Pre-treatment and oxidation behavior of sol–gel Co coating on 430 steel in 750 °C air with thermal cycling

Research and development efforts continue to improve oxidation resistance and electrical conductivity of solid oxide fuel cell (SOFC) interconnects. This research evaluates the performance of a ferritic stainless steel (AISI 430 — a candidate SOFC interconnect material) with and without a Co coating (via sol–gel dip coating technique) with various pre-treatments, during cyclic oxidation exposures up to 750 °C in laboratory air. A pre-treatment exposure of Co coated samples to a 750 °C reducing atmosphere (prior to oxidation exposures) led to the formation of an effective Co-based spinel oxide coating that lowers oxidation rates by more than 40 times, and substantially lowers area specific resistance (ASR) in comparison to uncoated specimens. The development of effective sol–gel Co coatings with reducing pre-treatments, and their evolution within simulated SOFC interconnect environments is presented and discussed.     

Lanthanum Effect on the Isothermal High Temperature Oxidation Behavior at 1,000 °C of a Phosphoric Acid-Treated AISI 304 Stainless Steel

Many studies have shown that a phosphoric-acid treatment improves the high temperature oxidation resistance in air of some alloys. Interestingly, though, the phosphoric-acid treatment generates a structural modification of the steel surface which is catastrophic for the high-temperature oxidation behavior at 1,000 °C. The aim of our work was to test the effect of a reactive element sol–gel coating on high-temperature oxidation resistance of phosphoric acid-treated AISI 304 steel. The oxide scale growth mechanisms were studied by exposing La-coated and uncoated phosphoric acid-treated 304 steel samples to high-temperature conditions in air. A phosphoric-acid treatment modified the structural composition and the surface morphology of the AISI 304 steel by the formation of a FeH₂P₃O₁₀ structure, leading to hematite formation and to a breakaway phenomenon. Lanthanum coating, after initial phosphoric-acid treatment, led to the formation of LaCrO₃ which limited through-scale cracking and reduced the growth of iron oxides.     

A room temperature cured sol–gel anticorrosion pre-treatment for Al 2024-T3 alloys

The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion protection system. Historically these corrosion protection systems have been based upon the use of chemicals containing Cr(VI) compounds. Organic–inorganic hybrid silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In the present study, a hybrid epoxy–silica–alumina coating with or without doped cerium nitrate has been prepared using a sol–gel method. The hybrid coatings were applied by a dip-technique to an Al–Cu alloy, Al 2024-T3, and subsequently cured at room temperature. The anticorrosion properties of the coatings within 3.5% NaCl were studied using electrochemical impedance spectroscopy (EIS), and conventional DC polarisation. An exfoliation test method involving immersion in a solution of 4 M NaCl, 0.5 M KNO₃ and 0.1 M HNO₃ was also used. The cerium nitrate doped sol–gel coating exhibited excellent anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The resistance to corrosion of the sol–gel coating was also evaluated by analysing the morphology of the coating before and after corrosion testing using scanning electron microscopy.     

Anion embedded sol–gel films on Al for corrosion protection

We report here on the successful incorporation of organic anions into a sol–gel film on Al as a means of enhancing the protection against corrosion. Following our previous study where we showed that hydrophobic sol–gel films provided pronounced corrosion inhibition, we studied the corrosion inhibition that phenylphosphonic acid (PPA) has when embedded inside a thin sol–gel coating on Al. The anion of this organic anion tends to stay inside a phenyltrimethoxysilane (PTMOS) based sol–gel film due to π-interactions. Our findings, which are derived primarily from potentiodynamic polarization measurements, electrochemical noise, scanning electron microscopy measurements and Auger electron spectroscopy (AES), clearly show that the organic phosphonate adds to the protection efficiency of the sol–gel film.     

An investigation of a CoNiCrAlY overlay coating and its oxidation behavior by means of imaging SIMS

A CoNiCrAlY overlay coating and its oxidation behavior have been studied by means of imaging SIMS. Maps of the coating and scale showed a highly uneven yttrium distribution which arose at least partly during the deposition of the coating. The implications of this for the resistance of the coating to oxidation and hot corrosion are discussed. There was no evidence of yttrium build-up at the scale/metal interface. The yttrium maps show the power of imaging SIMS for studying the distribution of elements which are present in very low concentrations. A preliminary experiment using¹⁸O^– as a tracer showed that the scale grew primarily at the scale/metal interface.A preliminary account of this work has been given in the Proceedings of the Conference on Microscopy of Oxidation, M. J. Bennett and G. W. Lorimer, eds., Institute of Metals, London, 1991, p. 252.     

