Short-term corrosion behavior of galvanized coatings in natural waters of the Greek territory

The corrosion behavior of hot-dip galvanized steel immersed in domestic and seawater was investigated after an exposure period up to 10 days. The examination of the coatings was accomplished with optical microscopy, scanning electron microscopy and X-ray diffraction. From this investigation it was deduced that the corrosion process in domestic water is slow and mainly proceeds through pitting corrosion, while the corrosion phenomena in seawater are more intense. The main mechanisms in this environment are pitting and intergranular corrosion. In both waters the Cl⁻ and the O²⁻ ions diffuse in the coating up to the Fe/Zn interface. Especially in seawater the Cl⁻ ions seem to be very aggressive.     

Enamel coating for protection of the 316 stainless steel against tribo-corrosion in molten zinc alloy at 460℃

Hot-dip galvanizing provides excellent corrosion and wear resistance for steels.However,the equipment itself,such as the steel roller,immerged in corrosive molten zinc suffers serious material loss during steel's production.Its protection has become the main technique problem in modern galvanizing line.In this study,an enamel coating was designed and prepared.Its tribo-corrosion in molten zinc alloy(Zn-0.2 wt%Al)at 460℃was investigated in comparison with the traditional WC-12 Co composite coating and the 316 stainless steel.Results indicate that the steel suffers serious material damage.Various corrosion products of Fe₂ Al₅ Znx form at the worn surface and the wear scar has reached 200μm deep after merely 5 h tribo-corrosion.Though the two coatings provide an improved tribo-corrosion resistance,for the WC-12 Co coating,its chemical reaction with the molten zinc increases brittleness and promotes cracking.The synergistic wear and corrosion cause its degradation.The enamel coating performs better during tribo-corrosion.It is chemically stable in molten zinc thus able to provide high corrosion resistance.In addition,the amorphous[Si04]network and the self-lubricating CaF₂ crystallite help it to build up an intact amorphous glaze layer readily at surface on sliding,leading to a reduced wear loss.During the whole tribo-corrosion process,the enamel coating is completely free of cracking,and the Zn penetration is inhibited.     

微弧氧化TC4钛合金与金属电偶的腐蚀行为

为了减小钛合金与金属偶合的接触腐蚀,采用微弧氧化技术,在TC4钛合金表面生成了一层主要由Al2TiO5相、锐钛矿TiO2相及金红石TiO2相组成的多孔膜.研究了TC4钛合金微弧氧化前后与45钢、LY12铝合金和紫铜偶合的电偶腐蚀行为.结果表明:3种金属分别与TC4钛合金偶合时,45钢会发生严重的电偶腐蚀,LY12铝合会...     

热浸镀层的海水全浸腐蚀行为研究

在舟山站进行了锌铝合金镀层的海水全浸腐蚀试验,失重腐蚀测试结果表明,其腐蚀深度随全浸时间按抛物线规律变化。XRD和EPMA分析结果表明,镀层表面的钙质沉积层与镀层金属的腐蚀产物及其凝聚的粘土胶体一起构成的复杂的腐蚀产物层,抑制了镀层腐蚀的传质过程和受扩散控制的氧去极化作用,从而使锌铝合金镀层表现出优异的耐海水腐蚀性能。     

镍合金在天然海水中的腐蚀行为

目前,尚未见到关于镍合金在我国海域的腐蚀行为的报道,这给镍合金在海水中的应用带来不便,为此,通过天然海水暴露试验,获得了9种镍合金在青岛海域全浸区、潮汐区暴露1年、2年、4年、7年的腐蚀结果,讨论了它们在海水中的腐蚀行为.结果表明,镍合金在海水中的耐蚀性相差很大.NS334,NS335,NS336,GH3128在海水中有很好的耐蚀性,Hastelloy-G和GH181有好的耐蚀性.镍合金在潮汐区的耐蚀性比全浸区好.机械划伤、加工残余应力增加镍合金对点蚀的敏感性.海生物污损对NS112和NS312在海水中的腐蚀有明显影响.添加Cu,Cr,Mo,W能提高镍合金在海水中的耐蚀性.该试验结果为实际应用提供了参考.     

