Corrosion Behavior and Characterization of Plasma Nitrided and Borided AISI M2 Steel

This study investigates the corrosion behaviors of plasma nitrided (PN) and borided AISI M2 steels. The PN process was carried out in a dc-plasma system at a temperature of 923K for 6 h in a gas mixture of 80%N 2 -20%H 2 under a constant pressure of 5 mbar. The boriding process was carried out in Ekabor-II powder at a temperature of 1223K for 6 h. X-ray diffraction analysis on the surface of the PN and borided steels revealed the presence of FeB, Fe 2 B, CrB, MoB, WB, FeN, Fe 2 N, Fe 3 N and Fe 4 N compounds. Corrosion surfaces of the samples were analyzed using a SEM microscopy and X-ray energy dispersive spectroscopy EDS. The corrosion resistance of PN and borided AISI M2 steel is higher compared with that of untreated AISI M2 steel. Corrosion resistances of the PN and borided AISI M2 steel increased the 4-6fold.


Introduction
Plasma nitriding and boriding are a surface hardening and advanced surface modification technology which has recently become industrially important 1 .It is used for increasing wear resistance, fatigue strength, surface hardness and corrosion resistance of industrial components for a wide variety of applications [1][2][3] .Nitriding of steel results in formation of a compound layer and nitrogen diffusion zone beneath the compound layer.The diffusion layer is composed of nitrogen saturated ferrite together with dispersed precipitates of iron and alloy element nitrides, but the compound layer is mainly a combination of iron nitrides 4 .
Boriding is a diffusion surface treatment, which is defined as the enrichment of the surface of a workpiece with boron by means of thermo-chemical treatment [5][6][7] .Thermal diffusion treatments of boron compounds used to form iron borides typically require process temperatures of 973 and 1273K.The process can be carried out in laser, solid, liquid, gaseous or plasma media.Boron atoms, because of their relatively small size (atomic radius 90 pm) and very mobile nature can diffuse into substrate material (atomic radius 126 pm).When iron, the most current substrate metal, boron atoms dissolve in iron interstitially, but can also react with it to form boride layer.Boron atoms due to their relatively small size and very mobile nature can diffuse easily into ferrous alloys forming FeB and Fe 2 B intermetallic, nonoxide, ceramic borides.The diffusion of B into steel results in the formation of iron borides (FeB and Fe 2 B) and the thickness of the boride layer is determined by the temperature and time of the treatment [8][9][10][11][12][13][14] .
Boriding and plasma nitriding are used in numerous applications in industries such as the manufacture of machine parts for plastics and food processing, packaging and tooling, as well as pumps and hydraulic machine parts, crankshafts, rolls and heavy gears, motor and car construction, cold and hot working dies and cutting tools.The corrosion behavior of plasma nitrided and borided steels has been evaluated by a number of investigators [15][16][17][18][19][20][21] .However, there is no information about the corrosion behaviors of plasma nitrided and borided AISI M2 steel.The main objective of this study was to investigate the corrosion behaviors of plasma nitrided and borided AISI M2 steel.Structural and corrosion properties were investigated using optical microscopy, microhardness tests, XRD, SEM and EDS analysis.

Plasma nitriding, boriding and characterization
Table 1 gives the composition of untreated AISI M2 steel.The test samples were cut into Ø25x8mm dimensions and ground up to 1000G and polished using diamond solution.The PN process was carried out in a dc plasma system at a temperature of 923K for 6 h in a gas mixture of 80%N 2 -20%H 2 under a constant pressure of 5 mbar.Boriding heat treatment was carried out by using a solid boriding method with commercial Ekabor-II powders.Boriding heat treatment was performed in an electrical resistance furnace under atmospheric pressure at 1223K for 6 h followed by cooling in air.
The microstructures of the polished and etched crosssections of the samples were observed under an Olympus BX-60 optical microscope.The presence of nitrides and borides formed in the coating layer was detected by means of the X-ray diffraction equipment (Shimadzu XRD 6000) using Cu Kα radiation.The diffusion zone on the nitrided sample and the thickness of borides were measured by means of a digital thickness measuring instrument attached to an optical microscope (Olympus BX60).Micro-hardness measurements were performed from the surface to the interior along a line to see variations in the hardness of the nitride and boride layer, transition zone and matrix, respectively.The micro-hardness of the boride layers was measured at 8 different locations at the same distance from the surface by means of a Shimadzu HMV-2 Vickers indenter with a load of 50 g and the average value was taken as the hardness.

Corrosion test
The acid solution used was 4% M HCI.The aforementioned cylindrical borided specimens and nonborided specimens were weighted before immersion, with an accuracy of 0.01 mg.At specific time intervals the specimens were withdrawn from the solutions and weighted without any additional treatment.Thus, the weight loss in relation to the initially exposed surface was continuously recorded.The immersion tests were repeated 10 times and mean values were used for the acquisition of weight loss curves.Before and after each corrosion test, each sample was cleaned with alcohol.

