*2.1. Materials*

Steel samples were cut from a chain link (R4, Φ120 mm) used in the North Sea at a depth of 85 m for 13 years, provided by a project partner. The nominal composition of the steel is presented in Table 1. The dimensions of steel samples were 25 × 20 × 10 mm3. A copper wire was connected with the steel sample for an electrical connection. Steel samples and connections were embedded in epoxy resin and polished using sandpaper up to 1200 grit. Then the samples were cleaned ultrasonically in ethanol for 2 min and blow dried in air.

**Table 1.** Nominal composition of the grade R4 steel in wt.% (the rest is Fe).

