Passivation of Copper in Acid Medium C. Alonsol and P. O m Departamento de Eiectroquirnica. Facultad de Ciencias Universidad Aut6noma de Madrid, Cantoblanco, 28049 Madrid. Spain
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Coooer is a freauentlv used metal because of its " eood resistance to corrosion and mechanical properties, excellent electrical and thermic conductivity, and easy solderability with soft or strong soldering. In the emf scale i t is unreactive with respect to hydrogen and thermodynamically stable, without tendency to corrode in water and nonoxidizing acids, free from dissolved oxvgen. .. In oxidizing - acids and ionic solutions exposed to air, which form complexes of copper, the corrosion can he intensive. The potential-pH diagrams for copper as shown in Figure 1 define regions where copper is soluble notably as Cu2+, HCu02-, or Cu022- ions and where it exists as condensed phases of the pure metal or its compounds. If the p H and potential are such that the metal-environment systems exist + stahle, then copper may dissolve in a region where C U ~ is until an eauilibrium Cu2+ concentration is attained. This dissolution-is simply corrosion. Thus, while the metal-environment system exists in the region - of soluble ions, it mav he expected to corrode. If, on the other hand, it exists in the region of potential and p H where the metal is stahle (the more negative direction potential), then the metal will not corrode or will he immune to corroding. In the other regions ofnoncorrosion, the solid stable form is not the metal but rather an oxide. a hvdroxide. or a salt (passivation domain). The metal then t e n i s to becbme coated with these, which can, according to the circumstances, form on the metal either a nonporous film practically preventing all direct contact between the metal itself and the solution (in which case protection against corrosion is perfect) or a porous deposit, which only partially prevents contact between the metal and the solution (in which case the protection is only imperfect). Understood in this way, passivation thus does not necessarily imply the absence of corrosion. Experimental Approximately 200 mL of 70%phosphoric acid is placed in a 250mL beaker. The anode consists of a metallic copper strip (area = 0.82 cmZ).The anode must previously be filed, freed from grease with acetone, and washed with distilled water in order to ensure a clean surface. A strip of capper with a surface considerably larger than the anode is used as the counter-electrode. The potential of the working electrode is measured in relation to the saturated calomel electrode IHgMgzC1~lKCl(s)J in the same solution. The solution of 70%Hap01 was prepared from Merck reagents and bidistilled water (passed through a Millipare-Milliq system). The temperature was maintained at 25 OC f 0.1 OC. The measurements were takea in an aerated medium. The selected potential was applied by means of an Amel model 549 votentiostat. The intensitv readine was taken usine a Hewlett~ackardmodel R.IR,Adigital ~ultime~er.Thepotentiu~tatiecirruit used ir shown in Figure I . Results The potential of the working electrode was measured in open circuit against SCE in the solution of 70% HzPOa. The potential obtained was 0.075 V. This potential was imposed on the working electrode and
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Figure 1. PotentiaCpH equllibrlum diagram for the system copper-water at 25 OC.
I Figure 2. Patentiostatic circuit.
it was confirmed from the readout. From 0.075 V the potential was increased in intervals of 10 mV toward more anodic values with the objective of obtaining the potentiostatic curve. The polarization curve (Fig. 3) thus obtained shows three very different characteristic zones. Zone I: Increase in the intensity of current up to a critical value of 15 mA cm-l. Zonp Il: The current falls ma valueof magnitude of approaimately 13 mA rm-?during an interval of potential 1.3 V. Zone 111: From a potential of 1.600 VISCE, the current density grows progressively. Volume 64 Number 5 May 1987
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Values Used to Produce PolarIration Curve EISCE
log i
EISCE
-
Figure 3. Polarization curve of
log i
EISCE
-(mv)
109 i (ilrn~&~)
copper in 70% solution of phosphoric acid.
The values of i and E corresponding to the said curve are shown in the table. Discussion
According to the potential in an open circuit (0.075 V vs SCE) obtained from the phosphoric acid solution (pH = 0.551, the system is a t its potential next to its equilibrium ~ o t e n t i a Cu-Cu2+ l as is shown in Fieure 1. As the i m ~ o s e d potential is made more anodic, theconcentration of cu2+ increases in the ~roximitvof the electrode. On the other hand, if the (oln&xes arc.-formed, the continuouv impoveri~hnienrof Cu?' favors the dissolvinpof the metal and rherefore the corrosion. At this point a trail of blue is observed that could he due to the formation of (POa)zCurnHzO, which is soluble in an acidic medium. For apotential of 0.240 V the critical i is reached (15 mA cm-% and an increase of the imposed potential to 0.300 V involves a decrease in the ~ . implies density of the current to a value of 13mA ~ m - This the existence of a protective oxide; but, in the pH zone where the system is located, the formation of the said film accordine to Pourbaix's diagram is imorobahle. Onlv a t hieher .. DH . valurs is ir possiblr to find thr it~rmarionof 0xidt.a eurh as Cu,O (reddish) and CuO rhlack) that are thermodvnamically stable. Experimentally (Fig. 3) in the range of potential (0.300-1.600 V vs. SCE) corresponding to the passivity zone, the fall of small blackish particles is observed. The aforementioned oxides require a pH of approximately 4 for their formation. This leads one to suppose that in the base of the pores in the oxide layer, the pH is very different from the pH of the solution. In the said margin of potential, the density of the current remains constant although its value compared with the critical i does not vary greatly. This indicates that the deposit of oxides is porous, and therefore i t is not a perfect protector. Thus passivation does not necessarily mean passivity. If the anodic and cathodic potentials are less than the potential indicated by line b (Fig. 1) in relation to the equilibrium of the reaction
a reduction of dissolved oxygen given that the medium is aerated, is mainly produced. The reaction of ieduction decreases the concentration of H t and therefore the acidity of the solution. If the potential of the cathode is ereater than the uotential indicated hv line b (Fig. I), the release of 0 2 is produced in accordance with the reaction
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460
Journal of Chemical Education
The potential of the above reaction is E = 0.98 - 0.059 pH
+ 0.05914 logP(0,)
For partial pressure values of oxygen in solution between 0.03 and 1.00 atm (inclusive), the potential values can vary from 0.74 V to 0.76 V for pH = 4. This has been calculated assuming that the atmospheric pressure is 760 mm Hg. This same reaction occurs on the anode for potentials greater than 1.600 V (zone of transpassivity) provoking an increase in the density of the current. The difference between the calculated value and the ex~erimentalcould he due to the over-tension of activation fo; said reaction. Furthermore, in the trans~assivitvzone the oxides of higher oxidation number can be formed such as, a hydrated CuzOs according to
After the electrode is subjected to polarization, the area introduced into the solution remains polished with a shiny appearance. However, the nonanodized area is similar to metallic copper, providing perfect differentiation. General Bibliography Byasmas. A. de S. NACE Basic Corrosion Courss: National Assoelation of Corraion
