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.     

Optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion

The main objective of the EU OPTICORR project is the optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion, the development of a life‐cycle approach (LCA) for the materials in energy production, particularly for the steels used in waste incinerators and co‐fired boiler plants. The expected benefits of this approach for safe and cost effective energy production are: ‐ control and optimisation of in‐service performance of boiler materials, ‐ understanding of high‐temperature corrosion and oxidation mechanisms under service conditions, ‐ improvement of reliability to prevent the failure of components and plant accidents and ‐ expanding the limits of boiler plant materials by corrosion simulations for flexible plant operation conditions (steel, fuel, temperature etc.). The technical aim of the EU OPTICORR project is the development of modelling tools for high‐temperature oxidation and corrosion specifically in boiler conditions with HCl‐ and SO₂‐containing combustion gases and Cl‐containing salts. The work necessitates thermodynamic data collection and processing. For development and modelling, knowledge about the corrosion mechanisms and exact data are needed. The kinetics of high‐temperature oxidation and corrosion are determined from laboratory thermo‐gravimetric tests (TG) and multi‐sample exposure tests. The materials studied are typical boiler tubes and fin‐steels: ferritic alloys, the austenitic steel T347 and the Ni‐based alloy Inconel 625. The exposure gases are dry air, air with 15 vol‐% H₂O, and with 2000 ppm HCl and 200 ppm SO₂. The salt deposits used are based on KCl‐ZnCl₂ and Ca, Na, K, Pb, Zn‐sulfates. The test temperatures for exposures with deposits are 320 and 420°C and, for gas exposures, 500 to 600°C. At present the tools being developed are ChemSheet based programmes with a kinetic module and easy‐to‐use interface and a more sophisticated numerical finite‐difference‐based diffusion calculation programme, InCorr, developed for prediction of inward corrosion and internal corrosion. The development of modelling tools for oxidation and high‐temperature corrosion was started with thermodynamic data collection for relevant systems and thermodynamic mappings. Further, there are needs to develop the simulation model and tool for salt‐induced hot corrosion based on the ChemSheet approach. Also, the work on modelling and simulating with the InCorr kinetic modelling tool will be continued to demonstrate the use of the tool for various steels and alloys in defined combustion environments.     

Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

The oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.     

Hochlegierte korrosionsbeständige Stähle für Chemie‐, Energie‐ und Meerestechnik – Rückblick und Ausblick

High‐alloyed corrosion resistant steels for the chemical process industry, power engineering and marine technology – past and future Today's most common high‐alloyed corrosion resistant steels are in their majority characterised by very low contents of carbon and sulphur and, in many cases, by substantial amounts of nitrogen as an alloying constituent. Their broad use in the chemical process industry, power generation and marine technology has become possible when new metallurgical processes for steel making had been introduced in the 1960s. The time before had seen mainly stabilised grades, being highly alloyed with copper in many cases, which have disappeared to a large extent in our days. The superferritic grades (ferritic steels with ≥ 25% chromium) had been the materials of great expectations in the 1970s, but have found a very limited application only in the chemical industry since then, e.g. for the handling of hot concentrated sulphuric acid, due to the high risks of low ductility cracking of these materials at greater wall thickness. These risks can be managed better if the highly alloyed ferritic phase is present in a finely dispersed compound with an austenitic phase where the ferritic part is adding its advantages, higher strength and resistance to stress corrosion cracking, to the duplex compound. This can result in low weight and corresponding cost saving. The application of the corrosion resistant duplex grades will expand further as much as users will better learn the special requirements of manufacturing of these materials and to take advantage of their unique properties. However, the most important alloy developments since the 1960s have been seen in the field of the austenitic stainless steels being highly alloyed with chromium, molybdenum and nitrogen. Especially the austenitic 6% Mo grades as e.g. X1NiCrMoCuN25‐20‐7 – alloy 926 (1.4529) have found many applications in chemical process industry, power generation and marine technology. Higher alloyed grades as e.g. X1NiCrMoCu32‐28‐7 – alloy 31 (1.4562) are excelling in extraordinary resistance to corrosion by acids and pitting attack. In addition today's upper limits of alloying austenitic corrosion resistant grades have been explored with grade X1CrNiMoCu33‐32‐1 – alloy 33 (1.4591) for chromium additions up to about 33% and with grade X1NiCrSi24‐9‐7 – alloy 700 Si (1.4390) for additions of silicon up to about 7%, providing a high corrosion resistance mainly in oxidising acids. When considering the prospects of further development of the corrosion resistant duplex grades the ferritic phase within these materials is both offering chances and setting limits. The high‐alloyed austenitic corrosion resistant steels have a potential being unexplored so far in the alloy range where molybdenum and nitrogen are becoming more prominent compared to the chromium content.     

