Cavitation erosion is a phenomenon that is well known with relation to diesel engines. Cavitation is the formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Erosion is the result of the formed vapor bubbles rapidly collapsing which produces a shock wave that can remove small amounts of metal from cylinder walls. As this process repeats pitting of the metal will occur and over time holes will form. Irregularities in the metal surfaces from casting or machining and the cavities from the erosion process itself encourage the formation of these vapor bubbles.
As coolant flows through an engine it will travel through passages and in places where the passages restrict such as in between cylinder walls and the cylinder block casting low pressure areas are induced. This is known as the Venturi effect: the reduction in fluid pressure that results when a fluid flows through a constricted path. These low pressure areas lower the boiling point of the coolant which encourages the formation of vapor bubbles or "cavities." In addition, the rapid flexing of the cylinder walls from compression and ignition induce alternating low and high pressure in the coolant against the cylinder wall. The low pressure waves further induce the formation of bubbles and the high pressure waves cause the bubbles to collapse. This creates a high velocity jet of water that removes a small amount of metal from the cylinder wall. Below is a depiction of the collapse of a vapor bubble in relation to pressure and the resulting jet of coolant.
Another way cavitation can occur is when a pump blade or vane moves faster than the fluid it is moving. This action creates a low pressure area where air pockets can form. When the fluid is hot enough as in the case of an engine cooling system, the coolant can boil in this low pressure area. As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may even damage the blades. On the bottom right is an example of a front engine cover that has been damaged by cavitation where the water pump is mounted.
It is possible to prevent this from happening with the use of chemical additives in the cooling fluid that form a protective layer on the cylinder wall. This layer will be exposed to the same cavitation, but rebuilds itself. When this occurs the implosion attacks the protective coating on the metal surface. This coating is a coolant additive commonly referred to as "DCA" (diesel coolant additive) or "SCA" (supplemental coolant additive) of which the most common types are nitrite or borate salt. Ford diesel equipped trucks require nitrite.
A wet cylinder liner from a Caterpillar diesel engine with severe erosion of the cylinder wall. Notice that the cavitation runs along a line from top to bottom where coolant pressure is lowest. The cavitation is also notably heavier hear the top where compression and ignition pressures are highest.
The parent bore cylinder wall of a 7.3L Power Stroke diesel engine with cavitation. Again note that the perforations in the cylinder wall are near the top and outboard edge of the cylinder closest to the outer cylinder block wall.
Pump cavitation that eroded the front engine cover of a 6.4L Power Stroke diesel engine. Though the result is the same, this type of cavitation erosion is pump induced. Improper system filling and bleeding is believed to be the cause where air pockets in the system affect system pressure.