ALSTOM Industrial Gas Turbines produces a range of engines from 2MW-25MW in size. The company depends heavily on the use of coatings throughout the engine to ensure adequate protection against, corrosion, oxidation, wear, seizure and tip clearance.

One of the most critical components in the engine is the gas turbine blade. The high pressure turbine blade operates under the most arduous conditions of temperature and stress of any component in the engine. Not only does the blade experience high temperature and direct stress, it also experiences rapid temperature transients at various points during the engine cycle. The hot gases surrounding the blade are highly oxidising and may contain high levels of contaminants such as sulphur and chlorine if low grade fuels are used. Erosion by combustion products and the outside environment can also be a problem. An ideal coating should be strain tolerant as well as be able to survive the harsh corrosive/erosive environment.

Increases in engine operating temperatures has meant that the traditional corrosion resistant turbine blade alloys such as IN738 and IN939 are no longer strong enough to last the expected 25,000 hour minimum life. This has meant that higher strength alloys such as Mar M247, CM186 and CMSX-4 are required for creep strength. These alloys have relatively poor hot corrosion resistance and hence have to rely on coatings to prevent severe and potentially life limiting damage.

Typically, turbine blade coatings for industrial use are either diffused nickel aluminides or "stand-alone" MCrAlY overlay coatings. The overlay coatings are usually applied by spray techniques such as plasma spraying and coating composition is based upon superalloys with small additions of reactive elements.

In order to ensure adequate corrosion protection ALSTOM Energy Technology Centre has developed a relatively simple corrosion test. The test involves placing coated pins in an ash which was devised in earlier COST activities and passing a mixture of SO₂/air through the ash. The corrosion resistance of the coatings is then evaluated at typical temperatures of 700° C, 800° C and 850° C for times up to 2000 hours. The average and maximum depth of corrosion penetration is then determined by optical metallography. The test is very severe and whilst the performance of the coatings cannot be directly related to engine life, the test is very useful as a ranking tool and it is known that coatings which last for 2000 hours in the test will last for 25,000 hours in the most severe engine environment.

ALSTOM Energy Technology Centre has acquired a lot of data on the performance of turbine blade coatings for use in severe corrosion environments. As a company ALSTOM is willing to offer a range of services to ensure the most suitable coatings are used. These services can take the form of either:

1) Advice on coating selection

2) Testing of coatings for suppliers and end users

For further information regarding coatings advice and testing please contact:

Mick Whitehurst - ALSTOM Energy Technology Centre
+44 (0) 116 2015628 (telephone)
+44 (0) 116 2015464 (fax)
mick.whitehurst@energy.alstom.com (E-mail)

ALSTOM also offers a complete range of metallurgical services such as mechanical testing and failure investigation, plus an extensive material property database. If you require further information on how ALSTOM can help you on any of the above, please contact:

Dr. P. Barnard - Head of Materials, ALSTOM Energy Technology Centre
+44 (0) 116 2015559 (telephone)
+44 (0) 116 2015464 (fax)
pete.barnard@energy.alstom.com (E-mail)