The corrosion of pure niobium in oxidizing,sulfidizing, and oxidizing-sulfidizing gas mixtures at 600–800°C

The corrosion of pure niobium has been studied at 600–800°C in various environments as part of a study of the corrosion resistance of its alloys with iron, cobalt, and nickel to atmospheres of low-oxygen and/or high-sulfur activities. The results have shown that not only the sulfidation but also the corrosion in mixed atmospheres and particularly the oxidation under low oxygen pressures of pure niobium are quite slow, with kinetics rather similar in the three types of gas mixtures used. The good corrosion resistance of niobium to attack by oxygen under low pressures is quite interesting because this element is corroded very rapidly by oxygen under high oxygen pressures, due to the formation of the nonprotective highest oxide Nb₂O₅ as a main corrosion product.     

Influence of solid-state CaS-CaO-CaSO4 deposits on corrosion of high-temperature alloys in simulated FBC environments

This study addresses questions concerning the likelihood of sulfidation attack of heat-exchanger alloys beneath deposits of sulfur-sorbent material in fluidized-bed combustors. Alloy specimens were exposed at 900°C in calcium sulfate-calcium oxide and calcium sulfide-calcium oxide mixtures, in environments in which the oxygen partial pressures were fixed at values corresponding to the equilibrium values for each solids mixture, using controlled ratios of CO and CO₂. The only source of sulfur in these systems was the calcium sulfate or sulfide. Sulfidation attack of nickel-base alloys occurred in both mixtures, the calcium sulfide-calcium oxide mixture being the more aggressive. Iron-base alloys were less susceptible to attack, although susceptibility increased with increasing nickel content. FeCrAlY-type alloys were resistant to attack. Comparison with corrosion behavior under conditions in which the oxygen and sulfur partial pressures were the same as those used here, but in which the sulfur source was in the gas phase, indicates that the form of the sulfidation attack is similar but that its progress is much slower under solid deposits.     

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.     

Laboratory corrosion tests for simulating fireside wastage of superheater materials in waste incinerators

Laboratory corrosion tests were performed to clarify the effects of relative amounts of fused salts in tube deposits on corrosion rates of superheater materials in WTE plants. All test exposures were at 550 °C and of 100 h duration. The nine synthetic ashes used as corrodents consisted of mixtures of chlorides, sulfates and oxides. The test materials were alloy steel T22, stainless steels TP347H, TP310HCbN, and alloys HR11N and 625. The gas atmosphere consisted of 500 to 3000 ppm HCl‐30 ppm SO₂‐10% O₂‐10% CO₂‐20% H₂O‐bal.N₂. Generally, the relative amount of fused salts in non‐fused ash constituents at 550 °C increased with increasing the chlorine content of the ashes. The corrosion rate of T22 steel did not depend directly on ash chlorine content, but for ashes of 7.7 wt.% Cl, the corrosion rate depended on the calculated amount of fused salt at 500 °C. The corrosion rates of TP347H steel and alloy 625 were maximum for ashes of 6–8 wt.% Cl. For ashes of 7.7 wt.% Cl, the corrosion rates of T22 steel, stainless steels, and alloys increased with ashes having higher amounts of fused salts. Increased HCl content of the gas caused higher corrosion of the stainless steels and high‐nickel alloys, but there was no clear corrosion‐exacerbating effect with T22 steel.     

Sulfidation behaviour of nickel aluminides

The sulfidation behaviour of four nickel aluminium alloys containing 25 to 45 at.% Al was studied over the temperature range of 750 to 950°C in a gas mixture of H₂-H₂S (0.1 to 10 vol.%). The sulfidation kinetics were determined using a continous weight gain system. The corrosion products were examined by SEM, EDX and XRD. Sulfidation in H₂-H₂S gas mixtures formed bilayered scales consisting of an outer layer of Ni₃S₂ and an inner layer of NiAl_3,5S_5,5 on all alloys regardless of the different aluminium contents. In H₂-H₂S gas mixtures the sulfidation kinetic generally followed the parabolic rate law for all alloys. The influence of aluminium content on corrosion rate was relatively low. The influence of low oxygen partial pressure on sulfidation was investigated in H₂-H₂S-H₂O mixtures. In these atmospheres the corrosion mechanism is completely different. Severe attack by rapid internal oxidation destroyed all the alloys except Ni25Al (25 at.%Al). The internal oxidation zone consisted of a mixture of γ-Ni₃Al and Al₂O₃. On the alloys containing 36 and 45 at.% Al local attack occurred, fast growing pocks were observed after an incubation period. Nickel aluminides show this corrosion phenomena only in H₂-H₂S-H₂O mixtures. An interruption of the H₂S gas flow stops the running internal oxidation. In flowing H₂-H₂O atmospheres no internal oxidation was observed. These facts prove that H₂S is necessary for starting and maintaining the internal oxidation of the nickel aluminides.     

