腐蚀产物MgO对Fe基合金在固态和熔融NaCl-MgCl2 中高温耐蚀性能的影响

熔融氯化盐是下一代聚光式太阳能热发电站（第3代CSP）候选传热和储热介质,含MgCl₂的熔融氯化盐对金属传热管道和储热容器腐蚀后在其表面形成MgO,MgO对管道耐腐蚀性能影响尚不清楚。通过对比碳钢和3种Fe-Cr-Ni合金在固态（345 ℃）和熔融NaCl-MgCl₂（445和545 ℃）中的腐蚀行为,分析了MgO对4种试样在不同温度下的腐蚀行为机理。结果表明,在固态NaCl-MgCl₂中,碳钢表面MgO壳致密且连续,可以保护试样免受腐蚀。在熔融NaCl-MgCl₂中,4种试样表面也生成了致密的MgO壳,但它因热应力作用而开裂和剥落,熔融盐沿着氧化膜裂纹渗入MgO/基体界面,发生化学-电化学联合腐蚀反应,不能保护试样免受该熔盐腐蚀。     

Fe-14Cr-Mn合金在不同温度熔融共晶NaCl-MgCl_2中腐蚀行为及机理研究（英文）

熔融共晶NaCl-MgCl_2作为太阳能中高温相变储热介质,运行状态下其温度常在熔点附近波动。但不同温度下该熔融盐对金属容器材料腐蚀行为不清楚。以Fe-14Cr-Mn合金为例,采用浸盐法研究了718、768和818 K共晶熔融NaCl-MgCl_2对该合金的腐蚀行为,探讨了腐蚀机理。结果表明:腐蚀速率随温度增加略微增加,3种温度下试样腐蚀动力学曲线服从线性规律(斜率k≈–4.806×10~(-4))。腐蚀初期,试样表面形成泡状腐蚀产物,腐蚀80 h后形成腐蚀坑洞,主要是Fe、Fe-Cr和MgO,也检测到含微量Fe和Ni的Mg-Fe-Ni的氧化物。腐蚀机理主要是MgCl_2吸潮后以H2O形式引入的氧原子和溶解在熔融盐中的微量氧作为阴极去极化剂,合金元素Cr和Mn与Cl-反应生成氯化物,氯化物吸附水分子,形成具有低熔点的含水氯化物(如CrCl3·6(H_2O)和MnCl_2·n(H_2O))逃逸腐蚀体系。另外,在熔盐表面形成了由MgCl_2·(H2O)6,NaCl和MgO组成的盐壳,而在熔融盐内部,NaCl与NaMgCl_3共存。本研究为研发耐熔融NaCl-MgCl_2腐蚀的新合金奠定了基础。     

原位自生MgO对氯化物熔盐腐蚀性的影响

熔融氯化物盐是优异的传热流体和热能储存介质,在太阳能发电领域具有广泛的应用价值。但在实际工作环境下,熔融氯化物盐对金属管道（Inconel 625合金）有较强的腐蚀性,Inconel 625焊丝通常作为修补材料在太阳能管道修补上使用。为了解决熔融氯化物盐对Inconel 625熔敷金属的强腐蚀性问题,采用MAG焊堆焊出Inconel 625熔敷金属,探究在KCl-MgCl₂熔盐中加入纳米级MgO颗粒与MgCl₂·6H₂O原位自生MgO对KCl-MgCl₂盐腐蚀性的影响。结果表明：Inconel 625熔敷金属在KCl-MgCl₂、KCl-MgCl₂+5% MgO和KCl-MgCl₂+5% MgCl₂·6H₂O（质量分数）中经过72 h的腐蚀后,试样的质量损失分别为0.00714、0.00512和0.00308 g·cm^-2。Inconel 625熔敷金属在原位自生MgO熔盐中的腐蚀速率同比降低56.86%和39.85%。熔盐中加入纳米MgO颗粒虽然能够缓解氯化物熔盐的腐蚀性,但熔盐发生团聚或沉降现象,导致MgO和MgCr₂O₄保护壳层分布不均匀;在熔盐中添加MgCl₂·6H₂O使其原位生成MgO,生成的MgO和MgCr₂O₄保护壳层更加均匀,能够阻碍腐蚀性介质侵蚀,是降低氯化物熔盐腐蚀性的有效方法。     

