What causes porosity in brazed joints?

• A. Inadequate cleaning, contamination, insufficient flux, moisture, or overheating causing gas entrapment.
• B. Excessive flux removal.
• C. Proper flux concentration prevents porosity.
• D. Cooling too slowly prevents porosity.

Answer: (A)

Gas porosity in a brazed joint happens when gas gets trapped in the molten braze as it solidifies. The most common causes are surface contaminants and inadequate cleaning—leaving oils, oxides, or scale that release gas or prevent proper wetting. If flux isn’t applied correctly or is insufficient, the protective shielding isn’t enough to keep air and oxides out, so gases can form and remain trapped. Moisture on flux or the parts turns to steam in the heated joint, creating bubbles. Overheating can also break down flux and promote gas formation or oxide growth, which traps gas in the braze. All these factors lead to gas entrapment and porosity.

Excessive flux removal removes the protective layer, which promotes oxidation rather than porosity, while proper flux concentration helps prevent porosity. Cooling slowly doesn’t directly create porosity; it affects other aspects of the joint but not the gas trapping that causes porosity.