Engineering, 2010, 2, 322-327
doi:10.4236/eng.2010.25042 Published Online May 2010 (http://www.SciRP.org/journal/eng)
Corrosion Behavior of Laser Remelted CoNiCrAlY Based
Composite Coatings
Dragos Utu1, Gabriela Marginean2, Viorel-Aurel Serban1, Cosmin Codrean1
1University “Politehnica” Timisoara, Faculty of Mechanical Engineering, Timisoara, Romania
2University of Applied Sciences Gelsenkirchen, Gelsenkirchen, Germany
Received November 27, 2009; revised February 5, 2010; accepted February 12, 2010
Abstract
The corrosion behavior of High-Velocity Oxygen Fuel (HVOF) sprayed MCrAlY coatings obtained from
CoNiCrAlY particles (wt. 8% Al) mechanically doped with Al2O3 nanopowder was investigated before and
after laser remelting. The latter process was applied in order to achieve a homogeneous structure as well as
better mechanical properties for the coating (reduced brittleness offered by the presence of the Al2O3
nanoparticles). Another important task of the laboratory investigations was the investigation of the corrosion
behavior of the modified coatings. The results obtained from the potentiodynamic polarization measurements
carried out in a chloride environment revealed an enhanced corrosion resistance of the laser remelted coat-
ings comprising a refined microstructure. Microhardness measurements of the modified coatings revealed
lower values in comparison with that of the samples in as-sprayed status. This observation leads to the as-
sumption that a concomitant improvement of coatings ductility occurred as well.
Keywords: Laser Remelting, Conicraly Coatings, Corrosion Behaviour
1. Introduction
In the turbine blades section of the engines, the overall
operating conditions became progressively more hostile
in terms of temperature and mechanical environment. A
solution in order to solve this problem is applying of
protective thermal barriers consisting of a ceramic insu-
lating layer bonded to an oxidation resistant MCrAlY
coating. The latter one belongs to the family of high
temperature coatings (around 850-1200C), where M is
selected from one or a combination of iron, nickel and
cobalt [1,2]. Cr and Al are present in the MCrAlY
chemical composition because they are able to form
highly tenacious protective oxide scales [3], whilst Y
promotes formation of these stable oxides [4]. Their pro-
tection role is given by the formation of a compact, sta-
ble, and adherent oxide layer (usually α-A2O3) on the
surface, which exhibits any interaction between the base
material and the corrosive medium. Without this protec-
tive scale, the coating and ultimately the substrate, would
come under rapid oxidation and/or corrosion attack [5].
The durability or service life of the MCrAlY coating
depends mainly on the stability of the formed alumina
scale [6].
The microstructure of the grown oxide scales depends
strongly on the coating properties, the manufacturing
process and the operating conditions.
In a previous research work it has been demonstrated
that the mechanical alloying of MCrAlY powders with
nano-Al2O3 leads o a better high temperature oxidation
behavior of the HVOF-sprayed coating in comparison
with the conventional MCrAlY coating. This conclusion
is based mainly on the reduced oxidation rate of the
Al2O3 doped MCrAlY coating, which is a very important
parameter concerning the kinetics of the oxide scale
growth [7,8]. Therefore, another supplementary task
should be settled in the coatings investigation, namely
their behavior under mechanical loadings. Doping of the
MCrAlY coatings with ceramic particles which are uni-
formly distributed along the grain boundaries between
the MCrAlY particles, showed the main disadvantage
concerning its negative influence on the coating ductility
(due to the presence of brittle compounds).
Rapid melting and solidifying of the MCrAlY coatings
achieved using a laser beam can offer good mechanical
behavior of the whole system (coating-substrate).

