Role of platinum in the Na2SO4-Induced hot corrosion resistance of aluminum duffusion coatings

Electrochemical corrosion measurements have been carried out with Pt-containing and Pt-free Al-diffusion coatings on IN 738 LC in a 90Na₂SO₄+ 10K₂SO₄ (mol%) melt at 1173 K. Pt improves the resistance to basic fluxing while there are no significant differences between both coating types in their resistance to acidic fluxing. The corrosion resistance of the Pt-containing coating is also higher in the passive potential region where protective scales rich in Al₂O₃ are formed. The reason for the different behavior of both coating types appears to be related to the high corrosion resistance of the Pt-rich surface layer of the coating and an increased Al₂O₃ content in the scale of the Pt-containing type.     

Effects of SO2 and SO3 on the Na2SO4 induced corrosion of nickel

The effects of SO₂ and SO₃ in the environment on the hot corrosion behavior of Ni in the temperature range 750–950°C has been studied. Below the melting point of Na₂SO₄ (884°C), rapid corrosion takes place by formation of a Na₂SO₄-NiSO₄ melt, which can penetrate the porous oxide scale and give rise to sulfide information by coming in contact with the metal. The distribution of the sulfides depends on the SO₂ level in the ambient gas. Continued corrosion occurs by a sulfidation-oxidation mechanism. At temperatures above the melting point of Na₂SO₄, accelerated degradation occurs via dissolution of the surface scale, followed by reprecipitation of the oxide in a nonprotective form.Deceased     

Na2SO4- and NaCl-Induced hot corrosion of six nickel-base superalloys

The short-time hot-corrosion behavior of six industrial nickel-base superalloys was investigated with static deposits of Na₂SO₄ or NaCl or both in still air. The oxidation kinetics and scale morphologies were measured with traditional laboratory techniques-thermobalance, metallography, electron microprobe, and x-ray analyses. Susceptibility to hot corrosion was found to be correlated to the type of scale produced during simple oxidation. Alloys forming an A1₂O₃ scale were found to be susceptible to Na₂SO₄ deposits, independent of their chromium content. The quantity of Na₂SO₄ deposit dictated the nature of the attack and, under certain conditions, the refractory element alloy additions appeared to play an essential role. Alloys containing Cr₂O₃ or TiO₂ in the simple oxidation scale proved to be sensitive to NaCl attack. Again, the severity of the attack within the susceptible alloy group was not related to the chromium or titanium content. Although less intensive than the Na₂SO₄ -induced hot corrosion, NaCl contaminations provoked extensive spalling. All of the hotcorrosion types encountered in this study were interpreted in the light of existing theories.Supported by the Délégation Générale à la Recherche Scientifique et Technique.     

热浸镀层在青岛站的海水腐蚀行为对比(Ⅰ)——全浸区

开展了热浸镀锌（GI）、锌-5%铝-稀土（GF）和锌-55％铝-1．6％硅（GL）镀层钢板在青岛站的全浸区海水腐蚀测试，并利用质量损失腐蚀测试、显微结构分析和电化学测试，研究了3种镀层钢板的海水腐蚀行为。结果表明：GI镀层的腐蚀电流密度最大，在流动海水中以很快的速度溶解而又难于形成稳定的保护性腐蚀产物膜，耐海水腐蚀性能最差；GF和GL镀层的腐蚀电流密度较小，腐蚀产物具有抑制保护性，表现出较为优异的耐海水腐蚀性能。对位于海水全浸区的钢材基体提供1年保护期所需的镀层最小厚度分别为：GI镀层45pm；GF镀层18pm；GL镀层25pm；3种镀层在海水全浸区的耐蚀性强弱顺序依次为：GL〉GF〉GI；如果考虑到局部腐蚀，那么GF镀层可能会比GL具有更强的综合优势。     

热浸镀层在青岛站的海水腐蚀行为对比(Ⅱ)——潮差区

测试了热浸镀锌（GI）、锌-5%铝-稀土（GF）和锌-55%铝-1.6%硅（GL）镀层钢板在青岛站的潮差区海水腐蚀行为,并利用腐蚀质量损失测试和显微结构分析,首次研究了3种镀层钢板的海水潮差区腐蚀行为。结果表明：3种镀层在潮差区均出现不同程度的生物污损,与全浸区相比其腐蚀速度明显降低;GI镀层溶解速度最快,氧化膜保护效果不佳,耐腐蚀性能最差;GF镀层的耐蚀性明显提高,潮差区的充气条件又促进了镀层的钝化,表现出较为优异的耐腐蚀性能;由于保护性的锌的腐蚀产物被滞留在富铝的枝晶网络中,比较充分的充气条件又促进了镀层富铝相的钝化,GL镀层在海水潮差区表现出最佳的腐蚀性能。对位于海水潮差区的钢材基体提供1 a保护期所需的镀层最小厚度分别为：GI镀层25μm;GF镀层12μm;GL镀层3μm;GF和GL镀层在潮差区的耐蚀性分别是厚度相当的GI的3倍和5倍。     

