S.W. Banovic

Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Abstract Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information was used to determine the partition coefficients k of the elements in each dissimilar weld. The dilution level was found to decrease as the ratio of volumetric filler metal feedrate to net arc power increased. Reasons for this behaviour are discussed in terms of the distribution of power required to melt the filler metal and base metal. In addition, the segregation potential of Mo and Nb was observed to increase (i.e. their k values decreased) as the Fe content of the weld increased. This effect is attributed to the decreased solubility of Mo and Nb in austenite with increasing Fe additions. Since the Fe content of the weld is controlled by dilution, which in turn is controlled by the welding parameters, the welding parameters have an indirect influence on the segregation potential of Mo and Nb. The results of the present work provide practical insight for corrosion control of welds in super austenitic stainless steels.

Growth of Nodular Corrosion Products on Fe-Al Alloys in Various High-Temperature Gaseous Environments

The mechanisms for nodular corrosion-product development were investigatedin various high-temperature gaseous environments. Fe–Al alloys, with5–20 wt.% Al, were exposed in both oxidizing and sulfidizing[p(S2)=10−4 atm, p(O2)=10−25 atm] atmospheres at 700°Cfor times up to 100 hr. The corrosion kinetics were monitored by theuse of a thermogravimetric balance and the morphological developmentthrough light-optical and scanning-electron microscopies,energy-dispersive spectroscopy, electron-probe microanalysis,and quantitative-image analysis. Under both conditions, theelimination of nodule formation was observed by increasing thealuminum content of the alloy, above 5 and 7.5 wt.% Al for oxidizingand sulfidizing environments, respectively, which promoted the growthand maintenance of a continuous surface scale of alumina. For thosealloys that were observed to develop nodular corrosion products, theirmorphological appearance was similar in nature regardless of thecorroding species. The nodules typically consisted of an outeriron-rich product, either sulfide or oxide, that was randomly dispersedacross an alumina scale. Samples from the oxidizing atmosphere displayeda single growth-rate time constant from the kinetics data, suggesting thatthe nodule growth mechanism was by the simultaneous or codevelopment oftwo different (Fe and Al) oxides from the onset of exposure. Measurementof nodule planar diameter and depth of penetration into the alloyindicated that growth occurred through diffusional processes. Kineticsdata from the development of sulfide nodules in the reducingenvironment revealed a different type of mechanism. Multiplegrowth-rate time constants were found due to the localized mechanicalfailure of an initially formed surface scale. At early times in thesulfidizing atmosphere, a low corrosion rate was recorded as acontinuous-alumina scale afforded protection from excessive productdevelopment. However, with the mechanical failure of the scale, sulfurwas able to attack the underlying substrate through a short-circuitdiffusion mechanism that resulted in rapid weight gains from nonprotective,iron sulfide growth. The sulfide morphologies observed were very complex ascontinued growth of the nodule did not solely depend upon the diffusingspecies through the previously formed corrosion products, but also,continued mechanical failure of the oxide scale. It is suggested that thedifference in development mechanisms between the two environments may liein the relative growth rates of the nonprotective, Fe-base corrosionproducts formed.

High temperature sulfidation behavior of low al iron-aluminum compositions

Iron-aluminum weld overlay coatings are currently being considered for enhanced sulfidation resistance. The performance of these alloys in reducing atmospheres far exceeds other conventional materials presently used. However, the application of iron-aluminum alloys is currently limited due to hydrogen cracking susceptibility subsequent to welding. A direct correlation between the severity of embrittlement and the amount of aluminum in the alloy has been observed, specifically when the composition resides in the ordered region of the Fe-Al phase diagram (above 10 wt% Al). Higher Al overlays have been deposited, but only with extensive pre-heat and post-weld heat treatments which are not always feasible. In addition to the welding problem, detailed corrosion studies indicating the sulfidation behavior of these binary alloys has not been fully conducted. Previous research has utilized gas compositions and temperatures which would be considered very aggressive (temperatures above 700 C and P{sub S2} {ge} 10{sup {minus}7} atm), especially when compared to industrial boiler environments. Less assaulting conditions could not be found in the open literature. Therefore, research has been initiated to evaluate the sulfidation behavior of weldable iron-aluminum compositions, in the range of 5--10 wt% Al, in moderately reducing environments.

Corrosion behavior of weldable Fe–Al alloys in oxidizing–sulfidizing environments

AbstractThe objective of the present study was to investigate the corrosion behavior of weldable Fe–Al alloys in environments representative of low NOx gas compositions, i.e., high partial pressures of sulfur [p(S2)] and low partial pressures of oxygen [p(O2)]. Using thermogravimetric techniques, binary alloys with 0–12.5 wt% Al were exposed in oxidizing–sulfidizing environments [p(S2) = 10–4 atm and p(O2) = 10–25 atm] at 500–700°C for various times up to 100 h. Post-exposure characterization consisted of surface and cross-sectional microscopy in combination with energy dispersive spectroscopy and/or electron probe microanalysis. It was found that the Fe–Al alloys exhibited three different stages of corrosion behavior: inhibition, breakdown, and steady-state. Observance and/or duration of these stages was directly related to the aluminum content of the alloy. The inhibition stage was characterized by growth of a thin, gamma alumina scale that suppressed rapid degradation of the underlying substrate for al...

Metallographic preparation and degradation of the τ-phase (FeAl2S4) formed after high-temperature oxidation–sulfidation of Fe–Al alloys

Abstract The stability of corrosion products formed after high-temperature exposure of an Fe–5 wt.% Al alloy in an oxidizing–sulfidizing environment was investigated both during metallographic preparation and subsequent exposure to the ambient environment. The primary phases formed were an outer layer of iron sulfide (Fe1−xS) and an inner layer composed of τ-plates (FeAl2S4) and iron sulfide particles. No difficulties were found concerning the stability of the iron sulfide phases, but it is known that the τ-phase is easily hydrolyzed by water. Therefore, standard metallographic procedures where water is used as a lubricant and/or cleansing solution during preparation could not be exercised. Using scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis, the effect of the use of various lubricants and/or cleansing solutions was examined in order to produce good quality, polished cross-sections of the corrosion scales. The best results were obtained using 200-proof dehydrated ethyl alcohol as the lubricant and cleansing solution. It was also observed that post-exposure of polished samples to the ambient environment degraded the microstructure with time. It is believed that moisture from the air reacted with the τ-phase, resulting in the evolution of hydrogen sulfide gas.

Investigation of Iron Aluminide Weld Overlays

Conventional fossil fired boilers have been retrofitted with low NO(sub)x burners in order for the power plants to comply with new clean air regulations. Due to the operating characteristics of these burners, boiler tube sulfidation corrosion typically has been enhanced resulting in premature tube failure. To protect the existing panels from accelerated attack, weld overlay coatings are typically being applied. By depositing an alloy that offers better corrosion resistance than the underlying tube material, the wastage rates can be reduced. While Ni-based and stainless steel compositions are presently providing protection, they are expensive and susceptible to failure via corrosion-fatigue due to microsegregation upon solidification. Another material system presently under consideration for use as a coating in the oxidation/sulfidation environments is iron-aluminum. These alloys are relatively inexpensive, exhibit little microsegregation, and show excellent corrosion resistance. However, their use is limited due to weldability issues and their lack of corrosion characterization in simulated low NO(sub)x gas compositions. Therefore a program was initiated in 1996 to evaluate the use of iron-aluminum weld overlay coatings for erosion/corrosion protection of boiler tubes in fossil fired boilers with low NO(sub)x burners. Investigated properties included weldability, corrosion behavior, erosion resistance, and erosion-corrosion performance.