Biological Influences

Interaction of vitamin D and its metaboUtes with sex hormones has been demonstrated, particularly ia birds ia which the egg-laying functions combine calcium needs and reproductive activity. The metaboUtes of vitamin D behave as hormones. As such, they play an active role ia the endocrine system, along with other hormones, to maintain the various body functions. Several biological influences of metaboUtes of vitamin D have been studied, including effects related to cancer (193—197), skin diseases (198—201), immunomodulatory effects (202,203), and Alzheimer s disease (204—206) (Fig. 9). 

See Chap. 5, Oxygen Corrosion Chap. 6, Biologically Influenced Corrosion and Chap. 17, Graphitic Corrosion. ... 

The presence of biological material is not as critical in making a diagnosis of biologically influence attack as are other factors. All the following conditions must exist ... 

The importance of biologically influenced corrosion (particularly micro-biologically influenced corrosion) has been underestimated for many years. Recently, more attention has been paid to biological forms of corrosion— yet more attention does not always mean an improved situation. 

Many forms of corrosion resemble each other. However, it is just as true that each form of corrosion produces a unique fingerprint by which it can be differentiated from all other forms of attack. Biologically influenced corrosion is no exception. 

The presence of organisms (large or small) in proximity to corrosion by itself is not proof of biologically influenced corrosion, any more than a correlation of lunar phases with stock market fluctuations establishes a lunar-financial connection. It should be stressed vigorously that all evidence must be consistent with any single corrosion mode before a definitive diagnosis can be made (see Critical Factors above). Further, all alternative explanations must be carefully examined. 

The triggering mechanism for the corrosion process was localized depassivation of the weld-metal surface. Depassivation (loss of the thin film of chromium oxides that protect stainless steels) can be caused by deposits or by microbial masses that cover the surface (see Chap. 4, Underdeposit Corrosion and Chap. 6, Biologically Influenced Corrosion ). Once depassivation occurred, the critical features in this case were the continuity, size, and orientation of the noble phase. The massive, uninterrupted network of the second phase (Figs. 15.2 and 15.21), coupled... 

---