Paper: ASAT-13-MS-14
th
13 International Conference on
AEROSPACE SCIENCES & AVIATION TECHNOLOGY,
ASAT- 13, May 26 – 28, 2009, E-Mail: asat@mtc.edu.eg
Military Technical College, Kobry Elkobbah, Cairo, Egypt
Tel : +(202) 24025292 – 24036138, Fax: +(202) 22621908
Effect of Plasma Sprayed Alumina Coating
on Corrosion Resistance
*
**
N. Hegazy , M. Shoeib , Sh. Abdel-Samea
***
, H.Abdel-Kader
†
Abstract: Plasma sprayed alumina coatings on the surface of metals change greatly the
corrosion law of metals in strong acid solutions and enhance effectively their corrosion
resistance property. In this study, the plasma spray process was employed on substrate of
AISI 304 stainless steel with deposition of AL2O3 ceramic coatings with and without Ni5%AL as bond layer. In order to enhance the corrosion resistance, a post-treatment has been
carried out using polyester sealant. The porosity of the coating was measured by optical
methods before and after sealing. The effectiveness of the type of coatings, bond layer, and
sealing treatment on the corrosion behavior of the coatings were determined through static
immersion test in 5%HCL. High corrosion resistance was obtained in the sealed condition
where minimum porosity was occurred.
Keywords: Plasma spray, AL2O3 ceramic coating, Sealing, Bond coat, Corrosion
1. Introduction
Plasma spray coatings of component surfaces are used to prevent the corrosion of certain
substrate. Thermal spray coatings have been highly successful in industry due to their
versatility. As far as anticorrosion and antiwear applications are concerned, the most
frequently used coating materials are oxide ceramic coatings [1] .Aluminum oxide, AL2O3,
more often referred to as alumina, is an exceptionally important ceramic material which has
many technological applications. It has several special properties like high hardness, chemical
inertness, wear resistance and a high melting point. Alumina ceramic can retain up to 90% of
their strength even at 1100 oC. Because of excellent properties of alumina ceramics, they are
widely used in many refectory materials, grinding media, cutting tools, high temperature
bearings, a wide variety of mechanical parts, and critical components in chemical process
environments, where materials are subject to aggressive chemical attack, increasingly higher
temperature and pressures [2]. It is reported that the corrosion resistance of alumina coatings
are higher than that of cermet and metallic coatings [5]. Ceramic coatings usually are
characterized by a relatively high open porosity which is deleterious when the coatings have
*
Ministry of Military Production - Factory 18 (main author)
Metallurgical Research Center-Tiben
***
Ministry of Military Production -Factory 270
†
Prof. Faculty of Engineering- Helwan-Helwan University
**
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Paper: ASAT-13-MS-14
to perform in an aggressive environment. The porosity allows a path for electrolytes from the
outer surface to the substrate [3,4,5]. There are also adhesion problems between the oxide
coating and metallic substrate. A viable solution is to insert a metallic "bond coat" between
the substrate and the coating [1].
The effects of bond coat (Ni-5%Al) on the properties of AL2O3 and AL2O3 – 13%Ti O3
coatings were investigated by S. Yilmaz [6]. The results indicated that application of bond
coat layer in the plasma spraying of AL2O3 and AL2O3 – 13%Ti O3 has increased the hardness
and bonding strength of coatings.
The current research is focused on stainless steel components coated with alumina. The effect
of bond layer on the corrosion resistance of alumina coating substrate will be discussed.
Sealing to reduce porosity, using polyester sealant, will be used in bond-coat specimens.
2. Experimental Work
2.1 Material
Substrates of 75 x 45 x 1mm were cut from AISI 304 stainless steel sheet. The chemical
composition of the substrates is listed in Table 1. Alumina coating was used with and without
bond coating. The Al2O3 powder used (METCO101) was cracked grain. Bond layer of Ni5%Al (METCO 450 NS) powder with spherical particles was used, Fig. 1. The phases of the
two powders obtained from x-ray diffraction are shown in Fig. 2.
