"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)