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Some Properties of the Anodized Aluminium Surface Z. Bolanca, Faculty of Graphic Arts, University of Zagreb, Croatia A. Hladnik, Institute for Cellulose and Paper, Ljubljana, Slovenia Contact

SUMMARY

This work presents, how the formation conditions of the anodic oxide layer and composition, concentration and pH value of the solution which is in contact with the anodized aluminium can influence on the charge and hydrophilic ability of the surface.
The obtained results are discussed in relation to the amphoteric dissociation of the oxidized aluminium in presence of the solution and energetic situation on the interphase boundary oxide layer / solution and its close proximity.
Apart from the certain scientific contribution the investigations have the application in the field of printing technology. The applied methods showed as reliable ones for investigating the hydrophilic ability of the surface of the oxidized aluminium and could be applied for studying process on the printing form in offset printing as well as for monitoring its quality.

INTRODUCTION

Oxide layers of different properties can be formed by anodic oxidation of aluminium in dependence on the conditions of electrochemical process, firstly on the kind and temperature of electrolyte, current density, voltage and duration of anodizing (1). The conditions of the anodic layer formation influence many surface properties of aluminium, such as surface hardness, adsorption ability, electric conductivity of surface, adhesion ability etc. (2, 3). The presence of oxide layer has a very strong influence on corrosive behaviour of aluminium. The speed of aluminium corrosion does not depend only on the thickness of the oxide layer, its compactness of surface covering or crystallographic structure, but the corrosion process is also influenced by the electric charge of the surface (4).

It is known, that the charge on the surface of the oxidized aluminium is the consequence of the interaction of the oxide and the surrounding. The results of the former investigations of the anodic oxidized aluminium have shown that the surface charge depends on the oxidation process, on the after treatment of the oxide as well as on the composition and pH value of the solution, which is the oxidized aluminium in contact with (5).

In the mentioned works, was found that the hydrophilic ability of the surface of the oxidized aluminium is the function of the surface charge and it changes in dependence on the change of the mentioned factors. The phenomenon of the hydrophilic ability of the anodized aluminium surface can be explained by the interaction on the boundary phase solid/liquid, caused by adhesion existence. According to the quantitative expression, the wetting will take place when the total free surface energy is positive and when the surface energy of the solid is greater than the sum of the free surface energy of the liquid and the interphase tension between them. In other words, the wetting will appear when the adhesion between the liquid and the solid is greater than the cohesion in the liquid (6).

The influence of anions in the solution on the surface charge and the hydrophilic ability of the oxidized aluminium is interesting. The investigation results have shown that the oxalates present in KCl solution, which the oxidized aluminium is in contact with, move the value of the surface charge towards the negative values (7). For better understanding of the anion influence from the solutions on the behaviour of the surface of the oxidized metal, the ways of their bindings with the solid phase should be investigated. There are whole series of possibilities, from the relative loose adsorptive bond to the formation of the more stabile chemical compounds and formation of the crystal structure.

The interaction between the anodized aluminium and phosphate ion as well as the mixtures of the phosphates and sulphates are in the centre of attention in this work. Their influence on the surface charge and the hydrophilic properties of the anodic oxidized aluminium have been investigated.

EXPERIMENTAL

Two methods were used for investigation of electrochemical and surface processes on anodized aluminium. Zeta potential was determined by means of electrochemical method. Using the values of zeta potential and basic idea of the boundary oxide layer/electrolyte, the charge density was evaluated and the thickness of double-layer was identified with the value ae^-1 from the diffuse layer theory. The contact angle was measured by optic goniometer and Dynamic Adsorption Tester FIBRO 1100. There is the detailed description of the methods in earlier works (8).

Samples made from refined aluminium were used in investigations. The mass part of iron of 0,05% and aluminium of 99,944% was determined by chemical analysis. All samples were degreased in 10% NaOH solution heated up to 40^oC, then washed with water, neutralized in 30% HNO₃ solution and washed with water again. Anodization of aluminium was done under different conditions. In the first test series 10% solution of sulphuric acid was used as electrolyte. Anodization was performed at the temperature of 15^şC, current density of 100 Am^-2 and time of 900 s (sample 1). Anodization is exothermic process, which leads to electrolyte heating and special attention is paid to maintaining the temperature at determined level. Through strong mixing, the local overheating of electrolytes near the anode was prevented. In the second series of the tests, 4% solution of phosphoric acid was used as electrolyte. Anodization was performed at the current density of 110 Am^-2 in the period of 600 s (sample 2). Electrolyte used in the third test series was 15% solution of sulphuric acid and 2% solution of phosphoric acid at temperature of 24^şC
(sample 3).For measuring the contact angle and zeta potential, 10^-4 mol/dm³ solution of KCl with the addition of 5x10^-5mol/dm³ KH₂PO₄ (solution A), 5x10^-5mol/dm³ K₂SO₄ (solution B). Except that, the solutions obtained by mixing 5x10^-5 mol/dm³ KH₂PO₄ (solution A) and 5x10^-5mol/dm³ K₂SO₄ (solution B) in the ratio of 1:1 (solution C), in the ration of 1:9 (solution D) and in the ration of 9:1 (solution E). The solutions of different pH values were obtained by addition of HCl and NaOH:

