenum disulfide which when combined with moisture and heat can form a corrosive sulfuric acid.
- Certain oils, especially silicones and greases, tend to migrate as temperature changes, which may result in contamination of coatings, attraction of dust, and the possibility of holding moisture.
Perspiration and other factors
Often overlooked causes of corrosion are the salts and oil found in human perspiration. Wearing protective gloves may result in electrostatic discharge (ESD) damage so situational awareness is an important factor when handling sensitive components.
Avionics components are subject to dust and lint accumulation. This condition is generally evident when the equipment has been installed for long periods of time and can become more severe with forced cooling air. In addition to the dust and lint accumulation from the movement of air, dust and lint can be attracted by magnetic fields from electric currents surrounding wiring and equipment. Dust can also accumulate on both the external and internal surfaces of components when ventilating holes and louvers are incorporated. The problem with accumulation of dust and lint is it will trap and hold moisture which can provide the electrolyte for corrosion and fungus growth, plus dust and lint can degrade avionics equipment by being a conductor or an insulator. When dust and lint act as a conductor in the presence of moisture, they can provide a path for a current flow to either ground as a short or an unwanted circuit path between components. When dust and lint act as an insulator the avionics equipment can overheat causing premature failure. This can be compounded as excessive lint accumulation on air inlet screens or filters will significantly reduce cooling air flow.
Antenna systems are exposed to severe environmental situations. Without adequate corrosion protection numerous failures including shorts, open circuits, loss of dielectric strength, signal attenuation, poor bonding, or electromagnetic interference (EMI) may be experienced. Structural damage to the aircraft can also result. Antennas mounted on the fuselage require openings in the aircraft skin to route the various cables to the antenna plus provide structural attachment. The area around the antenna mounting is susceptible to moisture intrusion from rain, de-icing fluids/materials, condensation, aircraft wash, and internal fluids (i.e., fuel, oil, lavatory and galley products, etc.). Antennas mounted on the lower fuselage are particularly susceptible.
It should go without saying that we are working in a field where there is frequent exposure to hazardous chemicals and processes. Anyone involved with an unfamiliar chemical or product should always investigate what types of personal protective equipment need to be employed. It is important to remember that corrosion is a material in the process of returning to its natural state and inhaling or even absorbing through the skin may have some disastrous effects to a person’s health.
Corrosion and environmental conditions are natural phenomena that adversely affect avionics equipment. Although they can never be totally eliminated, the problems these phenomena cause can be minimized so that they are more manageable. This can be achieved only by understanding the equipment failure mechanisms, implementing a preventive maintenance program, and using corrosion control techniques and materials.
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