The Hot Corrosion of Fe-Mn-Al–C Alloy with NaCl/Na2SO4 Coating Mixtures at 750°C

The high-temperature corrosion behavior of Fe-30.1Mn-9.7Al-0.77C alloy initially coated with 2 mg/cm² NaCl/Na₂SO₄ (100/0, 75/25, 50/50, 25/75 and 0/100 wt.%) deposits has been studied at 750°C in air. The result shows that weight-gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 hr, while the kinetics with salt deposits all display multi-stage growth rates. The corrosion morphology of the alloy with 100% Na₂SO₄ coating is similar to that of simple oxidation. NaCl acts as the predominant corrosion species for Fe-Mn-Al-C alloy, inhibiting the formation of a protective oxide scale. For the alloy coated with over 50% NaCl in salts, NaCl induces selective oxidation of manganese and results in the formation of secondary ferrite in the alloy substrate as well as void-layers with different densities of voids layer by layer in the secondary-ferrite zone.     

Sulfate-induced hot corrosion of nickel

Nickel specimens with layers of Na₂SO₄ deposited on the metal surface have been reacted in O₂+4% SO₂ in the temperature range 660–900°C. At temperatures from 671°C (the eutectic temperature of Na₂SO₄+NiSO₄ liquid solutions) to 884°C (the melting point of Na₂SO₄), molten Na₂SO₄+NiSO₄ is formed in the scales above critical pressures of SO₃, and the molten sulfate causes accelerated hot corrosion of nickel. The rapid hot corrosion is preceded by an incubation period during which Na₂SO₄+NiSO₄ solid solutions and eventually molten sulfate are formed. The critical SO₃ pressures for formation of molten sulfate as a function of temperature have been delineated through experimental observations, and these are in agreement with theoretical estimates. When only solid solutions of Na₂SO₄+NiSO₄ can be formed, the reactions are slower than specimens with no Na₂SO₄ layer. The reaction mechanism is concluded to involve inward transport of SO₃/NiSO₄ and of oxygen through the molten sulfate distributed as a network in the NiO layer of the outer part of the scale. Beneath the NiO/molten sulfate layer, the scale consists of NiO with a network of Ni₃S₂. Sulfur, present as (Ni-S)_liq, is enriched at the metal/scale interface. Nickel diffuses outward through the Ni₃S₂ network in the inner layer to the boundary of the NiO/molten sulfate layer, where it reacts with the inwardly diffusing oxygen and SO₃/NiSO₄. The enrichment of sulfur next to the metal is concluded to be due to inward sulfur transport in the NiO+Ni-sulfide layer.     

Effect of Na2SO4 and Water Vapor on the Corrosion of Ti3SiC2

The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the presence of Na₂SO₄ deposit and water vapor at 900°C and 1000°C. The mass gain per unit area of the samples superficially coated with Na₂SO₄ exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces. The main corrosion phases on the sample surface were identified by XRD as TiO₂, Na₂Si₂O₅ and Na₂TiO₃. After Ti₃SiC₂ corroded in the presence of the Na₂SO₄ deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti₃SiC₂ beneath the Na₂SO₄ film without water vapor. Because water vapor penetrated the corrosion layer and then reacted with SiO₂ to form volatile Si(OH)₄, an intermediate porous and TiO₂-enriched layer formed in the corrosion layer.     

Effect of pre-oxidation on the hot corrosion of CoNiCrAlYRe alloy

The effect of pre-oxidation on the resistance to hot corrosion was examined by corroding the CoNiCrAlYRe alloy at 900 °C in molten Na₂SO₄. Preoxidized specimens featured strong adhesion of oxide scale with uniform multi-layered structure. The time of pre-oxidation was crucial for controlling Al content sufficient for subsequent hot corrosion. However, direct corrosion yielded a defective and non-protective oxide scale, which allowed detrimental penetration of sulfur into substrate. Sulfur migrating along phase boundary was trapped by yttrium to diminish slightly sulphidation. Thus, two advantages of proper pre-oxidation treatment were presented, as keeping repairing for Al₂O₃ scale and inhibiting sulfur penetration.     

Corrosion of metals and alloys in sulfate melts at 750°C

The corrosion of Ni, Co, Ni-10Cr, Co-21Cr, and IN738 was studied at 750°C in the presence of molten sulfate mixtures (Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄) and in an atmosphere consisting of O₂+0.12% SO₂-SO₃. The corrosion was observed to be similar for both Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄ melts. The corrosion of Ni and Co tookplace by the formation of a mixed oxide plus sulfide scale, very similar to the corrosion in SO₂ or SO₃ alone. The initial stage for the corrosion of Ni-10Cr involved the formation of a thick NiO+Ni₃S₂ duplex scale, and Cr sulfide was formed during the later stages. A pitting type of morphology was observed for both Co-21 Cr and IN738. The pit was Cr sulfide at the beginning, and subsequently the sulfides oxidized to Cr₂O₃. A base-metal oxide layer was present above the pit, and this was observed to be formed very early in the corrosion process. A mechanism is proposed to explain this. In general, the formation of sulfides appears to be the primary mode of degradation in mixed sulfate melts.     

