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242   C h a p t e r   7              C o r r o s i o n   F a i l u r e s ,   F a c t o r s ,   a n d   C e l l s    243


                                                                   Hydrated
                                                                   alumina


                          7




                         Relative volume  5




                          3




                          1


                                 Al         Al O 3    Al O ·H O    Al(OH) 3
                                              2
                                                        2
                                                          3
                                                            2
                                                  Oxide type
                      FIGURE 7.35  Relative volume of aluminum corrosion products.

                      The  buildup  of  voluminous  corrosion  products  when  hydrated
                      alumina  is  formed  can  produce  an  undesirable  increase  in  stress
                      levels near critical fastener holes (Fig. 7.36) and subsequent fracture
                      due to the high tensile stresses resulting from the “pillowing.”

                      7.4.7  Surface Film Cells
                      The  surface  of  a  metallic  object  can  be  easily  reacted  in  normal
                      atmospheres  making  it  behave  quite  differently  from  a  piece  of
                      unreacted metal, often resulting in a situation resembling dissimilar
                      metal corrosion. A film may be formed which is invisible, actually
                      only a few molecules in thickness, but which may have a potential
                      as much as 0.3 V different from the unfilmed metal. Naturally, such
                      a potential difference is enough to create an active corrosion cell.
                      Steels in soil have a tendency to film with time. “Old” steel, that is,
                      steel  which  has  been  in  the  ground  for  several  years  may  then
                      become cathodic with respect to “new” steel, even when the two are
                      identical in bulk composition (Fig. 7.37). It is thus strictly a surface
                      phenomenon [21].
                         A  common  occurrence  of  surface  film  cell  is  found  in  older
                      distribution piping systems where a section of pipe has been replaced
                      because of corrosion damage. The new piece of pipe, exposed to the
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