A brazed joint made without flowing nitrogen through the tubing develops a pinhole leak after a season of operation, and the system also shows a restricted metering device. What did the missing nitrogen contribute to?
Correct Answer
A) Oxide scale formed inside the hot tubing, leaving contamination in the joint and loose flakes circulating in the system
Copper heated in the presence of air grows a black oxide scale on the inside of the tube. Some of that scale stays at the joint, where it can leave a path for a slow leak to develop, and the rest flakes off and travels with the refrigerant and oil until it lodges in a screen or a metering device. Flowing a small amount of dry nitrogen through the tubing during brazing displaces the oxygen so the scale never forms. It is a two-minute step that prevents both problems.
Why This Is the Correct Answer
Oxygen plus hot copper produces cupric oxide scale on the inside surface. That scale contaminates the joint area where the filler is trying to bond and then breaks loose as the system runs, which is why a no-purge braze often shows up as both a leaky joint and a plugged screen or metering device.
Why the Other Options Are Wrong
Option B: Nitrogen flowing through the tube would have cooled the joint fast enough for the filler metal to harden properly
The purge is a trickle of gas rather than a quench, and cooling a brazed joint rapidly is undesirable in any case. Nitrogen protects the bore of the tube while it is hot; it does nothing to the filler metal.
Option C: Without a nitrogen purge the filler metal cannot be drawn into the joint by capillary action at all
Filler metal flows into a properly cleaned, fitted and heated joint whether or not nitrogen is moving through the line. The purge protects the inside of the tube rather than the capillary action at the joint.
Option D: The missing nitrogen let the copper absorb refrigerant vapor during brazing, which weakened the tube wall right at the joint
Copper does not absorb refrigerant, and the tubing should be empty of refrigerant before a torch comes near it. What hot copper does take up in an unpurged tube is oxygen, and that is what forms the scale.
Memory Technique
No purge, black scale. The flakes plug the metering device and the joint weeps.
Reference Hint
Flowing dry nitrogen through tubing during brazing prevents internal oxide scale; leak repair and verification requirements are in 40 CFR Part 82, Subpart F.
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