BS in Materials Engineering
Materials Engineering Department
Samples of two Thermal Spray Coating (TSC) alloys on a low carbon steel substrate were obtained: alloy C276 and Nicko-Shield 200. Specimens of each alloy were subjected to heat treatments at temperatures at 1100° for 60 minutes and 1200° for 10 minutes, with some specimens left in the as-sprayed condition. Three replicates were prepared for each condition. Thin 1” strips were cut using a diamond wafering blade for porosity measurements and 2” x 1” specimens were cut for corrosion testing using a SiC abrasive saw. The porosity specimens were mounted in acrylic resin, polished, and examined using SEM. From these images, area percent porosity was calculated using automatic image analysis software. The average porosity of the untreated specimens was 2.51% for C276 and 2.2% for Nicko-Shield 200; these values were not statistically different. Heat treatment resulted in no significant change in area percent porosity. The corrosion specimens were mounted in acrylic so that only the coating surface was exposed to the environment. These specimens were immersed in 20% H2S04 at 65°C for 200 hrs. Each specimen was weighed before and after immersion to measure mass loss due to corrosion. The average mass loss of as-sprayed Nicko-Shield 200 was 6.405 mm/yr. The heat treated specimens of Nicko-Shield 200 showed improved corrosion resistance, with an average mass loss rate of 0.92 and 0.54 mm/year after treatments at 1100° for 60 minutes and 1200° for 10 minutes, respectively. C 276 failed corrosion testing under all conditions. Overall, heat treating was found to increase corrosion resistance without effecting porosity. Visual evidence suggests that the change in corrosion behavior was caused by the formation of an oxide layer during heat treatment.