Pores are the most common defects in welds made by different fusion welding methods. The intensity of formation of gas nuclei in the weld pool can be increased as a result of stirring of the metal melt. As nucleation of a bubble in a liquid is associated with overcoming of inter molecular adhesion forces, this bubble will develop more intensively toward that phase which offers less resistance to its growth. As the temperature of the slag at the interface with the molten electrode metal is always higher than that at the interface with the weld-pool metal, the total gas content of the weld metal should be determined primarily by the processes, which occur at the droplet stage. In addition to the metal–slag and metal–molten non-metallic inclusion interfaces, the weld pool also contains interfaces between solid non-metallic inclusions and metal melt, as well as between the metal melt and crystalline grains.