i Sea Water Service

Aluminium bronzes are used in marine engineering for deck and underwater fittings ranging from fasteners to propellers. Pumps, valves and heat exchangers in ships and in land-based installations using sea water for cooling all employ aluminium bronzes.

• Marine propellers

• Other underwater fittings

• Sea water pumps

• Valves

• Heat exchangers

• Pipework

• Resistance to General Corrosion, Crevice Corrosion and Impingement attack in sea water

Marine Propellers

The requirements for materials for marine propellers are high resistance to corrosion fatigue, corrosion/erosion and cavitation erosion, a high strength-to-weight ratio, good castability and tolerance of welding and local working for repairing damage sustained in service. The choice of alloys for the manufacture of large propellers essentially reduces to nickel aluminium bronze, manganese aluminium bronze and high tensile brass ("manganese bronze").

Results of corrosion fatigue tests on cast material always show a considerable scatter and the values obtained depend upon such factors as the size of specimen and the frequency of loading. Most published results, however, agree in showing the corrosion fatigue strength of nickel aluminium bronze in sea water to be approximately twice that of high tensile brass, with manganese aluminium bronze falling about midway between these two (See Table 6). Among the ferrous alloys spheroidal-graphite cast iron has a corrosion fatigue strength approximately equal to that of high tensile brass. The incorporation of nickel and chromium in the austenitic grade produces no improvement in corrosion fatigue strength although the general corrosion resistance of the material is considerably improved. Cast austenitic stainless steel is also reported to have a fatigue strength approximately equal to that of high tensile brass.

Note, however, that since results of corrosion fatigue tests are dependent on factors such as test bar design and size, and test frequency, comparisons between results from different sources should only be made with caution.

Nickel aluminium bronze shows higher corrosion/erosion resistance than high tensile brass - in which this form of attack tends to be selective upon the beta phase - and its resistance to cavitation erosion is greater than that of high tensile brass by a factor of about eight. Manganese aluminium bronze offers corrosion/erosion resistance approximately equal to that of nickel aluminium bronze with somewhat inferior resistance to cavitation erosion.

Repair welding of high tensile brass propellers can introduce a corrosion hazard since this alloy is susceptible to stress corrosion cracking in sea water and is, therefore, liable to suffer stress corrosion in the weld and heat-affected zone, where residual stresses remain, unless a stress relief heat treatment is carried out after welding. Manganese aluminium bronze also shows susceptibility to stress corrosion cracking, although to a considerably smaller extent, and must also be given a stress relief heat treatment after welding. Nickel aluminium bronze requires more care in welding to avoid formation of cracks in either the weld or the parent metal but, since it is not subject to stress corrosion in sea water, no subsequent stress relieving treatment is required.

The blades, hub body, hub cone and bolts of variable pitch propellers can be made from nickel aluminium bronze or from stainless steel. Stainless steels have higher resistance to corrosion/erosion but are considerably more susceptible to cavitation damage and are more prone to damaging crevice corrosion. Consequently nickel aluminium bronze is usually the preferred material.

Table 6 Corrosion Fatigue Properties of Marine Propeller Alloys

(1) (a) Todd, B., Trans. I . Mar. E., 1968, 80.

(b) Webb, A. W. O., Eames, C. F. W., and Tuffrey, A., Symposium "Propellers 75", Soc. Naval Architects and Marine Engineers, USA 1975.

(c) British Cast Iron Research Association.

(d) Stone Manganese Marine Limited.

(e) Bulletin Sea Horse Institute, May 1963.

(2) Samples cut from propellers. All other results for cast test pieces.

(3) Nickel aluminium bronze from this source is the proprietory alloy "Nikalium".

Tables of corrosion fatigue results from various sources are presented in Chapter 8, "Propellers", by A. W. O. Webb and H. Capper in "Materials for Marine Machinery", Ed. S. H. Frederick and H. Capper, The Institute of Marine Engineers/Marine Media Management Limited, London, 1976. Data taken from these are combined in Table 6.

Other Underwater Fittings

Nickel aluminium bronze, manganese bronze and high tensile brass are all used for cast underwater fittings such as propeller shaft brackets and rudders and in conditions of free exposure to sea water are satisfactory. For fittings to be used under conditions where deposits of silt or mud may form on them, high tensile brass and manganese aluminium bronze are both liable to show dealloying of the beta phase. The nickel aluminium bronze may show slight attack of this type in the alpha phase adjoining the lamellar kappa but to a very much smaller extent. High tensile brass and manganese aluminium bronze are not suitable for underwater fasteners because of their liability to stress corrosion cracking. Nickel aluminium bronze CA104 or DGS 1043, aluminium silicon bronze DGS 1044, phosphor bronze or Monel are used. Phosphor bronze and Monel are, however, of lower strength than nickel aluminium bronze.

