Keronite_on_Al_alloys_struct_corr

MICROSTRUCTURAL AND CORROSION CHARACTERISTICS OF PEO TREATED UNS
A97075, A92099 and A92195 ALLOYS
*S. Shrestha and S. Hutchins
Keronite International Ltd
Cambridge CB21 6GP
United Kingdom
ABSTRACT
Microstructure characteristics of coatings prepared on some aerospace grade alloys UNS
A97075, A92099 and A92195 by plasma electrolytic oxidation (PEO) have been
investigated. Some comparisons have been made with a sulfuric acid hard anodized
coating. Electrochemical corrosion plots showed the PEO coatings to be highly passive in
salt solutions and are in agreement with standard salt fog exposure results. These results
suggest that the PEO process can offer these high strength aluminum alloys with excellent
and improved corrosion resistance. In addition, extreme hardness of the PEO ceramic
coatings due the formation of crystalline Al2O3 phases have potentials to offer enhanced
wear resistance and multifunctional characteristics required in extreme environments of
aerospace and defense applications. The improved corrosion resistance of the PEO coating
compared to a hard anodized coating is related to the formation of a highly inert oxide layer
as well as its structure and integrity at corners.
Keywords: plasma electrolytic oxidation, ceramic coating, aluminum alloys, corrosion
INTRODUCTION
Hard anodizing of high strength aluminum alloys such as 2xxx, 7xxx series and others,
including casting alloys with high Cu and Si content, presents considerable challenge. For
these higher Si- and Cu-content alloys, the anodized layer tends to be highly porous and of
low hardness. For example, the hard anodized coating on alloyed aluminium such as
A92024/A97075 will have coating hardness of about 250-400HV compared to 700HV for
the less alloyed materials [1, 2]. Thus, anodized high strength alloys of 2xxx and 7xxx
series, which are of primary interest as structural materials in aerospace and defense
applications, have poorer corrosion and wear (impact, fretting, and abrasion) resistance due
to pores and defects in the anodized layer. For space applications, additional coating
characteristics such as black color to achieve thermo-optical properties e.g. solar
absorptance, thermal emittance and improved surface characteristics to resist against cold
1
welding are considered necessary. The black anodizing process is often used for space
applications following the ESA PSS-01-703 [3]. However, this process is considered
environmentally undesirable due to the use Co or Ni sulfide. In addition, the European
Space Agency is currently having problems with these black anodized coatings due to their
relatively poor performance [4]. Thus, there exists a strong need to identify new coating
processes that can meet not only the environmental requirements but also the continual
demand by aerospace and defense hardwares for improved coating performance with
multifunctional properties to work under extreme environmental exposures (Fig.1).
Wear e.g. fretting,
impact, sliding,
Cold welding / friction etc
seizure
Pre-treatment for
paints / adhesives
Dielectric / Electrical
conductance
Aluminum
alloys
Corrosion e.g. general,
SCC, galvanic, fatigue
Environmental
degradation e.g. humidity,
thermal shocks, UV,
atomic oxygen in LEO
Debris and
Outgassing
leading to
contamination at
sensitive and
operational
surfaces
Thermal
conductance /barrier
Thermo-optical properties e.g.
solar absorptance, thermal
emittance
FIGURE 1 - Multifunctional properties required from surface coatings on aluminum alloys
for use in aerospace applications
PEO is an emerging technology and offers versatility in terms of coating most aluminum
alloys including high Cu, Zn and Si containing alloys and Al-MMCs in all forms such as cast,
extrusions and sheets. Details of the PEO process are described elsewhere (5).
While there are a number of published papers on PEO coatings on aluminum, reporting e.g.
process characterization (5,6), physical and mechanical properties (7,8), tribological
properties (9-14) and thermo-optical properties (15-16), there has not been any concerted
attempt to compare PEO with hard anodized coatings in relation to corrosion resistance.
The aim of this paper is to undertake corrosion tests on PEO coatings on high strength AlZn and Al-Li alloys by standard salt fog and electrochemical test methods and compare with
a hard anodized coating.
EXPERIMENTAL
Materials and Test Specimens
Three high strength aerospace grade aluminum alloys: UNS A97075 (5.6Zn 2.5Mg 1.6Cu
0.3Cr Bal. Al), A92099 (2.5Cu 1.6Li 0.8Zn 0.3Mg 0.3Mn 0.1Ti Bal. Al) and A92195 (4Cu
1.0Li 0.4Mg 0.3Ag 0.15Zr Bal. Al) were chosen in this study as substrate materials.