D. UTU ET AL.323
2. Experimental Procedures
CoNiCrAlY coatings (280-350 µm) with wt. 8% Al
content (Co-32Ni-22Cr-8Al-0.5Y) and mechanically mixed
with wt. 2% Al2O3-nanopowder were sprayed onto an
alloy 617 substrate (5 mm thick) using the HVOF (High
Velocity Oxygen-Fuel)-spraying technique.
The equipment used was a CJS Gun of the company
Thermico, Germany, which is operated with a hydrogen-
stabilized liquid fuel oxygen combustion.
The coatings were remelted using a CO2 laser from
TRUMPF company, by applying an unfocussed beam for
a witdth about 100 mm. The remelting treatment was
performed using argon as shielding gas. The optimized
parameters used during the laser treatment process are
presented in Table 1. The beam power, P and the
working distance, d were kept constant during the
treatment, while advancing velocities, v were varied.
In order to determine the corrosion resistance of the
coatings before and after laser remelting electrochemical
measurements were also carried out. The tests were per-
formed in a 5% H2SO4 solution containing 58 g/L NaCl,
using an electrochemical corrosion cell and a potentio-
stat/galvanostat PGP201 from Radiometer.
Polarization curves were recorded in the positive di-
rection starting from free potential at room temperature
in a three electrode cell using calomel electrode (SCE) as
reference. The applied potential was varied between
–1000 and 1000 mV using a rate of 50 mV/min.
Table 1. Experimental conditions for laser remelting.
Material
Coating
thickness
[μm]
Remelting
parameters
P
[kW]
d
[mm]
v
[mm/s]
Penetration
depth [μm]
5 239
CoNiCrAlY
8% Al 280-350
3 310 10 110
3. Results and Discussions
3.1. Coatings Morphology
SEM-investigation of the as-sprayed coating (Figure 1)
shows the presence of oxides in the structure of the ma-
terial. It can be seen that the deformation degree of the
CoNiCrAlY particles during spraying was not very pro-
nounced. The insulating Al2O3 ceramic nanoparticles
form a thermal barrier for the MCrAlY powders which
are exposed to a reduced thermal energy during the
HVOF spraying process. This phenomenon is demo-
nstrated by the presence of partially molten particles.
Depending on the parameters of the laser remelting
treatment (see Table 1), different penetration depths were
obtained (compare Figure 2(a) with 3(a)). Increasing the
advancing velocity of the remelting process led to a re-
duced penetration depth as well as to a finer coating mi-
crostructure (Figure 2(b) respectively 3(b)). In both
Magn DEt 50μm
500× BSE
Figure 1. SEM micrograph (cross-section) of the as-sprayed MCrAlY+2% Al2O3 coating [8].
Identify applicable sponsor/s here. (sponsors)

D. UTU ET AL. 324
200μm
Magn Det
100× BSE
342μm
239μm
(a)
20μm
Magn Det
1000× BSE
(b)
Figure 2. SEM micrographs of the laser remelted coatings (v = 5 mm/s).
cases the remelted zone was free from pores and oxides.
3.2. X-ray Diffraction Measurements
The X-ray diffraction tests were performed on a Philips
X’Pert X-ray diffractometer using a Cu-Kα radiation, in
order to determine the phase composition of the coatings
before and after laser remelting.
X-ray diffraction patterns (Figure 4) show for all the
investigated samples the presence of a phase-mixture
consisting of -Ni/’-N i3Al, -NiAl, -Al2O3 and Cr2O3.
The XRD patterns from Figure 4 indicate that in the
case of the laser remelted samples the Al2O3 oxide phase
was partially dissolved in solution increasing the matrix
content -Ni/’-Ni3Al. The appearance of the -NiAl
phase can be also noticed (phase precipitation during the

D. UTU ET AL.325
laser remelting process – see the dark-grey cellular structures
on the SEM-micrographs Figure 2(b) respectively 3(b)).
3.3. Microhardness Tests
The microhardness of the coatings was measured with a
Vickers tester from Wolpert applying a 0.1 kgf load. The
reported values (Figure 5) represent the indentations
made along the coating cross-section, where P0 is the
as-sprayed coating and P1 and P2 are the laser remelted
coatings using v = 5 mm/s respectively v = 10 mm/s.
The hardness curves evidence that the laser remelting
200μm
Magn Det
100 × BSE
282μm
110μm
(a)
20μm
Magn Det
1000
× BSE
(b)
Figure 3. SEM micrographs of the laser remelted coatings (v = 10 mm/s).

