Nickel Aluminium Bronze alloys are overlooked compared to duplex and super duplex steels, yet they offer interesting properties in corrosive environments. .
Casted Aluminium Bronze is manufactured according to many different international standards ( EN 1982, ASTM B148, Inoxyda proprietary alloys …) and are a combination of copper, aluminium, nickel, iron, manganese which are combined to meet the best compromise between improved mechanical strength, corrosion resistance, weldability and machinability.
These additions produce a unique set of properties (Ref 1) which are useful in valve components :
Some examples of mechanical properties reached by Nickel Aluminium Bronze (NAB) alloys :
For sea water applications the castings can be heat treated at 675°C for six hours to improve the corrosion resistance and for maximum corrosion resistance it is also recommended that aluminium should be less than Al % < (8.2 +Ni /2 )
Other typical valve applications include components in submarines and surface vessels within the Defence Marine sector, firefighting equipment on oil rigs and desalination plants.
Typical properties within these sectors include.
Many design engineers, by lack of data, consider copper based alloys as being low strength. Below chart gives a general comparison between different alloys :
Bronze alloys are known for friction properties when Aluminium Bronze or Nickel Aluminium Bronze (NAB) alloys are much less known; yet, they offer unique corrosion characteristics as required by pumps, especially in sea water corrosion environments, along with interesting biofouling properties. Those 2 properties make them a preferred solution for all marine related applications (pumps, valves, heat exchangers, propulsion systems).
Aluminium Bronze is a family of alloys including copper, aluminium, nickel and iron ; Each individual component can be adjusted to improve strength, corrosion resistance and/or machinability.
Those combinations allow a unique set of properties (Ref 1) which are usefull in many pump application :
With a main property coming from the protective oxide surface film which has the ability to self- repair, it outstands steel coated applications in the long run.
Optimum corrosion protection is reached when :
Some design engineers, by lack of data often consider copper based alloys of being low strength but again correct selection can lead to high mechanical properties such as with Inoxyda 154 grade.
Very well known to all pump specialist, this results from small water vapour bubbles are formed from the turbulence at low pressure points and these can migrate to high pressure points where they collapse violently on the surface of the component causing eventually erosion at the point of contact. Nickel Aluminium Bronze has excellent resistance to this phenomenon and as such is still the predominant alloy for propellers in shipbuilding industry.
A main requirement for cryogenic pumps is the resistance at low temperatures. Unlike most steels, copper based alloys do not embrittle and have a good dimensional stability at low temperatures down to -190°C . A main issue to be addressed in those applications is the possible presence of oxygen which reduces to scope to bronze alloys; for other application aluminium bronze are regularly used, such as for example the Inoxyda 3 Cryo alloy .
Biofouling can be a serious problem with any systems handling sea water, particularly at elevated temperatures with warm climates.
High copper alloys have considerable resistance to the growth of crustaceans, sea weeds and other marine organismsThe slow dissolution of copper ions inhibits their growth and prevents mass population. The adherence of crustaceans on Nickel alloys and stainless steels can have a massive impact on their corrosion resistance due to crevice corrosion caused by differential aeration associated with these growths.
NAB alloys have 2 main advantages compared to Duplex and Super Duplex alloys :
More detailed technical information available :