The ionization balance in the welding fume plasma is studied for shielded metal and gas metal arc welding. The presence of condensed particles causes the ionization of gas atoms on the particle surface and leads to ionization balance displacement in the space charge layer around particle. Therefore, the average electron and ion number densities are changed, when charged particles are present in the plasma. It leads to the nucleation process change and, accordingly, influences the fume formation rate. The measured particle formation rates for different potassium additives in the shielding gas are presented. The calculated nuclei number densities for gas metal arc welding with potassium additive, which correlates well with experimental data, are demonstrated.