- Spall are flakes of a material that are broken off a larger solid body. One common form of spalling occurs due to moisture freezing inside cracks in rock, cracking off the outer surfaces. Spalling can occur on a concrete surface if exposed to salt or if improperly finished.
- Spalling is also used to describe the flakes of metal chipped off the inside of armor plating on tanks and other armored fighting vehicles due to attacks on the outside creating a shock wave that travels through the armor and breaks the softer metal on the inside. Spalling is an intended effect of the high explosive squash head anti-tank round and of many other munitions which may not be powerful enough to pierce the armor of a target. This spall is dangerous to crew and equipment, and may result in a partial or complete kill of a vehicle. Many AFVs are equipped with spall liners for protection.
- In corrosion, spalling occurs when a metal sheds tiny particles of corrosion products as the reaction progresses. These corrosion products are not soluble or permeable, but, as opposed to passivation, they fail to adhere to the metal surface or form a barrier to further corrosion. This happens as the result of a large volume change during the reaction. In the case of actinide metals (most notably the depleted Uranium used in some types of ammunition), the material expands so violently upon exposure to air that a fine powder of oxide is forcibly expelled from the surface. This property, along with these elements' inherent toxicity and (often to a lesser extent) radioactivity, make them very dangerous to handle in metallic form.
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