Oxidation protection of 20Cr-25Ni-Nb stainless steel at 1300°C

Enhanced environmental protection of chromia-forming advanced metallic alloys at normal operating temperature, typically 900°C, may be provided by two well-established approaches—incorporation of reactive elements into the protective oxide scale or an amorphous ceramic coating acting as a diffusion barrier. The continued effectiveness of such approaches, namely by cerium and yttrium ion implantation and with a vapor deposited amorphous silica coating, in reducing oxidation of 20Cr-25Ni-Nb stainless steel in a carbon-dioxide-based environment has been examined during 0.5 and 1 hr transients to 1300°C. The influence of pre-oxidation of the ion-implanted, silica-coated, and uncoated steel for extended periods in the same environment at 825–900°C has also been established.     

Oxidation Behavior of a Ti3Al–Nb Alloy with Surface Thin Oxide Films

Thin films of aluminum, cerium, and yttrium oxides were applied onto the surfaces of Ti₃Al–11Nb samples using an electrodeposition technique. The oxidation behaviors of the Ti₃Al–Nb alloy, with and without these surface-applied films, were studied in air at 800–1000°C. The results showed that the oxidation rate of the alloy can be reduced by Ce oxide and Y oxide films, and the greatest improvement comes from oxidation of the Y oxide-coated specimens at 800°C. With increasing oxidation temperature, the difference between the Co-oxide and Y-oxide films becomes smaller. The results also indicated that the Ce-oxide and Y-oxide films can significantly improve the oxide scale-spallation resistance. On the other hand, Al-oxide films result in detrimental effects on the oxidation and scale-spallation resistance of the Ti₃Al–Nb alloy. Based on the experimental results, the effects of the different surface films on the oxidation mechanism are discussed.     

304不锈钢表面冷喷涂TC4钛合金涂层性能研究

采用冷喷涂技术在304不锈钢表面制备了TC4钛合金涂层,通过扫描电子显微镜观察了涂层的形貌、组织结构,并利用电化学方法研究了涂层的腐蚀电化学特征。研究结果表明,冷喷涂制备的TC4钛合金涂层致密性存在较为明显的梯度现象,靠近基体的涂层密度明显高于表面;涂层喷涂过程没有出现明显氧化现象,与基体的结合强度可达20 MPa左右;涂层的耐腐蚀性能优于304不锈钢,可大大提升不锈钢材料在海洋环境中的耐点蚀性能。     

A superficial coating to improve high-temperature-oxidation resistance of a plain-carbon steel under nonisothermal conditions

The nonisothermal oxidation behavior of 0.16% carbon steel at a heating rate of 7 K·min^–1 up to a temperature of 1400 K in dry air ( =21.27 kPa) is reported. It was envisaged to develop a cheaper and easily applicable superficial coating material for minimization of scale loss during high-temperature exposure. The coating material consisted of an aqueous slurry of bentonite and calcium silicide mixture. Such a coating material when applied superficially on the steel surface not only enhances oxidation resistance but also helps in forming an adherent oxide scale to the alloy substrate even up to a third cycle of exposure to 1400 K. Postoxidation analyses of the alloy-scale combination using XRD, SEM, and EDS revealed the formation of a barrier layer comprised of complex silicates and aluminoferrite that provided improved oxidation resistance.     

Protecting stainless steel by glass coating during slab reheating

A glass coating was prepared onto AISI 304 stainless steel by a slurry-spraying technique and its effects on oxidation behavior of this steel was investigated at 1250 °C in air. Results were compared with those for bare specimens. The glass coating decreased the weight gain by 98% after oxidation for 9 h, and oxidation kinetics of 304 stainless steel was changed to linear behavior due to the coating. Steel loss of the coated specimens was reduced by 1-2 orders of magnitude. Hot stage microscopic, optical, XRD and SEM-EDX analyses revealed that the as-received coating melted at high temperature, to form self-healing, homogeneous glass coating which acted as a perfect oxygen diffusion barrier and prevented the breakaway oxidation of 304 stainless steel. During cooling, the glass coating spalled because of CTE mismatch between coating and the steel. This glass coating by low-cost and easy handling method is potentially applicable during slab reheating of stainless steel.     

