The critical heat flux (CHF) is studied experimentally in vertical tubes heated directly using power current (direct current 2500 A, 15 V) and cooled with water at a low mass flow rate (0 to 0.2 Mg/m(2) . s) and at low pressure (0.1 to 0.8 MPa). A smooth tube and a tube with a porous coating layer sintered onto the inner surface were used. The tube and the porous coating layer are both made from INCONEL-600. The results (so far at moderate heat fluxes) are compared with each other and with correlations by Katto and by Weber. Enhancement of heat transfer was determined as well as a negative effect of the porous coating below the expected value of CHF.