Volume 7 Paper 5
The Use of SVET for Investigating Changes in the Corrosion Mechanism Induced by Forming Galvanised Steel Samples
B.P. Wilson, D.A. Worsley, H.N. McMurray and J.R. Searle
Keywords: 3-D SVET, forming, galvanneal
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JCSE Volume 7 Paper 5
Submitted 20th June 2004, full puplication 6th December 2004
The Use Of SVET For Investigating Changes In The Corrosion Mechanism
Induced By Forming Galvanised Steel Samples
B.P. Wilson 1, D.A. Worsley
2, H.N. McMurray 2 and J.R. Searle 3
1 Max-Planck-Institut f�r Eisenforschung, Max-Planck-Str.
1, 40237 D�sseldorf, Germany, mailto2('wilson','mpie.de');
2Department of Materials Engineering,
Works, Deeside, Flintshire,
§1 A novel three dimensional scanning reference
(3-D SVET) apparatus is described, which uses a bi-functional probe to record
topographical and current density data. This apparatus is used to investigate
the effects of forming on localised current distributions as they occur at the
surface of galvannealed (Zn-Fe alloy coated) sheet
steel freely corroding in near neutral, aerated, aqueous chloride electrolyte.
On flat samples areas anodic and cathodic currents are localised but anodic and
cathodic sites occur at random over the exposed sample surface during a 24-hour
immersion period. In formed samples cathodic activity remains strongly focussed
on the convex portions of the exposed sample surface and anodic activity
remains focused on concave portions of the exposed sample surface. This effect
is ascribed to iron exposure occurring through cracks in the Zn-Fe coating
layer on the convex portion of the sample surface and to geometrically
facilitated O2 mass transport in convex regions combining to promote
cathodic O2 reduction.
§2 Keywords: 3-D SVET, forming, galvanneal
§3 The automotive industry uses approximately 35
million tons of steel every year. The majority of which is galvanised then
formed into intricate shapes and joined in a number of ways including adhesives
 and resistance spot welding
With increasing demands for corrosion resistance in automotive steels it is
desirable to determine the effect of forming and joining on subsequent
localised corrosion behaviour. The scanning vibrating electrode technique
(SVET) is a well-established electrochemical technique that allows the spatial
distribution of the often highly localised electrochemical reactions of
corrosion to be investigated.
The SVET utilises a movable microtip electrode to
detect the potential gradients produced by localised ionic current fluxes
within the solution above a corroding surface. SVET has been used to investigate
a variety of substrates including organically coated architectural steels
and corrosion pathways including galvanic [7,8,9,10,11,12]
and pitting corrosion
However, to date, the use SVET has been largely restricted to planar samples,
which obviously limits its usefulness in the context of formed and joined
materials. The failure of SRET in non-planar applications arises from the
difficulties associated with obtaining a topographical profile of the sample,
which can be subsequently utilised to accurately manoeuvre the microelectrode
probe at a fixed distance above the corroding surface. Here we describe the
development and use of a non-planar, three-dimensional SVET apparatus, which is
simple to use and which can provide additional insight into corrosion processes
occurring on formed samples of zinc coated steel for automotive use.
§4 The material we have aimed to
investigate consists of a sheet steel substrate with galvanneal
zinc-alloy coating. The galvanneal coating is
produced by first passing the steel sheet through a molten zinc bath containing
~0.15% aluminium by weight to produce a hot-dipped zinc coating of controlled
thickness. The coated sheet is then annealed at temperatures sufficient to
break down the thin aluminium-rich intermetallic
layer formed during hot dipping and produce a coating consisting entirely of
 Galvanneal is primarily used for automotive
applications in both internal and external body panels. The galvanneal
coating have improved spot weldability and paint ability
due to the presence of iron in the coating surface. The major drawback of galvanneal is the brittle nature of the alloy coating,
which can lead to cracking, �dusting� and delamination of the coating during
steel of automotive grade was obtained from Corus
Strip Steel Products. The 0.8mm gauge mild steel strip substrate had been
coated on both sides and annealed to produce a ~10�m thick iron-zinc alloy
layer. All chemicals were obtained in analytical grade purity from the Aldrich
chemical company Ltd.
§6 Flat 50mm x 50mm square coupons
were cut from a galvannealed panel, degreased with
acetone, and polished with an aqueous slurry of 0.9�m aluminium powder to
remove any surface oxide layer. Coupons were finally rinsed with distilled
water, then acetone, and allowed to dry in air. A 10mm x 10mm square area for
SVET investigation was isolated in the centre of the sample surface using
insulating lacquer (Lacomit) followed by PVC
insulating tape, i.e. the only part of the coupon left uncoated was the area to
be scanned. Domed samples were prepared by an impact former from flat coupons
using a 2.5cm diameter bull nosed die with stored impact energy of 18J. After
forming, a 20mm x 10mm rectangle was isolated on the dome contour and the
remainder of the sample was again insulated using both Lacomit
and insulating tape.
