"Smart Overlay Coatings - Concept and Practice"

by

J R Nicholls and S Neseyif (Cranfield University )

H E Evans and M Taylor ( Birmingham University )

A Kempster (Diffusion Alloys Ltd )

G Ridgeway ( A T Poeton Ltd )

P Chandler ( C A Technology Ltd )

J E Oakey ( British Coal )

M Whitehurst ( GEC Alshom )

( This project was supported by a DTI LINK initiative and EPSRC )

Abstract

Smart overlay coatings are chemically graded overlay coatings, designed to provide optimal resistance to high temperature oxidation, type I and type II hot corrosion. The coating system has been designed for application to turbine components, within utility gas turbines.

The functionallly graded coating system is based on modified MCrAlY corrosion resistant alloys, where M is Ni, Co or a combination of Ni and Co. The coatings are fabricated through a combination of spraying techniques, plasma spraying and high velocity oxy-fuel, and chemical vapour deposition. Thus the coatings are chemically graded, enriched in aluminium and/or chromium at various positions throughout the coating.

This paper will present the concepts behind the smart overlay coatings and describe how chemical gradient compositions, coupled with the morphology of oxidation/corrosion attack, permits the coating to exhibit smart behaviour. Results will be presented on the application of CVD techniques to modify the chemical gradients within the coating, and the performance of the chemically grade coating in simulated gas turbine environments.

The Concept

The "Smart Overlay Coating" is a chemically graded coating enriched in aluminium, chromium ( and possibly silicon ) to different levels at various depths to provide a more optimumresponse to corroion attack observed within industrial gas turbines at different temperatures.

The behaviour is illustrated schematically in Figure 1.

 

Figure 1 Schematic representation of Response of Optimised Coating

 

Corrosion in Utility Turbines

Three forms of high temperature corrosion damage have been identified.

1. Accelerated high temperature oxidation

• Gaseous oxidation at <950°C

• Surface oxides plus sub-surface penetration of oxides and sulphides

• Alumina scales offer best protection

2. Type I Hot Corrosion

• Molten salt induced corrosion between 750-950°C

• Non protective oxide scales; internal sulphidation and depletion of scale forming elements ( Al and Cr )

• Alumina scales offer best protection

3. Type II Hot Corrosion

• Molten salt induced corrosion, in the presence of SO3 in the combustion gas, between 650-800°C

• A localised pitting form of corrosion attack

• Scales rich in Chromium or Silicon offer best protection

 

Coating Production

 

Chemically graded coating structures were produced through a combination of spraying technologies and chemical vapour depoition.

Production Route

Microstructure and Composition Profiles

The combination of plasma spraying plus diffusion aluminising results in a layered microstructure.

This chemically graded coating is rich in aluminium at the coating surface, with chromium rich compositions displaced inwards towards the cente of the coating.

 

Optimisation of Corrosion Resistant Composition

The influence of coating composition on type I and type II corrosion has been studied by mapping alloy corrosion rates onto a ternary diagram. Results at 750 ∞c are illustrated below.

 

Ternary maps at 650, 700, 750, 800 and 900 ∞c have been produced, by co-sputter deposition of model alloy compositions, using Ni, Cr and Al source materials and corroding them at the respective temperatures.

For best corrosion resistance

• At 650°C Alloys rich in Cr ( <40 wt % ) with Al at 5 - 10 wt %

• At 700°C Alloys rich in Cr ( <40 wt % ) but with Al at 20 - 40 wt %

• At 750 & 800 °C Alloys with composition close to Ni -33 wt %

Cr-33 wt %Al

• At 950 °C 1. Alloys 14-18 wt %Cr & Al levels upto 13 wt%, or

2. Alloys with high aluminium contents, should lie on the binary section Ni-16%Cr-13%Al to Ni-24%Cr-18%Al

 

Corrosion Tests

The coatings were evaluated using a salt recoat test procedure with 80mol%Na2SO4/20mol%K2SO4 and a depositing rate of 0.3mg/cm2/20h.

The test duration was 500 hours in an air /SO2 (400 vpm) gas.

Tests were completed at 700 and 800°C. The corrosion loss was measured statistically with maximum corrosion penetration reported.

Type II hot corrosion morphologies were observed for all coatings at 700 ∞c with type I hot corrosion observed at 800 ∞c.

Conclusions

• Ternary diagram studies of hot corrosion behaviour have shown that type II corrosion resistant coatings require high Cr and Al contents, with otimum behaviour close to a Ni-33%Cr-33%Al composition.

• Graded coatings, rich in Cr and Al can be produced by argon shrouded plasma spray, or HVOF, deposition of a base alloy composition followed by pack aluminising.

• Hot corrosion studies at 700 and 800°C have shown that type II and type I corrosion morphologies are produced.

• At 800°C, the structure coatings offer a significant advantage over RT22 against type I hot corrosion after 500h exposure.

• At 700°C , the aluminiseed MCrAlY compositions corrode at a faster rate than RT22. The HVOF Amdry 995 coating, plus aluminised, is more corrosion resistant than the Ar-plasma sprayed equivalent and was still protective after 500h at 700°C.

Acknowledgement

The authors would like to thank the LINK initiative and EPSRC for providing part of the funds to support this work.

 
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