In industrial water electrolysers, where stainless steel or others iron-based materials constitute the cell body and pipes, iron traces cannot be avoided in the electrolyte: iron present in the steel is dissolved over time, due to the drastic operating conditions (KOH high-concentration KOH, 30%wt, and high temperature, T = 80-90°C) 1. These metallic traces are bound to have two effects on the nickel electrodes of water electrolysers. The first is the catalytic enhancement of the anodic performance 2. The second is the electrodeposition of iron on the nickel cathode 3. This phenomenon is not so well documented and its effect on the hydrogen evolution reaction (HER) performance is unclear. On the one hand, it has been shown to hinder the HER kinetics on Ni electrodes 4. On the other hand, it could enhance the overall performance of the cathode, by roughening the surface and increasing the Electrochemical Surface Area (ECSA) 5. It is therefore essential to quantify the amount of iron in the electrolyte, study the deposition conditions of iron on nickel cathodes and evaluate its effect on the HER performance. To verify this, KOH 30%wt. was prepared and iron was deliberately introduced in the media. The solution was then electrolysed with different coulometries and potentials, and iron was deposited on the cathodes during the HER (Fig. 1). During these experiments, the electrolyte was sampled and analysed by ICP-MS, AAS and UV-Vis and the cathodes were characterised by XRD, XPS and AAS; their ECSA was assessed by double-layer measurement to help comprehend the performance changes induced by such a deposit. 1. Groot, M. T. De. Alkaline water electrolysis : with or without iron in the electrolyte ? Curr. Opin. Chem. Eng. 42, 100981 (2023). 2. Trotochaud, L., Young, S. L., Ranney, J. K. & Boettcher, S. W. Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 136, 6744–6753 (2014). 3. Bailleux, C. Advanced water alkaline electrolysis: a two-year running of a test plant. Int. J. Hydrogen Energy 6, 461–471 (1981). 4. Huot, J. & Brossard, L. Time dependence of the hydrogen discharge at 70°C on nickel cathodes. Int. J. Hydrogen Energy 12, 821–830 (1987). 5. Demnitz, M. et al. Effect of iron addition to the electrolyte on alkaline water electrolysis performance. Iscience 27, 108695 (2024).