Air-to-liquid intercoolers are by far the most common form of intercooler found on marine engines, given that a limitless supply of cooling water is available and most engines are located in closed compartments where obtaining a good flow of cooling air for an air-to-air unit would be difficult. Marine intercoolers take the form of a tubular heat exchanger with the air passing around a series of tubes within the cooler casing, and cooling water circulating inside the tubes. The source of water for the intercooler depends on the exact cooling system fitted to the engine. Most marine engines have fresh water circulating within them which is passed through a heat exchanger cooled by sea water. In such a system, the intercooler will be attached to the sea water circuit and placed before the engine's own heat exchanger to ensure a supply of cool water. The main materials used for this kind of application are meant to resist to sea water corrosion: Copper-Nickel for the tubes and bronze for the sea water covers.