i Uniform or General Corrosion

The amount of metal removed by general corrosion in sea water or fresh water is insufficient to cause significant damage to components in any of the non-ferrous metals or alloys in normal commercial use. In some aggressive waters, pure copper and some of the high copper alloys such as bronzes and gunmetals can, however, introduce sufficient copper into the water to cause increased corrosion of galvanized steel or of aluminium alloys downstream of the copper alloy components. Aluminium bronzes are virtually unaffected by cupro - solvent waters and no problems from copper pick-up are experienced in connection with aluminium bronze components.

Acidic solutions cause relatively rapid dissolution of many copper alloys but aluminium bronzes are very little affected by non-oxidizing acids and are widely used for handling sulphuric acid, for example.

ii Pitting

Pitting corrosion is important because of its localised character which can result in perforation of the wall of a valve, pump casting, water tube or other vessel in a relatively short time. All common metals and alloys are subject to pitting corrosion to a greater or less extent under certain conditions of service. Stainless steels form deep pits of very small cross section in waters of high chloride content.

Pitting in copper alloys is not directly associated with chloride content and they do not normally show significant pitting, for example, in sea water service. Sulphide pollution of the sea water may cause pitting in most copper alloys including aluminium bronzes; for polluted waters copper alloys containing tin are usually best.

iii Crevice Corrosion

Practically all metals and alloys suffer accelerated local corrosion either within or just outside crevices or "shielded areas" where two components or parts of the same component are in close contact with one another but a thin film of water can penetrate between them. The effect is greatest for stainless steels which depend upon free access of oxygen to the wetted surfaces to maintain the protective oxide film on which their corrosion resistance depends. Crevice corrosion of stainless steels usually takes the form of severe pitting within the crevice and this is a serious limitation on the uses to which these alloys may be put. Crevice corrosion of copper-nickel alloys takes a different form, resulting in a narrow trench of corrosion at the edge of the crevice often with some deposition of copper on the metal surface within the crevice. Most other copper alloys show similar crevice corrosion but to a greater or lesser extent. Crevice corrosion of aluminium bronzes tends to occur within the crevice and usually takes the form of selective phase attack and dealloying as discussed in Section 3(iv) and Section3(v). The effects are, therefore, related to the metallurgical structure of the particular aluminium bronze concerned and are least in the single phase alloys. None of the aluminium bronzes, however, is seriously affected by crevice corrosion in the way that stainless steels may be, since the attack does not produce pitting or serious roughening of the surface.

iv Selective Phase Attack

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.

v Dealloying

A form of corrosion affecting some copper alloys results in selective removal of the principal alloying element leaving a residue of copper. This residue has a porous structure and very low strength but it retains the shape and approximate dimensions of the original alloy. Consequently the depth to which the attack has penetrated is very difficult to assess except by destructive methods such as the preparation of metallographic sections. The most common example of dealloying is provided by the duplex brasses in which the selective phase attack on the beta phase takes the form of dezincification with effective removal of zinc and formation of a weak copper residue. A similar type of corrosion known as dealuminification occurs when selective phase attack takes place in aluminium bronzes. The conditions under which it occurs are those under which selective phase corrosion is experienced, as described in Section 3(iv). It can be very largely prevented under most conditions of service by ensuring that the alloy used is free from gamma 2 phase.

Selective phase corrosion in the CMA alloys takes the form of dealloying of the beta phase. The conditions under which it occurs have already been discussed in Section 3(iv). The susceptibility of CMA alloys to selective phase dealloying corrosion is less than that of the duplex brasses but much greater than that of the aluminium bronzes with low manganese content which should always be used in preference to CMA for applications involving static or shielded area conditions in sea water and for acidic environments.

vi Corrosion/Erosion

All common metals and alloys depend for their corrosion resistance on the formation of a superficial layer or film of oxide or other corrosion product which protects the metal beneath from further attack. Under conditions of service involving exposure to liquids flowing at high speed or with a high degree of local turbulence in the stream, this protective film may be prevented from forming or may be eroded away locally exposing unprotected bare metal. The continued effect of erosion, preventing permanent formation of a protective film, and the corrosion of the bare metal consequently exposed can lead to rapid local attack causing substantial metal loss and often penetration. This type of attack is known as corrosion/erosion or impingement attack. (See Table 1, Table 2 and Table 7.)

The highest resistance to corrosion/erosion is shown by alloys on which the protective film reforms very rapidly if it should suffer mechanical damage and on which the film itself is resistant to erosion. Stainless steels are particularly resistant to this type of attack. Unalloyed copper is relatively poor but all copper alloys are substantially more resistant than copper itself and nickel aluminium bronze is among the most resistant of all the copper alloys.

