Field test of superheater corrosion in a CFB waste boiler: Part II – Scale formation characteristics

This study concerns the scales formed on the steels T22 , Alloy 310, Alloy 28 and the nickel‐based Alloy 65 in a superheater test coil at 460–540°C in a CFB waste boiler. The methods used for the characterisation of the scales included SEM, EDX, Auger spectroscopy and XRD. The deposits on the tubes consisted mainly of alkali chlorides and calcium sulphate. The scales formed consisted of Fe₂O₃ and Fe₃O₄ on the T22 steel, NiFe₂O₃ and Cr₂O₃ on Alloy 310 and Alloy 28, and Cr₂O₃ and NiO on Alloy 65. Rapid corrosion on the steel T22 was associated with the growth of an open columnar iron oxide below a thick porous chlorine‐containing scale. Pitting corrosion on Alloy 310 occurred and it may be associated with selective corrosion, first following the grain boundaries then uniformly attacking the metal. The only protective oxide was observed on Alloy 28 that formed an inner chromium oxide separating the chlorides from the metal. Dense thin chromium oxides were observed in the scale on Alloy 28, but no major cracks were found perpendicular to the tube. Alloy 65 suffered from grain boundary attack and was locally attacked under thick porous chromium oxide with nickel chlorides in the advancing front. Molybdenum was enriched at the interface to the metal on both Alloy 28 and Alloy 65.     

Werkstoff- und Korrosionsprobleme in wäßrigen Lösungen bei hohen Temperaturen und Drücken

Materials and corrosion problems in aqueous solutions at high temperatures and pressures The present report, based on papers and discussions of an international conference on electrochemistry at high temperatures and pressures forms a survey of materials and corrosion problems at high temperatures and pressures. Among the topics of the conference there were, among others, corrosion problems in power stations with particular emphasis on high alloy chromium nickel steels and nickel base alloys which are preferentially used in nuclear power stations. Unalloyed and low alloy steels were lower on the scale of interests. A matter of particular interest, however, were investigations and findings on corrosion cracking failures, which may occur on chromium nickel steels and nickel chromium iron alloys under rather different conditions of heat treatment of the materials (solution annealed or sensitized) and of the corrosive medium. With reference to pertaining literature data a summary is presented of the present state of knowledge concerning the various type of cracking corrosion which may occur on the materials of interest, i. e. chromium nickel steels of the 18 8 type, Incoloy 800 und nickel base alloys of the Inconel 600 type.     

Influence of nickel additions on the corrosion behaviour of low nitrogen 22% Cr series duplex stainless steels

Four 22% Cr duplex stainless steel alloys containing varying nickel contents of approximately 5%, 7%, 9% and 13% were melted in a vacuum induction furnace, hot rolled, solution annealed and water quenched. Their corrosion behaviour was investigated in sulphuric acid and sodium chloride solutions using potentiodynamic polarization and chronoamperometric techniques. Increasing nickel contents have an appreciable influence on the observed corrosion behaviour. Polarization curves show similarities in the uniform corrosion behaviour of the alloys in the various solutions, but significant differences in their pitting resistance in chloride environments. Alloy 2209 (22% Cr - 9% Ni) generally exhibited the highest resistance to uniform corrosion, while alloy 2213 (22% Cr - 13% Ni) displayed the best pitting corrosion resistance. X-ray diffraction analysis of the surfaces of the alloys after corrosion in the different media revealed different corrosion products. Scanning electron micrographs revealed that the alloys were differently attacked by chloride ions, with different pit depths and morphologies. The corrosion behaviour exhibited by the alloys was attributed to the influence of nickel on phase modification, microstructural refinement, and the types of films formed in different environments. Uniform corrosion behaviour of the alloys was concluded to be predominantly controlled by phase composition and ratio, while pitting resistance was chiefly controlled by an alloy’s nickel contents.     

Einfluss der Halbzeugform auf die Korrosionsbeständigkeit hochlegierter Ni‐Cr‐Mo‐Werkstoffe

Effect of semi‐finished products on the corrosion resistance of high‐alloyed Ni‐Cr‐Mo materials The corrosion resistance of different semi‐finished products of six superaustenitic steels and nickel based alloys in the condition of delivery was investigated in some typical standard corrosion tests. The resistance of sheets, plates, strips, seamless tubes and welded tubes to intercrystalline corrosion was tested according to ASTM G 28 methods A and B, as well the resistance to pitting corrosion according to ASTM G 48 method C. The nickel based alloys 625, C‐276 and alloy 59 are resistant to the FeCl₃‐test according to ASTM G 48 method C and therefore a differentiation of these types in regard to their localized corrosion resistance was achieved only in the more aggressive ‘Green‐Death’‐solution. The laboratory experiments confirmed that the corrosion resistance is identical for all semi‐finished products and that it shows only a slight dependence of the surface condition of the materials tested. Additionally, some typical industrial and practical applications of the six high performance materials are presented to demonstrate the excellent corrosion resistance in the manufactured condition.     

