Lanthanum effects on microstructure and galvanic corrosion of Fe–20Cr–18Ni–6Mo–0.8Cu–0.2N stainless steel welded joints

Abstract

This study investigates lanthanum (La) effects on microstructure and galvanic corrosion of Fe–20Cr–18Ni–6Mo–0.8Cu–0.2N stainless steel and Q235 carbon steel welds. Tungsten inert gas welding was employed to achieve sound joint formation. Coarse grains are formed in the fusion zone, and σ-phases consisting of Cr₃Ni₂ and Fe–Cr–Mo are produced in the heat-affected zone by prolonged heating. In La-contained alloys, between the grain boundary precipitation phase presented and substrate surface existed large potential difference results in galvanic corrosion. However, the alloy contained 0 wt% La had a lower surface potential of 1.927 μV and 0.5 wt%, 1.5 wt%, 2.0 wt% La alloy more than 2.800 μV. This result demonstrated La plays a positive role in raising alloy surface potential to reduce the initially corrosion tendency. From electrochemical results, the alloy containing 0.5 wt% La demonstrated a stable open-circuit potential of − 0.518 V and largest passive film breakdown potential (E_b) of 0.45 V after 12 days corrosion, indicating minimization of galvanic corrosion tendency. Moreover, the 0.5 wt% La alloy (2.40 × 10⁶ cells/ml) was found inhibiting about 78.18% of SRB growth by bacteria count for 0 wt% La (1.10 × 10⁷ cells/ml).

Graphical abstract