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.     

In-situ corrosion investigation at Masnedø CHP plant – a straw-fired power plant

Various austenitic and ferritic steels were exposed on a water-cooled probe in the superheater area of a straw-fired CHP plant. The temperature of the probe ranged from 450–600°C and the period of exposure was 1400 hours. The rate of corrosion was assessed based on unattacked metal remaining. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. The ferritic steels suffered from corrosion mainly via material loss. The austenitic steels suffered from predominantly selective corrosion which resulted in depletion of chromium from the alloy. A clear trend was observed that selective corrosion increased with respect to the chromium content of the alloy.     

Hochlegierte nichtrostende Stähle und Nickelbasislegierungen im Chemie-Apparatebau

High alloy stainless steels and nickel-base alloys in chemical equipment construction The new ferritic chromium steels and ferritic-austenitic steels, high alloy special austenitic steels and nickel-base alloys presented in recent years have by now yielded some years' positive industrial experience in several applications. The group of austenitic steels containing 6 wt.-% molybdenum are at the threshold for large size application in chemical equipment construction. The results of the first comparative investigations with other high alloy steels are rather promising. The spectrum of nickel-base alloys will certainly be complemented by Hastelloy alloy C-22, so that this alloy will be used in addition to titanium alloys in contact with strongly oxidizing media.     

Oxidation kinetics of Y‐doped FeCrAl‐alloys in low and high pO2 gases

A model Fe‐20Cr‐5Al‐0.05Y alloy was oxidized in Ar‐20%O₂ and Ar‐4%H₂‐7%H₂O at 1200–1300 °C. Two‐stage oxidation experiments using oxygen isotope tracers showed that inward oxygen diffusion was predominant in both gases, but more isotope exchange was observed in the H₂/H₂O gas reaction. The alumina scales formed in both gases were composed of columnar grains, the lateral size of which increased linearly with depth beneath the scale surface. Thermogravimetric measurement of oxygen uptake revealed kinetics which were intermediate to parabolic and cubic kinetic rate laws. A model based on grain boundary diffusion control coupled with competitive oxide grain growth accounts satisfactorily for the results when the requirement for a divergence‐free flux within the scale is imposed. This treatment shows that the oxide grain boundary diffusion coefficient is lower when H₂O is the oxidant. It is concluded that hydrogen slows the grain boundary diffusion process by altering the nature of the diffusing species.     

Temperatur- und Konzentrationsabhängigkeit der Korrosion nichtrostender austenitischer und ferritischer Werkstoffe in 20- bis 75%iger Salpetersäure

Corrosion resistance of austenitic and ferritic stainless alloys in 20 to 75% nitric acid as a function of temperature and concentration A series of stainless austenitic and ferritic materials was exposed for 100 days to boiling nitric acid which contained no corrosion products; the corrosion rates and depths of the grain boundary attack were observed. Provided the structure is precipitation-free, the following are suitable for long-term exposure; the austenitic steels X 2 CrNi 1912, X1 CrNi 25 21, X1 CrNiMoN 25 222 and X1 NiCrMoCu31274, the practically Mo-free and Cu-free development steel X1 NiCr31 27, and the highly Mo-alloyed variant X1 NiCrMoCu 31275. In the case of alloy NiCr21 Mo it is advisable to limit the concentration and/or the temperature of the nitric acid. The “superferrite” X1CrNiMoNb2842, the Japanese steel X1 CrNiNb 30 2 and the austenitic steels X2 CrNiMoN 1713 3 and X1 CrNiMoN 25 22 2 in the version with high nickel content are unsuitable. Thus, as an alloying element, molybdenum does not always impair the resistance of stainless steels to nitric acid. The decisive factor affecting the corrosion rates is the chromium content of the material. The temperature-dependent function of the corrosion in azeotropic nitric acid conforms to Arrhenius relations. The concentration-dependent function of the corrosion in 20 to 75 (80)% nitric acid can be described by a hyperbolic equation. An exception is formed by X1 CrNiSi 1815; here the corrosion rate increases with the concentration of the acid until the azeotropic point is reached; then, owing to the formation of a surface film, it falls until the acid becomes highly concentrated.     

