Jothi Sudagar

High Corrosion Resistance of Electroless Ni-P with chromium-free Conversion pre-treatments on AZ91D Magnesium Alloy

Phosphate-manganese, tannic acid and vanadium conversion coatings were proposed as an effective pre-treatment layer between electroless Ni-P coating and AZ91D magnesium alloy substrate to replace the traditional chromate plus HF pre-treatment. The electrochemical results show that the chrome-free coatings plus electroless Ni-P coating on the magnesium alloy has the lowest corrosion current density and most positive corrosion potential compared with chromate plus electroless Ni-P coating on the magnesium alloy. These proposed pre-treatment layers on the substrate reduce the corrosion of magnesium during plating process, and reduce the potential difference between the matrix and the second phase. Thus, an electroless Ni-P coating with fine crystalline and dense structure was obtained, with preferential phosphorus content, low porosity, good corrosion-resistance and strengthened adhesion than the chromate plus electroless Ni-P.

Microstructure and Mechanical Properties of an Extruded Mg-2Dy-0.5Zn Alloy

Microstructure and mechanical properties of an extruded Mg-2Dy-0.5Zn (at.%) alloy during isothermal ageing at 180 °C were investigated. Microstructure of the as-extruded alloy is mainly composed of a-Mg phase, 14H long period stacking order (LPSO) phase and small amounts of (Mg, Zn) x Dy particle phases. During ageing, the 14H LPSO phase forms and develops and its volume fraction increases with increasing ageing time. Tensile test showed that the peak-aged alloy exhibits similar yield and ultimate tensile strengths and elongation to failure at room temperature, 100 °C and 200 °C, but excellent elevated temperature strengths at 300 °C as compared to the as-extruded and over-aged alloys. The analysis showed that the excellent elevated temperature strengths of the peak-aged alloy are attributed to the LPSO phase strengthening and the grain refinement strengthening, and the role of the LPSO strengthening is related to not only its amount, but also its morphology.

Electroless Ni–P deposition with vanadium based coating as pretreatment on AZ91D magnesium alloy

Abstract A vanadium-based conversion coating is proposed as an effective pretreatment for electroless Ni–P deposition on AZ91D magnesium alloy. The magnesium alloy substrate was immersed in a NaVO3 solution with various parameters such as the vanadium concentration, immersion time and bath temperature being varied. The results reveal the quality and corrosion protection performance of the coating increases with increasing treatment time up to a limit. However, excess treatment time induces the formation of cracks in the coating layer, leading to reduced corrosion resistance of the conversion coating. Increasing bath temperature decreases the crack density and increases the thickness of the conversion layer. Electrochemical polarisation results showed that the vanadium treatment in a bath containing NaVO3 of concentration 30 g L−1 at 80°C for 15–20 min gives good corrosion resistance. This optimum vanadium treatment was used as a pretreatment for the subsequent electroless Ni–P deposit and it had good passive parameters, showing better corrosion protection on AZ91D substrate than a chromium based treatment.

Influence Of Surfactants On The Corrosion Properties Of Chromium-Free Electroless Nickel Deposit On Magnesium Alloy

Influence of surfactants on the corrosion properties of chromium-free electroless nickel deposit were investigated on AZ91D magnesium alloy. The corrosion tests were carried out by immersion test (1 M HCl) and electrochemical polarization test (3.5 wt% NaCl). The surfactants in the electroless nickel bath increases the corrosion resistance properties of the deposit on the magnesium alloy. In addition, smoothness and amorphous plus nano-crystalline phase were increased and accounted for the significant corrosion resistance. As a consequence, the corrosion potential moved towards the positive direction and the corrosion current density decreased. The immersion tests also provided the same trend as that of electrochemical polarization test. On the whole, the study concluded that corrosion resistance was enhanced by including a surfactant in the electroless deposits on magnesium alloy.

A new method to prepare MgO and base for further electroless nickel deposition on magnesium substrate

The present research uses the safer hydrogen halide to prepare the MgO-MgI2/MgBr2 layers onto AZ91 magnesium alloy in alkaline solution. The water-soluble metal salts (MgI2/MgBr2) were not responsible for the corrosion protection; whereas Mg(OH)2 formed first and then converted to MgO by heat-treatments and this MgO was responsible for the corrosion protection. The electrochemical results showed that the development of MgO mitigated the corrosion process and improved the corrosion resistance. This new treatment was then used as a pre-treatment for further electroless nickel (EN) deposition and it showed the good corrosion current density and better corrosion potential on the magnesium substrate. This new treatment and EN depositions were characterised by XPS and SEM. The electrochemical polarisation and EIS test of these EN depositions were evaluated in 3.5 wt.% NaCl. In addition, pitting behaviour of these EN depositions were evaluated by linear sweep voltammetry test.