Hochkorrosionsbeständige Sonderstähle für Rauchgas-Entschwefelungsanlagen

Highly corrosion resistant special steels for flue gas desulfurisation plants Highly corrosion resistant stainless steel grades have been proved under the severe corrosion conditions existing in flue gas desulfurisation scrubbers (FGD). Besides general corrosion pitting, crevice corrosion and eventually stress corrosion cracking can occur. Thus highly alloyed special steels must be used. Steel grades with a minimum content of 2.75% Mo are essential. At higher chloride levels and decreasing pH-values higher alloyed stainless steels containing up to 6% Mo are necessary. Some of these special steels are described in view to their composition and mechanical properties; their corrosion behavior has been tested under laboratory and field conditions. The use of nitrogen alloyed grades has been shown of remarkable advantage. Nitrogen additions enhance the mechanical properties and structure stability. Furthermore the precipitation of deleterious intermetallic compounds during heat treatment will be delayed by nitrogen additions, thus e.g. multi layer weldings can be carried out with higher security in view to corrosion resistance and mechanical properties. Materials selection for the different scrubber systems will be illustrated by examples. Up to now experiences about stainless steel components in FGD plants are taken into consideration. Welding with distinctly higher alloyed filler metal at the medium-touched side has been well proved in view to adequate corrosion properties.     

New stainless steels for sea-water applications. Part II: Comparison of corrosion and mechanical properties of laboratory and commercial ELI ferritic and superaustenitic stainless steels

This paper represents a follow-up to the first part of the work on new stainless steels for sea-water service. Four laboratory ELI (Extra Low Interstitial) ferritic stainless steels (types 25 Cr-4 Ni-4 Mo), two commercial ELI ferritic stainless steels (types 25 Cr-4 Ni-4 Mo? Ti and 26 Cr-2.5 Ni-3 Mo? Ti) and two highly alloyed austenitic stainless steels (types 20 Cr-25 Ni-4.5 Mo? Cu and 20 Cr-18 Ni-6 Mo? N) have been investigated. With a view to establish the performance of these new alloys in chloride containing environments, systematic electrochemical and laboratory exposure tests have been carried out to define how various factors affect its susceptibility to intergranular, pitting, crevice and stress corrosion. Tension tests were also performed. From the comparison of the localized corrosion resistance and mechanical properties it has been concluded that the laboratory Ti, Ti + Nb or Nb stabilized ELI ferritic stainless steels and the commercial type 25 Cr-4 Ni-4 Mo? Ti of analogous composition could be a valuable alternative to the more expensive highly alloyed stainless steel type 20 Cr-25 Ni-4.5 Mo? Cu which has been especially developed and already used for industrial sea-water applications.     

Susceptibility of stainless steel alloys to crevice corrosion in ClO2 bleach plants

Stainless steels, including duplex stainless steels, are extensively used for equipment in pulp bleaching plants. One serious corrosion problem in chlorine dioxide bleach plants is crevice corrosion of stainless steels, which is frequently the factor that limits their use in bleach plants. Crevice corrosion susceptibility of alloys depends on various environmental factors including temperature, chemical composition of environment and resulting oxidation potential of system. Upsets in the bleaching process can dramatically change the corrosivity of the bleaching solutions leading to temperatures and chemical concentrations higher than those normally observed in the bleach process. When the environmental limits are exceeded the process equipment made of stainless steel can be severely affected. Environmental limits for crevice corrosion susceptibility of eight stainless steel alloys with PRE numbers ranging from 27 to 55 were determined in chlorine dioxide environments. Alloys used in this study included austenitic, ferritic-austenitic (duplex), and superaustenitic stainless steels. The performance of the different stainless steel alloys mostly followed the PRE numbers for the respective alloys. The 654SMO alloy with the highest PRE number of 55 showed the highest resistance to crevice corrosion in this environment. Under the most aggressive chlorine dioxide bleach plant conditions tested, even alloys Nicr3127 and 654SMO with PRE numbers 51 and 55 respectively were susceptible to crevice corrosion attack. The two factors that seem to contribute the most to crevice corrosion and pitting in the investigated environments are temperature and potential.     

