In avionics systems, why are active and dissimilar metals at risk of corrosion?

Active and dissimilar metals are at risk of corrosion primarily because they often have unprotected contacts. When two different metals come into contact, particularly if one is more anodic (active) than the other, there can be an electrochemical reaction that leads to corrosion. This is especially true if there is moisture present, as it can facilitate the flow of electrons between the two metals, creating a galvanic cell. In this galvanic reaction, the more active metal tends to corrode faster while the more noble metal is protected.

Unprotected contacts between dissimilar metals can result in increased corrosion rates due to a lack of any barrier or protective coating that might otherwise prevent electrochemical interactions. Proper coating and insulation can minimize these issues, but if the surfaces are directly touching with no protection, the risk of corrosion is significantly higher.

The other options are less relevant to the mechanism of corrosion between active and dissimilar metals. For instance, expansion properties and moisture absorption characteristics do not directly relate to the corrosion process in the context of dissimilar metal contact. Overall, recognizing the importance of unprotected contacts in dissimilar metal pairings is vital for managing corrosion in avionics systems.