Corrosion of aluminium,stainless steels and AISI 680 nickel alloy in nitrogen‐based fuels

Nitrogen‐based compounds can potentially be used as alternative non‐carbon or low‐carbon fuels. Nevertheless, the corrosion of construction materials at high temperatures and pressures in the presence of such fuel has not been reported yet. This work is focused on the corrosion of AISI Al 6061, 1005 carbon steel (CS), 304, 316L, 310 austenitic stainless steels (SS) and 680 nickel alloy in highly concentrated water solution of ammonium nitrate and urea (ANU). The corrosion at 50 °C and ambient pressure and at 350 °C and 20 bar was investigated to simulate storage and working conditions. Sodium chloride was added to the fuel (0–5 wt%) to simulate industrial fertilizers and accelerated corrosion environment. Heavy corrosion of CS was observed in ANU solution at 50 °C, while Al 6061, 304 and 316L SS showed high resistance both to uniform and pitting corrosion in ANU containing 1% of sodium chloride. Addition of 5% sodium chloride caused pitting of Al 6061 but had no influence on the corrosion of SS. Tests in ANU at 350 °C and 20 bar showed pitting on SS 304 and 316L and 680 nickel alloy. The highest corrosion resistance was found for SS 310 due to formation of stable oxide film on its surface.     

Application of High Temperature Corrosion-Resistant Materials and Coatings Under Severe Corrosive Environment in Waste-to-Energy Boilers

Corrosion-resistant materials (CRMs) and coatings are key technologies to increase power generation efficiency and reduce maintenance in waste-to-energy (WTE) plants. Corrosion environment became severe as steam temperatures have increased. The steam condition of more than 400 °C/3.9 MPa became possible in WTE boilers by using highly durable corrosion-resistant coatings, such as thermal spray of Al/80Ni20Cr alloy, HVOF-sprayed NiCrSiB alloy, Alloy 625 weld overlay for waterwall tubes and also superheater tubes. Also, the use of 310S type stainless steels and high Cr-high Mo-Ni base and high Si-Cr-Ni-Fe alloys have progressed because of a better understanding of corrosion mechanisms. Furthermore, high durability coatings using cermet and ceramic materials were applied to high temperature superheaters. This paper describes the major developments and the application of CRMs and coating technologies in the last 30 years in WTE plants, the corrosion mechanisms of alloys, the deterioration mechanisms of spray coating layers, and future subjects for the development of corrosion-resistant materials and coatings.     

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.     

Corrosion behavior of nickel alloys in wet hydrofluoric acid

Wet hydrofluoric acid at concentrations below approximately 60% is highly corrosive to glass, reactive metals, carbon steel and stainless steels. Nickel alloys offer moderate corrosion resistance over a wide range of acid concentration and temperature. The corrosion behavior of eleven commercial alloys was quantified through laboratory testing. Variables that were studied included testing time, acid concentration, temperature, vapor and liquid phases and the presence of residual stresses. Results show that the corrosion rate of a Ni‐Cu and a Ni‐Cr‐Mo‐Cu alloy increased with the acid concentration and the temperature. However, both for increasing acid concentration and temperature, the corrosion rate of the Ni‐Cu alloy increased faster than the corrosion rate of the Ni‐Cr‐Mo‐Cu alloy, especially in the vapor phase. Even in unstressed coupons, nickel alloys showed internal penetration in presence of wet HF; the mode of this internal penetration varied from alloy to alloy. Considering all the studied variables that influence corrosion, the highest ranked material for wet HF service was a Ni‐Cr‐Mo‐Cu alloy.     

Versuche zur Natriumkorrosion austenitischer Werkstoffe bei 1000 °C

Investigations into sodium corrosion of austenitic materials at 1000 °C A series of austenitic materials with sufficiently high strength for the use at high temperatures were examined in respect to their compatibility with liquid sodium at 1000 °C. Austenitic steels with nickel contents of 12% showed the highest stability, but materials with 32% nickel and 20% chromium suffered also moderate losses of weight, however, additional internal corrosion effects. Nickelbase alloys were most severely attacked. They are less applicable for the use with sodium of very high temperatures in spite of their high temperature strengths.     

