Low Temperature Hot Corrosion Research Articles

Influence of temperature on hot corrosion behavior of GH4169 superalloy subjected to Na2SO4–NaCl salts attack

This work studies the hot corrosion behavior of GH4169 Ni-based superalloy, deposited with a mixture of salts comprising 95 wt% Na2SO4 and 5 wt% NaCl, across three distinct temperatures (i.e., 650 °C, 800 °C and 950 °C). Corrosion and non-corrosive exposure experiments were compared, yielding data on mass loss and gain, respectively. Material characterization results revealed that the corrosion layer was mainly comprised of Cr2O3, Fe2O3, NiO, Al2O3, TiO2, NbS2 and MoS2. Notably, as the temperature ascended from 650 °C to over 800 °C, the corrosion mechanisms underwent a transition from pitting to uniform corrosion, corresponding to low-temperature hot corrosion and high-temperature hot corrosion, respectively. At 650 °C, a large number of semi-ellipsoidal corrosion pits manifested on the surface. Conversely, at 800 °C and 950 °C, the corrosion layer on the surface exhibited nearly uniform spallation. The pit growth model and spallation dynamics model were, respectively, developed based on the observed microstructure features. The models serve as tools for quantitative examination of the hot corrosion process of the superalloy at different temperatures.

Low-temperature hot corrosion of arc evaporated Ti1−xAlxN on Ni-Cr-Co based superalloys

Cathodic arc evaporated Ti1−xAlxN coatings were deposited on a Nimonic c-263 superalloy and tested under low temperature hot corrosion conditions. Treated with a MgSO4/Na2SO4 salt-mixture, all samples were annealed at 700 °C in a 2800 ppm(volume) SOx-rich atmosphere for 1, 5, 15 and 30 h. A significantly reduced corrosion severity was exhibited by all Ti1−xAlxN variants over the uncoated NiCrCo alloy. A synergistic fluxing mechanism was found to be the dominant factor for the coating breakdown. Depending on the relative Al-to-Ti content on the Ti1−xAlxN sublattice, differences in the coatings’ corrosion and scaling behavior were observed.

On the sulfation reaction of protective and transient oxides at 650 °C and its role in the low temperature hot corrosion mechanism

MxOy-Na2SO4 (with M = Ni, Cr, Fe, Al, Ti, Nb) and NiSO4-Na2SO4 mixtures were prepared and their reactivity was examined at 650 °C in air + SO2 (400 and 1000 ppm). The formation of a molten phase for NiSO4-Na2SO4 mixtures and the rapid sulfation of NiO in these conditions were demonstrated. NiO sulfation should be considered when explaining the Type II hot corrosion attacks. Cr2O3, Fe2O3 and Al2O3 also react with the gas phase showing these oxides dissolved according to an acidic-fluxing mechanism in air + SO2/SO3 (400 and 1000 ppm) environments. Al2O3 is more soluble than Cr2O3 in these conditions.

Hot-Corrosion and Particle Erosion Resistance of Co-Based Brazed Alloy Coatings

Tape brazing constitutes a cost-effective alternative surface protection technology for complex-shaped surfaces. The study explores the characteristics of high-temperature brazed coatings using a cobalt-based powder deposited on a stainless-steel substrate in order to protect parts subjected to hot temperatures in a wear-exposed environment. Microstructural imaging corroborated with x-ray diffraction analysis showed a complex phased structure consisting of intermetallic Cr-Ni, C-Co-W Laves type, and chromium carbide phases. The surface properties of the coatings, targeting hot corrosion behavior, erosion, wear resistance, and microhardness, were evaluated. The high-temperature corrosion test was performed for 100 h at 750 °C in a salt mixture consisting of 25 wt.% NaCl + 75 wt.% Na2SO4. The degree of corrosion attack was closely connected with the exposure temperature, and the degradation of the material corresponding to the mechanisms of low-temperature hot corrosion. The erosion tests were carried out using alumina particles at a 90° impingement angle. The results, correlated with the microhardness measurements, have shown that Co-based coatings exhibited approximately 40% lower material loss compared to that of the steel substrate.

