Formation of Manganese-Containing PEO Coatings on Aluminum Alloys
335
1 Experimental
Oxide coatings were applied to the surface of commercial Al alloys. The chemical
composition of Al alloys is given in Table 1.
For PEO alkaline (KOH) solutions with addition of diphosphate P 2 O 7
4– or
manganate (VII) MnO 4
– oxygen-containing anions (oxo-anions) of varied concentration were used [14]. Oxidizing was performed at anodic polarization with direct
current density of 5…20 A/dm
2 and a voltage 120…240 V. Oxidizing was forced
cooling for keeping up the temperature of electrolytes in the interval 25…30 °C.
Voltage time dependences as well as operating parameters were fixed by MTech PGP550S connected with computer. Stages of PEO were recorded visually and refined
after graphical processing of voltage chronograms and differential dependencies.
Polarization measurements were conducted close to the corrosion potential using
potentiostat Model IPC Pro-M. Potential scan rate was varied within the interval
s = 10
−3 …10
−1 V/s.
The coated with oxides samples were analyzed by scanning electron microscope ZEISS EVO 40XVP. Images of a surface were gained by means of recording
secondary electrons (BSE) by a scan of an electron stream on sample’s surface that
allowed with high resolving ability and good contrast range to explore topography of a
surface. Software environment SmartSEM was used for processing images/Chemical
composition of a surface explored using the analysis of a characteristic roentgen spectrum filled by means of energy-dispersive spectrometer INCA Energy 350. Stimulation of X-rays was carried out bombarding radiation of samples by the bundle of
electrons with energy 15 keV.
Topography of the surface oxide coating was studied by the AFM method.
Research was carried out using NT-206 microscope with the CSC-37 contact probe.
The cantilever has a lateral resolution of the order of 3 nm [15].
A diffractometer DRON-2.0 was used for the investigation of the coatings’ structure. The studies were carried out using monochromatic Co-Kα radiation with a
wavelength of λ = 1.7902A. The phases in the coatings were identified using the
PCPDFWIN electronic file system.
Corrosion resistance of samples (area of 1 cm
2 ) coated with mixed oxides was
investigated by methods of electrode impedance spectroscopy (EIS) and polarization resistance [16]. EIS of the electrodes with the coatings were registered in the
frequency range of 10
−2 –10
6 Hz at the free corrosion potential using the electrochemical module Autolab-30 was equipped with the frequency response analyzer module
(FRA-2). The module was controlled using the Autolab 4.9 program according to
the standard procedure with the subsequent processing by the Zview 2.0 package.
The corrosion tests were performed in 0.1 M Na 2 SO 4 and 0.01 M NaOH aqueous
solutions. Graphic-analytical method was applied to simulate the electric equivalent
circuits of interfacial boundary metal–metal oxide.
The catalytic properties of samples coated with mixed oxides were established
in the model reaction of CO conversion to CO 2 . The equipment specification and
335
1 Experimental
Oxide coatings were applied to the surface of commercial Al alloys. The chemical
composition of Al alloys is given in Table 1.
For PEO alkaline (KOH) solutions with addition of diphosphate P 2 O 7
4– or
manganate (VII) MnO 4
– oxygen-containing anions (oxo-anions) of varied concentration were used [14]. Oxidizing was performed at anodic polarization with direct
current density of 5…20 A/dm
2 and a voltage 120…240 V. Oxidizing was forced
cooling for keeping up the temperature of electrolytes in the interval 25…30 °C.
Voltage time dependences as well as operating parameters were fixed by MTech PGP550S connected with computer. Stages of PEO were recorded visually and refined
after graphical processing of voltage chronograms and differential dependencies.
Polarization measurements were conducted close to the corrosion potential using
potentiostat Model IPC Pro-M. Potential scan rate was varied within the interval
s = 10
−3 …10
−1 V/s.
The coated with oxides samples were analyzed by scanning electron microscope ZEISS EVO 40XVP. Images of a surface were gained by means of recording
secondary electrons (BSE) by a scan of an electron stream on sample’s surface that
allowed with high resolving ability and good contrast range to explore topography of a
surface. Software environment SmartSEM was used for processing images/Chemical
composition of a surface explored using the analysis of a characteristic roentgen spectrum filled by means of energy-dispersive spectrometer INCA Energy 350. Stimulation of X-rays was carried out bombarding radiation of samples by the bundle of
electrons with energy 15 keV.
Topography of the surface oxide coating was studied by the AFM method.
Research was carried out using NT-206 microscope with the CSC-37 contact probe.
The cantilever has a lateral resolution of the order of 3 nm [15].
A diffractometer DRON-2.0 was used for the investigation of the coatings’ structure. The studies were carried out using monochromatic Co-Kα radiation with a
wavelength of λ = 1.7902A. The phases in the coatings were identified using the
PCPDFWIN electronic file system.
Corrosion resistance of samples (area of 1 cm
2 ) coated with mixed oxides was
investigated by methods of electrode impedance spectroscopy (EIS) and polarization resistance [16]. EIS of the electrodes with the coatings were registered in the
frequency range of 10
−2 –10
6 Hz at the free corrosion potential using the electrochemical module Autolab-30 was equipped with the frequency response analyzer module
(FRA-2). The module was controlled using the Autolab 4.9 program according to
the standard procedure with the subsequent processing by the Zview 2.0 package.
The corrosion tests were performed in 0.1 M Na 2 SO 4 and 0.01 M NaOH aqueous
solutions. Graphic-analytical method was applied to simulate the electric equivalent
circuits of interfacial boundary metal–metal oxide.
The catalytic properties of samples coated with mixed oxides were established
in the model reaction of CO conversion to CO 2 . The equipment specification and