A method of preventing and controlling corrosion and build-up of scale in cooling-water systems at water temperatures below about 212.degree. F. The method comprises the use of an effective amount of hydrogen peroxide, i.e., 0.01 to 100 parts by weight per million parts of water, about 0.01 to 100 parts by weight of at least one organic triazole per million parts of water, 0.1 to 200 parts by weight per million parts of water of at least one polymerized ethylenically unsaturated carboxylic acid, e.g., polymerized maleic acid having average molecular weights ranging from about 250 to 500,000 and 0 to 200 parts by weight, e.g., 0.1 to 50 parts of at least one non-ionic surfactant.