Studies on the Formation of Protective Coating of Alumina over RAFM Steel Channels

Alumina (Al2O3) is a potential ceramic material which has capability to form impervious protective coating layers that can provide underlying substrate permeation barriers to toxic gases, excellent electrical insulation and high temperature anti-oxidation cum corrosion resistances. In proposed fusion reactors the coolant channels made of reduced activation ferritic martensitic steels (RAFMS) will carry liquid coolant of Pb-Li. These channels require protective coating layers to prevent following corrosion, conduction due to PB-Li flow and radioactive tritium permeation. Hence, alumina coating is highly desirable over reduced activation ferritic martensitic steels (RAFMS) as coolant channels in test blanket modules (TBM) of fusion reactors. The flow of liquid coolant Pb-Li creates resistance to flow due to conduction, also corrosive action of Pb-Li can damage the channels with a risk of leakage of highly radioactive tritium via permeation. However, alumina coating directly by pack cementation or other techniques is difficult to form due to contamination with iron aluminide type of intermetallics. In the present investigation, direct alumina coating was achieved plasma spray technique having average coating thickness of 100 µm. Alumina coatings obtained by adding sealant have good adhesion strength (6-7MPa) with porosity level 5% only. The coatings had high hardness values (800-900 VHN) for providing adequate wear resistance. The SEM images at the cross section show uniform layer of coating formation and the surface morphology indicates dense coating without any cracks.