GTAW of magnesium uses which polarity?

• A. DC+
• B. DC-
• C. AC
• D. AC+

Answer: (C)

Polarity in GTAW controls both where the heat goes and how the surface oxide is handled. For magnesium, the oxide film forms readily and can cause porosity and weak welds if it isn’t broken up during welding. Using alternating current provides a cleaning action that repeatedly disrupts and removes that oxide while still delivering heat to form a proper weld pool. The returning cycles also help balance heat input, which is important for magnesium’s high thermal conductivity.

So, the best approach is alternating current, because it both cleans the surface oxide and provides effective heat distribution for a sound magnesium weld. Fixed direct-current polarities lack the oxide-cleaning action and are more prone to weld defects in this metal.