The effect of ageing of AISI 347H FG austenitic steel on KCl-induced corrosion was investigated by comparing the corrosion attack on tube material previously service-exposed for 100000 h and that on an as-received tube. Cr-rich σ-phase precipitates were found mainly along grain boundaries in the microstructure of the service-exposed material, whereas no σ-phase was present in the as-received condition. Laboratory corrosion experiments were carried out using KCl-free and KCl-covered samples in a 15% (v/v) H2O (g) + 5% (v/v) O2 (g) + N2 (g) (balance) atmosphere at 600 °C for 168 h. Microstructural characterisation was performed before and after the corrosion experiments using light optical microscopy, X-ray diffraction spectroscopy, scanning electron microscopy with energy-dispersive X-ray analysis (SEM/EDX) and scanning transmission electron microscopy with energy-dispersive X-ray analysis (STEM/EDX). The presence of KCl resulted in increased attack, and the service-exposed material had increased attack compared to the as-received TP347H FG material; thus, the deepest attack was observed for KCl-covered previously service-exposed material where grain boundary internal attack occurred. The experimental investigations indicate preferential attack of the Cr-rich precipitates and the presence of chlorine at the corrosion front and Cr-rich precipitates (σ-phase) ahead of the corrosion front for KCl-covered service-exposed material. The findings suggest that service-exposed AISI 347H FG material may experience an increased KCl-induced corrosion attack caused by selective attack of the Cr-rich σ-phase.