What is the characteristic of atomic hydrogen welding?

Atomic hydrogen welding is characterized by its use of an electric arc produced between two tungsten electrodes in the presence of hydrogen gas. This process results in the disassociation of hydrogen into atomic hydrogen, which is highly reactive and can help clean the surfaces being welded as well as promote good fusion between them. The primary feature of atomic hydrogen welding is its reliance on this unique reaction to create heat for welding, making it effective for certain applications.

The assertion that atomic hydrogen welding is considered an obsolete process also underscores its historical significance and the evolution of welding techniques. Although it was once widely utilized, advancements in other welding methods, such as MIG and TIG welding, which are more efficient and easier to use, have led to a decline in the use of atomic hydrogen welding.

While the other options touch upon aspects related to welding processes, they do not specifically capture the unique nature or the historical context of atomic hydrogen welding as a method that relies on the reaction of atomic hydrogen to melt the base metals. Thus, the emphasis on its obsolescence is not only a technical fact but also reflects on the development and preference for newer welding techniques in modern applications.