High Temperature Sulphidation Research Articles

Failure Analysis of Inconel 601 Radiant Tubes in Continuous Annealing Furnace of Hot Dip Galvanizing Line

Metallurgical investigation was conducted to establish the genesis of crater-like defects and perforations in Inconel 601 radiant tubes used in Radiant Tube Heating/ Radiant Tube Soaking (RTH/RTS) section of the Continuous Annealing Furnace in Hot Dip Galvanizing Line (HDGL) of an integrated steel plant. The radiant tubes, several of them, had undergone catastrophic damage after only about 70 days of service under mixed gas firing inside the furnace. Visual examination, chemical analysis and secondary electron imaging (SEI) coupled with energy-dispersive x-ray spectrometry (EDS) were concomitantly utilized for the investigation of failed radiant tube samples. Based on the findings, the unprecedented failures were attributed to the high temperature sulphidation attack experienced by the tubes on their inner walls, owing to sulphur dioxide (SO2) resulting from combustion of hydrogen sulphide (H2S) in the mixed gas (2H2S + 3O2 → 2H2O + 2SO2). The investigation also pointed toward the apparent formation of low-melting nickel-nickel sulphide (Ni-Ni3S2) eutectic because of sulphidation, which led to unique spherical manifestations of the defects and served to highlight the deleterious influence of sulphur-bearing compounds on nickel-base alloys at high temperatures. Suitable alternative materials of construction are proposed for the radiant tubes operating under such environments.

Identifying active sulphur species responsible for high temperature sulphidation in condensate

Identifying active sulphur species responsible for high temperature sulphidation in condensate

Analysis on high-temperature corrosion characteristic of 0Cr18Ni9NbCu3BN steel

The high-temperature corrosion characteristic of 0Cr18Ni9NbCu3BN steel was analysed in this paper. The samples were selected from a super-heater with service time of 34 696 h in an ultra-supercritical boiler in China. Scanning electron microscopy, energy spectrum analysis and energy dispersive X-ray spectroscopy were adopted to analyse the element constitute, morphology and phase structure of the corrosion scale. The results indicated that corrosion scale can be divided into three layers, and the middle layer of corrosion scale is enriched with Cr. The Cr-rich layer could not set an efficient barrier to prevent sulphur from penetrating to the matrix. It actually can prevent O from penetrating to metal base effectively. The main corrosion forms are high-temperature oxidation and sulphidation, while the sulphidation rate was greater than the oxidation rate. The sulphidation is the major type of corrosion attack. Oxidation and sulphidation interactively cause the corrosion progress consistently.

High temperature sulphidation of HVOF sprayed CeO2 modified NiCrAlY coatings in H2/H2S atmospheres

High temperature sulphidation is one of the most deleterious forms of surface degradation. In the present work, the high temperature sulphidation behaviours of high velocity oxyfuel sprayed NiCrAlY and NiCrAlYCe coatings on high pressure steels in H2–H2S mixtures were investigated. Results showed that the NiCrAlYCe coating provided much better sulphidation resistance than the NiCrAlY coating in the given environment. After adding minor CeO2, the outer layer of scales became compact and showed good adhesion, which decreased the sulphur vapour pressure inside the scale and promoted the formation of more protective inner layer of sulphospinels (Cr, Ni)Al2S2.

High Temperature Sulphidation Behavior of Fe20Cr6Al-Y,Hf Steel

The high temperature corrosion behavior of Fe20Cr6Al-Y,Hf steel was studied in the range of temperature 800-1000oC in H2/H2S atmospheres at pS2=10-2 ,10-3 and 10-4 Pa sulphur vapor pressures. Kinetics depend on the temperature and sulphur vapor pressure. After 24 hours the whole specimen was practically consumed because the samples were 0,06 cm thick. Morphology of the scales have been performed by SEM techniques. Phase and chemical composition have been studied by EDX and XRD techniques. It was found that scale formed on Fe20Cr6Al-Y,Hf alloy was built with porosity sulphides layer. EDX analysis of the scale surface show that the any aluminum, hafnium and yttrium sulfides were found in the formed scale layer, however small amounts of Al2S3 was detected in scale/steel interface. Also internal sulphidation was observed. A phase analysis of the formed scale revealed that it is composed mainly of an FeS, Fe7S8 phases and CrS, Cr5S6. Result were compared with data obtained on the pure Fe and Cr samples.