The coating technology used both for bond and top coating is air plasma spray. A plasma
spray system (PLASMA- TECHNIK AG M-1000) apparatus was used. The apparatus
components are: Power source (PT-800), control console (M-1000), plasma torch-(F4),
powder feed unit (TWIN-10), water cooler system (T-500) and holding device for the work
piece, in addition to dust separator (MK-9) and wet separator (NA-100). The spraying
parameters have been optimised for each coating to obtain a coating quality as highest as
possible. The spraying parameters of ceramic coat and bond coat are listed in Table 2.
Prior to spraying, substrates were grit-blasted by corundum (20-65 mesh) to improve the
adhesion. Degreasing was followed in washing tanks using vapour solvent (trichlorethane and
trichlortrifluorethane). Ni-5%Al bond coat alloy has been applied and in the end the ceramic
coating (Al2O3) has been sprayed on the surface of the bond coat.
In order to enhance the corrosion resistance a post-treatment has been carried out using
polyester sealant. Coated specimens were ultrasonically cleaned in ethanol and dried at 60 oC
before sealing treatment. Sealant was applied by using a nylon brush, and was allowed to
impregnate into the coatings. After curing, specimens were ground to eliminate the residue of
the sealant left on he surface by using 120 grit Sic papers.
Table 1: The chemical composition of 304 stainless steel (wt %)
Element
%
C
Si
Mn
Cr
Mo
Ni
Co
Cu
Fe
0.0441 0.375 1.90 18.09 0.317 8.72 0.122 0.357 bal.
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Paper: ASAT-13-MS-14
(a)
(b)
Fig.1 Scanning electron micrograph of coating powders:
(a) AL2O3; (b) Ni-5%Al
350
AL2O3 powder
■
■-α AL2O3
■
300
■
250
Intensity
200
■
150
■
■
100
■
■
50
■
■
0
20
30
40
50
60
70
80
-50
2-Theta
(a)
800
◊- Nickel
◘- Aluminum
Bond powder
700
◊
600
Intensity
500
400
300
◊
200
100
◊
◘
◘
◘
0
-100
20
30
40
50
60
2-Theta
(b)
Fig. 2 X-ray results of the two powders
(a) Alumina, (b) Nickel - 5% aluminium.
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Paper: ASAT-13-MS-14
Table 2 The spraying parameters of ceramic and bond coat
Spraying parameters
Bond layer
Ceramic layer
Coating material
Ni-5%Al
Al2O3
Argon
55L/min
41L/min
9.5
14
Current
600 A
530 A
Voltage
60 V
72 V
Nozzle Torch
6 Ø mm
6 Ø mm
Powder gas (Ar)
3.5L/min
3.4L/min
Spray distance
140 mm
120 mm
H2
2.2 Corrosion test
The static immersion corrosion test was performed in 5%HCl solution, at pH=1. The degree
of corrosion of specimens was measured by the weight loss method.The coated test specimen
was located into a special cell with sealing rubber to prevent the HCl acid water solution from
penetrating into the stainless steel substrate. The weight of each specimen was measured
before and after immersion for each appointed immersion time, using an analytical balance
(0.1mg accuracy). Before measuring the weight, specimens were dried to 120 oC for 10
minutes.
2.3 Microstructures
The microstructure and morphology of the powder, as well as the coatings were examined by
means of a JEOL JSM 5410 scanning electron microscope (SEM) and optical microscope
(OM). The specimens were cut parallel to the spray direction and mounted in epoxy. They
were polished following standard metallographic techniques.
2.4 Porosity measurement
Porosity of the different coatings was measured by using image analyser attached to the
optical microscope. The average percentage porosity was determined using 10 fields, at four
different positions on the same specimen.
2.5 Microhardness
The hardness of coating layer and bond coat were measured on the metallographic specimens
using a Vickers microhardness tester with a load of 100 gf. Indenting time of the indenter was
applied for 30s.
