What defect is most commonly caused by insufficient cleaning of the base metal before GMAW?

• A. Cracking in the weld bead
• B. Lack of fusion at weld toe
• C. Porosity from oil, dirt, rust, or moisture in the weld area
• D. Excess weld metal buildup

Answer: (C)

When the base metal isn’t cleaned, oil, dirt, rust, or moisture can remain on the surface. During GMAW, the shielding gas protects the molten weld pool, but it can’t remove these contaminants. As the metal melts and solidifies, trapped gases from those substances form tiny voids, which appear as porosity in the weld. This is the most common defect tied to inadequate cleaning because it directly results from contaminants in the weld area. Other issues like cracking, lack of fusion at the toe, or excess weld metal buildup are usually due to heat/technique, joint fit-up, or deposition causes rather than surface cleanliness.