Elsevier

Corrosion Science

Volume 47, Issue 12, December 2005, Pages 3336-3367
Corrosion Science

The corrosion of nickel–aluminium bronze in seawater

https://doi.org/10.1016/j.corsci.2005.05.053Get rights and content

Abstract

Nickel–aluminium bronze (NAB) alloys show good corrosion resistance under marine conditions. The corrosion behaviour of cast and wrought NAB alloys is illustrated in this work through a range of electrochemical techniques including open-circuit potentiometry with time, oxygen reduction voltammetry, NAB dissolution voltammetry, potential step (or flow step) current transients and linear polarisation resistance. The galvanic coupling of NAB to stainless steel or copper is examined by zero resistance ammetery. The importance of using controlled flow working electrodes is illustrated by the use of a rotating disc electrode, a rotating cylinder electrode and a bimetallic (NAB/copper–nickel) rotating cylinder electrode. In addition to controlling the hydrodynamics, such electrodes allow charge transfer data to separate from those of mass transport control under mixed kinetic control. Longer term seawater immersion trials on planar coupons coupled to titanium or cupronickel are also reported. The relative contributions of erosion and corrosion attack are considered using a wall-jet electrode and the corrosion characteristics of NAB are compared to those of copper and copper–nickel in chloride media.

Introduction

Aluminium bronzes are copper-based alloys in which aluminium (<14 wt.%) is the main alloying element. In addition, some of the alloys contain iron, nickel, manganese or silicon. Both cast and wrought aluminium bronzes offer a good combination of mechanical properties and corrosion resistance. Consequently, aluminium bronzes have been widely used for decades in a variety of marine applications, including valves and fittings, ship propellers, pump castings, pump shafts, valve stems and heat exchanger waterboxes [1].

Nickel–aluminium bronze (NAB) alloys containing 9–12 wt.% aluminium with additions of up to 6 wt.% each of iron and nickel represent one of the most important groups of commercial aluminium bronzes. Increasing aluminium content results in higher strength, which is attributable to a hard face-centred cubic (fcc) phase which enhances the properties of castings as well as hot working in wrought alloys [2]. The other alloying elements also improve properties and alter microstructure. Nickel improves corrosion resistance, while iron acts as a grain refiner and increases tensile strength. Nickel also improves yield strength, and both nickel and manganese act as microstructure stabilisers [2]. Table 1 shows the composition of the main commercial alloys: CuAl10Fe5Ni5, CuAl10Ni5Fe4, CuAl11Ni6Fe6 and CuA19Ni5Fe4Mn. NAB alloys are metallurgically complex alloys with several intermetallic phases in which small variations in composition can result in the development of markedly different microstructures [2], [3], [4], which can result in wide variations in seawater corrosion resistance. The microstructures that result in optimum corrosion resistance can be obtained by controlling the composition and heat treatment [5]. For example, castings of CuA19Ni5Fe4Mn used for naval applications are given an annealing treatment at 675 °C for 2 to 6 h [2].

The microstructure of a sand cast NAB (Fig. 1a) consists of light etched areas of α-phase, which is a fcc copper-rich solid solution and dark etched martensitic regions (‘β-phase or retained β’-a high temperature phase), surrounded by lamellar eutectoid phases and a series of intermetallic κ-phases. The κI-phase is globular or rosette shaped and is reported to be iron-rich (based on Fe3Al). The κII-phase also takes the form of dendritic rosettes which are unevenly distributed at the α/β boundary and are smaller than the κI rosette. The κIII-phase can appear in either a lamellar or sometimes a coagulated or globular (degraded lamellar) form. It grows normal to the α/β boundary, as well as forming at the boundary of the large κI-phase, and is described as being nickel-rich (NiAl). The κIV-phase is a fine precipitate within the α-phase and is considered to be iron-rich. The microstructure perpendicular to the extrusion direction for the wrought NAB bar is shown in Fig. 1b). The microstructure has been influenced by the extrusion with little evidence of the β and κIII-phases. The heat treatment specified for the NES747 Part 2 British Naval standard results in the minimising or elimination of the more corrodible β-phase and an increased density of fine κ-phase precipitates in the α-phase, see Fig. 1c.

