What can cause porosity in a weld?

Porosity in a weld is primarily caused by the presence of impurities in the weld area, which can lead to trapped gas bubbles within the solidified weld. Rust on the metal is a significant contributor to this problem. When the base metal has rust, it introduces contaminants that can create hydrogen and other gases during the welding process. These gases can then get trapped in the weld as it cools, leading to porous connections that can weaken the integrity of the weld.

Moisture, while also a potential issue, specifically refers to water on the weld surface, which is more closely related to issues such as hydrogen embrittlement than to porosity directly caused by the metal's surface condition. Heat from welding can influence various aspects of the weld but doesn't directly lead to porosity in the absence of contaminants. The incorrect electrode type may affect the quality of the weld but typically would not lead to porosity unless paired with unsuitable conditions or contaminants in the weld area.