The surfaces of AISI 316L stainless steel were laser alloyed with ruthenium powder and a mixture of ruthenium and nickel powders using a cw Nd:YAG laser set at fixed operating parameters. The microstructure, elemental composition, and corrosion characteristics of the alloyed zone were analyzed using optical and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and corrosion potential measurements. The depth of alloyed zone was measured using the AxioVision program and found to be approximately 1.8 mm for all the alloyed specimens. Hardness profile measurements through the surface-substrate interface showed a significant increase from 160 HV for the substrate to a maximum of 247 HV for the alloyed layer. The sample laser alloyed with 80 wt% Ni-20 wt% presented the most noble corrosion potential
Minor additions of ruthenium to the bulk volume of steels resulted in a significant improvement of corrosion resistance in many reducing environments [
Laser surface modification techniques have been extensively studied for selective improvement of surfaces for wear, hardness, and corrosion [
In this study, laser surface alloying of AISI 316 with Ru and with (Ru + Ni) mixed powders was investigated. The effect of the amounts of Ru added into the alloyed layer was studied by selecting a mixture with the following compositions (80 wt% Ni + 20 wt% Ru) and (50 wt% Ni + 50 wt% Ru). The objective(s) has been to keep the Ru content low, while maintaining superior corrosion resistance. The microstructure, chemical composition, hardness, and corrosion behaviour of the alloyed layers were analysed using SEM, EDX, hardness tester, and corrosion potentials.
The AISI 316 stainless steel was cut into 10 × 5 × 0.5 cm rectangular plates. The surfaces of the plates were sandblasted and cleaned with acetone prior to laser surface alloying. Nickel and ruthenium were in the form of powders of commercial purity, 99.6 wt% and 99.9 wt%, respectively, were used to surface alloy AISI 316 stainless steel samples. The powders were mixed to specific Ru : Ni wt% proportions, and Table
Composition of the powders used on sample.
Alloy name | Powder composition | Substrate material | |
---|---|---|---|
Nickel wt% | Ruthenium wt% | ||
Alloy 1 | 0 | 100 | AISI 316 |
Alloy 2 | 80 | 20 | AISI 316 |
Alloy 3 | 50 | 50 | AISI 316 |
The laser surface alloying was performed with a Rofin Sinar DY044 continuous wave Nd : YAG laser. A 600
Metallographic specimens were prepared by cutting the samples transversally across the alloyed layer and mounting the pieces separately in a bakelite or/and lucite powder using a mounting press. The samples were etched electrolytically in 60 wt% nitric acid in distilled water at 1.5 V for 20 s. The microstructures and the elemental composition profile were evaluated using Zeiss Axiotech 25 HD microscope and JSM 5800 LV SEM with energy dispersive X-ray spectroscopy (EDS), respectively. The hardness was determined using a Future-Tech FM-700 Vickers Micro-hardness testing instrument.
The corrosion tests were carried out in 80% sulphuric acid solution which was kept at 60°C using a thermostat-controlled bath. The corrosion performance of the laser alloyed surfaces was evaluated by means of electrochemical polarization measurements using an Autolab potentiostat, which utilizes platinum as the opposite electrode and a saturated silver-silver chloride electrode as the reference electrode. Potentiodynamic polarization curves were obtained for each alloy. A scanning rate of 0.1 mV/sec was used to conduct all the measurements.
Figure
Typical cross section of the laser alloyed zone.
Figure
Optical microstructures of laser alloyed AISI 316 SS: (a) alloy 1 (9.6% Ru) and (b) alloy 2 (5.6% Ru).
The average chemical composition of the laser alloyed layer obtained by EDX analysis is shown in Table
EDX elemental composition of the laser alloyed layer (wt%) with an error of 0.2 wt%.
Alloys | Al | Si | Cr | Mn | Fe | Ni | Mo | Ru |
---|---|---|---|---|---|---|---|---|
Alloy 1 | 0.1 | 0.4 | 16.2 | 1.6 | 61.2 | 9.1 | 1.8 | 9.6 |
Alloy 2 | 0.1 | 1.3 | 17.1 | 1.3 | 50.3 | 22.8 | 1.5 | 5.6 |
Alloy 3 | 0.1 | 1.2 | 14.7 | 1.3 | 53.5 | 16.9 | 1.0 | 9.5 |
Microhardness measurements across the bead-substrate interface revealed a significant increase in hardness, varying from 158 HV for the AISI 316 to substrate 247 HV for the laser alloyed bead, for alloy 1, Figure
Variation of hardness with the distance from the bead/substrate interface in an AISI 316 stainless steel surface alloyed with Ru. The hardness measurements showed an error of approximately 3%.
Potentiodynamic polarisation curves of the samples tested in 80% sulphuric acid at 60°C are shown in Figure
Comparison of potentiodynamic polarization curves for AISI 316 SS substrate, 5.6 wt% Ru steel and 9.6 wt% Ru steel.
Higher Ru content on the surface does not necessarily give better corrosion behavior. The effect of Ru on the corrosion behavior of the surface depended on the amount of Ni present. Higher nickel contents showed more effective improvements in corrosion resistance. Thus, laser surface alloying with Ru and Ni together present an economical way of using the two elements, because the Ru amount can be kept optimally low for maximum corrosion enhancement. The nature of the alloyed surface was greatly affected by the variation in the composition of the preplaced powder, thus showing that the laser surface alloying is system dependent. Further investigations into the surface alloying with ruthenium and nickel, particularly to identify their optimal composition for maximum corrosion improvements on various steel surface, is highly recommended. The laser surface alloying with Ru and nickel can be applied to various alloys. The surface alloying approach might be particularly suitable for thick engineering components or plates which require better corrosion properties, although the alloying should be sufficient to ensure that all the elements are in solution.
The authors would like to thank the CSIR-National Laser Centre for using its facilities. The Department of Science and Technology and the National Research Foundation, South Africa, are thanked for funding and support.