The electrochemistry of iron, zinc and copper in thin layer electrolytes

Corrosion Science, Vol. 35, Nos 1-4, pp. 713-718, 1993 Printed in Great Britain.

0010-938X/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd

THE ELECTROCHEMISTRY OF IRON, ZINC AND COPPER IN THIN LAYER ELECTROLYTES S. H. ZHANG and S. B. LYON Corrosion and Protection Centre, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.

Abstract--An electrochemical cell, suitable for the study of electrochemistry in thin-layer electrolytes, has been designed and produced. Comprehensive electrochemical research on this electrochemical cell under various electrolyte thicknesses (100-1132/~m) has been carried out using a d.c. (potentiodynamic polarization) electrochemical technique. By this method, it is hoped to elucidate information concerning the mechanisms of reactions occurring in thin-layer electrolytes and to what degree the results obtained can be extended to research into atmospheric-corrosion phenomena.

INTRODUCTION

ATMOSPHERICcorrosion can be regarded as wet corrosion of materials under the thin

water film formed on the surface of the material. This kind of corrosion is usually considered to be of an electrochemical nature. So the general electrochemical laws which hold for metal corrosion in bulk electrolytes also hold for the special case of atmospheric corrosion. The conventional investigating method used in bulk solution electrochemistry can obviously be used in thin-layer electrolyte electrochemistry only after major modification. Although recently considerable effort~ have been made to obtain better understanding of mechanisms of atmospheric corrosion with the aid of the modified thinlayer electrolyte electrochemical c e l l 1 ~ there is still much uncertainty concerning the electrochemical nature of the electrochemical cell covered with the thin layer electrolytes, due to the difficulties and complexities in making electrochemical arrangements and measurements in very thin layer electrolytes (e.g. << 1000/~m). This work studies the cathodic 02 reduction current on Cu, Zn and Fe with a dilute electrolyte 100-1132/~m in thickness. EXPERIMENTAL

METHOD

The electrochemical cell

The configuration of the thin-layer electrolyte electrochemical cell used in this study is shown in Fig. 1. The sample material in the form of a rod 3 mm in diameter and 15 mm length was embedded in a perspex ring with epoxy resin vertically and centrally as a working electrode. Circular and straight slots were made around the working electrode on the upper face of the sample for the auxiliary electrode (a piece of platinum wire, dia. 0.5 mm) and a Luggin probe respectively. The thin electrolyte layer was formed by dropping a few droplets of de-ionized water on the surface of the sample. The uncompensated solution resistance R~, i.e. the resistance between the working electrode and the tip of the Luggin probe, is a serious problem in making electrochemical measurements due to the poor conductivity of the thin layer of the electrolyte. In order to reduce the effect of IR drop a small area of the working electrode was chosen. In addition, the relative positions of the counter electrode and working electrode in this cell permit just a fraction of the polarization current to pass through the uncompensated solution resistance R~. This also leads to a reduction in the IR drop in the electrochemical measurements. The probe and solution bridge were filled with 0.2 M Na2SO 4 solution in agar for the purpose of reducing the resistance of the reference electrode circuit, which is important for the stability and accuracy 713

714

S.H. ZHANGand S. B. LYON

Slot for the auxiliaryelectrode Slot for the Lugginprobe

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FIG. 1.

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The thin-layer electrolyte electrochemical cell.

of the measurement. This arrangement produced radial current flow at the working electrode and allowed open access of surrounding air to the top of the electrolyte meniscus.

Thicknesses of the electrolyte layer In this study, the thickness of the electrolyte layer on the surface of the working electrode is an important factor affecting the results of the experiments. The device used for the measurements of the thickness of the electrolyte layer used a needle micrometer and an ohmmeter to locate the position of the electrolyte meniscus and the sample to +5/~247m.

Experimental process The thin-layer electrolyte electrochemical cell was placed in a plastic box, the bottom of which was covered with 1 M Na2SO4 solution to keep a constant humidity in the box and prevent the change in the thickness of the water layer on the surface of the sample during test. Evaporation and condensation of the electrolyte layer on the surface of the working electrode was accomplished using a semiconductor heat pump underneath the sample by changing the polarity of the power source to provide a heating and a cooling source. Electrochemical polarization was performed using a "Ministat" precision potentiostat (H.B. Thompson) and linear sweep generator (Chemical Electronics Ltd). The corrosion potential of the sample was monitored using a 3478A multimeter (Hewlett-Packard). Potential current curves were automatically recorded on an x-y recorder (Linseis, LY17100). Unless otherwise stated, all polarization curves were obtained at a potential sweep rate of 30 mV min -1, and all potential reported refer to the saturated calomel electrode (SCE). The tests were conducted at room temperature and the exposed area of the working electrode was 0.0706 cm2. The working electrodes were wet polished with 600, 800, 1200, 4000 grade emery papers respectively, then washed in running water, degreased with acetone and further cleaned by de-ionized water prior to each electrochemical run. EXPERIMENTAL

