Application note #25

Multi Pitting Corrosion

I- Introduction
- 4 working electrodes made of steel,
The effect of the corrosion in our daily life can
- Ag/AgCl electrode as a reference
be underlined by its cost for the society. For an
electrode,
industrial country, the cost is estimated at 3-
- platinum wire as a counter electrode.
4% of gross domestic product.
Most metals are sensitive to corrosion in the
presence of oxygen and humidity. The general
equation of this oxidative process is described
as follows:
M → Mz+ + z e-
Corrosion is a general term which generalizes
various kinds of processes, such as:
- uniform corrosion: corrosive attack on the
entire surface area,
- crevice corrosion: localized corrosive
attack caused by oxygen,
- galvanic corrosion: corrosion caused by
two different materials in contact,
- pitting corrosion: localized corrosive attack
caused by aggressive anion.
In this note, only the pitting corrosion in
presence of chloride is investigated. For this
purpose, Linear Polarization (LP) and Fig. 1: N'Stat Box.
Multielectrode Potentiodynamic Pitting (MPP)
are performed.
II- Experimental conditions
Investigations are carried out with a VMP3 and
EC-Lab® software with 3% NaCl at room
temperature in aerated water. Before
measurements, the electrodes were polished
for 5 minutes using the PK-3 Electrode
Polishing kit and rinsed by distilled water and Fig. 2: “CE to ground connection” in
alcohol. “Advanced Settings” window.
A multi-working electrode set-up is required. In Note: All settings and raw data files presented
order to overcome the connection complexity hereafter are available in the Data Sample
due to the multielectrode investigation, N'Stat folder of EC-Lab® Software as LP_03.mpr,
Box is used (Fig. 1). Note that the utilization of LP_04.mpr, LP_05.mpr, LP_12.mpr,
such a device requires the “CE to ground” MPP_03.mpr, MPP_04.mpr, MPP_05.mpr,
connection (Fig. 2) which is described in and MPP_12.mpr,
technical note #23 [1]. Electrodes are as
follows:

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Web: www.bio-logic.info
III- Results LP_12.mpr
<I>, log spacing vs. Ew e
LP_01.mpr LP_03.mpr LP_04.mpr #

III-1 LP investigations 1e0

LP investigations are performed in the anodic

1e-1
area using slow scan rate (Fig. 3) in order to
be under steady-state condition [2]. Moreover, 1e-2

<I>/mA, log spacing

it is important to note that the E range is
1e-3
reduced from -2 V to 2 V in order to optimize
the potential resolution to 100 µV. Measured 1e-4

current is averaged in order to reduce noise.

1e-5

1e-6

1e-7

-1.5 -1 -0.5
Ewe/V

Fig. 4: Four-electrode LP curves.

An example of data determination is shown in

Fig. 5. All results are summarized in Table 1.
log ( <I> ) vs. Ew e
LP_03.mpr # LP_03_tafel.mpp

-1e0

-2e0
log ( <I>/mA )

-3e0

Fig. 3: “Parameters Settings” window for LP -4e0 Ep = -742 mV

measurement.
-5e0
From LP curves (Fig. 4), it is possible to
-6e0
determine the corrosion potential (Ecorr) and
other related parameters, such as corrosion -7e0
current (Icorr), cathodic (βc) and anodic (βc)
-8e0 Ecorr = -1,041 mV
Tafel constant and corrosion rate with the -1.5 -1 -0.5
“Tafel Fit” analysis tool. Moreover, the pitting Ewe/V
potential (Ep) can be directly determined by
tracking the cursor on the graph and by right
clicking on the pitting potential (Fig. 5).

Analysis of Table 1 shows similar Ecorr for the

four electrodes included between -1,152 and
-1,000 mV vs. Ag/AgCl with -1,078 mV as
mean. Concerning Ep, values are included
between -760 and -740 mV vs. Ag/AgCl with
an average of -747 mV vs. Ag/AgCl.

Fig. 5: LP curves of the electrode number 3 and

its corresponding “Tafel Fit” window.

Bio-Logic Science Instruments, 1 rue de l'Europe, F-38640 Claix - tel: +33 476 98 68 31 – Fax: +33 476 98 69 09 2
Web: www.bio-logic.info
 Table 1: Data resulting from LP investigations MPP_12.mpr
<I>, log spacing vs. Ew e
MPP_01.mpr MPP_03.mpr MPP_04.mpr #

(Potentials are given in mV vs. Ag/AgCl).

1e-1
Electrode 1 3 4 12
1e-2
Ecorr/mV -1,152 -1,035 -1,124 -1,000
1e-3

<I>/mA, log spacing

Icorr/nA 0.12 0.12 0.15 0.13
1e-4
βc/mV 265 306 290 323
1e-5
βa/mV 362 311 384 328
1e-6
Corrosion -3 -3 -3 -3
0.13x10 0.12x10 0.15x10 0.13x10
rate /mmpy 1e-7

Ep/mV -760 -742 -746 -740

1e-8

-1.5 -1
Ewe/V
III-2 MPP investigations and Statistics
Fig. 7: MPP curves of the four electrodes.
Parameters of MPP investigations are similar
to those of LP investigations (Fig. 6). Ep and Ecorr are determined simultaneously via
According to the results of LP investigations, the “MultiPitting Statistics” function (Fig. 8).
Ip, which is the limiting current, is set to 50 µA. Note that Eoc of the “Multi Pitting Statistics”
corresponds to Ecorr. Ep and Ecorr computed
thank to this statistics tool, are in agreement
with those obtained with Tafel Fit from LP
results. Indeed, Ecorr averages are -1,078 mV
and -1,202 mV vs. Ag/AgCl for LP and MPP
investigations respectively, and Ep averages
are -747 mV and -716 mV vs. Ag/AgCl for LP
and MPP investigations respectively.

Fig. 6: “Parameters Settings” window for MPP

measurements.

MPP curves (Fig. 7) display the current shift

due to the pitting process.

Fig. 8: “Multi Pitting Statistics” window in

®
“Analysis” menu in EC-Lab software.

Bio-Logic Science Instruments, 1 rue de l'Europe, F-38640 Claix - tel: +33 476 98 68 31 – Fax: +33 476 98 69 09 3
Web: www.bio-logic.info
IV- Conclusion References
This note exhibits some corrosion techniques [1] Technical Notes #23, http://www.bio-
(LP and MPP) and corresponding analysis logic.info/potentiostat/notes.html
processes (“Tafel Fit” and “MultiPitting [2] Traité des Matériaux, Vol. 12 Corrosion et
Statistics”). These tools are complementary. chimie de surfaces des métaux, D. Landolt, ed.
Presses Polytechniques et Universitaire
Indeed, “Tafel Fit” allows the user to obtain the
Romandes, 2003.
corrosion rate and other typical corrosion
parameters, whereas “MultiPitting Statistics” [3] Cinétique électrochimique, J.-P. Diard, B. Le
Gorrec, C. Montella, ed., Hermann (Paris) 1996.
allows the user to compute data
simultaneously from several data files.
Other investigations can be carried out with
EC-Lab® software such as Critical Pitting
Temperature with a thermostat controlled via
EC-Lab® software.

Bio-Logic Science Instruments, 1 rue de l'Europe, F-38640 Claix - tel: +33 476 98 68 31 – Fax: +33 476 98 69 09 4
Web: www.bio-logic.info