Anticorrosion Properties of the Low-Temperature Glow Plasma Nitriding Layer on AISI 904L Austenitic Stainless Steel in Hydrofluoric Acid Obtained at Various NH3 Pressures

Wei Shi,Jiaxu Wang,Song Xiang,Ruyi Jiang

doi:10.3390/coatings10121156

Abstract

A low-temperature (400 °C) glow plasma nitriding layer on AISI 904L austenitic stainless steel was obtained at various NH3 pressures and studied using electrochemical method, X-ray diffraction, and scanning Kelvin probe. The pressure of NH3 dominated the microstructure of the nitriding layer. The saturation degree of γN controlled corrosion performance and microhardness. Insufficient NH3 pressure (<100 Pa) resulted in discontinuous nitride caking coverage, whereas excessive NH3 pressure (>100 Pa) facilitated the transformation of the nitriding layer to harmful nitrides (CrN) due to a localized overheating effect caused by the over-sputtering current.

Highlights

Austenitic stainless steels (ASSs) are employed in many industrial fields due to their corrosion resistance, ease of formability, and weld ability [1,2,3]
The black pitting was the cathodic sputtering damage caused by the glow discharge [20,21], and the amount of such pitting decreased with increasing NH3 pressure
Compared with that of the 904L ASS matrix, the microhardness of all the specimens subjected to low-temperature glow plasma nitriding (LTN) at different NH3 pressures had greatly improved

Summary

Introduction

Austenitic stainless steels (ASSs) are employed in many industrial fields due to their corrosion resistance, ease of formability, and weld ability [1,2,3]. The corrosion resistance of ASSs in chloride solution or H2 SO4 /HF has been reported [4,5], whereas the corrosion of stainless steel in HF has received little attention. Polymer materials, such as polytetrafluoroethylene (PTFE), are more widely used in HF-related industries than stainless steel to prevent corrosion, metals are irreplaceable in some cases. The present research originated from an engineering problem in a heat exchanger in the phosphorite exploitation industry. The metal tubes of heat exchangers suffer from HF-containing solvent and mineral particle wear. Stainless steel with high surface hardness can be ideal tube materials

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Discussion

Conclusion