The corrosion of Ni3Al in a combustion gas with and without Na2SO4-NaCl deposits at 600–800°C

The corrosion behavior of Ni₃Al containing small additions of Ti, Zr, and B in combustion gases both with and without Na₂SO₄–NaCl deposits at 600–800°C has been studied for times up to four days. The corrosion of the saltfree Ni₃Al leads to the formation of very thin alumina scales at 600°C but of mixed NiO–Al₂O₃ scales containing also some sulfur compounds at higher temperatures, while the rate increases with temperature up to 800°C. The presence of the salt deposits considerably accelerates the corrosion rate, especially at 600 and 800°C. The duplex scales formed at 600°C are composed mostly of a mixture of NiO and unreacted salt in the outer layer and of alumina and aluminum sulfide with some nickel compounds in the inner layer. The scales grown at 700°C contain only one layer of complex composition, while those grown at 800°C are similar but have an additional outer layer containing similar amounts of nickel and aluminum. At 600 and 700°C NiSO₄ can be detected also in the salt layer. The samples corroded at 700°C and 800°C also show an Al-depleted zone containing titanium sulfide precipitates at the surface of the alloy. The hot corrosion of Ni₃Al involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures Ni₃Al shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales.     

High-temperature corrosion behavior of sputtered K38 nanocrystalline coatings with and without yttrium addition in molten sulfate at 900 °C

The high temperature corrosion behavior of sputtered Ni-based superalloy K38 nanocrystalline coatings with and without yttrium addition in molten sulfate (75 wt.% Na₂SO₄ + 25 wt.% K₂SO₄) was investigated at 900 °C in air. The results indicated that nanocrystallization significantly increased the corrosion resistance through the rapid formation of a protective oxide scale. The addition of yttrium in the nanocrystalline coating furthermore improved the corrosion resistance of the coating.     

Initial atmospheric corrosion of zinc in presence of Na2SO4 and （NH4）2S04

Initial atmospheric corrosion of zinc in the presence of Na₂SO₄ and (NH₄)₂SO₄ was investigated via quartz crystal microbalance(QCM) in laboratory at relative humidity(RH) of 80% and 25 °C. The results show that both Na₂SO₄ and (NH₄)₂SO₄ can accelerate the initial atmospheric corrosion of zinc. The combined effect of Na₂SO₄ and (NH₄)₂SO₄ on the corrosion of zinc is greater than that caused by (NH₄)₂SO₄ and less than that caused by Na₂SO₄. Fourier transform infrared spectroscopy(FTIR), X-ray diffractometry(XRD) and scanning electron microscopy(SEM) were used to characterize the corrosion products of zinc. (NH₄)₂Zn(SO₄)₂, Zn₄SO₄(OH)₆·5H₂O and ZnO present on zinc surface in the presence of (NH₄)₂SO₄ while Zn₄SO₄(OH)₆·5H₂O and ZnO are the dominant corrosion products on Na₂SO₄-treated zinc surface. Probable mechanisms are presented to explain the experimental results.     

On the reaction mechanism of nickel with SO2+O2/SO3

High-purity nickel has been reacted with 96% O₂+4% SO₂ at 700–900°C. The reaction has been studied at 700°C as a function of the total gas pressure (0.06–1 atm) and at 1 atm as a function of temperature (700–900°C). The reaction mechanism changes with the effective pressure of p(SO₃) in the gas. When NiSO₄ (NiO + SO₃ = NiSO₄) is formed on the scale surface, the scale consists of a two-phase mixture of NiO + Ni₃S₂; in addition, sulfur is enriched at the metal/scale interface. A main process in the reaction is rapid outward diffusion of nickel through the Ni₃S₂ phase in the scale; the nickel reacts with NiSO₄ to yield NiO, Ni₃S₂, and possibly NiS as an intermediate product. When NiSO₄ cannot be formed, the scale consists of NiO, and small amounts of sulfur accumulate at the metal/scale interface. It is proposed that the reaction under these conditions is primarily governed by outward grain boundary diffusion of nickel through the NiO scale, and in addition, small amounts of SO₂ migrate inward through the scale—probably along microchannels.     

