Aluminum alloy sacrificial anodes applied to protect the subsea immersed tunnels' steel shells are buried in an unfamiliar inhomogeneous backfilled stone/brine media. To verify its influence, the effect of the backfilled stone (10–12, 6–10, and 3–6 mm)/brine (40 Ω cm) media, denoted as Stone 1, Stone 2, and Stone 3, on the initial electrochemical performance of an Al─Zn─In─Si─Sn─Ti anode is investigated by electrochemical and surface analysis techniques. With decreasing the stone size, the Stones 1–3 media's resistivity increases to 100, 125, and 150 Ω cm, respectively. The backfilled stone/brine media inhibits the anode′s free corrosion and pitting process and accelerates its activation in the initial period. The anode′s capacity and current efficiency decrease slightly; however, the anode′s output current and its transmission are depressed more heavily by decreasing the backfilled stone size. Also, the backfilled stone shields parts of the cathode surfaces, decreases dissolved oxygen concentration, and blocks the diffusion of the anode's dissolution product in the media, resulting in the change of the anode's protective effect.

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