Corrosion Behavior of Laser Remelted CoNiCrAlY Based Composite Coatings

doi:10.4236/eng.2010.25042

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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

E-mail: [email protected]

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

[kW]

[mm]

[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.

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.