Der Einfluß von Cer,Zirkonium und Bor auf die Oxidationsbeständigkeit hitzebeständiger Stähle an Luft

Influence of cerium, zirconium and boron on the oxidation resistance of heat-resistant steels in air Isothermal and cyclic oxidation experiments were carried out in air to investigate the influence of the minor elements such as Cerium, Zirkonium and Boron on the oxidation resistance of heat resistant ferritic and austenitic steels like X 10Cr 18, X 10CrAl 18 and X 15 CrNiSi 20 12. In the case of cyclic experiments samples were exposed at constant temperatures for 100 h and then cooled to R. T. This cycle was repeated 10 times. The corrosion was determined as weight change and was continuously measured by a thermo-balance. The distribution of the alloying elements on the phase boundary scale/steel was examined by Scanning-Electron-Microscope. Addition of small amounts of Ce (0.3 wt-% max.) could reduce the oxidation rate in the case of isothermal and cyclic conditions. Zirkonium concentrations below 0.1 wt-% could have a beneficial effect, but at higher concentrations the oxidation rate increases with increasing amounts of Zr. Small Boron concentrations of 0.02 wt-% lead to catastrophic oxidation at temperatures above 1000°C.     

Selektive Korrosion an Verbindungsschweißen von Stählen

Selective corrosion at welded assemblies of steels The paper deals with the possible selective corrosion phenomena at the most important types of weld assemblies of steels, viz. those of unalloyed ferritic ship building steels, austenitic manganesechromium steals, aluminium-killed boiler steels an well as chemically resistant ferritic and austenitic chrom-nickel steels. With unalloyed ship building steels, a strong corrosion element is liable to occur between not fully descaled plates and scaleless welding material, where the latter is particularly liable to be attacked. But even where scalefree plates are welded, a local element may be set up, depending on the killing intensity and on the filler metals, between plate and welding material in such a way that, again, the welding material is liable to be dissolved first. In the case of austenitic manganese-chromium steels, exposed to sea water, intercrystalline corrosion takes place at a certain distance from the weld as a result of chrome-carbide segregation. In the case of welded assemblies of aluminium-killed steels, intercrystalline fissuration is particularly liable to occure in a more or less marked decarbonised zone at the transition. With ferritic chromium steals and austenitic chrom-nickel steels, the welds are decisively influenced by the segregation of chrome-carbides at the grain boundaries which results in a proneness to intercrystalline grain decomposition. Measures are discussed by which the segregation of chrom-carbides during welding can be avoided. Finally, mention is made of the effect of delta-ferrite on the corrosion behaviour of austenitic welding material.     

Nitridation in NH3‐H2O‐mixtures

Exposures were conducted of iron, nickel, ferritic 1‐18%Cr steels, austenitic 18%Cr‐9%Ni‐ and 20%Cr‐31%Ni‐steels and a 16%Cr‐Ni‐base alloy at 500°C in He‐30%H₂O and 70%H₂O‐30%NH₃, to compare the corrosion behaviour of these materials in water vapor as in conventional power plants with their behaviour in a NH₃‐H₂O mixture, i.e. under conditions of the “Kalina‐cycle”. After 50 h in He‐H₂O generally a dense oxide scale had grown on iron and on the steels, whereas the scale grown in NH₃‐H₂O was porous, due to initial formation of the γ′‐ and ε‐nitrides, which are converted to Fe₃O₄ later. The porous scale allows internal nitridation of the Cr‐steels, nitrogen is transferred into the metal phase and reacts to finely dispersed CrN‐precipitates. This process causes stresses in the material and formation of cracks. The higher the Cr‐content of the material, the worse is the damage of the materials surface. Least corrosion damage occurs for iron and the 1%CrMo‐steel, however, the inward penetration of nitridation is greatest, and after 5 years on the low Cr‐steel a layer of about 15 mm would be embrittled by internal nitridation, formation of γ′ and ε‐nitride layers and external oxidation. Nickel is strongly damaged by intermediate formation of instable Ni₃N, which causes internal stresses and cracking, but also pore formation by its decomposition. The surface region of the 15%Cr‐Ni‐base alloy is also destroyed by internal nitridation and extrusion of Ni‐particles, while for this material the inward penetration of nitridation is relatively slow due to the low solubility and diffusivity of N in Ni and Ni‐alloys.     

