Flow influenced electrochemical corrosion of nickel aluminium bronze – Part II.
Anodic polarisation and derivation of the mixed potential
G. KEAR
1
*, B.D. BARKER
2
, K. STOKES
3
and F.C. WALSH
4
1
Building Research Association of New Zealand (BRANZ) Ltd., Private Bag 50 908, Porirua City 6220, New Zealand
2
Applied Electrochemistry Group, Centre for Chemistry, University of Portsmouth, PO1 2DT, UK
3
DSTL Winfrith, Winfrith Technology Park, Dorchester DT2 8WX, UK
4
Electrochemical Engineering Group, School of Engineering Sciences, University of Southampton, Highfield SO17
1BJ, UK
(*author for correspondence, e-mail: garethkear@branz.co.nz)
Received 12 November 2003; accepted in revised form 13 July 2004
Key words: flow-enhanced corrosion, mass transfer, nickel aluminium bronze, rotating cylinder electrode, rotating
disc electrode, seawater
Abstract
The mixed charge and mass transfer influenced anodic current response of CA 104 nickel aluminium bronze (NAB)
is presented as a function of both laminar (rotating disc electrode) and fully turbulent (rotating cylinder electrode)
fluid flow. At low values of positive polarisation, the overall behaviour of the freshly polished material in filtered
and artificial seawaters is closely related to that of unalloyed copper. The primary anodic reaction in this case is the
selective dissolution of the copper component via a cuprous di-chloride complex anion. At large positive
polarisation, the solid solution mole fraction for the production of a discrete film of protective alumina (Al
2
O
3
)is
examined as a function of Reynolds number and discussed in terms of a new mechanism for the passivation NAB in
seawater. The polarisation data is used to replicate experimental Reynolds number dependent, corrosion potentials
and corrosion current densities over a wide range of electrode angular velocities.
1. Introduction
The addition of aluminium increases the corrosion
resistance of copper in seawater, sulphuric acid and
general salt solutions. The alloying element also pro-
vides good wear properties and resistance to high
temperature oxidation [1]. The corrosion resistance of
both aluminium bronze and nickel aluminium bronze
(NAB) alloys has been attributed to a sustainable
protective layer of alumina, which builds up quickly
on the alloy surface post-exposure to the corrosive
environment [2, 3]. Additions of nickel and iron enable
greater amounts of aluminium to be present in the alloy
(9–11%) before chemically and mechanically detrimen-
tal Cu
9
Al
4
phases are produced [1, 4].
The majority of literature on NAB corrosion has
centred upon marine cavitation damage [5] exposure
tests [6–8] and case histories [9]. From this work, it is
clear that the dissolution rate of the alloy can exhibit the
flow rate dependence commonly observed with other
copper-based alloys [10, 11]. General weight loss derived,
dissolution rates in NAB seawater pipe flow can range
from 0.05 mm y
)1
(2 lAcm
)2
) under static conditions
up to 1 mm y
)1
(36 lAcm
)2
) within turbulent and
impinging fluid regimes.
NAB linear polarisation resistance (LPR) measure-
ments made by Schussler and Exner in artificial seawater
[2, 3] were relatively high (5–10 lAcm
)2
) although
dissolution rates decreased with time of exposure to
reach around 0.5 lAcm
)2
. A mean anodic Tafel slope of
0.064 V decade
)1
was calculated for freshly polished
surfaces and the number of electrons exchanged in the
rate-determining steps of both reactions was determined
as 1. Schussler and Exner also proposed that after a
potential (E) step to )0.190 V vs saturated calomel
electrode (SCE), corresponding to a polarisation of
approximately +50 to 100 mV, an alumina-based,
protective film formed. Film formation corresponded
with a notable decrease in the apparent flow dependence
of the current response.
In Part I of this series of papers, the cathodic charac-
teristics of NAB were discussed [12]. In this work, the
electrochemical characteristics of the anodic dissolution
of NAB are examined using the rotating disc electrode
(RDE) and the rotating cylinder electrode (RCE) and
used in the derivation of the corrosion potential (E
corr
)or
mixed potential. The hitherto unexamined dependency of
the overall corrosion rate on fluid flow conditions will
also be determined using the high reproducible laminar
and turbulent fluid flow regimes resent at the RDE and
Journal of Applied Electrochemistry 34: 1241–1248, 2004.
1241
Ó
2004 Kluwer Academic Publishers. Printed in the Netherlands.