Influence of Alloying Elements on the Corrosion Properties of Various Plasma Nitrided Steels

Plasma nitriding is one of the surface hardening processes widely used for improving wear, fatigue and corrosion properties of steel due to its beneficial features such as good reproducibility and low distortion of the treated pans. It is known that depending on the type of metals and corrosion environment, nitriding can either increase or decrease the corrosion resistance [1 ]. In general, the corrosion resistance of stainless steels is usually adversely affected by nitriding, whereas, beneficial effect can be obtained for low alloy steels [2]. It is reported that the improvement in corrosion resistance is related to the presence of the g' - nitride and the predominance of CrN precipitation is responsible for the deterioration in corrosion. Ibendorf and Schroter measured the surface corrosion potential of ferrous metals and indicated that the e-phase exerted a strong passivation tendency. The present study investigates the effect of alloying elements on the corrosion behaviour of steels after plasma nitriding.

Four different steels with varying amount of alloying elements were selected for plasma nitriding process. They are AISI 1045,4340, H13 and 304 steels. The plasma nitriding process was performed with 35% of nitrogen and 65% of hydrogen gas mixture at 520 deg. C for 10 hours.

A typical microhardness depth profile for all steel samples are shown in Apparently the hardness increased after plasma nitriding. Plasma nitrided AISI 304 had a much higher hardness followed by AISI H13. AISI 4340 and AISI 1045. The profile shows that the hardness decreases gradually as a function of distance from the surface producing a diffused interface between case and core of the plasma nitrided steel in case of AISI 1045, AISI 4340 and AISI H13 steel.

The microstructure of the cross-sectioned plasma nitrided samples. A while layer of 8.8 microns and 6 microns in case of AISI 1045 and AISI 4340 steel was observed respectively with no evidence of diffusion layer. There was no while layer formation in case of AISI H13 and AISI 304 steel.

The electrochemical corrosion behaviour of AISI 1045, AISI 4340, H13 and AISI 304 without nitriding and after nitriding. The excellent corrosion resistance of stainless steel has been attributed to the presence of native passive layer of Cr2o3. In the present work the plasma nitiriding of SS led to CrN precipitation. The separation of Cr as a result of CrN precipitation led to the loss of its passivating property. The protection rates were highest in AISI 4340 followed by AISI 1045, AISI H13 and AISI 304 steel. Also, AISI 4340 and AISI H13 show the same trend of passivation after plasma nitriding process. This is due to the presence of alloying elements like Mo, Ni and V. It can be concluded that the low alloy steels had better corrosion resistance properties. The presence of thin white layer comprising of iron nitrides in case of low alloy steels AISI 1045 and AISI 4340 created a protective layer against corrosion. However potentiodynamic studies also indicated a better protection rate of AISI H13 steel compared to AISI 304. This increment could be due to the presence of alloying elements like Mo, Ni, and V, which extend die pre-passive potential to the noble direction.

Source: Facilitation Centre for Industrial Plasma Technologies