Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

Cr对钢耐海水腐蚀性的影响

获得了５种含铬低合金钢在海水中暴露１、２、４、８（７）年的腐蚀数据,讨论Ｃｒ对钢耐海水腐蚀的影响,铬钢的耐海水腐蚀性不仅与Ｃｒ的含量有关,还与其他复合合金元素有关。短期浸泡时,钢的耐海水腐蚀性随铬含量（无其他合金元素复合）增加而提高。长期浸泡,Ｃｒ对钢的耐海水腐蚀性有害,约１％Ｃｒ与Ｍｏ（－Ａｌ）复合对钢的耐海水腐蚀性的影响与Ｃｒ的影响没有左别大于２％Ｃｒ与Ｍｏ（－Ａｌ）复合大幅度提高钢在海水中短期浸泡的耐蚀性,并使耐蚀性逆转时间明显推迟.小于1%Cr与Mn-Cu、Cu-Si-V、Ni-Cu-Si、Ni-Mn等复合对钢的耐海水腐蚀性有害。     

Influence of shot peening and thermal ageing treatment on resistance to intergranular corrosion in shielded metal arc weld metal for type 600 nickel base alloy

Shielded metal arc weld metal for type 600 nickel base alloy (alloy 182) is used for weld components in nuclear power plants. To evaluate the intergranular corrosion resistance of alloy 182 after application of shot peening and subsequent thermal ageing treatment at 593–793 K, we conducted the corrosion test (immersed in boiled 16% sulphuric acid +5.7% copper sulphate aqueous solution at 57.6 ks) using specimens of alloy 182. The results show that the intergranular corrosion resistance of alloy 182 subjected to heat treatment at 893 K for 72 ks was improved by shot peening. Also, the intergranular corrosion resistance was not changed by thermal ageing treatment at 593–793 K subsequent to shot peening. Because remaining chromium depletion layers along grain boundaries were still observed by transmission electron microscope (TEM) after shot peening, disappearance of chromium depletion layers cannot be a factor in the improvement of the intergranular corrosion resistance. The results of measurement of surface residual stress by the X-ray diffraction method show that the compressive residual stress introduced by shot peening still remained on the surface of the specimens. Based on these observations, we assumed that chromium depletion layers along grain boundaries near the surface were dissolved by the environment of the corrosion test, the dissolved regions were closed by the compressive residual stress on the surface, and then the remaining chromium depletion layers were protected from the corrosive environment. This assumption explains why the intergranular corrosion resistance was improved although chromium depletion layers remained.     

Korrosionselemente durch Gefügeheterogenität in nichtrostenden Chrom-Nickel-Stählen

Corrosion elements set up by structure heterogeneity in stainless chromium-nickel steels Transformation processes and precipitation of carbides could be the reason for corrosion elements in austenitic stainless steels. Examples are the intergranular corrosion and the selective martensite corrosion. The intergranular corrosion is based on a change of the electrochemical properties of the grain boundary regions by impoverishment of chromium. This “classical intergranular corrosion” takes place at all potentials. Resulting from the impoverishment of chromium after colling to very low temperatures, or by cold deformation, an α′-phase can be produced. This phase is only attacked in the active region of the current density-potential-curve. Both the intergranular corrosion and the corrosion of martensite are caused by local anodes. A selective attack of grain boundaries is also observed in the transpassive region. The intensity of this attack increase with precipitation of carbides. During the sensitivisation cathodes are produced, which cause a polarisation in the intergranular corrosive regions. Therefore, this attack of grain boundaries is not only connected with the formation of local anodes but also influenced by the formation of local cathodes.     

