Tantalum CVD coatings : process, properties, performance

Abstract

Failure of current standard pipeline materials in chloride processing environments has a high environmental and economic impact. Tantalum (Ta) is resistant to attack in HCl environments due to its passive oxide layer. Solid and lined Ta components are, however, prohibitively expensive for large-scale deployment. Ta chemical vapour deposition (CVD) coatings on standard carbon steel pipeline materials offer an economic solution with superior properties but have been demonstrated to exhibit a dual phase nature with a metastable, hard, and brittle beta (β) phase, and a ductile alpha (α) phase, and also detrimental interfacial characteristics. This project describes the deposition of bulk α-Ta CVD coatings on standard 0.3 – 0.35 wt.%Cmax pipelines and components. \nThe potential for tailoring of CVD processes such coatings were investigated using a hybrid statistical design of experiment approach. Deposition rate, phase fraction and Knoop hardness of coatings was measured in response to varying deposition temperature, reagent gas flows, and sample position and finish. Statistical analysis identified deposition temperature, hydrogen (H2) reagent and argon (Ar) carrier gas flows as dominant parameters. Optimal levels for the desired properties were obtained and then applied to newly installed large-scale equipment. \nTa coatings on higher carbon content steels demonstrated reduced interfacial defects and a reduction in interdiffusion zone thickness. Optical micrographs using an alternate oxalic acid based etch and EBSD highlight the distribution of the deposited α/β-Ta phase within coatings. β-Ta shows preferential positioning at the surface and in the bulk, and α-Ta at the interface. XRD diffraction data is used to calculate approximate coating phase fractions that ranged from 5 – 96%. Heat treatments identified a β→α transition temperature of >950°C, correlated through exothermic peak detection in DSC analysis. \nCoating adhesion improved with the reduction of interfacial defects and the application of the heat treatment to minimise β-Ta content. Three-point bend testing investigated the performance of single/dual Ta coatings 20-80µm thick on A105 steel with varying post-deposition heat treatments. Single layer coatings, and those with singular heat treatments, suffered delamination on test. Samples with dual Ta coatings that had a heat treatment after both deposition processes had no visible or acoustic sign of failure after bending through 90° demonstrating ductility of the coating. Coating integrity was proved through HCl immersion testing with no measurable mass loss detected. \nInitial Ta coated mild steel had a corrosion rate of 11.4 MPY (0.26 mm/yr). Using the preferred two-stage deposition and heat treatment process C-ring corrosion testing of stressed A105 steel samples resulted in a ‘nil’ corrosion rate of 0.06 MPY (0.0014 mm/yr) equivalent to that of solid Ta.