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.     

The role of NaCl in the hot-corrosion behavior of Nimonic alloy 90

The influence of sodium chloride on the hot-corrosion behavior of Nimonic alloy 90 has been investigated by employing the half-immersion, crucible test. Nimonic 90 samples were hot corroded in the presence of NaCl between 700–900°C. The results showed that the weight-loss plots with both time and temperature were linear indicating the catastrophic nature of attack. An examination of the corroded samples by XRD, XRF, EPMA, SEM, and chemical analysis indicated that as the corrosion time increased, an increase in the depletion of alloying constituents like Cr, Al, Ti, and Co took place with a resultant enrichment of nickel on the alloy surface. The formation of CoCl₂ and Na₂CrO₄ was observed in all the tests. A few experiments were carried out in the presence of Na₂SO₄ and in a 1% NaCl mixture, in order to see the influence of NaCl on Na₂SO₄. The results indicated that Na₂SO₄ is innocuous when compared with NaCl. However, the severe attack was observed in the presence of the 1% NaCl mixture between 700–800°C, i.e., above the eutectic temperature and the m.p. of NaCl (800°C). The corrosion was minimum, when the salt mixture existed in the molten state. All the corroded samples were magnetic in nature. The role of NaCl on the hot-corrosion behavior of Nimonic 90 has been discussed in the light of the above crucible-test investigations.     

Morphological Development of Subscale Formation in Fe–Cr–(Ni) Alloys with Chloride and Sulfates Coating

Three types of stainless steel (430, 304, and 310) with a coating of NaCl, NaCl/AlCl₃, or NaCl/Al₂(SO₄)₃ are exposed at 750 and 850°C. Results show that NaCl has a major effect on corrosion and sulfur plays an important role in intergranular corrosion. After high-temperature exposure with a 100% NaCl coating, the morphologies of alloys 304 and 310 show typical uniform subscale attack the depths of attack increasing with temperature, while alloy 430 showed a planar attack. Alloy 310 has the highest chromium content and has the least metal loss. After high-temperature exposure with a NaCl/AlCl₃ coating, the corrosion morphologies and depths of attack are similar to those associated with an NaCl coating, but only voids are larger in the subscale. When coated with NaCl/Al₂(SO₄)₃, the alloys are attacked simultaneously by sulfur and chlorine at 750°C, resulting in a typical sulfur-attack intergranular corrosion. However, as the temperature increases to 850°C, the corrosion morphology changes to a uniform subscale attack.     

Mechanism of Degradation of Titanium Alloy IMI 834 and Its Protection Under Hot Corrosion Conditions

The excellent combination of high-temperature strength and lightweight properties makes titanium-base alloys attractive for high-temperature applications in aircraft engines. However, more hot corrosion of titanium alloys is a life-limiting factor, particularly when aircraft fly at low altitudes across the sea. In the present paper, an attempt has been made to understand the degradation mechanism of titanium alloy, IMI 834 under hot corrosion conditions at elevated temperatures. The hot corrosion studies were carried out by determining weight loss at different temperatures and in salts of pure Na₂SO₄, 90% Na₂SO₄+10% NaCl and 90% Na₂SO₄+5% NaCl+5% V₂O₅. Subsequently, the rate constants were evaluated. The depth of attack due to hot corrosion was compared with oxidation data. Finally, the degradation mechanism of the titanium alloy that leads to degradation of mechanical properties in aggressive environments has been discussed and suitable coatings suggested to enhance the operational life of engines by effectively preventing both oxidation and hot corrosion.     

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.     

High temperature corrosion behaviour of a new Ni-30Fe-10Al-Cr-Alloy

The high temperature corrosion behaviour of a new duplex nickel-base alloy containing about 30 mass% iron, 10 miss% aluminium and 8 mass% chromium was determined in both air and hot process gases containing methane/hydrogen, sulphur dioxide and hydrogen sulphide, respectively. It was found that the corrosion resistance against carburisation, sulphidation and oxidation was excellent due to the formation of a dense, protective alumina scale. The adherence of the alumina scale was increased by an addition of 0.1 mass% hafnium. The concentration of chromium was found to have a remarkable impact on the oxidation and high temperature corrosion resistance. Alloys without chromium showed increased corrosion rates in both air and sulphur-containing gas atmospheres due to the initial formation of nickel oxides. In sulphidising SO₂- and H₂S- containing gases at least 4 mass% chromium are required to stabilise the formation of alumina and to prevent the formation of nickel/sulphur compounds.     

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.     