The hot corrosion behaviour of HVOF sprayed MCrAlX coatings under Na2SO4 (+NaCl) salt films

The hot corrosion behaviour of two NiCoCrAlYTa and CoCrAlYSi HVOF sprayed coatings and a CoCrAlY VPS coating were investigated under laboratory conditions at 900°C using a synthetic gas atmosphere containing sulphur as an impurity. All the coatings tested showed good protection under Na₂SO₄ salt films. In the presence of NaCl in the Na₂SO₄ salt films, the corrosion rates of low Al containing coatings increased considerably but the NiCoCrAlYTa coating with higher Al content still revealed good performance. It is suggested that NaCl in the salt film causes premature failure of the protective scale and reduces the incubation period of corrosion in the coatings of lower Al content. Furthermore, it seems that the finely dispersed Al rich oxide particles in the sprayed and heat‐treated HVOF coating microstructure do not lead to internal corrosion. The experimental investigations include short‐term corrosion kinetic measurements and SEM analyses.     

Hot corrosion of B-1900 superalloy by simulated fluidized bed coal combustor deposits

Calcium sulfate deposits in fluidized bed coal combustors are thought to contribute to heat transfer tube hot corrosion observed in test beds. Although Na₂SO₄ caused catastrophic hot corrosion of B-1900 superalloy at 900°C in air, the corrosion due to CaSO₄/Na₂SO₄ mixtures resembled that of simple oxidation, both in scale appearance and kinetics. High scale calcium content suggested a solid state reaction with CaSO₄ that did not compromise the protective scale. Although trace vanadium pentoxide added to a high percentage CaSO₄ mixture was shown to be innocuous, trace lead oxide caused catastrophic hot corrosion. However, the mechanism is unknown. This suggests that the latter coal impurity warrants attention as a possible severe corrosive threat, although CaSO₄ itself appears innocuous in an oxidizing environment.Formerly graduate student at Rensselaer Polytechnic Institute in the Department of Materials Engineering, Troy, New York 12181     

The effect of K2SO4 additive in Na2SO4 deposits on low temperature hot corrosion of iron-aluminum alloys

To understand the effect of K₂SO₄ additive in an Na₂SO₄ deposit on low temperature hot corrosion, the corrosion behavior of Fe-Al alloys induced by Na₂SO₄+K₂SO₄ was compared to that by Na₂SO₄ alone, and sulfation of Fe₂O₃ in the presence of either Na₂SO₄ or Na₂SO₄+K₂SO₄ was studied. It was found that K₂SO₄ additive promoted the low temperature hot corrosion, but did not change the corrosion-mechanism. Experimental results refuted the prior suggestions that the accelerated hot corrosion resulted either from the formation of K₃Fe(SO₄)₃ or from the stimulation of sulfation of Fe₃O₃. The earlier formation of the eutectic melt caused the accelerated hot corrosion, or in other words, the K₂SO₄ additive shortened the induction stage of hot corrosion.     

The Inhibitive Effect of Traces of SO2 on the Oxidation of Iron

The oxidation of Fe was investigated at 500–700°C in the presence of O₂ with 0–1000 ppm SO₂. The exposures were carried out in a thermobalance and lasted for 24 h. The oxidized samples were investigated by grazing-angle XRD, SEM/EDX, GDOES and XPS. The rate of oxidation of pure iron is slowed down by traces of O₂ in O₂ below 600°C while SO₂ has no effect on oxidation rate at higher temperatures. Exposure to SO₂<600°C resulted in the formation of small amounts of sulfate at the gas/oxide interface. In addition, sulfur, probably sulfide, accumulated at the metal/oxide interface. The influence of SO₂ on oxidation rate is attributed to surface sulfate. The sulfur distribution in the scale is rationalized in terms of the thermodynamic stability of compounds in the Fe–O–S system. Exposure to SO₂ caused the formation of hematite whiskers.     

Study of molten salt corrosion of HK-40m alloy applying linear polarization resistance and conventional weight loss techniques

Corrosion rates from electro-chemical polarization resistance technique (LPR) and weight loss method (WL) of HK-40m alloy exposed to 80 mol% V₂O₅-20Na₂SO₄ at 600 and 700 °C were obtained at a maximum time of 10 days. Results were supported by X-ray diffraction and electron microscopy analysis. A comparison of corrosion rates from both techniques indicated that corrosion rates from LPR were higher than that from WL, being the values more or less in the same order of magnitude. At 600 °C corrosion rates values were twofold; whereas at 700 °C threefold. The difference in results from both techniques was mainly explained by the fact that V₂O₅ behaves as a semiconductor oxide, and even though Na₂SO₄ is totally ionic, the corrosion mechanism with this mixture may not display a purely electro-chemical process. Some qualitative characteristics were observed for both techniques.     