Microstructure and characterization of LVOF sprayed aluminium oxide-13% titanium dioxide and aluminium oxide-40% titanium dioxide coatings on ASTM 316 boiler steel

Mostly common issue in power generation plant is recognised as disintegration or degradation of steels used in boilers in recent days and main causes for this type of failure are erosion, low temperature hot corrosion, high temperature hot corrosion etc. Defensive or protective coatings have been found effective to solve this problem and showed good results. Thermally sprayed coatings are widely used in various industries in boiler components such as boiler tubes, super-heater tubes, boiler walls and Induced Draft (I.D.) fans in coal fired thermal power plants, automobile system etc. The coatings produced by LVOF method show good adherence to the substrate with lower oxidation rate. In present work LVOF Sprayed Al2 O3-13%TiO2 as well as Al2 O3-40%TiO2 Coatings were deposited on ASTM316 Boiler Steel. The characterization of coating was done by SEM, X-Ray diffraction for checking the phases formed on the surface, and elemental examination of the cross-section. The surface morphology of the above said both the coatings was examined with the help of Scanning Electron Microscopy with EDAX attachment, XRD examination and the X-ray mapping. The research investigation shows that the selected feedstock and coating technique have produced the successful coatings perfectly.

Effect of Air Cooling on Low Temperature Hot Corrosion Behavior of T12 Steel in Coal Ash

Hot corrosion experiments in coal-fired boiler environment in the lab were always conducted under constant temperatures of interest, while in the actual working conditions, boiler tubes are cooled down through water cooling from a relatively high fireside temperature, e.g., 1400oC. To accurately investigate the degradation regularity of boiler tubes in the actual working environment, it is necessary to probe the difference between these two conditions. Herein, electrochemical tests including open circuit potential (OCP), electrochemical noise (EN) and potentiodynamic polarization (PDP) have been adopted to compare the coal ash hot corrosion behavior of T12 steel in constant 400oC and air cooling down to 400oC from 1400oC using our proposed hot corrosion electrochemical sensor system.

Performance Comparison Between Isothermal Hot Corrosion And In Situ Cyclic Hot Corrosion of Nickel-Based Superalloys

Although a lot of work has been done to understand both major mechanisms of hot corrosion, namely type I (high-temperature hot corrosion) and type II (low temperature hot corrosion), there is very little information available on more representative cyclic performance in these regimes. This work addresses this by assessing the performance of isothermal (type I and type II) hot corrosion tests against combined (short and long) cyclic corrosion tests. Single-crystal alloy PWA 1484 and directionally solidified alloy MAR-M247 were assessed in all test regimes. Pre- and post-exposure dimensional metrology was used to quantify the corrosion damage and characterised using SEM/EDX. This paper highlights that the results of short cycle test conditions are more damaging compared to long cycle and standard isothermal type I and II test conditions. The cast nickel-based alloy MAR-M247 was found to be a better performer compared to PWA 1484 single-crystal alloy.

High temperature erosion behaviour of Type AISI 446 stainless steel under the combined effect of surface modification and pre hot-corrosion

In the present study, the effect of Surface nano crystallization (SNC), using Ultrasonic shot peening (USP), on pre-hot corrosion-erosion behaviour of Type AISI 446SS at elevated temperatures is studied. No phase change was revealed by X-ray diffraction (XRD), however, the microhardness is significantly increased in the shot peened region for USPed samples. The USPed samples were subjected to low-temperature hot corrosion (LTHC) tests under isothermal conditions for 20 h at 550, 650 and 750 ℃. The LTHC tests were carried out by spray deposition techniques using two salt mixture (Na2SO4/NaCl). Alumina (Al2O3) sand operated air-jet erosion tester was used to test the pre-hot corroded samples. The passive layer formed during LTHC suffered expulsion of metallic flakes and led to a significant change in erosion rate.

Analysis of combined static load and low temperature hot corrosion induced cracking in CMSX-4 at 550°C

A CMSX-4 3-point bend specimen was statically loaded under hot corrosion conditions and SEM, (S)TEM and EDX techniques were used to analyse the cracking generated. Sulphur, chlorine, sodium and oxygen were found at the crack tip, and an influence of loading on the corrosion mechanism’s preference to interact with either the γ or γʹ was observed. The microscopy analysis is in support of the corrosive mechanism being a combined stress and electrochemical corrosion linked with low temperature hot corrosion, where crack propagation occurs as a result of localised corrosion enhanced material degradation. High magnification EDX mapping identified W as segregating to the γʹ at room temperature.

Effects of Pt and Si on the low temperature hot corrosion of aluminide coatings exposed to Na2SO4-60 mol% V2O5 salt

Hot corrosion of pack (zero/medium) Pt-modified aluminide and slurry (zero/low/medium) Pt-modified silicon-aluminide coatings exposed to a Na2SO4-60 mol%V2O5 deposit at 700 °C was evaluated using the deposit recoat technique. Thickness loss metrology was utilized to study corrosion penetration into the coatings. The corrosion products and remaining coatings were characterized by means of SEM, EDX, XRD and EPMA. The aluminide coating and medium Pt-modified silicon-aluminide coating showed the worst and the best corrosion resistance, respectively. Enhanced hot corrosion resistance of the Pt-modified silicon-aluminide was attributed to microstructure of the coating with a two-phase PtAl2-NiAl outer layer and formation of refractory silicides.

