Douglas Singbeil

Corrosion Control of Type 316L Stainless Steel in Sulfamic Acid Cleaning Solutions

Abstract An investigation was conducted to better understand the corrosion behavior of Type 316L (UNS S31603) stainless steel in sulfamic acid (NH2HSO3) cleaning solutions and the relative merits o...

Corrosion of Bed Nozzle Alloys in a Wood-Waste Fluidized Bed Power Boiler

Abstract A field test was conducted to identify a more corrosion-resistant material than Type 310H (UNS S31009) stainless steel, from which to fabricate bed nozzles for a fluidized bed power boiler that burns salt-laden wood-waste (hogged) fuel. Test nozzles fabricated from Type 310H stainless steel, Alloys 556 (UNS R30556), 59 (UNS N06059), HR160 (UNS N12160), and 625 (UNS N06625) were installed, subsequently removed, and examined as a function of time. Of the various alloys tested, Alloys 625 and HR160 had the best performance in terms of the amount of thickness loss, and the extent of internal damage exhibited. However, those alloys were still susceptible to corrosion by the fireside (fluidized bed) environment and, therefore, will require replacement over time. Despite the observed corrosion, those two alloys significantly extended the nozzle life beyond that of Type 310H stainless steel. Nozzle design was found to have a strong influence on the apparent corrosion resistance of Alloy 625 and, therefor...

Corrosion of High-Alloy Superheater Tubes in a Coastal Biomass Power Boiler

Abstract Metallurgical examinations were conducted on a set of damaged high-alloy (Type 310H [UNS S31009] stainless steel and Alloy 625 [UNS N66250] weld overlay) superheater tubes removed from a c...

Cracking Susceptibility of Alternative Composite Tube Alloys in a Molten Sodium Sulfide Hydrate-Sodium Hydroxide Salt Mixture

Abstract The cracking of composite tube systems based on Type 304L stainless steel (SS) (UNS S30403) in the lower-furnace section of kraft recovery boilers continues to be a serious problem. One remedial approach used by pulp mills involves replacing the conventional system based on Type 304L SS with alternative composite tube systems based on Alloys 825 (UNS N08825) and 625 (UNS N06625). U-bend stress corrosion cracking (SCC) tests were conducted on these alloys in a sodium sulfide hydrate-sodium hydroxide (Na2S·xH2ONaOH) molten salt mixture at 180°C (simulated wash-water environment) to help rationalize the observed service performance and to help assist with the selection of the more resistant composite tube system. Attention was given to studying relative SCC resistance of those three alloys in relevant metallurgical conditions, which include mill-annealed, sensitized, cold-worked, and solution-annealed conditions. It was concluded that Type 304L SS is susceptible to cracking even with an ideal micros...

Sulphidation resistance of composite boiler tube materials

Abstract A lab-based testing program was undertaken to generate data to better define the sulphidation resistance of composite tubes installed in the lower-furnace section of black liquor recovery boilers. All composite tube cladding alloys tested were observed to have an acceptable corrosion rate at normal operating temperatures (up to 400°C) in the synthetic lower-furnace gaseous environment tested (1% H2S–99% N2). This acceptable corrosion resistance is due to the expected formation of a relatively protective chromium-rich inner sulphide scale. An increase in temperature up to 560°C was found to significantly increase the corrosion rate. Of the various alloys tested, Alloy HR11N exhibited the lowest corrosion rate at each of the three temperatures tested. Moreover, the corrosion rate was found not to be strongly dependent on the fabrication route (weld overlay versus co-extruded). To minimize corrosion, operating conditions that promote prolonged exposure to elevated temperatures in the lower-furnace section of black liquor recovery boilers should be avoided, regardless of the type of composite tube installed.

