For most mild steel, it is not necessary to preheat the steel, even in thick sections. Preheating, as well as maintaining interpass temperatures is sometime used when welding high-strength or high-performance steels. This reduces the likelihood of weld cracks. Mild steel is ductile enough that weld cracks aren't usually a problem. Preheating reduces the speed at which the weld cools and solidifies. in high-strength steels, this produces a more ductile microstructure in the weld and heat affected zone, thus reducing the possibility of hot and cold cracks. This also may improve some of the mechanical properties of the H.A.Z., such as impact toughness. The slower cooling rate allows more time for hydrogen to diffuse out of the weld, reducing the potential for hydrogen embrittlement. Hydrogen is produced when water vapor reacts with the steel at high temperatures, producing iron oxide and hydrogen gas. Some steels can be damaged by even relatively small amounts of hydrogen. Electrodes used in flux core arc welding and in shielded metal arc welding often contain fluxes which tend to absorb moisture from the air. Also, rust and mill scale contain water molecules which are chemically bound to the iron atoms. Note that hydrogen embrittlement is generally not an issue with mild steel, due to it's low carbon and alloy content. Preheating also reduces shrinkage stresses, due to the slower cooling rate. This is beneficial in parts which are heavily restrained, or where distortion is a particular concern.