A steel impeller placed in a water pump has been studied with the aim of understanding the surface erosion-corrosion phenomena responsible for reducing the pumping efficiency of water inside the cooling system. This experiment includes long-term (over about half a year) observations with a powered microscope and precise weight measurements. The experiments were carried out under different conditions of water using several mixtures of water and coolants with water contents of 25%, 50%, 75%, and 100% (i.e., pure tap water). The visual microscopy results reveal that most of the steel surface is pitted and clearly rusty, such that corrosion phenomena are noticeable as time passes. In addition, the amount of weight lost from the sample specimens submerged in static water increased linearly, whereas those placed in mixtures of water and coolant initially gained weight before reaching an almost constant weight. In order to see the dynamic effect of the impeller on the erosion-corrosion phenomena, surface observations of the steel impeller were also made at each time after suitable running conditions in water and water-coolant mixtures, namely around $3{sim}4$ hours of operation per day under changing rotation speeds. The results show that the rate of weight loss was linear and $2{sim}3$ times higher than the impeller in water under both static and dynamic conditions. However, when the impeller was submerged in the mixtures, the weight loss was initially insignificant due to the corrosion-proof ingredients of the coolant, but after around 2 months the weight loss substantially increased and gradually approached a linear curve.