铬含量对ECAP铜铬合金组织和显微硬度的影响

制备不同质量百分含量铬的铜铬合金试样,对合金试样进行等径角挤压(ECAP)处理。然后采用金相显微镜和硬度测试仪等进行合金微观组织和力学性能的研究。结果表明,铬合金元素在合金中以颗粒状均勺分布,ECAP加工的道次越多,晶粒细化程度越大。其中Cu-7.5wt%Cr和Cu-11.25wt%Cr合金经ECAP加工处理后晶粒细化效果最为明显。同时,经过ECAP后试样各表面的显微硬度值均有不同程度的增加。     

Steam oxidation resistance of new 12%Cr steels: Comparison with some other ferritic steels

The oxidation resistance in pure steam at the 600-650 °C temperature range of a newly developed 12%Cr steel has been investigated for long-term exposures (224 days = 5,376 h). The laboratory and industrial heats were tested in comparison with other ferritic 9-13% chromium steels. Corrosion rates were determined by direct measurements of mass losses obtained after a reducing descaling process. Weight loss and metallographic results confirm the good corrosion resistance in steam of the new steel and allow classing the tested steels in 2 families: one classical with average oxidation behaviour, “T91-type” and another one with low mass losses, varying very slightly with the temperature and the exposure time increasing. To have a better understanding of the observed phenomena, the possible influences of the main alloying elements (Cr, Si, Mn, Mo, W) of steels mentioned by different authors were reviewed and compared to the results obtained for the ten 9-13%Cr studied steels. It appears that the alloying elements cannot be considered separately: as a matter of fact they have not only a specific influence but also a joint influence on the steam corrosion behaviour of the 9-13%Cr ferritic steels.     

等通道转角挤压Zn-22Al合金在NaCl水溶液中的腐蚀行为

研究了等通道转角挤压 (ECAP) 工艺处理对Zn-22Al合金耐腐蚀性能的影响,考察了显微组织演变对Zn-22Al合金在3.5% (质量分数) NaCl溶液中的失重及电化学腐蚀行为的影响。结果表明：相比铸态Zn-22Al合金,经过ECAP处理后的试样组织明显细化,晶粒尺寸均匀。晶粒细化导致晶界大量增加及应力增大,试样表面缺陷增多,富铝相被优先腐蚀后,导致被富铝相包围的富锌相脱落,腐蚀速率加快,腐蚀失重随着挤压道次的增加而增加。电化学测试结果表明,随着挤压道次的增加,Zn-22Al合金腐蚀电流密度逐渐增加,腐蚀电位逐渐下降,合金的耐腐蚀性能随挤压道次的增加而逐渐降低。     

Effects of ECAP Extrusion on the Microstructure,Mechanical Properties and Biodegradability of Mg-2Zn-xGd-O.5Zr Alloys

Effects of equal channel angular pressing (ECAP) extrusion on the microstructure,mechanical properties and biodegradability of Mg-2Zn-xGd-0.5Zr (x =0,0.5,1,2 wt％) alloys were studied in this work.Microstructure analysis,tensile test at ambient temperature,immersion test and electrochemical test in Hank's solution were carried out.The results showed that Gd could further enhance the grain refinement during the ECAP extrusion.Both Gd addition and ECAP extrusion could improve the mechanical properties of the alloys,and the extrusion played the dominant role.Minor addition of Gd (0.5-1 wt％) could obviously enhance the corrosion resistance of the alloys.To some extent,ECAP extrusion improved the corrosion resistance of the alloys due to the change of second phases distribution and the refinement of grains.Further increase in extrusion pass was detrimental to the improvement of the corrosion resistance as a result of increment of the grain boundaries.     

Der Einfluß des Prüpotentials und der Wärmebehandlung auf das Korrosionsverhalten niob- und titanstabilisierter ferritischer Chromstähle mit rund 17% Chrom

The influence of test potential and heat treatment on the corrosion behaviour of ferritic chromium steels stabilized with niobium or titanium By means of corrosion-chemical, electrochemical, metallographical and electronmicroscopical investigations, the influence of the electrode potential and the heat treatment on the corrosion behaviour of steels of the X 8 CrTi 17 and X 8 CrNb 17 has been determined. With ferritic 17 pC chromium steels, a distinction must be made between two different types of grain boundary corrosion:

• (1) Grain boundary corrosion on steels which were quenched at high temperatures and therefore sensitized, unstabilized or understabilized, due to the segregation of chrome-rich carbides at the grain boundaries, causing a chromium reduction in their vicinity (typical inter-crystalline corrosion), and
• (2) Grain boundary corrosion on steels quenched at high temperatures but fully stabilized, due to the chemical dissolution of the type MX carbonitrides segregated at the grain boundaries during quenching (carbide corrosion).

