An overview of microbiologically influenced corrosion in aircraft

Most documented cases of microbiologically influenced corrosion (MIC) in aircraft are related to fungi. Fungi are dessicant-resistant microorganisms that can remain active down to a relative humidity of 65%. Fungi are on photosynthetic organisms, having a vegetative structure known as a hypha, the outgrowth of a single microscopic reproductive cell or spore. A mass of threadlike hyphae make up a mycelium (Figure 1).' Mycelia are capable of almost indefinite growth in the presence of adequate moisture and nutrients so that fungi often reach macroscopic dimensions. Yeasts are fungi that multiply by forming buds instead of mycelia. Fungi are ubiquitous in atmospheric and aquatic environments where they assimilate organic material and produce organic acids including oxalic, lactic, acetic, and citric. Spores, the nonvegetative dormant stage, can survive long periods of unfavorable growth conditions, e.g., drought and starvation. In the following sections fungal degradation of polymeric materials and fuels used in aircraft will be reviewed.