Cracking and Corrosion of Composite Tubes in Black Liquor Recovery Boiler Primary Air Ports

Black liquor recovery boilers are an essential part of kraft mills. Their design and operating procedures have changed over time with the goal of providing improved boiler performance. These performance improvements are frequently associated with an increase in heat flux and/or operating temperature with a subsequent increase in the demand on structural materials associated with operation at higher temperatures and/or in more corrosive environments. Improvements in structural materials have therefore been required. In most cases the alternate materials have provided acceptable solutions. However, in some cases the alternate materials have solved the original problem but introduced new issues. This report addresses the performance of materials in the tubes forming primary air port openings and, particularly, the problems associated with use of stainless steel clad carbon steel tubes and the solutions that have been identified

A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars

The Curiosity rover discovered fine-grained sedimentary rocks, inferred to represent an ancient lake, preserve evidence of an environment that would have been suited to support a Martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. C, H, O, S, N, and P were measured directly as key biogenic elements, and by inference N and P are assumed to have been available. The environment likely had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars

Multiple-interface tracking of degradation process in organic photovoltaics

The investigation of the stability in organic photovoltaics has been focused on individual components via localized and destructive analysis, which is limited to broken devices, instead of an operational OPV, and unable to obtain correlated information of degrading interfaces. DC biased AC impedance spectroscopy is employed here, to track multi-interface degradation without breaking the device. By varying DC bias, individual interface degradation is revealed via current density and capacitance versus voltage plots. While one of the impedance semicircles is linked to the interface of P3HT:PCBM, the other represented the interface between the mixture and metal electrode, involving metal oxide in an aged device. The results confirm that, more than one degradation process take place simultaneously at individual interfaces

Influence of Edge Effects on Local Corrosion Rate of Magnesium Alloy/Mild Steel Galvanic Couple

The effect of the insulator-mixed-material edge on the galvanic corrosion rate of magnesium alloy (AE44)–mild steel (MS) couple is experimentally studied using scanning vibrating electrode technique (SVET), profilometry, and classical electrochemistry. The local and average corrosion rates estimated from the experimental depth of anodic attack profile of AE44-MS couple are validated by 2D and 3D corrosion numerical models. Our study demonstrates experimentally and theoretically that the presence of the insulator edge increases the local current density, which enhances the corrosion rate. The extent of the local corrosion rate enhancement and its effect on the overall corrosion rate of the mixed material is discussed and depends on the mixed material’s geometry and the edge type

Localized Corrosion Behavior of AZ31B Magnesium Alloy with an Electrodeposited Poly(3,4 Ethylenedioxythiophene) Coating

Herein we report the characterization of localized corrosion of AZ31B magnesium alloy electrochemically coated with poly(3,4-ethylenedioxythiophene) (PEDOT) from ionic liquid electrolyte. Several scanning probe techniques including high resolution electron microscopy, SVET, SECM amperometric detection of H2 fluxes, potentiometric SECM detection of local pH and localized potentiodyanmic measurements were used to evaluate the microstructure of the coating and its corrosion protectiveness. In order to examine long-term durability of corrosion protection due to the PEDOT coating, these measurements were performed after different immersion times. It was observed that PEDOT coating appears to lose its protective ability after localized coverage of corrosion products. The results of this study provide important information considering the interest in this coating for use in biomedical implants and prior indications of beneficial passivation

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