SiO2 based hybrid inorganic–organic films doped with TiO2–CeO2 nanoparticles for corrosion protection of AA2024 and Mg-AZ31B alloys

Hybrid sol–gel coatings provide an approach as protective layers on metals. In this work, corrosion protection of aluminium and magnesium alloys by SiO₂-methacrylate coatings doped with TiO₂–CeO₂ nanoparticles was studied. The films show an improvement of the barrier properties at initial immersion. The reactivity of both alloys produces a deterioration of the protection with longer immersion, although TiO₂–CeO₂ nanoparticles let to observe signals of self-healing effect. Aluminium oxide/sol–gel interface was found to be stable. In combination with excellent paint adhesion on sol–gel films, these coatings can be a promising alternative pre-treatment for high strength aluminium alloys prior to painting.     

T91钢和TP304钢对含氯盐水蒸气的抗氧化性能

为比较T91钢与TP304钢在垃圾焚烧环境中的抗氧化性能,用含有微量KCl的水蒸气气氛模拟垃圾焚烧烟气,在530,600,670℃下对材料进行了氧化试验。结果表明:在含有微量KCl的高温水蒸气条件下,经过24h后T91钢的氧化质量增加是TP304钢的5倍,表明TP304钢对于高温含Cl-水蒸气的抗氧化能力优于T91钢的;在三种温度下氧化后,T91钢和TP304钢表面生成的片状氧化物成分与金属基体的Cr含量有关,Cr含量较低的T91钢表面生成了Fe2O3,Cr含量较高的TP304表面则生成了富含Cr的(Fe,Cr)2O3,(Fe,Cr)2O3氧化层的生成是TP304钢具有更高抗氧化性的原因。     

Influence of Yttrium-Alloying Addition on the Oxidation of Alumina Formers at 1173 K

The oxidation behavior of three commercial Fe–Cr–Al alloys, Kanthal APM, Kanthal A1, and Kanthal AF (containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been registered and the presence of yttrium does not seem to have a significant effect on the oxidation rate. On the contrary, the reactive element markedly influences the parabolic oxidation rate and the composition of the oxide scale. In situ X-ray diffraction (XRD) shows that yttrium promotes the transformation of transition alumina into -Al₂O₃, leading to the formation of a more protective oxide scale.     

Evidence for Chromium Evaporation Influencing the Oxidation of 304L: The Effect of Temperature and Flow Rate

The influence of temperature and flow rate on the oxidation of 304L steel in O₂/H₂O mixtures was investigated. Polished samples were isothermally exposed to dry O₂ and O₂+40% H₂O at 500–800°C at 0.02–13 cm/sec flow velocity, for 168 hr. The samples were analyzed by gravimetry, XRD, ESEM/EDX, and AES depth profiling. The oxidation of 304L in water vapor/oxygen mixtures at 500–800°C is strongly influenced by chromium evaporation. The loss of chromium tends to convert the protective chromia-rich oxide initially formed into a poorly protective, iron-rich oxide. The rate of oxidation depends on flow rate; high flow rates result in an early breakdown of the protective oxide. The most rapid breakdown of the protective oxide occurs at the highest temperature (800°C) and the highest gas flow (4000 ml/min=13 cm/sec). The oxide formed close to grain boundaries in the metal is more protective, while other parts, grain surfaces suffer breakaway corrosion. The protective oxide consists of a Cr-rich 50–200-nm thick M₂O₃ film, while the parts experiencing breakaway corrosion form a 10–30-m thick Fe-rich M₂O₃/M₃O₄ scale. The results show that chromium evaporation is a key process affecting the oxidation resistance of chromia formers and marginal chromia formers in O₂/H₂O mixtures.     

The effect of surface implantation of yttrium and cerium upon the oxidation behaviour of stainless steels and aluminized coatings at high temperatures

Assessments have been carried out on the extent to which the oxidation resistance of metals at high temperatures can be improved by surface implantation with yttrium or cerium. The elements were implanted to a concentration of 0.2–0.8%, to a depth of 0.2 μm. Yttrium implantation reduced the magnitude of oxidation and oxide spallation for a 20% Cr/25% Ni/Nb steel in carbon dioxide, at 800–850°C. The benefits approached those from a comparable yttrium alloy addition. The oxidation behaviour of a 15% Cr/4% Al steel, in air and carbon dioxide, at 1100–1200°C, was improved markedly by a 0.86% yttrium alloy addition but yttrium implantation into the steel was without significant influence. The oxidation behaviour of aluminized coatings on three nickel based alloys in air, at 1100°C, was also unaffected by implantations of either yttrium or cerium. The mechanisms associated with the results are discussed in detail.     

