Study on salt spray corrosion of Ni–graphite abradable coating with 80Ni20Al and 96NiCr–4Al as bonding layers

Two kinds of Ni–graphite abradable coating systems (coating system A with 80Ni20Al bond coating and coating system B with 96NiCr–4Al bond coating) were prepared. The top and the bond coatings were independently deposited by different techniques of flame and plasma spraying. Corrosion behavior of the coating systems was investigated via the electrochemical and salt spray corrosion tests. Open-circuit potential and polarization tests were carried out in an oxygen-saturated 5 wt.% NaCl neutral solutions at 35 °C. It was found that the 80Ni20Al coating exhibited the lowest corrosion resistance. Results of the salt spray corrosion (SSC) tests support a corrosion mechanism in the Ni–graphite coating being a crevice corrosion, which leads to loose corrosion products accumulated on the surface of the coating systems. For coating system A, analysis of the corrosion damage of the bonding layer showed a prevalent localized attack on intersplat boundaries. For coating system B, the bonding layer exhibited less corrosion, but the matrix of the top coating close to the interface corroded heavily due to a galvanic coupling between these two different coating layers. Experimental results after 960 h SSC tests showed that the bonding strength decreased to 3.5 MPa and 2.6 MPa for coating systems A and B, respectively.