Experiences with a high-alloyed stainless steel under highly corrosive conditions

The paper emphasizes experiences from the chemical industry with a highly alloyed stainless steel, Avesta 254 SMO. A number of installations are reported where this 6 Mo austenitic stainless steel has been selected since conventional stainless steels have shown inadequate corrosion resistance. These installations are at the production of

• – aluminium fluoride
• – chlorate, sodium and potassium
• – diamines
• – synthetic fibres
• – soda and at the recovery of
• – solvents, viz. chlorinated hydrocarbons
• – sulphur dioxide

Further the paper summarizes some experiences from the fabrication point of view.     

AVESTA 654 SMO™ – A new nitrogen-enhanced superaustenitic stainless steel

The properties of a new, very highly alloyed stainless steel, Avesta 654 SMO, are compared with those of other stainless steels and nickel-base alloys. The new steel has a very high strength but still possesses a high elongation. The corrosion properties have been investigated in accelerated laboratory tests and in environments representing the expected main application areas, sea water, pulp bleaching and flue gas cleaning. The tests clearly show that the new steel has appreciably better corrosion resistance than the superduplex and superaustenitic 6 Mo steels and that the resistance in many cases matches that of the best nickel-base alloys.     

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.     

Korrosionsbeständigkeit von Verbindungen von Rohren aus nichtrostenden Stählen in Wässern

Corrosion of joints for stainless steel tubes in water The most important commonly used joining techniques for stainless steel tubes which are used for the transport of water and gases are welding and brazing. With corrosion attack by dry gases, both connections are resistant against corrosion. However, in water and aqueous condensates limits of application exist with regard to the corrosion resistance. The corrosion resistance of weld connections with stainless steel tubes is diminished by

• – annealing colours (oxide films) and scale layers in the weld area;
• – changes in the microstructure adjacent to the welds (sensitization of the stainless steel material);
• – surface finish of weld seams after welding;
• – welding faults resulting from bad handling and workmanship.

Type and extent of corrosion damage occurring on weld connections with decreased corrosion resistance depend on the composition of the water and condensates, mainly on their chloride content. Typical examples for the causes of degraded corrosion resistance of weld connections, and possible types of corrosion attack, namely pitting, crevice corrosion, and stress corrosion cracking and their mechanisms are described. Furthermore, measures are shown by which the corrosion resistance of weld connections with stainless steel tubes can be increased. Joints of stainless steel tubes by hard soldering with capillary fittings are endangered by knife line attack at the phase boundary between the stainless steel and solder (interfacial corrosion). Knife line attack means in this context the loss of adhesion between steel and hard solder. The severity of the corrosion risk, in particular the incubation time until the occurrence of the corrosion damage, depends on the water quality, mainly on chloride concentration, and pH. The press fitting with non-metallic gasket is a relatively new joining system, and it is used in the cold and warm water domestic installation. This joining technique is described. For domestic water distribution, an installation system with tubes and press fittings made of steel grade AISI 316 SS has been developed. This system is resistant to corrosion attack in potable water of usual composition, and it is already applied in-service in a considerable extent. Other joining systems are stainless steel weld fittings, threaded screw fittings, and compression couplings with cutting or clamping rings. They are used mainly in industrial installations.     

Zum Korrosionsverhalten nichtrostender austenitischer Chrom-Nickel-(Molybdän,Kupfer)-Stähle in nahezu wasserfreier Essigsäure

Corrosion of stainless austenitic steels in almost anhydrous acetic acid As-welded samples and looped specimens from 5 differently alloyed stainless steels were tested for up to 246 days in 99,5% to 99,95% acetic acid at 118°C (boiling temperature/normal pressure) and at 150°C; the chloride content was varied between < 1 and 100 ppm. Pitting corrosion – of shallow depth, however (approx. 0,1 mm) – was already observed at surprisingly low chloride concentrations. Only the following were found to be resistant to pitting corrosion:

• – stainless steels 1.4439 and 1.4539, containing approx. 4,5% molybdenum, in 99,5% acetic with < 1 ppm chloride at 118 and 150°C,
• – stainless steels 1.4439 and 1.4539 in 99,9% acetic acid with < 1 ppm chloride at 118°C, and
• – special stainless steel X 2 CrNiMoCuN 20 18 6, containing approx. 6% molybdenum, in 99,5% acetic acid with > 3, < 10 ppm chloride at 118 and 150°C.

