Volume 6 Preprint 19
Erosion corrosion of copper-10%nickel alloy revisited
T. Hodgkiess and G. Vassiliou
Keywords: Cu-Ni alloys, corrosion, hydrodynamics, saline water
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Volume 6 Paper C038
Erosion corrosion of copper-10%nickel alloy
T. Hodgkiess and G. Vassiliou
Department of Mechanical Engineering, University of Glasgow, Glasgow
Q12 8QQ, UK,
This paper describes the findings of a study of the corrosion behaviour
of copper-10% nickel-base alloy when subjected to impingement
erosion, at jet velocities between 2.4-86 m/s, by 3.5% NaCl solution at
19°C. Corrosion monitoring utilised DC polarisation techniques. After
short exposures (0.5 and 4 hours), the corrosion rates, obtained by
Tafel extrapolation, were determined as a function of impingement
velocity. The resulting corrosion-rate/velocity relationships were
complex but were more appropriately interpretable in terms of the
Reynolds number. Additionally, the corrosion rates were studied as a
function of time (up to 72 hours) at a fixed impingement velocity of
intermediate severity (17 m/s) and the corrosion rates were observed
to be cycling between high and low values. The findings have been
discussed in terms of alternating regimes of active corrosion of an unfilmed metal surface (with corrosion under mass transfer control of the
anodic reaction) and corrosion processes modified by the presence of
a corrosion product film.
Cu-Ni alloys, corrosion, hydrodynamics, saline water
The extensive use of copper-nickel alloys in marine engineering
industries is founded on their generally good durability in most marine
conditions. This property, taken together with relatively high thermal
conductivity, accounts for one of the most important applications of
these materials, i.e. for tubes in heat transfer equipment. Another
related application is for other types of pipework such as fire mains.
These applications have formed the basis of investigations of the
behaviour of copper-nickel alloys in flowing saline water over a period
of several decades. A particular issue of interest has been that of the
seawater flow conditions that can be tolerated without loss of
corrosion resistance and these studies have led to the emergence of
guidelines for upper velocity limits which have been summarised in a
recent general review of copper-nickel alloys for seawater corrosion
resistance and antifouling [#ref1]. Thus, for heat exchangers, velocity
limits of around a few metres per second are widely quoted depending
upon actual alloy composition but these figures are almost certainly
conservative even for such applications and more-so for other marine
uses of copper-nickel alloys. Powell and Michels [#ref1] quote
experiences of short-term flows of 12-15 m/s without problems in
fire mains and Glover [#ref2] reported successful performance of
90/10 Cu/Ni for ships hulls at speeds up to 19 m/s and also as
cladding on the rudder of a ship operating at 12 m/s with severe
turbulence induced by the adjacent propeller.
Despite the above-mentioned activities aimed at obtaining estimates
of "flow-rate performance envelopes" and also considerable study of
the constitution of the protective films on the surface of copper-nickel
alloys, there has been less effort devoted to the detailed mechanisms
of erosion corrosion of these materials. This situation represented
part of the impetus behind a recent research study [#ref3] into the
erosion corrosion behaviour of copper-nickel alloys under liquid
impingement conditions in saline water. Some of the findings from
the study are described and discussed in this paper.
The material, whose erosion corrosion behaviour is described herein, was
copper-10%Ni conforming to UNS:C70600 of composition: 11.4%Ni,
1.9%Fe, 0.7%Mn. Test samples, of size 20 x 20 mm, were cut from plate of
thickness 2.5 mm. Electrical connecting wires were attached to the back
faces of the samples prior to their encapsulation in epoxy resin followed by
abrasion on silicon carbide papers and diamond polishing to 5-micron
finish, washing in methanol and air-drying. The specimens were placed in
a liquid impingement rig (Fig. 1) in which the saline solution was
continuously re-circulated via a low-pressure pump and a high-pressure
pump to produce a liquid jet of 1-mm or 4-mm diameter impinging on the
specimens at 90°. Both the jet nozzle and specimen were submerged and
the nozzle-specimen distance was 5 mm The impinging velocity was
controlled by a pressure regulating valve and the velocities utilised were
2.4-86 m/s, i.e. pertinent to the practical use of copper-nickel alloys but
also extended to much higher velocities for the purposes of mechanistic
Fig 1: Schematic diagram of liquid jet impingement rig incorporating
electrochemical monitoring; A = auxiliary electrode, R = reference
electrode, W = working electrode
Experiments were carried out at 19 ± 2°C in 3.5% NaCl solution made from
analytical-grade NaCl and distilled water. Anodic and cathodic polarisation
potentiodynamic scans were undertaken during impingement at 15
mV/minute using a computer-driven potentiostat, platinum counter
electrode and a saturated calomel reference electrode (SCE). The
polarisation scans were undertaken, at a range of velocities after exposure
periods of 30 minutes and 4 hours, whilst maintaining the jet
impingement. Only a single pair of (cathodic and anodic) polarisation
scans was made on any one sample at the termination of the test.
