Formation of Manganese-Containing PEO Coatings on Aluminum Alloys
351
The sparking voltage and operating one which corresponds to the formation of
oxide films in manganese-containing electrolytes are lower than in solutions based on
KOH. Such results can be explained by the inclusion of more conductive manganese
oxides in the composition of the growing oxide layer, while in alkali almost pure
dielectric alumina is formed [14].
It agrees with the previously proposed scheme and sequence of stages of mixed
conversion oxide coatings formation in alkaline electrolytes. Aluminum alloys
contain alloying components of various nature and intermetalides. This leads to
certain features of the PEO process of such materials. Some micro-additives (for
example, manganese, zinc, and silicon) can form soluble salts when the alloy is
oxidized in an alkaline electrolyte. In this case, the surface layer is depleted in these
components and it becomes possible to form a complete and perfect alumina film.
The oxidation of copper alloyed alloys (for example, D16, AK12M2MgN) in an
alkali solution is difficult due to the inability to remove copper from the surface
layers. So, PEO for such alloys is not feasible to conduct in solutions of pure alkali.
The oxidizing occurs with the formation of anodic oxide film containing copper, and
the breakdown and sparking potentials in the PEO process are not achieved.
The addition of KMnO 4 to electrolyte leads to oxidizing of impurity alloy components. This contributes to the fact that the anodic oxidation process goes into modes
of sparks and micro-arc discharges. As a result, mixed oxide films doped with Mn
grows on aluminum alloys (Fig. 14).
It should be noted that at low current densities (i ≤ 10 A/dm
2 ), sparking is almost
absent, and the thickness of the forming film changes slowly, while the content of
manganese in the coating changes slightly. To produce coatings that are enriched
with Mn, it is necessary to maintain the current density of oxidizing in the range
15–20 A/dm
2 .
The PEO of aluminum alloy AK12M2MgN with high silicon content is characterized by certain features [27, 28]. The high current densities (about 20 A/dm
2 ) are
necessary to remove the alloying components from the outer layer and for an efficient
conduction of the oxidizing.
(a)
(b)
(c)
Fig. 14 Morphology of surface of Al|Al 2 O 3 , MnO x coatings on alloys of aluminum: A99 (a), AMn
(b), D16 (c). Magnification × 500 and 1000
351
The sparking voltage and operating one which corresponds to the formation of
oxide films in manganese-containing electrolytes are lower than in solutions based on
KOH. Such results can be explained by the inclusion of more conductive manganese
oxides in the composition of the growing oxide layer, while in alkali almost pure
dielectric alumina is formed [14].
It agrees with the previously proposed scheme and sequence of stages of mixed
conversion oxide coatings formation in alkaline electrolytes. Aluminum alloys
contain alloying components of various nature and intermetalides. This leads to
certain features of the PEO process of such materials. Some micro-additives (for
example, manganese, zinc, and silicon) can form soluble salts when the alloy is
oxidized in an alkaline electrolyte. In this case, the surface layer is depleted in these
components and it becomes possible to form a complete and perfect alumina film.
The oxidation of copper alloyed alloys (for example, D16, AK12M2MgN) in an
alkali solution is difficult due to the inability to remove copper from the surface
layers. So, PEO for such alloys is not feasible to conduct in solutions of pure alkali.
The oxidizing occurs with the formation of anodic oxide film containing copper, and
the breakdown and sparking potentials in the PEO process are not achieved.
The addition of KMnO 4 to electrolyte leads to oxidizing of impurity alloy components. This contributes to the fact that the anodic oxidation process goes into modes
of sparks and micro-arc discharges. As a result, mixed oxide films doped with Mn
grows on aluminum alloys (Fig. 14).
It should be noted that at low current densities (i ≤ 10 A/dm
2 ), sparking is almost
absent, and the thickness of the forming film changes slowly, while the content of
manganese in the coating changes slightly. To produce coatings that are enriched
with Mn, it is necessary to maintain the current density of oxidizing in the range
15–20 A/dm
2 .
The PEO of aluminum alloy AK12M2MgN with high silicon content is characterized by certain features [27, 28]. The high current densities (about 20 A/dm
2 ) are
necessary to remove the alloying components from the outer layer and for an efficient
conduction of the oxidizing.
(a)
(b)
(c)
Fig. 14 Morphology of surface of Al|Al 2 O 3 , MnO x coatings on alloys of aluminum: A99 (a), AMn
(b), D16 (c). Magnification × 500 and 1000