In arc welding, the base metal is melted by externally applying heat from an electric arc constituted between the base plates and electrode. In order to produce electric arc, the electrode is given one polarity and the work material is given another and sufficient potential difference is maintained between the two. The potential difference is maintained with the help of a power source. Such power source provides either AC or DC power; however, depending on the connection made, DC power source can provide two different polarities also—Straight Polarity and Reverse Polarity.
Direct Current Straight Polarity (DCSP), also called Direct Current Electrode Negative (DCEN) occurs when electrode is connected with the negative terminal of the power source and base metals are connected with the positive terminal. When the connection is made opposite, it is termed as reverse polarity. So in Direct Current Reverse Polarity (DCRP), also called Direct Current Electrode Positive (DCEP), base metals are connected with the negative terminal of the power source and electrode is connected with the positive terminal. You may like read the following articles also.
All three polarities have their respective pros and cons and a suitable polarity must be selected prior to welding in order to get desired result. In terms of filler deposition rate, direct current reverse polarity or electrode positive provides best result. However, there exist a number of factors to judge suitable polarity; deposition rate is just one of such factors. You may read: Comparison among DCSP, DCRP, and AC polarities in arc welding.
Direct Current Electrode Positive (DCEP) or Direct Current Reverse Polarity (DCRP) provides maximum deposition rate in arc welding, especially when the electrode is consumable. Alternating Current (AC) polarity also provides good deposition rate; however, Direct Current Straight Polarity (DCSP) gives least deposition rate. In terms of consumable electrode deposition rate, welding polarities can be arranges in the following manner.
DCRP—Maximum electrode melting and deposition rate.
AC—Moderate electrode melting and deposition rate.
DCSP—Least electrode melting and deposition rate.
Why DCRP provides maximum deposition rate?
In case of arc welding, electrons liberate from negative polarity and accelerated towards positive polarity in the external circuit in presence of sufficient potential difference. Flow of such avalanche of electrons basically constitutes the electric arc (prime source of heat in arc welding). Although the length of the arc column is short (2-5mm), electrons are accelerated at a very high rate and thus the striking velocity becomes very high. When high velocity electrons strike the solid surface its kinetic energy is converted into thermal energy and consequently heat is generated.
In direct current reverse polarity, electrode is made positive and work is made negative. So avalanche of electrons liberate from work surface and strike the electrode at a very high velocity. As a result, more heat is generated at the vicinity of the electrode. As a thumb rule it is considered that around 66% of total arc heat is generated near electrode; whereas rest of the heat is generated near base plates. In such scenario, if the electrode is consumable one, then it melts down at a faster rate because of higher heat and ultimately deposit on the base plate. Thus reverse polarity increases the electrode deposition rate. It is to be noted if the electrode is non-consumable type (as in case of Tungsten Inert Gas welding) then reverse polarity may not provide higher deposition rate. In such cases position of the filler tip (if used at all) and filler feeding rate will determine filler deposition rate.