Investigation on galvanic corrosion behavior of Q235 in deep-sea water/sediments of the South China Sea via manned deep diving

To investigate the corrosion behaviors of metals in deep-sea water and sediments, in 2021, we conducted a systematic set of deep-sea water and sediment corrosion tests for the first time in the South China Sea at a depth of 3404.0 m via a Chinese-manned submersible Shen Hai Yong Shi. In this study, as one of the commonest failure causes, the galvanic corrosion of Q235 mild steel in deep-sea water and sediments were comprehensively analyzed. Various techniques were employed to characterize micro-/macro-morphologies, elemental composition, crystal structure and chemical state of each specimen and its corresponding corrosion products to elaborate the corrosion reaction and mechanism. The results reveal that the galvanic corrosion rate of Q235 increases when being placed in the deep sea, when compared with that in the shallow sea. In the deep-sea water, rather than being slowed down by the low temperature and dissolved oxygen level, the galvanic corrosion reaction of Q235 is accelerated by the high hydrostatic pressure, which can change the passivation film and electrical double-layer to shift the electrode potentials and increase the potential difference of the galvanic couple. Furthermore, in deep-sea sediments, although the mass transfer is slowed down to accumulate more corrosion products, its galvanic corrosion rate is still higher than that in shallow sea. This study provides important and insights into the galvanic corrosion behaviors of Q235 in deep-sea water/sediments provide valuable guidance for material design and selection ensuring safety during service life.