Specimens of either ∅25mm x 5mm thickness or 20mm x 20mm x 5mm were machined
and wet ground to 600 grit finish prior to the surface treatment. The edge of the specimens
was just slightly de-burred to remove any sharp edge. An M3 x 4mm deep threaded hole
was prepared on the side face of each specimen for electrical connection required during
the surface treatment process. The machined specimens were lightly degreased in a nonsolvent based low-alkaline cleaner Gardoclean T5378 (Chemetall product) followed by
rinsing for 30s in tap-water and 30s in de-mineralized water immediately prior to the surface
treatment.
Surface Treatment
PEO coatings were prepared by Keronite International Ltd using commercial Keronite PEO
equipment and proprietary low-concentration alkaline electrolytes, free of any heavy metals
2
and Cr+6. A schematic of the PEO process and a typical appearance of components during
processing are shown in Fig.2. Nominal coating thicknesses were 50-60µm on the 7075
alloy and 30-40µm on 2099 and 2195 alloys.
Hard-anodized coatings were prepared using the sulfuric acid hard-anodizing process. The
coated specimens were immediately rinsed in cold water followed by hot water sealing.
Nominal coating thickness of 50-60µm was prepared on the 7075 alloy. Typical process
parameters during the PEO and hard anodizing / black anodizing of AA7075 alloy are given
in Table 1.
a)
b)
Modulated electrical voltage
Spark
discharge
Cooler
Electro
Electrode
Sample
Sample
Electrolyte
∼
∼
Filter
Pump
FIGURE 2 - Schematic of a PEO process (a); and plasma discharge (glow) surrounding the
surface of a component during PEO processing (b)
Coating Characterization
A coated sample was sectioned to a size of 10x20mm using an abrasive cutting wheel and
the cut specimen was mounted in cold mounting compound (VariDur 3000 from Buehler).
Standard metallographic preparation techniques such as grinding on abrasive papers and
polishing to 1µm diamond suspension were used to prepare the cross-section. The
obtained cross-section was then examined under either an optical microscope or a
scanning electron microscope (SEM). Determination of the various crystalline phases
present in the coating was undertaken using the X-ray diffraction (XRD) technique. The
XRD technique used CuKα radiation to collect spectra from the coating material. Coating
surface roughness was measured using a Surfcom 130A profilometer using 0.25 cut-off and
10mm evaluation length. Coating hardness was measured on the polished cross section by
Vickers micro-indentation with a load of 50g using the procedure described in ASTM E38499.
Salt Fog Testing
For the salt fog tests, specimen sizes 20mmx20mmx5mm or ∅25mmx5mm were used
depending upon the available feedstock material. The salt fog exposure was carried out
following the guidelines described in the ASTM standard B117-2003 ‘Standard practice for
operating salt-spray (fog) apparatus’ and using a CorrosionBoxe salt fog chamber
manufactured by CO.FO.ME.GRA. srl, Italy. This test comprised of exposing the coating
surface to a 5wt% NaCl solution atomized to create a fog within an enclosed chamber. The
coating was supported 15-20° from the vertical. Tests were carried out for various exposure
durations up to 360 hours. The chamber temperature was maintained at 35±1°C. Changes
to the coating surfaces were recorded following periodic observations.
Anodic Polarization
Specimens were fixed to the bottom of the electrochemical test cell sealed with an ‘O’ ring
with the exposed area being 2cm2. The threaded hole used for electrical connection during
3
the coating process was used also for electrical connection during electrochemical
corrosion tests. The coatings were tested in as-prepared condition (i.e. without polishing or
post-sealing) and only after degreasing with alcohol. Testing of the uncoated specimen was
performed on a surface abraded with 600-grit sandpaper.
Electrochemical corrosion tests were performed using a three-electrode electrochemical
corrosion test cell arrangement attached to a computer-controlled potentiostat as shown in
Fig.3. Experiments were carried out following the guidelines described in the ASTM
standard G61. The electrolyte was a 5wt% NaCl solution (made from distilled water) with a
pH value of 6.8-7.0 and solution conductivity of 85mS.cm-1 and in static condition. The
temperature was maintained at 20±1°C.
