The effect of chromate and molybdate on the corrosion inhibition of copper and steel has been investigated in two eutectics solution at room temperature. We used stationary electrochemical methods and optical microscopy for the characterization of these materials. For steel, the molybdate inhibitor appears less efficient than the chromate one; for copper the polarization curves present an anodic passivation domain only in the chromate inhibited medium. The possibilities of galvanic coupling between them were simulated. This simulation has shown that, between copper and steel, the risk is more important in chromate medium than in molybdate one

Florina CONSTANTIN

Jean-Pierre MILLET

Marioara ABRUDEANU

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
 
UTILISATION OF ELECTROCHEMICAL METHODS FOR 
CHARACTERIZATION OF SOME CORROSION INHIBITORS FOR 
STEEL AND COPPER 
 
Florina CONSTANTIN1, Marioara ABRUDEANU1, 
Jean-Pierre MILLET2 
1University of Piteşti, Materials Science and Engineering Laboratory 
2Université de Lyon, MATEIS- RI2S CNRS UMR 5510 – F69621 Villeurbanne CEDEX, France 
email: abrudeanu@upit.ro, jean-pierre.millet@insa-lyon.fr 
 
ABSTRACT 
 
The effect of chromate and molybdate on the corrosion inhibition of copper 
and steel has been investigated in two eutectics solution at room temperature. We 
used stationary electrochemical methods and optical microscopy for the 
characterization of these materials. For steel, the molybdate inhibitor appears less 
efficient than the chromate one; for copper the polarization curves present an 
anodic passivation domain only in the chromate inhibited medium. The possibilities 
of galvanic coupling between them were simulated. This simulation has shown that, 
between copper and steel, the risk is more important in chromate medium than in 
molybdate one. 
 
KEYWORDS: molybdate, chromate, corrosion inhibitors, cooper, steel 
 
1. Introduction 
 
We can controlled corrosion of metals by 
reducing the oxidation tendancy, by isolating the 
substrate from the environment or by reducing the 
aggressiveness of the environment. The use of 
corrosion inhibitors is a good method to minimize or 
to eliminate the corrosion. They can act either by 
forming a passive layer or by forming an insoluble 
compound that clogs the areas susceptible to 
corrosion or by inhibiting one stage of the redox 
system (oxidation or reduction reactions) [1]. For 
many years we used chromium for a variety of 
industrial application such as leather tanning, 
metallurgy electroplating, catalysis for corrosion 
inhibitors and wood preservatives. Under extremes 
conditions chromates protect more metals and alloys 
than any other commercial corrosion inhibitor, 
butbecause of its environmental toxicity and the risk 
to human health, chromates are being replaced by less 
toxic inhibitors, [2-5]. Attention was focused on the 
ion molybdate because of the chemical structure 
similarity and expected behavior between chromate 
ions and the elements of the VI group of periodic 
table[6-7]. Molybdate present a low toxicity and for 
that it is used to protect many materials, including 
copper and steel. The aim of this work is a 
comparative study between chromate and molybdate 
inhibitors in two eutectic solution at room 
temperature. 
2. Experimental details 
 
This study was carried out with two metals: steel 
DD11 (0.07%C, 0.2%Mn, 0.011%P, 0.011%S, 
0.009%Si, 0.029%Al) and copper DHP 99.93% 
(99.95%Cu + Ag and 0.024%P). Table 1 presents the 
mechanicals properties of these materials. These 
metals have two shapes: plate (1.2*1.2cm2) for steel 
and tube (1.5cm and Ф1.5) for copper.  
The plate samples were polished with abrasive 
paper (until granulometry 1000) and the tubes were 
filled with resin, but because of there shape, they are 
not polished, but only degreased in ultrasounds bath 
for 1-2 min, then dried with hot air. The eutectic 
solutions contain 12-13wt% NH4Cl and 27-
28wt%NaNO3. They are different by te nature of 
corrosion inhibitors, one of them contains the 
chromate and the other one contains the molybdate. 
The used volume of these solutions was 110cm3. 
Electrochemical tests were carried out using a 
classical three electrodes cell with graphite as counter 
electrode, saturated calomel electrode SCE (+0.242V 
vs. SHE) as reference electrode and the samples as 
working electrode. The potentiodynamic polarization 
curves were obtained using an EG&G potentiostat 
model 263 at a constant voltage scan rate of 
0.166mV/s. The surface morphologies were observed 
with a digital microscope Hirox KH7700. 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
Table 1. Mechanical properties of metals 
 