Comparison of corrosion modes of oxidation and sulphidation of weld metal with rare earth oxides

Through addition of rare earth oxides into the coating of electrodes, the high temperature oxidation and sulphidation resistance of weld metal were studied. The weight loss test results showed that the addition of rare earth oxides could improve the high temperature resistance of weld metal, especially in sulphidation environment. The reasons for the difference of corrosion rate between oxidation and suphidation were studied with the application of SEM. It was believed that different corrosion modes were the main reasons for that: oxidation rate was mainly decided by the Cr content of the weld metal and sulphidation rate was mainly decided by the inner corrosion occurring in the grain boundary. The marked effect of rare earth oxides in sulphur environment was related to the corrosion mechanism.

High Temperature Sulphidation of Cast Austenitic Alloy Cr25-Ni32-Nb with Microadditives Ti, Zr and Ce

High Temperature Sulphidation of Cast Austenitic Alloy Cr25-Ni32-Nb with Microadditives Ti, Zr and Ce

Effect of alloying additions and preoxidation on high temperature sulphidation resistance of iron–chromium alloys

High temperature corrosion of structural alloys in sulphur bearing environments is many orders of magnitude higher than in oxidising environments. Efforts to increase sulphidation resistance of these alloys include addition of alloying elements. This paper reports the effect of yttrium, aluminium and yttrium plus aluminium addition on the sulphidation behaviour of an Fe–20Cr alloy in H2–2%H2S at 700 and 800°C. The effect of preoxidation of these alloys on sulphidation behaviour was also investigated. The initial stages of sulphidation of these alloys were observed and overall sulphidation kinetics was determined. In the presence of yttrium, the sulphidation rate of the alloy decreased by an order of magnitude, and the sulphide layer on the FeCrY alloy was significantly thinner than that formed on the FeCr alloy. Aluminium addition increased sulphidation resistance even more. The FeCrAl and FeCrAlY alloys exhibited parabolic sulphidation kinetics at the two temperatures. On both alloys, initial chromium sulphide (Cr2S3) formation at grain boundaries and Al2S3 and FeCr2S4 formation over the grains was followed by formation of an outer Cr2S3 layer over a mixed sulphide layer. Preoxidation of the two alloys resulted in an ‘incubation period’ with practically no sulphidation during subsequent exposure to S bearing environments. After this incubation period, the sulphidation behaviour of the FeCr alloy was similar to that when the alloy was not preoxidised. The incubation period of the FeCrAl alloy was significantly longer, over 45 h, compared with 2 h for the Al free alloy. Based on thermogravimetric as well as SEM, energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) data, sulphidation mechanisms have been proposed for the alloys before and after preoxidation.

High Temperature Sulphidation of Composite Coatings on Cast Iron

A cast iron retort used for producing carbon disulphide in the Charcoal-Sulphur process usually has a lifespan of less than one year due to high-temperature corrosion. In order to improve the lifespan, two thermal-sprayed coating systems were investigated. One was an inner coating of a Fe-Cr-Al alloy surmounted by a top coating of aluminum, and the other was a single aluminum coating. After 576 hours exposure under the operating conditions, the specimens with arc-sprayed composite coatings showed a good resistance to sulphur attack. On the other hand, the results of the specimens with a single aluminum coating were found not to be satisfactory. The coatings and the coating/substrate interface were characterized by SEM, EPMA and X-ray diffraction. The results showed the existence of a reliable bonding between the composite coatings and the substrate with formation of the intermetallic compound AlCrFe2 which was beneficial against the sulphur attack.

Effect of tin on high-temperature sulphidation of Fe–Cr alloys in H 2/H 2S atmospheres

The high temperature sulphidation behaviour of Fe–46Cr– xSn ( x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H 2/H 2S mixtures with different sulphur vapour partial pressures of 10 −1, 10 −3 and 10 −5 Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe–46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10 −5 Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe x Sn y inclusions with tin concentrations of up to 12 at.%.

