During the GTAW of a chromoly pipe, the welder notices the root pass is discoloured a dark grey to black on the inside diameter despite using a purge. The flowmeter reading at the purge line is consistently 8 L/min. What is the most likely cause for this issue?

Question: During the GTAW of a chromoly pipe, the welder notices the root pass is discoloured a dark grey to black on the inside diameter despite using a purge. The flowmeter reading at the purge line is consistently 8 L/min. What is the most likely cause for this issue?

Answer options:

• The travel speed is too fast for the heat input. ✅ The purge gas is not pure argon, or there is an air leak in the purge system.
• The tungsten electrode has been contaminated during welding.
• The welding current is too high, causing excessive melt-through.

Correct answer: The purge gas is not pure argon, or there is an air leak in the purge system.

Explanation: Dark grey to black discolouration on the root of chromoly pipe indicates severe oxidation on the backside, typically caused by inadequate purging. This points to either impurities within the purge gas supply, an insufficient flow rate for the volume being purged, or more commonly, a leak in the purge dam or connections allowing atmospheric oxygen to enter the purged area.