This study investigates corrosion inhibition of steel using thiourea and cations such as aluminium, calcium and magnesium under cathodic control in a 3.5% NaCl solution and in seawater. Steel protection in a 3.5% NaCl solution is normally incomplete under a cathodic potential less electronegative than -1.100 V. However, the protection can be enhanced by nearly 50% by adding either 50 ppm aluminium ion or 75 ppm thiourea in solution, and by almost 90% by the combined use of these additions. This study also analyzes how combining inhibitors and cathodic control may be used to protect steel. Moreover, this investigation monitors pH in the solution, measures zero-resistance current between the graphite-steel couple, as well as analyses cathode reaction products. A mechanism is also proposed to interpret the combined effects of inhibitors and cathodic control on the protection of steel.