In arc welding, the base metal is melted with the help of an electric arc, constituted between the base plates and electrode. In order to produce electric arc, the electrode is given one polarity while the work material is given another and sufficient potential difference is maintained between the two electrodes. This 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 can provide two different polarities, 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.
- Direct Current Straight Polarity (DCSP) in Arc Welding
- Direct Current Reverse Polarity (DCRP) in Arc Welding
- Alternating Current (AC) Polarity in Arc Welding
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. Reverse polarity or electrode positive provides better arc cleaning action. However, there exist a number of factors to judge suitable polarity; arc cleaning is just one of such factors. You may read: Comparison among DCSP, DCRP, and AC polarities in arc welding.
What is oxide cleaning action in arc welding?
During arc welding, the surfaces of the base metals must be cleaned properly in order to get satisfactory and sound weld joining. Dirt, oxides, coating, foreign loose particles, oil or grease, etc. must be removed from the surface or work material; otherwise such items will mix with the molten metal and will remain embedded within the weld bead. Thus the result will be a defective welding. Presence of contamination on the work surface may also result in requirement of higher potential difference to initiate electric arc. It can also destabilize arc. Hence removal of such items is indispensably necessary prior to welding.
However, dirt, oil, coating, etc. can be easily removed by following proper steps, but removing oxide layer is quite difficult, especially if the base metal is prone to oxidation (such as aluminum or magnesium). Welding polarity inherently helps to rupture such oxide layer present on the base metal surface; and such action is called oxide cleaning by arc. It is also called arc cleaning or ionic cleaning. Suitable polarity can improve arc cleaning action.
Which polarity provides better oxide cleaning in arc welding and why?
Direct Current Reverse Polarity (DCRP) is best in terms of oxide cleaning action. In DCRP connection, the workpiece is given negative polarity and the electrode is given positive polarity. So in the external circuit, electrons liberate from the work surface (negative polarity) and accelerated towards electrode (positive polarity) due to the presence of sufficient potential difference. Flow of avalanche of electrons basically constitutes the electric arc (prime source of heat). When avalanche of electrons liberate from work surface, they break the thin non-conductive oxide layer deposited on work surface to make a free passage for flow. This improves oxide cleaning action by arc and consequently reduces chances of inclusion defects.
In alternating current (AC) polarity when workpiece becomes negative and electrode becomes positive (in half cycle) it acts similar to DCRP polarity. To some extent AC polarity provides oxide cleaning action. However, direct current straight polarity (DCSP) provides no oxide cleaning action because electrons liberate from electrode (negative polarity) and strike the work surface. In fact, DCSP tends to embed oxide particles into the weld metal pool leading to inclusion defects. Therefore, on the basis of oxide cleaning action, welding polarities can be compared as follows.
- DCRP—Best oxide cleaning.
- AC—Moderate oxide cleaning.
- DCSP—Poor oxide cleaning.