The mechanism of scale adhesion on sputtered microcrystallized CoCrAl films

The oxidation mechanisms of sputtered microcrystalline Co–30Cr–5Al coatings were investigated by an acoustic emission technique, scatch test, transmission electron microscope (TEM), which was compared with CoCrAlY alloy. The results indicated that the beneficial effects of microcrystallization on the scale adhesion of Co–30Cr–5Al alloy are as follows: (1) The sputtered CoCrAl coating possesses a columnar structure, and oxidation along the columnar grains may form many micropegs which can anchor the scale to the metallic substrate, enhancing bonding of the scale. (2) The grain size of the sputtered coating is several orders of magnitude smaller than that of the cast alloy, and the grain size of oxide scales formed on the former is finer than that of the latter. The finer oxide scale may relieve the growth stresses during isothermal oxidation and partial thermal stresses during cooling by plastic deformation through grain sliding. The microcrystalline coating is more plastic than the cast alloy, which may relieve a certain amount of thermal stresses of the oxide scales. On the basis of oxide adhesion and plasticity, microcrystallization is more beneficial than the addition of reactive elements.     

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.     

Effect of nano‐particulate sol–gel coatings on the oxidation resistance of high‐strength steel alloys during the press‐hardening process

The need for lighter constructional materials in automotive industries has increased the use of high‐strength steel alloys. To enhance passenger's safety press hardening may be applied to steel parts. However, as the steel parts are heated up to 950 °C during this process they have to be protected by some kind of coating against the intense oxide formation usually taking place. As the coating systems used so far all have certain disadvantages in this work the ability of nano‐particulate thin coatings obtained by the sol–gel process to improve the oxidation resistance of 22MnB5 steel is investigated. The coatings obtained from three sols containing lithium aluminum silicate and potassium aluminum silicate showed the best performance against oxidation. The structural properties of the coating materials were characterized using different methods like XRD and differential thermal analysis. Comparison of the oxidation rate constants proved the ability of the coatings to protect against oxidation at temperatures up to 800 °C. Press‐hardening experiments in combination with investigations on the thermal shock resistance of the coated samples also showed the ability of the coatings to stay intact during press hardening with only slight spalling of the coatings in the bending areas. The absence of any secondary intermetallic phases and layer residues during laser beam welding experiments on coated samples proves the suitability of the nano‐particulate coatings for further industrial processing.     

Effect of a NiAl Coating on the Oxidation Resistance of a NiAl–TiC Composite

The effect of a microcrystalline NiAl coating prepared by magnetron sputtering on the high-temperature oxidation resistance of a NiAl–TiC (20 vol.%) composite was investigated in air at 1000 and 1100°C. It was found that the isothermal as well as the cyclic oxidation resistance of the NiAl–TiC composite were greatly improved by the microcrystalline NiAl coating. The oxide scale formed on the NiAl–TiC composite was composed mainly of TiO₂. On the contrary, the scales formed on NiAl–TiC coated with microcrystalline NiAl were only Al₂O₃.     

Microstructure and oxidation resistance of reactive plasma clad Cr7C3 /γ-Fe ceramic composite coating

A new type oxidation resistance in situ Cr7C3/γ-Fe ceramic composite coating was fabricated on hardened and tempered grade C steel by reactive plasma clad with Fe-Cr-C alloy powders. The oxidation resistance of the ceramic composite coating was investigated under the test condition of 900℃ and 50 hours. The results indicate that the coating has a rapidly solidified microstructure consisting of blocky primary Cr7C3 and the inter-blocky Cr7C3/γ-Fe eutectics and is metallurgically bonded to the hardened and tempered grade C steel substrate. The high temperature oxidation resistance of the coating is up to 1.9 times higher than that of grade C steel. The oxidation kinetics curve of the coating is conforming to the parabolic-rate law equation. The excellent oxidation resistance of the coating is mainly attributed to the continuous oxide films which consist of Cr2O3 and Fe2O3. The continuous oxide films can prevent the inner part of the coating from being further oxidized.