Platinum
auxiliary
electrode
(AE)
Reference electrode
(SCE)
Electrolyte
solution 4L
Coated sample exposing 2cm
(Working electrode, WE)
2
Electrical connection
FIGURE 3 - Schematic diagram of three-electrode corrosion test cell
The specimens were electrically connected to the Gamry potentiostat. The coated surface
was exposed to the electrolyte and the rest potential ‘Ecorr’ (also known as free corrosion
potential) was stabilized for 15 minutes prior to the anodic polarization. This potential was
measured using a reference saturated calomel electrode (SCE). The area exposed to the
electrolyte was then anodically polarized from its rest potential to more positive potential at
a rate of 10mV.min-1. This was continued until a current density of 1mA.cm-2 or potential of
1000mV wrt to Ecorr was measured in the external circuit. A plot of the corrosion current
density as a function to the polarization potential was recorded.
The collected polarization plots were used to compare the corrosion behavior of the alloys
in the uncoated condition and with the coating. The following features were used to
compare the corrosion behavior:
•
Breakdown potential ‘Eb’ – At potentials above Eb, the current rapidly increases
with further increase in potential. The constant current region below Eb is referred
to as a passive region where little or no corrosion occurs. At potentials above Eb,
rapid corrosion is occurring. The presence of a passive region and a more positive
value for Eb indicates better resistance to corrosion attack.
•
Rest potential ‘Ecorr’ – a more negative Ecorr representing greater susceptibility to
corrosion attack.
4
RESULTS AND DISCUSSION
Visual Surface Appearance
Visual examination of the PEO coatings on 7075, 2099 and 2195 showed uniform and
smooth appearance with no visible defects and irregular features. The appearance was
dark grey / light black. Such darker appearance comes from Cu content in the alloys. The
hard anodized coating also displayed a uniform and smooth appearance with no visible
irregular features. Surface roughness (Ra) measured on the as-prepared surface for the
7075, 2099, 2195 and the hard anodized 7075 were 0.9, 0.7, 0.8 and 0.5±0.1µm
respectively with hard anodized surface showing the lower roughness values. When a light
polish was given to the PEO layers, surface roughness < 0.3µm with further polishing to
0.04±0.01µm was achieved.
a)
b)
c)
FIGURE 4 - SEM images of PEO coatings on:
a) 7075 b) 2099 and c) 2195 showing abundance of smaller irregular and larger pancake
shaped alumina particles of various sizes corresponding to plasma discharges. These
particles show central discharge channels and shrinkage cracks typical from the PEO
process.
Microstructure Characteristics
Secondary electron images of as-prepared surface of the PEO coatings on the three alloys
are shown in Fig.4 that show abundance of spherical and irregular shaped particles-like
structures that were formed during the PEO process. The presence of a number of small
holes of corresponding discharge channels and shrinkage cracks of larger pancake shape
particles are also visible from the surfaces. Although it is difficult to differentiate the three
coating surfaces from the SEM examination, it is clear that the PEO coating on the 7075
alloy with least Cu content has abundance of smaller irregular features on the surface. The
5
Al-Li alloys with increasing Cu content, i.e. from 2099 to 2195, show increasing abundance
of larger pancake shaped features. These features have not yet been analyzed in this work
but would be interesting to investigate if these correlate to the alloying variations and
subsequent PEO process.
a)
b)
Keronite
FIGURE 5 - Cross sectional micrographs of:
a) PEO coating on A97075 showing a uniform coating coverage and good edge retention
b) Hard anodized coating on A97075 showing extensive truncated V-shaped through
thickness cracks extending down to the substrate at corners.
a)
b)
FIGURE 6 - Cross sectional micrographs of:
a)
PEO coated A92099 showing denser microstructure
b)
PEO coated A92195 showing more open microstructure
Optical micrographs of the cross sections of the PEO coating and the hard anodized
coatings in Fig.5 show very distinct edge characteristics of the coatings. The PEO coating
displays a highly uniform coverage and no evidence of through thickness cracks despite of
the presence of micro-scale porosity, which is not seen as interconnected. The hard
6
anodized coating clearly shows truncated ‘V’ opening cracks that extend down to the
substrate. These features of the hard anodized coatings are well known but rarely
presented in publications.
Cross sectional images of the PEO coating on the 2099 and 2195 substrates in Fig.6
clearly show an increasingly open coating microstructure with higher Cu content although a
highly dense PEO coating that could be achievable on even higher Cu containing 2219
alloy had been reported in the past [15].