Effect of chloride ions on electrochemical properties of anodized <Emphasis Type="Italic">AV</Emphasis> and <Emphasis Type="Italic">D</Emphasis>16 aluminum alloys in aqueous and glycerin-containing aqueous sodium sulfate solutions

The effect of chloride ions (0.01 N NaCl) on the electrochemical properties of anodized (in chromic anhydride or sulfuric acid) AV and D16 aluminum alloys in aqueous sulfate (0.5% Na₂SO₄) and glycerin-containing aqueous sulfate (0.5% Na₂SO₄, 33% glycerin) solutions is studied. Depending on the conditions of anodizing and the composition of the alloy and environment, currents on the anodized alloys in the passive range are shown to be smaller by one to four orders of magnitude compared to those on nonanodized alloys. Anodizing increases the resistance of alloys against pitting corrosion. Alloys anodized in sulfuric acid and then treated in dichromate are not susceptible to pitting corrosion. Alloys anodized in chromic anhydride are less resistant against pitting.     

Hot Corrosion of a Single Crystal Ni-Base Superalloy by Na-Salts at 900°C

The hot corrosion behaviors of a single crystal (SC) Ni-base superalloy coated with Na₂SO₄ and 75 wt.% Na₂SO₄-25 wt.% NaCl mixture were studied in air at 900°C. The results showed that the corrosion productions were laminar structure, porous and easily spalled. And sulfides formed quickly in the deep SC superalloy under the corrosion production. The addition of NaCl into Na₂SO₄ considerably accelerated the corrosion of the SC superalloy, and the corrosion scale became more porous. The hot corrosion process was explained based on sulfide formation and its subsequent oxidation.     

Effect of enamel coating on oxidation and hot corrosion behaviors of Ti-24Al-14Nb-3V alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na₂SO₄+15%K₂SO₄ and 15%NaCl+85% Na₂SO₄ molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al₂O₃+TiO₂+AlNbO₄ scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na₂SO₄+K₂SO₄ and NaCl+Na₂SO₄. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl⁻ anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.     

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.     

The salt-induced corrosion behaviour of FeCr alloys at elevated temperatures—II. Alloys rich in chromium

The oxidation behaviour of two Na₂SO₄-coated, chromia-forming, iron-based alloys at 900°C has been studied thermogravimetrically, and the reaction products examined in detail by metallographic and E.P.M.A. techniques. Na₃SO₄ coatings markedly enhance the oxidation rates of both alloys and result in the formation of thick, compact, stratified scales. On the basis of subsequent experiments designed to characterize the singular roles of sodium oxide and sulphur, and in the absence of scale fluxing, it is postulated that the formation of sulphides in the alloy substrate and the mechanical failure of scale are responsible for the enhanced oxidation. Sodium chromate, a feature of the hot corrosion reactions of Na₂SO₄-coated chromia-forming, Ni- and Co-based binary alloys, is shown not to be a by-product of the corrosion reaction of equivalent iron-based alloys. Instead salt/scale reactions result in the formation of sodium-iron oxide, which is capable of assisting in the corrosion reaction, albeit in a minor way. The role of NaCl additions during Na₂SO₄ induced corrosion is also examined.     

Aluminizing and boroaluminizing treatments of Mar-M247 and their effect on hot corrosion resistance in Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>−NaCl molten salt

The effect of surface modifications of Mar-M247 superalloy on hot corrosion resistance was examined in Na₂SO₄−NaCl molten salt. The Mar-M247 was aluminized and boroaluminized by pack cementation in Ar and underwent a cyclic hot corrosion test in Na₂SO₄−NaCl molten salt. The XRD results showed that a Ni₂Al₃ phase was formed between the aluminized layer and the substrate when the surface modification temperature was below 1273 K. However, a NiAl phase formed when the temperature was above 1273 K. The intensity of the XRD peak in the NiAl phase increased after post heat treatment. Hot corrosion resistance increased for the specimens containing NiAl rather than Ni₂Al₃ phase. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247, which underwent surface modification at 1273 K and above. In the boroaluminized Mar-M247 specimens, corrosion resistance decreased as a result of the blocking of outward diffusion of Cr by boron and decreased cohesion between the oxide scale and the aluminized layer during thermal cycling.     

The salt-induced corrosion behaviour of FeCr alloys at elevated temperatures—I. Alloys dilute in chromium

The effect of sodium sulphate coatings on the oxidation behaviour of pure iron and iron-5% chromium in oxygen at 1 atm. pressure has been investigated using a conventional thermobalance technique. The oxidation kinetics and the scale morphology of pure iron were relatively unaffected by the presence of Na₂SO₄, whereas Fe-5Cr was found to undergo an initial enhanced oxidation. However, when the limited supply of salt was exhausted the rate became similar to that of the uncoated alloy. In the absence of porous scale it is concluded that fluxing processes, such as those postulated for the appropriate Ni and Co based alloys, are not a feature of the Na₂SO₄ induced corrosion of the present alloy. Experiments designed to investigate the singular roles of sodium oxide and sulphur indicate that it is sulphide formation which is the dominant cause of the initial accelerated oxidation. There is also evidence to suggest that reactions involving scale and salt may contribute to the acceleration in rate. Additions of sodium chloride do not drastically influence the overall level of attack of either of the Na₂SO₄-coated materials.     