Failure analysis of ruptured pipe connected to natural gas pre-heater

Failure Analysis was made on a ruptured low carbon steel pipe that was connected to a natural gas pre-heater in a chemical plant. The pipe has been in service for about 30 years. The analysis indicated that failure of the pipe was due to thin-lip rupture, resulting from high temperature sulphidation. The rate of sulphidation was much faster at the bend areas where turbulent flow was occurring. The pipe also suffered from graphitization due to decomposition of the pearlite phase as a result of long service time at moderately high temperature. It was recommended to replace the pipe with a new one and to regularly monitor the pipe thickness with special emphasis on bend areas. The plant was also advised to minimize the ingress of humidity and oxygen into the piping system during shutdowns.

A study of high temperature sulphidation products of pure and Sn-doped Fe–Cr alloys

Sulphidation products, i.e. scales and metallic cores resulting from high-temperature sulphidation carried out in an atmosphere of H 2/H 2S mixture on pure and Sn-doped quasi-equiatomic Fe–Cr alloys in α and σ phases were investigated with scanning electron microscopy, X-ray diffraction and Mössbauer spectroscopy. The scales were found to have a two-layer structure; the outer and the inner scale. Iron was found to exist in various forms within the scales; magnetic and non-magnetic (Fe,Cr) 1− x S (the latter obviously due to fine-grains), α-Fe (mostly within the inner scales) and (Fe,Cr)S 1+ x (only in case of Sn-doped σ-phase alloys). Metallic cores were found to have Fe- and Sn-enriched outer mantel whose thickness was determined to be ∼45 μm and grain-like inner core with Cr-enrichment in form of CrS within the grain-borders.

An examination of the cause of extensive corrosion of the shell of a 3CR12 conditioning tower in a cement plant

The failure of a conditioning tower at a cement plant was investigated. It was found that the structure failed due to a combination of high temperature oxidation and sulphidation. This was aggravated by the presence of sodium and potassium salts in the exit gases. It was concluded that a stainless steel with a higher chromium content than the 3CR12 would be more suitable for this application.

Investigation of scales resulted from a high-temperature sulphidation of Fe–Cr alloys

Scales resulted from a sulphidation of a series of Fe100−xCrx alloys (x≤45) at T=1073 K in an atmosphere of H2S/H2 with the partial pressure of sulphur pS2=10−3 Pa were investigated by means of Mössbauer spectroscopy, X-ray diffraction, electron- and light-microscopy as well as electron-probe microanalysis (EPMA). The results obtained gave evidence that morphologically all the scales, excepting the one grown on the most concentrated alloy, were macroscopically homogenous (single-layer). The exceptional scale had a two-layer structure. Crystallographically, all the scales were single-phase and had a structure of the FeS except the one grown on Fe90Cr10 alloy which was, in addition, composed of Fe7S8. The structure of the duplex scale was not determined. Mössbauer spectra recorded both in a transmission geometry as well as in a CEMS mode gave clear evidence that the single-layer scales were microscopically not uniform. The spectra characteristic of the scales grown on the alloys with x≤10 consisted of two subspectra: one had spectral parameters similar to those of F1+δS, while the other subspectrum had a broad distribution of the hyperfine field. Based on the X-ray diffractograms and EPMA-profiles the latter could have been identified as originated from the (Fe,Cr)1−δ. The relative amount of (Fe,Cr)1−δS was found to increase with x, and for x≥15 the entire scale was composed of it. A relationship between the kinetics of sulphidation, which was determined from the weight gain of test pieces, the morphology and the structure of the scales formed is discussed, too.