低温镁热还原TiO2制备金属钛粉的热力学研究

本文针对TiO₂镁热还原产品氧含量高的问题进行了热力学研究,计算了反应体系的吉布斯自由能和绝热温度,绘制了反应优势区图,结果表明当反应温度高于1681 K时TiO无法被镁还原。针对这一问题,提出添加稀释剂的思路以实现对还原过程温度的抑制,计算了不同NaCl、MgCl₂添加量和初始温度对反应绝热温度的影响,指出了钛氧化物充分还原的热力学条件。最后分别以NaCl和NaCl-MgCl₂共熔盐作为稀释剂进行了实验研究,实现了钛氧化物的充分还原。     

STUDY ON HOT CORROSION RESISTANCE OF A NEW DIRECTIONAL SOLIDIFICATION Ni–BASED SUPERALLOY

研究了新型定向凝固镍基高温合金DZ68的抗热腐蚀性能, 并与K438合金进行了比较. 结果表明: 热处理态DZ68合金组织中几乎没有(γ+γ') 共晶, 碳化物尺寸小,其整体组织比较均匀; 在热腐蚀过程中发生比较均匀的腐蚀, 其外腐蚀层的腐蚀产物主要是(Ni, Co)Cr₂O₄, 内腐蚀层的腐蚀产物主要是Al₂O₃. 热处理态K438合金组织中存在较多的 (γ+γ'共晶和数量较多、尺寸较大的长条状碳化物, 组织均 匀性较差; 在热腐蚀过程中发生不均匀腐蚀, 其外腐蚀层的腐蚀产物主要是NiO, 内腐蚀层的腐蚀产物主要是CrS.两种合金中Ti元素的偏析有促进其它元素偏析的倾向, 使合金组织的均匀性恶化,热腐蚀均匀性变差. 在本实验条件下, DZ68合金的抗热腐蚀性能略好于K438合金.     

Corrosion Behavior of Cr, Fe and Ni Based Superalloy in Molten NaCl

熔融NaCl作为相变储热介质对容器材料具有强烈腐蚀性。通过对Cr基、Ni基和Fe基合金在850℃熔融NaCl中192 h的腐蚀实验,绘制了其腐蚀动力学曲线;利用SEM、EPMA、EDS、XRD等方法分析了腐蚀后试样横截面特征、元素分布特点及试样表面的腐蚀产物,讨论了腐蚀行为。结果表明:3种合金均满足线性腐蚀规律;Cr元素与熔融NaCl等物质反应,是导致这3种合金质量损失率增大的主要原因。Cr基合金耐蚀性最差,明显表现出晶界腐蚀特征,并且晶界为熔融NaCl渗入合金基体提供通道,促进了反应进程。Ni基合金和Fe基合金试样表面均形成了较厚的腐蚀层,为Cr2O3提供附着基体,降低了Cr元素扩散驱动力,提高了合金耐蚀性。另外,Fe基合金表面形成了尖晶石结构的腐蚀产物,进一步抑制了腐蚀过程,其耐蚀性最好。     

铁铬镍基高温合金耐熔融NaCl腐蚀性研究(英文)