Korrosionsverhalten von Gasturbinenwerkstoffen unter strömenden Heißgasbedingungen

Corrosion behaviour of gas turbine alloys under high velocity burnt fuels The aim of alloy development in the field of nickel based superalloys for flying and land based gas turbines is to enhance significantly the mechanical properties at high temperatures thus leading to a higher temperature capability. The higher temperature capability of the structural elements of gas turbines results in an increased efficiency, a lowered fuel consumption and less emissions. To achieve an increased high temperature capability, however, surface degradation of the material must be adjusted adequately, hence corrosion resistance has to be improved. Additional to the isothermal and cyclic oxidation tests which are performed in stagnant air the oxidation behaviour of alloy 2100 GT and alloy C‐263 was investigated by means of burner‐rig‐experiments under high velocity burnt fuels. In the burner rig test facility the sample is exposed to a hot gas stream of burned natural gas with gas velocities in the range of 60 m/s to 150 m/s. The metal temperature of the sample can be adjusted in the range of 900°C to 1200°C. In the tests described in this paper the gas velocities were chosen to be 60 m/s, 100 m/s and 140 m/s. The test duration was 1 h and 10 h. The test temperature was kept constant at 1000°C. After 1 h of testing both alloys showed mass gain which was significantly higher for alloy C‐263. After 10 h of testing the mass loss of alloy C‐263 was enhanced with increasing gas velocity. Alloy 2100 GT showed only at the highest gas velocity a mass loss. The examinations by means of SEM and light‐optical microscopy of the oxide scale and of the microstructure showed that alloy 2100 GT has a dense adherent alumina scale and suffers no internal oxidation even under burner‐rig‐test conditions. Alloy C‐263 forms a mixed chromia and Cr‐Ti‐mixed oxide scale. The chromia is evaporated with increasing gas velocity, leaving (Cr‐Ti)O₂‐needles on the surface. In the isothermal and cyclic oxidation tests alloy 2100 GT shows an excellent oxidation behaviour up to 1200°C with a corrosion rate of less than 0.1 mm/a. The aluminium content of app. 3 wt.‐% which is remarkably high for a wrought alloy leads to the formation of a thin dense and adherent alumina scale. Alloy C‐263 is a chromia former which is not suitable for temperatures higher than 1000°C.     

Evaluation of Alloy 146, 279, 900, and 926 sensitization to intergranular corrosion by means of electrochemical methods and image analysis

Properties of stainless steels are affected by improper heat treatment. When that happens, the material can become sensitive to intergranular corrosion. This loss of properties is due to a microstructural changes. In this paper, electrochemical potentiokinetic reactivation tests (single and double loop) were used to evaluate steel sensitization to intergranular corrosion. Four alloys, Alloy 146 (UNS 1.4331) and 279 (UNS 1.4429) (austenitic stainless steels), Alloy 900 (UNS 1.4462) (duplex stainless steel), and Alloy 926 (UNS N08926) (high alloyed stainless steel), were employed as working electrodes. The specimens were sensitized at different temperatures (600–825 °C) for periods of 1 or 2 h in argon atmosphere. Electrochemical tests were conducted using the electro-optical devices P-200002525 and P-200002526. These devices permit to obtain images of the electrode surface and electrochemical data simultaneously. In addition, these results were compared to standardized etching with oxalic acid (ASTM A-262), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Two alloys showed sensitization to intergranular corrosion, Alloy 900 and Alloy 926 at 725 and 825 °C. Additionally, the single and double loop methods showed different passivation results. In the double loop test, the formation of a layer over the entire electrode surface was visible during the process of activation. The film remained until the end of experiment. However, in the single loop method no significant variations were observed on the electrode surface during passivation process, but if material is sensitized a film was formed during reactivation branch.     

The corrosion behavior of several heat resistant materials in air + 2% Cl2 at 300 to 800 °C. Part 2 – Nickel base alloys

The corrosion behavior of the alloys 59, C‐2000 and HR‐160 was investigated in dry air and in air with 2% Cl₂ at temperatures of 300 to 800 °C. Up to 500 °C Alloy 59 and Alloy C‐2000 do not exhibit any significant attack. At 650 °C in particular HR‐160 is subjected to a marked increase of the corrosion rates. In the latter case the higher amount of C compared to the other two alloys seems to decrease corrosion resistance. At 800 °C the resistance of C‐2000 is inferior to that of Alloy 59 which is attributed to differences in the microstructure consisting of Cr/Mo precipitates in the Ni‐base matrix which are primarily attacked.     

