Die technischen Aspekte der Bildung von Oxidschichten und deren Verhütung

Industrial aspects of the formation of oxide layers and their prevention The corrosion losses acceptables in steam generators of nuclear power plants are about 2.25 mm in 30 years; this is why unalloyed or low alloy steels can be used only to a limited extent in hot carbon dioxide (during the first 2000 to 3000 hours the oxidation follows a parabolic law, afterwards breakaway is possible). Silicon killed steel are superior to rimmed steel. Low sulfur contents are favourable, while surface treatment with dichromate solution is generally deleterious because the dichromate formed is decomposed during oxidation and the resulting chromium enrichment facilitates oxygen diffusion and enhances oxidation. Tubing suspensions may produce tensile stresses in crevices, so that screws are even torn out. Likewise, fretting may occur under simultaneous oscillating loads. In the case of ZrCu alloys the oxidation is slower than that of pure zirconium, probably because the large atomic volume of copper makes oxygen diffusion more difficult.     

The influence of surface oxide composition and structure on carburization in carburization in carbon monoxide – carbon dioxide

The oxidation and carburization of two 20 Cr 30 Ni alloys with nominally 0 and 0.5 wt % Si have been investigated by cyclic exposure to carbon monoxide and carbon dioxide at 1000°C. In carbon monoxide a chromium oxide film is formed which inhibits further carbon deposition. In carbon dioxide an iron chromium oxide of the spinel type is formed which during subsequent exposure to carbon monoxide acts as a catalyst for carbon deposition. In some areas a breakthrough of the initially formed chromium oxide layer occurs during carbon dioxide treatment. Extensive carbon penetration takes place in these breakthrough areas. The spinel formation and the breakthrough is retarded in the silicon-containing steel owing to the formation of a thin continuous silica layer at the metal-chromium oxide interface. This has a beneficial effect on the carburization resistance. The results also imply that it is possible to lower the catalytic effect of the tube material on carbon deposition or even on coke formation by controlling the surface oxide composition and structure.     

Elektrochemisches Verhalten und Deckschichtbildung hochlegierter Manganstähle

Electrochemical behaviour and scaling of high alloy manganese steels Passivating surface layers are considered to be one of the indispensable requirements for stress corrosion cracking of metallic materials. It is shown by potentiostatic and potentiokinetic current density-potential curves that the steel X 40 MnCrN 19 in neutral aqueous chloride solutions has a passive potential region. The passivation behaviour Of precipitation hardened samples is in agreement with the chromium depletion theory. The effect of alloying on the passivation behaviour of low carbon Mn steels is studied in 3 % NaCl solution at 20 and 100 °C Increasing proportions ε-martensite reduce the passivation of susceptibility. Increasing the Mn content has the same effect. The vital factor concerning passivation behaviour, however, is chromium content. Increasing the temperature of the corrodent results in an increased tendency to form scales of steels containing less than 8 % Cr. Long-term corrosion tests have shown, that increasing the Cr content produces a continuous transition from general localized and even pitting Corrosion. Tests made without applied current in aerated solutions have shown, that the variation in time of corrosion potentials depends from the tendency to be passivated of the materials and from the oxygen content of the solutions. In oxygen containing solutions passivable steels exhibit a pronounced corrosion in the pitting region, because with such alloys anodic dissolution current densities equal to those of the limiting diffusion current of oxygen reduction are obtained only at potentials above the pitting potential.     

铬铝多元共渗层组织和性能的研究

用固体铝热法对碳钢和中低合金工具钢进行铬铝多元共渗，对其渗层组织，相组成，硬度及耐蚀性，抗氧化性等进行研究。结果表明：铬铝多元共渗的外层组织均由含铬的碳化物，金属间化合物组成，具有较高的硬度和较优的耐蚀性及抗氧化性；铝量影响共渗的扩散层，硅量影响共渗层的致密性和脆性，它们都能使共渗层层厚显著增加；扩散层主要由铝铁化合物组成，硬度较低，它们也具有优良的耐蚀性和抗氧化性。因此，碳钢或中低合金钢及热作模     

含铬低合金钢在海水中的腐蚀研究

研究了含铬低合金钢在海水中的腐蚀行为 ,分析了其腐蚀率随时间的变化及铬元素对钢海水腐蚀行为的影响 .对铬钢的耐蚀性逆转和铬元素对钢海水腐蚀的影响提出了新的看法 .含铬低合金钢在海水中短期浸泡的耐蚀性比碳钢好 ,长期浸泡的耐蚀性比碳钢差 .碳钢的腐蚀率随时间逐渐下降 ,而含铬低合金钢的腐蚀率随时间逐渐上升 .铬元素使钢在海水中的初始腐蚀率降低 ,但它同时改变了钢的腐蚀率随时间下降的性质 ,使钢的腐蚀率逐渐上升 ,从而导致含铬低合金钢在海水中长期暴露的耐蚀性与碳钢发生逆转 .     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

