Corrosion & Environmental Degradation		Table of Contents
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	Biological Corrosion · Living organisms can provide local environments in which materials will corrode by some of the paths discussed above. The example is given is important for in-ground ferrous pipes. · Bacteria in either an aerobic (oxygen present) or anaerobic (little or no oxygen) environment can use chemicals in their environment for metabolic purposes and generate corrosive waste products. · Aerobic bacteria can oxidize sulphur in the environment to produce sulfuric acid in the sulfur oxidizing reaction: 2S + 3O2 + 2H2O -> 2 H2SO4 . Dryer soils permit ready oxygen uptake. The acid product will corrode ferrous metals and can be produced with local concentrations of 5% by weight.  · Anaerobic bacterial processes that reduce sulfates to sulfite in the reaction:                                                        SO42- + 4H2 -> 4 H2O + S2- obtain hydrogen from cellulose or other organic products in the soil. Wet environments favor this reaction. · The two reactions can be active at different times of the year as soil water content changes. This cyclic behavior promotes severe corrosion conditions that can cause damage to unprotected steel underground pipes.