Engineering - Royal Australian Navy

Naval Engineering Bulletin • June 2001
Hot Corrosion of Marine Gas
Turbine Blades
An Engineering Plant Component in a Hostile Environment
By LCDR Andrew Fysh, RAN
Introduction
Gas turbine engines are in widespread use today, because
of their combination of performance, lightweight, low maintenance
requirement, and efficiency. Developed primarily
for the aircraft industry, they have found more recent application
in warships (and, more recently still, in commercial
shipping). Many internal components of the gas turbine
are subjected to temperatures well in excess of 500 0 C, necessitating
careful design and in-service management to
ensure reliability and to avoid expensive repairs. This report
outlines the corrosion effects of these temperatures, in
the marine operating environment, with specific reference
to the General Electric LM2500 gas turbine in use in the
Royal Australian Navy’s surface fleet.
Development of Gas Turbine
Hot Section Materials
The Hot Section Environment
The performance requirements of gas turbine engines has
increased considerably since the Whittle engine of the 1930s,
to the end that turbine entry temperatures have more than
doubled. Of all turbine engine components, it is undoubtedly
the high-pressure turbine (HPT) 1 blades, which operate
under the most arduous conditions:
• high temperature (up to 1000 0 C)
• high rotational forces and direct stress
• rapid temperature transients (especially in aircraft
engines)
• hot corrosive combustion gases
• erosive particles entrained in the gases (combustion
by-products such as carbon, airborne particles such
as sand 2 and salt)
The combination of direct stress and temperature encourages
blade creep, while the rapid temperature transients
ultimately cause thermal fatigue. It was these phenomena
that first dictated material requirements for HPT blades.
The Whittle engine commenced service using austenitic
steel; this was found to have insufficient creep strength.
Development of cobalt-based alloys followed, leading to the
now widespread use of cobalt-based and nickel-based
superalloys. Nickel-based superalloys can be routinely used
at temperatures up to 0.8 times melting temperature, and
can have a service life of up to 100 000 hrs at slightly lower
temperatures.
Nickel-Based Superalloys
Table 1 shows the various alloying elements used in nickelbased
superalloys for gas turbine hot-section components,
and their principal characteristics. The composition of two
common alloys is shown for comparison. Inconel® 718 is
an earlier alloy (1960s), and is still used widely in the LM2500
engine for HPT rotor and stator structural components.
René 80 (introduced by General Electric in 1980) has superior
properties designed for use in the LM2500 first-stage
HPT blades. In addition to changes in composition of nickelbased
superalloys since their introduction, further improvements
in properties have arisen from optimisation of
melting, casting and forming methods, hot-working processes
and heat treatments.
In the pursuit of even greater engine performance efficiency,
the stress/temperature environment in some engines is
such that first-stage (and, more recently, second-stage) turbine
blades also require air cooling, by convection through
radial passages. Later blade stages, though less likely to
1 The ‘hot section’ components of a gas turbine engine are the combustor, turbine inlet nozzles, turbine rotor blades, and associated structure and assemblies.
The HPT, consisting of one or more stages of nozzles and rotor blades, receives the hot pressurised gases from the combustor to provide power to
drive the compressor section (enabling the engine to be self-sustaining after start-up). Downstream of the HPT is a propulsion jet (in aircraft engines), or
a mechanically independent low-pressure turbine to drive a propeller shaft (marine engine) or alternator rotor (power generation). The HPT inlet temperature
represents the hottest part of the gas turbine cycle.
2 While predominantly a problem for land-based or marine gas turbines, airborne sand particles have been known to exist at an altitude of 30 000 ft in the
Middle East!
43