<p>Sodium gluconate is one of the corrosion inhibitors of steel in cooling water, and it is low cost. To improve its low inhibition ability, a composite corrosion inhibitor composed of sodium gluconate (SG) and hexamethylenetetramine (HMTA) was proposed and examined in this paper. Immersion, scanning electron microscope (SEM), atomic absorption spectrometry (AAS), and electrochemical tests were used to estimate the influence of HMTA on the inhibitor performance of SG. The results showed that the addition of hexamethylenetetramine can remarkably increase the inhibition performance of sodium gluconate. When the concentration of sodium gluconate was fixed at 0.2 wt% and the concentration of hexamethylenetetramine was 1.2 wt%, the best performance of the composite corrosion inhibitor was obtained.</p> Graphical abstract <p></p>