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99059 CORROSION RESISTANCE OF WELDABLE Fe-Al ALLOYS IN REDUCING ENVIRONMENTS

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Product Number: 51300-99059-SG
ISBN: 99059 1999 CP
Author: S.W. Banovic, J.N. DuPont and A.R Marder
Industry: Energy Generation
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Low aluminum Fe-Al alloys are being investigated for use as protective coatings in reducing environments. Intermetallic compositions have already demonstrated excellent high temperature corrosion resistance in both oxidation and Sdtidatim edronm ents, however, they are limited in use due to low room temperature ductility and poor weldability arising from hydrogen embrittlement. Using Gas Tungsten Arc and Gas Metal Arc Welding process without the use of pre-heat and post weld heat treatments, a weldability regime below approximately 10 wt% Al in the overlay has been found. The high temperature sulfidation behavior of alloys located near this crack/no crack boundary (5 to 12.5 wt% Al) was examined using a thermogravimetric balance in a reducing gas mixture of 0.1% H2-1.0% H2S-balance Ar gas at 700°C. Light optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were used to analyze the surfaceand cross-sectional morphologies. It was seen that the Fe-Al alloys exhibited lower weight gains than bare water wall tubes (carbon steel) and 309 stainless steel. Aluminum additions to the iron were found to extrinsically affect the iron sulfide reaction product morphology. Low carbon steel, with no aluminum, formed a continuous, and somewhat dense, iron sulfide scale that provided some protection. Small additions of aluminum (5-7.5 wt % Al) resulted in the growth of iron sulfide as nodules or a porous surthce scale. Larger additions (above 10 wt %) were found to limit the growth of iron sulfide and significantly reduce corrosion rates. These results indicate that Fe-Al claddings containing 5 to 10 wt% Al show potential promise for applications requiring a combination of weklability and sulfidation resistance in moderately reducing environments.
Low aluminum Fe-Al alloys are being investigated for use as protective coatings in reducing environments. Intermetallic compositions have already demonstrated excellent high temperature corrosion resistance in both oxidation and Sdtidatim edronm ents, however, they are limited in use due to low room temperature ductility and poor weldability arising from hydrogen embrittlement. Using Gas Tungsten Arc and Gas Metal Arc Welding process without the use of pre-heat and post weld heat treatments, a weldability regime below approximately 10 wt% Al in the overlay has been found. The high temperature sulfidation behavior of alloys located near this crack/no crack boundary (5 to 12.5 wt% Al) was examined using a thermogravimetric balance in a reducing gas mixture of 0.1% H2-1.0% H2S-balance Ar gas at 700°C. Light optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were used to analyze the surfaceand cross-sectional morphologies. It was seen that the Fe-Al alloys exhibited lower weight gains than bare water wall tubes (carbon steel) and 309 stainless steel. Aluminum additions to the iron were found to extrinsically affect the iron sulfide reaction product morphology. Low carbon steel, with no aluminum, formed a continuous, and somewhat dense, iron sulfide scale that provided some protection. Small additions of aluminum (5-7.5 wt % Al) resulted in the growth of iron sulfide as nodules or a porous surthce scale. Larger additions (above 10 wt %) were found to limit the growth of iron sulfide and significantly reduce corrosion rates. These results indicate that Fe-Al claddings containing 5 to 10 wt% Al show potential promise for applications requiring a combination of weklability and sulfidation resistance in moderately reducing environments.
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