Table 1 Corrosion/Erosion Attack in Jet Impingement Tests at 9.3 m/s.

Jet impingement tests on cast nickel aluminium bronze DGS 348 and 70/30 copper-nickel condenser tube CN107 have been carried out at 10°C and 15°C by BNF Metals Technology Centre using natural sea water at a water jet velocity of 9.3m/s. The extent of corrosion/ erosion occurring at the jets in 28-day tests is given in Table 1. The diameter or depth of attack recorded is the average for four specimens.

Table 2 Resistance of Copper Alloys to Impingement Attack and General Corrosion in Sea Water

The data in Table 2 are taken from a paper, "The Resistance of Copper Alloys to Different Types of Corrosion in Sea Water", by Sigmund Bog of the Ship Research Institute of Norway, presented at the 7th Scandinavian Corrosion Congress, Trondheim, 1975.

vii Cavitation Damage

Under water flow conditions even more severe than those responsible for corrosion/erosion, cavitation damage may occur. This is the result of formation of small vapour bubbles (cavitation) in the water in regions where the flow conditions produce low pressures and subsequent violent collapse of these bubbles on the surface of the metal in neighbouring areas where the local pressure is higher. The stresses generated by the collapse of cavitation bubbles are much greater than those associated with corrosion/erosion and are often sufficient not only to remove protective corrosion product films but actually to tear out small fragments of metal from the surface - usually by fatigue. The metal freshly exposed as a result of this action will of course be subject to corrosion and the resultant damage is due to a combination of corrosion and the mechanical forces associated with the bubble collapse. In view of the magnitude of the mechanical forces associated with cavitation damage the contribution made by corrosion is, however, relatively small.

Cavitation damage is a serious problem principally in high duty pump impellers and marine propellers but can occur sometimes in restricted waterways of valves working at high flow rates. It can be reduced by correct hydrodynamic design of the propeller or pump but it is not usually possible to produce a design which will ensure freedom from cavitation under the full range of operating conditions that have to be covered. Nickel aluminium bronze shows exceptionally high resistance to cavitation damage and is for that reason the alloy most commonly used for production of large marine propellers and high duty pump impellers. (See Table 3a and Table 3b.)

Table 3a Cavitation Erosion in 3% NaCl Solution

Published data for resistance to cavitation erosion generally refer to tests carried out using equipment in which the specimen is vibrated at 20 kHz. Such tests in 3% sodium chloride solution at an amplitude of ±0.025 mm reported by A Tuffrey in "Vibratory Cavitation Erosion Testing", National Engineering Laboratory Report No. 149, April 1964, produced the following depths of attack:

Table 3b Cavitation Erosion Rates in Fresh Water

I. S. Pearshall (Chartered Mechanical Engineer, July 1974) gives the following cavitation erosion rates from tests at 20 kHz in fresh water:

viii Stress Corrosion

Stress corrosion is a highly localised attack occurring under the simultaneous action of tensile stress and an appropriate environment. The total amount of corrosion is very small but cracking occurs in a direction perpendicular to that of the applied stress and may cause rapid failure. The environments conducive to stress corrosion cracking vary for different types of alloy. Stainless steels suffer stress corrosion cracking particularly in hot chloride solutions. Most copper alloys show susceptibility to stress corrosion cracking in the presence of ammonia or ammonium compounds and in some moist sulphur dioxide environments. They vary, however, in their degree of susceptibility, the brasses being the most susceptible and copper-nickel alloys the least susceptible. Aluminium bronzes are much superior to brasses, though not as good as copper-nickel in this respect. (See Table 4 and Table 5.)

The possibility of stress corrosion cracking can be reduced to a minimum by ensuring that components are given a stress relief heat treatment to remove internal stresses arising from working or welding and by keeping assembly stresses in fabricated equipment as low as possible by accurate cutting and fitting of the component parts. Service stresses are, however, frequently unavoidable and where these are likely to be high the low susceptibility of the aluminium bronzes, and especially of the nickel aluminium bronzes, to stress corrosion is an important consideration.

Stress corrosion cracking may follow a transgranular or intergranular path depending upon the alloy and the environment. In the presence of ammonia, stress corrosion cracking of aluminium bronzes follows a transgranular path. Intergranular stress corrosion cracking can occur, however, in the single phase alloys such as CA106 ( "Alloy D ") in high pressure steam service. Research in USA showed that susceptibility to this type of attack can be eliminated by the addition of 0.25% tin to the alloy. This is not provided for in British Standards at present but the American UNS Designation 61300 covers "Alloy D" with the appropriate tin addition.

