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 Striatum edronm ents, however, they are limited in use due to low room temperature ductility and poor weld ability arising from hydrogen embrittlement. Using Gas Tungsten Arc and Gas Metal Arc Welding process without the use overheat and post weld heat treatments, a weld ability regime below approximately 10wf% Al in the overlay has been found. The high temperature sulfidation behavior of alloys located near this crack boundary (5 to 12.5wt% Al) was examined using a thermo gravimetric balance in a reducing gas mixture of O.l% H2-1.0% H-h-Balance Ar gas at 700C. Light optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were used to analyze the cross- section almorphologiea. It was seen that the FeAl 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 Yowl) resulted in the growth of iron sulfide as nodules or a porous surthcescale. Larger additions (above10 w 0/0)were to limit the growth of iron sulfide and significantly reduce corrosion rates. These results indicate that Fe-Alcl@lings containing 5to 10wt?% Al show potential promise for applications requiring a combination of weld ability and sulfidation resistance in moderately reducing environments.

Weld overlay coatings are being used to protect low alloy steel water-wall panels of coal-fired boilers with low NOX burners. By depositing an alloy that offers better sulfidatiort resistance than the underlying tubes, the accelerated wastage ?4 that is presently experienced can be decreased. A material system presently under consideration fm use as a cladding is iron-aluminum. While data are indicating the excellent corrosion resistance of iron aluminizes in reducing environments 2,thesealloy aren?t readily weld able. The intermetallic compositions have been shown to be susceptible to hydrogen (cold) ?*?5 This problem has been observed to be more prevalent as the aluminum content of the cracking subsequent to deposition . demit is increased 18. Recentwork?9has shown that when depositing under the conditions normally used to produce overlay - in a M= (no PXKJIa or post-weld ha torment), a maim of 10wt???Al can be tolerated in the cladding before cracking occurs (Figure 1). Other investigations ?4mhave shown that by using pre-heat and post weld heat treatment$ the severity of the hydrogen cracking problem can be reduce@ thus allowing for higher aluminum content overlays to be produced crack &e. However, the use of such extensive treatments is usually not possible when coating large scale structures. Therefore, alloys with lower aluminum contents are being investigated fm weld overlay coatings.

Studies conducted on time alloys have shown that even small additions of aluminum to iron can decrease suffixation rates by an order of magnitude 21-X.However,the conditions used in these studies (temperatures above 700Cand PHz>107 atm) we resume what more aggressive than environments bind in fossil-fired boilers with low NOX burners =7. Recent work x has shown that weldable compositions of Fe-Al alloys performed reasonably well in moderately reducing environments (temperatures below700C, P~,<10?atm and Pm> 10-~atm). Compositions with 5-10 wt% Al were s

This content is only available via PDF.
You can access this article if you purchase or spend a download.