Low Temperature Hot Corrosion Research Articles

ChemInform Abstract: The Mechanism of Sodium Sulfate‐Induced Low Temperature Hot Corrosion of Pure Iron

Abstractis studied in a SO3‐SO2‐O2 mixed atmosphere at 600‐800 °C. A layer of Na2SO4 is deposited on the iron sample prior to the corrosion test.

The mechanism of sodium sulphate-induced low temperature hot corrosion of pure iron

The Na 2SO 4-induced corrosion kinetics of pure iron between 600 and 800°C was studied using a SO 3-SO 2-O 2 mixed atmosphere. At any temperature accelerated oxidation of the metal was observed: this phenomenon is attributed to the formation of a liquid sulphate phase from the initially solid Na 2SO 4 deposit. A corrosion mechanism based on the dissolution of iron and iron oxides on the scale surface is proposed. The attack occurs through different stages: first a liquid FeSO 4-Na 2SO 4 phase is probably formed from the initially pure solid Na 2SO 4 layer; then accelerated oxidation goes on by SO 3 inward diffusion and iron outward diffusion through the molten salt. Transport models and quantitative estimates of the fluxes of the reacting species are presented.

High-temperature corrosion of alumina-forming coatings for superalloys

The results of recent experiments on the cyclic oxidation and hot corrosion of diffusion aluminide and CoCrAlY overlay coatings on nickel-base superalloys are described. It is shown that platinum additions to aluminide coatings produce a substantial increase in resistance to cyclic oxidation and high temperature (Type 1) hot corrosion but have a rather small effect on resistance to low-temperature (Type 2) hot corrosion. The corrosion of aluminide coatings is also shown to be influenced by the composition of the underlying superalloy. Mechanisms for the above behavior are proposed. The effects of yttrium and hafnium on the cyclic oxidation of CoCrAl are illustrated. The results of scanning and transmission electron microscope characterization of the hot corrosion morphologies of CoCrAlY coatings are discussed and a mechanism to account for the microstructures is briefly described.

Sub-melting point hot corrosion of alloys and coatings

Sub-melting point hot corrosion is referred to as the hot corrosion occurring at temperatures well below the melting point of the original salt deposit. From the point of view of the melts formed, two types of sub-melting point hot corrosion are often encountered. One is related to the formation of a binary or ternary sulphate resulting from sulphation of the base metal oxide. The other is associated with the formation of a melt by the reaction between some refractory metal oxide and the sulphate deposit. The phenomena and mechanism of sub-melting point hot corrosion of nickel and cobalt base alloys including MCrAlY coatings (namely low-temperature hot corrosion or Type II hot corrosion) caused by an Na 2SO 4 deposit are reviewed. The sub-melting point hot corrosion of iron, AISI 304 stainless steel, and FeCr alloys caused by a K 2SO 4Na 2SO 4 deposit is reported and its mechanism is proposed in the light of which the hot corrosion is attributed to the dissolution of iron as cations at the oxide-melt interface and precipitation of Fe 2O 3 as porous particles at the melt-gas interface. The hot corrosion behaviour of B-1900 alloy in the presence of an Na 2SO 4 coating was studied at 827 °C as well as at 750 °C, and the alloy was found to experience catastrophic oxidation in this case. The experimental results lead to a proposition that MoO 3 (probably as Mo 2O 7 2−) takes part in the reduction reaction. The sub-melting point hot corrosion of a number of superalloys containing molybdenum coated with CaSO 4 has been studied. The resistance to sub-melting point hot corrosion of CoCrX (X ≡ Si, Hf or Y) coatings and aluminium and chromium diffusion coatings is briefly discussed.

Evaluation of the hot corrosion protection of coatings for turbine hot section components

A program was conducted to evaluate the relative corrosion resistance of commercially available coatings in laboratory furnace testing. Three turbine section superalloys were selected, MAR-421, IN-738LC and FS-414, and coating types ranged from simple diffusion aluminides to modified aluminides and a CoCrAlY overlay. Results indicate that the coatings were more protective to IN-738LC than to MAR-M421, with a silicon aluminide and platinum-chromium aluminide affording the best protection overall. The simple aluminide was surprisingly quite resistant in the temperature regimes studied, relative to the more complex aluminides, particularly with IN-738LC. The recommendations for corrosion-resistant coatings (for low temperature and high temperature hot corrosion environments) are as follows: silicon aluminide and platinum-chromium aluminide for MAR-M421; simple aluminide, platinum-chromium aluminide and silicon aluminide for IN-738LC; and platinum-chromium aluminide and silicon aluminide for FS-414.