热浸镀层在青岛站的海水腐蚀行为对比(Ⅲ)——飞溅区

测试热浸镀锌（GI）、锌-5％铝-稀土（GF）和锌，55％铝-1．6％硅（GL）镀层钢板在青岛站的飞溅区海水腐蚀行为，并利用腐蚀质量损失测试和显微结构分析，研究3种镀层钢板的海水飞溅区腐蚀行为。结果表明：3种镀层在飞溅区均未发生生物污损，腐蚀速度在3个海水区带中最低；GI镀层由于腐蚀电流密度最大，氧化膜保护效果不佳，耐海水腐蚀性能最差；GF镀层由于腐蚀电流大幅度降低，飞溅区充分的充气条件促进了镀层的钝化，因此表现出较为优异的耐海水腐蚀性能；由于保护性的锌的腐蚀产物被滞留在富铝的枝晶网络中，充分的充气条件又促进了镀层富铝相的钝化，所以GL镀层在海水飞溅区表现出最佳的腐蚀性能。对位于海水飞溅区的钢材基体提供la保护期所需的镀层最小厚度分别为：GI镀层14μm；GF镀层8μm；GL镀层4μm；GF和GL镀层在飞溅区的耐蚀性分别是厚度相当的GI的2倍和4倍。     

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.     

A method for investigation of hot corrosion by gaseous Na2SO4

A novel in situ exposure method for investigating hot corrosion of aluminde coatings in the absence of an alkali sulphate melt is presented. The samples are exposed to Na₂SO₄(g) at a temperature above the dew point of the gas. Results from 100 and 500 h exposures at 900 °C of Ni-based In792 covered with SIF232 aluminide coating are presented. Already after 100 h of testing, attack characteristic for Type I hot corrosion is observed. The reactions taking place during the testing are discussed and the results are compared with those from the well-known ex situ salt hot corrosion test.     

Influence of laser‐glazing on hot corrosion resistance of yttria‐stabilized zirconia TBC in molten salt mixture of V2O5 and Na2SO4

Plasma‐sprayed 8YSZ (zirconia stabilized with 8 wt% yttria)/NiCoCrAlYTa thermal barrier coatings (TBCs) were laser‐glazed using a continuous‐wave CO₂ laser. Open pores within the coating surface were eliminated and an external densified layer was generated by laser‐glazing. The hot corrosion resistances of the plasma‐sprayed and laser‐glazed coatings were investigated. The two specimens were exposed for the same period of 100 h at 900 °C to a salt mixture of vanadium pentoxide (V₂O₅) and sodium sulfate (Na₂SO₄). Serious crack and spallation occurred in the as‐sprayed coating, while the as‐glazed coating exhibited good hot corrosion behavior and consequently achieved a prolonged lifetime. The results showed that the as‐sprayed 8YSZ coating achieved remarkably improved hot corrosion resistance by laser‐glazing. Changes in the coatings were studied by scanning electron microscopy (SEM) to observe the microstructure and X‐ray diffraction (XRD) technique to analyze the phase composition. XRD results showed that the reaction between yttria (Y₂O₃) and V₂O₅ produced yttrium vanadate (YVO₄), leaching Y₂O₃ from YSZ and causing the progressive destabilization transformation from the tetragonal (t) to monoclinic (m) phase. The external dense layer produced by laser‐glazing restrained the penetration of the molten salt, to a certain extent, into the coating, which led to a relatively low m‐ZrO₂ content in the coating after the hot corrosion test. Additionally, the segmented cracks in the coating surface induced by laser‐glazing were helpful to the improvement of strain tolerance of the coating. The two factors were important contributions to the significant enhancement of hot corrosion resistance of the as‐glazed YSZ coating.     

超音速喷涂45CT涂层在KCl+K2SO4+Na2SO4熔融盐中的热腐蚀行为

目的研究超音速喷涂45CT涂层抗KCl+K_2SO_4+Na_2SO_4熔融盐腐蚀性能,了解45CT涂层在熔融盐中的防护及失效机制。方法将试样置于KCl+K_2SO_4+Na_2SO_4混合熔盐中,获得样品在熔盐中不同温度下的腐蚀动力学规律。采用XRD、SEM、EDS对高温腐蚀产物成分、结构、形貌进行分析。结果 45CT涂层在熔融盐中的主要腐蚀产物为Cr_2O_3、NaCrO_2。在600℃和650℃下,涂层能较好地保护基体,涂层上腐蚀产物层较薄;而在700℃下,涂层退化严重,涂层内有大量金属硫化物,基体中的Fe向涂层扩散。结论在高温条件下,涂层易发生硫化,导致涂层与氧化膜的弱结合。随着测试温度升高,涂层在KCl+K_2SO_4+Na_2SO_4熔融盐中的热腐蚀越严重,涂层失效加快。     

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.     