Experiments were carried out at least in duplicate for each condition
Additionally the effect of time was studied over a period of 72h at a fixed
impingement velocity of 17 m/s with potentiodynamic sweeps conducted at
30 min, 4, 8, 48 and 72h - again involving a single pair of scans per
specimen. In order to obtain more frequent indications of the trends in
corrosion rate as a function of time, a number of 22-mV anodic scans were
undertaken on a single specimen to yield values of the polarisation
resistance, Rp, from the gradients of the linear potential/current plots.
Effect of impinging velocity
Anodic and cathodic polarisation scans, on specimens subjected to
impingement for 30 minutes from the 4-mm nozzle, are shown in Fig. 2.
There was a substantial negative shift in free corrosion potential (Ecorr) for
all velocities compared to the static conditions. The anodic polarisation
plots display active dissolution above the free corrosion potential, Ecorr.
The cathodic polarisation scans reveal a (not unexpected) large
depolarisation of the cathodic reaction under impingement conditions.
Fig. 2: Anodic (upper) and cathodic (lower) polarisation scans after 30
min. impingement with 4-mm nozzle
An objective was to obtain values of the corrosion current density, icorr,
using the conventional "Tafel extrapolation" approach, i.e. back
extrapolation of the linear portions of the E/log i plots back to Ecorr. In this
respect, it was recognised that, under the severe hydrodynamic conditions
involved in the experiments, the measured anodic polarisation current
densities, im, were affected to a significant extent by residual cathodic
currents, ic. Consequently, the measured anodic polarisation plots were
corrected graphically using the following expression for the "corrected
anodic current density", ia.
ia = i m + i c
An example of this approach is shown in Fig. 3 in which the full lines are
the measured anodic and cathodic polarisation plots (with Ecorr set to zero
on the graph for convenience of plotting), the solid circles represent the
corrected anodic current densities (using the above equation) and the
dashed line is the corrected anodic polarisation Tafel line.
Fig. 3: Measured polarisation scans, and corrected anodic plot, after 30
min. impingement at 17 m/s with 1-mm nozzle
Table 1 presents the summarised results, of the calculated corrosion
rates after a 30-minute impingement, from the entire sequence of
experiments involving the two nozzle diameters. The final column in
the table gives the corrosion rates converted to instantaneous rates of
weight loss using Faradays' laws. The results are plotted in Figs. 4 - 6
and display a rather complex dependence of corrosion rate on
impinging velocity that will be discussed later. Also, there is a
different relationship obtained from the two nozzle diameters.
A similar, but somewhat restricted, set of experiments was also carried
out after four hours liquid impingement and the summarised results
are given in Table 2.
Table 1: Corrosion rates after 30 minutes liquid impingement
Velocity, m/s Nozzle diam. Measured
icorr, µA/cm2 icorr, µA/cm2 rate, mg/h
0.5, 0.5, 0.5 0.5
4.5, 3.4, 4.5 4.13
6.2, 8.3, 9.5 8.0
icorr , µA/cm 2
Fig 4: Scatter graph of corrosion rate as a function of impingement
velocity after 30 min. with 1 mm nozzle
icorr , µA/cm 2
Fig. 5: Scatter graph of corrosion rate as a function of impingement
velocity after 30 min. with 4 mm nozzle
Corrosion rate, mg/h
Fig 6: Average values of corrosion rate as a function of impingement
velocity after 30 min.