Cavitation behavior of active thermal-sprayed coatings

As a first stage in the development of active coatings to improve both localized corrosion resistance and cavitation resistance in sea water, we studied flame-sprayed coatings of aluminum, zinc and Zn-15 wt.% Al applied to steel substrates. Cavitation was carried out in distilled water and in 3.5 wt.% NaCl solution, and was characterized by weight loss and by scanning electron microscopy. Open-circuit potentials in 3.5 wt.% NaCl solution were measured in an attempt to monitor internal fracture, which would allow seepage of solution through the coating to the more noble steel substrate.Cavitation damage rates were found to be significantly higher in the saline solution for the alloy and the aluminum coatings, whereas little sustained change in damage rate was found for the zinc coatings. The data indicate that the strength of the particle matrix essentially governs the overall strenght of the coating. However, in saline solution the effects of alloy strengthening are slightly reduced by the tendency of localized corrosion attack to initiate coating flaws and eventual failure.     

Corrosion properties of thermal sprayed aluminium (TSA) coatings deposited by powder flame spraying

Thermal sprayed aluminium (TSA) coatings are widely used for the protection of steels from aqueous corrosion, including offshore structures and components that are exposed to seawater immersion, tidal and splash zone environments. In this paper, the influence of deposition parameters of aluminium coating on its corrosion properties was studied. For different application parameters of TSA coatings, the thickness, the microstructure and corrosion properties were determined. Determination of thickness was performed according to EN ISO 2064:2000 relating to the definition and determination of the thickness of metallic and other inorganic layers. Microstructure was determined using a Olympus GX50 optical microscope. The corrosion properties of the coatings were compared using an accelerated electrochemical corrosion test method. Electrochemical studies were carried out in accordance with the requirements of ASTM standard G 102 – 89. The TSA coating performance was quantified in terms of its corrosion potential and the corrosion rate, which was estimated from Tafel polarisation resistance measurements.     

Corrosion resistance of a silane/ceria modified Mg--Al-layered double hydroxide on AA5005 aluminum alloy

The present work aimed at assessing the electrochemical behavior and the corrosion inhibition performance of Mg--Al-layered double hydroxide (LDH) coatings modified with methyltrimethoxysilane (MTMS) and cerium nitrate on AA5005 aluminum alloy. The chemical compositions and surface morphologies of the coatings were investigated by XRD, FT-IR and FE-SEM, while their corrosion resistance was evaluated by electrochemical and immersion tests. An optimum corrosion resistance of the composite coatings was obtained by adding 10⁻² mol·L⁻¹ cerium nitrate. An excess addition of cerium nitrate resulted in a loose structure and poor corrosion resistance of the coating. The corrosion mechanism of the composite coatings was proposed and discussed.     

Electrochemical characteristics of pipeline-steel with planar-defect epoxy-coating in simulated solutions of Ku''erle soil

An X70 pipeline steel and a low-carbon steel with different types of coating defects were studied using corrosion tests in Ku’erle simulated soil solution containing 0.6000 mol/L Cl⁻. Electrochemical characteristics of the X70 pipeline steel with planar-defect epoxy-coating in simulated solutions of Ku’erle soil were tested. The results show that in 0.6000 mol/L Cl⁻ simulated solution, for low-carbon steel with coating defects (the area ratio is 4.91%), at free corrosion potential and with immersion time, the corrosion with disbond was more severe than that with a break or with break and disbond. In Ku’erle soil simulated solution, when the coating defect area ratio was increased gradually (0.39% → 1.57% → 6.28%), the corrosion of the X70 steel under coating with break was promoted gradually, and the influence of immersion time on the corrosion decreased. The influence of the immersion time on the corrosion was weakened when the defect area ratio increased to a certain degree.     

M38合金表面防护涂层的热腐蚀行为

通过粉末包埋渗的方法在M38高温合金表面制备了4种改性铝化物涂层：NiCr-CrAI、Al-Si、Al-Ti和Co-Al涂层,对比研究了4种涂层在900℃下的涂盐（25％NaCl＋75％Na2SO4质量分数）热腐蚀行为。结果表明,4种改性的铝化物涂层中Al-Ti涂层抗热腐蚀性最好,腐蚀产物连续、致密;Al-Si涂层与Al-Ti涂层抗热腐蚀性相当,但腐蚀表面有局部剥落现象、氧化较严重;Co-Al涂层和NiCr—CrAl涂层抗热腐蚀性能依次降低,其中Co-Al涂层裂纹较严重,NiCr-CrAl涂层氧化较严重。