For many decades corrosion resistant Copper Alloys have been utilized in sea water applications for their unique combination of properties anti-biofouling non-sparking resistance to cavitation corrosion excellent wear properties and low magnetic permeability.One disadvantage has been lack of mechanical strength and Copper Nickel Chromium (CuNi30Cr2) is a unique alloy developed to overcome this. It does not rely on a precipitation hardening mechanism for increasing the mechanical properties but “spinodal” decomposition which is a chemical strengthening mechanism. Additionally the alloy is single phase and hence it does not suffer from selective phase corrosion which has been problematic with some copper based alloys in marine salt water environments.This paper evaluates the wrought version of CuNi30Cr2 which can now be manufactured to Def Stan 02-886 and offers significant scope for design engineers with much higher mechanical properties increased toughness and increased internal integrity. It provides the findings of a four year corrosion program with comparisons against 26 other alloys involving a range of copper alloys Monel® Inconel® Super Duplex Titanium and Stainless Steels. General corrosion galvanic and crevice corrosion in a sea water environment were all examined as well as mechanical properties.

In marine applications aluminium bronze is used extensively. The alloy IACS W24 Cu3/ CuAl10Fe5Ni5-C-GS is the most frequently used alloy for large ship propellers.Under certain conditions the material surface of ship propellers is damaged due to cavitation erosion. Cavitation erosion can be described as a hydrodynamic phenomenon which is related to the formation and collapse of gas bubbles in a liquid. The cyclic mechanical load on the material surface causes plastic deformation and material erosion. The addition of corrosive conditions can increase the material erosion.The aim of the following investigations is the understanding of the cavitation erosion behavior of aluminium bronze in artificial seawater with focus on the role of the complex microstructure.Towards this purpose vibratory cavitation tests in artificial sear water were carried out and the damage of the material surface was observed by scanning electron microscopy (SEM). In addition the corrosion behavior was investigated by exposure tests and registration ofcurrent density-potential-curves.The specimens for the investigations were taken from different parts of a large cast ship propeller: the propeller tip the center and near the hub. Within the propeller there is a significant difference regarding the grain size which is smallest in the tip and four times bigger in the hub. Due to higher strength in the propeller tip in comparison this area shows the smallest cavitation erosion damage.SEM observations showvarious mechanism of damage of the microstructure which includes the Cu-rich α-phase and different intermetallic κ -phases consisting of Al with Ni and Fe.Next step of the investigation was the variation of the standard alloy composition with focus on the Fe/Ni-relation and the Mn-content whereby the composition stays within the limits of the international standard requirements.The different alloy compositions show a significant influence on the formation of the complex microstructure of Aluminium bronze. The specimens with a high Ni-content show mainly the lamellar κ-phases between the α-grains and no round precipitations. Increasing the Fe-content leads to more round iron κ-phases and only a few proportion of eutectoid.The differences of the microstructure influence the mechanical strength and toughness the corrosion resistance and the cavitation erosion resistance of the alloy. The SEM-observations show a selective cavitation erosion and corrosion of the phases.Keywords: cavitation cavitation erosion cavitation corrosion aluminium bronze ship propellers microstructure

Aluminum-based, iron-based and nickel-based alloys of known microstructures were exposed to cavitation erosion conditions using a vibratory cavitation testing apparatus in seawater as a testing medium. The cavitation tests were made at a frequency of 20 KHz as per ASTM-G30-90 and at a temperature of 25oC. Cavitation made the surface of these alloys very rough, exhibiting large cavity pits in the middle region of the attacked area as revealed by the scanning electron microscope (SEM). The cavitation damage initiated at interfaces of high energy such as grain boundaries, twins, Stacking faults and second phase particles. Hydrodynamic, electrochemical and metallurgical factors played a major role in loss of metals for these alloys as a result of the cavitation action.