Microstructural characteristics of the oxide scale formed on 304 stainless steel in oxygenated high temperature water

The microstructural characteristics of oxide scale formed on type 304 stainless steel in oxygenated high temperature water have been investigated. From outer to inner layer, the oxide scale consists of faceted spinel particles, irregularly shaped hematite particles and a compact layer of nano-sized spinels. Some outmost spinels formed on top of other particles are depleted in Cr, while the hematite particles tightly embedded into the inner layer contain more Cr in the inner than in the outer part. The inner nano-sized oxide grow inwards directly from the bottom of outer particles. The related oxidation mechanism is discussed.     

Effect of alternately changing the dissolved Ni ion concentration on the oxidation of 304 stainless steel in oxygenated high temperature water

Characteristics of oxide films formed on 304 stainless steel under alternately changing Ni²⁺ concentrations in oxygenated high temperature water were examined. Oxides preformed under low Ni²⁺ concentration evolve from hematite to spinel after subsequent immersion under high Ni²⁺ concentration. Meanwhile, Ni content in the surface layer of oxide rises up while Fe content drops. Oxide films preformed under high Ni²⁺ concentration show little change in phase composition after subsequent immersion under low Ni²⁺ concentration. Fe contents in surface layer rise up while the change of Ni contents depends on the original phase composition and whether residual Ni²⁺ is present.     

Corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

The corrosion behavior of an austenitic stainless steel (UNS S30400) has been characterized in a 10,000 h test conducted in hydrogenated, ammoniated water at 260 °C. The corrosion kinetics were observed to be parabolic, the parabolic rate constant being determined by chemical descaling to be 1.16 mg dm⁻² h^−1/2. X-ray photoelectron spectroscopy, in combination with argon ion milling and target factor analysis, was applied to provide an independent estimate of the rate constant that agreed with the gravimetric result. Based on the distribution of the three oxidized alloying constituents (Fe, Cr, Ni) with respect to depth and elemental state, it was found that: (a) corrosion occurs in a non-selective manner, and (b) the corrosion film consists of two spinel oxide layers--a ferrite-based outer layer (Ni_0.2Fe_0.8)(Fe_0.95Cr_0.05)₂O₄ on top of a chromite-based inner layer (Ni_0.2Fe_0.8)(Cr_0.7Fe_0.3)₂O₄. These compositions agree closely with the solvi phases created by immiscibility in the Fe₃O₄-FeCr₂O₄ binary, implying that immiscibility plays an important role in the phase separation process.     

Crack initiation model for sensitized 304 stainless steel in high temperature water

To investigate the initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel in high temperature water, a constant load SCC test method combined with in situ crack observation technique was employed. The in situ crack observation system allowed us to detect small cracks of at least 100 μm. As a result, a fracture time decreased with an increase in an applied stress. The first cracks were observed at most 3 h before the specimen was fractured under all the stress conditions. After that, many cracks were initiated in a short time to fracture. The fracture was caused by coalescence of multiple cracks rather than by growth of some primary cracks. The simulation model for surface crack initiation was newly developed using a Monte Carlo method, which was based on damage mechanics and stress analysis around the existing cracks. The simulation could represent the empirical results of changes in the crack distribution and the cumulative number of cracks during the SCC tests. It was concluded, therefore, that the crack initiation process should be considered in simulating the life prediction of the material in this SCC system.     

The effect of roll-processing orientation on stress corrosion cracking of warm-rolled 304L stainless steel in oxygenated and deoxygenated high temperature pure water

Stress corrosion crack growth rates of 304L stainless steel after one-directionally warm-rolled along the longitudinal (L) direction were measured in oxygenated and deoxygenated pure water at 288 °C. The fracture mode is mainly intergranular in the Transverse-longitudinal (T-L) orientation specimen, while it is occasionally intergranular in the Longitudinal-transverse (L-T) orientation specimen. The crack growth rates in the T-L orientation are higher than those in the L-T orientation in both oxygenated and deoxygenated environments, which is related to the effective cracking growth path as the result of microstructural anisotropy and applied loading direction.     

