Final answer:
Rust on the metal is a common cause of porosity in a weld, as it leads to contaminants in the weld pool that can create pores as the metal cools and solidifies.
Step-by-step explanation:
Porosity in a weld can occur for various reasons, and among the options provided, rust on the metal is a typical cause. When a metal surface that is supposed to be welded is rusty, it can lead to impurities within the weld pool as the rust (iron oxide) and other contaminants are not conducive to creating a clean weld. These impurities can get trapped in the cooling metal, creating cavities or pores. On the other hand, welding too slowly can lead to excessive heat input, which can also introduce porosity by creating excessive gas bubbles that fail to escape from the molten pool. However, too short of an arc has less to do with porosity and more with weld quality concerning stability and penetration. So, the correct answer would be a. Rust on the metal.
The provided reference information gives an insight into the corrosion process where protective coatings on iron are breached, leading to rust formation. This rust can promote porosity if the affected areas are welded without proper cleaning and preparation, as rust introduces contamination in the welding process.