Mechanical properties 
Rp  Rm A50  Type of metal 
[MPa] [%] 
Steel DD11 283 383 35.2 
Cu DHP 99,93% 82 256 47.6 
 
Table 2. Values of free potential (mV/SCE) and corrosion current density (μA/cm2) 
 
Solutions 
with chromate with molybdate 
E  I  E  I  Metals 
[mV/SCE] [μA/cm2] [mV/SCE] [μA/cm2] 
Steel plate - 480 1 – 2 - 580 837 
Cu tube - 190 31.0 – 32.0 - 340 21 – 23 
 
3. Results and discusions 
 
As shown in table 2, we can observed that, for 
steel alloy, in the eutectic solution inhibited with 
chromate the values of free potential reach the 
stability value around -480 mV/SCE and in the 
eutectic solution inhibited with molybdate the values 
of free potential reach the stability value around -
580mV/SCE. For copper, in the eutectic solution 
inhibited with chromate, these values grow up to -
190mV/SCE, but when the eutectic solution contains 
molybdate inhibitor the values of free potential are 
about two times more negatives (-340mV/SCE). In 
the cae of steel, the values of corrosion current 
density calculated by QuickCalc software M352 from 
polarization curves, show between the both inhibited 
solution a difference of a factor 800, and on the 
contrary for copper theses values are approximately 
the same, wich means that a combination of both 
corrosion inhibitors is not accepted. 
The corrosion potential values deduced from the 
potentiodynamic polarization curves, (fig.1) have not 
the same magnitude in both cases; the steel plate is a 
little less noble in the solution inhibited with 
molybdate. The values of current density in the 
molybdate solution are smaller than in the chromate 
(800 times). We also observed that for steel in both 
cases (in chromate or molybdate environment) 
appears not the anodic passivation plateau. According 
to the polarization curves, in the case of copper, the 
current densities for the two environments, chromate 
and molybdate, have approximately the same values. 
At the opposite, the corrosion potential is lower in the 
case of molybdate than in the chromate environment. 
Regarding the polarization curves for the chromate 
inhibitor we can observe a plateau of anodic 
passivation. This plateau means the period necessary 
for the protective films formation. 
The optical microscopy images for all the metals 
in the two solutions (with inhibitor chromate or 
molybdate) are presented in Fig.2. These images are 
carried out after an important anodic polarization. It 
appears that in the chromate or molybdate 
environment the steel is attacked uniformly. At the 
other hand, for copper we can see in the case of 
chromate inhibitor a uniform corrosion but also an 
oxides film (green) on the surface of sample. The 
copper sample used for testing the efficiency of 
molybdate was also corroded. 
In a galvanic coupling (bimetallism or 
differential aeration) the current intensities that are 
interesting are not the corrosion current densities. The 
coupling current intensity for the galvanic coupling is 
given by the intersection of the anodic part of the less 
noble metal curve with the cathode part of the nobler 
metal curve, taken into account the area ratio. The 
anode is the less noble metal or the less aerated 
metallic zone and the cathode is the noblest metal or 
the most aerated metallic zone. The anode/cathode 
area ratio is very important, because a small anodic 
area relative to a large cathodic one leads to that the 
current density of corrosion is entirely located on a 
small area and that can be very penalizing. 
To see if there is a real risk of galvanic coupling 
between these materials in the investigated media we 
simulate these risks. We distinguished the two 
solutions (with chromate and molybdate inhibitors). 
For our study, we considered the anode and cathode 
areas are equal (1cm2). 
It can be observed that there are no really risks 
of galvanic couplings in the molybdate solution. It 
appears that, in medium with chromates, the galvanic 
coupling between steel and copper presents an 
important risk and this is much more important than 
the anode/cathode ratio areas is more little, (steel is 
the anode and copper is the cathode). 
 
