How to prevent backside oxidation during stainless steel pipe welding

1. Introduction 
During welding of stainless steel pipes in petrochemical construction, the weld surface and heat-affected zone are prone to oxidation and discoloration. To ensure the corrosion resistance of stainless steel, pickling and passivation treatment are performed on the weld after welding to form a dense oxide film on its surface. However, the inner wall of the stainless steel pipe often cannot be pickled and passivated, severely reducing its corrosion resistance. General welding processes and construction measures make it difficult to guarantee the welding quality of the back-side weld and heat-affected zone. Therefore, it is necessary to improve the welding process and take measures to prevent backside oxidation and discoloration.

2. Reasons for easy oxidation of the back side and surface of stainless steel pipe welds
1) When the back side is purged with argon gas, the air inside the stainless steel pipe is constantly in a vortex-like movement, making it difficult to completely remove. Even with extended replacement time, the oxygen content can reach <0.01% (volume fraction) in a fully sealed state. However, after opening the bevel and starting welding, the oxygen content will rise to 0.05% (volume fraction) or even higher. This high oxygen content ultimately leads to oxidation of the stainless steel pipe surface on the weld, turning it blue or purple.
2) During on-site construction, welders typically use simple TIG welding torches with a scratch-and-ignite technique. After the arc is interrupted, the high-temperature weld bead instantly loses its argon protection, leading to easy oxidation and discoloration at the joint after each arc termination.
3) At each arc initiation, air enters the inner weld bead through the open weld groove. At this time, the oxygen content in the inner weld bead is high, making it prone to localized oxidation.
4) During filler and capping layer welding, high welding current, high heat input, or high interpass temperature can also cause oxidation and discoloration on the back of the weld bead.
5) Low argon purity and high oxygen content reduce the protective effect on the weld.
6) Stainless steel has poor thermal conductivity, only 1/3 that of steel. During welding, heat cannot dissipate effectively, resulting in high temperatures in the weld bead and heat-affected zone, making them more susceptible to oxidation.

3. What are the improvements to the welding process for stainless steel pipes?
1) Oxygen in the shielding gas at the inner weld bead is the main cause of oxidation and discoloration in the weld bead and heat-affected zone. Therefore, eliminating oxygen from the shielding gas can solve the oxidation problem. After process improvement, 99.999% high-purity argon is used as the welding gas, and a mixed gas (5% H2 + 95% Ar) is used as the back shielding gas. The chemical reaction between hydrogen and oxygen at high temperatures reduces the oxygen content inside the tube.
2) Argon is denser than air. When replacing the air inside the tube, a low-fill, high-displacement principle is adopted, and the filling and replacement time is appropriately extended.
3) A welding machine and high-frequency arc-starting torch with functions such as current slow-increase, current decay, early gas supply, and delayed gas stop are used. At the moment of arc ignition, the burning arc first burns away the oxygen in the shielding gas, preventing oxidation of the weld joint during arc initiation. After the arc is extinguished, the high-temperature weld remains under effective argon protection, preventing localized oxidation and discoloration of the joint.
4) The nozzle diameter is increased to φ10～φ12mm to increase the protection range.
5) High-quality aluminum foil tape with good sealing performance is used to seal the bevel; paper tape cannot be used.
6) Reduce the argon gas flow rate at the final joint. Ideally, maintain a constant argon gas flow rate while opening the vent on one side for optimal welding results.
7) Control the interpass and interpass temperatures to no more than 60℃. During welding, cooling methods such as wrapping circulating water copper pipes around the weld sides or using damp cotton cloths can be employed to reduce the high-temperature dwell time of the weld.
8) Use low-heat-input TIG welding for the first three layers, while continuing argon gas protection at the inner rim. Once a certain thickness is achieved, then use electrode welding. Whether using TIG welding or electrode arc welding, keep the current as low as possible, employing a straight-line or slightly oscillating arc movement method. While ensuring weld quality, the welding speed should be as fast as possible to reduce heat input and prevent excessively high weld temperatures and oxidation.

4. Summary 
In the construction of a chemical plant, stainless steel pipes cannot be pickled and passivated after welding. Furthermore, oxidation of the inner surface of the weld is not permitted during welding, requiring the weld and heat-affected zone to be silver-white. After welding using the above-mentioned process measures during construction, the design and owner's requirements were met, ensuring the corrosion resistance of the stainless steel pipes.