RESULTS AND DISCUSSION

Cathodic polarization curves The cathodic polarization curves for iron, copper and zinc covered with different t h i c k n e s s l a y e r s o f w a t e r ( f r o m a b o u t 100 t o 1200 t i m ) u n d e r n o r m a l c o n d i t i o n s ( l a b o r a t o r y a i r ) a r e s h o w n i n F i g s 2, 3 a n d 4. For iron and copper, the cathodic polarization curves reveal the obvious

Fe, Zn and Cu in thin-layer electrolytes

715

characteristics of concentration polarization with a limiting diffusion current range appearing. The limiting diffusion current densities increase with a decrease of the thickness of water layer, as would be expected if the cathodic reaction rate were controlled by the reactant oxygen diffusion to the surface of the electrode. If the cathodic process is controlled only by the diffusion of the reactant to the surface, the cathodic current density is now inversely proportional to the thickness of the water layer ~ according to Fick's law:

io: = 4FDo~ Co2. 6

(1)

By substituting the value of oxygen solubility in water (2.645 x 10 -y mol cm-2) 5 and of the oxygen diffusion coefficient in water (2.10 x 10 -5 cm 2 s - l ) 6 into this equation, the following expression can be obtained. ia = 21278.25

(2)

where i is the limited diffusion current density ~uA c m - : ) and ~ the thickness of diffusion layer ~um). The dependence of the limited diffusion current [at a potential of - 0 . 9 5 V (SCE)] on the reciprocal of water layer thickness on the surface of the copper sample is shown in Fig. 5. An excellent linear relation between the limited diffusion current density and the reciprocal of the water layer thickness is observed. The slope of the curve is 22210.06, very close to the theoretical value 21278.25 calculated above. The experimental value of the oxygen diffusion coefficient (2.196 × 10 -5 cm 2 s -~ calculated from the slope is also close to the data published in the literature (2.10 × 10 -5 cm 2 s-l). 6 From the zinc cathodic polarization curves, it is not easy to determine any regular behaviour. This may be attributed to its too negative corrosion potential, which make the hydrogen evolution process easier when the electrode potential is polarized to a more negative value. If the cathodic process of zinc covered with a thin water -04

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Cathodic polarization curves for iron covered with different thickness layers of water (from 100 to 1118/~m) under normal conditions (laboratory air).

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layer is dominated by hydrogen evolution rather than oxygen diffusion (which has relation to the thickness of the water layer) it is understandable that the zinc cathodic process under the condition of a thin water layer is independent of the thickness of the water layer and not sensitive to the wet and dry cycles of the environment. Figure 6 shows the change of oxygen reduction current with the change of the thickness of the water layer covering the surface of iron, which was realized with the evaporation of the water layer on the surface by heating the sample with the semiconductor heating pump. As the time heating the sample increased, the water layer on the surface of the sample became thinner and the oxygen reduction current increased first. If the sample was heated further, the surface of the sample became -09

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dry and the oxygen reduction current declined sharply. This process may be explained as follows: in the wet condition, the sample is covered by a water layer (e.g. thickness 800/~m) and the oxygen reduction rate is controlled by the diffusion rate of oxygen through this water layer. When the water layer becomes thinner by evaporation the oxygen reduction rate increases and reaches a maximum value at a certain thickness of water layer at a nearly dry condition. When the surface of the sample is dried further the oxygen reduction rate declines sharply because there is not enough electrolyte to maintain the proper electrochemical process for the reduction of oxygen.

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The change of oxygen reduction current with the change of the thickness of water layer indicated by the time spent to heat the iron sample.

718

S.H. ZHAN6and S. B. LYON

SUMMARY In this investigation an electrochemical cell, which was covered with different thicknesses of thin electrolyte layers and can realize the process of evaporation and condensation of thin water layer on the surface, has been designed and produced. The cathodic process of materials (iron, copper and zinc) covered by a thin water layer has been studied using this specially designed electrochemical cell. The results show that when the thicknesses of thin water layer covering the surface are greater than 100 pm, the cathodic processes for iron and copper reveal a limiting diffusion current, whose value depends on the magnitude of the water thickness. The relations between the oxygen reduction current [at - 0 . 9 5 V(SCE)] and the reciprocal of the thickness of water layer shows a good agreement with Fick's diffusion law. However, when the water layer is below 100pm, as in most cases of atmospheric corrosion, the electrochemical behaviour of the cathodic process is now dominated by activation control. For zinc, due to its too negative corrosion potential the cathodic process is not sensitive to the thickness of the water layer on the surface. During the drying period for iron, the oxygen reduction current experiences a maximum value and then declines down to almost zero at a completely dry condition. REFERENCES S. G. FISHMANand C. R. CROWE,Corros. Sci. 17, 27 (1977). M. STRATMANNand H. STRECKEL,Corros. Sci. 30,689 (1990). M. STgATMANNand H. STRECKEL,Corros. Sci. 30,697 (1990). M. STRATMANNand H. STRECKEL,Corros. Sci. 30,715 (1990). 5. Annual Book of A S T M Standards, Vol. 11.01, p. 454 (1989). 6. CRC Handbook of Chemistry and Physics, P f-49. CRC Press, Boca Raton, FL (1990).

1. 2. 3. 4.