Interaction of low temperature hot corrosion and creep

Investigations have been carried out to study the effect of hot corrosion on the creep rupture properties of type 304 stainless steel in the temperature range 600–700°C. The corrodents used were 100% Na2SO4 and a mixture of 75% Na2SO4 and 25% NaCl. The creep life decreases in the presence of the corrodents and the mixture of Na2SO4 and NaCl has been found to exert the worst attack in all cases. The corrosion products on the fractured surface has been analysed and, based on the data, a model for creep rupture life has been proposed under the interaction condition.

Thermodynamics of the Na2SO4-K2SO4-CoSO4 system and their relevance to low-temperature hot corrosion

Thermodynamic calculations and experiments were performed to determine the SO3 partial pressures and temperatures at which K2SO4-CoSO4 binary mixed liquid phases form on CoO and Co3O4 in the presence of K2SO4. The calculations and experiments are in excellent agreement. Similar calculations were also made of the compositions at the liquidus surface and the associated SO3 partial pressures for the K2SO4-Na2SO4-CoSO4 ternary system. These calculations show that the presence of K2SO4 substantially reduces the SO3 partial pressures required to stabilize a liquid salt phase on the surface of oxidized cobalt alloys at 600–800°C. Consequently, at these temperatures the hot corrosion in coal-fired systems, where K levels are high, is expected to be worse than in oil-fired systems, where K levels are low. This prediction was confirmed by experiments in a pressurized fluidized bed coal combustor and in an atmospheric pressure burner rig.

Structure and 900 °C hot corrosion behavior of chromium-modified platinum aluminide coatings

It is well known that the addition of platinum to diffusion aluminide coatings has demonstrated great success in producing protective coatings which delay the onset of high temperature hot corrosion attack in the range 800 – 1000 °C. Recently, it has been found that the addition of chromium to diffusion aluminide and platinum aluminide coatings improves low temperature (700 °C) hot corrosion resistance on IN738 nickel-base superalloy substrates. Therefore, a program was initiated to study the effect of the combined addition of chromium and platinum with varying processing sequences on aluminides in accelerated hot corrosion environments at 900 °C. In this investigation, ten coating types on the nickel-base superalloy substrate IN738 were tested at 900 °C for 200 h. Of these ten coatings, two coatings showed excellent high temperature (900 °C) hot corrosion resistance: (i) Pt-Al (produced by an intermediate temperature, intermediate activity (ITIA) process) and (ii) Cr-Pt-Al (process D). The average measured depth of attack rate was about 0.025 μm h -1 which is about one order of magnitude slower than that of published low temperature (700 °C) hot corrosion data. Scanning electron microscopy was used to examine the microstructures of the coatings. The ITIA Pt-Al coating has a thin (3 μm) surface layer of PtAl 2, with an outer intermediate zone of profuse PtAl 2 particles dispersed in an NiAl matrix and an inner intermediate zone of substrate-element-rich precipitates and carbides in a β-NiAl matrix over an interdiffusion zone. The Cr-Pt-Al (process D) coating has a thick (18 μm) continuous surface layer of PtAl 2, with an outer intermediate zone of a small volume fraction of α-Cr and other substrate-element-rich precipitates dispersed in a β-NiAl(Pt, Cr) matrix and an inner intermediate zone of α-Cr-rich precipitates, refractory metal carbides and NiAl over a typical interdiffusion zone. It appears that an outer dense layer of the PtAl 2 phase backed up with a fairly high level of chromium and platinum near the surface is beneficial in protecting the superalloy substrate against high temperature corrosion attack.

The mechanism of the low-temperature hot corrosion of pure iron and manganese at 600–800°C

The corrosion of pure iron and manganese in simulated combustion gases at 600–800°C in the presence of sodium sulfate deposits is faster than the same reaction without salt deposits. This effect is due to the formation of a liquid solution between sodium sulfate and the sulfates of the corroded metals. The nature of these liquid solutions is examined together with the possible mechanisms for the transport of matter through the liquid and the dissolution-reprecipitation of solid metal compounds in the liquid salt. Finally, the reasons for the formation of sulfides in mixture with oxides in the inner solid region of the scale and the effects produced by the presence of sulfides on the overall reaction rate are considered.

Hot corrosion behavior of cast Co-22Cr-11Al alloy and Co-22Cr-11Al-0.3Y electron beam PVD coating on in 738

The initiation and early stages of propagation of low temperature hot corrosion (LTHC) attack at 700 °C have been studied on a cast CoCrAl coating alloy and on a commercially prepared electron beam physical vapor deposited (EBPVD) CoCrAlY coating on a drop-cast IN 738LC substrate. To evaluate the role of active elements such as yttrium and hafnium on the LTHC degradation process, some specimens of cast CoCrAl alloy were partially ion-implanted with either yttrium or hafnium or cobalt. The specimens of cast CoCrAl coating alloy were pre-oxidized for 1 h at 700 °C before exposure to conditions that simulate the LTHC attack typically observed on hot-end components of gas turbine engines operated in a marine environment. Specimens of the EBPVD CoCrAlY coating were not given the 1 h pre-oxidation at 700 °C before exposure to the LTHC environment. Hot corroded specimens were subsequently examined by scanning and transmission electron microscopies and by energy-dispersive X-ray analysis. The LTHC results obtained on the cast CoCrAl coating alloy and on the commercially prepared EBPVD CoCrAlY coating on IN 738LC substrate are presented and compared. Fundamental mechanisms of initiation and propagation of hot corrosion attack for this class of coatings have been formulated that are consistent with the experimental results.

