Thorium oxide tungsten should not be used with which current type?

• A. DCEN
• B. DCEP
• C. Pulsed DC
• D. AC

Answer: (D)

Thoriated tungsten is designed for direct current TIG welding because it emits electrons readily and stands up well to DC arc conditions. With alternating current, however, the electrode heats and cools on each half-cycle, which causes faster wear and can destabilize the arc as thorium oxide forms and is eroded. That makes this type unsuitable for AC. All the DC modes—DCEN, DCEP, and pulsed DC—are compatible with thoriated tungsten, since they maintain a steady polarity or controlled heat. If you need AC TIG, a different electrode such as pure tungsten or lanthanated/ceriated types is typically used instead.