熔融NaCl作为相变储热介质对容器材料具有强烈腐蚀性。通过对Cr基、Ni基和Fe基合金在850℃熔融NaCl中192 h的腐蚀实验,绘制了其腐蚀动力学曲线;利用SEM、EPMA、EDS、XRD等方法分析了腐蚀后试样横截面特征、元素分布特点及试样表面的腐蚀产物,讨论了腐蚀行为。结果表明:3种合金均满足线性腐蚀规律;Cr元素与熔融NaCl等物质反应,是导致这3种合金质量损失率增大的主要原因。Cr基合金耐蚀性最差,明显表现出晶界腐蚀特征,并且晶界为熔融NaCl渗入合金基体提供通道,促进了反应进程。Ni基合金和Fe基合金试样表面均形成了较厚的腐蚀层,为Cr2O3提供附着基体,降低了Cr元素扩散驱动力,提高了合金耐蚀性。另外,Fe基合金表面形成了尖晶石结构的腐蚀产物,进一步抑制了腐蚀过程,其耐蚀性最好。     

AZ91D合金表面铈转化膜成膜工艺及其耐腐蚀性的研究

  通过正交试验确定了AZ91D 镁合金在CeCl₃H₂O₂体系中的最佳成膜条件.利用扫描电镜对转化膜进行形貌观察和成分分析,利用浸泡集气和电化学方法了膜层的耐腐蚀性能.结果表明,在最佳成膜条件下,形成的膜层均匀、致密.在3.5% NaCl溶液和0.15 mol/L H₃BO₃/0.05 mol/L Na₂B₄O₇缓冲溶液中的动电位极化测试表明,最佳条件下铈盐转化处理可使合金的腐蚀电位正移,腐蚀电流密度明显降低,起到一定的防护作用.     

介质对GH3625合金高温腐蚀中氧化膜的影响机制

本文采用XRD、SEM、EDS、增重法测量和热力学计算等手段研究了铸态GH3625合金在900 ℃下不同腐蚀介质中(Air、75 wt.% Na₂SO₄+25 wt.% NaCl和2% SO₂+H₂O+Air)腐蚀120 h后的高温腐蚀行为,探讨了不同腐蚀介质下氧化膜的形成及破坏机制。研究表明,GH3625合金在不同介质中腐蚀后,从合金最表层到合金内部结构均分为疏松氧化层、致密氧化层、贫Cr区和合金基体,其氧化层主要由NiO、Cr₂O₃和NiCr₂O₄组成。同时,合金在熔盐和高温酸性气氛中腐蚀后的贫Cr区深度(约14 μm和11μm)均大于在空气中氧化后贫Cr区深度(约8 μm),腐蚀介质对合金表层氧化膜的破坏程度从大到小依次为熔盐介质、高温酸性气氛介质、空气介质。合金的腐蚀主要是不同介质下合金表层氧化膜的形成与溶解的相互竞争,当GH3625合金在熔盐介质中腐蚀时,腐蚀机制主要是氧化膜与Cl^_-反应生成气相Cl₂和与Na₂O反应生成铬酸盐Na₂CrO₄；而当GH3625合金在酸性气氛介质中腐蚀时,腐蚀机制主要是Cr、Ni元素与SO₂、O₂的连续硫化和氧化反应。     

Fe-RE(Y、La和Ce)合金中相稳定性和固溶度的第一性原理研究

合金元素的固溶度对于新型合金的设计和合金动力学过程的研究具有重要的作用。本文利用密度泛函理论,计算了Fe-RE二元化合物的基态,确定了Fe-Y和Fe-Ce体系的稳态和亚稳态结构。计算结果表明,Fe-Y的稳定结构为Fe₁₂Y.tI26、Fe₁₇Y₂.hP38和Fe₂Y.cF24,Fe-Ce的稳定结构为Fe₁₇Ce₂.hP38、Fe₁₉Ce₅.hR24和Fe₂Ce.cF24。基于稀格子气统计热力学理论,利用第一性原理计算得到了稀土元素在α-Fe中随温度变化的固溶度曲线。结果显示,稀土Y、La和Ce元素在α-Fe中的固溶度为S_La>S_Ce>S_Y,这一趋势对应于三者在α-Fe中的溶解形成焓关系H_sol(La)< H_sol(Ce)< H_sol(Y).     

