plasma electrolytic oxidation of magnesium alloys for automotive applications

116 Pages·2014·22.25 MB·English

PLASMA ELECTROLYTIC OXIDATION OF
MAGNESIUM ALLOYS FOR AUTOMOTIVE
APPLICATIONS

A thesis submitted to The University of Manchester for the degree of
Master of Philosophy
in the Faculty of Engineering and Physical Sciences

2013

by MILENA URBAN

SCHOOL OF MATERIALS
CONTENT
List of Figures ................................................................................................. 4
List of Tables .................................................................................................. 9
Abstract......................................................................................................... 10
Declaration .................................................................................................... 11
Intellectual Property Statement ...................................................................... 12
Acknowledgements ....................................................................................... 13
1 Introduction ..................................................................................... 14
2 Literature review .............................................................................. 16
2.1 Magnesium and magnesium alloys ................................................... 16
2.1.1 Classification ................................................................................... 16
2.1.2 Applications ..................................................................................... 17
2.2 Plasma electrolytic oxidation ........................................................... 19
2.2.1 Introduction ..................................................................................... 19
2.2.2 The microdischarge models .............................................................. 20
2.2.3 Microdischarge characteristics ......................................................... 22
2.2.4 Anodising stages during PEO ........................................................... 24
2.2.5 Phase formation during PEO ............................................................ 25
2.2.6 The key elements influencing PEO process ...................................... 26
2.2.7 Power supply ................................................................................... 26
2.2.8 Substrate .......................................................................................... 28
2.2.9 Solution ........................................................................................... 29
2.3 Characterisation of PEO coatings on magnesium alloys ................... 31
2.3.1 Coatings morphology ....................................................................... 31
2.3.2 Porosity ........................................................................................... 33
2.3.3 The growth rate and thickness .......................................................... 35
2.4 Properties of the PEO coatings ......................................................... 37
2.4.1 Mechanical properties ...................................................................... 37
2.4.2 Corrosion resistance ......................................................................... 38
3 Research methodology ..................................................................... 41
3.1 Materials .......................................................................................... 41
3.2 Sample preparation .......................................................................... 42
3.3 PEO process..................................................................................... 42
3.4 Sol-gel ............................................................................................. 44
3.5 Characterisation techniques .............................................................. 47
3.5.1 SEM/EDX ....................................................................................... 47
3.5.2 XRD ................................................................................................ 48
3.5.3 Electrochemical measurements ........................................................ 49
3.5.4 Potentiodynamic polarization ........................................................... 50
3.5.5 Electrochemical noise measurements ............................................... 50
4 Results ............................................................................................. 52
4.1 Materials characterization ................................................................ 52
4.1.1 Elektron 21 ...................................................................................... 52
2
4.1.2 AZ31 ............................................................................................... 55
4.2 Optimisation of the PEO process parameters .................................... 56
4.2.1 The influence on different values of current density ......................... 56
4.2.2 The influence of different charges .................................................... 67
4.3 Investigation of the coatings properties ............................................ 77
4.3.1 XRD ................................................................................................ 77
4.3.2 Corrosion resistance ......................................................................... 79
4.4 Sol-gel treatment .............................................................................. 87
4.4.1 Coatings morphology ....................................................................... 88
4.4.2 Corrosion resistance ......................................................................... 92
4.4.3 Electrochemical noise measurements ............................................... 93
4.5 AZ31 ............................................................................................... 95
4.5.1 Voltage-time responses .................................................................... 96
4.5.2 Microstructure ................................................................................. 97
4.5.3 XRD ................................................................................................ 99
4.5.4 Corrosion test ................................................................................... 99
5 Discussion ..................................................................................... 101
5.1 The influence of a solution composition on the PEO process .......... 101
5.2 The influence of a current density on the PEO process and coatings101
5.3 The influence of a time on the PEO coatings .................................. 102
5.4 The PEO coatings on Elektron 21................................................... 103
5.5 Sol-gel treatment ............................................................................ 103
5.6 The PEO coating on AZ31 ............................................................. 104
6 Conclusions ................................................................................... 105
7 Future work ................................................................................... 106
8 References ..................................................................................... 107

