AbstractThe present study investigates the influence of partial cement substitution with biomass ash on the corrosion behaviour of steel embedded in the mortar. To evaluate this influence, corrosion parameters of steel in mortar exposed to tap water and 3.5 wt% NaCl solution were monitored over a period of 50 days. The electrochemical cell consisted of a steel plate as a working electrode, covered with a layer of mortar that was exposed to the testing solution. Open‐circuit potential and linear polarisation were used to monitor changes in corrosion behaviour, whereas electrical impedance spectroscopy was used to evaluate the conductivity of the matrix. Additional to corrosion parameters, the chloride migration coefficient was tested after 28 days. On the basis of the results, it was observed that mortars with biomass ash provide equal or better protection for embedded steel, compared with the reference mortar. Mortars that performed slightly better than the reference mortar and other mortars were prepared with ashes with a higher amount of pozzolanic oxides.