Sea Water Pumps

Nickel aluminium bronze is widely used for impellers in centrifugal pumps due to its excellent resistance to both corrosion/erosion and cavitation damage. For the most severe applications or where long life and reliability are particularly important the pump body can also be made of nickel aluminium bronze together with the shaft and the fasteners but the body is often made of gunmetal. Gunmetal impellers may be used in pumps operating under relatively low speed conditions. Monel impellers may be used in high duty pumps but these do not normally offer any advantage over nickel aluminium bronze and are usually more expensive. Cast austenitic stainless steel impellers may be cheaper than nickel aluminium bronze but do not provide the same strength or resistance to cavitation damage. These materials are also less reliable for shafts because of their liability to pitting corrosion in the gland area during shut-down periods.

Some sea water pumps have cast iron bodies with impellers of gunmetal, nickel aluminium bronze, Monel or austenitic stainless steel. During the early life of such pumps the cast iron provides some sacrificial protection to bronze or Monel impellers but the corrosion of the cast iron takes the form of selective phase corrosion leaving a surface that is essentially graphite. A heavily graphitized pump body is strongly cathodic to non-ferrous impeller materials and can cause accelerated attack on them. Consequently the use of aluminium bronze or other non-ferrous impellers is not recommended for sea water pumps with cast iron bodies if long service is expected. Cast austenitic steel impellers are preferable in those circumstances.

Valves

Valves in salt water systems with steel or galvanized steel pipework are usually also of ferrous material but are protected internally by non-metallic coatings. The discs and seats are usually of cast nickel aluminium bronze or Monel and the stems of wrought nickel aluminium bronze or phosphor bronze but sometimes of Monel or 70/30 copper-nickel. High tensile brass is not suitable for the valve stems because of its liability to dezincification nor are stainless steels because of their liability to pitting in crevices.

For systems using copper alloy pipework, gunmetal valves are most commonly used - frequently with nickel aluminium bronze stems - but for high integrity systems valves made entirely from aluminium bronze or from copper-nickel are used. Nickel aluminium bronze has the advantage of greater strength and is usually less expensive than copper-nickel for this purpose.

Heat Exchangers

Steam condensers, oil coolers and other heat exchangers operating on sea water usually have tubes of aluminium brass or of copper-nickel. Aluminium bronze CA102 is manufactured in tube form but is not very often used in heat exchangers. There is an increasing tendency to use titanium tubes for condensers and heat exchangers where conditions are very severe or where extended trouble-free life is essential - for example, drain coolers in ships and main condensers in electricity generating stations.

The "traditional" material for heat exchanger tubeplates is rolled naval brass and this is usually satisfactory with tubes of aluminium brass or 90/10 copper-nickel. Experience of deep dezincification in naval brass tubeplates used with 70/30 copper-nickel tubes in main condensers led the British Navy, several years ago, to change to aluminium bronze CA106 for tubeplates. As a result of subsequent experience of occasional local dealloying corrosion of CA106 a further change was made to CA105 which has proved quite satisfactory. CA106 is, however, very widely used with copper-nickel tubes in sea water cooled condensers and heat exchangers without problems.

Apart from titanium-clad steel, aluminium bronze CA105 or CA106 is the only satisfactory material for tubeplates in heat exchangers using titanium tubes. CA105 is preferred since its reliability from the corrosion point of view is higher, but it is harder and somewhat more difficult to drill. Waterboxes and covers for heat exchangers and condensers are usually of rubber-coated cast iron or steel but, in desalination plant, waterboxes fabricated by welding 90/10 copper-nickel are often used. CA106 (UNS Alloy 61400) or the corresponding alloy with addition of tin (UNS Alloy 61300) are suitable provided that appropriate care is taken over welding (Section 3(xii) and Section 3(viii)).

Pipework

Sea water piping is often of steel or galvanized steel if first cost is a ruling factor and corrosion failures will not result in serious loss or damage. For higher quality systems 90/10 copper-nickel is used, 70/30 copper-nickel being occasionally employed where maximum strength and corrosion resistance are required. These properties can be obtained also from aluminium bronze and there is consequently interest in the use of pipes made from aluminium bronze by welding. Such pipework should show resistance to corrosion and corrosion/erosion equal to or better than 90/10 copper-nickel but with a better strength/weight ratio.

Table 7 Resistance to General Corrosion,Crevice Corrosion and Impingement Attack in Seawater.

