Why must base metal be clean before welding and what methods are common?

• A. Contaminants cause porosity and inclusions; clean with wire brushing, grinding, degreasing, solvent cleaning, and rust removal.
• B. Cleaning is optional and does not affect weld quality.
• C. Cleaning only removes rust; other contaminants do not affect welds.
• D. Base metal cleanliness is not important if shielding gas is used.

Answer: (A)

Keeping the base metal surface clean before welding is essential because any oil, grease, dirt, rust, paint, or oxide on the surface can vaporize or trap gases when heated, preventing proper fusion and leading to porosity and inclusions in the weld. A clean, oxide-free surface allows the filler metal to wet the base metal and fuse evenly, producing a stronger, defect-free weld.

Common methods to achieve cleanliness include mechanical cleaning with a wire brush or grinding to remove rust and scale, and chemical cleaning or degreasing to remove oils, grease, and hydrocarbons. Solvent cleaning, rust removal, and ensuring the surface is dry are typical parts of this process, often used in combination.

Other choices miss this point: cleaning is not optional, because contaminants directly affect weld quality; it’s not only about removing rust, since oils and other coatings also harm the weld; and shielding gas does not replace the need to clean the surface.