The high temperature oxidation behaviour of austenitic stainless steels

High temperature annealing in a dynamic vacuum has been utilised to induce the growth of duplex oxide over the whole surface of stainless steel specimens. It is found that duplex oxide grows at a rate which does not obey a simple power law. The oxidation kinetics and oxide morphology have also been studied for a series of ternary austenitic alloys which cover a range of composition between 5 and 20% chromium. A model has been developed from the observations to describe the formation of duplex oxide and the subsequent formation of a “healing layer” which virtually causes the oxidation process to stop. This phase tends to form at grain boundaries and a relationship has been derived for the reaction kinetics which relates the reaction rate with grain size of the substrate.     

Oxidation behaviour of Super 304H stainless steel in supercritical water

Oxidation of Super 304H steel in supercritical water at 600°C and 25?MPa was investigated, for the oxidation time of up to 1000?h. The oxidation kinetics approximately followed a parabolic law. The composition and microstructure of the oxide were investigated using the SEM equipped with EDS, EBSD and XRD. The phase compositions of the oxide evolve with increasing time. Cr₂O₃ was present at the initial stage and not detected while Fe₂O₃ appeared at longer time. EBSD indicated that continuous Cr₂O₃ formed at the interface of oxide and matrix for HR3C at 600°C for 40?h but only scattered Cr₂O₃ can be observed for 200?h. After the formation of a thin Fe–Cr–Ni oxide film on the Super 304H steel surface, the Fe-rich outer nodular oxide begins to grow and forms a continuous layer after 1000?h. The growth process of Super304H steel in supercritical water was discussed.     

Electropolishing effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

The corrosion rate of electropolished 304 stainless steel surfaces (UNS S30400) is found to be lower by more than a factor of three relative to that determined previously for machined surfaces in mildly alkaline, hydrogenated water at 260 °C. This favorable result is attributed to significant changes in nanocrystallinity of the corrosion oxide layer caused by the removal of surface microstrain, which had been imparted during the machining process. In the absence of microstrain, a low-porosity, protective, corrosion layer forms that is composed of extremely small and uniformly-sized spinel oxide crystals. Application of scanning electron microscopy (FEG-SEM), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) in conjunction with ion milling and target factor analyses, found the corrosion layer to consist of micrometer-size crystals of a ferrite-based spinel oxide (non-protective) over-laying nanometer-size crystals of a chromite-based spinel oxide (protective). Composition of both phases is unchanged from that previously observed on corroded, machined surfaces and is representative of solvus phases in the immiscible Fe(Fe_1−nCr_n)₂O₄ spinel binary. The smaller size (10 vs. 26 nm) and greater surface density (∼10,000 vs. 835 μm⁻²) of the chromite-based crystals relative to those formed on machined (i.e., cold-worked) surfaces, however, is consistent with the absence of preferred high energy nucleation sites on strain-free surfaces. Therefore, electropolishing, which removes surface microstrain induced by cold-working, represents a preferred reference surface condition.     

Zinc treatment effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 °C were found to lower the corrosion rate of austenitic stainless steel (UNS S30400) by about a factor of five, relative to a non-zinc baseline test [S.E. Ziemniak, M. Hanson, Corros. Sci. 44 (2002) 2209] after 10,000 h. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed that miscibility gaps in two spinel binaries—Fe(Fe_1−mCr_m)₂O₄ and (Fe_1−nZn_n)Fe₂O₄—play a significant role in determining the composition and structure of the corrosion layer(s). Although compositions of the inner and outer corrosion oxide layers represent solvus phases in the Fe₃O₄-FeCr₂O₄ binary, zinc(II) ion incorporation into both phases leads to further phase separation in the outer (ferrite) layer. Recrystallization of the low zinc content ferrite solvus phase is seen to produce an extremely fine grain size (∼20 nm), which is comparable in size to grains in the inner layer and which is known to impart resistance to corrosion. Zinc(II) ion incorporation into the inner layer creates additional corrosion oxide film stabilization by further reducing the unit cell dimension via the substitution reaction

0.2Zn²⁺(aq)+Fe(Fe_0.35Cr_0.65)₂O₄(s)?0.2Fe²⁺(aq)+(Zn_0.2Fe_0.8)(Fe_0.35Cr_0.65)₂O₄(s)     

Whisker growth morphology of high temperature oxides grown on 304 stainless steel

During the high temperature oxidation of as-cast Type 304 stainless steel there is a complex growth pattern of the oxide layers which is shown to be dependent on; alloy composition and local segregation, surface finish, temperature and atmosphere. Of interest is the appearance of whisker growth morphologies which grow in excess of 10 μm in length and at random orientations to the sample surface. This paper illustrates these features and discusses the formation mechanism in terms of the oxide microstructure.     