The influence of CO2, AlCl3 · 6H2O, MgCl2 · 6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na₂SO₄, AlCl₃ · 6H₂O and MgCl₂ · 6H₂O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO₂ was 350 ppm or <1 ppm and the exposure time was four weeks. Under the experimental conditions all four salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corroded surfaces were studied by ESEM, OM, AES and FEG/SEM equipped with EDX. The corrosion products were analyzed by gravimetry, IC and grazing incidence XRD. In the absence of CO₂, the corrosivity of the chloride salts studied increases in the order MgCl₂ · 6H₂O < AlCl₃ · 6H₂O < NaCl. Sodium chloride is very corrosive in this environment because the sodium ion supports the development of high pH in the cathodic areas, resulting in alkaline dissolution of the alumina passive film and rapid general corrosion. The low corrosivity of MgCl₂ · 6H₂O is explained by the inability of Mg²⁺ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl₃ · 6H₂O and especially, NaCl, while it is slightly corrosive in the presence of MgCl₂ · 6H₂O. The corrosion effects of CO₂ are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO₂, Na₂SO₄ is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO₂ is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.     

Effects of Chlorate on the Hot Corrosion Behaviors of Cr0.82Al0.18N Coating Deposited on a Ti3Al Based Alloy

为了研究氯化物熔盐对Cr-Al-N涂层热腐蚀性能的影响,采用反应磁控溅射的方法在Ti3Al合金表面制备了Cr0.82Al0.18N涂层,研究了涂层在Na2SO4+25%K2SO4和Na2SO4+25%Na Cl(质量分数)2种熔盐中的热腐蚀行为,采用XRD和SEM分析了涂层热腐蚀产物的相组成和形貌。结果表明:涂层在含有氯化物盐的熔盐中腐蚀时表现为失重而在硫酸盐中腐蚀时表现为增重;Cr0.82Al0.18N涂层在2种熔盐中的腐蚀都主要由Cr2O3和θ-Al2O3组成。氯化物熔盐和腐蚀产物的挥发导致了在含有氯化物熔盐中腐蚀时的失重现象。当含有氯化物盐时会增加熔盐中氧离子的活度,从而加速Cr2O3在熔盐中的溶解。此外,热腐蚀过程中产生的氯气也会参与腐蚀反应,从而加速涂层的消耗。     

Aluminizing and boroaluminizing treatments of Mar-M247 and their effect on hot corrosion resistance in Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>−NaCl molten salt

The effect of surface modifications of Mar-M247 superalloy on hot corrosion resistance was examined in Na₂SO₄−NaCl molten salt. The Mar-M247 was aluminized and boroaluminized by pack cementation in Ar and underwent a cyclic hot corrosion test in Na₂SO₄−NaCl molten salt. The XRD results showed that a Ni₂Al₃ phase was formed between the aluminized layer and the substrate when the surface modification temperature was below 1273 K. However, a NiAl phase formed when the temperature was above 1273 K. The intensity of the XRD peak in the NiAl phase increased after post heat treatment. Hot corrosion resistance increased for the specimens containing NiAl rather than Ni₂Al₃ phase. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247, which underwent surface modification at 1273 K and above. In the boroaluminized Mar-M247 specimens, corrosion resistance decreased as a result of the blocking of outward diffusion of Cr by boron and decreased cohesion between the oxide scale and the aluminized layer during thermal cycling.     

