Tough-pitch copper (copper containing about 0.02 to 0.05% oxygen, mostly dispersed as Cu₂O) can become brittle when it is heated in a reducing atmosphere containing hydrogen or is gas welded or brazed. Which gives the correct cause and remedy?

Answer: A. Hydrogen that enters reacts with Cu₂O to form steam, which creates high-pressure cavities and cracks at grain boundaries, so oxygen-free copper or phosphorus-deoxidized copper is used

Hydrogen that diffuses into copper reacts with Cu₂O at grain boundaries and elsewhere as Cu₂O + H₂ → 2Cu + H₂O, and because water molecules cannot diffuse through copper, they create high-pressure cavities at grain boundaries that lead to cracking. This is called hydrogen embrittlement of copper (hydrogen sickness). The remedy is to use oxygen-free copper, which contains almost no oxygen, or phosphorus-deoxidized copper. Residual phosphorus lowers the conductivity of phosphorus-deoxidized copper, so oxygen-free copper is used where high conductivity is needed. Copper does not form stable hydrides, so the option blaming hydrides is wrong. Nor is the embrittlement caused by melting of Cu₂O; adding more oxygen only increases the source of steam.