A new austenitic alumina forming alloy: an aluminium‐coated FeNi32Cr20

The FeCrAl alloys owe their low oxidation rate to the formation of a slow growing α‐aluminium oxide scale. Therefore they are used, for example, as a substrate material in metal‐supported automotive catalytic converters. Increasing exhaust gas temperatures mean that, in addition to the oxidation properties, high temperature mechanical properties should also be improved. Compared to the ferritic FeCrAl alloys, austenitic alloys possess the required high mechanical strength at higher temperatures. However for most commercially available materials the oxidation resistance is not sufficient due to a low aluminium content. High aluminium contents are avoided in austenitic alloys, since they cause severe workability problems, even at aluminium contents, which are below the necessary amount to get a pure alumina scale. The newly developed material Nicrofer 3220 PAl (coated FeNiCrAl) consists of an austenitic FeNi32Cr20 alloy coated with aluminium on both sides. It combines the outstanding oxidation resistance of an alumina forming FeCrAl alloy with the advantage of the high temperature strength of an austenitic alloy. Additionally the oxidation is even lower than the oxidation of the commercial grade Aluchrom YHf (FeCr20Al6)—conventional homogenous FeCrAl. Aluminium coated FeNiCrAl can easily be formed into its final shape. Prior to service, an in situ heat treatment is recommended in order to optimize the properties.     

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.     

The Effect of Temperature on the Formation of Oxide Scales Regarding Commercial Superheater Steels

This study addresses the surface changes of three commercial steels (a low alloy ferritic 10CrMo9-10 steel, a Nb-stabilized austenitic AISI347 steel, and a high alloy austenitic Sanicro 28 steel) by comparing the oxide scale thicknesses, chemical compositions, and surface morphologies of samples after pre-oxidation at 200, 500 and 700 °C with different exposure times (5 and 24 h) under humid or dry conditions. With all three steels, the oxide scale thickness increased as functions of temperature and exposure time, the effect of temperature being more prominent than the effect of exposure time. The presence of water resulted in thicker oxide scales at the studied low alloy ferritic steel, whereas in the two austenitic steels, the presence of water increased chromium diffusion to the oxide scale rather than the scale thickness. The oxide layers characterized and analyzed in this paper will be further studied in terms of their abilities to resist corrosion by exposing them under corrosive conditions. The results regarding the corrosion resistance of the steels will be published in a sequel paper.     

Oxidation resistance of sintered stainless steels: effect of yttria additions

The porosity of sintered stainless steels modifies their oxidation behavior, as compared to that of wrought stainless steels. This work studies the oxidation behavior of three sintered stainless steels: one ferritic (AISI 434L) and two austenitic (AISI 316L and 304L). 304L with yttria additions is also been studied to explore the possibility of reducing the oxidation rate of austenitic stainless steels by using this reactive element. The results demonstrate the influence of the formation of NiFe₂O₄ on the high-temperature behavior of sintered austenitic stainless steels and the effectiveness of yttria additions in increasing the oxidation resistance at 800 °C.     

Untersuchungen der Hochtemperatursulfidierung von Stählen in H2/H2S- und H2/H2S/H2O-Gasgemischen

