ICMCTF2012 Session A1-1: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

The results from a novel method for hot corrosion testing by gaseous Na₂SO₄ attacks will be presented. In this hot corrosion test, sodium sulphate vapour is continuously supplied to the surface of the exposed samples through salt evaporation from a crucible placed inside the furnace. The temperature of the furnace and the position of the test coupons are chosen so that there is no condensation of the salt on the specimen’s surfaces during the exposures. In this work NiAl diffusion coatings with and without Pt were exposed at 900° C up to 1000 h and investigated using different high-resolution techniques. It will be shown that already after 100 hours of testing there is an observable corrosion attack, characterized by a penetrated protective oxide and the formation of sulfides at the oxide/metal interface. The possible scenario of reactions taking place during the testing is presented and the obtained results are compared with those from testing using a well-established ex-situ salt hot corrosion method.

Under conditions of carbon activities higher than one and low oxygen partial pressures steels and nickel base alloys can be destroyed by a corrosion mechanism called metal dusting, in the temperature range of 400 to 900°C. A carbon activity higher than one means that the gas phase is beyond equilibrium and a strong driving force exists for the deposition of carbon from the gas phase onto the metal surface. This process is promoted by the catalytically triggered disintegration of hydro carbons or carbon oxides in the gas environment. As a consequence of the carbon deposition on the metal surface, carbon can diffuse into the material and precipitate as graphite at preferred crystallographic orientations in the metal subsurface causing total metal disintegration and the formation of ‘metal dust’.

Metals catalytically triggering this type of corrosive attack are iron, nickel and cobalt. In order to inhibit the catalytic promotion of carbon oxide or hydro carbon disintegration and by this the deposition of carbon a catalytic poisoning concept was developed. Tin was shown to occupy the catalytically active centers of nickel on the metal surface via the formation of an intermetallic Ni₃Sn₂-phase with the latter suppressing any catalytic effect of the surface.

The performance of this phase was studied for 3000 hrs for several conventional alloys. In the work described in this paper the influence of alloy composition on the stability of the intermetallic phase will be discussed.

For technical applications, the performance of the intermetallic phase at the surface under oxidizing conditions is also a point of interest as start up and shut down processes usually occur in air. The influence of oxidation during these periods on the stability of the intermetallic phase was investigated as well and the results will be reported here.

A third part presented in this paper deals with possibilities to further improve the protection concept.

An addition of rare earth oxides to superalloy coatings could enhance the high temperature properties of the coatings in the actual service condition due to the purification of its microstructures. High temperature oxidation studies of D-Gun sprayed MCrAlY coatings (Where M = Ni or Fe) with minor addition of CeO₂ is scarce in the literature. Therefore, the present work was focused to study the effect of rare earth oxide (CeO₂) on the high temperature oxidation behavior of D-Gun sprayed NiCrAlY coatings on Ni- and Fe- based superalloys. Cyclic oxidation tests were carried out for 100 cycles at 900 ^oC to study the oxidation kinetics of un-coated and coated superalloys. The corroded products obtained during cyclic oxidation of coatings were subjected to XRD, FE-SEM/EDAX and X-ray mapping analysis to elucidate the high temperature oxidation mechanisms.

Keywords: Detonation-Gun process; Superalloys; Coatings; Oxidation; Rare earth, thermal

spray coating

Simple β-(Co,Ni)Al coatings, Hafnium (Hf)-modified β-(Co,Ni)Al coatings, Pt-diffused coatings, Pt-modified β-(Co,Ni,Pt)Al + ξ-PtAl₂ coatings, and Hf-Pt-modified β-(Co,Ni,Pt)Al coatings on Haynes-188 substrates were tested at 1050°C in air for up to 4670h. Surface morphology and cross-section microstructure of the tested specimens were inspected and compared by using Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that the Pt-diffused Haynes 188 specimen failed first in 600h; the testing of other four coatings was stopped at 4670h. HfO₂ particles were observed in the oxide scale on Hf-modified or Hf-Pt-modified aluminide coatings. The difference between coatings with or without Hf, as well as the oxidation performance difference between Ni-base superalloy substrate and Co-base superalloy substrate will also be discussed.

Key Words: Hafnium, aluminide coatings, Co-base alloys, oxidation testing