电沉积CeO2薄膜对TP304H钢高温水蒸气氧化机制的影响

研究了TP304H钢表面电沉积CeO2薄膜在610-770℃水蒸气中的氧化行为的变化.结果表明,沉积CeO2薄膜可有效减缓氧化速度.SEM和EDX分析显示,氧化膜表面形貌由多层结构转化为单层,Cr元素扩散进入CeO2膜层而形成Cr-Ce复合氧化物膜层.表面形成的氧化膜中Cr浓度显著提高,CeO2膜处于外层氧化膜与基体之间,内氧化消失.根据CeO2特性和试验结果,由于CeO2中Ce离子具有处于富氧/贫氧环境中具有Ce^4＋/Ce^3＋两价和高氧空位浓度特性,认为沉积CeO2薄膜对氧向内扩散具有一定的阻挡作用,限制了CeO2薄膜/基体界面处的氧分压,使得Cr优先扩散穿过CeO2薄膜在CeO2薄膜/气体界面处氧化,从而推迟了Fe氧化物形成的时间.     

Yttrium sol–gel coating effects on the cyclic oxidation behaviour of 304 stainless steel

The present results reveal the interest of sol–gel coating technique to improve 304 steel high temperature oxidation resistance. An yttrium sol–gel coating appears to enhance the oxidation resistance during isothermal oxidation test, to decrease widely the oxide weight gain and to reduce the initial transient oxidation stage generally observed in the case of blank steels. Moreover, the experimental results confirm that yttrium sol–gel coating also plays a significant role on the cyclic oxidation behaviour of the 304 steel. In fact, the yttrium addition promotes remarkably the prolongation of the period during which the oxide scale still remains adherent to the substrate.     

Effect of prior conversion treatment on high temperature oxidation behaviour of ceramic coated 17Cr ferritic stainless steel

Abstract

Convers(on coatings modified by deposits of alumina or zirconia have been studied as a way of improving the resistance to thermal oxidation of a 17Cr ferritic stainless steel. Conversion coatings, characterised by high porosity, facilitate the electrochemical deposition of alumina or zirconia layers and enhance their adhesion to the substrate. The resulting coats, after heating, possess compositional gradients which are favourable to strong adhesion. Alumina coatings are more effective than zirconia coatings in suppressing thermal oxidation of the steel.     

Influence of Zn injection on characteristics of oxide film on 304 stainless steel in borated and lithiated high temperature water

The oxide film on 304 stainless steel exposed to the hydrothermal environments at 573.15 K up to 20 days without/with 10 ppb Zn injection has been investigated ex situ by X-ray photoelectron spectroscopy (XPS). ZnFe₂O₄ and ZnCr₂O₄ were found to be formed in the oxide film at the initial stage of immersion by substitution reaction between Zn²⁺ and Fe²⁺, and ZnCr₂O₄ became dominant after long-term immersion. The calculations of potential-pH diagrams, solubilities and crystallographic features of spinels have been done to evaluate the oxide film structure and the inhibition mechanism caused by Zn injection.     

The corrosion behaviour of AISI 304L stainless steel in 0.1 M HCl at room temperature—I.

A consideration of the critical potential and the protection potential of localized corrosion is given. From a hypothesis about when a fully developed localized attack can occur a definition of the field of protection potential has been attempted. The potentiostatic method was used to define this field relative to solution heat treated AISI 304L stainless steel in 0.1 M HCl at 20°C, with test times of ? 150 h. The results obtained differ significantly from what can be deduced from the potentiodynamic technique, and they confirm the formulated hypothesis and definitions. In the active loop of potentiodynamic curves, the nucleation and development of pitting, together with general corrosion, is documented by SEM. The hypothesis can be advanced that such pitting corrosion is a direct consequence of inhibition phenomena resulting from products formed during overall corrosion.     

