Page 254 - Corrosion Engineering Principles and Practice
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228    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    229


                                               Type of join
                                               • Welded
                       Electrode potentials    • Rivets
                       • Thermodynamics        • External connection
                       • Scaling                              Geometry
                       • Deposits                             • Surface area ratio (Sa/Sc)
                                                              • Surface shape
                                                              • Separation distance
                        Reaction kinetics
                       • Polarization                           Bulk solution
                        Characteristics                         Properties
                       • Complexing reactions  Metal A  Metal B  • Oxygen content
                       • Precipitation reactions                • pH
                                                                • Conductivity
                         Alloy composition
                         • Main phases                         Mass transport
                         • Microstructure                      • Flow rate
                         • Impurities  Surface characteristics   • Turbulence
                                       • Nature of protective film  • Presence of bubbles
                                       • Presence of holidays
                                       • Solution dependence

                      FIGURE 7.17  Factors affecting galvanic corrosion [14].




                  Metal                                Potential V (CCSRE)
                  Commercially pure magnesium              –1.75
                  Magnesium alloy (6% Al, 3% Zn, 0.15% Mn)  –1.6
                  Zinc                                     –1.1
                  Aluminum alloy (5% zinc)                 –1.05
                  Commercially pure aluminum               –0.8
                  Mild Steel (clean and shiny)             –0.5 to –0.8
                  Lead                                     –0.5
                  Cast iron (not graphitized)              –0.5
                  Mild steel (rusted)                      –0.2 to –0.5
                  Mill scale on steel                      –0.2
                  High silicon cast iron                   –0.2
                  Copper, brass, bronze                    –0.2
                  Mild steel in concrete                   –0.2
                  Platinum                              0 to –0.1
                  Carbon, graphite, coke c                 +0.3


                   Typical potentials normally observed in neutral soils and water, measured in relation to
                  a copper/copper sulfate reference electrode.

                 TABLE 7.5  Practical Galvanic Series for Metals in Neutral Soils and Water [15]
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