The heat treatment conditions conducive to carbide corrosion were determined, and the correlation of this type of corrosion with the potential was ascertained by potentio-static tests in sulphuric acid and compared with the behaviour of synthetic carbides. The different corrosion behaviour of the ferritic chromium steels quenched at high temperature and stabilized with niobium and titanium, respectively, is attributed to the different chemical dissolution rates of the carbo-nitrides segregated. The findings also provide an explanation of the corrosion behaviour of sensitization-annealed, stabilized austenitic chrome-nickel steels in acid solution.     

Influence of microstructure on mechanical properties and corrosion behavior of 3%Cr steel in CO2 environment

Corrosion behavior and mechanical properties of two 3%Cr pipeline steels were studied which have same chemical compositions but different microstructures, bainite‐ferrite and ferrite‐pearlite microstructures. Analysis of chemical compositions and surface morphology of the corrosion scale were performed using X‐ray diffraction and scanning electron microscopy in conjunction with energy dispersive X‐ray spectroscopy. The semi‐passive character of corrosion scale was investigated using polarization curves measurements. The influence of microstructure on the chromium enrichment in the corrosion scale for different test periods was investigated. The microstructure was observed to affect the corrosion behavior even when the steel surface has been covered with corrosion scales and continue to have an effect until the corrosion scale possessing semi‐passive character. Possessing both good strength and corrosion resistance, 3%Cr steel with bainite‐ferrite microstructure was very promising for pipeline steel applications.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Untersuchungen über den Einfluß der Wärmebehandlung auf die Korrosion titanstabilisierter ferritischer Chromstähle in siedender konzentrierter Salpetersäure

Investigations into the influence of the thermal treatment on the corrosion of titanium-stabilized ferritic chromium steels in boiling concentrated nitric acid Titanium-stabilized ferritic chromium steels with about 17% Cr are, after quenching from high temperature, susceptible to grain boundary corrosion in boiling nitric acid; this corrosion is attributable to the chemical dissolution of the titanium carbonitrides coherently segregated at the grain boundaries. This carbide corrosion can be reduced by heat treatment within the temperature range around 850°C. In this process, the segregated carbo-nitrides are formed-in (in-formation annealing). The influence of quenching (0.5–5 hours, 900–1200°C/W[L]) and subsequent in-formation annealing (1–30 hours, 850°C/W) on the corrosion rate in boiling 65% nitric acid has been investigated in detailed on two steels of types X 8 Cr Ti 17 and X 8 Cr Mo Ti 17. In addition, the corrosion behaviour of titanium-stabilized and unstabilized ferritic chromium steels with about 17 pC chromium have been compared with each other.     

用EPR法评价奥氏体不锈钢的敏化程度——材质因素及敏化条件的影响

用电化学方法(EPR 法)研究了不同材料和各种敏化条件对奥氏体不锈钢敏化程度的影响,并采用摸拟贫铬区的 Fe-11%Ni-Cr(6～18%)钢,研究了 EPR 法的特性.发现对不含 Mo 的钢而言,贫铬区的铬含量在16%以下时,就发生再活化溶解,因此,EPR 法比 Strauss 法更灵敏。试验后试样表面的金相观察发现:在晶界或夹杂物周围,首先发生方向性侵蚀点,然后,连结成腐蚀沟.     

纯镁切应力变形后的微观组织演变、力学性能、腐蚀行为

纯镁为密排六方结构,具有较少的独立滑移系导致其塑性较差。研究了纯镁变形后的微观组织演变、力学性能、腐蚀行为。结果表明,纯镁经过等径角挤压（ECAP）变形后晶粒明显细化以及基面织构发生了弱化,导致纯镁的塑性得到了显著地提高。等径角挤压变形后纯镁强度降低主要是因为基面织构弱化影响大于晶粒细化。此外,等径角挤压变形后纯镁自腐蚀电位和腐蚀电流密度明显增加,纯镁的抗腐蚀性能显著提高。纯镁的腐蚀机理可能从局部腐蚀向均匀腐蚀转变,从而减少了样品在标准模拟体液浸泡中的腐蚀脱落,确保了试样的完整性。     

