Russian scientists have been exploring the causes of corrosion inside spacecraft (translation of original article). An initial (somewhat alarming) observation that manned spacecraft suffered from corrosion more than unmanned spacecraft prompted further investigation.
The corrosion in question is of the aluminium alloys widely used in spacecraft because they are lightweight and highly resistant to corrosion - normal chemical corrosion. Biocorrosion is the term coined to cover corrosion caused by living organisms growing on the metal surface (biofilm). The organisms can secrete acids and other corrosive chemicals such as ammonia which cause the damage.
Aspergillus versicolor has (amongst others) been isolated from spacecraft even though growth conditions are harsh, and it has been found that the conditions of high humidity and ultrasonic irradiation found in a spacecraft allow these organisms to grow well. They grow readily on most surfaces of the spaceship and contribute to microscopic caverns appearing on and in equipment/work surfaces along with Penicillium expansum, Cladosporium cladosporioides and others.