Final word count: 27 277
3
List of Figures
Fig. 1. Examples of automotive components made from magnesium: (a) steering wheel
core, (b) seat frame, (c) rear transfer case, (d) cam cover [5]. ......................... 18
Fig. 2. The typical arrangement of equipment for PEO, (1) window, (2) mixer,
(3) connecting wires, (4) ventilation, (5) grounded case, (6) power supply unit,
(7) work piece, (8) cooling system, (9) bath, (10) insulating plate [11]. ......... 19
Fig. 3. The two kinds of a current-voltage profiles for the plasma electrolytic process,
discharge phenomena takes place (a) in the near-electrode area, (b) in the oxide
film on the electrode surface [11]. ................................................................. 21
Fig. 4. Schematic illustration of the discharge models for the PEO process for an Al
sample [19]. .................................................................................................. 22
Fig. 5. A voltage-time response during an anodising under a current control mode [6].
...................................................................................................................... 25
Fig. 6. SEM micrographs of cross section of the coating on MA8 magnesium alloy
formed in an electrolyte containing 15 g/l of Na SiO ·5H O and 5 g/l of NaF
2 3 2
under (a) a unipolar mode and (b) a bipolar PEO mode [28]. ......................... 27
Fig. 7. SEM micrographs of the plan view and cross sections of PEO coatings formed
in (a,c) 10 g/L Na SiO + 1 g/L KOH, (b,d) 10 g/L Na PO + 1 g/L KOH,
2 3 3 4
under a constant current density 60 mA/cm2 [58]. .......................................... 31
Fig. 8. Scanning electron micrograph of the AZ91D and X-ray Si elemental map after
PEO at 650mA/cm2 for 900s [23]. ................................................................. 32
Fig. 9. Cross-section TEM images of microstructure of the dense layer near the
coating/metal interface (left) and porous top layer (right) [62]. ...................... 33
Fig. 10. Dependence of (1) the specific electrical strength, (2) chemical resistance and
(3) porosity against initial current density for coatings formed by an AC PEO
process on Mg alloys during 60 min [38]. ...................................................... 35
Fig. 11. Schematic diagram of dimension changes after PEO (a) and variation of the
proportion of coating thickness growing inwards and outwards (b) [32]. ....... 36
Fig. 12. The average hardness (a) and wear rate (b) of pure magnesium (99.96%)
without and with PEO coatings produced with different current densities for
30 min in a silicate or phosphate solution [56]. .............................................. 38
Fig. 13. Schematic representation of the microstructure (a) and the corrosion (b) of a
coated on magnesium alloy [6]. ..................................................................... 39
Fig. 14. OCP versus immersion time in 0.1 M NaCl solution; (a) PEO coating in
silicate solution, (b) PEO coating in phosphate solution [54]. ........................ 40
Fig. 15. The PEO set-up: (1) voltage attenuator, (2) function generator, (3) SCXI signal
conditioning block, (4) AC power supply. ..................................................... 43
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Fig. 16. The PEO set-up – the double-walled glass beaker: (1) working electrode
(sample), (2) counter electrode (stainless steel plate), (3) magnetic stirrer. ..... 44
Fig. 17. Schematic representation of the emission processes resulting from electron
bombardment [89]. ........................................................................................ 48
Fig. 18. Schematic representation of Bragg-Brentano diffractometer [88]. ................. 49
Fig. 19. Schematic representation of an electrochemical noise measurement
configuration [91].......................................................................................... 51
Fig. 20. Optical images of Elektron 21 (etching reagent: 5ml acetic acid, 6g picric acid,
10ml H O, 100ml ethanol (95%)). ................................................................. 52
2
Fig. 21. SEM micrographs of Elektron 21 after polishing (a) plan view, (b) zirconium
enriched grain, (c) the first type of particle, (d) the both types of particles (e)
the second type of particle present in the alloy, (f) defects in alloy (void) ...... 53
Fig. 22. X-ray diffraction data of Elektron 21 magnesium alloy. ................................ 54
Fig. 23. SEM micrographs of AZ31. .......................................................................... 55
Fig. 24. X-ray diffraction data of AZ31 magnesium alloy........................................... 56
Fig. 25 Voltage-time response of Elektron 21 followed PEO at constant current density
420 mA/cm2 in the standard solution ............................................................. 58
Fig. 26. Voltage-time response during PEO process in the standard solution for
different current densities. ............................................................................. 59
Fig. 27. Current density-time response during PEO processes in the standard solution
for different current densities. ........................................................................ 59
Fig. 28. Voltage-time response during the PEO process in the KF solution for different
current densities. ........................................................................................... 60
Fig. 29. Current density-time response during PEO processes in the KF solution for
different current densities. ............................................................................. 61
Fig. 30. Backscattered electron micrographs of cross-sections of Elektron 21 following
PEO in the standard solution at a charge of 300 C/cm2, (a,b) 160 mA/cm2,
(c,d) 220 mA/cm2, (e,f) 420 mA/cm2, (g,h) 640 mA/cm2. .............................. 62
Fig. 31. Backscattered electron micrographs of plan views of Elektron 21 following
PEO in the standard solution at a charge of 300 C/cm2, (a,b) 160 mA/cm2,
(c,d) 220 mA/cm2, (e,f) 420 mA/cm2, (g,h) 640 mA/cm2. .............................. 63
Fig. 32. Backscattered electron micrographs of cross-sections of Elektron 21 following
PEO in the KF solution at a charge of 300 C/cm2, (a,b) 220 mA/cm2, (c,d)
420 mA/cm2, (e,f) 640 mA/cm2. .................................................................... 64
5
Fig. 33. Backscattered electron micrographs of plan view of Elektron 21 following
PEO in the KF solution at a charge of 300 C/cm2, (a,b) 220 mA/cm2, (c,d)
420 mA/cm2, (e,f) 640 mA/cm2. .................................................................... 65
Fig. 34. Relationship between coating thickness and current density during PEO
treatment. ...................................................................................................... 67
Fig. 35. Voltage-time response during the PEO process at 420 mA/cm2 in the standard
solution for different charges Q supplied to the sample surface. ..................... 68
Fig. 36. Voltage-time response during the PEO process for 640 mA/cm2 in the standard
solution for different charges Q supplied to the sample surface. ..................... 68
Fig. 37. Voltage-time response during the PEO process for 420 mA/cm2 in the KF
solution for different charges Q supplied to the sample surface. ..................... 69
