Study on the corrosion behaviour of a three-metal coupling system under temperature variation

This study focuses on a typical three-metal pair (20# carbon steel/H62 brass/6063 aluminum alloy) commonly used in marine engineering. The weight loss method, electrochemical detection and wire beam electrode (WBE) technique were employed to investigate temperature-induced corrosion effects on this three-metal pair and establish a corrosion model. From the polarisation curves of single metals, it was found that under the dual effects of temperature on the corrosion process, the corrosion rate of 6063 aluminum alloy showed an inflection point at 20 °C. Results from polarisation curve superposition and galvanic current tests of the three-metal system indicated that the system followed the mixed potential theory: E(6063 aluminum alloy) < E(20# carbon steel) < E(mixed potential) < E(H62 brass) and |Ig (H62 brass)| = |Ig(20# carbon steel)| + |Ig(6063 aluminum alloy)|. However, a ‘galvanic effect amplification’ phenomenon was also observed, which does not exist in bimetallic systems. The weight loss method revealed that as temperature increased, the corrosion weight loss of both 6063 aluminum alloy and 20# carbon steel in the coupling system increased. Notably, the corrosion rate of the aluminum alloy was more significantly affected by temperature. The WBE technique showed that with rising temperature, the electrode potentials of the three metals decreased to varying degrees. This indicated a reduction in the activation energy of the corrosion process and a decline in the corrosion resistance of the coupling system. Among the three metals, the 6063 aluminum alloy had the lowest electrode potential (lower than that of 20# carbon steel and H62 brass), so it acted as the main anode in the coupling system and suffered corrosion. At 30 °C, 20# carbon steel underwent anodic/cathodic polarity reversal. By integrating the above analysis results, a corrosion model for the three-metal coupling system was established. This model fills the gap of the classical mixed potential theory in multimetal–seawater temperature coupling scenarios.