The corrosion resistance of NAB has been attributed to a protective layer, perhaps 900 to 1000 nm thick, containing both aluminium and copper oxides [6], [7]. The oxide layer is aluminium-rich adjacent to the base metal and richer in copper in the outer regions. There are also oxides of nickel and iron, together with trace amounts of copper salts and copper hydroxychlorides, e.g., Cu2(OH)3Cl and Cu(OH)Cl, which form after longer exposure times to seawater. The oxide layer adheres firmly to the base metal and consequently provides corrosion protection reducing the corrosion rate by a factor of 20–30 [7]. This protection has been attributed to both a decrease of the anodic dissolution reaction which hampers the ionic transport across the oxide layer, as well as a decrease in the rate of the cathodic reaction on the oxide layer [6]. In quiet, tidal or flowing seawater, provided the flow velocity does not exceed a certain limit, the protective film corrosion resistance continues to improve until it achieves a long-term steady-state of 0.015–0.05 mm y−1 (0.6–2.0 μA cm−2) [1], [2]. NAB is the most resistant of the readily available copper-based alloys to flow-induced corrosion. However, under conditions of service involving exposure to seawater flowing at high speed, or with a high degree of turbulence, damage can occur to the protective oxide layer, locally exposing the unprotected bare metal. NAB is vulnerable to such attack in unpolluted seawater at flow speeds in excess of 4.3 m s−1 and the degree of attack is reported to vary logarithmically with velocity: from 0.5 mm y−1 (20 μA cm−2) at 7.6 m s−1 to 0.76 mm y−1 (31 μA cm−2) at 30.5 m s−1, and even at 7.6 m s−1 corrosion rates could rise locally to 2 mm y−1 [8].

In contrast to the many studies of the corrosion of copper [9] and its alloys [10] in chloride media, there have been few studies devoted to electrochemical kinetics at NAB surfaces. Schüssler and Exner [6], [11] studied the dissolution of cast NAB in seawater but they considered a fixed velocity of electrolyte and performed a limited kinetic analysis. Kear et al. have considered both cathodic [12] and anodic [13] polarisation during NAB corrosion. In aerated seawater, the anodic dissolution of NAB shows many featured in common with the dissolution of copper, a simplified anodic reaction being the formation of the dichlorocuprous anion:Cu-e-+2Cl-CuCl2-The predominant cathodic reaction is the four electron reduction of dissolved oxygen:O2 + 2H2O + 4e  4OHAt the mixed potential, the anodic kinetics are influenced by charge transfer and mass transport contributions. Consequently, the rate and mechanism of corrosion (which are dependent on film formation) are very flow sensitive [12], [13] as is the bimetallic corrosion of NAB when coupled to copper–nickel or copper [14].

The aim of this paper is to illustrate the corrosion characteristics of NAB in seawater using both electrochemical and traditional exposure techniques. The short term potentiometric and voltammetric techniques are complemented by bimetallic coupling, erosion–corrosion studies and the monitoring of corrosion rates over extended periods of up to twelve months. It is important to consider flow conditions during corrosion studies of NAB as mass transport is an essential component of the anodic kinetics; the protective films formed are also flow sensitive. Rotating disc electrode (RDE), rotating cylinder electrode (RCE), wall-jet disc electrode (WJDE) and bimetallic rotating cylinder electrode (BRCE) geometries have been used as complementary, hydrodynamic working electrodes (Fig. 2). The smooth RDE and RCE provide well-defined surfaces for laminar and turbulent flow studies, respectively, while a twin electrode, bimetallic RCE allows galvanic studies under well-controlled turbulent flow conditions. The WJDE facilitates a comparison of NAB dissolution rates under corrosion, erosion and combined erosion–corrosion conditions using the jet impingement geometry. The longer-term corrosion rates of NAB (including galvanic effects) have also been considered using planar coupons under tidal conditions. A range of experimental time scales, electrode geometry, flow conditions and NAB alloys have been used to quantify the corrosion characteristics of NAB over a broad range of operational conditions.