Three Dimensional SVET
§7 The custom built 3D-SVET
apparatus is shown schematically in figure 1. The position of the SVET probe is
controlled using stepper motors driving an orthogonal arrangement of linear
bearings (Time and Precision Ltd.) The SVET signal was detected and digitised
using an EG&G Model 5120 lock-in amplifier. Probe movement and data logging
are carried out under microcomputer control. The SVET probe assembly is shown
schematically in figure 2. The probe itself consists of a 125 �m diameter
platinum wire sealed in a glass tube of 250 �m external diameter, with the end
polished flat to expose a 125 �m diameter platinum microdisc.
The probe is mounted on a moving coil electromagnetic driver (vibrator)
incorporating a re-entrant magnetic core. The electromagnetic driver is housed
in a mu-metal enclosure to prevent magnetic flux
leakage. The probe assembly was used in two modes: firstly as a contact
detector (displacement mode) and secondly as an SVET electrode (SVET mode).
§9 Figure 1. Schematic representation
of the three dimensional SVET apparatus
§10 Displacement mode
§11 Vertical displacement of the
probe electrode, as shown in figure 2B, forces the moving coil further into the
re-entrant magnetic core, which has the effect of increasing coil inductance.
By making the drive coil part of an inductance bridge circuit interrogated at
900Hz it is possible to reliably detect vertical probe displacements of 2 �m.
The amplitude of the 900Hz interrogation signal is such that the amplitude of
probe vibration in displacement mode is <1 �m. Thus, acquisition of
topographical data is carried out by probe contact with the sample surface in
air, i.e. before introduction of the immersion electrolyte.
§12 Sample height at a given set of X
and Y co-ordinates is measured by lowering the probe in 2�m increments from a
known (reference) Z co-ordinate until contact is detected. On contact, the X, Y
and Z co-ordinates of the probe are stored in a surface array and the process
repeated for the next set of X and Y co-ordinates. It is this surface array
that is used to control the position of the probe during the course of
subsequent SVET scans i.e. topographical data is acquired once, in air, and
used repeatedly to control SVET scans in the presence of electrolyte. This
means that the time consuming process of surface profiling is carried out
before the corrosion experiment commences and no time penalty is incurred
during SVET scans. Furthermore, the restoring force associated with probe
displacement is minimal, which ensures that profiling does not damage the
§14 Figure 2. Schematic
representation of bi-functional probe showing inductance change on vertical
§15 SVET Mode
§16 Samples were immersed in near
neutral (pH6.5) 0.86 mol dm-3 aerated, unstirred, aqueous NaCl at 25�C. The concentration of dissolved oxygen in the
bulk solution was assumed to be constant at 2 x 10-5 mol dm-3,
the same as for air saturated water at this temperature.
The SVET probe vibration frequency was 140Hz and the vibrational
amplitude was ~40�m. All scans were conducted with the probe maintained at a
constant 100�m above the corroding surface. For the formed samples the height
values here adjusted using the active Z component of the triaxial
micromanipulator, which was controlled using the previously stored surface
§17 For flat samples, an array of 40
x 40 SVET measurements were taken per scan to produce a mesh of 1600 data
points across the 10mm x 10mm exposed sample surface. For the formed an array
of 80 x 40 SVET measurements were taken per scan to produce a mesh of 3200 data
points across the 20mm x 10mm exposed sample surface. For both flat and formed
samples the experimental period lasted for 24 hours in which the SVET apparatus
performed a scan every hour.
Results and Discussion
§18 The most probable electrochemical
processes associated with corrosion of the galvannealed
surface in near neutral, aerated, aqueous chloride electrolyte are anodic zinc
dissolution (1) and cathodic oxygen reduction (2).
§19 Figure 3 shows a series of iso-current contour plots of the normal component of
current density in the plane of scan derived from SVET scans above freely
corroding flat galvanneal samples. The lighter areas
these plots denote regions of net anodic activity and darker areas denote
regions of net cathodic activity. It may be seen from figure 3 that anodic
current distributions are highly localised, with one or more centres of anodic
activity evident in every scan. Conversely, cathodic current remains more
generalised throughout the course of the experiment. Localised corrosion
activity of this sort is typical of galvanised steel surfaces exposed to
aerated aqueous chloride electrolytes.
§20 It is also evident from figure 3
that the location of anodic activity changes over time. This behaviour is
consistent with the hypothesis that progressive dezincification of the
zinc-iron alloy coating at anodic sites will lead to their eventual passivation. Anodic activity would then resume in a fresh
(un-dezincified) area of the coating. Cross sections
through the unformed galvanneal coating layer before
and after corrosion driven de-zincification are shown in figures 4a and 4b
respectively. Nevertheless, no particular pattern was observed in the evolution
of anodic activity on the flat galvanneal surfaces.
