PSI - Issue 42

A. Laureys et al. / Procedia Structural Integrity 42 (2022) 1458–1466 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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on an etched background. This material showed a higher resistance to local corrosion compared to the other materials. At 130 °C and higher temperatures deposits were visible on the surface of the G35 samples (Fig. 9). EDX analysis showed that these are phosphate deposits forming on top of the original oxide film. The deposits formed by precipitation of dissolved metal species with phosphate species. Literature states that a phosphate film can act as a protective layer, which retards further corrosion (Iken et al. (2007), Escrivà-Cerdán et al. (2012), Reffas et al. (2009)). This is, however, not directly reflected in the calculated corrosion rates. BenSalah et al. (2014) observed that the corrosion resistance of Sanicro28 at low temperature increased with immersion time due to the formation of iron phosphate and/or chromium phosphate. At higher temperature, phosphate formed a porous polyphosphate film identified by Raman. Abdel-Kader et al. (2008) claimed that a strong protective layer forms, which consists mainly of chromium, manganese, and molybdenum phosphates. The phosphates measured here are possibly also present on the other samples, but non-identifiable by SEM. Performing Raman or XPS in future investigation could deliver more clarity on the topic. 100 C 110 C 120 C

140 C

160 C

130 C

120 C

25 µm

25 µm

25 µm

25 µm

Figure 8: SEM images of corroded Hastelloy G35 surface after testing for 48 h in 85 wt% FGPA at various temperatures.

160 C

Deposit

Matrix

130 C

140 C

Wt% 40.6 25.2

σ

Wt% 55.2 32.0

σ

Ni Cr

0.3 0.2 0.2 0.1 0.2 0.1 0.1

Ni Cr

0.3 0.2 0.2 0.3 0.1

O 19.0

Mo

7.2

P

7.9 5.6

C

5.3 0.3

Mo

Fe

Si

1

K 0.7

25 µm

25 µm

Figure 9: Deposits formed on the surface of Hastelloy G35 tested in 85 wt% FGPA for 48h identified by SEM/EDX analysis.

3.3. Role of chemical composition The corrosion resistance as a function of temperature is generally linked to the stability of the protective oxide layer with temperature. To explain the corrosion behavior of the different materials, the differences in chemical composition are investigated. Table 1 gave an overview of the chemical composition of the different materials. Until 140 °C Sanicro35 and Hastelloy G35 show the best corrosion resistance. They both exhibit the highest amount of molybdenum, which improves resistance to pitting and general corrosion by modification of the passive film composition. An insoluble molybdenum oxide (MoO 3 ) is incorporated in the outer layer of the passive film of the materials. As such, it accelerates the repair of the passive film, which prevents localized corrosion (Abdel-Kader et al. (2008), Pardo et al. (2000), Wegrelius et al. (1999), Guenbour et al. (1988), Sanchez-Tovar et al. (2012)). Moreover, the Hastelloy G35 steel surface stayed etched rather than being consumed by corrosion, as was the case for all the other materials. The phosphates observed on the surface could act as an additional protective layer, inhibiting severe attack (Escrivà-Cerdán et al. (2012), Reffas et al. (2009)). The corrosion rate of Sanicro35 and Hastelloy G35 increased strongly when reaching temperatures higher than 140 °C. Sanicro28, Hastelloy G30 and 316Ti did not exhibit such a strong rise in corrosion rate at those higher