"CORROSION PERFORMANCE OF LASER CLAD OVERLAYS AND THERMAL SPRAY COATINGS: A COMPARISON" PART 4 OF 8

Eight part series on the relative effectiveness of laser cladding and HVOF spraying for corrosion resistance. Part 4 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 4

Gravimetric Test

The gravimetric test results are commented in terms of visible observations and gravimetric weight measurement data. No visible signs of corrosion were observed on any of the laser clad coatings. Thermal sprayed SS316 was clearly the most corroded coating. Bright red rust residues were visible in the solution and hard adherent red rust was visible on the specimen surface. Note that traces of surface corrosion remained on the thermal sprayed SS316 sample even after each cleaning operation. In the case of Stellite 6, a thin layer of corrosion product was observed, which was easily removed during each cleaning operation. 

The gravimetric data shown in Table 4 and Fig 6 are consistent with the visual observations. Notably, the laser clad coatings sustained none or negligible weight loss.  Among the thermal sprayed coatings, SS316 sustained by far the greatest weight loss, followed by Stellite 6 and IN-625. It is interesting to note that in the case of SS316, the greatest weight loss increments for all three samples occurred in the first week of exposure. This was followed by the lowest weight loss increments during the second week of exposure.

Generally, it can be stated that the gravimetric test results were in agreement with the potentiodynamic test results. 

Table 4: Gravimetric test data represented in terms of weight loss measured for each interval period, and total weight lost

Fig. 6: Gravimetric data – total weight loss for each sample

2.5 Gravimetric test:

The gravimetric corrosion test consisted of immersing the samples in 3.4% NaCl solution for 28 days. The samples were held at room temperature at 72˚F (22˚C) in glass jars (Fig 3) containing approximately 30 fl oz (900 ml) of solution sealed using an air tight lid. Periodic weight measurements were taken every 7 days to determine the corrosion rate by weight loss.

The test methodology was based on ASTM G31, Standard Practice for Laboratory Immersion Corrosion Testing of Metals.

The samples were cleaned before and after each weight measurement. More specifically, the cleaning sequence is as follows: each sample was wiped with soft Kimwipe tissue paper then immersed in acetone, ultrasonically cleaned for 15 min, wiped again with wet Kimwipe tissue paper, and finally blow dried using hot air.

 

Fig. 3: Gravimetric test continuous exposure setup