Investigations of high-temperature sulfidization of steels in H₂/H₂S- and H₂/H₂S/H₂O gas mixtures Various ferritic steels, one with aluminium as an alloying compound, and various austenitic steels were corroded in H₂S-H₂- and H₂S? H₂O? H₂-gas mixtures in the temperature range 300 to 700 °C. The overall pressure always was 1 bar and the H₂O partial pressure 0,1 mbar, whereas H₂S partial pressures were varied between 1 and 200 mbar. In thermogravimetric investigations the rate laws were found to be nearly linear for all ferritic steels and to a large extent parabolic for the austenitic steels. The sulfidization rate of all ferritic steels decreased with increasing chromium content, up to 18 wt% somewhat less significantly than for higher contents. The sulfidization rates of austenitic steels were decreased only moderately by increasing nickel content. The aluminium content of the steel X7 CrAl 15 5 reduced the constant of the linear rate law by more than one order of magnitude, as compared with aluminium-free steels. The improved corrosion resistance was caused by selective oxidation of aluminium, but the protection by Al₂O₃ was destroyed by creep deformation under mechanical stress. Cracks along grain boundaries were formed by stresses below 80 Mpa and for more than 80 Mpa transcrystalline cracks occurred. Metallographic examination revealed double layers on all steels. The outer layer of the ferritic steels predominantly consist of iron sulfide FeS, the inner layer, however, of a mixture of ironchromium sulfide. The outer sulfide layer of the austenitic steels contained apart from FeS chromium and nickel sulfides with low contents, too. In the inner sulfide layer a heterogeneous mixture of iron-chromium-spinells, iron-nickel-spinells and Ni₃S₂ was identified. Mechanisms for layer growth are proposed.     

Behaviour of NiAl APS‐coatings in chlorine‐containing atmospheres

Due to their thermal expansion coefficient being close to those of ferritic steels, NiAl atmospheric plasma spraying (APS)‐coatings have been chosen to be tested in chloridizing atmosphere. A new type of quasi‐stability diagram which couples thermodynamic and kinetic approaches was developed to define the stability domains of the pure metals aluminium and nickel in dynamic chlorine‐ and oxygen‐containing atmospheres. NiAl APS‐coatings (300 µm) with low porosity on steels were applied and tested in an atmosphere containing 0.2 v/o chlorine and 3 ppm oxygen. After 280 h of exposure one quarter of the outer coating was transformed into a protective alumina layer. Much nickel depletion, due to the evaporation of nickel chlorides, was observed.     

Grain-Size Effects on the High-Temperature Oxidation Behaviour of Chromium Steels

Oxidation of two low-Cr (Cr content 1.5 wt% and 2.25 wt%) and three high-Cr (Cr content 9 wt%, 12 wt% and 18 wt%) boiler steels was investigated at temperatures between 550 °C and 830 °C in laboratory air. Thermogravimetry (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied for evaluation of the oxidation kinetics, oxides phase identification and scale structure analysis. Particular attention was paid to the phenomenon of inward oxidation and its relationship with the Cr content of the steels under investigation. The results showed that the inward oxidation of the Cr steels is governed by grain boundary diffusion. Two different Cr-depending tendencies concerning the effect of alloy grain size on the inward oxidation were observed. For low-Cr steels (less than 2.25 wt% Cr), an increase in the grain size improved the oxidation resistance, while steels with high Cr content (18 wt% Cr) can form a thin and protective chromia scale on the surface more easily at finer grain size. In the latter case an increase in grain size deteriorates the oxidation resistance by the formation of a thicker scale composed of both an outer and an inner Fe-oxide-based layer.     

Der Einfluß des Titangehaltes auf die Aktivkorrosion ferritischer Cr-Stähle und austenitischer Cr-Ni-Stähle in Schwefelsäure

The influence of the Titanium content on the active corrosion of ferritic Cr steels and austenitic Cr-Ni steels in sulphuric acids Non-stabilized and Ti-stabilized steels of types X8Cr17, X8Crti17, X5CrNi189, X10CrNiTi189 and X5CrNiMoTi25 25 are compared with each other in respect of their behaviour in the potential range of active corrosion in sulphuric acid. With ferritic 17 per cent. chromium steels, no Ti influence was detected. With more highly alloyed austenitic Cr-Ni or Cr-Ni-Mo steels, the titanium had the effect of narrowing down the potential range of active corrosion and reducing the corrosion rate which facilitates the passivation capacity of the steels. These phenomena are particularly marked with the steel known as X5CrNiMoTi25 25. With this steel, the corrosion rate in the active-passive transition range with titanium contents from 0.41 to 0.66 per cent. and about 0.66 per cent. C ist increased in the sensitivized state. Within a narrow potential range of about 200 mV, intercrystalline corrosion is encountered which decreases with increasing Ti content and is completely prevented if the C content is reduced below 0.03 per cent. The potential range of the inter-crystalline corrosion is more negative than the test potential of the Strauss test normally used for testing the grain disintegration resistance.