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TECHNICAL PAPERS

Using In Situ Atomic Force Microscopy to Investigate the Kinetics of Corrosion of WC–Co–Cr Cermet Coatings Applied by High-Velocity Oxy-Fuel

[+] Author and Article Information
V. A. Souza

Tribology and Surface Engineering Research Group, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom

A. Neville1

Tribology and Surface Engineering Research Group, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdoma.neville@leeds.ac.uk

1

Corresponding author.

J. Eng. Mater. Technol 129(1), 55-68 (May 19, 2006) (14 pages) doi:10.1115/1.2400258 History: Received March 05, 2005; Revised May 19, 2006

Most of the early applications of thermal spray coatings were focused toward providing a remedy to excessive wear degradation. However, as the introduction of such coatings into wider industrial sections increases there is also exposure to other potential degradation processes—aqueous corrosion is one such process. The complex microstructures in cermet coatings have been shown to translate to complex modes of corrosion attack. In this paper an electrochemical test methodology to probe the local/microaspects of corrosion initiation and propagation will be described. A new electrochemical cell has been devised in which the corrosion can be followed “live” and in “real-time.” The surface is subjected to in situ imaging by atomic force microscopy which shows that not only the binder (Co, Cr) corrodes in high-velocity oxy-fuel thermal spray coatings but also the hard phase, with oxidation and dissolution of WCW2C taking place. Also potentiostatic tests indicated that the corrosion of WC-based coatings follows an Arrhenius relationship enabling the determination of activation energy (Ea) for the corrosion of WC and demonstrating that the oxidation and dissolution of WC are temperature, particle size, potential, and pH related

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 9

The ICP analysis for tungsten, cobalt, and chromium and charge generated (Q) during anodic polarization

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Figure 10

Anodic polarization forward curve with AFM scanning performed in parallel and anodic polarization no AFM insitu scanning

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Figure 11

AFM image scanned on forward curve between 0.504V and 0.564V(SCE)

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Figure 12

AFM 2D image scanned on forward curve between 0.775V and 0.833V(SCE)

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Figure 13

AFM 2D scanned between 0.761V and 0.7V(SCE)

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Figure 14

AFM 3 D image scanned between 0.761V and 0.700V(SCE)

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Figure 15

AFM line profile scanned between 0.761V and 0.700V(SCE)

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Figure 16

2D AFM image scanned between 0.546V and 0.47V(SCE)

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Figure 17

Line profile scanned between 0.546V and 0.47V(SCE)

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Figure 18

2D AFM image scanned between 0.32V(SCE) and 0.289V(SCE) on the reverse anodic polarization curve

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Figure 31

Ratio of current density at 90°C and 20°C at different potentials.

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Figure 32

ICP analysis of the solution after 44days HVOF WC–Co–Cr coatings immersion test

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Figure 1

Association of accelerated techniques and corrosion in real conditions

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Figure 2

Schematic representation of an electrochemical three-electrode cell

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Figure 3

Configuration of insitu AFM imaging apparatus and electrochemical cell.

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Figure 24

Exposure of different surfaces due to mechanical or chemical removal of WC: (A) exposure of binder (Co, Cr); (B) exposure of WC; and (C) exposure of WC and the binder.

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Figure 25

Images taken at the forward curve; (a) at 0.092V(SCE) and (b) at the end of anodic polarization

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Figure 26

Potentiostatic tests at 0.0, 0.3, 0.5, and 0.7V(SCE)

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Figure 27

SEM images after potentiostatic tests; (a) performed at 0.0V and (b) performed at 0.5V

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Figure 28

Arrhenius plot for the potentiostatic test

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Figure 29

Regression analysis to determine correlation coefficient (R-squared) value for the potentiostatic tests

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Figure 30

Activation energy (Ea) as a function of potential for HVOF WC–Co–Cr

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Figure 4

SEM image of the as-polished WC–Co–Cr coating surface: (a) 2400× magnification and (b) 10438× magnification.

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Figure 5

AFM image of as-polished surface under dry conditions: (a) 2D image, and (b) the line profile of the dotted line presented in (a)

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Figure 6

The 3D AFM image of as-polished WC–Co–Cr HVOF coating under dry conditions

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Figure 7

The 2D AFM image of as-polished HVOF WC–Co–Cr sample under liquid

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Figure 8

SEM image after anodic polarization at 18°C: (a) low magnification; and (b) high magnification (59.79K×)

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Figure 19

3D AFM image scanned between 0.32V and 0.289V(SCE)

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Figure 20

Line profile scanned between 0.32V(SCE) and 0.289V(SCE)

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Figure 21

2D AFM image scanned between −0.196V and −0.210V(SCE)

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Figure 22

3D AFM image scanned between −0.196V(SCE) and −0.210V(SCE)

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Figure 23

Line profile scanned between −0.196V and −0.210V(SCE)

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