Fig. 38. Backscattered electron micrographs of cross-sections of Elektron 21 following
PEO in the standard solution at a constant current density of 420 mA/cm2, (a,b)
113 C/cm2, (c,d) 300 C/cm2, (e,f) 500 C/cm2, (g,h) 700 C/cm2. ...................... 70
Fig. 39. Backscattered electron micrographs of plan views of Elektron 21 following
PEO in the standard solution at a constant current density of 420 mA/cm2, (a,b)
113 C/cm2, (c,d) 300 C/cm2, (e,f) 500 C/cm2, (g,h) 700 C/cm2. ...................... 71
Fig. 40. Backscattered electron micrographs of cross-sections of Elektron 21 following
PEO in the standard solution at a constant current density of 640 mA/cm2, (a,b)
300 C/cm2, (c,d) 500 C/cm2, (e,f) 700 C/cm2. ................................................ 72
Fig. 41. Backscattered electron micrographs of plan views of Elektron 21 following
PEO in the standard solution at a constant current density of 640 mA/cm2, (a,b)
300 C/cm2, (c,d) 500 C/cm2, (e,f) 700 C/cm2. ................................................ 73
Fig. 42. Backscattered electron micrographs of cross-sections of Elektron 21 following
PEO in the KF solution at a constant current density of 420 mA/cm2, (a,b) 113
C/cm2, (c,d) 200 C/cm2, (e,f) 300 C/cm2, (g,h) 500 C/cm2. ............................ 74
Fig. 43. Backscattered electron micrographs of plan views of Elektron 21 following
PEO in the KF solution at a constant current density of 420 mA/cm2, (a,b) 113
C/cm2, (c,d) 300 C/cm2, (e,f) 500 C/cm2, (g,h) 700 C/cm2. ............................ 75
Fig. 44. Relationship between coating thickness and charge supplied to the metal
surface during PEO treatment. ....................................................................... 77
Fig. 45. XRD data for Elektron 21 following PEO at 420 mA/cm2 for 4.5 min in the
standard solution. .......................................................................................... 78
Fig. 46. XRD data for Elektron 21 following PEO at 420 mA/cm2 for 500 C/cm2 in the
KF solution. .................................................................................................. 78
Fig. 47. Open circuit potential vs time of Elektron 21 magnesium alloy with and
without PEO coating in 3.5% NaCl solution at room temperature. ................. 79
6
Fig. 48. Polarization curves of Elektron 21 magnesium alloy with and without PEO
coating after 20 min of immersion in 3.5% NaCl solution at room temperature.
...................................................................................................................... 80
Fig. 49. Variation of potential (top) and current (bottom) during electrochemical noise
measurements for 3 days in 3.5% NaCl at room temperature. ........................ 81
Fig. 50. Variation of noise resistance R during electrochemical noise measurements in
n
3.5% NaCl at room temperature. ................................................................... 82
Fig. 51. Variation of noise impedance Z during electrochemical noise measurements in
3.5% NaCl at room temperature. ................................................................... 82
Fig. 52. Surface appearance of Elektron 21: (a) immediately after immersion in 3.5%
NaCl and after (b) 1.5 h, (c) 5 h and (d)13.5 h. .............................................. 83
Fig. 53. SEM micrographs of (a) plan view and (b,c) cross-section of Elektron 21 after
the electrochemical noise measurements ........................................................ 84
Fig. 54. Surface appearance of Elektron 21 followed PEO treatment at 420 mA/cm2 in
the standard solution: (a) immediately after immersion in 3.5% NaCl and after
(b) 1.5 h, (c) 5 h and (d) 13.5 h. ..................................................................... 85
Fig. 55. SEM micrographs of (a,b) plan views and (c,d) cross-sections of Elektron 21
following PEO in the standard solution, after the electrochemical noise
measurements. ............................................................................................... 85
Fig. 56. Surface appearance of Elektron 21 following PEO treatment at 420 mA/cm2 in
the KF solution: (a) immediately after immersion in 3.5% NaCl and after (b)
1.5 h, (c) 5 h and (d) 13.5 h. .......................................................................... 86
Fig. 57 SEM micrographs of plan views of Elektron 21 following PEO in the KF
solution, after the electrochemical noise measurements. ................................ 87
Fig. 58. SEM micrographs of plan views of (a,b) SG_1_104_3h, (c,d) SG_2_104_3h
(e,f) SG_PEO_1_104_3h and (g,h) SG_PEO_2_104_3h. ............................... 89
Fig. 59. SEM micrographs of cross-sections of (a,b) SG_1_104_3h, (c,d)
SG_2_104_3h (e,f) SG_PEO_1_104_3h and (g,h) SG_PEO_2_104_3h. ....... 90
Fig. 60. Elemental distribution map of oxygen, silicon, phosphorus and magnesium in
Elektron 21 following the PEO and sol-gel treatment (SG_PEO_1_104_3h). 91
Fig. 61. Open circuit potential vs time of Elektron 21 magnesium alloy with and
without the PEO and sol-gel coating in 3.5% NaCl at room temperature. ....... 92
Fig. 62. Polarization curves of Elektron 21 magnesium alloy with and without the PEO
and sol-gel coatings after 20 min of immersion in 3.5% NaCl at room
temperature. .................................................................................................. 93
Fig. 63. Variation of potential (top) and current (bottom) during electrochemical noise
measurements for 3 days in 3.5% NaCl at room temperature. ........................ 94
7
Fig. 64. Variation of noise resistance (R ) during electrochemical noise measurements
n
in 3.5% NaCl at room temperature. ............................................................... 95
Fig. 65. Variation of noise impedance (Z) during electrochemical noise measurements
in 3.5% NaCl at room temperature. ............................................................... 95
Fig. 66. Voltage-time response during the PEO process at 420 mA/cm2 for 4.5 min in
the standard solution. ..................................................................................... 96
Fig. 67. Voltage-time response during the PEO process at 420 mA/cm2 for 4.5 min in
the KF solution. ............................................................................................. 97
Fig. 68. Backscattered electron SEM micrographs of cross-sections of AZ31 following
PEO at a constant current density of 420 mA/cm2 in (a-b) the standard and (c-
d) the KF solution.......................................................................................... 98
Fig. 69. SEM micrographs of plan views of AZ31 magnesium alloy following PEO at a
constant current density of 420 mA/cm2 in (a-b) the standard and (c-d) the KF
solution. ........................................................................................................ 98
Fig. 70. XRD data for AZ31 following PEO at 420mA/cm2 for 4.5min in the standard
solution. ........................................................................................................ 99
Fig. 71. Open circuit potential vs time of AZ31 magnesium alloy with and without the
PEO coating in 3.5% NaCl at room temperature. ......................................... 100
Fig. 72. Polarization curves of AZ31 magnesium alloy with and without the PEO
coating after 20 min of immersion in 3.5% NaCl at room temperature. ........ 100