Effect of Prehot Corrosion on Erosion Behavior of High Chromium Ferritic Steel for Heat Exchangers

This study presents the prehot corrosion effect on erosion behavior of AISI 446 SS in simulated heat exchanger environment at elevated temperature. Samples were spray deposited using two salt mixture (Na2SO4/NaCl). Subsequently, low-temperature hot corrosion tests were carried out at 550, 650, and 750 °C for 20 h. Chlorination followed by sulfidation was mainly responsible for the passive layer formation during the process of hot corrosion. The prehot corroded samples were subjected to air-jet erosion test using alumina as the erodent, at impact velocity of 100 m/s and flux rate of 4.2 g/min, with variable impingement angles of 30 deg, 60 deg, and 90 deg. The passive layer formed during corrosion underwent detachment of metallic flakes through cracking during the impact of erodent, and was responsible for a significant change in erosion rate. Cutting, plowing, lip formation, and particle embedment were identified as the operative mechanisms during erosion.

Corrosion Damage Mechanism of TiN/ZrN Nanoscale Multilayer Anti-Erosion Coating

TiN/ZrN multilayers can effectively improve the erosion resistance of metals, particularly titanium alloys employed in aero engines. To explore the corrosion damage mechanism of TiN/ZrN nanoscale multilayers (nanolaminate), a novel [TiN/ZrN]100 nanolaminate coating was deposited on Ti-6Al-4V alloys by multi-arc ion plating method. Salt spray corrosion tests and hot corrosion experiment were carried out to evaluate the corrosion resistance of the coating. The corrosion and damage mechanisms were explored with the help of detailed microstructure, phase composition and element distribution characterizations. The salt spray corrosion tests showed that the [TiN/ZrN]100 nanolaminate coating possessed good corrosion resistance, which protected substrate against the corrosion. The low temperature hot corrosion tests showed that the oxidation occurred on the surface of the coating, which improved the oxidation resistance of the sample. However, the oxidized droplets squeezed the coating, and destroyed the oxidized layers. As a result, the coating was peeled off from the substrate. The research highlights the corrosion resistance of the novel TiN/ZrN nanolaminate coating and offers a support for their application in engine compressor blade.

Type II Hot Corrosion Screening Tests of a Cr2AlC MAX Phase Compound

Low-temperature hot corrosion tests were performed on bulk Cr2AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high-temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina formers, it contains no Ni or Co that may react with Na2SO4 salt deposits needed to form corrosive mixed (Ni,Co)SO4–Na2SO4 eutectic salts active in Type II hot corrosion. Cr2AlC samples coated with 20K2SO4–80Na2SO4 salt were exposed to 300 ppm SO2 at 700 °C for times up to 500 h. Weight change, recession, and cross-sectional microstructures identified some reactivity, but much reduced (< 1/10) compared to a Ni(Co) superalloy baseline material. Layered Al2O3/Cr2O3 scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr7C3 impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr2O3 scales, with Cr–S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr2AlC. This indicates some potential for Cr2AlC as LTHC-resistant coatings for superalloys.

Low-temperature hot corrosion effects on the low-cycle fatigue lifetime and cracking behaviors of a powder metallurgy Ni-based superalloy

In this paper, low-cycle fatigue (LCF) experiments were conducted to analyze the influence of low-temperature hot corrosion on the LCF behavior of the powder metallurgy Ni-based superalloy FGH96 at 700 °C. The LCF lifetime of the salt-coated samples was drastically lower than that of bare samples. The low-temperature hot corrosion would change the crack initiation mode (from subsurface defects to corrosion pits) and early propagation modes (from transgranular to intergranular). Moreover, the degradation mechanism of hot corrosion in the LCF lifetime was connected with early crack initiation resulting both from the deteriorated surface because of corrosion pits and the accelerated fatigue crack propagation due to weakened grain boundaries.

