Fungal growths cause detrimental effects on materials through direct attack, indirect attack, and physical interference. Direct attack occurs when the fungus breaks down a nonresistant material and uses it as a nutrient source, resulting in deterioration that affects the physical properties of the material. Some examples of nonresistant materials are animal- and vegetable-based 2 adhesives, certain polyurethanes, and paints or varnishes that contain susceptible constituents. Indirect attack causes damage to materials when fungal growth on surface deposits of dust, grease, perspiration, and other contaminants accumulates on materials during manufacture or in service. Damage is caused to the underlying material, even though that material may be resistant to direct attack. Damage to optical systems results primarily from indirect attack. Physical interference occurs on electrical or electronic systems where the damage may result from either direct or indirect attack. For example, fungi can form undesirable electrical conducting paths across insulating materials or may adversely affect the electrical characteristics of critically adjusted electronic circuits. Fungi can adversely affect light transmission through the transparency, block delicate moving parts, or change non-wetting surfaces to wetting surfaces, leading to a loss in performance.
In addition, metabolic waste products (i.e., organic acids) excreted by a fungus cause corrosion of metals, etching of glass, or staining or degrading of plastics and other materials. Fungus on materials can cause physiological problems (e.g., allergies) or be so aesthetically unpleasant that the users will be reluctant to use the material.
Even legacy systems that are primed with chromate primers have experienced fungal issues. The H-53 Fleet Support Team (FST) at Cherry Point has reported persistent mildew in the bilge area. Additionally, microbial-induced corrosion has been reported in the F-16 cockpit area and mold has been found in the cargo compartment under insulation blankets on some C-130 aircraft. These instances show that even chromate systems do not totally control fungal growth in certain environments. Looking forward, beach testing has shown that fungi grow more readily on some newer non-chromate coatings, suggesting that constituents other than the pigments in the new coatings are more susceptible to fungal attack. The potential for increased susceptibility to fungi will undoubtedly be a concern with the transition of more environmentally friendly, non-chromate coatings into routine use, especially on interior surfaces.