When is it appropriate to use reverse polarity in welding?

Using reverse polarity in welding is appropriate when the goal is to concentrate heat on the electrode. This method allows the current to flow from the workpiece to the electrode, which results in a higher heat concentration in the electrode. This is particularly useful in certain types of welding processes, such as MIG (Metal Inert Gas) welding, where the heat needs to be applied more directly to ensure effective melting and fusion of the materials being welded.

When reverse polarity is applied, the electrode's tip gets hotter while the workpiece receives less heat, which can be beneficial in various applications, especially when dealing with certain types of steel. This approach aids in improving the weld penetration and can help in achieving a cleaner weld due to the less intense heat applied to the workpiece.

The other choices are less relevant because they either limit the context in which reverse polarity can be applied or misinterpret the fundamental principles of polarity in welding. Therefore, understanding that reverse polarity is primarily used for enhancing the heat on the electrode helps clarify its appropriate application in welding processes.