In duplex and multiphase alloys the phases have different electrochemical potentials and there is consequently always a tendency for the most anodic phase to be corroded preferentially.

The extent to which this occurs depends upon how great the potential difference is between the anodic phase and the surrounding phases and upon the distribution and intrinsic corrosion resistance of the anodic phase. The most commonly encountered examples of selective phase corrosion are in the duplex brasses such as free machining brass, diecasting and hot stamping brasses, Muntz metal, naval brass and the high tensile brasses commonly called manganese bronzes. The beta phase in all these alloys is anodic to the alpha and forms a continuous network providing a continuous path of low corrosion resistance by which attack can penetrate deeply into the alloy.

The danger of selective phase attack occurring on the gamma 2 phase in aluminium bronzes has already been discussed in Section 1 where it was also explained that the formation of this phase can be avoided by suitable control of composition and/or cooling rate. Under free exposure conditions in fresh waters or sea water, aluminium bronzes free from gamma 2 phase do not show selective phase corrosion but, under crevice conditions, beneath deposits or marine growths or under the influence of galvanic corrosion or of electrical leakage corrosion, selective phase attack can occur. In the alpha-beta alloys this takes the form of slightly preferential attack on the beta phase. In the nickel aluminium bronzes selective phase attack may affect small amounts of residual beta phase if any is present but is more likely to affect the narrow band of alpha phase immediately adjoining the lamellar kappa and to spread from that into the kappa phase itself. This selective phase attack in aluminium bronzes is not usually of great significance and occurs only when they are subjected to particular severe service conditions. For such conditions of service it can be beneficial to apply to nickel aluminium bronze castings the heat treatment required in DGS Specification 348 (six hours at 675�C + 15�C followed by cooling in still air). This is, however, only necessary if the rate of cooling of the casting from about 900�C has been too rapid for formation of the normal alpha-plus-kappa structure.

The copper manganese aluminium alloys CMA1 and 2 are essentially of alpha-beta structure but the beta phase is of different composition from that in the aluminium bronzes of low manganese content and is more susceptible to selective phase corrosion. This does not occur, however, to any significant extent under free exposure and rapidly flowing water conditions such as exist on marine propellers. In static sea water service - especially under shielded area conditions or under the influence of galvanic coupling to more noble alloys - severe selective phase corrosion of the beta phase can occur and, since the beta phase is continuous, can cause serious deterioration.