 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
 
a. 
 
 
b. 
 
 
c. 
 
 
d. 
Fig.1. Evolution of open circuit potential versus time and potentiodynamic polarization curves for 
steel plate (a-b) and copper (c-d) in the eutectic solutions inhibited with chromate or molybdate. 
 
        
         
a. 
      
        
c. 
       
         
b. 
      
        
d. 
 
Fig.2. The optical microscopy images (x20) for steel (a. effect chromate inhibitor, b. Effect molybdate 
inhibitor) and for copper tube (c. effect chromate inhibitor and d. effect molybdate inhibitor). 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
 
a. 
 
b. 
Fig.3. Simulation of galvanic coupling in eutectic solution with: a. chromate inhibitor and  
b. molybdate inhibitors. 
 
4. Conclusions 
 
For the two metals the evolution of open circuit 
potential versus time varies a lot for the copper in 
molybdate environment where the potential is less 
noble than in the chromate environment. 
In the case of steel, the corrosion potentials do 
not have the same order of magnitude, the current 
densities vary widely. In chromate environment, the 
current density is slightly smaller compared with the 
molybdate environment. 
For copper, the corrosion potentials are slightly 
smaller in the case of molybdate versus the case of 
chromate. But the values of current density are 
comparable. Copper is the only metal that has 
presented a plateau of anodic passivation in chromate. 
It was found that all metals are corroded; this 
corrosion was observed after a significant anodic 
polarization. 
Even if the results obtained in molybdate 
inhibited media are not as good as those obtained with 
chromate inhibitor, nevertheless the use of molybdate 
does not seems such a bad possibility of substitution 
in the case of copper. 
 
References 
 
[1]. Zamfir, S., Vidu, R., Brinzoi, V. - Coroziunea materialelor 
metalice, ed. Didactica si pedagogica, R.A., Bucuresti, 1994; 
[2]. Landolt, D. - Corrosion et chimie de surfaces des métaux, 
Presses Polytechniques et Universitaires Romandes, CH-Lausanne ; 
ISBN 2-88074-245-5, 1993; 
[3]. Liang, C., Guo, J., Hu, X. - Effects of Mo Composites on the 
Corrosion Behaviors of Low Alloys Steel, Journal of Wuhan 
University of Technology-Mater., Sci. Ed., vol. 22, no. 1, 2007; 
[4]. Wang, J., Inhibitor effects on the anodic behaviour of iron 
electrodes, Chin. J. Oceanol. Limnol.,vol. 15, no. 2, 1997; 
[5]. Scendo, M. - Inhibition of copper corrosion in sodium nitrate 
solutions with nontoxic inhibitors, Corrosion Science, no. 50, pp. 
1584-1592, 2008; 
[6]. Hazzazi, O.A. - Corrosion inhibition studies of copper in 
highly concentrated NaCl solutions, J. Appl. Electrochem, no.37, 
pp. 933-940, 2007; 
[7]. Maximovitch, S., Barbal, G., Le Cras, F., Claudet, F. - The 
electrochemical incorporation of molybdenum in the passive layer 
of a 17%Cr ferritic stainless steel. Its influence on film stability in 
sulphuric acid and on pitting corrosion in chloride media, 
Corrosion Science, vol. 37, no. 2, pp. 271-291, 1995. 
 
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Utilisation of Electrochemical Methods for Characterization of same Corrosion Inhibitors for Steel and Copper

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Utilisation of Electrochemical Methods for Characterization of same Corrosion Inhibitors for Steel and Copper

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