Figure 7a shows coating hardness values measured by Vickers indentation method. All the
PEO coatings had hardness in excess of 1400HV, which is more than an order of
magnitude of the base alloy hardness. The hard anodized coating on the 7075 alloy had
hardness measured at about 400HV compared to 1620HV of the PEO coating on the same
alloy.
Vickers hardness, HV0.05
XRD patterns collected from the PEO and hard-anodized coatings on the 7075 alloy are
shown in Fig.7b. Sharp peaks of crystalline γ-Al2O3 were detected from the PEO coating.
The hard-anodized coating shows a small ‘hump’ between 25 to 35 degrees 2θ near the
base line trace, which is typical of amorphous material. It is believed that the aluminum
peaks are from the underlying substrate while the anodized coating was an amorphous
alumina. The presence of such crystalline alumina phases explains the high hardness of
the PEO coating. Although not measured here, the phase contents of the PEO coatings on
the Al-Li alloys, various reports [6-8, 15, 16] suggest that these do contain similar crystalline
structures that are responsible for such high hardness values.
2000
a)
PEO coating
Hard anodised coating
b)
1500
1000
500
0
A92195
A92099
A97075
FIGURE 7 - Micro-indentation hardness of PEO and hard anodized coatings (a) and XRD
spectra collected from PEO and hard anodized coatings on 7075 (b). The PEO coating on
7075 alloy shows the presence of crystalline γ-Al2O3 and hard anodized coating shows the
presence of amorphous alumina
Anodic Polarization
Anodic polarization plots from the electrochemical tests taken on flat faces of the uncoated
7075, the PEO coated 7075 and the hard anodized 7075 alloy are shown in Fig.8a. The
uncoated 7075 alloy displayed an immediate rapid increase of the current density on
slightly increasing the potential from its rest potential ‘Ecorr’. This immediate rise of the
anodic current density over a small positive potential range from ‘Ecorr’ is indicative of very
rapid corrosion occurring at the surface of the uncoated alloy. Both the PEO-coated and the
hard anodized 7075 alloy displayed a large potential range about 500mVsce (passivity)
positive from the Ecorr, where the current density was stable and recorded low at <1μA.cm-2.
This is indicative of very little corrosion occurring on the coating/substrate system and a
good level of barrier protection afforded to the underlying substrate. Until about –200mVsce,
7
both the PEO and the hard anodized specimens displayed rather similar behavior albeit up
to this potential the PEO-specimen displayed somewhat better corrosion resistance as
indicated by the polarization trend with reduced corrosion current. Above –200mVsce, higher
currents are recorded by the PEO coated specimen possibly due to now larger pores,
opening via shrinkage cracks and discharge channels resulting in corrosion initiation by
pitting mechanism. Meanwhile, very fine-scale columnar cracks in the hard anodized
coating may now have been further sealed by the corrosion product itself. It is noteworthy
that the PEO coating on this instance was not sealed whereas the hard-anodized coating
had already been sealed.
Significantly measurable differences in the coating/substrate system were observed in
Fig.8b when the corners of the coated specimens were involved during the corrosion tests.
Exposure of such corners and edges would be inevitable in real component situations. The
PEO coated system exhibits a small initial passivity of about 60mVsce from the Ecorr while
the anodized system shows no initial passivity at all. After the initial breakdown ‘Eb’ of the
unsealed PEO surface, there was a rapid increase in the corrosion current until (although at
very low level) about 150mVsce above the Ecorr when the corrosion began to slow down
indicating semi-passivation and filling of the pores by the corrosion product. The sealed
hard anodized coating in Fig.8b shows rapid increase in the corrosion current until it
reached approximately 100µA/cm2 and the potential of 200mVsce above the Ecorr. After this,
passivation started probably after filling of the large cracks by the corrosion product. Such
changes in the corrosion performance due to the exposure of the corners, particularly in the
hard anodized system can be explained due to the presence of the through thickness
opening cracks described above in Fig.5b.
a)
b)
Sealed hard
anodised
7075
0
-200
-400
200
Unsealed
PEO coated
7075
-600
-800
-1000
1.0E-05
Potential, mVsce
Potential, mVsce
200
untreated 7075
1.0E-03
1.0E-01
1.0E+01
-200
-400
Sealed hard
anodised
7075 with
corner
-600
-800
-1000
1.0E-05
1.0E+03
Unsealed
PEO coated
7075 with
corner
0
Current density, µA/cm2
1.0E-03
1.0E-01
1.0E+01
1.0E+03
Current density, µA/cm2
FIGURE 8 - Anodic polarization plots of uncoated UNS A97075, unsealed PEO and sealed
sulfuric acid hard anodized coatings on UNS A97075 in 5 wt% NaCl solution
a) flat surfaces showing passive performance
b) with corners exposed showing a small passive region for the PEO and a highly active
performance shown by the hard anodized system.
