What is Yellow fog weld defect? how to determine

What is Yellow fog weld defect? how to determine

“Yellow fog” is not a standard term in welding to describe a specific weld defect. However, based on the description, it may refer to a situation where the shielding gas used in gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW) appears yellow or discolored during the welding process. This discoloration can be indicative of contamination or impurities in the shielding gas or weld environment, which can have detrimental effects on weld quality.

Identification and Determination:

  1. Visual Inspection: During welding, operators may observe the shielding gas around the weld area appearing yellow or discolored, resembling a foggy haze.
  2. Effects on Weld Appearance: The yellow fog may affect the appearance of the weld bead, potentially indicating the presence of contaminants or impurities that have infiltrated the shielding gas.

Potential Causes:

  1. Contaminated Shielding Gas: Presence of contaminants such as oil, moisture, or particulate matter in the shielding gas supply can cause discoloration and affect gas performance.
  2. Welding Environment: Factors such as poor ventilation or inadequate gas coverage due to wind or drafts can lead to the entrainment of atmospheric gases or impurities into the shielding gas, resulting in discoloration.

Prevention Strategies:

  1. Gas Quality Assurance: Ensure that shielding gas cylinders are stored, handled, and maintained properly to prevent contamination. Regularly check gas cylinders, regulators, and hoses for leaks, damage, or signs of contamination.
  2. Environmental Control: Minimize exposure to atmospheric contaminants by welding in a clean, well-ventilated environment, free from drafts or air currents that can introduce impurities into the shielding gas.

Impact on Weld Quality:

  1. Porosity and Contamination: Contaminated shielding gas can lead to the formation of weld defects such as porosity, oxidation, or inclusion of foreign materials, compromising weld integrity and mechanical properties.
  2. Incomplete Fusion: Inadequate shielding due to contaminated gas may result in incomplete fusion or lack of penetration, reducing weld strength and increasing the risk of weld failure under load.

Real-Life Example:

Consider a welding operation in a manufacturing facility where aluminum components are being welded using gas metal arc welding (GMAW). During welding, operators notice that the shielding gas appears yellow or discolored, indicating potential contamination. Visual inspection of the welds reveals signs of porosity and discoloration in the weld bead, prompting further evaluation using non-destructive testing techniques such as radiographic testing.

Radiographic Process:

In radiographic testing (RT) for weld inspection:

  • X-rays or gamma rays are directed through the weld joint onto a film or digital detector.
  • Any internal defects or anomalies caused by contaminated shielding gas, such as porosity or lack of fusion, may be detected as variations in density or clarity in the radiographic image.


Yellow fog, indicative of contaminated shielding gas, can have detrimental effects on weld quality by promoting the formation of defects such as porosity or incomplete fusion. By ensuring gas quality assurance and environmental control measures, welders can minimize the risk of contamination and maintain consistent, high-quality welds in various welding applications. Regular inspection and testing help identify and address potential issues early, ensuring the integrity and reliability of welded components.

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