Looped specimens and ground as-welded samples showed no sensitivity to transcrystalline, chloride-induced stress corrosion cracking at any of the concentration ranges. High surface-removal rates can be expected if air has access to the specimens; under this condition pitting corrosion and general corrosion may overlap. Contamination of acetic acid with chlorides must be prevented under all circumstances.     

Study of the influence of welding parameters on the stress corrosion resistance of AISI 304 steel

Austenitic stainless steel has been welded to coat pressure vessels in petrochemical plants. The material is highly susceptible to stress corrosion in chloride environments, which can damage the weld and lead to the rupture of the component. In this work we did the evaluation of the influence of welding parameters on the stress corrosion resistance of AISI 304 steel exposed to a magnesium chloride solution. AISI 304 sheets were manually welded using three different coated electrodes (AWS E309-16, E308L-16, E316L-16) and two heat inputs (5.0 and 9.0 kJ/cm). The welded samples were analysed by tensile strength tests, optical microscopy and corrosion tests carried out according to ASTM G36-73 guidelines. The results showed that the AWS E309-16 electrodes produced the best results due to the microstructure of the resulting weld metal. The presence of a network of ferrite particles in an austenitic matrix acts as a barrier to crack propagation, thus enhancing the resistance to stress corrosion of the material. This effect is associated to the morphology and distribution of the phase rather than its contents. Welding in a direction parallel to the stress axis using a relatively high heat input improved the stress corrosion of the material even further. The HAZ (Heat Affected Zone) of AISI 304 steel was highly susceptible to stress corrosion in chloride solution. The presence of carbide precipitates in the austenite grain boundary deteriorated the corrosion resistance of the steel, as they promoted anodic dissolution and the development of stress corrosion cracks.     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

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.     

选区激光熔化MnSi2增强316L不锈钢复合材料的表面质量及耐蚀性能

为改善316L不锈钢在海洋环境下的耐腐蚀性能,通过MnSi₂增强316L不锈钢基体,采用选区激光熔化(SLM)制备MnSi₂/316L不锈钢复合材料。利用Image-Pro Plus软件、光学显微镜、扫描电镜(SEM)及电化学工作站研究了激光功率对316L不锈钢金属基复合材料致密度及耐腐蚀性能的影响,通过Tafel极化曲线和阻抗谱表征其耐腐蚀性能的强弱,并通过点蚀形貌揭示了其腐蚀机理。结果表明:添加MnSi₂是提高316L不锈钢耐腐蚀性能的有效途径。随着激光功率的增大,耐腐蚀性能呈现先提高后降低的趋势,当激光功率达到190 W时,2%MnSi₂/316L不锈钢复合材料的致密度为99.80%,其腐蚀电位为－0.053 V (vs SCE)。同时,2%MnSi₂可以显著改善316L不锈钢的成形质量,提高其耐腐蚀性能,其腐蚀形式为氯离子诱导氯化物生成的点蚀,且点蚀产生位置主要集中在孔隙边界处。     

Performance of galvanized and stainless steel rebars in concrete under macrocell corrosion conditions

In the recent past, the damage caused by rebar corrosion in concrete structures has been considered as one of the major durability problems affecting the service life of concrete structures. In order to prevent corrosion of steel reinforcement, various types of protective methods have been adopted. One of the protective measures is using galvanized reinforcement in concrete to enhance the service life. In the present study various types of galvanized rebars namely bare – Cold Twisted Deformed (CTD), bare – Thermo Mechanically Treated (TMT), galvanized – Cold Twisted Deformed, Galvanized – Thermo Mechanically Treated, galvanized and chromated – Cold Twisted Deformed, galvanized and chromated – Thermo Mechanically Treated and stainless steel rebars have been evaluated for their corrosion resistance in M30 grade concrete under macro cell corrosion condition over a period of one year. From the studies it has been observed that TMT bars performed better when compared to CTD bars. Among the rebars tested, stainless steel rebar has shown negligible corrosion in chloride contaminated concrete.     

