What kind of joint should not be used for piping in contact with soil due to corrosion risks?

The correct answer highlights that all types of joints listed—socket welded, welded, and threaded—should not be used for piping in contact with soil due to potential corrosion risks. When gas piping systems are placed underground, moisture and soil can contribute to corrosion over time.

Socket welded joints, while often strong, can trap moisture and create pockets where corrosion can initiate. Similarly, welded joints might also introduce issues, as the heat from welding can change the properties of the metal and any exposed surfaces can be susceptible to rusting and corrosion when in prolonged contact with soil. Threaded joints present another concern as the threads can create crevices that can hold moisture, leading to accelerated corrosion.

Choosing materials and joint types that minimize corrosion risk in buried applications is essential for maintaining the integrity and safety of the piping system. It's standard practice to use specialized coatings, materials resistant to corrosion, or other protective measures in these situations to prevent failures and leaks.