Über die transpassive Auflösung von Nickel-Basis-Legierungen und Edelstählen in sauerstoff- und chloridhaltigem Hochtemperatur-Wasser

Transpassive dissolution of nickel-base alloys and stainless steels in oxygen and chloride containing high-temperature water The corrosion behaviour of different nickel base alloys and stainless steels (2.4605 [alloy 59; NiCr23Mol6Al], 2.4633 [alloy 602 CA; NiCr25FeAlY], 2.4819 [alloy C-276; NiMol6Crl5W], 2.4856 [alloy 625; NiCr22Mo9Nb], 2.4606 [alloy 686; NiCr20Mu16], 2.4646 [alloy 214; NiCrl6AlFe] and 1.4401 [UNS S 31600; X5CrNiMol7122]) was investigated in oxygen and chloride containing high-temperature water (temperatures up to 600°C; pressures up to 38 MPa; oxygen concentration 0.48 mol/kg; chloride concentrations up to 0. 1 mol/kg). All alloys show a similar corrosion behaviour, depending on temperature. At temperatures below about 150°C, only slight intergranular corrosion was observed. At higher temperatures (between about 150 and 300°C) pitting was detected. Most of the original surface in this temperature range remained unattacked. At higher temperatures, morphology of pitting changed towards shallow pitting and the whole surface is penetrated. The high general corrosion observed in these areas can be attributed to transpassive dissolution of the alloys' protecting chromium oxide layers with following dissolution of the alloy. At supercritical temperatures, corrosion decreased drastically, and only transpassive intergranular corrosion was detected. The observed decrease of ion-induced corrosion phenomena can he attributed to the change of physical and chemical properties of water (solubility of ions). Corrosion in neutral and alkaline solution was significantly less. Both pitting and transpassive dissolution shifted towards higher temperatures or were not detected respectively.     

Morphological Development of Subscale Formation in Fe–Cr–(Ni) Alloys with Chloride and Sulfates Coating

Three types of stainless steel (430, 304, and 310) with a coating of NaCl, NaCl/AlCl₃, or NaCl/Al₂(SO₄)₃ are exposed at 750 and 850°C. Results show that NaCl has a major effect on corrosion and sulfur plays an important role in intergranular corrosion. After high-temperature exposure with a 100% NaCl coating, the morphologies of alloys 304 and 310 show typical uniform subscale attack the depths of attack increasing with temperature, while alloy 430 showed a planar attack. Alloy 310 has the highest chromium content and has the least metal loss. After high-temperature exposure with a NaCl/AlCl₃ coating, the corrosion morphologies and depths of attack are similar to those associated with an NaCl coating, but only voids are larger in the subscale. When coated with NaCl/Al₂(SO₄)₃, the alloys are attacked simultaneously by sulfur and chlorine at 750°C, resulting in a typical sulfur-attack intergranular corrosion. However, as the temperature increases to 850°C, the corrosion morphology changes to a uniform subscale attack.     

Besondere Aspekte der Temperaturführung und Wärmebehandlung hochlegierter Werkstoffe für den Chemie-Apparatebau

Special aspects of temperature control during heat treatment of high alloyed materials for the Chemical Industry High alloyed materials require a special control of the temperature during any annealing and fabrication procedure to assure the corrosion resistance, which was the main criterion for the selection of the material in the application proposed. Due to this requirement the time-temperature sensitization behaviour with respect to isothermal and polythermal heat treatments has been determined for alloy Cronifer 1925 hMo (1.4529), Nicrofer 3127 hMo (1.4562) as well as for the nickel base alloys Nicrofer 5716 hMoW (2.4819) and Nicrofer 5923 hMo (2.4605). The application of the standard corrosion test according to SEP 1877 II on alloys alloyed with at least 20% chromium shows no correlation between all second phase precipitation which can affect the corrosion resistance under practical conditions. For alloy Cronifer 1925 hMo and Nicrofer 3127 hMo the determination of the critical pitting temperature in the ferric chloride test has been shown to be an effective procedure to evaluate changes in the microstructure due to improper heat treatments during fabrication. The nickel based alloy Nicrofer 5716 hMoW, which is alloyed with only 16% chromium shows already high corrosion rates in the proper solution annealed and quenched condition, when tested according to the strong oxidizing test solution of ASTM G 28 A. Furthermore, the corrosion rate will increase rapidly after sensitization. Due to the fact that the alloy has not been developed for strong oxidizing media, the test media according to ASTM G 28 B should be applied, since it is more appropriate to this alloy. Although alloy Nicrofer 5923 hMo is alloyed with 23% chromium and 16% molybdenum the alloy exhibits a precipitation behaviour which allows easy fabrication of heavy components. Testing according to ASTM G 28 A allows the identification of improper heat treatments.     