RESULTS AND DISCUSSION

The dependence of zeta potential on pH value of the 10^-4mol/dm³ KCl solution, which is in contact with anodized aluminium, is presented in figure 1. It is visible that the dependence is linear and the slope and position of the straight-line change in dependence on the conditions of the formation of the oxide layer. The intersection of the straight line with the abscissa, which determines the isoelectric point, i.e. the point of zero charge net, shifts towards the lower pH values as follows: electrolyte at anodization of aluminium is the solution of phosphoric acid > electrolyte at anodization of aluminium is the solution of sulphuric and phosphoric acid > electrolyte at anodization of aluminium is the solution of sulphuric acid.

Fig 1: Dependence of zeta potential on pH solution that is in contact with anodized aluminium and anodization conditions

Zeta potential of the anodized aluminium sample 1 in dependence on pH and composition of the solution which it is in contact with (solutions A, B, C, D and E) is presented in figure 2. The investigated dependence is linear in all the cases. The straight lines, which present the dependence of zeta potential on pH in contact with the solutions B and D, do not intersect the abscissa and only negative values are associated with zeta potential. It can be noticed that straight lines representing the dependence of zeta potential on pH, referring to the solutions A and E which the anodized aluminium is in contact with, intersect the abscissa in the same value, i.e. they have the same isoelectric point. The intersection of the straight line with the abscissa, in case when the anodized aluminium is in contact with the solution C, has been shifted for about one and half pH unit towards the acid area in relation to the solutions A and E.

Fig 2: Dependence of zeta potential on pH and composition of the liquid phase, which is in contact with the anodized aluminium

Figure 3 presents the dependence of the contact angle of the anodized aluminium of the sample 1 in contact with the solutions A, B, C, D and E. The contact angle generally presents the quantitative expression for wetting. It is the inner angle, which is formed by the drop of liquid with the surface of the solid phase. The volume change of that drop base for the investigated period of time from 0,01 to 50 s is also given.

Fig 3: The change of the contact angle of the volume and the base of the drop of the time

As it is visible from the graphic presentation, the achieving of the value balance of contact angle depends on the composition of the liquid phase. Two types of curve changes of the contact angle with time can be noticed. In one curve type, the contact angle is decreased in the whole observed period of time in the first ten seconds very quickly. The systems of the anodized aluminium sample 1 in contact with the solutions B, D and C are associated to that type of dependence. It is characteristic for the second curve type, that very quickly in 10 seconds, the constant values of the contact angle are achieved. The systems of anodized aluminium sample 1 in contact with the solutions A and E are associated with this type of curves. It was noticed that in all investigated systems the increase of the drop base is proportional to the increase of the hydrophilic ability of the oxidized aluminium surface

In the upper part of the figure 4 the dependence of the surface charge density on pH for systems anodized aluminium sample 1 in contact with the solutions A, B, C, D and E is presented, while the belonging contact angles are presented in the lower part of the figure. It is visible from the figure that the maximum of the contact angle is in the pH area, in which the straight line of the dependence of density charge on pH intersects the abscissa. For the systems anodized aluminium sample 1 of the solutions B and D, the dependence curve of the contact angle on pH has somewhat different flow. Within the experimental conditions, there is only one part of the curve, the one in which the value of the contact angle decreases with the increase of pH. It is characteristic for these systems that the straight line representing the dependence of the charge density on pH does not intersect the abscissa.

Fig 4: Dependence of surface charge and density and contact angle on PH and composition of the liquid phase,which is in contact with the anodized aluminium

The value of the contact angle for the anodized aluminium samples 2 and 3 changes with pH of A, B, C, D and E solutions so, that with the increase of pH, the value of the contact angle increases at first and after achieving the maximum it decreases. The position of the curves and maximums on the curves is different for aluminium sample anodized in phosphoric acid sample 2 as well as in the mixture of phosphoric and sulphuric acid sample 3 in relation to the observed solution.
However, it is true that with the increase of the charge density on the surface of the anodized aluminium the contact angle has less and less value regardless of positive or negative charge. In the area of isoelectric point in which the net charge is zero, the value of the contact angle is the greatest one.