The corrosion of Nb-modified Ti3Al in a combustion gas with and without Na2SO4−NaCl deposits at 600–800°C

The corrosion behavior of a Nb-modified Ti₃Al intermetallic compound containing 11 at.% Nb in a simulated combustion gas with and without deposits of a Na₂SO₄–NaCl mixture was examined at 600–800°C for times up to four days. In the absence of salt deposits the corrosion rates were rather low and increased only slightly with temperature, producing very thin scales of mixed oxides of Ti, Al, and Nb without sulfides. The presence of the salt deposits produced higher weight gains during an initial stage of one to two days at 600 and 700°C, after which the reaction stopped. A more important and longlasting effect was observed instead at 800°C, when the kinetics of hot corrosion became nearly linear. The scales formed by hot corrosion were complex mixtures of various corrosion products at all temperatures and showed a porous outer region containing a mixture of unreacted salts with oxides (mainly TiO₂), an intermediate region of a mixture of variable composition of oxides of the three metals, and a TiO₂-rich layer beneath it. At 800°C the scales tended to form a thin, discontinuous Al₂O₃-rich layer in the middle and contained an additional innermost region presenting a large concentration of sulfur, very likely as Nb and Ti sulfides. The high rate of hot corrosion at 800°C is attributed to the appearance of sulfides in the inner region of the scale and to a more efficient scale fluxing.     

Accelerated oxidation of iron induced by Na2SO4 deposits in oxygen at 750°C—A new type low-temperature hot corrosion

Na ₂ SO ₄ -induced accelerated corrosion of iron in oxygen at 750°C was observed. EDX, XRD, SEM, EPMA and some chemical examinations were carried out to understand the corrosion mechanism. The accelerated oxidation was attributed to the formation of abundant sulfide which has a highly defected lattice and allows rapid diffusion of iron ions. The sulfide resulted in turn from the formation of a liquid phase which was a eutectic melt of Na ₂ SO ₄ and Na ₂ O. The formation of and other possible effects of the melt were discussed. The accelerated oxidation was compared with the usual low-temperature hot corrosion, showing that it has most of the characteristics of low-temperature hot corrosion except that it occurred under basic conditions developed by the removal of sulfur from the sulfate deposits instead of the usual acidic conditions established by the SO ₃ in the atmosphere.     

Na2SO4- and NaCl-Induced hot corrosion of six nickel-base superalloys

The short-time hot-corrosion behavior of six industrial nickel-base superalloys was investigated with static deposits of Na₂SO₄ or NaCl or both in still air. The oxidation kinetics and scale morphologies were measured with traditional laboratory techniques-thermobalance, metallography, electron microprobe, and x-ray analyses. Susceptibility to hot corrosion was found to be correlated to the type of scale produced during simple oxidation. Alloys forming an A1₂O₃ scale were found to be susceptible to Na₂SO₄ deposits, independent of their chromium content. The quantity of Na₂SO₄ deposit dictated the nature of the attack and, under certain conditions, the refractory element alloy additions appeared to play an essential role. Alloys containing Cr₂O₃ or TiO₂ in the simple oxidation scale proved to be sensitive to NaCl attack. Again, the severity of the attack within the susceptible alloy group was not related to the chromium or titanium content. Although less intensive than the Na₂SO₄ -induced hot corrosion, NaCl contaminations provoked extensive spalling. All of the hotcorrosion types encountered in this study were interpreted in the light of existing theories.Supported by the Délégation Générale à la Recherche Scientifique et Technique.     

Influence of pre-oxidation on the hot corrosion of Ti3SiC2 in the mixture of Na2SO4-NaCl melts

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO₂ would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.     

The reaction of preoxidized nickel in SO2 at high temperatures

The reaction between preoxidized nickel and SO₂ was studied at 800–1000° C. The experimental methods consisted of thermogravimetry, metallography, scanning electron microscopy, and electron microprobe analysis. Sulfur accumulates in the surface layer of the metal during the SO₂ exposure. It is concluded that this is due to transport of SO₂ through the oxide scale. This leads in turn to formation of a Ni-S liquid solution which eventually ruptures the scale and causes an accelerated, breakaway reaction behavior. The induction period for the accelerated reaction increases with increased preoxidation and furthermore is influenced greatly by the microstructure of the oxide scale.     

Reactions of chromium in SO2-containing atmospheres

The morphology and composition of scales formed on unalloyed chromium in atmospheres containing SO₂ at high temperatures have been studied. SEM images show chromium sulfides formed at the metal/scale interface. The results indicate that SO₂ may penetrate the scale as molecules, but the rate of this penetration is low. At 600°C formation of Cr₂(SO₄)₃ is also observed by SEM on the scale surface formed in SO₂+O₂/SO₃ mixtures.