It has been very recently observed that, at high tensile stress, CA106 ("Alloy D") can undergo intergranular stress corrosion cracking in hot brine also. Laboratory tests indicate that the UNS 61300 alloy resists stress corrosion under these conditions. It is, therefore, suggested that the alloy containing tin should be used not only for super-heated steam but probably also for hot brine if the operating or fabrication stresses are high. Since, however, the resistance of alloy 61300 to stress corrosion in hot brine has so far been demonstrated only in laboratory tests it is recommended that its performance under service conditions should be checked before deciding finally upon its use.

The results in Table 5 were obtained using very severe test conditions, i.e., a high ammonia content in the atmosphere and very high stress levels (including plastic deformation) in the samples. Under normal service conditions aluminium bronzes very rarely show stress corrosion cracking.

Table 4 Atmospheric Stress Corrosion Tests on Copper Alloys

The results in Table 4 were obtained from atmospheric exposure tests of U-bend specimens exposed to industrial environments (J. M Popplewell and T. C. Gearing, Corrosion, 1975, 31, 279).

Table 5 Comparison of Stress Corrosion Resistance of Brasses, Aluminium Bronzes and Copper-Nickel Alloys

Stress corrosion tests were carried out by D. H. Thompson (Mater Res. & Std., 1961,1, 108) using loop specimens of sheet material exposed to moist ammoniacal atmosphere. The ends of the loops were unfastened once every 24 hours and the extent of relaxation from the original configuration was measured. This is a measure of the progress of stress corrosion cracking on the outside surface of the loop. Table 5 gives the time to 50% relaxation for various alloys tested.

ix Corrosion Fatigue

Metals and alloys can fail by fatigue as a result of the repeated imposition of cyclic stresses well below those that would cause failure under constant load. In many corrosive environments the cyclic stress level to produce failure is further reduced, the failure mechanism then being termed corrosion fatigue. The relative contributions to the failure made by the corrosion factor and the fatigue factor depend upon the level of the cyclic stress and upon its frequency, as well as upon the nature of the corrosive environment. Under high frequency loading conditions such as may arise from vibration or rapid pressure pulsing due to the operation of pumps, etc., the corrosion resistance of the alloy is of less importance than its mechanical strength but under slow cycle high strain conditions both these properties become important. Because of their combination of high strength with high resistance to normal corrosive environments, aluminium bronzes, and particularly the nickel aluminium bronzes (which are the best in both these respects), show excellent corrosion fatigue properties under both high frequency and low frequency loading conditions.

Figure 2. High strain/low cycle corrosion fatigue results for cast nickel aluminium bronze DGS 348.

Taken from Ship Department Publication 18 "Design and Manufacture of Nickel Aluminium Bronze Sand Castings", Ministry of Defence (PE), 1979, Figure 2 presents results of corrosion fatigue tests carried out in sea water at 32°C. The tests employed flat specimens strained by bending about a zero strain mean position.

x Galvanic Corrosion

When two metals or alloys are used in contact with one another in an electrolyte such as water they affect one another's resistance to corrosion. Usually one of the pair - the more "noble" - will cause some degree of accelerated corrosion of the other and will itself receive a corresponding degree of protection. A useful guide to interactions at bimetallic contacts is provided by the British Standards Institution "Commentary on Corrosion at Bimetallic Contacts and Its Alleviation" PD 6484: 1979. This groups together all varieties of aluminium bronze and the silicon bronzes and the information that it gives is, therefore, of somewhat limited value but differences between aluminium bronzes with respect to galvanic corrosion are usually negligible.

Galvanic corrosion tests are usually carried out in sea water since this and fresh water are the environments in which mixtures of metals are most frequently encountered. Such tests show most aluminium bronzes to be slightly more noble than other copper alloys with the exception of 70/30 copper-nickel. The differences are, however, small and the additional corrosion of aluminium bronzes produced by coupling to copper-nickel or of other copper alloys by coupling to aluminium bronze is usually insignificant.

Stainless steels and titanium are both more noble than aluminium bronzes but the degree of acceleration of attack produced by coupling to these materials is normally only slight. Tubeplates of aluminium bronze are commonly used for heat exchangers with titanium tubes and experience has confirmed that galvanic attack on the tubeplate is negligible.

