IJUP08 - Universidade do Porto

Evaluation of primer coatings on steel subtracts by a low-cost,
fast electrochemical technique
V. Gonçalves 1, 2 , A. Mendes 1, 2 , J. Machado 3 , F. Oliveira 3 , J. Nogueira 2, 3 , H.
Aguilar Ribeiro 1
1 LEPAE – Laboratory for Process, Environmental and Energy Engineering, Department of
Chemical Engineering, Faculty of Engineering, University of Porto, Portugal.
2 Rede de Competência em Polímeros, Faculty of Engineering, University of Porto, Portugal.
3 CIN – Corporação Industrial do Norte, Maia, Portugal.
Companies producing anticorrosive coatings for steel structures often need to test newly
formulated products, to determine their actual behavior before large-scale production. In
the coating industry, the most widely spread technique for assessing anticorrosive
properties of a coating is still salt fog spray test. However, they give very subjective
information and are very time and cost demanding. Modern electrochemical techniques,
such as the electrochemical impedance spectroscopy (EIS), have made testing of the
corrosion degradation behavior of painted metal systems a relatively simple matter, but the
time to obtain an indication of the paint quality was still too long (1 week to 1 month) [1].
In the present work we propose the use of an electrochemical test, known as “AC/DC/AC
method” [1], to estimate the barrier properties of three different coatings, containing zinc
particles as a corrosion inhibitor agent, applied to steel substrates (Fig. 1). As any other
protocol designed to assess coating quality, this one includes a mechanism to stress the
coating and induce its failure; this is achieved by combining impedance results (AC) and
cathodic polarizations (the sample is cathodically polarized to generate an alkaline
environment and stimulate delamination at the metal/coating interface). In the present
study, the cathodic polarization (DC) was carried out for 2 minutes at a constant voltage of
-2 V, followed by a relaxation step to reach a new equilibrium stage at the normal
corrosion potential. The impedance of the stressed system was again evaluated. This
sequence stress/impedance was repeated six times, for about 2 hours, to assess
anticorrosive coatings performance, while similar conclusions were obtained after more
than 800 hours by salt fog spray tests. The results of this study yield a performance ranking
of the three different coatings, and show the ability of the AC/DC/AC technique to
evaluate properly and much faster the anticorrosive properties of paint coatings.
Impedance, IZI / Ω
100,000
10,000
1,000
100
10
1
1 10 100 1,000 10,000 100,000
Frequency, f / Hz
Fig. 1 – Left: Picture of the experimental set-up; Right: Level I Bode Level IIplot (impedance Level III
modulus vs.
frequency) for different coatings applied on metal substrates after exposure to electrolyte.
References:
[1] García, S.J. and Suay, J. (2006), Application of electrochemical techniques to study the effect
on the anticorrosive properties of the addition of ytterbium and erbium triflates as catalysts on a
powder epoxy network, Progress in Organic Coatings, 57, 273-281.
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