This project focuses on the experimental study to establish the relationship between the hardness of different types of water that forms lime scales on metal surfaces and the corrosion rates of carbon steel, expressed in penetration rates per unit time. 48 square metal samples were cut from carbon steel sheets with known metallurgical composition fabricated by Libyan Iron and Steel Company in Misurata and supplied by Brega Company. Four groups of aerated water were used to immerse all samples groundwater from North-Ajdabiya region and diluted seawater with distilled water in different ratios (50%, 25%, and 12.5%) respectively. Each group was separately warmed and controlled at 100 Co, after which the12 samples were immersed in each of the four types of water described above for four periods. The chemical analysis of every kind of water was conducted in the laboratory at Hawari GMMR headquarters in Benghazi. In the top of the list was diluted seawater with (12.5%) distilled water which has a total hardness (17415.6 mg/l); followed by diluted seawater (25%) with a total hardness of (11009.9 mg/l); in third place was diluted seawater (50%) with total hardness (5404.8 mg/l); while groundwater had a lowest value of hardness (1601.4 mg/l). For more than 40 days, all samples were subjected to water aerated by a pumped stream of air. Corrosion rates (in mpy) were determined as weight loss of metal as result of corrosion. Although the groundwater has a lower value of CaCO3, the result showed that samples immersed in heated groundwater and then exposed to the same type of water has the lowest corrosion rate. In contrast, the lowest rates of corrosion were in diluted seawater (12.5% and 25%) respectively due to the high concentration of CaCO3. Therefore, the highest rate of corrosion was noted in diluted seawater (50%) because of a decrease in CaCO3 content....................... Keywords: ...........Corrosion rates, groundwater, diluted seawater, hardness of water.