Metallography Investigation of Dry Corrosion Boiler Tube

In-situ (on-site) metallography has been proven as a method to assess metallurgical condition of material without damaging the material (Non Destructive Testing). However in this case, in-situ metallography must be completed together with several analysis to prevent metallography misinterpretation. This paper also provides an overview of inspection technique continuity, together with result from in-situ (on-site) and laboratory analysis.Water wall tube in-situ metallography was conducted during periodic inspection of a 200 MW Steam Power Plant Boiler. It was found another phase beside ferrite and pearlite. The phase is in horizontal direction and suspected as micro crack, delta ferrite or retained austenite.The further investigation was carried out by performing laboratory analysis including: metallography, scanning electron microscopy (SEM) equipped with EDX (Energy-dispersive X-ray spectroscopy) and hardness test.Result of the investigation showed that the phase was oxide as a corrosion product derived from tube surface and hot gas reaction during combustion. The oxide is identical as dry corrosion oxide, without liquid electrolyte. The oxide was only found on fireside tube surface. It also can be described that the fireside tube was over exposed to high temperature as shown the change lamellar pearlite to globular spheroidized pearlite.