Einfluß von Chrom,Molybdän und Stickstoff auf die Korrosionsbeständigkeit des Ni-freien,austenitischen Edelstahles Macrofer 2515MoN (W.-Nr.: 1.4653)

Influence of chromium, molybdenum and nitrogen on the corrosion resistance of the Ni-free, austenitic stainless steel Macrofer 2515MoN (German Alloy No. 1.4653) Nitrogen alloyed, Ni-free, austenitic stainless steels comprising of more than 1 wt.-% nitrogen are a new group of alloys with promising properties. They show a very interesting combination of high strength and toughness with a high corrosion resistance. This combination of properties make the alloys not only suitable for fasteners but also for parts for medical and dental applications. This work shows the influence of chromium, molybdenum and nitrogen on the corrosion resistance of Fe25Mn-alloys in media typical for the above mentioned applications. According to these results Fe25Mn-alloys with appr. 20 wt.-% chromium, about 3 wt.-% molybdenum and appr. 1,3 wt.-% nitrogen have an excellent corrosion resistance in Ringer solution, artificial saliva and artificial sweat. The critical pitting temperature (CPT) as well as the critical crevice temperature (CCT) with 61°C respectively 37°C tested according ASTM G 48A provided significantly higher temperatures when compared to the commercially well established Ni-austenite X6CrNiMoTi17-12-2 (German Alloy No. 1.4571).     

Improving the surface properties of A286 precipitation-hardening stainless steel by low-temperature plasma nitriding

Plasma nitriding over a wide range of treatment temperatures between 350 and 500 °C and time from 5 to 30 h on A286 austenitic precipitation-hardening stainless steels has been investigated. Systematic materials characterisation of the plasma surface alloyed A286 alloy was carried out in terms of microstructure observations, phase identification, chemical composition depth profiling, surface and cross-section microhardness measurements, electrochemical corrosion tests, dry sliding wear tests and corrosion-wear tests. Experimental results have shown that plasma nitriding can significantly improve the hardness and wear resistance of A286 stainless steels owing to the formation of nitrogen supersaturated S-phase; the surface layer characteristics (e.g. microstructure, case depth and hardness) of the plasma surface alloyed cases are highly process condition dependent and there are possibilities to provide considerable improvement in wear, corrosion and corrosion-wear resistance of A286 steel.     

Laboratory corrosion tests for simulating fireside wastage of superheater materials in waste incinerators

Laboratory corrosion tests were performed to clarify the effects of relative amounts of fused salts in tube deposits on corrosion rates of superheater materials in WTE plants. All test exposures were at 550 °C and of 100 h duration. The nine synthetic ashes used as corrodents consisted of mixtures of chlorides, sulfates and oxides. The test materials were alloy steel T22, stainless steels TP347H, TP310HCbN, and alloys HR11N and 625. The gas atmosphere consisted of 500 to 3000 ppm HCl‐30 ppm SO₂‐10% O₂‐10% CO₂‐20% H₂O‐bal.N₂. Generally, the relative amount of fused salts in non‐fused ash constituents at 550 °C increased with increasing the chlorine content of the ashes. The corrosion rate of T22 steel did not depend directly on ash chlorine content, but for ashes of 7.7 wt.% Cl, the corrosion rate depended on the calculated amount of fused salt at 500 °C. The corrosion rates of TP347H steel and alloy 625 were maximum for ashes of 6–8 wt.% Cl. For ashes of 7.7 wt.% Cl, the corrosion rates of T22 steel, stainless steels, and alloys increased with ashes having higher amounts of fused salts. Increased HCl content of the gas caused higher corrosion of the stainless steels and high‐nickel alloys, but there was no clear corrosion‐exacerbating effect with T22 steel.     

硫酸钙沉积导致铁基和镍基合金的热腐蚀

作者在几种铁基和镍基高温合金的试样上涂硫酸钙盐膜在1000℃及1150℃加热研究了硫酸钙对这些合金热腐蚀的影响。测定了这些合金的高温腐蚀动力学曲线。对腐蚀产物进行了 X 射线衍射、电子衍射、电子探针分析及金相检查。实验结果证明:由硫酸钙沉积导致的热腐蚀与合金成份有密切的关系。温度愈高,盐膜愈厚,腐蚀也愈严重。含钼又有足够铝含量以生成 Al_2O_3氧化皮的合金,由于 Al_2O_3被 MoO_3酸性熔融,与 CaSO_4反应可能生成液态的 CaMoO_4,会发生严重的腐蚀。合金的铬含量增高会提高抗 CaSO_4沉积腐蚀的能力,抗蚀性原来不好的合金渗铬后抗蚀性亦显著提高。     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.