Table 2: Corrosion rates after 4 hours liquid impingement
Velocity, m/s Nozzle diam. Measured
icorr, µA/cm2 icorr, µA/cm2 rate, mg/h
Effect of time
The effect of time was studied mainly by focusing on an impinging
velocity of 17 m/s but with more limited tests at 86 m/s; all
experiments being carried out with the 1-mm nozzle. Polarisation
plots are shown in Fig. 7 and the summarised corrosion rates,
obtained by Tafel extrapolation, are shown in Table 3.
Table 3: Corrosion rates after different periods of liquid impingement
Velocity, m/s Time, hours
icorr, µA/cm2 icorr, µA/cm2 rate, mg/h
4,5, 3,4, 4.5 4.13
Fig. 7: Anodic (upper) and cathodic (lower) polarisation scans after various
periods of impingement at 17 m/s and 86 m/s
In order to obtain more data to assist in the evaluation of trends in
corrosion rate with time, in two experiments at an impingement velocity of
17 m/s, a series of linear polarisation type monitoring exercises was
undertaken at times of 30 minutes, 2, 4, 8, 16, 24, 40, 48, and 72 hours.
The gradients, Rp, of the potential/current density plots, resulting from
the short (22 mV) anodic polarisation scans, were measured followed by
calculation of 1/Rp. It should be emphasised that the values of 1/Rp were
used simply as an approximate qualitative indication of the trends in
corrosion rate and that no attempt was made to quantify these values via
estimates of Tafel slopes (obtained from full polarisation scans in other
experiments) which would have been inappropriate in the conditions of
these experiments. The summarised data are shown in Table 4 from which
it is seen that the value of Ecorr shifted continuously in the positive
direction whereas the time dependence of the corrosion rate is apparently
rather complex. This is shown graphically in Fig. 8 which also reveals a
general correlation between the time trends of 1/Rp and of icorr
(calculated, Table 3, from Tafel extrapolation of full polarisation scans ) the only slight exception being the small reduction (< 10%) in icorr between
4 and 8 hours.
Table 4: Values of Ecorr, Rp and 1/Rp averaged form two experiment
Average 1 / Rp
(1 / kohm cm2)
icorr (µA/cm 2)
1 / Rp ( 1 / [kohm cm2])
Time ( hours)
Fig. 8: 1/Rp and icorr versus time for impingement velocity of 17 m/s
(Note: Full line refers to 1/Rp, solid-triangular points refer to icorr
calculated from full Tafel plots)
General trends in corrosion rate
As displayed in Fig. 6, the corrosion-rate/impingement-velocity
relationship appears to be rather complex and the results from the two
nozzle sizes fall into two populations. The data for both 30-minute and
four-hour exposures are re-plotted, in Figs. 9 and 10, in terms of the more
fundamental hydrodynamic parameter, the jet Reynolds number, Re,
defined as follows.
Re = vd/γ
where v = velocity (m/s) , d = characteristic length (here taken as the
diameter, in m, of the nozzle) and γ is the kinematic viscosity (taken as 1 x
10-6 m2/s for seawater at 20°C). Comparison of Figs. 6 and 9 shows that,
when plotted in terms of Re, the results from the two nozzles move closer
together and the step in the corrosion-rate/hydrodynamic- parameter
relationship is more accentuated. Moreover, the results from the four-hour
experiments (Fig. 10) provide a good indication of a single relationship
between instantaneous corrosion rate and the Reynolds number with a
distinct step in the plot at intermediate Re.
Corrosion rate, mg/h
Fig 9: Corrosion rate as a function of Reynolds number after 30 min.
