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                                          MATERIALS OF CONSTRUCTION
                   the more commonly used inexpensive materials, such as the carbon and low alloy steels,
                   a guide to what is considered acceptable is given in Table 7.3. For the more expensive
                   alloys, such as the high alloy steels, the brasses and aluminium, the figures given in
                   Table 7.3 should be divided by 2.
                                           Table 7.3.  Acceptable corrosion rates
                                                                 Corrosion rate
                                                               ipy         mm/y
                                  Completely satisfactory     <0.01        0.25
                                  Use with caution            <0.03        0.75
                                  Use only for short exposures  <0.06      1.5
                                  Completely unsatisfactory   >0.06        1.5


                     The corrosion rate will be dependent on the temperature and concentration of the
                   corrosive fluid. An increase in temperature usually results in an increased rate of corrosion;
                   though not always. The rate will depend on other factors that are affected by temperature,
                   such as oxygen solubility.
                     The effect of concentration can also be complex. For example, the corrosion of mild
                   steel in sulphuric acid, where the rate is unacceptably high in dilute acid and at concen-
                   trations above 70 per cent, but is acceptable at intermediate concentrations.


                   7.4.2. Galvanic corrosion
                   If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic
                   metal will be increased, as the metal lower in the electrochemical series will readily act as
                   a cathode. The galvanic series in sea water for some of the more commonly used metals is
                   shown in Table 7.4. Some metals under certain conditions form a natural protective film;
                   for example, stainless steel in oxidising environments. This state is denoted by “passive”
                   in the series shown in Table 7.4; active indicates the absence of the protective film. Minor

                                           Table 7.4.  Galvanic series in sea water
                          Noble end
                          (protected end)  18/8 stainless steel (passive)
                                           Monel
                                           Inconel (passive)
                                           Nickel (passive)
                                           Copper
                                           Aluminium bronze (Cu 92 per cent, Al 8 per cent)
                                           Admiralty brass (Cu 71 per cent, Zn 28 per cent, Sn 1 per cent)
                                           Nickel (active)
                                           Inconel (active)
                                           Lead
                                           18/8 stainless steel (active)
                                           Cast iron
                                           Mild steel
                                           Aluminium
                                           Galvanised steel
                                           Zinc
                                           Magnesium
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