Page 276 - Corrosion Engineering Principles and Practice
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250    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    251


                         The following are general recommendations to avoid the initiation
                      of SCC due to the formation of corrosion cells in service.
                          •  Residual stresses may be minimized by careful thermal stress
                             relief appropriate to the alloy
                          •  Compressive stresses may be introduced by controlled shot
                             peening
                          •  Stress raisers should be eliminated or at least minimized
                          •  Open crevices should be eliminated, as well as areas where
                             deposits can accumulate and build up tremendous internal
                             pressure
                          •  Conditions  conducive  to  evaporation  and  concentration  of
                             corrosive species should also be minimized

                 7.6  Visualizing Corrosion Cells

                      The existence and location of anodes and cathodes in a corrosion
                      cell can be demonstrated by the changes in color of certain reagents.
                      Such  color  changes  have  been  very  helpful  in  the  early  days  of
                      corrosion science to study the local interplay of local anodes and
                      cathodes on apparently homogeneous steel surfaces exposed to a
                      corrosive environment. As noted by Cushman and Gardner in their
                      1910  textbook,  it  is  a  matter  of  common  observation  that  iron
                      usually corrodes rapidly at certain weak points in an effect known
                      as pitting [24].
                         The  interest  in  using  color  changes  to  reveal  subtle  corrosion
                      mechanisms is still quite modern as attested by two papers recently
                      published in a reputable corrosion journal. In one paper, the corrosion
                      of aluminum and aluminum alloys in chloride-containing agar gels
                      was studied by using a broad-range pH indicator [25]. Distinct changes
                      in pH were observed at low-pH anodic sites and at high-pH cathodes.
                      There was a definite edge effect that occurred when gels were placed
                      directly on the metal, which dominated the pattern of corrosion. Wet-
                      abraded surfaces initially showed a general type of corrosion, whereas
                      dry-abraded surfaces showed localized corrosion under the gel.
                         In a second paper published recently, paint systems containing
                      color-change or fluorescing compounds were found to be sensitive to
                      underlying  corrosion  processes  by  reacting  to  the  pH  increase
                      associated  with  the  local  cathodic  reaction  [26].  The  sensitivity  of
                      acrylic-based  coating  systems  for  detection  of  cathodic  reactions
                      associated  with  corrosion  was  determined  by  applying  constant
                      cathodic current and measuring the charge at which color change or
                      fluorescence was detected.
                         As  explained  in  much  detail  in  Chaps.  2  and  3,  the  cathodic
                      reaction in a corrosion process generally produces an increase in the
                      concentration of hydroxyl ions as a result of removal of hydrogen
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