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3. Results and Discussions
3.1 Microstructures
The cross-sectional micrographs of the alumina coating without and with bond coat before
corrosion test are shown in Fig. 3. at different magnifications. Also, the sealed alumina/bond
coat/substrate is shown in Fig.3(c). It could be observed that a good adhesion between (a) the
coating and substrate, (b and c) the coating, bond layer and substrate. The interfaces between
top ceramic layer and bond coat and between bond coat and substrate are firm and almost
totally free of material lacks or cracks. A slight oxide formation can be observed along the
bond coat–substrate interface, Fig.3(b). Porosity is clearly observed in the plasma- sprayed
alumina coating, Figs. 3(a) and 3(b). The measured porosity for the alumina coating was
about 8.5% which is a normal value. As reported in literature [6] the porosity in the thermal
spray coating in a range between 6 and 9%. After the sealing treatment, porosity was about
2.96%. This means that sealing decreases porosity in the alumina coating.
(a)
(b)
(c)
Alumina
Alumina
Bond coat
304 SS
304 SS
(a)
(b)
Alumina
Porosity
Alumina
Bond coat
304 SS
(a)
(c)
Porosity
(c)
(b)
No porosity
Bond coat
Fig.3 SEM of cross-section view for the AL2O3 coatings at different magnifications:
(a) without bond coat, (b) with bond coat, and (c) sealed AL2O3 with bond coat.
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3.2 Microhardness
The microhardness profiles on the cross section of the spray coating belonging to (a)
alumina/substrate, (b) alumina/bond layer/substrate, and (c) sealed alumina/bond
layer/substrate are shown in Fig. 4. Irrespective of coating condition, results indicated that
alumina coat has higher hardness values than the stainless steel substrate. The hardness of the
stainless steel was about 196 HV0.1( lowest value ) and the hardness of the coat is nearly 4.5
times higher than that of the substrate. This result is in agreement with the reported in
reference 6. The hardness variation along the coat thickness is not uniform which may be
attributed to the porous structure of alumina (alumina coating has porosity of about 8.5%).
The hardness of the bond coat is nearly similar to the substrate.
Microhardness( Hv0.1)
1200
1000
800
Substrate
600
400
200
alumina coating
0
0
100
200
300
400
distance( μm)
500
600
(a) Al2O3/304SS
bond coat
Microhardness( Hv0.1)
1000
800
600
Substrate
400
200
alumina coating
0
0
100
200
300
400
500
distance (μm)
(b) Al2O3/Ni-Al/304SS
Fig. 4 Microhardness profiles evaluated on transverse
section of all specimens before corrosion tests
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Paper: ASAT-13-MS-14
Microhardness( Hv0.1)
1200
1000
800
Substrate
600
400
200
Sealed alumina coating
0
0
100
200
bond
layer
300
400
500
600
distance (μm)
(c) Sealed Al2O3/Ni-Al/304SS
Fig. 4 (continued) Microhardness profiles evaluated on transverse
section of all specimens before corrosion tests
Figure 5 shows the hardness profiles of both sealed and unsealed Alumina/ bond
layer/substrate coat. Comparing these results it concluded that the sealed alumina has higher
hardness values along the coat thickness which may be due to the low porosity. This means
that sealing improves the hardness by reducing porosity (sealed alumina coating has porosity
of about 2.96%).
3.3. Corrosion
Results of the immersion corrosion tests in 5% HCl are shown in Fig. 6 for the different
coating conditions. Comparing these results, at the different immersion times, it is clearly
observed that the weight losses of the sealed specimens were much lower than that of the
other conditions.
The specimens having bond coat show higher values at the different times and less than the
substrate (stainless steel). Experimental results revealed that peeling was observed at the
interface between the AL2O3 coating and substrate after 72h. On other hand no peeling was
observed until 120h for the AL2O3/bond layer/substrate. This may be due to the nickelaluminum bond coat which ensures a strong bonding between the oxide ceramic coating and
metallic substrate.
Figure 7 shows the corrosion rate of the different specimens. As could be observed the sealed
specimens showed the lowest corrosion rate while the substrate (stainless steel) shows higher
values as 115 times the corrosion rate of the sealed alumina. The corrosion rate of the other
conditions were within the sealed and substrate (Alumina applied pealed after 72h).