The anodic polarization plots of the PEO-coated 2099 and 2195 alloys are presented in
Fig.9 both of which again demonstrate the passivity offered by the PEO ceramic coatings.
Both uncoated alloys show actively corroding surface in the salt solution. A large range of
passivity indicated that the PEO coatings are capable of offering significant corrosion
protection to these high strength alloys.
a)
b)
8
PEO treated 2099
800
PEO treated 2195
600
200
0
-200
-400
untreated 2099
-600
-800
1.0E-04
1.0E-02
1.0E+00
1.0E+02
Potential, mVsce
Potential, mVsce
400
400
200
0
-200
-400
untreated 2195
-600
-800
1.0E-05
1.0E+04
Current density, µA/cm2
1.0E-03
1.0E-01
1.0E+01
1.0E+03
Current density, µA/cm2
FIGURE 9 - Anodic polarization plots in 5 wt% NaCl solution:
a) Uncoated UNS A92099 and PEO coated A92099
b) Uncoated UNS A92195 and PEO coated A92195
800
Eb
Eb
Passivity
400
200
0
Eb
Eb
-200
-400
Ecorr
-800
Ecorr
Ecorr
7075
2099+PEO
2099
2195+PEO
2195
-1000
7075+Hard
anodising
-600
7075+PEO
Potential (E), mVsce
600
FIGURE 10 - Free corrosion potential ‘Ecorr’, typical breakdown potential ‘Eb’ and passive
range of un-coated, PEO coated and hard anodized alloys in 5 wt% NaCl solution.
The Ecorr and Eb values for the tested coating / substrate systems are presented in Fig.10. It
was noticed that the 7075 alloy had a more negative Ecorr than 2099 followed by 2195
alloys. This can be explained by the presence of higher amount of elements such as Zn and
Mg in the 7075 alloy that have lower electrode potentials. As such, poorer corrosion
resistance of this alloy would be expected in the uncoated condition. Coatings on all
substrates appeared to have resulted in the Ecorr moving towards a more positive value
which clearly indicates the improvement in corrosion initiation (susceptibility). While a large
Eb-Ecorr range indicated that these coatings have improved resistance to corrosion attack.
One common feature with the PEO coated and the hard anodize coated systems when
excluding corners (Fig. 10) is that they all appear to have similar passivity range. A more
interesting observation is that the Ecorr for the higher-Cu/lower-Li containing alloy such as
2195 appeared to have ennobled to higher Ecorr values perhaps indicating improved
resistance to corrosion initiation while at the same time with the higher Eb it can be well
susceptible to localized pitting initiation and further propagation. Detailed further study on
the phase composition with the alloying of Li would be required to better understanding
these electrochemical parameters.
9
The PEO coated 7075 specimens after 360 hrs of salt fog exposure are presented in
Fig.11. As discussed above, the more negative Ecorr of the Zn, Mg containing alloy
appeared to show extensive general corrosion of this alloy (Fig.11a) although this would be
difficult to compare with other PEO coated alloys in Fig.12 which have done only 106 hrs.
However, it is clear from the salt fog tests that general mechanism of corrosion of the PEO
coated 7075 was by minor pitting attack as seen in Fig.11b. The more active polarization
plot with the presence of corners on hard anodized coating is further substantiated by
Fig.11c which shows enhanced corrosion attack at edges after the salt fog exposure.
a)
Pin point
corrosion
b)
c)
Corrosion
at the edge
Uncoated alloy
Unsealed PEO
Sealed hard anodised
FIGURE 11 -Surface of: a) uncoated; b) PEO coated and c) hard anodized 7075 alloy after
360 hrs of salt fog exposure
a)
b)
Pitting
corrosion
Uncoated 2099
PEO coated 2099
Uncoated 2195
PEO coated 2195
FIGURE 12 - Surface of: uncoated and PEO coated specimens after 106 hrs of salt fog
exposure:
a) 2099
b) 2195
The nobler Ecorr of the Li alloys are shown to have reduced general corrosion attack on the
uncoated surfaces. A slightly reduced corrosion of the 2099 alloy could be due to its less
Cu / higher Li contents. While on the coated (unsealed) surfaces, this was also reflected
with the presence of a corrosion pit.