The nanostructure and microstructure of steels: Electrochemical Tafel behaviour and atomic force microscopy

The influence of chemical composition and heat treatment on a low-carbon steel, chromium steel and high speed steel has been examined by polarisation curves and electrochemical parameters deduced from the Tafel plots. The electrochemical corrosion resistance, which is small between the as-received steels become greater after heat treatment, following the order: carbon steel < chromium steel high speed steel. To explain these differences, the nano- and microstructure of the steels has been characterized by the ex situ techniques of atomic force microscopy and optical microscopy, before and after surface etching with Nital (a solution of 5% HNO₃ in ethanol). This causes preferential attack of the ferrite phases showing the carbide phases more clearly. From these nanostructural studies it was possible to better understand why the passive films formed on chromium steel and high speed steel have superior protective properties to those formed on carbon steel.     

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。     

Korrosionsbeständigkeit von CrNi-Stahlguß (Typ 18/9 bzw. 18/13) mit erhöhtem Siliciumgehalt in konzentrierten Salpetersäurelösungen

Corrosion resistance of chromium nickel cast steel (types 18/9 and 18/13) within increased silicon content in concentrated nitric acid solutions The corrosion resistance of CrNi cast steel, types 18/9 and 18/13, with different carbon (0.035-0.12%) and silicon (appr. 4%) contents was investigated in nitric acid (27-96%) eventually containing Cr⁶⁺ ions between 25 and 100°C. In this connection mechanical properties and structure of the above cast steels were determined, too. It has been found that in the case of 80% nitric acid solutions and of nitric acid solutions of higher or lower concentrations but containing Cr⁶⁺ ions the cast steel must have a silicon content of appr. 4% together with a carbon content as low as possible. The results of the laboratory tests have been confirmed unter industrial conditions.     

Impact of oxyfuel atmospheres H2O/CO2/O2 and H2O/CO2 on the oxidation of ferritic–martensitic and austenitic steels

In future power plant technologies, oxyfuel, steels are subjected to steam rich and carbon dioxide rich combustion gases. The effect of simulated combustion gases H₂O/CO₂/O₂ (30/69/1 mol%) and H₂O/CO₂ (30/70 mol%) on the corrosion behavior of low alloyed steels, 9–12% chromium steels and an austenitic steel were studied. It was discovered that the formation of protective chromium rich oxides is hampered due to the carburization of the base material and the formation of chromium rich carbides. The kinetics of corrosion and carburization are quantified. The effect of temperature and the effect of gas pressure are analyzed statistically.     

Wasserstoffinduzierte Spannungsrißkorrosion an unverzinkten und verzinkten Baustählen

Hydrogen induced stress corrosion cracking of non galvanized and galvanized construction steels The processes of atmospheric corrosion and corrosion in collected water which may lead to hydrogen induced stress corrosion cracking of high-strength reinforcing steels in casing tubes before injection with concrete are discussed. Hydrogen uptake during corrosion occurs in weakly acid solutions as well as in neutral or alkaline aqueous solutions. The hydrogen uptake by proton discharge in acid solutions decreases with increasing pH of the electrolyte. Hydrogen can also be absorbed in neutral to weakly alkaline solutions if steels are plastically deformed and water reacts with the fresh iron surface. In alkaline solutions, hydrogen uptake is possible if, at the generally passivated steel surface, localized corrosion (pitting or crevice corrosion), local galvanic cells and a sufficient decrease in the pH of the pit electrolyte occurs. In the case of galvanized steels with damaged zinc layers, hydrogen uptake may result from the cathodic polarization of the free steel surface by zinc dissolution. The absorbed hydrogen interacts with the microstructure of the steels and weakens the bonds between the iron atoms. The influence of the microstructure of high-strength steels on the fracture behaviour is discussed on the basic of the so-called decohesion theory.     

