Aluminide coatings modified via electroless platinum plating for high temperature applications: manufacturing and comparison with standard electrodeposited Pt-aluminides

Introduction Aluminide coatings are widely used to improve long-term oxidation resistance of Ni-based superalloy components. Pt modification considerably increases their protective capabilities when a 4-7 μm Pt layer is deposited onto the superalloy, prior to aluminizing. The widespread method for Pt deposition employed at industrial scale is electroplating, which presents several limitations when coating complex shaped components due to the edge-effect of the electric field. To overcome such limits, this work aims to develop a stable electroless solution for platinum plating, in order to manufacture Pt-modified coatings and compare their microstructure and protective capabilities with standard electrodeposited Pt aluminides. Material and Methods The proposed electroless plating solution is based on hydrazine-monohydrate as the reducing agent. Deposition parameters (temperature, pH and bath loading) are investigated in terms of bath stability, plating rate and quality of the obtained coatings. 4.5 μm of Pt are deposited on René N4 superalloy discs and compared to industrial state-of-art electrodeposited Pt layers obtained from Q-salt® solutions. Diffusion aluminides are produced by slurry aluminizing and microstructure was compared by SEM, EDS and XRD. Protective capabilities are investigated performing long-term isothermal oxidation tests at 1093°C for 1000h. Results The best deposition parameters were found to be pH=0.85, at 55°C with a bath loading of 10.8 ml/cm2, achieving high bath stability and plating rate of 3.3 mg/cm2/h. Electroless Pt layers exhibit uniform thickness along the geometry, whereas thickness of electrodeposited coatings is highly variable due to edge-effect of the electric field. Such variability is also present in Pt-aluminide coatings and, despite the same phase composition mainly made by β-(Ni,Pt)Al, electroless aluminides show more uniform Pt distribution across the geometry. Oxidation tests demonstrated comparable performance after 1000h exposure. Discussion pH was the main factor affecting stability of the electroless plating solution and the decrease of hydrazine reduction potential at low pH allows controlled reduction of Pt2+. Deposition temperature was selected determining the best compromise between plating rate and coating quality, since too high temperature leads to fast and uncontrolled reactions that negatively affect deposition. Eventually, bath loading effect was investigated to determine its relationship with plating rate and deposition efficiency. Tuning of these parameters guarantees a deposition process that complies with the practical necessities of industry. High uniformity of Pt electroless coatings, comparable phase composition between electroless and electrodeposited Pt-aluminides and similar protective capabilities demonstrate that electroless process can be an effective alternative to electroplating for high temperature applications.