Effects of nitrogen content on pitting corrosion resistance of non-magnetic drill collar steel

High-nitrogen (N) austenitic stainless steel (Cr–Mn–N series) is commonly used for non-magnetic drill collars, which exhibits excellent mechanical properties and corrosion resistance. The effects of N content (0.63 to 0.86 wt.%) on the pitting corrosion behavior of the experimental non-magnetic drill collar steel were investigated using the electrochemical tests and immersion tests. Besides, X-ray photoelectron spectroscopy was used to analyze the constitution of the passive film. The results show that with the enhancement of N content from 0.63 to 0.86 wt.%, the metastable pitting corrosion sensitivity of the tested materials in 3.5 wt.% NaCl solution decreased and the pitting corrosion resistance increased. Meanwhile, the corrosion rate in 6 wt.% FeCl₃ solution at 30 °C decreased from 10.40 to 4.93 mm/a. On the other hand, nitrogen was concentrated in the form of ammonia (NH₄⁺ and NH₃) on the outermost surface of the passive films. The contents of Cr₂O₃ and Fe₂O₃ raised in the passive films, together with the content of CrN, at metal/film interface increased as N content increased from 0.63 to 0.86 wt.%, which facilitated protective ability of the passive films, thus contributing to higher pitting corrosion resistance.