Corrosion of carbon steels in high-temperature water studied by electrochemical techniques

Electrochemical methods were used to study the corrosion behavior of five Fe-Cr alloy steels and 304L stainless steel in high-temperature water. Passivity can be achieved on A-106 B carbon steel with a small content of chromium, which cannot be passivated at room temperature. The formation rate and the stability of the magnetite film increased with increasing Cr content in the steels. A mechanistic model was developed to describe the corrosion processes of steels in high-temperature water. The crack growth rate on steels was calculated from the maximum current of the repassivation curves according to the slip-oxidation model. There was a high crack growth rate on 304L stainless steel in high-temperature water. Of the four Fe-Cr alloys, the crack growth rate was lower on 0.236% Cr- and 0.33% Cr-steels than on 0.406% Cr steel and 2.5% Cr-1% Mo steel. The crack growth rate on 0.33% Cr steel was the smallest over the tested potential range.An increase in Cr content in the steel is predicted to reduce the corrosion rate of steel at high temperatures. However, this increase in Cr content is predicted not to reduce the susceptibility of steel to cracking at high temperatures.     

Influence of chromium,molybdenum and cobalt on the corrosion behaviour of high carbon steels in dependence of heat treatment

High chromium martensitic steels are designed to provide high corrosion resistance in combination with high strength. Some of these steel grades contain primary carbides for improving the wear resistance, e.g. the steel 440C. The present paper mainly deals with the effect of chemical composition and microstructure on the corrosion properties. Different experimental alloys were produced in the shape of small ingots. The influence of the alloying elements chromium, molybdenum, cobalt, and carbon on the corrosion properties was studied. The results can be summarized as follows: Chromium and molybdenum improve the corrosion resistance, however, only the content of these elements in solid solution in the steel matrix is effective. In case of cobalt the corrosion resistance decreases. The reason is the interaction between cobalt and carbon and its effect on the chromium content in the steel matrix. The calculated pitting resistant equivalent number of high chromium martensitic steels is only limited valid, because there is a major effect of carbide precipitation on the corrosion behaviour. Further investigations were focused on the heat treatment. Especially the effect of the tempering temperature of these steels was studied. The tempering temperature is most relevant for secondary hardening carbide precipitation, which lowers the chromium content of the matrix with detrimental influence on the corrosion properties. The carbide precipitation and chromium distribution was characterized by means of energy filtered transmission electron microscopy (EFTEM).     

Silicon surface treatment via CVD of inner steel tube surfaces

Inner surfaces of steel tubes should be protected against corrosion and oxidation by a coating in order to assure optimised application in power plants and the chemical industry. Since the silicon content in chromium alloyed steels improves the oxidation protection. Chemical vapour deposition (CVD) of Si has been investigated for martensitic 9% chromium steel (T91). The surface treatment was performed as a combined process of CVD at atmospheric pressure with heat treatment in a chemical atmosphere with high frequency heating (HF), automated with a Simatic PCS7. The Si out of SiCI₄ diffused into the steel lattice of the substrate surface. As a result, the Si‐content near the surface layer increased as a ferrite layer which did not transform to martensite during heat treatment. The excess of Si was converted to Si₃N₄ using NH₃. This improves the anti‐oxidation behaviour. The physical and chemical properties of the modified surfaces were investigated and characterised using the SEM, EDX, electron microprobe and X‐ray diffraction methods. The deformation capacity of a coated specimen was proven by means of hot tensile tests. It could be determined that in the ferritic layer Fe is substituted by Si with a maximum concentration of approx. 5%, however the Cr content was almost constant.     

Resistance to Oxidation at Elevated Temperatures of a Number of Alloy Steels

Abstract

A wide range of steels, extending from a plain carbon steel to high chromium-nickel, austenitic types and some nickel-based alloys, have been subjected to a cyclic oxidation test in natural gas combustion products at various temperatures. Resistance to oxidation can be expressed in terms of a breakdown temperature which provides a convenient parameter by means of which the behaviour of steels and alloys may be compared. On this basis, the beneficial effects of additions of chromium of up to 30% are confirmed, as are additions of silicon and/or aluminium to chromium steels. Cobalt, copper, molybdenum, vanadium, tungsten, niobium, titanium and nitrogen have comparatively little effect. Manganese appears to be detrimental when added to chromium steels and carbon can be in some circumstances. Nickel can enhance oxidation resistance in some circumstances but is detrimental in others. The presence of sulphur in free-machining, stainless steels appears to counteract the effect of a higher-thannormal manganese content.     

Beitrag zur Methodik der Messung von Lochfraßpotentialen an austenitischen Chrom-Nickel-Stählen

Contribution to the methodology of pitting corrosion potential measurement on austenitic chromium nickel steels Reproducible values of the pitting potential and the incubation period are obtained only when definite techniques are applied to the specimen preparation. When the specimens are to be masked for the purpose of suspension they must be passivated without this mask because otherwise there would occur some type of crevice corrosion in the boundary zone between mask and open specimen surface. The corrosion potential in this is more negative (by 150 mV for a steel AISI 316) the the pitting potential as such. Preferential dissolution occurs in this zone when the pitting potential is attained. When, on the other hand, the passivation is effected without the mask, uniform pitting is achieved across the whole free surface and the pitting potential may be measured without difficulty. This potential turns out to be independent from the duration of passivation, while the incubation period distinctly increases with passivation time.     