The data in Table 7 are taken from Defence Standard 01/2 "Guide to Engineering Alloys Used in Naval Service: Data Sheets". The figures for general corrosion rate and crevice corrosion were determined using samples fully immersed beneath rafts in Langstone Harbour for one year. General corrosion results were provided by freely exposed specimens; crevice corrosion results by specimens held in Perspex jigs providing crevice conditions between the metal sample and the Perspex. The corrosion/erosion resistance tests were carried out using the Brownsdon and Bannister test, the specimens were fully immersed in natural sea water and supported at 60°C to a submerged jet, 0.4 mm diameter placed 1 - 2 mm away, through which air was forced at high velocity. From the minimum air jet velocity required to produce corrosion/erosion in a fourteen-day test, the minimum sea water velocity required to produce corrosion/erosion under service conditions was estimated on the basis of known service behaviour of some of the materials.

Note (1)

The Defence Standard Data Sheets from which the figures in Table 7 are taken give ''up to 0.5 mm/year" as the crevice corrosion rate and 14 ft/sec as the corrosion/erosion resistance limit for nickel aluminium bronze AB2 or CA104 and 8 ft/sec for aluminium silicon bronze DGS 1044. Ship Department Publication 18 "Design and Manufacture of Nickel-Aluminium-Bronze Sand Castings", Ministry of Defence (PE), 1979, gives the following corrosion data:

''Self-corrosion rate: For general long-term use over several years a reasonable design value is 0.05 mm/year but under ideal conditions for nickel aluminium bronze in sea water a black film slowly forms which reduces the corrosion rate in accordance with an equation of the form: Corrosion rate ~ (time)-0.2

Crevice corrosion: After the initiation period which can be about 3 - 15 months with negligible corrosion the crevice corrosion propagates at about 1 mm/year.

Impingement resistance: 4.3 m/s is an appropriate design value in clean flowing sea water."

Crevice corrosion in nickel aluminium bronze takes the form of selective phase dealloying as described in Section 3 (iii), Section 3 (iv) and Section 3 (v) and is usually of little practical significance since it has only a minimal effect on the surface finish. Crevice corrosion of austenitic stainless steels 304 and 316 - although shallower - takes the form of pitting with consequent serious deterioration of surface finish.

The Defence Standard Data Sheets suggest slightly higher corrosion/erosion resistance for aluminium bronze AB1 and CA103 than for nickel aluminium bronze AB2 and CA104 and much lower resistance for aluminium silicon bronze. Practical experience indicates, however, that the nickel aluminium bronzes are superior and aluminium silicon bronze only marginally inferior to other aluminium bronzes in this respect. It is perhaps significant that the Defence Standard Data Sheet figures for corrosion/erosion resistance were derived from Brownsdon and Bannister test results. Table 2 compares other Brownsdon and Bannister test results with those of jet impingement tests which are considered to be more representative of service behaviour.

ii Water Supply

The principal use of aluminium bronzes in the water supply industry is for pumps of the centrifugal and axial flow types but it is also employed for valve trim - especially for valve spindles.

Pumps

Valves

 

Pumps

For pumps handling fresh waters the corrosive conditions are obviously less severe than in sea water systems but impeller tip velocities are nevertheless usually too high for gunmetals, and aluminium bronzes are, therefore, specified. AB1 is sometimes used but the higher resistance of AB2 to corrosion/erosion makes it a better choice - even for pumps where the expected flow conditions are such that AB1 might be considered adequate. Very long working lives are expected from pumps in the water supply industry and the service conditions often change, usually making greater demands on the pumps.

Centrifugal pump bodies are sometimes of cast iron but more frequently of gunmetal or aluminium bronze to avoid contamination of the water by corrosion of the cast iron. The shrouds of axial flow pumps can be made of cast iron since they can be effectively protected by non-metallic coatings. Pump spindles are usually of nickel aluminium bronze or stainless steel. Stainless steel rarely suffers pitting attack in this type of pump since the pumps are normally operated continuously.

Valves

The larger size valves used in the water supply industry are of coated cast iron or steel but with internal trim in non-ferrous material or stainless steel. Monel and aluminium bronze AB1 or AB2 are often used, with aluminium bronze CA103 or CA104 for spindles; either will give satisfactory corrosion resistance but CA104 has higher strength. High tensile brass is sometimes used for valve spindles and is permitted by the relevant British Standards but aluminium bronze is much to be preferred because of the liability of high tensile brass to selective phase dealloying (dezincification) in some waters.

Aluminium bronze is not generally used in small stopvalves for domestic water installations except for valves that are to be installed underground. BS 5433 requires these to be made from materials immune to dezincification and permits forged aluminium bronze CA104 for all parts of the valve except the body, which is cast gunmetal.

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