PSI - Issue 42

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

1463

6

140 C

160 C

110 C

120 C

130 C

25 µm

25 µm

25 µm

25 µm

Figure 5. SEM images of corroded Sanicro35 surface after testing for 48 h in 85 wt% FGPA at various temperatures.

140 C

160 C

SEM analysis of Hastelloy G30 (Fig. 6) showed uniform and minor local corrosion starting from 110 °C, with mainly increasing pitting corrosion with temperature. Nevertheless, the microstructure is still clearly discernable at all temperatures, this in contrast with the previously discussed observation for austenitic stainless steels. The corrosion attack seems to occur more locally, with deep pits forming on the surface. As seen for Sanicro28, localized attack starts by the formation of triangular and square indent shaped pits. Local corrosion damage is not well represented by the calculated corrosion rates through weight loss, as the used formula typically determines the amount of uniform corrosion. However, even though the calculated corrosion rate did not increase substantially between 120 and 160 °C for G30, deep pits were observed on the surface. Such corrosion can be even more critical than uniform corrosion, as perforation of material can occur without strong global attack of the material. 100 C 110 C C 2

120 C

170 C

160 C

110 C 2

120 C C

140 C

100 C 1

25 µm

25 µm

25 µm

25 µm

Figure 6. SEM images of corroded Hastelloy G30 surface after testing for 48 h in 85 wt% FGPA at various temperatures.

160 C 7

140 C 6

170 C

When studying the formed corrosion pits in more detail, particles were observed in the corrosion pits (Fig. 7). EDX analysis showed that these particles contain high amounts of copper. It is unclear if these particles were inclusions present in the original microstructure or were formed during corrosion of the material. Abdel-Kader et al. (2008) stated that copper dissolves from the alloy during corrosion and is re-deposited on the active corrosion sites, as such stifling local damage propagation.

Matrix

Inclusion

Wt% 30.6 21.6 16.8

σ

Wt% 42.3 28.2 12.5

σ

Cu Ni Cr

0.4 0.3 0.2 0.5 0.2 0.1 0.4 0.1 0.4 0.1

Ni Cr Fe

0.4 0.3 0.2 0.3 0.2 0.4 0.1 0.2 0.1

C 11.8

C

6.8

S

8.0

Mo

4

Fe

6.8

W 2.5

Mo Co

2.2 0.8

Co Cu Mn

1.6 1.2 1.0

W 0.8

1 µm

Mn

0.6

Figure 7: Hastelloy G30 tested at 140 °C in FGPA for 48h. Inclusions are identified by EDX within corrosion pits.

Hastelloy G35 shows little corrosion susceptibility under 130 °C (Fig.8). Increasing uniform corrosion is observed starting from 130 °C, with only very limited localized corrosion. Corrosion manifested as a limited amount of indents