Shipbuilders regularly specify NAB valves for areas such as: cooling systems, scrubbers, ballast water treatment systems, fire-fighting, and desalination.

NAB – an attractive alloy for shipbuilding and offshore valves

Materials selection is of course of major importance in any marine environment such as shipbuilding, offshore facilities, desalination plants, etc, because seawater can be highly corrosive. Well-known and reliable solutions are available, such as cathodic protection, coated metals or stainless steels. However, another category of alloys – the nickel aluminium bronze (NAB) family – should also be considered as they have beneficial properties for use in seawater.
 
^ Shipbuilders regularly specify NAB valves for areas such as: cooling systems, scrubbers, ballast water treatment systems, fire-fighting, and desalination.

Article by Olivier Gouriou, Inoxyda
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Properties of nickel aluminium bronze alloys

The nickel aluminium bronzes, also called NAB alloys, are a range of copper based alloys containing nickel, aluminium and iron. By altering the relative proportions of these and other elements, developers have created a whole family of NABs with a range of useful properties. Most of these alloys are very well documented in various norms like ASTM B148, EN 1982, BS14000 or even in the technical literature such as the excellent Guide to Nickel Aluminium Bronze for Engineers, written by Ivan Richardson and available through the Copper Development Association.

Reading such literature clearly shows that NAB’s seawater corrosion resistance is due to its self-repairing film which outlasts most steel-coated solutions. Moreover, NABs have other interesting properties such as:

  • High strength
  • Density (5% lighter than steel)
  • Non sparking
  • Low magnetic permeability (of <1.03μ in selected grades)
  • High corrosion resistance
  • Good cryogenic properties 
  • High resistance to biofouling
  • Lower cost than titanium

In addition to which we can add readily availability, very good machinability and ease of weld repairs. These characteristics means that NAB materials can be easily transformed into useful products such as pumps and valves via well-established manufacturing processes.

NAB manufacturing processes

As a material which is already widely used to manufacture engineered products, NAB alloys are available in a range of different forms. All of these are of course supported by international standards to be used as reference point. Common basic product forms and applicable manufacturing techniques include:

  • Continuous casting. Creates billets which can then be forged or directly machined to create small mechanical components.
  • Sand casting. Enables larger and complex engineered parts to be made. These are typically finish-machined prior to assembly
  • Centrifugal casting. Used to produce cylindrical parts, again typical finish-machined before assembly.
  • Forging. Based on cast billets
  • Plates. Welded together to create larger equipment
  • Drawn bars. Used for various axles and very small machined parts

Engineered applications

Over the years, NAB alloys have been successfully used to create components and equipment for use in seawater environments. The author has personal experience in producing components for use in gate, globe, ball, butterfly and check valves, for example. The list below includes just a short selection of ‘tried and trusted’ areas of use:

  • Pumps: impellers, casings, discharge elbows
  • Valves: bodies, discs, wedges, bonnets
  • Heat exchangers: channels, tubesheets, floating heads, covers
  • Propulsion: blades, hubs, bearings
  • Renewable energy: Kaplan blades, ball bearing cages

Developers have in addition successfully created customized NAB alloys for use in areas where standard alloys may be less satisfactory. Just to list a few examples:

  • Mine hunter propulsion lines (using an NAB with a low magnetic permeability)
  • Underwater electronic casings and couplings (NAB with high mechanical properties, eg, TS > 750 MPa)
  • Cryogenic valves and pumps (using an NAB with optimized low-temperature resistance)

Quality controls

All applications in the shipbuilding industry fall under the very strict control of major classification societies such as DNVGL, Lloyds, Bureau Veritas, RINA, RMRS, etc, and as such are subject to certifications which go beyond the basic ISO 9001 certification, to include PED (Pressure Equipment Directive 2014/68/EU) and in some cases end-user qualifications.

Classification societies regularly require several controls to guaranty the quality of the parts, among which:

  • Inspection and test plans
  • Dye penetrant test
  • Dimensional and pressure test
  • UL thickness measurement
  • X-rays ( in particular for high pressure applications)

All of which are of course applicable to all nickel aluminium bronze manufacturing processes mentioned above.

Summary

Due to its properties, nickel aluminium bronze is an option to be considered in comparison to other coated metals or stainless steels alloys when confronted with seawater corrosion, biofouling or non-sparking issues. If the cost of those alloy seem to be significant, remember that 60% of the raw material cost can be recovered when items are scrapped, thus further improving the TCO.
 

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