Section snippets

Electrochemical measurements at RDE and RCE electrodes

Rotating electrode studies were carried out using discs or cylinders machined from wrought NAB, supplied by Stone Manganese Marine Ltd. in rod form to BS 2874: 1986: CA 104 and having the composition Cu balance, Fe 4.43, Si 0.05, Mn 0.14, Pb 0.02, Al 9.31, Ni 4.65, Mg 0.01 max, Zn 0.11 wt.%. Copper (99.9% wt./wt.) and 90-10 Cu–Ni (BS 2874: 1986: CN 102) were also used for comparison purposes. A Pine Instruments Company, model AFMSRX analytical rotator was used to control the rotation speed to

Open-circuit potential (OCP) for corrosion

The open-circuit potential vs. time characteristics of nickel–aluminium bronze were examined in both filtered and BS 3900 artificial seawaters as a function of electrode rotation rate. Fig. 3a shows the typical behaviour of the alloy in artificial seawater over a rotation speed range of 200–9500 rpm. After initial immersion, the measured potential became more negative with time until a steady potential was achieved. The time taken to reach a steady OCP always decreased with increasing electrode

Conclusions

  • (i)

    The corrosion characteristics of NAB alloys have been studied using short term electrochemical techniques, impingement studies and longer term immersion trials.

  • (ii)

    A range of electrochemical techniques have been used to examine the electrochemistry of freshly polished NAB. It was observed in this case that a potential step current transient method gave superior quantitative reproducibility in relative to linear sweep voltammetry. Overall corrosion rates measured with both Tafel extrapolation and

Acknowledgments

The authors are grateful to Dstl (Defence Science and Technology Laboratory) UK and QinetiQ-Haslar, UK for financial contributions to the research programme. The wrought NAB alloy for the electrochemical studies was supplied by Stone Manganese Ltd. The remainder of the copper alloys were supplied by Dr. Clive Tuck from Meighs Ltd. The contents of this paper include material subject to © Crown Copyright 2004 Dstl. Some of the electrochemical data in this paper was obtained during Gareth Kear’s

References (65)

  • A. Schüssler et al.

    Corros. Sci.

    (1993)
  • G. Kear et al.

    Corros. Sci.

    (2004)
  • A. Schüssler et al.

    Corros. Sci.

    (1993)
  • J.B. Zu et al.

    Wear

    (1990)
  • G. Faita et al.

    Corros. Sci.

    (1975)
  • S.R. de Sanchez et al.

    Corros. Sci.

    (1982)
  • G. Kar et al.

    Corros. Sci.

    (1973)
  • P.A. Lush et al.

    Corros. Sci.

    (1979)
  • R.J.K. Wood et al.

    Corros. Sci.

    (1990)
  • L.E. Eiselstein et al.

    Corros. Sci.

    (1983)
  • G.P. Power et al.

    Electrochim. Acta

    (1981)
  • G. Bianchi et al.

    Corros. Sci.

    (1973)
  • H.A. Videla et al.

    Int. Biodeterior. Biodegrad.

    (1992)
  • J.P. Busalmen et al.

    Electrochim. Acta

    (2002)
  • S. Ceré et al.

    J. Electroanal. Chem.

    (1999)
  • J.P. Busalmen et al.

    Electrochim. Acta

    (2002)
  • M.V. Vazquez et al.

    J. Electroanal. Chem.

    (1994)
  • F. King et al.

    J. Electroanal. Chem.

    (1995)
  • R.C. Barik et al.

    Wear

    (2005)
  • K.S. Tan et al.

    Wear

    (2003)
  • F.B. Mansfeld et al.

    Corros. Sci.

    (1994)
  • F.P. Ijsseling

    Corros. Sci.

    (1974)
  • A.H. Tuthill

    Mater. Performance

    (1987)
  • H.I. Meigh

    Cast and Wrought Aluminium Bronzes—Properties, Processes and Structure

    (2000)
  • P.R. Howell, On the Phases Microconstituents in Nickel–aluminium Bronzes, Copper Development Association Inc.,...
  • M. Cook et al.

    J. Inst. Met.

    (1951)
  • H.S. Campbell, Aluminium Bronze Corrosion Resistance Guide, Publication 80, Copper Development Association, UK, July...
  • B.G. Ateya et al.

    Corrosion

    (1994)
  • J.P. Ault, Erosion Corrosion of Nickel–aluminium Bronze in Flowing Seawater, Corrosion 95, Paper No. 281, NACE...
  • G. Kear et al.

    J. Appl. Electrochem.

    (2004)
  • G. Kear et al.

    J. Appl. Electrochem.

    (2004)
  • G. Kear et al.

    J. Appl. Electrochem.

    (2004)
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