Corrosion rate, mg/h
1 mm nozzle
Fig 10: Corrosion rate as a function of Reynolds number after 4 hours
Fig. 11 displays a schematic representation of the observed trends, in this
study, of instantaneous corrosion rate as a function of hydrodynamic
severity (e.g. Re) for the first few hours of exposure. A schematic curve with
some resemblance to Fig. 11 has been suggested [#refs4,5] to be
representative of the behaviour of (unspecified) copper-base alloys in
seawater but without any actual experimental examples being presented or
referenced. In terms of previous experimental demonstration of such a
complex relationship, results of the corrosion rates of Cu-10%Ni at 1-10
m/s, with rather similar trends to those displayed in Fig. 11, were reported
in a very short Russian paper [#ref6] published some 30 years ago. It is
also possible to interpret some other extremely limited data [#ref7] as
indicating a partial correspondence to Fig. 11 since the data appeared to
demonstrate increasing impingement attack of Cu-10%Ni in seawater in the
approximate range 1-3 m/s with possibly similar rates of attack at 3-5
m/s. This paper [#ref7] did not include any mechanistic interpretation of
the findings. With respect to other copper-base materials, some workers
[#ref8] using a rotating cylinder found that, at high Re (up to 37000), the
corrosion rate of copper, brass and tin-bronze was virtually independent of
velocity; these findings appear to be analogous to region B in Fig.11.
Proposed mechanisms, to account for the different regions, A, B, C of Fig.
11, are presented below.
Fig.11: Schematic representation of relationship between corrosion rate of
Cu-10%Ni and hydrodynamic severity in the conditions of this
Region A of Fig. 11
It is relevant to commence the discussion on corrosion mechanisms of Cu10%Ni in the "low-flow" situation by considering corrosion of pure copper
in saline solutions. Workers over a long period of time have either
demonstrated or argued [#refs9-12] that the corrosion of this metal,
under quiescent or mild hydrodynamic conditions is under mixed charge
transfer/diffusion control involving
Cu --> Cu+ + eand
O2 + 2H2O + 4e- -->
4(OH)as the anodic and cathodic reactions respectively and further reaction of
the product, Cu+ , of the anodic reaction: Cu+ + 2Cl- --> CuCl2- with
transport of the copper complex ion, CuCl2-, from the metal surface into
the bulk solution representing the diffusion control process. The influence
of increased velocity is to accelerate the ion transport process. It has been
postulated [#ref10] that, as the velocity increases, the balance between
diffusion and activation control shifts to the latter - this trend being
reflected as a gradual levelling-off of the corrosion-rate/velocity
It is suggested that the influence of impingement on the corrosion
behaviour of Cu-10%Ni in region A of Fig.11 is essentially along the lines
as that (summarised above) for pure copper. Thus the corrosion rate in
static and moderate hydrodynamic conditions is under diffusion control of
the anodic reaction with the important step being outward diffusion of
metal ions, or complexed ions (e.g. CuCl2-) from anodic sites on the metal
surface. The increasing corrosion rate with Re is due to accelerated
outward diffusive flux of the anodic products through a boundary layer that
decreases in thickness with increasing hydrodynamic severity.
Experimental support for the above postulated mechanism, involving
anodic control, comes, firstly, from the consistent findings in this study of
more-negative Ecorr values under impingement conditions than in static
NaCl. Additionally, as illustrated in Figs. 2 and 12, the cathodic
polarisation plots do not demonstrate concentration polarisation effects.
Even under static conditions near to Ecorr, Fig. 12 reveals that the gradient
of the cathodic polarisation plot, over a range of about 50 mV from Ecorr, is
very similar to that of the other plots relating to increased hydrodynamic
severity. This observation for Cu-10%Ni that the cathodic reaction is under
activation (rather than diffusion) control is:•
contrary to suggestions (without evidence) by one author [#ref13] that
the corrosion rate of copper-nickel alloys in flowing seawater up to
about 1 m/s is under diffusion control of the oxygen-reduction cathodic
• but is in accord with the findings and arguments of others [#refs 8-12]
that the diffusion control process for pure copper does not involve the
oxygen-reduction cathodic reaction because this reaction is under
The gradients of the anodic apparent Tafel slopes after 30 minutes and 4
hours exposure in this study were in the range 50-75 mV/decade which
are similar to values obtained in studies of corrosion of pure copper in
seawater [#ref9] and in aqueous bromide solutions [#refs 14-15] and is
close to the value (60 mV/decade) typical [#refs 14-15] of a metal
dissolution reaction that is under diffusion control of dissolution products
from the metal surface into the bulk solution. This represents further
support for the postulation of anodic diffusion control of the corrosion of
Cu-10%Ni in moderate hydrodynamic conditions.
