Formation of Manganese-Containing PEO Coatings on Aluminum Alloys
355
The PEO of Al alloys in electrolytes with KMnO 4 addition leads to the producing
of mixed oxide systems Al|Al 2 O 3 ·MnO x . The incorporation of the manganese oxides
into the alumina matrix reflects on X-ray pattern (Fig. 17, red line). After that PEO Al
and Si lines on X-ray pattern disappear. In the same time, the lines of the MnO x are
manifested. We note that along with manganese dioxide MnO 2 , non-stoichiometric
oxides Mn 3 O 4 are present in the surface layers. Moreover, the higher intensity of
these oxides’ replicas is in the region of a fairly wide halo (at angles 2θ~20°). This is
due to both the amorphous structure of oxides and PEO nonequilibrium conditions.
We can conclude that the obtained data for both X-ray and AFM analysis coincides
with results for other oxide coatings on valve metals including Al alloys [14, 26].
The chemical composition, surface topography and morphology of the mixed
conversion oxide coatings on alloys of aluminum are a prerequisite for different
functional properties [5, 10].
4 Corrosion Properties and Catalytic Activity for Mixed
Manganese and Aluminum Oxides
Quantitative parameters for corrosion of samples coated with mixed oxides Al 2 O 3 ,
MnO x namely corrosion current density at open circuit potential j c and appropriative
depth index k h (Table 7) were calculated using parameter R3 which reflects the polarization resistance in the electric equivalent circuit of studied systems. The corrosion
resistance of mixed oxides is about 2–3 orders of magnitude higher as compared
with conversion coatings Al 2 O 3 obtained during traditional oxidizing in sulfate acid.
Open circuit (corrosion) potentials of samples coated with mixed oxides become
more stable and positive than that for samples with Al 2 O 3 coatings. Braking of the
corrosion process is achieved at long time exposition in model aerated solutions
(Table 3) due to low porosity of mixed oxides which does not exceed 5%.
The phase composition of mixed oxide coatings Al 2 O 3 ·MnO x, as well as branched
micro-porous globular surface with a high content of non-stoichiometric manganese
oxides will predetermine catalytic properties in heterogeneous reactions of oxygen
reduction/oxidation or destruction of the bond “oxygen—element.”
The model reaction of CO flameless oxidation on the surface of mixed oxide
Al 2 O 3 ·MnO x in the reactor at air flow starts at the temperature 345 K which is
positioned as ignition temperature. This parameter is lower as compared not only
than pure alumina but also than platinum catalyst (Table 8). The complete conversion
temperature T c which corresponds to the 100% transformation of CO to CO 2 at the
surface of samples with mixed oxides is also indicative of their higher catalytic
activity.
So the mixed oxide system, Al 2 O 3 ·MnO x exhibits outstanding catalytic activity
not inferior even to the contacts that contain precious metals. Such behavior is due to
some properties of non-stoichiometric manganese oxides. Firstly, among them are
MnO x acceptor affinity to electron reach intermediates such as double bond O = O
355
The PEO of Al alloys in electrolytes with KMnO 4 addition leads to the producing
of mixed oxide systems Al|Al 2 O 3 ·MnO x . The incorporation of the manganese oxides
into the alumina matrix reflects on X-ray pattern (Fig. 17, red line). After that PEO Al
and Si lines on X-ray pattern disappear. In the same time, the lines of the MnO x are
manifested. We note that along with manganese dioxide MnO 2 , non-stoichiometric
oxides Mn 3 O 4 are present in the surface layers. Moreover, the higher intensity of
these oxides’ replicas is in the region of a fairly wide halo (at angles 2θ~20°). This is
due to both the amorphous structure of oxides and PEO nonequilibrium conditions.
We can conclude that the obtained data for both X-ray and AFM analysis coincides
with results for other oxide coatings on valve metals including Al alloys [14, 26].
The chemical composition, surface topography and morphology of the mixed
conversion oxide coatings on alloys of aluminum are a prerequisite for different
functional properties [5, 10].
4 Corrosion Properties and Catalytic Activity for Mixed
Manganese and Aluminum Oxides
Quantitative parameters for corrosion of samples coated with mixed oxides Al 2 O 3 ,
MnO x namely corrosion current density at open circuit potential j c and appropriative
depth index k h (Table 7) were calculated using parameter R3 which reflects the polarization resistance in the electric equivalent circuit of studied systems. The corrosion
resistance of mixed oxides is about 2–3 orders of magnitude higher as compared
with conversion coatings Al 2 O 3 obtained during traditional oxidizing in sulfate acid.
Open circuit (corrosion) potentials of samples coated with mixed oxides become
more stable and positive than that for samples with Al 2 O 3 coatings. Braking of the
corrosion process is achieved at long time exposition in model aerated solutions
(Table 3) due to low porosity of mixed oxides which does not exceed 5%.
The phase composition of mixed oxide coatings Al 2 O 3 ·MnO x, as well as branched
micro-porous globular surface with a high content of non-stoichiometric manganese
oxides will predetermine catalytic properties in heterogeneous reactions of oxygen
reduction/oxidation or destruction of the bond “oxygen—element.”
The model reaction of CO flameless oxidation on the surface of mixed oxide
Al 2 O 3 ·MnO x in the reactor at air flow starts at the temperature 345 K which is
positioned as ignition temperature. This parameter is lower as compared not only
than pure alumina but also than platinum catalyst (Table 8). The complete conversion
temperature T c which corresponds to the 100% transformation of CO to CO 2 at the
surface of samples with mixed oxides is also indicative of their higher catalytic
activity.
So the mixed oxide system, Al 2 O 3 ·MnO x exhibits outstanding catalytic activity
not inferior even to the contacts that contain precious metals. Such behavior is due to
some properties of non-stoichiometric manganese oxides. Firstly, among them are
MnO x acceptor affinity to electron reach intermediates such as double bond O = O