Today virtually all castings that are required to withstand pressure and contain a gas or liquid are impregnated as an essential part of the manufacturing process. This is partly due to the increased use of aluminium and magnesium castings in the automotive industry with more complex shapes, thinner wall sections and tighter leak specifications. High volume automotive casting production often justifies 100% impregnation of all castings to reduce the risk and the high cost of an engine or other component failing in service due to the leakage of fuel, oil or water through micro porosity. Whatever the volume or application, impregnation is a widely established rectification process for castings that fail on leak test in the automotive, defence, aerospace and general engineering industries.
Sectioned cylinder block showing the typical range of porosity that can be found in cast metal:
Red: through porosity.
Blue: blind porosity.
Green: enclosed porosity
See process animation
Machining can turn both blind and enclosed porosity into through porosity causing a leak path, which will need impregnation to render the part leak free and recover the high cost of casting and machining.
US Department of Defense Standard MIL-STD-276A for the impregnation of porous structurally sound castings and powder metal components in aluminium, magnesium, copper, iron and zinc alloys.
US Department of Defense/US Navy MIL-I-17653C / QPL 17563 qualified list of impregnation sealants for cast and powder metal components.