Cathodic polarisation plots after 30 minutes impingement
(dashed line shows the gradient of the plot relating to static conditions in
the potential region immediately negative to Ecorr)
Region B of Fig. 11
There was evidence (Figs 9 and 10) in this study of a levelling-off of
the corrosion-rate/Re relationships at intermediate flow conditions and
similar trends have been observed [#refs8,10] in relation to the
corrosion of pure copper in respect to which it has been suggested
[#refs8,10] that this gradual flattening of the corrosion-rate/velocity
relation is due to a shift from predominant diffusion control to charge
transfer control. Such a transformation to pure charge transfer control
would be consistent with the corrosion rate being independent of
velocity (as in region B of Fig. 11) but it is concluded that such a
scheme does not account for the behaviour of Cu-10%Ni alloy in the
present investigation. The rationale for this assertion is as follows.
First, it is difficult to reconcile a scheme resulting in pure charge
transfer control at intermediate hydrodynamic severity with the return
to increasing corrosion rates at enhanced hydrodynamic severity
(region C of Fig. 11).
Secondly, there was evidence in the study of the production of corrosion
product films on the surface of specimens. Thin patchy films (upper
photograph in Fig. 13) were observed after impingement at Re = 45009500, i.e. around the conditions where the corrosion rate starts to become
independent of Re. At higher Re values (17000), the films were more
prominent but the grain structure of the alloy was clearly visible beneath
the film (lower photograph in Fig. 13) - indicating that the alloy was
undergoing general-surface, active corrosion, i.e. that the films were not
especially protective - as indeed the computed corrosion rates
Fig.13: Corrosion product films on surface of alloy; upper photograph, 4
hours at Re = 9500; lower photograph, 4 hours at Re = 17000;
Although the chemical composition of the films were not determined in this
investigation, this non-protective behaviour might indicate that they
consist of atacamite, Cu2(OH)3Cl, rather than the more protective Cu2O
It is thus suggested that the change in corrosion-rate/Re relation from
region A to B is associated with the formation of a film on the surface of
the alloy. The film will form when the corrosion rate has increased to such
an extent that the rate of production of copper ions and (OH)- ions have
reached sufficient magnitude to exceed the solubility of some copper-base
compound. This film is stable over the Re range of region B and its
presence leads to a change of rate control mechanism to one of transport
of anodic reaction products through the film. If the film is uniform in
coverage and thickness over the relevant Re range, the corrosion-rate/Re
profile would be as indicated by region B in Fig. 11. If, however, the film
coverage varies from partial-to-complete over a range of Re, then region B
in Fig. 11 might exhibit a slightly decreasing corrosion rate with Re; this is
perhaps a more likely scenario but was not detectable in the experimental
results (Figs. 9-10).
Region C of Fig. 11
In this hydrodynamic regime, Cu-10%Ni exhibited rapidly increasing
corrosion rate with increasing Re. The plausible explanation of this
transformation is that, as the hydrodynamic severity increases, the
film, present in conditions represented by region B on Fig. 11,
becomes unstable. Strong evidence for this situation was provided by
the absence of any films on the alloy after impingement at 86 m/s.
The evidence is that the corrosion rate in region C again becomes
under mass transfer control with the higher corrosion rates at higher
Re being due to increasing de-stabilisation of the surface film. As
stated earlier, the apparent anodic Tafel slopes were around 55-70
mV/decade, i.e. indicative of a mass transfer controlled process.
The postulation above, of the transformation to region C being
initiated by instability of surface films, is in accord with the general
suggestion by Lotz [#ref6] that a steeply rising curve like that in region
C is associated with a transition from mass transfer hampered by the
presence of corrosion product scale towards erosion of the scale.
The breakdown of corrosion product films on this copper-nickel alloy
is likely to be related to increasing hydrodynamic severity promoting
enhanced mass transfer and hence film dissolution [#ref17]. Some
time ago, the breakdown of films on copper-nickel alloys in severe
hydrodynamic conditions was attributed [#ref18] to the action of shear
stresses. This view was later modified to the suggestion of the role of
shear stress on corrosion being due to a linkage between shear stress
and mass transfer processes [#ref19] and even this influence of shear
stress on corrosion processes is disputed by Poulson [#ref17].