Q235上电弧喷涂Zn55Al涂层在3.5wt%溶液中的腐蚀行为研究

本文使用电弧喷涂通过包套挤压+拉拔的方法制备的Zn55Al伪合金丝材成功的在Q235钢上喷涂出了Zn55Al涂层。通过扫描电镜和微区XRD研究了Zn55Al 伪合金丝材的显微结构。通过浸泡腐蚀实验和电化学方法研究了Zn55Al涂层、Zn15Al涂层和 Al涂层的腐蚀行为,并对比了三种涂层之间的差异。结果表明Zn55Al伪合金丝材由纯锌和纯铝组成,在整个成型过程中没有产生合金化。Zn55Al涂层由层片状的富锌相和富铝相组成。经过20天的浸泡实验,Zn55Al涂层形成了一层致密的钝化膜,比其他两种涂层有更好的耐腐蚀性。Zn55Al涂层的自腐蚀电位大约是-1.25v,高于Zn15Al涂层低于纯Al涂层和Q235基体.电偶腐蚀实验表明,Zn55Al涂层比Zn15Al涂层具有更好的点虎穴保护作用。这些结果说明Zn55Al涂层具有更好的耐腐蚀性和可以给Q235基体提供更强的电化学保护.本文也讨论了Zn55Al涂层的的腐蚀机理。     

温度对高速电弧喷涂FeNiBSi/Cr_3C_2复合涂层热腐蚀性能的影响

采用高速电弧喷涂技术在45钢基体上制备出Fe Ni BSi/Cr3C2复合涂层.采用光学显微镜和扫描电镜分析涂层的显微结构,以及X射线衍射分析涂层相组成.试验选用摩尔比为7∶3的Na2SO4+K2SO4饱和水溶液为腐蚀介质涂刷于试样表面,并以20G钢作对比材料,在550~750℃下研究了涂层的抗热腐蚀性能.结果表明,涂层具有典型的层状结构特征,致密且连续.涂层的抗热腐蚀性能明显优于20G钢.在550和650℃下涂层表面形成了致密的氧化膜(Fe2O3,Cr2O3和Fe Cr2O4)阻碍了熔盐的侵入,提高了涂层抗热腐蚀性能.在750℃下涂层氧化膜变得不完整,不能有效地阻隔熔盐,此时涂层的抗热腐蚀性能较差.     

Corrosion behaviour and mechanism of 6082 aluminium alloy in NaCl and Na2SO4 etchants

The corrosion behaviour of 6082 aluminium alloy was studied by measuring the electrochemical impedance spectra and electrode polarization curves. After the electrochemical tests, a microstructural analysis of the samples was conducted by using optical microscopy and electron scanning microscopy techniques to determine the corrosion mechanism. The results show that the Nyquist plot of the electrochemical impedance data in the NaCl solution consists of high- and low-frequency capacitive impedance loops. When ions are added to the NaCl etchant, the Nyquist plots of the electrochemical impedance data are composed of two different curves: a high-frequency capacitive impedance loop and a low-frequency inductive impedance loop. The corrosion current density increases with increasing concentration, and as a result, the corrosion resistance of the aluminium alloy decreases. The microstructures of 6082 aluminium alloy consist of Mg₂Si secondary particles in a large α-Al matrix. Pitting corrosion initially occurs at the boundary between the matrix and secondary particles because the electrode potentials of the matrix and secondary particles are different. Then, corrosion paths develop along the network-like grain boundaries, and finally, massive network-like corrosion occurs throughout the entire alloy.     

Slurry erosion–corrosion behaviour of high-velocity oxy-fuel (HVOF) sprayed Fe-based amorphous metallic coatings for marine pump in sand-containing NaCl solutions

Corrosion and slurry erosion–corrosion (E–C) behaviour of FeCrMoMnWBCSi amorphous metallic coatings (AMCs) compared with 304 stainless steel was investigated by static electrochemical measurements and weight loss tests under rotating conditions in simulated seawater. E–C rates of the AMCs increased with flow velocity, particle size, sand content and NaCl concentration. The AMCs are preferentially attacked at coating defects. The superior E–C resistance of AMCs can be attributed to the high micro-hardness, alloying elements and amorphous microstructure. The AMCs with better slurry E–C resistance may be good candidates to solve the E–C problems of marine pumps in sand-containing seawater.     

An investigation on hot corrosion behavior of YSZ-Ta2O5 in Na2SO4 + V2O5 salt at 1100 °C

This study compares the hot corrosion performance of yttria stabilized zirconia (YSZ), and YSZ-Ta₂O₅ (TaYSZ) composite samples in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1100 °C. For YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For TaYSZ, minor amounts of NaTaO₃, TaVO₅ and Ta₉VO₂₅ are formed as the hot corrosion products with only traceable amounts of YVO₄. Due to the synergic effect of doping of zirconia with both Y₂O₃ and Ta₂O₅, the TaYSZ sample has a much better hot corrosion resistance than YSZ.     

Initial atmospheric corrosion of zinc in presence of Na_2SO_4 and (NH_4)_2SO_4

1 Introduction In view of the widespread use of zinc, as metallic sheet or zinc coatings, it was desirable to study its corrosion behaviour in the wide variety of atmospheres. The atmospheric corrosion of zinc has been studied in field exposures as well …     

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 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.