Final Report, Materials for Industrial Heat Recovery Systems, Tasks 3 and 4 Materials for Heat Recovery in Recovery Boilers

The DOE-funded project on materials for industrial heat recovery systems included four research tasks: materials for aluminum melting furnace recuperator tubes, materials and operational changes to prevent cracking and corrosion of the co-extruded tubes that form primary air ports in black liquor recovery boilers, the cause of and means to prevent corrosion of carbon steel tubes in the mid-furnace area of recovery boilers, and materials and operational changes to prevent corrosion and cracking of recovery boiler superheater tubes. Results from studies on the latter two topics are given in this report while separate reports on results for the first two tasks have already been published. Accelerated, localized corrosion has been observed in the mid-furnace area of kraft recovery boilers. This corrosion of the carbon steel waterwall tubes is typically observed in the vicinity of the upper level of air ports where the stainless clad co-extruded wall tubes used in the lower portion of the boiler are welded to the carbon steel tubes that extend from this transition point or “cut line” to the top of the boiler. Corrosion patterns generally vary from one boiler to another depending on boiler design and operating parameters, but the corrosion is almost always found within a few meters of the cut line and often much closer than that. This localized corrosion results in tube wall thinning that can reach the level where the integrity of the tube is at risk. Collection and analysis of gas samples from various areas near the waterwall surface showed reducing and sulfidizing gases were present in the areas where corrosion was accelerated. However, collection of samples from the same areas at intervals over a two year period showed the gaseous environment in the mid-furnace section can cycle between oxidizing and reducing conditions. These fluctuations are thought to be due to gas flow instabilities and they result in an unstable or a less protective scale on the carbon steel tubes. Also, these fluctuating air flow patterns can result in deposition of black liquor on the wall tubes, and during periods when deposition is high, there is a noticeable increase in the concentrations of sulfur-bearing gases like hydrogen sulfide and methyl mercaptan. Laboratory studies have shown that chromized and aluminized surface treatments on carbon steel improve the resistance to sulfidation attack. Studies of superheater corrosion and cracking have included laboratory analyses of cracked tubes, laboratory corrosion studies designed to simulate the superheater environment and field tests to study the movement of superheater tubes and to expose a corrosion probe to assess the corrosion behavior of alternate superheater alloys, particularly alloys that would be used for superheaters operating at higher temperatures and higher pressures than most current boilers. In the laboratory corrosion studies, samples of six alternate materials were immersed in an aggressive, low melting point salt mixture and exposed for times up to 336 h, at temperatures of 510, 530 or 560°C in an inert or reactive cover gas. Using weight change and results of metallographic examination, the samples were graded on their resistance to the various environments. For the superheater corrosion probe studies, samples of the same six materials were exposed on an air-cooled corrosion probe exposed in the superheater section of a recovery boiler for 1000 h. Post exposure examination showed cracking and/or subsurface attack in the samples exposed at the higher temperatures with the attack being more severe for samples 13 exposed above the first melting temperature of the deposits that collected on the superheater tubes. From these superheater studies, a ranking was developed for the six materials tested. The task addressing cracking and corrosion of primary air port tubes that was part of this project produced results that have been extensively implemented in recovery boilers in North America, the Nordic countries and many other parts of the world. By utilizing these results, boilers are being operated under more demanding conditions with fewer outages and reduced safety concerns about boiler explosions resulting from failed tubes in the lower boiler. The results of studies on the mid-furnace tube corrosion task and the superheater corrosion and cracking task have become available much more recently, and, consequently, the extent of implementation is far more limited. However, boiler operators are utilizing some of the operational information to minimize corrosion in the mid-furnace area. With construction of new boilers that are designed to operate at higher temperatures and pressures, the guidance on selection of superheater tube materials will almost certainly influence the selection of materials for the more critical areas of the superheaters.

Corrosion of Alloys in a Simulated Wood-Waste Fluidizing Bed Power Boiler Environment

Abstract Rapid corrosion of Type 310H (UNS S31009) stainless steel fluidizing bed nozzles was observed shortly after start-up of a fluidizing bed power boiler designed to combust a mixture of salt-...