Hot-corrosion resistance of dissimilar AISI 4340 and AISI 304L weldments in the molten salt environment at 600°C

High temperature corrosion resistance of welded components in power plant industry, hot sections of gas turbines, boilers, industrial waste incinerators, metallurgical furnaces and petrochemical installations is one of the serious necessity. The failure of hot sections of welded component is due to the deposition of molten salt which accelerates the hot corrosion reactions in the weld joint. In this research work, the oxy-chlorination of dissimilar weldments between AISI 304L austenitic stainless steel and AISI 4340 alloy steel at 600°C in eutectic mixture of K₂SO₄?+?60% NaCl under molten salt conditions was investigated. Corrosion kinetics for this dissimilar joints calculated from the thermogravimetric charts were represented. Corrosion on the dissimilar joint employing E309L filler in pulsed current gas tungsten arc welding process is found to be very aggressive, due to the fragmented delta ferrites at numerous locations resulting in the formation of the micro electrochemical cells, which act as sites for corrosion initiation and thus leading to higher corrosion rate. However, in the case of continuous current, the joint is able to protect itself better in this environment due to lower amounts of delta ferrites in weld zone, which is attributed to the higher temperature available in continuous mode leading to austenisation of delta ferrites in the multipass welding.     

Novel multilayer Mg–Al intermetallic coating for corrosion protection of magnesium alloy by molten salts treatment

A novel multilayer Mg–Al intermetallic coating on the magnesium alloy was obtained by AlCl₃–NaCl molten salt bath treatment. The molten salt was treated at 400 °C, which is lower than the treatment temperature of solid diffusion Al powder. The thick Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ multilayer Mg–Al intermetallic coating forms on the magnesium alloy. The corrosion resistance of AZ91D alloy with and without coating by multilayer of Mg-Al intermetallic compound was evaluated by electrochemical impedance spectroscopy measurements in 3.5% (mass fraction) NaCl solution. The polarization resistance value of the multilayer coating on the magnesium alloy by molten salt bath treatment is greater than that of the uncoated one, which is attributed to the homogenously distributed intermetallic phases.     

High-temperature corrosion of titanium-, niobium-, and manganese-rich Fe-25Cr alloys in H2-H2O-H2S gas mixtures

The simultaneous sulfidation and oxidation of Fe-25Cr, Fe-25Cr-4.3Ti, Fe-25Cr-7.5Nb, and Fe-25Cr-9.0 Mn alloys were studied at 1023, 1123, and 1223 K, respectively, in H₂-H₂O -H₂S gas mixtures. The influences of titanium, niobium, and manganese on the transition from protective oxide formation to the formation of sulfide-rich corrosion products of Fe-25Cr alloys have been investigated. It has been found that additions of titanium and niobium can improve the scaling resistance of Fe-25Cr alloys against sulfidation in H₂ -H₂O -H₂S gas mixtures at high temperatures. However, the addition of manganese does not increase the resistance to sulfidation of Fe-25Cr alloy. The oxide Cr₂Ti₂O₇, which can suppress sulfide formation, formed on the Fe-25Cr-4.3Ti alloy. The addition of manganese to Fe-25Cr does not form more stable and protective oxides than Cr₂O₃ which formed on Fe-25Cr. Thermodynamic stability diagrams are used to explain the experimental results.     

Corrosion behaviour of mild steel in chloride solutions with Na2HPO4 additive and the development of antifreeze

ABSTRACT

Chloride salts can prevent roads from freezing in cold regions, together with severe corrosion on steel constructions. To develop an ecological and low-corrosive antifreeze, di-sodium hydrogenphosphate (Na₂HPO₄) was chosen as the additive into chloride salts. The addition of Na₂HPO₄ into either of the antifreezes of natural salt (N/S), NaCl, CaCl₂ or MgCl₂ can suppress the corrosion reaction of mild steel in the 3.0% antifreeze solution. Moreover, the addition of CaCl₂ or MgCl₂ into the antifreeze containing NaCl and 2.0% Na₂HPO₄ significantly decreased the corrosion rate. According to polarisation and XPS analyses, it is evident that the obtained low corrosion rates on mild steel are related to the suppressed cathodic and anodic reactions and the formation of a protective film containing Fe, P, O, Ca or Mg.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).