Characterization of plasma nitriding and boride coatings
The cross-sections of the optical micrographs of the plasma nitrided and borided M2 steel are given in Figure 1.A typical optical photograph of the cross-sectional microstructure of the plasma nitrided M2 sample is given in Figure 1a.This photograph shows the formation of the     diffusion zone on the sample.However the formation of a continuous compound (white) layer on top of the nitrided layer of the sample is not visible in Figure 1a.After etching with 3% nital reagent, the nitrided (diffusion) layer appears as a dark zone on the sample.As can be seen in Figure 1b, the boride layer was formed on the M2 steel borided by a solid boriding method.Depending on the process time, temperature and chemical composition of substrates, the depth of the boride layer was found to be 55 µm.
The X-ray diffraction patterns of the nitrided and borided AISI M2 steel are given in Figure 2. XRD results showed that nitrided AISI M2 steel contained FeN, Fe 2 N, Fe 3 N, Fe 4 N and M 6 C phases (Figure 2a).XRD results showed that the boride layers formed on the AISI M2 steel contained FeB, Fe 2 B, CrB, MoB and WB phases in Figure 2b.The corrosion resistances of these nitride and boride layers are to a large extent known by the help of these phases.
Micro-hardness measurements were carried out on the cross-sections from the surface to the interior along a line in Figure 3.The hardness of the nitrided AISI M2 steel varied between 995 and 1286 HV 0,05.The hardness of the boride layer on the AISI M2 steel varied between 1378 to 1814 HV 0,05 .On the other hand, Vickers hardness values were 230 HV 0,05 for the untreated AISI M2 steel.When the hardness of the nitride and boride layer is compared with the matrix, the nitride layer hardness is approximately five times greater and the boride layer hardness is approximately eight times greater than that of matrix.Corrosion Behavior and Characterization of Plasma Nitrided and Borided AISI M2 Steel

Corrosion behaviors
Figure 4 shows the SEM and EDS (Figure 4c) analysis of the untreated AISI M2 steel before (Figure 4a) and after (Figure 4b) the corrosion test.Pores and pits were detected on the surface of the specimen after the corrosion test.In addition, oxides were observed on the surface of the specimen after the corrosion test.Figures 5a and 6a show the SEM microstructures of the specimens before the corrosion process while Figures 6b and 7b show the SEM microstructures of the specimens after the corrosion process.Oxides were observed on the surface of the specimen after the corrosion test.Oxide peak intensities formed on the surface of the specimens were found to decrease after the plasma nitriding and boriding treatments (Figure 5c and Figure 6c).After the corrosion test, porosity and pits were observed to form on the nitrided specimen.The corrosion resistance of a nitrided steel usually depends on the characteristic features of coatings such as the number of porosities (Figure 6b).These porosities negatively affect the adhesion of coatings and significantly reduce the corrosion resistance.The number of these voids is associated with the microstructure of the coating [22][23][24] .Figure 7 shows the surface roughness values of the untreated, plasma nitrided and borided specimen, before and after corrosion tests.For the untreated, plasma nitrided and borided AISI M2 steel, it was observed that surface roughness values increased the after corrosion tests (Figure 7).This result indicates that the plasma nitriding and boriding treatment have an effect both on the surface roughness and the corrosion resistances of the steel.At the end of these tests, the time-dependent variation of weight loss was obtained and given in a Figure 8 for the 4% M HCI solution.Weight loss in the untreated specimen after the corrosion was observed to increase rapidly with the increase in the processing time.It was detected that while the weight loss in the corrosion test solution during 120 h was 358.74 mg, the corrosion weight loss of the raw specimen increased to 561.82 mg. after 240 h for untreated AISI M2 (Figure 8a).In the plasma nitrided specimen after the corrosion test, it was detected that the weight loss in the corrosion test solution during 120 h was 83.72 mg while the corrosion weight loss of the plasma nitrided specimen increased to 154.18 mg. after 240 h.The lowest weight loss was observed in the borided AISI M2 specimen (93.21 mg.) at the temperature of 1223K for 6 h.While the weight loss of the untreated specimen was 561.82 mg., this value dropped to 93.21 mg. as a result of the boriding process for AISI M2 steel.The solubility of corrosion in the untreated specimen increased 6 times.As shown in the corrosion graphic curves, the solubility of corrosion decreased with the plasma nitrided and borided treatment.In addition, it was observed that the decreased solubility of corrosion in the borided specimens led to a decrease in the amount of material loss.It was observed that the solubility   of corrosion of the borided specimens and that of the plasma nitrided specimens were 6 times and 4 times lower than that of the untreated specimen, respectively.This significant decrease in corrosion rate and increase in corrosion resistance might be attributed to the formation of a protective surface coating treatment.As result of the corrosion resistances of the AISI M2 steel increased with the plasma nitriding and boriding treatment.

Conclusions
The following conclusions may be derived from the present study.
• The multiphase nitride coating which thermochemically occurred on the AISI M2 steel was constituted by the FeN, Fe 2 N, Fe 3 N, Fe 4 N and M 6 C phases while the boride coating was constituted by the FeB, Fe 2 B, CrB, MoB and WB phases; • The surface hardness of the nitrided AISI M2 steel was in the range of 995-1286 HV 0,05 , the surface hardness of the borided AISI M2 steel was in the range of 1378-1814 HV 0,05 and the untreated AISI M2 steel substrate was 230 HV 0,05 ; • The surface roughness values of the untreated, plasma nitrided and borided AISI M2 steel increased after corrosion tests; • The corrosion resistances of the plasma nitrided and borided AISI M2 steel increased 4-6-fold compared to that of the untreated AISI M2 steel; • The superior properties of the AISI M2 steel as well as poor corrosion properties were improved by the plasma nitriding and boriding process.

Figure 3 .
Figure 3. Variation of hardness depth in plasma nitrided and borided AISI M2 steel.

Figure 4 .
Figure 4. SEM and EDS analysis of the unborided AISI M2 steel a) before corrosion, b) after corrosion, c) EDS analysis.

Figure 5 .
Figure 5. SEM and EDS analysis of the plasma nitrided AISI M2 steel a) before corrosion, b) after corrosion, c) EDS analysis.

Figure 6 .
Figure 6.SEM and EDS analysis of the borided AISI M2 steel a) before corrosion, b) after corrosion, c) EDS analysis.

Figure 8 .
Figure 8. Weight loss of immersion tests of the plasma nitrided and borided AISI M2 steel in 4% M HCI solution.