Low temperature hot corrosion of CoCrAl alloys

Hot corrosion of CoCrAl alloys occurs rapidly at temperature of 600–750°C because of the formation of the low melting Na 2SO 4CoSO 4 eutectic. In this paper the mechanisms and solutions of this low temperature hot corrosion (LTHC) are reviewed. Rapid degradation results from the dissolution of the more noble metal (cobalt) and its oxides (CoO and Co 3O 4) on the surface. On the basis of this mechanism, we have developed CoCr coatings containing about 37.5 wt% Cr or more which are resistant to LTHC and provide adequate resistance against the normal high temperature hot corrosion. Theoretical calculations have also been performed to predict the conditions under which binary K 2SO 4CoSO 4 and ternary Na 2SO 4K 2SO 4CoSO 4 eutectics can form. These calculations suggest that the presence of K 2SO 4 can aggravate the LTHC caused by Na 2SO 4.

Reactions Occurring during the Sulfation of Sodium Chloride Deposited on Alumina Substrates

The reaction between solid and air containing 1% has been studied between 500° and 700°C. The reaction product, , forms not only on the surface of the but also on surrounding areas of the substrate due to the volatility of the at these temperatures. At the higher temperatures, the vapor pressure of is so high that the majority of the reaction product is distributed on the substrate. Above 625°C, the reaction product is a liquid solution of and that exists only so long as is supplied from the original crystal source. Eventually, the liquid solidifies by constitutional solidification is the is converted to . While it exists, the liquid solution is shown to be highly corrosive to and, on a scale of growing on alloy HOS 875, particularly attacks the grain boundaries of the scale at preferred sites where chromium and iron oxides and sulfides rapidly develop. This is proposed as one mechanism by which deposition contributes to the initiation of low temperature hot corrosion.

Zinc Sulfate Reactions in Hot Corrosion

To further understanding of the potential effects of zinc containing contaminants or fuel additives in gas turbine hot corrosion, the corrosion of (a gas turbine blade coating alloy) spray coated with 50 mole percent was compared with that of spray coated with alone at 700° and 850°C in air containing controlled partial pressures. Thermobalance experiments showed that delayed initiation of low temperature (700°) hot corrosion of , but did not significantly change the total amount of corrosion, nor inhibit LTHC once the propagation stage of corrosion was reached. At 850°, gave somewhat more corrosion than pure , presumably because allowed formation of a liquid sulfate phase. Molten was shown to follow regular solution behavior at 850°, with the standard free energy for formation of in being −9.34 kcal/mol. The overall results suggest that zinc would be of doubtful benefit as an inhibitor for gas turbine hot corrosion.

Low-Temperature Hot Corrosion in Gas Turbines: A Review of Causes and Coatings Therefor

In about 1975 an apparently new form of hot corrosion attack of gas turbine airfoils was identified during low-power, low-metal-temperature operation of a marine gas turbine. The rate of this corrosion was substantially greater at about 700° C than that usually observed for sulfate-induced hot corrosion at 800° to 1000° C. The same type of hot corrosion has been subsequently reported to occur in ground-based gas turbines, and is similar in principle to fireside corrosion of boiler tubes. This paper presents a review of probable mechanisms of this so-called low-temperature hot corrosion, of test methods for its laboratory and rig simulation, and of coatings in use or in advanced development for protection of gas turbine airfoils operating in this corrosion regime.

Coating service experience with industrial gas turbines

Abstractield experience with high–temperature coatings on land–based gas turbine blades and vanes is reviewed. The article covers all relevant coatings such as the chromium type, precious metal aluminides, and overlay coatings of the NiCrSi and MCrAlY (M = Fe, Co, Ni) types. Long term service behaviour, allows, various guidelines to be drawn for the selection of a coating appropriate to particular operating conditions. In the low–temperature hot–corrosion regime, high chromium containing coatings appear to be the most effective and the aluminides less so. At higher temperatures, aluminides are superior and offer protection comparable to that given by MCrAlY overlays. The advantage of these overlays lies in a higher compositional flexibility, especially when deposited by low–pressure (vacuum) plasma spraying. Service performance can be significantly affected by the mechanical integrity of the coatings; some ductility is required to accommodate thermal stresses. A general lifetime prediction for coatings is...