D. UTU ET AL. 326
Figure 4. XRD diffraction patterns: a-as sprayed sample, b-laser remelted v = 5 mm, c-laser remelted v = 10 mm.
process led to decreasing of the values from 450 HV to
almost 200 HV. This result has a positive effect on the
ductility of the material. The values of the measured
hardness along the coating thickness correlate very well
with the structures shown in the Figures 2(a) and 3(a).
In the domain where the Al2O3 particles were dissociated
by the laser energy, the coating has a lower hardness in
comparison with the zones where the oxide particles are
steel present.
3.4. Corrosion Tests
The polarization curves obtained for the tested materials
(P0, P1 and P2) are presented in Figure 6.
0
100
200
300
400
500
050100 150 200 250 300 350
Coating Depth,[µm]
Microhardness, [HV 0.1]
P0 P1 P2
Figure 5. Microhardness curves of the tested materials.

D. UTU ET AL.327
Figure 6. Polarization curves of the samples exposed in 5% H2SO4 with 58 g/L NaCl.
Table 2. Values of the measured corrosion potential and
current density.
Electrochemical data
Sample icorr (µA/cm2) Ecorr (mV)
P0 95.4 -685.9
P1 17.5 -550.4
P2 1.71 -592.2
Comparing the determined results for the corrosion
current density (icorr) it can be seen that the values of the
laser remelted coatings (Table 2) were shifted to lower
values in comparison with P0 (from 95.4 µA/cm2 to 17.5
respectively 1.71 µA/cm2 ) which means an improving of
the corrosion behavior in chloride environment com-
pared with the as-sprayed sample.
4. Conclusions
The investigations performed show a general improve-
ement of the coating properties due to the advantageous
microstructure of the remelted composite powder ob-
tained by applying a CO2 laser beam.
The corrosion behavior in a 5% H2SO4 solution
containing 58 g/L NaCl of the HVOF sprayed CoNi-
CrAlY coatings (wt. 8% Al) doped with Al2O3 nano-
powder was investigated before and after laser remelting.
The experimental results demonstrated that the laser
treatment had a positive effect on the corrosion resis-
tance of the coatings because of the structure refining
(free from pores and oxides).
Moreover, the ductility of the tested CoNiCrAlY
coatings mechanically doped with Al2O3 nanopowder
was improved by laser irradiation. It has been found a
hardness decreasing of the refined structure.
5. References
[1] T. A. Taylor and D. F. Bettridge, “Development of Al-
loyed and Dispersion-Strengthened McralyCoatings,”
Surface and Coatings Technology, Vol. 86-87, No. 1,
1996, pp. 9-14.
[2] A. Weisenburger, G. Rizzi, A. Scrivani, G. Mueller and
J. R. Nicholls, “Pulsed Electron Beam Treatment of
Mcraly Bondcoats for EB-PVD TBC Systems Part 1 of 2:
Coating Production,” Surface and Coatings Technology,
Vol. 202, 2007, pp. 704-708.
[3] L. Lelait, S. Alpérine and R. Mévrel, “Alumina Scale
Growth at Zirconia-Mcraly Interface: A Microstructural
Study,” Journal of Materials Science, Vol. 27, 1992, pp. 5-12.
[4] L. Russo, M. Dorfman and K. Lapierre, “Superalloy
HVOF Powders with Improved High Temperature Oxi-
dation, Corrosion and Creep Resistance,” European Pat-
ent EP1272301, Sulzer Metco Us Inc., Westbury, 2003.
[5] “NMAB: Coatings for High-Temperature Structural Ma-
terials: Trends and Opportunities,” National Academy
Press, Washington, D.C., 1996.
[6] H. Al-Badairy, G. Tatlock, S. Fawcett, P. Beahan and J.
Hunt, “FEG-SEM Investigation of Alumina Scales
Formed on Fecraly Alloys Oxidised at 1200°C,” Journal
de Physique IV France, Vol. 124, 2005, pp. 17-24.
[7] D. Maghet, G. Marginean, I. Mitelea, A. Davidescu and
W. Brandl, “Comparison of Oxidation Behaviour of
Various Thermally Sprayed MCrAlY Coatings”, The
European Corrosion Congress, Breisgau, 2007.
[8] D. Maghet “Morphology and Properties of HVOF-MCrAlY
Sprayed Coatings,” Ph.D. Thesis, Politehnica University of
Timisoara, Timisoara, 2007.