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    Monday
    Feb142011

    "CORROSION PERFORMANCE OF LASER CLAD OVERLAYS AND THERMAL SPRAY COATINGS: A COMPARISON" PART 3 OF 8

    Eight part series on the relative effectiveness of laser cladding and HVOF spraying for corrosion resistance. Part 3 of 8

    Index of Parts

    1.  Introduction
    2.  Open circuit potential measurement
    3.  Potentiodynamic test
    4. Gravimetric Measurement
    5. SEM
    6. XRD
    7. Vickers
    8. Conclusion

    Part 3 

    Potentiodynamic Test

    Potentiodynamic tests were conducted following OCP measurements under the same cell conditions to generate polarization curves (Tafel plots) and to determine the corrosion current density for each coating material. The applied potential range was from -1 V to +2 V. Data was collected at a scan rate of 0.166 mV/s.

    The results of the potentiodynamic tests are presented in Table 3 in terms of corrosion potential Ecorr, corrosion current Icorr, and corrosion penetration rate (CPR). CPR is calculated as a direct function of Icorr

    The corrosion potential (Ecorr) results indicate that laser clad coatings have higher corrosion potentials (i.e. less anodic) than thermal spray coatings. Among laser clad coatings, corrosion potential values are very similar. Among thermal sprayed coating, SS316 is most anodic, followed by Stellite 6 and IN-625.

     Table 3: Potentiodynamic test results presented in terms of corrosion potential Ecorr, corrosion current Icorr, and corrosion penetration rate (CPR)

    Figure 5 represents the corrosion rate data using a log scale on the Y axis. This representation confirms that among laser clad coatings, corrosion rate values are very similar and extremely low corrosion rates. On the other hand, thermal sprayed coatings exhibited significantly greater corrosion rates, with SS316 having by far the highest corrosion rate, followed by Stellite 6 and IN-625. However, the corrosion resistance of thermal sprayed coatings can be improved by sealing the coating with polymers [1].

     

    Fig. 5: Corrosion penetration rate (CPR) data represented in log scale

    References

     [1] D. Chidambaram, C.R. Clayton, M.R. Dorfman, Surface Coatings & Tech 176 (2004) 307-317.

     

    Sunday
    Jan232011

    "CORROSION PERFORMANCE OF LASER CLAD OVERLAYS AND THERMAL SPRAY COATINGS: A COMPARISON" PART 2 OF 8

    Eight part series on the relative effectiveness of laser cladding and HVOF spraying for corrosion resistance. Part 2 of 8

    Index of Parts

    1.  Introduction
    2.  Open circuit potential measurement
    3. Potentiodynamic test
    4. Gravimetric Measurement
    5. SEM
    6. XRD
    7. Vickers
    8. Conclusion

    Part 2

    Open circuit potential measurement

    As a general observation, OCP readings of the thermal sprayed coatings did not stabilize, even after 24 hours of exposure. Furthermore, the thermal sprayed coatings exhibited the common feature of an initial drop of OCP voltage, followed by a slow and very jagged increase over time. This can be explained by initial corrosion (oxidation), followed by a slow and inconsistent buildup of passive oxide layer. This hypothesis was tested by an additional experiment on thermal sprayed Inconel. In this case OCP readings were recorded for 168 hours, during which voltage values steadily climbed before appearing to reach a degree of relative constancy after 158 hours.

    In comparison, OCP readings for laser clad coatings were less variable, and appeared to have reached near constant values at the end of 24 hours. The laser clad coatings did not exhibit the initial drop that was observed with thermal sprayed coatings. This might be explained by the fact that the laser clad coatings were less anodic (Fig.4), even from the start of OCP measurement; however, when the passivation layer formed, it was rapid and continuous thereby mitigating further corrosion. OCP potential measurement readings of IN-625 were a close match to wrought IN-625 [1].

     

    Fig. 4: OCP values measured at 24h of exposure

    Open circuit potential measurement

    OCP measurements were conducted in a corrosion cell configured as shown in Fig.2.

    OCP measurements were conducted in 3.4% NaCl electrolyte, and held for 24 hours to allow for adequate stabilization. In one instance, for thermal sprayed Inconel, an additional test was conducted for a period of 168 hrs to establish a point of stabilization.

       

    Fig. 2: Corrosion cell configuration

    5. References

    [1] P.Vuristo, J. Vihinen, Maintenance Research, (2002)

     

    Sunday
    Jan092011

    "Corrosion Performance of Laser Clad Overlays and Thermal Spray Coatings: A Comparison" Part 1 of 8

    Eight part series on the relative effectiveness of laser cladding and HVOF spraying for corrosion resistance.

    Index of Parts

    1.  Introduction
    2. Open circuit potential measurement
    3. Potentiodynamic test
    4. Gravimetric Measurement
    5. SEM
    6. XRD
    7. Vickers
    8. Conclusion

    Part 1

    Introduction

    This series is based on the work of Sunil Musali, George Kim Ph.D., Peter Longobardi and Michael Breitsameter of FW Gartner, Houston Texas. The conclusions of this research were presented at the TSS NACE symposium in San Paulo Brazil Nov. 8 – 10 2010.

    As technology has progressed, the ability to modify the corrosion resistance of the surface of components has allowed the production of cheaper, stronger and/or more easily manufactured components. With the rapid improvements in materials and application techniques, there is an ongoing need to produce empirical evidence of the efficacy of the applications systems. Selecting the right coating for an application  requires the balancing of process capability, cost and desired service life. The focus of the researchers in this study is on relative performance of the two application methods outlined below. The data derived from this study will enable more informed consideration of application method used to produce coatings for corrosion applications.

    The researchers investigated the relative corrosion resistance of three commercially available wear/corrosion protection materials, 316 Stainless Steel, Stellite® 6 and Inconel 625 (In 625). They used both laser cladding (1kw Precitec Fiber coupled Laser) and the High Velocity Oxygen Fuel (Jetkote® JK3000, HVOF) thermal spray process.  This resulted in 6 sample coupons for testing.

    The corrosion resistance of each coating was determined by measuring the Corrosion Penetration Rate (CPR) using three different experimental methods: (1) open circuit potential measurements, (2) potentiodynamic tests, and (3) gravimetric measurements of anodic corrosion rate. In addition, coating properties like surface morphology, phase analysis, structure and hardness are examined using SEM, XRD and Vickers micro hardness tester respectively.

    Results show that laser cladding generates superior corrosion resistant coatings in comparison to thermal spray coatings. It is evident from the results that the laser clad overlays show corrosion resistance equivalent to wrought material.

     

    Fig. 1: (A) 1kW Laser, (B) Jet Kote 3000 HVOF gun

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