氮对高纯奥氏体不锈钢耐晶间腐蚀性能的影响

采用电化学、化学浸渍、俄歇电子能谱分析、物理化学相分析等方法研究了氮对高纯奥氏体不锈钢000Cr19Ni14耐敏化态晶间腐蚀和非敏化态晶间腐蚀性能的影响,并探讨了其作用机理.结果显示,高纯奥氏体不锈钢加氮合金化(≤0.20%)基本上消除敏化态晶间腐蚀,敏化处理不会引起晶界贫铬;当氮含量不超过0.087%时,对非敏化态晶间腐蚀影响很小,而超过0.087%时,由于在晶界氮元素的偏聚以及氮化铬的析出加速非敏化态晶间腐蚀.     

Effect of microstructural features on the corrosion behavior of severely deformed Al–Mg–Si alloy

The aim of this study was to determine the influence of severe plastic deformation processing and the changes in microstructure resulting therefrom on the corrosion resistance of an Al–Mg–Si alloy. The alloy was processed using incremental equal channel angular pressing, which caused a reduction in grain size from 15 to 0.9 µm. The grain refinement was accompanied by an increase in the number of grain boundaries and dislocations, and by changes in grain orientation. However, there was no change in the size and number of intermetallic particles, which presumably resulted in a constant number of galvanic couplings. Electrochemical experiments revealed only slight differences between the samples before and after processing. Higher potential transients/oscillations upon immersion and increased corrosion currents in the vicinity of corrosion potential point to slightly higher reactivity of the most refined material. This indicates that intermetallic particles are the most crucial microstructural elements in terms of corrosion resistance. Their impact exceeds that of grain boundaries, in particular, at the stage of corrosion initiation. The development of corrosion attack is controlled more by the microstructure of the matrix as the grain refinement resulted in a less pronounced corrosion attack in comparison with the coarse-grained sample.     

Improving corrosion resistance of Al‐11mass%Si alloy through a large number of ECAP passes

Ultrafine‐grained (UFG) Al‐11mass%Si alloy, processed by multi‐pass equal‐channel angular pressing (ECAP) at 573 K, was investigated on corrosion behavior in 0.6 M NaCl solution. Potentiodynamic polarization tests and scanning electron microscopy observation showed that a large number of ECAP passes resulted in lower corrosion current density, more positive corrosion potential, and rather smooth corroded surface with shallow corrosion pits. The uniform distribution of fine secondary‐phase particles on UFG Al matrix weakened the susceptibility to pitting corrosion while inhibited general microgalvanic reactions. The present results indicate that grain refinement of aluminum matrix to the UFG state and uniform redistribution of broken particles (including eutectic silicon and secondary phases), via severe plastic deformation at elevated temperature undergoing dynamic recrystallization, can significantly improve the corrosion resistance of Al alloys, besides the known exceptional mechanical advantages. The simple and effective ECAP procedure makes UFG Al alloys more attractive for high strength structural application in corrosive environment.     

Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).     

On the contribution of carbides and micrograin boundaries to the creep strength of tempered martensite ferritic steels

The objective of the present study is to find out how much micrograin boundary carbides contribute to the creep strength of tempered martensite ferritic steels. For this purpose we compare the creep behaviour and the microstructural stability of a tempered martensite ferritic steel (German grade: X20) with that of a strongly deformed binary Fe10Cr alloy. The binary Fe10Cr alloy was subjected to equal channel angular pressing (ECAP). The ECAP parameters are adjusted such that the Fe10Cr alloy develops an ultrafine grain microstructure which is very similar to the microstructure of X20 with one important difference: there are no carbides on micrograin boundaries. We perform creep experiments for both materials and compare the creep curves as well as the evolution of microstructures during creep. The results show that boundary particles keep ultrafine grain structures from recrystallising. They moreover slow down micrograin boundary migration and impede reactions between dislocations and subgrain boundaries. It is concluded that micrograin boundary carbides reduce creep rates by several orders of magnitude.