In reverse polarity, the flow of electrons is from the plate to the electrode, thus causing a greater concentration of heat at the electrode. See figure 15-43. The intense heat at the electrode tends to melt off the end of, the electrode and may contaminate the weld. Hence, for any given current, dc reverse polarity requires a larger diameter electrode than dc straight polarity. However, if reverse polarity is used with this amount of current, the tip of the electrode will melt off.
Polarity also affects the shape of the weld.
Straight polarity produces a narrow, deep weld, whereas reverse polarity with its larger diameter electrode and lower current forms a wide and shallow weld. Therefore, dc straight polarity is used for welding most metals because better welds are achieved. With the heat concentrated at the plate, the welding process is more rapid, and there is less distortion of the base metal. Unfortunately, oxides, scale, and moisture on the work piece often tend to prevent the full flow of current in the reverse polarity direction. If no current whatsoever flowed in the reverse polarity direction during a welding operation, the partial or complete stoppage of current flow would cause the arc to be unstable and sometimes go out.