奥氏体不锈钢铁污染的来源以及防护措施

奥氏体不锈钢作为核工业的重要钢材,其必须具有非常优良的耐腐蚀性能.然而在核主泵的生产流程中不规范的加工、运输以及安装会使奥氏体不锈钢的耐腐蚀性能降低,其中最严重之一的影响因素就是奥氏体不锈钢的铁污染.为使奥氏体不锈钢达到生产核电设备的严格要求以及核电站能够安全运行[1],对奥氏体不锈钢铁污染的来源的掌握很重要.本文对污染源进行总结,并针对不同污染源阐述了相应的防护措施.     

Corrosion‐technical properties of high‐strength stainless steels for the application in prestressed concrete structures

Experience with prestressed concrete over about half a century has indicated that the corrosion resistance of conventional prestressing steel does not always satisfy, especially the prestressing steels are susceptible to chloride attack (de‐icing salts) and hydrogen (hydrogen‐induced stress corrosion cracking). On the other hand corrosion agents, such as chloride, condensation water, can penetrate in the concrete and arrive at the surface of steels. Hence, corrosion damage of prestressing steels can happen and, in the extreme cases, the prestressed concrete structure collapsed resulting from the failure of the tendon. In this paper, consideration is made to use high‐strength stainless steels as prestressing tendon with bond in concrete. The high‐strength stainless steels of qualities 1.4301 (X5CrNi18‐10), 1.4401 (X5CrNiMo17‐12‐2), 1.4436 (X3CrNiMo17‐13‐3) and 1.4439 (X3CrNiMoN17‐13‐5) with sequence of increasing austenite stability were investigated. For application in prestressing tendon with bond in concrete the cold‐drawn high‐strength stainless steel of quality 1.4401 is an optimal proposition regarding its satisfactory resistance against pitting corrosion and stress corrosion cracking (SCC) in structure‐related corrosive conditions. The lower alloyed steel 1.4301 has an insufficient resistance against the chloride‐induced corrosion because of the lack of molybdenum and the content of deformation martensite due to the strong cold‐drawing of its unstable austenitic structure.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

Untersuchungen über den Einfluß des Gefügezustandes auf das Korrosionsverhalten hochlegierter,rost- und säurebeständiger Stähle in reiner sowie in chloridhaltiger Schwefelsäure

Corrosion Properties of High Alloyed Stainless Steels in Pure as well as in Chloride Containing Sulfuric Acid The corrosion behaviour of the high alloyed stainless steels material no. 1.4439 (X3CrNiMoN17135), 1.4539 (X2NiCrMoCu25205), 1.4503(X3NiCrMoCuTi2723) as well as the reference materials AlSI 316 L and alloy 825 was tested in diluted sulfuric acid (5, 10, 20 and 50%) at 50, 100 and 150°C. The test solutions additionally contained impurities as chlorides and cupric ions. On the material side the effect of various microstructures was checked as well: material as received (commercial production), solution annealed under laboratory conditions, cold deformed and for two selected steels electroslag remelted. Corrosion testing methods are: the immersion test will sheet coupons and the measurement of the weightloss; electrochemical testing, i.e. Current potential-and free corrosion potential-time-curves. No pitting corrosion is observed in the presence of chloride ions. In some cases the general corrosion rate is lowered if chloride ions are present. This beneficial effect of chloride ions, however, is observed only at low chloride concentrations (500 ppm). Annealing under laboratory conditions as well as electroslag remelting does not generally improve the corrosion resistance. A negative effect by cold deformation is only observed for standard stainless steel AlSI 316. Cupric ions added to the 20% sulfuric acid solution improve the corrosion resistance of all steels investigated to that extent, that they can be used in practice up to 100°C provided that the concentration of cupric ions in the solution is sufficiently high (2000 ppm). Electrochemical test results indicate that the positive effect of cupric ions is due to the shift of the free corrosion potential into the potential range of stable passivity. Copper alloyed stainless steels show the highest corrosion resistance.     