Surface modifications of oxide layer formed in isothermal high temperature conditions of AISI 304 stainless steel

Abstract

Chromia forming steels are excellent candidates to resist to high temperature oxidising atmospheres because they form protective oxide scales. To understand the oxidation mechanisms of the AISI 304 stainless steel in air at 800°C, in situ X-ray diffraction (XRD) has been used not only during high temperature oxidation, but also during and after cooling. The in situ XRD analyses carried out during high temperature AISI 304 steel oxidation in air at 800°C reveal the growth of iron containing oxides such as haematite Fe₂O₃ and iron chromite FeCr₂O₄, after 35 h of the oxidation test, whereas the initial nucleation of the oxide layer shows the single growth of chromia. Iron containing oxides develop over the initial layer and these oxides appear to be poorly adherent and spall off during cooling between 200 and 50°C. Protection against high temperature oxidation would be increased when the initial nucleation of manganese spinel compound is delayed in the oxide scale.     

304奥氏体不锈钢低温盐浴渗氮处理

采用430℃低温盐浴对304奥氏体不锈钢进行渗氮处理,研究了渗氮时间对渗氮层组织和性能的影响。利用XRD衍射仪、光学显微镜、表面显微硬度计和带能谱仪(EDS)的扫描电镜(SEM)分别分析渗氮层的相组成、厚度、表面硬度和显微组织。结果表明:304奥氏体不锈钢在430℃渗氮不同时间后,渗氮层厚度和表面硬度都随着时间的延长而增加。渗氮时间为1 h时,渗氮层仅为单一的S相,随着渗氮时间的增加,渗氮8 h时开始有少量CrN生成,渗氮16 h时,渗氮层由大量CrN+S相两相混合。用电化学极化的方法评价耐蚀性能的结果表明:盐浴渗氮处理后耐Cl-点蚀性能得到了一定的改善,在430℃渗氮4 h,其耐蚀性能是最好的,优于没经过渗氮的试样,而在所有的渗氮试样中,渗氮8 h、16 h的试样耐点蚀性能较差。     

Effects of temperature on the oxide film properties of 304 stainless steel in high temperature lithium borate buffer solution

The paper mainly investigated the effects of temperature on the oxide film properties of 304SS in lithium borate buffer solution by electrochemical measurements and XPS analysis. As temperature increased, the protective property of the film degraded and structure varied from a single layer to a double-layer. Whatever the temperature, the oxide film exhibited an n-type and p-type semiconductor in the potential range above and below the flat band potential, respectively. The electronic properties were assigned to a Fe–Cr spinel inner layer and a defective Fe–Cr oxide outer layer. The related growth mechanisms of the oxide film were also discussed.     

304L stainless steel oxidation in carbon dioxide: An XPS study

From the early beginning of the oxidation of 304L stainless steel in carbon dioxide at 1273 K (1 min, for a weight gain of 0.02 mg cm⁻²), the surface of the alloy was entirely covered by oxides: magnetite Fe₃O₄, chromia Cr₂O₃ and traces of wüstite Fe_1−xO. Later on, for weight gains approaching 1 mg cm⁻², magnetite remained at the outer interface, with traces of hematite (Fe₂O₃), above a thick layer of wüstite Fe_1−xO. Magnetite and wüstite may favour adhesion of thermal plasma protective coatings such as alumina.     

Predicting crack growth rate vs. temperature behaviour of Type 304 stainless steel in dilute sulphuric acid solutions

The coupled-environment fracture model for intergranular stress corrosion cracking of Type 304 stainless steel in boiling water reactor primary (water) heat transport circuits containing relatively pure water has been extended to incorporate the effects of sulphuric acid additions to the coolant and to include thermal activation of the crack tip strain rate. These extensions allow comparisons to be made between theoretically estimated and experimentally determined crack growth rates (CGRs) over a considerable temperature range after calibration at a single temperature. The model predicts, in agreement with experiment, that the CGR passes through a maximum with increasing temperature at a temperature of about 180 °C. The temperature dependence of the CGR is attributed to the competing effects of temperature on the thermally activated processes that occur at the crack tip and the properties (including ECP and conductivity) of the external environment.