Corrosion And Wear Behaviour of Plasma Electrolytic Oxidation And Laser Surface Alloy Coatings Produced on Mg Alloys
In the present investigation, surface coatings employing laser surface alloying (LSA) and plasma electrolytic oxidation (PEO) processes have been prepared on Mg alloys. The coatings have been investigated for corrosion and wear behaviour. Two important Mg alloys based on Mg–Al system were selected namely, MRI 230D and AM50 as substrates. LSA coatings have been prepared employing Al and Al2O3 as precursors using different laser scan speeds. PEO coatings were prepared in standard silicate and phosphate based electrolytes employing unipolar, pulsed DC. Hybrid coatings using a combination of the two processes were also produced and investigated for corrosion and wear behaviour. Hybrid coatings of LSA followed by PEO (LSA+PEO) were investigated for effectiveness of sealing the cracks in the LSA coatings by subsequent PEO process and consequent improvement in the corrosion resistance. Hybrid coatings of PEO followed by LSA (PEO+LSA) were prepared with an objective of sealing the pores in the PEO coating LSA treatment. In an attempt to produce more compact PEO coatings, electrolyte containing montmorillonite clay additives was employed for the PEO process of AM50 Mg alloy. The coatings were produced employing different current densities and the effect of current density on the microstructure and corrosion behaviour of coating was investigated. Electrochemical corrosion tests of uncoated and coated alloys were carried out in 3.5 wt.% (0.6M)NaCl, neutral pH, solution with an exposed area of 0.5 cm2 for a time duration of 18.5 h. For the PEO coatings with clay additives, corrosion tests were conducted additionally in 0.5 wt.% (0.08 M) NaCl, neutral pH, solution for a time duration of 226.1 h. Wear behaviour of LSA coatings was analyzed by employing a pin on disc tribo–tester conforming to ASTM G–99 standard at ambient conditions with ground EN32 steel disc of hardness Rc 58 as the counterface. Tests were conducted under dry sliding conditions for a sliding distance of 1.0 km at a sliding velocity of 0.837 m/s employing normal loads of 10, 20, 30 and 40 N. Friction and wear behavior of PEO and PEO+LSA coatings were analyzed at ambient conditions by employing a ball−on−flat linearly oscillating tribometer conforming to ASTM G–133 standard. AISI 52100 steel ball of diameter 6 mm was employed as the friction partner. Wear tests were conducted under dry sliding conditions for a total sliding distance of 100 m at normal loads of 2 N and 5 N with oscillating amplitude of 10 mm and mean sliding speed of 5 mm/s. LSA coatings could not improve the corrosion resistance of MRI 230D Mg alloy. This was attributed to the presence of cracks in the LSA coating, which resulted in the accelerated galvanic corrosion of the substrate. LSA coatings improved the wear resistance at all loads. The improved wear resistance was attributed to β (Mg17Al12) phase and Al2O3 particles in the coating which increased the hardness of the LSA layer. No trend in corrosion and wear resistance with laser scan speed was observed for LSA coatings. PEO coatings improved the corrosion resistance of the MRI 230D Mg alloy significantly. The improved corrosion resistance was attributed to the enhanced barrier protection provided by dense barrier layer formed at the substrate/coating interface and to the insoluble phase constituents in the coatings. PEO coating was effective in improving the wear resistance at low loads/contact pressures. At higher loads, the coating underwent micro–fracture as a result of the porosity in the coatings. Hybrid coatings of LSA followed by PEO (LSA+PEO) in silicate based electrolyte improved the corrosion resistance of LSA coatings. However, the corrosion resistance was not improved to the extent of PEO coatings on as–cast alloy as a result of cracks in the primary coatings, which were not fully sealed by the plasma conversion products. No trend in corrosion resistance with laser scan speed was observed for LSA+PEOcoatings. In hybrid coatings of PEO followed by LSA (PEO+LSA), primary PEO coating was completely melted and mixed with applied precursor to form a single composite LSA layer. The corrosion resistance of the hybrid coatings was observed to be lower than that of the as–cast alloy. The presence of solidification cracks reduced the barrier properties and resulted in the accelerated galvanic corrosion of the substrate similar to LSA coatings. Hybrid (PEO+LSA) coatings exhibited improved wear resistance as compared to as–cast alloy at lower loads as a result of increase in the hardness due to β (Mg17Al12) phase and oxide/ceramic particles in the hybrid layer. At higher loads, hybrid coatings exhibited higher wear rate as compared to as–cast alloy and PEO coatings. This was attributed to three–body abrasive wear as a result of dislodged hard oxide/ceramic particles in the wear tracks. No trend in corrosion and wear resistance with laser scan speed was observed for PEO+LSA coatings. PEO coatings on AM50 Mg alloy by employing clay additives in the electrolyte resulted in the reactive uptake of clay particles producing a predominantly amorphous coating at low current density. Clay additives were effective in improving the compactness of the coating at lower current density. At higher current densities, the porosity of the coatings increased. The clay particles got re–constituted producing increasing amount of crystalline phases with increase in current density. Long term impedance measurements showed that clay addition as well as increased current density employed for the PEO process was not effective in improving the corrosion resistance of the coatings. At low current density, even though the coating with clay additives was more compact, it was deficient in MgO and consisted predominantly of an amorphous phase, which underwent fast dissolution in electrolyte thereby resulting in an early loss of barrier properties. At higher current densities, even though the coatings consisted of increased amount of MgO and crystalline phases, which resist dissolution in the electrolyte, the increased porosity and defective barrier layer resulted in easy permeation of the electrolyte into the substrate/coating interface, which resulted in much earlier loss of barrier properties and inferior corrosion resistance.
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