Electrochemical tests

Figs. 6-7 show polarisation curves of C/Al2O3/Al absorption surfaces and aluminium substrate at solutions pH 3.5(a), 5.5(a) and 5.5(b) (see Table 1). The conductivity of the solution pH 5.5(a) was too low for polarisation measurements without sulphate addition. The anodic current densities of the C/Al2O3/Al absorption surfaces and the aluminium substrate show passive behaviour in acid rainwater (pH 3.5a) in wide potential area up to -200 mV (Fig. 6a). The anodic current density of the C/Al2O3/Al surface is ca. decade lower than that of aluminium substrate probably indicating lower corrosion rate of the C/Al2O3/Al absorber surface. The current densities of the C/Al2O3/Al surface and aluminium substrate are higher in acidic rainwater at pH 3.5(a, b) than at neutral rainwater at pH 5.5(b). This indicates higher expected general corrosion rate of the C/Al2O3/Al surface and the aluminium substrate at pH 3.5 than at pH 5.5. The current densities of aluminium substrates are slightly higher than those of the C/Al2O3/Al surfaces in neutral rainwater at pH 5.5 and in acid rainwater at pH 3.5, thus indicating possibly better corrosion resistance of graphite coated aluminium compared to an aluminium substrate.

Aging time [h]

Fig. 3. Changes in solar absorptance for samples exposed to simulated acid rain immersion tests. Samples analyzed with EIS denoted as A and B.

0 20 40 60 80 100 120 140

Aging time [h]

Fig. 5. PC values for samples exposed to simulated acid rain immersion tests. Samples analyzed with EIS denoted as A and B.

Figs. 8 — 9 show the EIS impedance spectra of an C/Al2O3/Al absorption surface and an aluminium substrate at the beginning of the test, after 1 and 5 days at pH 3.5(a) and after 1 and 14 days at pH 5.5(a), respectively. The passive metal in a passive state is characterised by a wide capacitive area, i. e. linear plot in the Bode impedance diagram. The faradaic charge transfer reaction associated with corrosion gives rise to a finite resistive element at low frequencies. Therefore the impedance at low frequency area decreases as a result of the initiated corrosion. Indications of material degradation are also a decrease of the resistance element in the high frequency region and the change from a largely capacitive to a resistive behaviour. Generally, the constant impedance value in low frequency area i. e. the polarisation resistance is inversely proportional to the corrosion rate of the material.

In neutral rainwater (pH 5.5(a), Fig. 8) an C/Al2O3/Al absorber surface exhibited mostly capacitive behaviour during the 14 days of immersion. The corrosion resistance of an aluminium substrate seemed to be lower than that of the C/Al2O3/Al surface. Reason for this is unclear, but possible causes are minor failures in the nolan lacquer insulation of the samples. Generally, the corrosion probability of aluminium is lower at pH 5.5 than at pH

3.5 (Pourbaix, 1966).

At higher potentials the impedance is probably distorted due to the low conductivity of the solution (a) (Table 1). The C/Al2O3/Al samples were lightened after immersion. The impedance of an C/Al2O3/Al absorber surface between 1 and 1000 Hz (Fig. 9) decreased during the immersion test high frequency area indicating possible decrease in graphite coating quality. However, this sample did not exhibit typical optical degradation (Sample B in Figs. 3-5).

In acidic rainwater (Fig. 9) the impedance of the C/Al2O3/Al surface and the aluminium substrate in low frequency area are relatively low and therefore expected corrosion rate is higher than at pH 5.5(a). The increase in the impedance values after 1 days indicates some kind of passivation of the surfaces. However, after 5 days the impedance at low

SHAPE * MERGEFORMAT

al., 2003a) on the surface is oxidated through chemical reactions forming CO, CO2 and other compounds (Hihara and Latanision, 1994). The revealed A^O3 layer subsequently probably follows typical alumina-aluminium corrosion mechanisms. We used FTIR — spectroscopy for determining the hydration level of the absorber substrate, consisting of thicker than naturally formed heterogeneous alumina layer on 0.5 mm thick aluminium substrate of 99.5 % purity.

The combined results are shown in Figs. 3-5. All samples inside dotted boxes contain identified Al2O3 hydroxides i. e. pseudoboehmite and/or boehmite, possible other forms of hydroxides as well (e. g. bayerite and gibbsite). Detailed analyses of the results show that hydroxides are identified in all samples exposed to pH 5.5 except one sample. The most degraded samples (PC > 0.18) have absorption bands related to both pseudoboehmite and boehmite (see Fig. 5 in (Konttinen et al, 2004)), whereas the rest of the hydrated samples do not have the characteristic absorption band of pseudoboehmite.