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.     

Hot Corrosion Behavior of a Ni3Al-Based IC21 Alloy in a Molten Salt Environment

Ni₃Al-based alloys have become important candidates for hot components in turbine engines, owing to their low densities and outstanding mechanical properties in service environments. The hot corrosion behavior of a Ni₃Al-based IC21 alloy in a molten salt environment of 75 wt% Na₂SO₄ and 25 wt% NaCl at 900 °C was studied, via oxidation kinetics analyses, scanning electron microscope observations and energy dispersive as well as diffraction analyses by X-ray. A multilayer corrosion oxide scale and dendritic morphology internal corrosion zone formed after hot corrosion, and inter-phase selective corrosion phenomena were also observed. Salt fluxing and oxidation-sulfidation processes were inferred to be the essential hot corrosion mechanisms of the alloy. Moreover, additions of Cr and Y proved to be beneficial to the hot corrosion resistance of the IC21 alloy, while the Mo content should be strictly controlled.     

The influence of iron and cobalt on the type II hot corrosion behavior of NiCr model alloys

The type II hot corrosion behavior of the alloys NiCr20, NiCr20Co10, and NiCr20Fe10 is investigated at 700°C in synthetic air + 0.5% SO₂ for up to 300 hr. Pure Na₂SO₄ and a eutectic mixture of MgSO₄–Na₂SO₄ are applied as deposits. The kinetics are investigated via dimensional metrology and correlated to the microstructural progression of the corrosion by examining the cross-sections. All alloys exhibit two-stage corrosion kinetics, with initially low and subsequently increased metal losses. Independent of the deposit composition, the metal loss after the longest exposure time is increased by the alloying element cobalt, whereas it is decreased for the iron-containing alloy. All alloys show increased metal losses when exposed to the MgSO₄–Na₂SO₄ deposit. The time to the propagation stage is similar for all tests. During the stage of low metal loss, all alloys develop a chromia scale and internal chromium sulfides. When the propagation stage is reached, chromium and nickel can be found along with oxygen and sulfur within the pit. Nickel is dissolved into the deposit, where it precipitates.     

Hot corrosion behaviour of a cobalt-base super-alloy K40S with and without NiCrAlYSi coating

A NiCrAlYSi coating was deposited by arc ion plating on a cobalt-base super-alloy K40S to improve its hot corrosion resistance at 1173 K in air. The K40S suffered from accelerated corrosion and formed non-protective scale with poor adherence when its surface was beneath Na₂SO₄ and Na₂SO₄ containing 25 wt.% NaCl salt deposits. After the K40S was coated with NiCrAlYSi coating, a protective α-Al₂O₃ scale was formed on the coating. Although the NiCrAlYSi coating changed into NiCoCrAlYSi during corrosion processes, it still possessed good corrosion resistance. In addition, the corrosion mechanisms were discussed on a basis of basic fluxing model.     

Hot corrosion behaviour of Nbss/Nb5Si3 in situ composites in the mixture of Na2SO4 and NaCl melts

Hot corrosion behaviour of Nb–16Si–24Ti–6Cr–6Al–2Hf (at.%) in the mixture of Na₂SO₄ and NaCl melts at 900 °C was studied. The results show that the corrosion kinetics of the alloy fit parabolic law. The oxides consist of a loose and porous outer layer and an internal oxidation zone. Outer oxides are mainly composed of TiO₂, TiNb₂O₇, Nb₂O₅, CrNbO₄ and SiO₂ while the internal oxidation zone is composed of TiO₂. Hot corrosion mechanism of the alloy in the presence of Na₂SO₄ and NaCl deposits is discussed.     

Hot corrosion Type II of FeCr‐based model alloys for boiler and heat exchanger applications

The hot corrosion Type II of the alloys FeCr20, FeCr20Ni10, FeCr20Ni20, and FeCr20Co10 is investigated at 700°C in air + 0.5% SO₂ with deposits consisting of Na₂SO₄ and a eutectic mixture of Na₂SO₄ and MgSO₄ for 24, 100, and 300 h. The alloying elements nickel and cobalt have a positive influence when tests are conducted using a MgSO₄‐Na₂SO₄ deposit. In this case, they reduce the metal loss and increase the time to the propagation stage. In contrast, when the alloys are exposed with a Na₂SO₄ deposit, these alloying elements increase the metal loss and allow for the transition to the propagation stage because they can form molten phases with the Na₂SO₄. During the incubation stage an oxide scale forms on the FeCr20 alloy, which is thicker than the one formed during exposure without a deposit, and iron oxides are observed, which precipitate in the deposit. The propagation stage occurs by a dissolution and precipitation mechanism forming localized pitting attack. Iron is the main species that dissolves and precipitates, while chromium remains mainly as an oxide beneath the initial surface. The additional elements are found in the pit and in the salt deposit.