STUDY ON THE CORROSION AND MECHANICAL PROPERTIES OF FJ316 STAINLESS STEEL-GLASS COMPOSITE

1.IntroductionTheeconomicproductionofcorrosionresistantmaterialsisofgrowingimportance.Recently)powdermetallurgyhasreceivedconsiderableattentionduetoitsmanyadvantagesoverthefusionmetallurgy.Ithasbeenfoundthat[1],thecorrosionresistanceofsinteredstainlesssteelsdependsontheprocessingofthematerials,onitsporosityandontheenvironmenttowhichitisexposed.Malhotraetal.[2]studiedtheeffectofsinteringparametersonporemorphologyandcorrosionresistanceof316Lstainlesssteel.Theyreportedfineporesalldbettercorrosio…     

Qualifizierung metallischer Werkstoffe für die Eindampfung von Abwässern aus der Rauchgasentschwefelung

Qualification of metallic materials for evaporation of waste water from flue gas desulfurization plants The ecologically-minded processing of waste water from the wet scrubbing of flue gases of coal-fired power plants to produce environmentally acceptable products is carried out in a two-step evaporater operating in closed loop mode. The evaporating process leads to high concentration of chlorides in the two evaporation steps: up to about 100 g/l in the 1st step and up to about 350 g/l in the 2nd step. Therefore in case of metallic design of the evaporation equipment materials of construction with exceptional resistance to chloride induced pitting are required. The corrosion resistance of the high-alloyed stainless steel Alloy 31 (X1NiCrMoCu32-28-7 – UNS N 08031) and of the NiCrMo-alloys Alloy C-276 (NiMo16Cr15W – UNS N 10276) and Alloy 59 (NiCr23Mo16Al – UNS N 06059) including their weldments were to be tested for this application both in the laboratory and in field tests. In addition the behaviour of Alloy 59 heat exchanger tubes had to be determined in field tests under heat-transfer service conditions. The critical pitting corrosion temperatures of the 3 materials after having been GTAW welded under uniform conditions with FM 59 (ERNiCrMo–12) filler were determined in potentiostatic tests in model solutions imitating concentrated waste water products as they may occur in practice, using 5 K temperature intervals. As to be expected the critical corrosion resistance limits of the materials lie at 85 °C at chloride concentrations of 100 g/l Cl⁻ for the Alloy 31 and of 300 g/l Cl⁻ for both the Alloy 59 and the Alloy C–276 respectively. Field tests in waste water evaporation units of flue gas desulfurization plants of coal-fired power stations are carried out as immersion tests with the welded materials and as heat-exchange experiments using longitudinally welded tubes of Alloy 59 (2.4605). The immersion tests over a period of 32 months show the Alloy 31 (1.4562) to be a corrosion resistant construction material for tubes and containers in the first evaporation step, whereas the Alloy 59 (2.4605) and the Alloy C–276 (2.4819) have to be used for the second evaporation step, where the chloride contents are much higher. The Alloy 59 is to value as the most resistant material according to its lower tendency to crevice corrosion. The heat-exchange experiments over a test period of 9 months cause to expect the Alloy 59 to be a suitable construction material for heat-exchanger tubes in both evaporation steps in comparison to graphite which is more succeptible to mechanical destroying.     

HCl-Induced High Temperature Corrosion of Stainless Steels in Thermal Cycling Conditions and the Effect of Preoxidation

Gaseous HCl released during combustion is one reason for the severe materials degradation often encountered in power generation from waste and biomass. In this study, three stainless steels (the low alloyed EN 1.4982, the standard EN 1.4301 and the higher alloyed EN 1.4845) were tested by repeated thermal cycling in an environment comprising N₂?C10%O₂?C5%H₂O?C0.05%HCl at both 400 and 700 °C. The materials were exposed with ground surfaces and preoxidised at 400 or 700 °C. A positive effect of preoxidation is evident when alloys are exposed at 400 °C. Oxide layers formed during preoxidation effectively suppress chlorine ingress for all three materials, while chlorine accumulation at the metal/oxide interface is detected for surface ground specimens. The positive effect of preoxidation is lost at 700 °C and corrosion resistance is dependent on alloying level. At 700 °C metal chloride evaporation contributes significantly to the material degradation. Based on the results, high temperature corrosion in chlorinating environments is discussed in general terms.