Corrosion Testing of some Austenitic Stainless Steels and of Inconel Alloy at 25°c in Acidic Chloride Solutions

Abstract

The corrosion of austenitic stainless steels types AISI 304, 310 and 316, and of Inconel alloy, was studied at 25°c, in 5% NaCl solution at an initial pH value of 2·5, and in 5% FeCl³ at pH 1·2. The resistance of the alloys in both corrosive environments was in the order: 310 > 316 > 304 > Inconel. Pre-treatment of the specimens with bubbling chlorine gas increased the subsequent corrosion rates of the alloys. Intermittent bubbling of gas mixtures such as Cl₂, N₂, and/or H₂S, increased the corrosion rate of Inconel alloy when Cl₂ was present, but decreased the corrosion rate when H₂ was present. Heat treatment of austenitic stainless steels increased the subsequent corrosion rates, whereas 16% pre-straining of annealed specimens slightly reduced the rates. Addition of trisodium phosphate to the corrosive solution reduced the corrosion rates and pitting tendency for all three types of austenitic stainless steel.     

Mixed oxidant corrosion in nonequilibrium gasifier environments

The major use of high-temperature steel alloys in gasifiers is in heat exchangers for cooling hot syngas, consisting mainly of CO and H₂ with lesser amounts of H₂O and CO₂ and minor quantities of H₂S and HCl. Metal temperatures range from 250 to 600°C, gas temperatures from 250 to 1200°C. Because of rapid cooling the composition of the gas does not change with temperature. Therefore the gas is not in equilibrium at the metal surface. Calculations show that such gases have lower oxygen and sulfur pressures than equilibrated gases at the same temperature. This makes the results of previous laboratory studies less appropriate for predicting mixed oxidant corrosion in gasifiers. For this reason the present study was carried out using nonequilibrium gas mixtures, similar to gases, produced in entrained-slagging gasifiers. Most corrosion experiments were carried out at 540°C, as this is a common temperature for superheaters and hot-gas cleanup systems. Iron-base model alloys containing 35% Ni, 20% Cr, and various minor alloying additions were studied. Three corrosion regimes were identified over the range of conditions studied, depending on the sulfur-to-oxygen pressure ratio of the gas and the alloy composition. At high P_{S 2}/P_{O 2} ratios a somewhat protective FeCr₂S₄ scale formed on all alloys. Below this layer internal oxidation and sulfidation occurred at a slow rate. At lower P_{S 2}/P_{O 2} ratios nonprotective Fe(Ni, Cr)S external scales formed. These allow rapid internal oxidation of the chromium in the alloy, resulting in high corrosion rates. Under the same conditions very low corrosion rates are obtained when silicon is added to the alloy, because the presence of SiO₂ precipitates makes the internal-oxidation layer protective. Thus, the same corrosion model is operative in all three corrosion regimes: external sulfidation of iron and nickel, together with internal oxidation of chromium and other strong-oxide formers.     

Impact of Iron-Sulfide Deposits on Oxidized Austenitic Steels as Simulation of Corrosion and Fireside-Tube Wastage in Coal Combustion

The sulfidation effect of molten iron sulfides was studied on oxidized austenitic steels as a simulation of furnace-wall corrosion in PC combustion environments. The test coupons were oxidized to produce an external oxide scale and pyrite was placed on the oxide and thermally treated in an inert atmosphere to decompose the pyrite into pyrrhotite. DSC-TGA and XRD indicated that FeS interacts with the Fe₂O₃ oxide layer, even at 700°C if the contact is good, changing the oxidation state of iron and the physical structure. On the other hand, the interaction of FeS with Cr₂O₃ between 1100 to 800°C, 24 hr in the inert atmosphere, consisted of the formation of a chromium sulfide layer beneath the oxide scale. SEM-EDX showed that the diffusion of sulfur in the steel matrix can be 30 m deep, indicated by small particles of chromium sulfide. It is demonstrated that iron sulfide deposits could be responsible for sulfidation of the alloys.     

Influence of Cu and Sn content in the corrosion of AISI 304 and 316 stainless steels in H2SO4

This paper addresses the influence of Cu and Sn addition on the corrosion resistance of AISI 304 and 316 stainless steels in 30 wt% H₂SO₄ at 25 and 50 °C. The corrosion process was evaluated by gravimetric tests, DC measurements and electrochemical impedance spectroscopy (EIS). The corrosion products were analysed by SEM, X-ray mapping and XPS before and after accelerated tests. The behaviour of both AISI 304 and 316 stainless steels in sulphuric acid solution was greatly improved by increasing Cu concentration and the synergic effect of Cu and Sn. Addition of Sn increased corrosion resistance, but less than addition of copper.     