Discussion

Total-immersion test results were compared to standard condensation test results (Konttinen and Lund, 2003) in order to determine correlation between the two methods. Samples degraded to PC=0.05 in total-immersion tests exhibit generally similar p*. as the samples degraded to PC=0.05 in standard condensation tests (for details, see (Konttinen et al, 2004)). Still, when comparing all the optical results (Figs. 3-5) we can conclude that the total-immersion method (Fig. 1) without controlled solution movement turned out not being optimal as the samples did not exhibit clearly detectable temperature and time dependencies. This method was chosen instead of e. g. rotating disk method (Magaino, 1997) because impedance spectroscopy (EIS) tests required the use of a liquid electrolyte in contact with the sample. We assumed that the gas feed and natural convection would rotate the solution inside the flask sufficiently enough for reproducible test results. It seems that this was not the case at least at pH 5.5. A controlled solution or sample movement combined with controlled temperature and gas feeding rate should be implemented in the future tests.

Semi-integrating device attached to the FTIR-spectrometer increases interference thus disturbing the exact determination of precise absorption peaks. Therefore we had to make rougher analyses of absorption bands.

Hihara and Latanision (Hihara and Latanision, 1994, pp. 251-252) noted in their study about corrosion of C/Al2O3/Al metal matrix composites (MMC) that proton reduction on graphite will polarise aluminium to noble potentials, explaining the negligible galvanic corrosion rates in de-aerated solutions for MMC. Their results are not directly comparable to ours as the MMC materials differ in structure and purpose of use from the C/Al2O3/Al absorber surfaces. Although our results for samples A-B do not generally match to those for MMC (Hihara and Latanision, 1994) with regard to aeration, it is possible that similar polarising phenomena occurs on the absorber surfaces as well under similar conditions, and may corrode the C/Al2O3/Al absorber surfaces during long-term natural exposure.

As a reference we exposed three C/Al2O3/Al absorber samples to non-aerated total — immersion at the room temperature, one at each pH. The samples exposed to pH 5.5 and pH 4.5 were hydrated approximately to the level of PC « 0.5-0.6 after 30 and 80 days, respectively. Sample exposed to pH 3.5 did not show any degradation after 160 days by visual inspection.

Conclusions

Tests reported in this paper have provided with a general picture on the degradation effect of acid rain (pH 3.5 or 4.5) and neutral rain (pH 5.5) rain on the rough C/Al2O3/Al solar absorber surfaces at 60, 80 and 99°C. In order to provide more accurate temperature — and time-dependent results the total-immersion test method used needs to be further
developed to include controlled movement of solution or sample. Similar to standard tests (draft proposal ISO/CD 12592,2) the main degradation mechanism has been found to be hydration of aluminium oxide (especially at pH 5.5). It is possible that pH 3.5 is too acidic for aluminium hydroxides to be formed, thus preventing further corrosion of the C/Al2O3/Al surface.

For aluminium/aluminium oxide containing samples FTIR-spectroscopy can be used for determining stages of hydration. In our tests we have observed similar degradation for another type commercial aluminium substrate based absorber surface as well.

According to the electrochemical measurements the corrosion rate of aluminium substrate is faster at pH 3.5 than at pH 5.5. At pH 3.5 the anodic current density of the C/Al2O3/Al surface is smaller compared to aluminium substrate. Electrochemical measurement results for aluminium substrate do not deviate significantly from the results for the C/Al2O3/Al surfaces, indicating that the electrochemical measurements measure corrosion characteristics of aluminium to a large extent. Optical degradation of C/Al2O3/Al surface is mainly due to hydration of aluminium oxide, and this phenomenon was not clearly detectable in these electrochemical measurements. With another type of EIS test system setup, it may be possible to obtain more coherent results.

All these results indicate that unglazed solar absorber surfaces based on aluminium substrate need to be well protected against rain diffusion onto the substrate in order to prevent degradation caused by hydration of aluminium oxide.

Acknowledgements

We wish to thank Mr. Mikko Mikkola for setup of the gas distribution system and Mr. Iwao Nitta for translating reference (Takahashi et al., 1987) into English.

References

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