Effect of exposure time on corrosion processes
Thus far, the discussion has focused on the influence of hydrodynamics on
the corrosion rate after short periods (0.5, 4 hours) of exposure. It is well
known [#refs1,20] that corrosion product films on copper-nickel alloys
become more protective with the passage of time.
The discussion is now extended to the influence of time on the corrosion
behaviour under constant hydrodynamic conditions. In this study,
investigations on this aspect were undertaken largely at 17 m/s (Re =
17000) which was around the region of the change in mechanism at short
times (Figs 9 and 10).
As demonstrated in Tables 3 and 4 and Fig. 8, there was a complex
relationship between corrosion rate and time in the jet impingement
conditions with an overall impression that the corrosion rate was cycling
between lower and higher values over the 72-hour test period. There was
generally good correlation between the corrosion rates determined by Tafel
extrapolation of full polarisation scans and those indicated by the 22-mV
(Rp) polarisation scans; the only slight exception to this (see Tables 3 and 4
and Fig. 8) being between 4 and 8h where the Tafel extrpolations yielded a
very small decrease in icorr in contrast to the "step" in the 1/Rp plot. As
shown by the polarisation curves plotted both in semi-log form in Fig. 7
and in linear form (Fig. 14), the anodic polarisation curves were very similar
after 4h and 8h at 17 m/s and there was a clear additional degree of
polarisation evident in the curve after 72h compared to 48h – again in good
agreement with the trends in Fig. 8. There was also an obvious feature of a
depolarisation of the anodic reaction at 86 m/s compared to 17 m/s. Most
of the cathodic polarisation plots displayed similar gradients confirming
the expected view that the corrosion process is not under cathodic control
in these severe hydrodynamic conditions. Microscopical examination of
specimens did not reveal any distinctive differences after impingement at
17 m/s for 4, 8, 48 hours; the constant feature was of a fairly deeply
etched structure over the entire surface.
Fig. 14: Linear anodic polarisation curves at various times under impinging
velocities of 17 and 86 m/s
It is of interest to note that linear polarisation data produced by Ijsseling et
al [#ref21] exhibited somewhat similar fluctuations with time for copper10%Ni in RDE experiments conducted in seawater at Re of about 30000 and
A suggested explanation of these cycling corrosion rates involves the idea
of progressive formation and breakdown of corrosion product films on the
specimen surface. It is postulated that, at 17 m/s impinging velocity, the
corrosion rate upon initial exposure is under anodic control via diffusion of
products of the anodic reaction across a diffusion boundary layer. As
corrosion proceeds, the concentration of the anodic products at the surface
continually increases and thus produces a steadily increasing
concentration gradient across the boundary layer. This facilitates an
increasing corrosion rate with time in this initial exposure period. Fairly
quickly (within the first hour), the concentration of anodic products attains
a level corresponding to the solubility limit of some copper-base
compound that thus forms a film on the metal surface. At this point in
time, corrosion control switches from diffusion of soluble ions across a
diffusion boundary layer to diffusion across a solid film; i.e. the corrosion
rate stabilises or reduces. It is further suggested that the films grown are
irregular and there is an increasing degree of surface roughness on a
micro-scale that promotes local turbulence and hence accelerates mass
transfer processes. It is thus postulated that, after a further period of time,
the film is destabilised and the corrosion rate starts to increase again and
the next ‘cycle’ of behaviour is commenced. Some support for this notion
of the role of increasing surface roughness is provided by limited
measurements of surface roughness on Cu-10%Ni specimens obtained
using Talysurf profiling equipment and reproduced below.
Exposure time at 17 m/s
Zero (initial surfac 4 hours
Roughness average, Ra ( µm)
It should be emphasised that the corrosion rates, even at the lower values
in Tables 3 and 4, were not typical of the establishment of fully protective
films on copper-nickel alloys [#ref20]. However, the above-suggested
mechanism might account for the generally observed behaviour of this
alloy at lower velocities than the 17 m/s considered here. Thus, at lower
velocities, although there may be an extended period for the formation of
the surface film to become established, thereafter, the film might be
expected to exist for long times because it is more stable in milder
hydrodynamic conditions – perhaps for sufficient time for a more protective
film (and hence lower corrosion rates) to become established.
