Materials for mid furnace environments of kraft recovery boilers
Kraft recovery boilers experience localized accelerated corrosion in mid furnace areas. Corrosion patterns may vary from one boiler to other depending upon the operation parameters and general design of the boiler. Local gaseous environments were characterized in areas with corrosion and equivalent areas without accelerated corrosion in a selected boiler over the period of two years. Results from this study indicated that in the areas with accelerated corrosion there was a large variation in the local gas composition at the waterwall surface. Reducing and sulfidizing gases were found in the areas with accelerated corrosion whereas the gas compositions were more oxidizing in the low corrosion areas. Observations during field tests have indicated that the local waterwall temperature may also vary in the mid furnace due to localized combustion and air flow patterns. Mid-furnace environments were simulated in laboratory to evaluate the effect of gas composition and fluctuations on corrosion kinetics of carbon steel tube material. Laboratory tests with SA-210 carbon steel samples with gas composition cycling between oxidizing and reducing/sulfidizing led to an unstable scale formation on the surface. Extent of accelerated corrosion was found to depend upon the extreme partial pressure values of sulfur and oxygen. Carbon steel samples were coated with chromized or aluminized diffusion coatings in the laboratory and were tested under sulfidizing conditions. Both type of diffusion coatings had superior performance compared to the carbon steel samples under similar conditions. Midfurnace corrosion can be controlled by making gaseous environments in the mid furnace more stable. However, more practical solution seems to be to use waterwall tubes with protective outer layer. This may be done by using composite tubes (stainless steel outer layer), thermal spray coatings of resistant alloys or diffusion barrier coatings like chromizing or aluminizing treatment.
Alloy degradation under oxidizing-sulfidizing conditions at elevated temperatures
Exposure of high-temperature alloys to a low-PO2 oxidizing-sulfidizing environment above about 550 °C is often characterized by an initial period of protective oxidation followed by more rapid corrosion. This paper will provide a brief overview of alloy corrosion in oxidizing-sulfidizing atmospheres, with specific consideration given to the thermodynamic and kinetic aspects of the process, resulting modes of degradation, and alloy selection. The concept of a "critical microstructure" in the subsurface of the alloy as a prerequisite to initiating protective scale breakdown will also be discussed. The paper will conclude with a specific example of alloy development for the filtration of hot oxidizing-sulfidizing gases.
High temperature corrosion of nickel‐chromium alloys containing refractory sulphide‐forming elements in sulphur/oxygen environments – theoretical considerations
Despite the fact that combustion atmospheres in gas turbines are normally strongly oxidizing, microexamination of corroded alloys often not only reveals an anticipated external oxide scale and a degree of internal oxidation, but also a significant amount of sulphide phase(s), particularly after turbine operation in marine situations. Thus a degree of selective sulphidation as well as oxidation occurs. While the exact method of the further propagation of the corrosion product(s) is subject to doubt, it is possible that subsequent oxidation of these sulphide particles may be a factor. It is clear, however, that a necessary prerequisite for the reduction of this complex form of attack is the elimination of or control oh the sulphidation which occurs concurrently with oxidation. The theory underlying the possible development of sulphidation resistant alloys via. selective sulphidation of specific alloying element additions is reviewed. On this basis a number of alloying elements, which can form so-called refractory sulphides of high melting point, and which hitherto have been largely ignored, are put forward in discussion as being potentially useful as additions to Ni-based alloys for the inhibition of attack in sulphur/oxygen environments. In this assessment thermodynamic data for these refractory sulphides have been calculated and are quoted. 
 
 
 
Hochtemperaturkorrosion von Nickel-Chrom Legierungen mit hochtemperaturbestandigen, Sulfide bildenden Zusaitzen in Schwefel/Sauerstoff-Atmospharen -Theoretische Betrachtungen 
 
 
 
Obwohl die in Gasturbinen auftretenden Verbrennungsgase normalerweise stark oxidierend wirken, zeigt die Mikrountersuchung korrodierter Legierungen haufig nicht nur den erwarteten auseren Oxidzundern und ein gewisses Mas an innerer Oxidation, sondern auch betrachtliche Mengen Sulfidphase(n), insbesondere wenn die Turbine unter marinen Bedingungen betrieben wurde. Es kommt daher in gewissem Umfang zu selektiver Sulfidierung und auch Oxidation. Wahrend die genaue Art und Weise der Ausbreitung einmal vorhandener Korrosionsprodukte noch zweifelhaft ist, kann die spatere Oxidation von Sulfidteilchen ein wesentlicher Faktor sein. Es ist jedoch klar, das eine notwendige Voraussetzung fur die Verringerung dieser komplexen Angriffsform die Ausschaltung bzw. Behinderung der gleichlaufend mit der Oxidation auftretenden Sulfidierung ist. Die Theorie, auf welche sich die Moglichkeit der Entwicklung von sulfidierungsbestandigen Legierungen uber den Zusatz von selektiv sulfidierbaren Legierungszusatzen grundet, wird untersucht. Auf dieser Grundlage werden einige Legierungselemente, die sogenannte hitzebestandige Sulfide hohen Schmelzpunktes bilden und bisher weitgehend ignoriert wurden, in die Diskussion eingefuhrt und als mogliche nutzliche Zusatze zur Inhibierung des Angriffs von Nickellegierungen in Schwefel/Sauerstoff-Systemen vorgestellt. In diesem Zusammenhang wurden auch thermodynamische Daten dieser hitzebestandigen Sulfide berechnet.