For such behaviour of the oxidized aluminium, the situation in the double layer on the boundary phase oxidized aluminium/electrolyte is responsible. The amphoteric dissociation appears on the oxidized aluminium, in presence of the solution, so that positive charge prevails in more acid medium on the surface and negative charge prevails in more alkaline medium. Depending on the surface charge the oriented adsorption of water dipole will appear, with the charge opposite to the surface charge. The presence of the ions such as sulphate and phosphate ones can essentially change the situation in the double layer, which has the consequence the change of zeta potential straight line slope in dependence on pH, i.e. on isoelectric point. Generally speaking, if the solid is in contact with the solution, double layer is formed on its surface as the consequence of interaction of the species from both phases and ions, molecules that is dipoles take part in that process.

The obtained results could be explained by energetic situation on the interphase boundary and its close proximity. Such case is presented in the diagram of energetic condition. It gives the quantitative picture of the energy system replacement during adsorption, i.e. disorption and the bond between the binding strength of the free energy and activation energy.

This profile can be influenced by the change of the energetic condition on the surface of the solid phase or in the solution close to the surface of the solid phase. In adsorption / disorption process it is necessary to master the energetic barrier which presents in principle the activation energy, and the difference of the energies of the initial and final state D G presents the affinity for adsorption.
In these investigations pH is the factor which essentially influences the density and the arrangement of the charge and the strength of the adsorption bonds, i.e. the intensity of the interaction with the solid phase. But, pH is not the only factor, which is active in the solid phase. In this case, the adsorption of the species from the solution is additionally active, which means that it is possible to achieve more complicated binding of water with solids by the adsorption of other species from the solution.
If we observe the system from the solution side that is close to the solid phase in the area in which there are disorbed molecules of the species, then this area can be influenced by the change of the composition and solution concentration.
Apart from the certain scientific contribution the investigations have the application in the field of printing technology. Such investigations of the boundary surfaces including the surface charge, manage with many processes in offset printing, which are based on phenomena bound to the interphase boundaries metal/solution, metal/printing ink and printing ink/solution.

CONCLUSION

This work presents, how the formation conditions of the anodic oxide layer and composition, concentration and pH value of the solution which is in contact with the anodized aluminium can influence on the charge and hydrophilic ability of the surface.
One of the applications of these investigations is the offset printing. If the presented considerations are applied to the offset printing form, it can be concluded that the hydrophilic ability of the non-printing areas is the consequence of the process and condition on the boundary phase in the area of double layer.
The applied investigating methods gave characteristic results. They showed as reliable ones for investigating the hydrophilic ability of the surface of the oxidized aluminium and could be applied for studying process on the printing form in offset printing as well as for monitoring its quality.

LITERATURE

1. Z. Bolanca, N. Barišic, M. Mikota, Some Surface Abilities of the Anodized Aluminium in Dependance on the Anodizing Conditions, Acta Graphica 4, (4), 1992., 193
2. Z. Bolanca, N. Barišic, S. Bolanca, Investigation of Electric and Dielectric Characterization of Anodic Barrier Layers on Aluminium, Proceedings on International Conference MATEST '97, Rovinj, 1997., 129
3. Z. Bolanca, N. Barišic, T. Bolanca, Dielectric Characteristics of the System Aluminium / Oxide Layer / Electrolyte, Proceedings in the 7^th European Conference on Non-Destructive Testing, Copenhagen, 1998., 1143
4. O.Korelic, B.Lovrecek, Z.Bolanca, 1984, Zaštitni oksidni slojevi na aluminiju - površinski naboj, Savjetovanje o dostignucima i tendencijama razvoja na podrucju zaštite materijala i industrijskog finiša, Zbornik radova str. 11, Zagreb
5. B. Lovrecek, Z. Bolanca, O. Korelic, Investigation of the Interphase Oxidized Aluminium / Electrolyte, 35^th Meet ISE, Berkeley, 1984., 876
6. P.W. Atkins, Physical Chemistry, Oxford University Press, Oxford, 1994.
7. O. Korelic, B. Lovrecek, Z.Bolanca, Die Rolle Oberflächenladung in hydrophilen Eigenschaften der Oxidierten Aluminium, Papier und Druck, 36, 1987., 177
8. Z. Bolanca, A. Hladnik, G. Perkovic, Contact Angle in Function of the Ecologically More Favourable Wetting Liquid, Proceedings, Advanced NDT Techniques, Cavtat 1999., 201

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