Atmospheric corrosion of carbon steels pre‐corroded by different pollutants

Carbon steels were pre‐corroded in atmospheres containing certain pollutants such as sulfur dioxide or hydrogen sulfide or nitrogen dioxide for certain period of time. Then the pre‐corroded samples were further exposed at two central test stations in the country to evaluate the influence of pre‐corrosion on the behavior of long‐term atmospheric corrosion of carbon steel by gravimetric measurement and rust examinations with FT‐IR and XRD. The results showed that the corrosion rates of every kind of carbon steel at Jiangjin station were two times higher than that at Beijing station. The influence of different pre‐corrosion conditions on the long‐term atmospheric corrosion of carbon steel was analyzed. It followed that the pre‐corrosion conditions made the long‐term atmospheric corrosion performance of three kinds of carbon steels different, but the differences were small. Among the three pollutants, NO₂ showed the strongest influence to the process. As the exposure time was going on, all the samples corroded slowly at Beijing and Jiangjin stations.     

The factor time at the formation and development of the passive layer in the atmosphere

Passive layers on high alloyed chromium nickel steels protect the metal from corrosion. The protective effect of the passive layer, however, does not immediately exist after the final surface treatment. Some time is necessary to form the protective layer. The exposure conditions are also decisive for the formation and further development of the passive layer besides the surface treatment. Samples of different chromium nickel steels are pickled and active passivated before they are exposed under laboratory conditions and in outdoor climates at different locations. The state of the corrosion resistance of the surface is determined with electrochemical noise measurements after different exposure times. The decreasing activity of the surface with an increasing exposure time indicates the further formation process of the protective qualities of the passive layer. An index which quantifies the state of the surface at the time of the measuring is calculated from the measured potential and potential noise signals. With these results, the development of the passive layer can be assessed and the corrosion resistance of the metal can then be estimated.     

Interkristalline Korrosion ferritischer Chromstähle mit rd. 17 Gew.-% Chrom nach Glühen im Temperaturbereich um 500 °C

Intergranular corrosion of ferritic 17% chromium stainless steels after heat-treatment in the 500 °C temperature range After stabilizing heat-treatment at 750°C, the non-stabilized, ferritic 17% chromium stainless steel Mat.-Nr. 1.4016 (X8Cr17) still contains a sufficient high concentration of carbon dissolved in solid solution, that after heat-treatment in the 500 °C temperature range carbides rich in chromium of the M₂₃C₆-type are precipitated, causing a relatively weak pronounced susceptibility of the steel to intergranular attack. The susceptibility to this type of attack can be detected by testing specimens in the sulfuric acid-copper sulfate-test with increased concentration of sulfuric acid as compared with the DIN standard 50914, followed by metallografic examination of the specimens. The susceptibility to intergranular corrosion of the material investigated occurring after heat-treatment in the low temperature range, which until now is unknown, is described in terms of a Rollason-diagram as it is commonly used for austenitic chromium-nickel stainless steels. As it is to be expected, the stabilized 17% chromium steels Mat. No. 1.4510 (X8CrTi17), 1.4511 (X8CrNb17) and 1.4523 (X8CrMoTi17) are resistant to intergranular corrosion after heat-treatment at low temperatures.     

The Corrosion Behavior of Four Commercial Steels in Reducing Atmospheres Containing HCl At 773–873 K

The corrosion of four Fe–Cr commercial steels with different chromium contents in a simulated waste-gasification atmosphere containing 0.5 vol. H₂, 0.5 vol. HCl, balance CO₂ has been investigated at 773 and 873 K. The same materials have also been tested in the same gas mixture free from HCl at both temperatures for comparison. The results show that the materials with low-chromium content (2.25CrMoV and NF616) undergo accelerated corrosion in the presence of HCl, while the stainless steel SS304 suffers very little corrosion. On the contrary a steel containing 12 wt. Cr (12CrMoV) corrodes rather rapidly at 773 K but quite slowly at 873 K. The beneficial effect of chromium on the corrosion resistance of the steels increases with the chromium content at both temperatures. The steels tested show corrosion rates generally decreasing with time, having kinetics which are approximately parabolic at 773 K but intermediate between parabolic and linear at 873 K. Only little or even no chlorine can be detected at the scale/metal interface at both temperatures for all materials corroded in HCl-containing atmospheres. The corrosion mechanism can be explained by the so-called active-oxidation model.