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Corrosion detected in turbine failure

TWI was asked to identify the mechanism and cause of failure of an industrial gas turbine and to suggest remedial action.

A first inspection by the gas turbine operator revealed a shaft failure, and that all the high pressure compressor turbine blades were broken at the airfoil root, except one. Damage was also found on the guide vanes and in the power turbine and compressor section.

The mangled failure debris, including dozens of blades which had failed at the airfoil root, were despatched to TWI where answers to the how and why were sought.

Visual examination and fluorescent penetrant inspection were undertaken in search of initiating defects. Fractographic studies in a scanning electron microscope were carried out and metallographic sections were taken. Chemical compositions were analysed and the mechanical properties of the shaft evaluated using mechanical testing and hardness measurements.

TWI's conclusions were that fatigue failure of one blade initiated the catastrophic failure. This was immediately followed by overloading and fracture of the blade ligament, causing impact damage and failure of the remaining blades in an avalanche manner.

However no evidence of a manufacturing fault or foreign object damage was found. The primary initiation site was thought to be on the low pressure surface of one blade near an area containing fine longitudinal and transverse cracking in the coating and consequently hot corrosion of the parent material. This later proved crucial in the final analysis.

No microstructural variations were found and the parent material conformed to the manufacturer's specification. Even the chemical composition and microstructure of the failed blade were exactly as specified.

The crucial finding was that evidence of corrosive attack of the blades existed. Damaging elements in the fuel most likely caused hot corrosion and cracking of the blades where the coating was damaged. In turn it was concluded thatthis led to initiation and propagation of the fatigue crack in the failed blade.

TWI recommended to its client that, mindful of the damaging operating environment it was imperative to improve the integrity of the blade's protective coating and initiate a programme of frequent deposit removal.

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