Performance comparison of plasma spray and physical vapor deposition BC23 coatings in the LM2500

Operation of U. S. Navy Component Improvement Program LM2500 engines on the GTS Callaghan has provided an at-sea test bed for evaluation of hot corrosion on gas turbine engine components. In a presentation at a previous International Conference on Metallurgical Coatings, (ICMC), comparison of several coatings applied by physical vapor deposition (PVD) techniques were compared after LM2500 service. A three-step coating BC23 consisting of PVD CoCrAl followed by codeposition of hafnium and aluminum and an electroplated platinum layer was identified as the most corrosion-resistant coating of several PVD coatings tested. In this paper, a plasma sprayed version of BC23 is compared to the three-step BC23 coating on LM2500 Stage 2 HPT blades from the GTS Callaghan. The plasma sprayed coating PBC23 was produced by an argon-shrouded plasma spray process using prealloyed CoCrAlHfPt powder. The results showed that in this application, where low temperature hot corrosion (type 2) is the predominant mode of corrosion, the plasma sprayed coatings had less attack. These results are compared to laboratory hot corrosion tests and microprobe analysis to explain the improved corrosion resistance of PBC23.

Kinetics of the Low Temperature Hot Corrosion of Co‐Cr‐Al Alloys

Hot corrosion rates have been measured on many Co‐Cr, Co‐Al, and Co‐Cr‐Al alloys coated with 2.5 mg/cm2 of and exposed in at 750°C. The hot corrosion resistance of the binary alloys increases with the chromium and aluminum content of the alloys. However, the additions of aluminum to Co‐Cr and of chromium to Co‐Al alloys have deleterious effects. These results can be explained on the basis of a mechanism similar to one previously proposed in the literature (1, 2). Additional experiments have been conducted as a function of temperature and the content of oxygen to extend the proposed mechanism.

High chromium cobalt-base coatings for low temperature hot corrosion

Accelerated oxidation tests have been conducted on a number of cast Co-Cr-Al alloys coated with a thin film of Na 2SO 4 and exposed in an O 2-0.15%(SO 2-SO 3) gas mixture at 750°C. These tests show that the corrosion resistance of binary Co-Cr alloys increases with the chromium content of the alloys and that even small concentrations of aluminum have detrimental effects on their corrosion resistance. Tests on Co-Cr coatings on a René 80 substrate under the same conditions indicate that there is a transition from a high to a low corrosion rate at a chromium level of about 37.5 wt.%. These results can be explained on the basis of a mechanism that we have already proposed in the literature. Based on our laboratory studies, we have developed new Co-Cr coatings that provide excellent resistance to low temperature hot corrosion and are also acceptable for conventional, high temperature, hot corrosion.

Inhibition of Low Temperature Hot Corrosion by Zn, Na Mixed Sulfates: Relation Between Sulfation and Corrosion Reactions

Zinc effects in low temperature hot corrosion (LTHC) were investigated by examining the sulfation of and by in melts at 700°C. Sulfation was substantially reduced as compared to sulfation in pure , especially for . Corrosion tests with deposited on BC‐21 composition , showed LTHC to be inhibited under concentrations up to 500 ppm. The results demonstrate that the corrosive properties of melts are significantly affected by . LTHC inhibition is postulated to result from reduced solubility of in the melt, and/or formation of “barrier” films on the oxide and metal surfaces.

Intermediate Temperature, Low-Cycle Fatigue Behavior of Coated and Uncoated Nickel Base Superalloys in Air and Corrosive Sulfate Environments

Low temperature hot corrosion effects on the low cycle fatigue (LCF) life and fracture morphology of a Udimet 720 alloy and the effectiveness of a protective coating in minimizing life degradation are reported. Strain controlled, continuous cycling (1 cpm) and 1 h hold (at maximum tensile strain) tests at 1350°F (732°C) in air revealed only small changes in life that could be attributed to creep during the 1 h hold. The presence of a corrosive salt environment, however, resulted in more than an order of magnitude degradation in life. In air and salt, the fracture was initially transgranular, but in salt it quickly changed to an intergranular mode, particularly at cyclic strains above 0.65 percent. These differences are attributed to reductions of 95 and 28 percent in the respective ductility and fatigue strength coefficients in the Coffin-Manson and Basquin relationships between cyclic strain and life. At cyclic strains ≤ ± 0.5 percent, a diffusion aluminide coating was protective in 1350°F (732° C) salt, with lives equivalent to those for the coated alloy in air; but at higher strains, coating failure occurred. Insignificant differences in fatigue life in air were noted for Udimet 720 and Udimet 710 alloys.