Discerning different and opposite effects of hydrogenase on the corrosion of mild steel in the presence of phosphate species

•Hydrogenase increases the corrosion process of mild steel in phosphate medium.•Hydrogenase that was deactivated by air revealed a protective effect.•In contrast, hydrogenase denatured by heating led to a thick deposit with deep cracks.•The iron-sulphur clusters of hydrogenase were suspected to exacerbate corrosion.

Mild steel coupons were exposed to hydrogenase in a 10 mM phosphate solution. Control coupons were covered by a layer of vivianite. The injection of hydrogenase caused a fast increase in the open circuit potential; this increase depended on the amount of hydrogenase injected and increased from 8 mV for 30 μL hydrogenase to 63 mV for 80 μL. The presence of enzyme resulted in a thicker deposit: high amounts induced the accumulation of corrosion products. Hydrogenase that was deactivated by air revealed a protective effect: non-degradation was observed. In contrast, hydrogenase that was denatured by heat provoked an important deposit of corrosion products with a heterogeneous, cracked structure. The study showed that the action of hydrogenase is not linked to its regular enzymatic activity but to a balance between the protective effect of its protein shell and the electrochemical action of its iron-sulphur clusters. Depending on the operating conditions, hydrogenase can either enhance or mitigate the formation of a corrosion layer on mild steel.