Chloride pitting and water chemistry control in cooling or boiler circuits

The effect of different anions (bicarbonate, phosphate, sulphate, hydroxyl) on the pitting corrosion by chloride ions has been investigated for a number of ferritic or austenitic steels and nickel alloys at different temperatures up to 175°C. For most of the alloys the normal order of inhibition is PO₄³ > SO₄^2- HCO₃, but some anomalies have been encountered for stainless steels at lower temperatures (<100°C), where the inhibitive action of SO₄^2- maybe more pronounced that the inhibition which can be obtained by adding equal amounts of phosphate. In the absence of strongly passivating alloying elements, such as chromium, high SO₄² concentrations may have a deteriorative effect. The same result may also be found under cathodic polarization conditions, where SO₄^2- and HCO₃ seem to stimulate reductive dissolution of the protective surface layer and may even cause some kind of pitting-type attack.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

Oxidation Behavior of ODS Fe–Cr Alloys

Four experimental oxide dispersion strengthened (ODS)Fe-(13–14 at. %)Cr ferritic alloys were exposed for up to 10,000 hr at 700–1100 °C in air and in air with 10vol.% water vapor. Their performance has been compared to other commercial ODS and stainless steel alloys. At 700–800°C, the reaction rates in air were very low for all of the ODS Fe–Cr alloys compared to stainless steels. At 900°C, a Y₂O₃ dispersion showed a distinct benefit in improving oxidation resistance compared to an Al₂O₃ dispersion or no addition in the stainless steels. However, for the Fe-13 %Cr alloy, breakaway oxidation occurred after 7,000 hr at 900°C in air. Exposures in 10 % water vapor at 800 and 900°C and in air at 1000 and 1100°C showed increased attack for this class of alloys. Because of the relatively low Cr reservoirs in these alloys, their maximum operating temperature in air will be below 900°C.     

The role of morphology and structure in the kinetic evolution of iron-sulfide films on Fe-base alloys

Sulfidation corrosion of 4130 steel in CH₃SH was studied in the temperature range 250–550°C. The rate of sulfidation attack was found to be a function of temperature and sulfur activity. Investigations of the corrosion process led to the proposal of two mechanisms of sulfidation, dependent on temperature. Cation diffusion through the iron sulfide corrosion product is the rate-determining step at higher temperatures (>370°C), while a surface reaction was identified as the rate-limiting step at lower temperatures. The corrosion scale has preferred orientation as determined by X-ray diffraction and morphological observations. The lower-temperature corrosion product is made up of columnar grains of pyrrhotite crystals with the c-axis aligned nearly perpendicular to the steel substrate. At high temperatures, a whisker morphology developed with the whiskers having variable texture with respect to the steel substrate. A preformed-surface-oxide layer on 4130 steel does not appear to significantly reduce sulfidation corrosion.     

Influence of gas phase composition on the Hot Corrosion of steels and nickel‐based alloys beneath a (Ca‐Na‐K)‐sulfate mixture containing PbSO4 and ZnSO4

The corrosion of steels and of nickel‐based alloys was studied in exposure experiments at 600 °C beneath a molten CaSO₄‐K₂SO₄‐Na₂SO₄‐PbSO₄‐ZnSO₄ sulphate mixture in N₂‐5 vol.% O₂ with and without additions of 1000 vppm HCl, 1000 vppm SO₂ and 1000 vppm HCl in combination with 250 vppm SO₂. In the N₂‐5 vol.% O₂ atmosphere, the corrosion products are iron‐ and nickel‐rich but chromium‐free precipitates of oxides in the solidified melt. Additionally, pits filled with layered corrosion products are formed, growing into the metal substrate. These layers consists of less soluble chromium‐rich oxides, containing varying amounts of zinc, (ZnCr₂O₄) alternating with potassium‐rich sulfates, most probably K₂S₂O₇. The addition of 1000 vppm SO₂ leads to a seperation of the melt in a K₂S₂O₇ part close to the metal surface and a Ca‐rich part on top in contact with the gas atmosphere. Compared to the N₂‐5 vol. % O₂ atmosphere accelerated corrosion was observed. In the K₂S₂O₇ part of the melt dissolved iron and nickel are identified, whereas in the Ca‐rich part iron‐ and nickel‐oxide precipitates are formed. Underneath the solidified salt, thin layers of sulfides are detected. In the N₂‐5 vol.% O₂‐1000 vppm HCl containing gas, the corrosive attack is also accelerated compared to the N₂‐5 vol.% O₂‐atmosphere. Much more oxide precipitates are found in the melt on every sample and the inward growth of the zinc‐free chromium‐rich oxides is significantly enhanced. Underneath the inward growing oxide small amounts of metal‐chlorides are detected. Compared to the SO₂ containing gas, the corrosive attack is enhanced for the iron‐based materials, but retarded for the nickel‐based alloys. In the 1000 vppm HCl‐250 vppm SO₂ containing gas, the corrosive attack is similar to the atmosphere containing only 1000 vppm HCl. In addition, sulfides are formed next to chlorides at the metal/scale interface.     