8
List of Tables
Table 1 Density of common structural materials [1]. ................................................... 16
Table 2 ASTM designation system for magnesium [5]. ............................................... 17
Table 3 Meaning of third letter of the designation of magnesium alloys. ..................... 17
Table 4 Reported electron concentrations and plasma temperatures. ............................ 23
Table 5 The percentage of porosity in a PEO coating on an aluminium alloy, reported in
[64]. .............................................................................................................. 34
Table 6 Summary of reported growth rates for PEO process on magnesium alloys. ..... 36
Table 7 Impedance values reported for magnesium alloys before and after PEO in 3.5%
NaCl.............................................................................................................. 40
Table 8 Chemical composition of Elektron 21 [78]. .................................................... 41
Table 9 Chemical composition of AZ31 [79]. ............................................................. 41
Table 10 Composition of PEO solutions and the nomenclature used in the present work.
...................................................................................................................... 43
Table 11 The composition of sol solutions, aging parameters and the nomenclature used
in the present work. ....................................................................................... 46
Table 12 Results of EDX chemical composition analysis of the grain depicted in........ 54
Table 13 Results of EDX chemical composition analysis of the first type of particle
(Fig. 21 (c)) in Elektron 21 magnesium alloy. ............................................... 54
Table 14 Results of EDX chemical composition analysis of the second type of particle
(Fig. 21 (e)) in Elektron 21 magnesium alloy. ............................................... 54
Table 15 Results of EDX chemical composition analysis of the particles in AZ31
magnesium alloy. .......................................................................................... 55
Table 16 PEO process parameters for Elektron 21 magnesium alloy. .......................... 57
Table 17 The thickness of the PEO coatings formed on Elektron 21 at a charge supplied
to the sample surface of 300 C/cm2, but for different current densities. .......... 66
Table 18 The PEO process parameters for Elektron 21 magnesium alloy. ................... 67
Table 19 Thickness of PEO coatings in the standard solution under a constant current
density of 420 mA/cm2. ................................................................................. 76
Table 20 EDX analysis of Elektron 21 following PEO treatment in the standard
solution, after the electrochemical noise test. Areas of point analysis are shown
in Fig. 55 (d). ............................................................................................... 86