These results indicate that the corrosion resistance of the sealed AL2O3/bond layer/substrate
coating was very high comparing with the other conditions. This is may be due to the
reduction of porosity (2.96%) as a direct influence of sealing treatment. The sealing also
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Paper: ASAT-13-MS-14
isolates the substrate away from the acid. Optical micrographs of AL2O3/bond layer/substrate
coated material, after corrosion test, are shown in Fig. 8. Spots of corrosion product on bond
layer and substrate are clearly observed. This was due to the interconnected porosity that
provides access of aggressive environment to the metal. SEM micrograph of the sealed
AL2O3/bond layer/substrate is shown in Fig. 9. No corrosion products were observed in the
bond coat or substrate. This is due to the sealing treatment which blocked the open pores and
reduces porosity (2.96%). Thus the sealant resin was able to protect the materials against
corrosion.
Microhardness( Hv0.1)
1200
bond
coat
1000
sealed AB
AB
800
Substrate
600
400
200
0
0
100
200
300
Distance (μm)
400
500
600
Fig. 5 Comparison between microhardness of sealed and unsealed
alumina/ bond coat /substrate
weight loss g/cm2
0.012
0.01
0.008
0.006
SS
A
AB
Sealed AB
0.004
0.002
0
0
50
100
150
hours
Fig.6 Weight loss Vs. corrosion time for stainless steel substrate (SS),
Alumina/substrate (A), alumina/ bond layer/ substrate (AB),
and sealed alumina/ bond layer/ substrate (( Sealed A+B).
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corrosion rate mm/y
2.5
2
1.5
1
0.5
0
SS
AB
A
ABS
Fig. 7 Corrosion rate in 5% HCl for, alumina/ substrate (A), alumina/ bond layer/
substrate (AB), and sealed alumina/ bond layer/ substrate (ABS).
(b)
(a)
C
Coorrrroossiioonn
PPrroodduucctt
C
Coorrrroossiioonn
PPrroodduucctt
Fig. 8 Optical micrograph of AL2O3/bond layer/substrate
(after corrosion test)
(a)
(b)
Fig.9 SEM micrograph of sealed AL2O3/bond layer/substrate
(after corrosion test)
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4. Conclusions
The main conclusions drawn from the present study are:
-
-
SEM reveals a good adhesion between the coating and substrate, the coating, bond layer
and substrate. The interfaces between top ceramic layer and bond coat and between bond
coat and substrate are firm and almost totally free of material lacks or cracks.
Sealing decreases porosity in the alumina coating from 8.5% to 2.96% and improves the
hardness.
The hardness of the alumina coat is nearly 4.5 times higher than that of the substrate.
Sealing treatment greatly improved the corrosion resistance of alumina coating in 5% HCl
solutions.
5. References
[1]
[2]
[3]
[4]
[5]
[6]
M.Rosso., A.Scrivani et al., “Corrosion resistance and properties of pump pistons
coated with hard materials", Refractory Metals and Hard Materials19 (2001), pp. 45-52
L. Curkovic, M. Fuduric et al, “Corrosion behavior of alumina ceramics in aqueous
HCL and H2SO4 solution”, Corrosion Science (2007)
Y.Dianran., H. Jining et al, “The corrosion behavior of a plasma spraying AL2O3
ceramic coating in dilute HCL solution”, Surface and Coating Technology 89(1997),
pp. 191-195.
E.Celik, I.Sengil, “Effect of some parameters on corrosion behavior of plasma-sprayed
coatings”, Surface and Coating Technology 97(1997), pp. 355-360.
E.Celik, I.Ozdemir et al, “Corrosion behavior of plasma sprayed coatings”, Surface and
Coating Technology 193(2005), pp. 297-302.
S.Yilmaz, “An evaluation of plasma-sprayed coatings based on AL2O3 and AL2O313wt. %TiO2 with bond coat on pure titanium substrate”, Ceramics International (2009)
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