General Discussion
The results presented above clearly show that PEO ceramics in general tend to have their
unique structures formed during high voltage plasma discharge events and by subsequent
formation of molten pool of alumina, and rapid quenching in cooled electrolyte. These pore
features can be easily sealed to achieve significantly improved corrosion protection [15, 16].
In addition, the low roughness and very high hardness of these coatings can offer potential
benefits in extreme environments where very high corrosion together with high wear
10
resistance are required. Furthermore, components having difficult shapes and corners can
be uniformly coated using the PEO process, thus making this process attractive for highly
demanding applications where hard anodized coatings would struggle to meet requirements
e.g. resistance to fretting and impact wear occurring e.g. in a space environment [15, 17],
high hardness and corrosion protection and the requirements of thermo-optical properties
[15, 16].
CONCLUSIONS
The following main conclusions can be drawn from the present work:
1. The surface morphology of PEO coatings is characterized by existence of circular
pancake and smaller structures, the center of each structure representing the
discharge channel surrounded by circular rings of rapidly solidified molten pool of
alumina. The resulting surface is relatively smooth with Ra < 1µm in as- prepared
condition, which can be further polished if required down to Ra 0.05µm.
2. The PEO process gives particularly good coverage of edges and corners compared
to a hard anodizing process. The PEO process is suitable for the surface
modification of Cu and Li containing high-strength aluminum alloys.
3. Such surface conversion process involving plasma discharges result in the formation
of crystalline alumina with very high hardness of minimum 1400HV on A97075,
A92099 and A92195 alloys which is about 3.5 to 4 times harder than a hard
anodized coating.
4. Anodic polarization confirmed the PEO coatings to maintain high passivity as shown
by their high breakdown potential ‘Eb’ and ennobled ‘Ecorr’ even when unsealed, thus
these coatings minimizing susceptibility to corrosion initiation and offering extremely
good resistance to corrosion attack of the underlying high strength Al-Zn and Al-Li
alloys. This was further demonstrated by exposures to salt fog environment.
5. Anodic polarization was able to indicate the weaknesses at corners in particular of
hard anodized coating which, even after hot water sealing, allowed immediate
ingress of salt solution to the substrate thus displaying an active behavior and this
was further supported by the presence of corrosion attack accentuated at corners
after the salt fog exposure.
ACKNOWLEDGEMENTS
The authors would like to thank Dr V. Samsonov for the preparation of coated specimens.
Dr B D Dunn from European Space Agency is acknowledged for the supply of Al-Li alloys.
REFERENCES
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Dong ed. Abington Publishing.
11
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12
TABLE 1
TYPICAL PROCESS PARAMETERS DURING PEO AND HARD ANODIZING
Coating
process
Alloy
UNS
Pre-treatment
A97075
PEO
(AC)
A92195
A92099
Hard
Anodizing
(DC)
Black
Anodizing
(DC)
A97075
A97075
Degrease in
alkaline solution
Coating
electrolyte
Proprietary
alkaline
free of Cr, V, Ni
or other heavy
metals
Degrease and
chemical clean
Sulfuric acid
in alkaline solution
H2SO4
Vapor degrease in
C2HCl3, etch in Na3PO4 +
Sulphuric acid
Na2CO3 at 90-95°C,
H2SO4
rinsing, deoxidising in
HNO3, rinsing
Total salt Typical
content
pH
(%)
Nominal
thickness
(µm)
50-55
PostCoating Voltage Current Process
rate
density temp treatment
2
(V) (A/dm )
(°C)
(μm/min)
Silicate
sealing
30-40
<4
10-12
None
1-2
30-40
15 - 20
12-18
<3
50-60
<3
25-35
13
0.8 - 1
<1
300 900
Up to
30
14 to 20
45 - 50
1-2
-10 to 0
< 50
1-2
Coating
formation
method
Surface
appearance
Plasma
oxidation
Grey to
charcoal
black
Oxidation
without
plasma
Bluish
black
Oxidation
without
plasma
Black
obtained by
inorganic
dye
None
25±1
Hot water
sealing
Cobalt
acetate or
Nickel
acetate
solutions