Analysis of Combined Static Load and Low Temperature Hot Corrosion Induced Cracking in a Ni-Based Single Crystal Superalloy

Analysis of Combined Static Load and Low Temperature Hot Corrosion Induced Cracking in a Ni-Based Single Crystal Superalloy

High Temperature Corrosion of Ni Based Alloy Inconel 740H in Simulated Coal-ash and Flue-gas Environments

The corrosion behavior of the Ni-based superalloy Inconel 740H exposed to synthetic coal-ash/flue-gas at both 750°C and 780°C for 5000h was studied. The microstructures of samples were characterized by using XRD, SEM with EDS. The results indicate that the 740H alloy exhibits stable microstructure after long time aging. The overall corrosion of specimens was divided into two successive stages. In initial stage, the Cr2O3 protective film formed on the surface of the samples and became thicker slowly and had a good adherence to the matrix. In propagation stage, the Cr2O3 was dissolved by the composite sulfate. This resulted in the low temperature hot corrosion of alloy. The corrosion layers were composed of inner sulfide layer, oxide layer and outer loose layer. The alloy corrosion at 780°C was more severe than that of at 750°C.

Hot corrosion behavior of the spray-formed nickel-based superalloy

An investigation of low temperature hot corrosion is carried out on a spray-formed nickel-based superalloy FGH100 pre-coated with Na2SO4-NaCl at 700 °C for 100 h. Mass gain measurement, x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy are used to study the corrosion behavior. Results reveal that corrosion behavior follows a sequence, that is, first rapidly proceeding, then gradually slowing down, and finally forming an outer layer composed of different types of oxides and an inner layer mainly comprised of sulfides. In-depth analysis reveals that the hot corrosion of FGH100 is a combined effect of oxidation-sulfidation and transfer of oxides.

Pit morphology and depth after low-temperature hot corrosion of a disc alloy

Hot corrosion of the low solvus, high refractory (LSHR) disc alloy was studied at 700 °C. The purpose of this study was to determine the conditions which result in a discrete, isolated pit morphology and to examine the influence of SO2 gas additions and various salt concentrations on the depth of those pits. Three salts, pure Na2SO4 and two Na2SO4–MgSO4 compositions, were used. It was found that with a eutectic Na2SO4–MgSO4 salt, there was no significant increase in pit depth between 0 and 30 ppm SO2 when O2 was also present in the gas stream. Gas flow was observed to affect pit formation, but the variation in the position of the corrosion mounds/pits on the sample surface was unexpected. There was limited evidence that pit nucleation was not associated with grain boundaries or grain triple point junctions. An evolution from single, isolated pits, to coalesced pits, to overlapping pits on a single sample was observed. At higher SO2 concentrations, the extent of attack increased, resulting in a uniform type of attack morphology with significant metal loss across the sample surface. It was concluded that hot corrosion attack by pit formation for these conditions is not easily explained or predicted.

The Effect of Different SO2/SO3 Catalytic Media on High-Temperature Corrosion Processes (Hot Corrosion, Fireside Corrosion, Sulfidation–Oxidation)

In many laboratory hot corrosion (HC) experiments and other experiments involving sulfur commercial catalytic converters are used to catalyze the $${\text{SO}}_{ 2} + { 1}/ 2 {\text{O}}_{ 2} \to {\text{SO}}_{ 3}$$ reaction. The conversion rate of SO2 to SO3 highly depends on the catalyst used and thus the HC results, especially in low-temperature HC conditions, may vary significantly depending on the catalyst selected. Different elements present in the catalyst affect the SO2 to SO3 conversion and with it the degree of HC induced attack. Modern automotive catalysts have low activity for the oxidation of SO2 to SO3. The conversion of SO2 to SO3 can be enhanced by modifications of the catalyst, e.g. by using a Pt-gauze or dip-coating the catalyst in a Fe2O3 solution.

Hot Corrosion of Metals and Alloys

When metals and alloys are used at high temperatures, especially in combustion processes, deposits often accumulate on the metal surfaces and affect the oxidation processes. This paper is concerned with deposit-induced accelerated corrosion, or hot corrosion, of metals and alloys. Initially, the characteristics of hot corrosion are identified for Na2SO4 deposits in terms of the factors that influence the reaction process. It is shown that hot corrosion consists of initiation or incubation and propagation stages. During the initiation or incubation stage, the deposit is shown to not have a significant effect on the corrosion processes, but it is causing conditions to develop whereby the propagation stage characteristics are determined with attendant large increases in the corrosion rates. Type I, high temperature hot corrosion and Type II, low temperature hot corrosion are then described in terms of historical mechanistic perspectives. The dependence of Type I and Type II hot corrosion on temperature and SO3 partial pressure is discussed along with future work that is needed in order to more completely understand these hot corrosion processes along with the effects of some elements such as Cr, Al, Mo, Co and Pt.