That is to say, anodic sites appeared to appear and disappear at random over
the exposed surface and not to become concentrated in any one part of that
§22 Figure 3. Iso-current contour plots of the normal component of
current density derived from SVET scans above flat galvanneal
samples freely corroding in 0.86 mol dm-3 aerated aqueous NaCl
§24 a) Flat sample �
§26 b) Flat sample �
§28 c) Domed sample �
§30 d) Domed sample �
§31 Figure 4a-d)
Backscatter images of galvanneal in cross-section (at
§32 Figure 5 shows a series of iso-current contour plots of the vertical component of
current density passing through the 3D-SVET scan envelope, i.e. a hypothetical
surface 100 mm above the sample surface as defined by the topographical data.
For greater clarity, the current density data in figure 5 has been mapped onto
the sample surface profile but it should be noted that this is not
surface current density.
§34 Figure 5. Iso-current contour plots of the vertical component of
current density derived from 3D-SVET scans above domed galvanneal
samples freely corroding in 0.86 mol dm-3 aqueous NaCl at 20oC.
§35 It can been seen from figure 5
that, as in the case of the flat galvanneal sample,
anodic and cathodic current distributions remain localised throughout the 24
hour experimental period. Furthermore current density values observed for the
formed sample are similar to those observed in the case of the flat sample.
However, in contradistinction to the flat case, the domed galvanneal
sample exhibits a definite pattern in the evolution of anodic and cathodic
current distributions. Figure 5 shows that the convex crown of the dome remains
strongly cathodic throughout and anodic activity is concentrated on the concave
dome sides. Thus it would appear that forming has significantly influenced the
distribution of electrochemical activity on the sample surface.
§36 The results shown here are of a
preliminary nature and their explanation is necessarily speculative.
Nevertheless, two phenomena suggest themselves as likely contributors to the
observed effects. The first of these derives from the nature of the galvanneal coating itself and changed induced in its
structure by the forming process. Figures 4c and 4d show cross sections through
the convex portion of the domed galvanneal sample
before and after corrosion respectively.
§37 It may be seen from figure 4c
that cracks have become initiated in the coating, normal to the sample surface,
and that these cracks have propagated to the coating-substrate interface and in
some cases into the substrate itself. This type of cracking was mainly observed
on the convex portion of the formed surface. Iron is well known to exhibit a
lower cathodic overpotential for reaction 2 than does
zinc. Thus, increased iron exposure in convex portions of the formed sample
surface may result in enhanced cathodic oxygen reduction in these regions, as
shown schematically in figure 6A.
§39 Figure 6. A)
Schematic illustration of the types of cracking induced in the galvanneal coating by the forming process B) illustration
of the geometry of oxygen diffusion at convex and concave portion of the sample
§40 However, for purely geometric
reasons, mass transport of dissolved O2 is also likely to be more
rapid to convex portions of the formed surface than to concave portions. This
effect is illustrated in figure 6B, which shows lines of oxygen diffusion flux
normal to the sample surface. It may be seen from figure 6B that lines of flux
converge onto the convex surface but diverge onto the concave surface. Thus, it
is also possible that differential aeration may contribute to the
characteristic patterns of electrochemical activity seen in figure 5.
§41 In addition, there is the
possibility that the localised dissolution of zinc and oxygen reduction on the
exposed iron substrate leads to a increase in the levels of pH within the
region of the dome apex. This could then lead to a stabilisation of the
distribution of the anodic and cathodic areas.
§42 We have shown that the
determination of current density distributions immediately proximal to the
surface of non-planar corroding samples is possible using 3-D SVET apparatus.
3-D scanning may be facilitated by using an electromagnetically driven probe,
which may be used both as a displacement (contact) detector and SVET electrode.
Thus, topographical data obtained in air may be used to scan the SVET probe
accurately at a fixed height above the sample when immersed in electrolyte
solution, without the need for probe re-alignment.
§43 Using this apparatus we have
shown that forming may significantly affect the distribution of localised
anodic and cathodic electrochemical activity occurring on Zn-Fe alloy coated
sheet steel freely corroding in near neutral, aerated aqueous chloride
electrolytes. Anodic activity tends to become focused on concave portions of
the formed surface and cathodic activity tends to become focused on convex
portions of the formed surface.
§44 This effect is consistent with
the promotion of cathodic oxygen reduction on convex portions of the exposed
sample surface as a consequence of increased iron exposure at open cracks in
the coating layer. However, increased rates of oxygen diffusion to the convex
surface, occurring simply as a result of the geometry of the diffusion problem,
may also contribute to the effect. Additionally a pH differential across the
surface, caused by the localised nature of the zinc dissolution and oxygen
reduction, could lead to the stabilisation of the observed anodic and cathodic
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