Effect of nitrogen in argon as a shielding gas on tungsten inert gas welds of duplex stainless steels

The effect of nitrogen content in the argon shielding gas of tungsten inert gas (TIG) welding on the corrosion resistance of a duplex stainless steel (DSS; AISI 2205) was investigated using potentiodynamic and cyclic polarization tests in lithium bromide (Liar) heavy brine solution. The weldability of DSS is related to the possibility of maintaining a microstructure with a ratio of ferrite to austenite phase of about 50:50. The ferrite-austenite ratio in DSS welding metals was analyzed in terms of quantitative metallography. Research work has also been carried out on the composition of the filler metal. Microstructural examination showed that the austenite phase in the weld increases with the presence of nitrogen in the shielding gas. Besides nitrogen content in the mixture Ar-N-2 also changes the morphology of the welds from an acicular to globular form of the austenite when it increases in the shielding gas. From the in-situ observation of the initiation and progress of corrosion in the welding metals, no difference was noticed between the samples welded in the presence or absence of nitrogen in the shielding gas. However, from electrochemical data it has been observed that subsequent additions of nitrogen in the shielding gas enhanced the pitting resistance of the welds. Nitrogen presence shifts pitting potentials of the welds toward higher values and broadens the passive region of those samples. On the other hand, the passive current of the electrodes welded under an atmosphere of Ar-N2 increases with the nitrogen content in that shielding gas. The higher austenite content in the welds with nitrogen, which have less molybdenum and chromium, produces higher passive current in these samples. The only advantage of welding under pure argon is the narrowest hysteresis loop of the welds. Pure argon reduces the appearance of nitride formation in the weld, which is responsible for the difficult repassivation of the samples welded in the presence of nitrogen.