The absence of significant galvanic effects under these conditions depends partly upon the effective exposed area of the titanium tube ends being not greatly in excess of that of the aluminium bronze. In situations where aluminium bronze is used in contact with and in close proximity to much larger areas of more noble materials such as titanium, stainless steel or nickel-copper alloys of the Monel type appreciable accelerated attack may sometimes be experienced. This usually takes the form of selective phase attack and dealloying as described in Section 3(iv) and Section 3(v).

xi Electrical Leakage Corrosion

Situations are sometimes met in service where aluminium bronze components are inadvertently exposed to electrical leakage currents either as a result of electrical faults resulting in current passing to earth, via a submerged pump for example, or as a result of incorrect positioning of impressed current cathodic protection equipment resulting in current passing from the water on to the metal equipment at one point and leaving it again at another. These conditions will accelerate attack of practically all metallic materials whether the current concerned is DC or AC. Aluminium bronze under these conditions will show local corrosion in the region affected by the current leakage, the corrosion usually taking the form of selective phase dealloying. The avoidance of this type of attack is obviously a matter of correct design and maintenance of the electrical equipment concerned.

xii Corrosion Associated with Welds

Welding can adversely affect the corrosion resistance of many alloys and in different ways. Galvanic corrosion can result from differences in composition or of structure between the filler and the parent metal. The metallurgical structure of the heat-affected zone adjoining the weld may be changed for the worse especially in multipass welding in which the time at elevated temperature is relatively long. Welding under conditions of restraint can also introduce stresses in the weld metal and in the heat-affected zones of the parent metal which may lead to stress corrosion cracking.

The most widely recognized harmful effect of welding on corrosion resistance is observed in stainless steels which are not either of very low carbon content or stabilised by the addition of titanium or niobium. Diffusion of chromium and formation of chromium carbides in the heat-affected zone leaves chromium-depleted material which is readily corroded, resulting in a line of attack close to the weld - commonly known as "weld decay". A reduction of corrosion resistance in the heat-affected zones of welds may occur, to a smaller extent and for different reasons, in some of the aluminium bronzes.

The aluminium bronzes most commonly used under conditions where welding is required are the single phase alloy CA106 ("Alloy D"), and the nickel aluminium bronze alloys CA105 and AB2. The welding of aluminium bronzes is dealt with in the CDA Aluminium Bronze Advisory Service publication, "Guidance Notes for Welding Aluminium Bronze Alloys", and only those aspects directly concerned with corrosion resistance will be discussed here.

Since problems of weld cracking can arise in welding CA106 with a matching filler unless the impurity levels in both the filler and parent metal are closely controlled it is common practice to use a duplex alloy filler containing ~ 10% Al. To avoid selective phase corrosion of the beta phase in the filler on subsequent service in sea water or in acid solutions, it is recommended that an overlay with a composition matching the parent metal should be applied on top of the duplex filler. If a matching filler rod is not available an overlay of nickel aluminium bronze is used.

The possibility of tensile stresses and consequent increased susceptibility to stress corrosion cracking arising as a result of welding under conditions of restraint has already been noted. A further factor to be watched in welding CA106 is the formation of microfissures in the heat-affected zone during welding which can act as stress raisers and so further increase the danger of stress corrosion cracking in subsequent service.

No serious corrosion problems are introduced in welding CA105. The use of an approximately matching filler ensures that galvanic effects between the filler and parent metal are reduced to a minimum although the aluminium content of the weld bead will usually be higher than that of the parent metal. The good high-temperature ductility of CA105 also means that there is little likelihood of microfissuring occurring and the level of stress in the heat-affected zone arising from welding under restraint is also likely to be less than in CA106 welded under similar conditions.

Nickel aluminium bronze castings may be welded to repair small areas of casting porosity, etc., or in the manufacture of large components or water circulating systems. The welding is usually carried out using a filler with approximately the same composition as the parent metal but welds made under conditions of severe restraint require a duplex filler to avoid weld cracking. A nickel aluminium bronze overlay must then be applied to avoid corrosion of the filler. In sea water service, selective phase dealloying corrosion of the alpha phase immediately adjacent to lamellar kappa sometimes occurs in the outer regions of the heat affected zones of welds in nickel aluminium bronze. The attack is sometimes accelerated by the presence of internal stresses in the casting which produce cracking in the porous copper produced by the dealloying corrosion and accelerate the rate of penetration of attack into the alloy.

Welding of nickel aluminium bronze castings can also reduce the corrosion resistance of the material by the presence of beta phase retained in the weld bead as a result of its cooling rapidly from the temperature at which conversion to alpha-plus-kappa begins. Beta phase may also be reformed from the alpha-plus-kappa in the heat-affected zone of parent metal nearest the weld.

Welded nickel aluminium bronze which has had no post-weld heat treatment is widely used in seawater and other environments without difficulty. Under severe service conditions, however, the beta phase formed by either of these mechanisms can suffer selective phase dealloying.

This possibility can be eliminated by the application of a post-weld heat treatment. The treatment laid down in the requirements of DG Ships Specification 348 is six hours at 675°C ± 15°C followed by cooling in still air. This ensures conversion of retained beta to alpha-plus-kappa and also modifies the lamellar kappa and greatly reduces the possibility of selective phase attack on the adjacent alpha.

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