Multifinger caliper runs in selected Halfdan producing wells have shown penetrations located in the top completions. Future mitigation requires identifying the factors controlling the corrosion process.The current mechanistic understanding is based on samples from retrieved tubing with slightly oxidized corrosion products. From these samples an attempt has been made to track back what chemistry prevailed when the tubing was in operation. The corrosion issues observed downhole in the producing wells may in some cases relate to partially coverage of iron sulfide film but the typical impression is pitting corrosion occurring under mixed deposits.The Halfdan corrosion mechanism described in this paper involves sequentially the evolution of the corrosion cell as the rust layers are building up. Microbial growth of sulfate-reducing prokaryotes (SRP) and methanogens may increase the ionic conductivity in the deposit and ensure reducing and acidic conditions by locally forming H2S and CH4 thereby causing microbiologically influenced corrosion (MIC). Central in the mechanism is furthermore the presence of a green rust GR(Cl-) layer characterized as an ionic water containing mineral as well as islands of e.g. magnetite and iron sulfide acting as cathodes in the deposit. The anode pits are kept active by an acidic chloride rich media.Keywords: Halfdan field Danish Sector of the North Sea underdeposit corrosion green rust magnetite iron sulfide sulfate-reducing prokaryotes (SRP) methanogens microbiologically influenced corrosion (MIC).