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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1. Introduction Finding materials which are corrosion resistant in phosphoric acid at elevated temperatures is a challenging task. Most metals which are suitable for relatively low temperatures, show unsatisfactory corrosion resistance at elevated temperatures (>120 °C) (Craig et al. (1995), Schillmoller (1988)). Austenitic stainless steels or nickel-based alloys with high concentrations of alloying elements, i.e. chromium, molybdenum and nickel, are considered as good choices for phosphoric media up to moderate temperatures. The corrosion resistance of materials strongly depends on their chemical composition, since alloying elements determine the stability of the passive film and its repassivation kinetics (BenSalah et al. (2014)). Corrosion can lead to substantial economic loss (Sanchez-Tovar (2012)) and should, therefore, be avoided as much as possible. Failure of an installation can be avoided by a better understanding of the limits of usefulness of the various materials. Wang and Turner (2008) observed that phosphate was incorporated into the outer part of the passive film of austenitic stainless steels during the passivation process in phosphoric acid. Phosphoric media favor the formation of iron phosphates, which form by precipitation of dissolved iron species with phosphates species (Escrivà-Cerdán (2012), Reffas (2009)) (Eq. 1 and 2). The passivation process in phosphoric acid solutions, consequently, is a combination of an inner chromium oxide layer together with an outer one consisting of soluble Fe(H 2 PO 4 ) 2 and the insoluble compounds FeHPO 4 and Fe 3 (PO 4 ) 2 . As such, an insoluble, porous phosphate layer can be produced, which has strong passivation characteristics (Abdel-Kader (2008), Moraes (2003)) . 6 3 4 + 3 → 3 ( 2 4 ) 2 + 3 2 (1) 3 ( 2 4 ) 2 → 3 ( 4 ) 2 ↓ + 4 3 4 (2) A gap exists in the state of art as so far only a limited amount of studies were carried out on corrosion of metals in concentrated phosphoric acid at temperatures above 100 °C. Therefore, the current work focusses on the interaction between various commercially available metals, i.e. austenitic stainless steels and nickel-based alloys, and phosphoric acid at elevated temperatures. The corrosion sensitivity was determined by weight loss measurements, scanning electron microscopy (SEM) (imaging of the corroded surfaces) and energy dispersive X-ray spectroscopy (EDX) (chemical analysis of the surface). 2. Materials and experimental procedure Several austenitic stainless steels and nickel-based alloys were taken as materials of study (316L, 316Ti, Sanicro28Sanicro28, Sanicro35, Hastelloy G30 and Hastelloy G35). The chemical composition of the alloys is given in Table 1. 316L is a Cr-Ni-Mo austenitic stainless steel, which contains Mo to increase resistance to pitting corrosion in chloride containing environments. 316Ti is a Cr-Ni-Mo-Ti austenitic stainless steel, which also contains Mo to increase the resistance to pitting and crevice corrosion. Titanium stabilizes the structure of the 316 at elevated application temperatures (<800 °C). Sanicro28 is a Ni-Cr-Mo-Cu stainless steel developed especially for phosphoric acid applications and shows high pitting and crevice corrosion resistance. Sanicro35 is a super austenitic steel with a high structural stability at high temperatures with high resistance to localized corrosion attack. Hastelloy G30 contains large amounts of chromium and nickel and is used when stainless steels such as 316L and Sanicro28 do not suffice. The alloy is often used to transport highly corrosive liquids such as phosphoric acid. Hastelloy G35 is a similar alloy as G30, but contains less iron and more molybdenum making it more resistant to localized corrosion.