Which statement about thorium oxide tungsten is incorrect?

• A. They work well with AC.
• B. They are very suitable for welding on steel, steel alloys, nickel alloys, and most metals other than aluminum and magnesium.
• C. They provide easier arc starting characteristic than pure tungsten.
• D. They reduce electrode tip temperature and erosion when 1-2% thorium is added.

Answer: (A)

Thorium-oxide (thoriated) tungsten electrodes are chosen because the thorium oxide increases electron emission, making arc starting easier and allowing good life under DC welding. They’re well-suited for DC welding of steels, nickel alloys, and most metals other than aluminum and magnesium, where other electrode types are preferred. The small addition of thorium also helps reduce tip heating and erosion, so with about 1–2% thorium the electrode lasts longer and wears less. However, they don’t perform as well with alternating current; AC TIG welding benefits from electrode types optimized for AC, because the rapid polarity changes can cause instability and faster degradation of thoriated tips. That’s why the statement claiming they work well with AC is not correct.