Influence of different hydrodynamic zones on impinged specimen
A submerged jet [#ref22] imposes a number of different hydrodynamic
regimes on the surface of a specimen that it strikes. High mass
transfer rates occur in the directly impinged region and mass transfer
rates fall off in the wall-jet region which begins at a distance of a few
nozzle diameters from the centre of the impinged specimen. The
intermediate zone between the directly-impinged and wall-jet regions
can represent a highly turbulent region – although the extent of such
elevated mass transfer rates are situation dependent [#ref22].
In this study, with the exception of the impingement velocity of 86
m/s (when severe mechanical erosion was observed in the directlyimpinged zone – see next section), the overall appearance of
specimens was essentially similar all over the specimen surface, i.e. in
all the above-summarised hydrodynamic zones. As mentioned earlier
in relation to Fig. 13, the consistent surface feature was of an etched
surface with surface films present in some conditions. Other evidence,
of similar attack over the specimen surface, was obtained from surface
profiling – see Fig. 15. This apparent insensitivity to hydrodynamic
regions on the specimen (which was also evident from measured,
very-small galvanic interactions between the central and outer zones
of an impinged specimen [#ref25]) is presumably due to the fact that,
in all conditions investigated in this research, the Cu-10%Ni alloy was
undergoing active corrosion.
Fig. 15: Surface profile on specimen after 48 hours at 17 m/s (1 mm
Relationship between corrosion rates and overall material loss during
It is appropriate to point out that, in this research study, estimates of the
summed weight loss by pure corrosion processes – measured by
electrochemical techniques described herein – were consistently lower than
the actual weight losses measured directly at the end of experiments
[#ref3]. It is of interest to note that other workers [#ref23,24] have also
reported similar differences between material losses obtained by the two
methods of measurement. In this research [#ref3], the difference, between
directly measured total weight loss and computed weight loss from
electrochemical monitoring, was found [#ref3] to be associated with
mechanical erosion and interactive synergy processes and the details of
these factors will be reported elsewhere.
Corrosion rates of copper-10%Ni-base alloy, measured in the early
stages (0,5, 4h) of exposure to impinging 3.5% NaCl solution, have
been found to have a complex dependency upon the hydrodynamic
severity with several different corrosion rate/hydrodynamic regimes.
The increasing corrosion rate, as impinging velocity or Reynolds number,
Re, increases from zero, has been rationalised in terms of mass transfer
control of the anodic reaction. This corrosion mechanism is rather similar
to that proposed in the past by other workers for the corrosion of pure
copper in aqueous halide solutions.
At intermediate values of Re, there is a transition to a situation of relative
insensitivity of corrosion rate with hydrodynamic conditions which is
attributed to the establishment of surface corrosion product films.
Even higher hydrodynamic severity leads to a return to increasing corrosion
rate with Re which is interpreted as being due to progressive breakdown of
surface films and re-initiation of mass transfer control of the anodic
At a fixed Re, of intermediate severity, the corrosion rate was observed to
cycle between high and low values as a function of exposure time and this
behaviour has been attributed to alternate sequences of formation and
breakdown of corrosion product films on the surface of the alloy.
The provision of laboratory facilities and of support for one of the
authors (G.V.), by Professor J.W. Hancock, Head of the Department of
Mechanical Engineering, University of Glasgow, is acknowledged.
!ref1 ‘Copper-nickel for seawater corrosion resistance and antifouling – a
state of the art review’, C.A. Powell and H.T. Michels, Paper No. 00627,
Corrosion/2000, Orlando, 2000.
!ref2 ‘Copper-nickel alloy for the construction of ship and boat hulls’, T.J.
Glover, Brit Corr J., 17, 4, pp155-158, 1982.
!ref3. ‘The erosion corrosion behaviour of copper-nickel alloys’, G.
Vassiliou, Ph.D Thesis, University of Glasgow, 2001.
!ref4 ‘Velocity effects in flow induced corrosion’, U. Lotz, Paper No. 27,
Corrosion/90, Baltimore, 1990.
!ref5 ‘Flow induced corrosion: 25 years of industrial research’, J. Weber,
Brit Corr J., 27, 3, pp193-199, 1992.
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