Which statement is NOT true when using GTAW with alternating current?

The statement that high frequency is for arc starts only is not accurate. In GTAW (Gas Tungsten Arc Welding) with alternating current, high frequency is utilized to start the arc, but it is also crucial in maintaining a stable arc during the welding process, particularly when working with aluminum and magnesium, which are commonly welded with AC. High frequency allows for better arc stability, preventing the tungsten electrode from touching the workpiece, which can cause contamination and other welding defects.

The other statements are indeed true: the use of alternating current does enhance the cleaning action on the oxide layers found on certain materials, particularly aluminum, which improves weld quality. Additionally, GTAW with AC can effectively weld thicker materials due to its ability to provide good penetration and a stable arc. This technique is essential in various applications where high-quality and precise welds are required.