Oxidation and sulfidation behavior of Fe-20Cr-16Ni-4Al-1Y2O3 oxide-dispersion-strengthened alloy

High-temperature oxidation and sulfidation studies were conducted on an oxide-dispersion-strengthened alloy of composition Fe-20Cr-16Ni-4Al-1Y₂O₃. The oxidation studies were conducted in air and low-PO₂ environments over a temperature range of 650–1200°C. Results are also reported on the sulfidation resistance of preformed oxide scales and the influence of reoxidation of preformed sulfide scales. Detailed microstructural results and x-ray diffraction analysis data are presented to substantiate the corrosion behavior of the alloy over the wide range of conditions anticipated in fossil-energy systems. Data are also presented on the corrosion behavior of the alloy in oxygen/sulfur mixed gas atmospheres, and the results are used to compare the corrosion behavior of the present alloy with other chromia- and alumina-forming alloys.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Besonderheiten im Korrosionsverhalten hochchrom- und molybdänhaltiger Legierungen in heißer 92, 5%iger Schwefelsäure

Peculiarities in the corrosion behaviour of high chromium and molybdenum containing alloys in hot 92.5% sulfuric acid In laboratory tests at temperatures above 50°C unusual high corrosion rates of passivating stainless steels and nickel alloys containing more than 26% Cr were observed in 92.5% sulphuric acid. In order to investigate the cause of this phenomenon further corrosion tests and additional chemical analyses were performed. The H₂SO₄ concentration tested displays a relative maximum of the electrical conductivity, the reason being a stronger dissociation of the sulfuric acid. Electrochemical investigations revealed an enhanced activity of the cathodic reactions which lead to higher corrosion rates. The cathodic reactions are strongly dependend on alloy constitution with special emphasis on the contents of Cr, Ni and Mo. Mo containing stainless steel show potential oscillations (of the open circuit potential) between ?50 and +550 mV_H. These alloys corrode under development of SO₂ (reduction of H₂SO₄ molecules) and formation of several sulfur compounds with different oxidation numbers (6+ and 2?). Alloys with chromium contents above 26% develop additionally hydrogen gas due to a lower hydrogen overvoltage of these alloys. With increasing nickel content the overvoltage for the reduction reaction of H₂SO₄ molecules will be lowered. This fact results in an elevation of the exchange current density for the Alloy NiCr45 and therefore to the highest corrosion rate observed. Alloy B-2 shows the best resistance, i.e. very low corrosion rates. Obviously high levels of molybdenum can compensate the influence of nickel on the overvoltage of the reduction reaction or even hinder the cathodic reaction.     

Comparative corrosion resistance of plasma-based low-energy nitrogen ion implanted austenitic stainless steel

A high nitrogen face-centered-cubic phase (γ_N) was obtained on the nitrided surface of 1Cr18Ni9Ti austenitic stainless steel by plasma-based low-energy nitrogen ion implantation. No pitting corrosion for the γ_N phase was confirmed by electrochemical polarization measurement in 3% NaCl solution. The protective passive film with a duplex character, iron hydroxide/oxides in the outer region and chromium hydroxide/oxides and iron oxides accompanying chromium and iron nitrides in the inner region, was by 2-3 times thicker than that of original stainless steel. The thick iron hydroxide/oxides region formed on the chromium hydroxide/oxides region due to the increase of alkalinity in the solution, leading to barrier against penetration of localized attack of the aggressive ions. The equivalent general corrosion resistance for the γ_N phase was observed in 0.5 mol/l H₂SO₄ solution relative to the original stainless steel. The passive film formed on the γ_N phase in 0.5 mol/l H₂SO₄ solution was similar to that of original stainless steel. The different role of nitrogen was proposed in pitting corrosion resistance and general corrosion resistance of austenitic stainless steel.     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.