Environmental Engineering Reference
In-Depth Information
Figure 3.60
Radiograph of boiler tube showing many fine transverse corrosion fatigue
cracks.
Figure 3.60 is the radiograph of a marine boiler tube that failed by corrosion
fatigue [9]. The presence of a large number of cracks are immediately noticeable.
The material is mild steel. The corrosion has been attributed to overheating. On
surfaces with an excessive rate of steam generation the net effect was that of
concentrating the boiler water, which then became corrosive to steel. Cyclic
stresses of the order of several 10,000 psi were considered to be produced by
the expansion and contraction of the tube under restrain.
3.10 BIOLOGICALLY INFLUENCED CORROSION
Biologically influenced corrosion refers to the degradation of metals caused by
the activity of living organisms. Both micro- and macroorganisms have been
found to contribute to corrosion in a variety of environments including soil,
groundwater, seawater, domestic and industrial freshwasters, natural petroleum
products, and oil-emulsion cutting fluids. It does not represent any special form
of corrosion. The aggravation of corrosion under environmental conditions
whereby the corrosion rates are expected to be low is often traced back to the
presence of bioorganisms. The biological activities may directly influence the
anodic or cathodic reactions, produce end products that are corrosive, or influence
protective surface films. The corrosion may also aggravate under the deposits
formed by the organisms or their bioactivity products.