9
Abstract
The present work concerns the plasma electrolytic oxidation (PEO) of
magnesium alloys. The first part focuses on the optimisation of the PEO process
parameters on Elektron 21 magnesium alloy in order to produce a protective coating on
the metal surface. Two electrolytes were employed, the first one contains 11 g/L
Na SiO , 10 g/L Na P O *10H O and 2.5 g/L KOH, while the second one additionally
2 4 4 2 7 2
consists of 8 g/L KF. The influence of the solution composition, current density and
time during the PEO process on the coating morphology and thickness are discussed.
An addition of KF in the electrolyte, an increase of a current density and prolongation
of the time of the process resulted in an increase of the thickness of the PEO coatings.
Longer times of the process led to a transition to the so-called “soft-spark” regime.
However, their occurrence caused the generation of a non-uniform coating in the
solution without KF. On the other hand, the prolongation of the time in the KF
containing electrolyte led to a thicker and more uniform coating. XRD analysis revealed
the presence of amorphous and crystalline phases, with the latter including MgO,
Mg (PO ) and MgSiO for treatment in the solution without KF, and MgO and MgSiO
3 4 2 4 4
for the PEO process in the electrolyte with the addition of KF. The PEO coatings have
protective properties and reduce the corrosion current density by two orders of
magnitude. Moreover, an increase of noise resistance and impedance at low frequency
by up to two orders of magnitude were recorded for the PEO coated material. The
second part of the work was focused on application of a post-treatment to fill the
porosity and defects present in the PEO coatings. A sol-gel technique was chosen. It
was found that a sol can easily penetrate through the PEO layer and fill the pores. The
sol-gel coating was found to act as a barrier layer for the penetration of the corrosive
solution and decreases the corrosion current density by one order of magnitude in
potentiodynamic polarization tests. In the last part of the work the PEO process was
carried out on AZ31 magnesium alloy. The PEO coating produced at a current density
of 420 mA/cm2 in the solution without addition of KF has a uniform and relatively
compact structure. The potentiodynamic polarization test showed that the coating has
protective properties and results in a reduction of the corrosion current density by two
orders of magnitude.

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