The proportional constant k of the Stern and Geary equation, for zinc corrosion in near-neutral chloride solution, has been experimentally estimated using i_corr values determined from differential pressure and weight loss methods. In the initial stage, the k value has been found to be 13mV. However, when the surface was covered with a thick corrosion product, the k value decreased down to 4mV, and then maintained a constant value. The k value estimated from the weight loss method agreed with the one estimated from the differential pressure meter (DPM). Experimentally, it was revealed that a 10mHz impedance could not be taken as the polarization resistance if the surface is covered with a thick corrosion product. However, a proportional relation between corrosion rate and reciprocal of impedance at 10mHz (Z _10mHz ⁻¹) still exists, even though the surface is covered with a thick corrosion product.