Use of PVD deposited TiN coating in retarding high temperature sulphidation

TiN coated Inconel 600 and Nimonic PE11 alloys were exposed to an atmosphere comprising a high sulphur potential (pS 2 ∼ 10 −1 Pa) and a low oxygen potential (pO 2 ∼ 10 −18 Pa) at 750 °C for periods up to 72 h. The sulphidation kinetics, determined by a discontinuous gravimetric method, demonstrated that the TiN coating greatly enhanced the sulphidation resistance of the substrates, particularly during the early stages of exposure. For TiN coated Inconel 600 the post-exposure analysis by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and glancing angle X-ray diffraction, showed the formation of an outer layer containing Ni 3S 2 on the surface of the TiN coating while an inner layer consisting of Cr 2S 3 developed at the coating/substrate interface. The double layered scale formed on uncoated Inconel 600 consisted of Ni 3S 2 and Cr 2S 3. For the TiN coated Nimonic PE11, sulphide nodules consisting of three layers — (Fe,Ni) 9S 8 (outmost) Cr 2S 3 MoS 2 (innermost) were observed to develop and the TiN coating was sandwiched between (Fe,Ni) 9S 8 and Cr 2S 3. The portion of the TiN coating which was enveloped by those sulphide nodules became unstable after long-term exposure and subsequently dissociated thereby causing the loss of environmental protection. Moreover, the coating was damaged mechanically by the growth of the sulphide nodules. The scale formed on the uncoated Nimonic PE11 showed the formation of a similar structure in a similar sequence.

High Temperature Oxidation and Sulphidation and Its Prevention by Ceramic Coatings

High Temperature Oxidation and Sulphidation and Its Prevention by Ceramic Coatings

Evaluation of high temperature sulphidation protection provided by amorphous Si/SiO2 coatings produced by ArF excimer laser chemical vapour deposition

Evaluation of high temperature sulphidation protection provided by amorphous Si/SiO2 coatings produced by ArF excimer laser chemical vapour deposition

Improved SiO2-coatings against high temperature sulphidation by internal stress reduction

Alloys such as AIS1 304 and AIS1 321 stainless steels and Incoloy 800H can be protected against high temperature corrosion by means of amorphous SiO2-coatings deposited by metal organic chemical vapor deposition (MOCVD). The coated alloys are only attacked locally after exposure to a 19% H2, 1% H2S, 1.5% H2O, Ar bal. environment at 823 K (pS2 = 1.2 x 10-9bar and pO2 = 9.3 x 10-29 bar). The crack density observed after sulphidation is identical for silica deposited onto all metallic substrates. Decohesion of amorphous silica coatings from AISI 304 is observed, when the coating thickness exceeds 1.7 µm, while coatings on Incoloy 800H are adherent up to 4 µm. These phenomena can be explained by the fact that the adhesion is a function of the chemical interactions between the metal and the coating, whereas crack formation is only a function of the internal stresses in the coating. Two methods are studied to reduce the coating failure by means of a reduction of internal stresses in the coating, the first is changing the deposition process and the second is reducing the thermal mismatch between the coating and the metal.

A new thermobalance for studies of the high-temperature sulphidation of metals and alloys

A new thermogravimetric apparatus for studying the kinetics of metal sulphidation in a H2/H2S gas mixture is described. The main difference between this device and other equipment is the application of hydrogen to obtain a H2/H2S mixture at suitable sulphur partial pressures at a total mixture pressure of 1 atm.

The sulphidation properties of titanium-, manganese- and niobium-bearing Fe25Cr alloys in H 2H 2S mixtures at 800°C

An investigation of high-temperature sulphidation properties of 4 wt% Ti-, 9 wt% Mn- and 8 wt% Nb-bearing Fe25Cr alloys has been carried out in H 2-H 2S mixtures of sulphur partial pressure in the range of 10 −3 < pS 2 < I Pa at 800°C. On the whole, the sulphidation kinetics of all alloys obeyed the parabolic law after the initial period of reaction. In some cases, fluctuations in the weight gain-time curves arose due to cracking of sulphide scales. Compared with the weight gain of Fe25Cr alloy, the additions of alloying elements improved the sulphidation resistance. The effect of 8 wt% Nb was relatively major, but the effects of 4 wt% Ti and 9 wt% Mn were minimal. The addition of these elements did not change the surface morphologies or improve the structure of sulphide scales. Combining the sulphidation kinetics and the analysis of sulphide scale structure, the sulphidation mechanisms of these Fe25Cr-base alloys have been proposed and the effects of titanium, manganese and niobium have been discussed.