Synergistic air port corrosion in kraft recovery boilers

Localized hot corrosion can occur on the cold-side of air-ports in Kraft recovery boilers. Depending on the basicity of the molten salt, either acidic or basic fluxing takes place, with a solubility minima at the transition between the two reactions. For stainless steel, if the basicity of the fused salt is between the iron and chromium oxide solubility minima, then a synergistic effect can occur that leads to rapid corrosion. The products of one reaction are the reactants of the other, which eliminates the need for rate-controlling diffusion. This effect can explain why stainless steel is attacked more readily than carbon steel

Hot Corrosion at Air-Ports in Kraft Recovery Boilers

Hot corrosion can occur on the cold-side of airports in Kraft recovery boilers. The primary corrosion mechanism involves the migration of sodium hydroxide and potassium hydroxide vapors through leaks in the furnace wall at the airports and their subsequent condensation. It has been reported that stainless steel is attacked much faster than carbon steel in composite tubes, and that carbon steel tubing, when used with a low-chromium refractory, does not exhibit this type of corrosion. For hot corrosion fluxing of metal oxides, either acidic or basic fluxing takes place, with a solubility minimum at the basicity of transition between the two reactions. For stainless steel, if the basicity of the fused salt is between the iron and chromium oxide solubility minima, then a synergistic effect can occur that leads to rapid corrosion. The products of one reaction are the reactants of the other, which eliminates the need for rate-controlling diffusion. This effect can explain why stainless steel is attacked more readily than carbon steel

Oxidation in a temperature gradient

The effects of a temperature gradient and heat flux on point defect diffusion in protective oxide scales were examined. Irreversible thermodynamics were used to expand Fick's first law of diffusion to include a heat flux term--a Soret effect. Oxidation kinetics were developed for the oxidation of cobalt and for nickel doped with chromium. Research in progress is described to verify the effects of a heat flux by oxidizing pure cobalt in a temperature gradient above 800 C, and comparing the kinetics to isothermal oxidation. The tests are being carried out in the new high temperature gaseous corrosion and corrosion/erosion facility at the Albany Research Center

State-of-the-art review of electrochemical noise sensors

There are a number of different techniques capable of being used to measure corrosion within equipment. The most simple, the use of metal coupons, usually causes the process to be shut down, is manpower intensive, and has a time delay in getting the required corrosion information. Electrical Resistance (ER) techniques are often used but their response is very sensitive to temperature and they cannot differentiate between general and localized corrosion. Electrochemical techniques, such as linear polarization resistance (LPR), electrochemical noise (EN), electrochemical impedance spectroscopy (EIS), harmonic distortion analysis (HDA), and electrochemical frequency modulation (EFM), have the capability of solving most of those drawbacks. Electrochemical probes can be mounted permanently in most equipment, give regular measurements of the intensity of corrosion, and some can detect localized corrosion. Of all of the electrochemical techniques, EN has the most potential for being used successfully to measure general and localized corrosion rates of equipment. The EN technique was studied in the late 1970s and early 80s as a means of detecting localized (stochastic) corrosion phenomena, such as occurs with pitting, crevice and cavitation attack. EN measurements are based on fluctuations in electrochemical potential and corrosion current that occur during corrosion. Electrochemical potential is related to the driving force (thermodynamics) of the reaction, while corrosion current is related to the rate of reaction (kinetics) of the reaction. The idea is that random electrochemical events on the surface of a corroding metal will generate noise in the overall potential and current signals. Each type of corrosion (for example general corrosion, pitting corrosion, crevice corrosion, and stress corrosion cracking) will have a characteristic “fingerprint” or “signature” in the signal noise. This “fingerprint” can be used to predict the type and severity of corrosion that is occurring. By comparison, conventional electrochemical techniques such as LPR, EIS, HDA and EFM rely on a steady-state analogy for the determination of general corrosion rates. Early studies were carried out using potential EN measurements, using time domain, statistical and frequency domain analyses to characterise the electrochemical response of systems undergoing localised corrosion. Current EN measurements followed quickly using zero resistance ammetry to study the current noise between two identical electrodes. For general corrosion processes, EN has been demonstrated independently by several workers to provide information similar to LPR. Noise technology has been used to study systems undergoing very low to very high rates of corrosion, for example, coatings performance, passive systems undergoing pit initiation/propagation, condensing systems, systems undergoing stress corrosion cracking, and general corrosion through to the very high corrosion rates experienced during chemical cleaning processes. This review will describe: state of the art methods and probes used to measure EN, data acquisition requirements, theory to analyze the signal and to relate the signal to corrosion rates and types, the results of EN field trials, and laboratory results in environments similar to gaspipelines

High temperature electrochemical corrosion rate probes

Corrosion occurs in the high temperature sections of energy production plants due to a number of factors: ash deposition, coal composition, thermal gradients, and low NOx conditions, among others. Electrochemical corrosion rate (ECR) probes have been shown to operate in high temperature gaseous environments that are similar to those found in fossil fuel combustors. ECR probes are rarely used in energy production plants at the present time, but if they were more fully understood, corrosion could become a process variable at the control of plant operators. Research is being conducted to understand the nature of these probes. Factors being considered are values selected for the Stern-Geary constant, the effect of internal corrosion, and the presence of conductive corrosion scales and ash deposits. The nature of ECR probes will be explored in a number of different atmospheres and with different electrolytes (ash and corrosion product). Corrosion rates measured using an electrochemical multi-technique capabilities instrument will be compared to those measured using the linear polarization resistance (LPR) technique. In future experiments, electrochemical corrosion rates will be compared to penetration corrosion rates determined using optical profilometry measurements

Corrosion probes for fireside monitoring in coal-fired boilers

Corrosion probes are being developed and combined with an existing measurement technology to provide a tool for assessing the extent of corrosion of metallic materials on the fireside in coal-fired boilers. The successful development of this technology will provide power plant operators the ability to (1) accurately monitor metal loss in critical regions of the boiler, such as waterwalls, superheaters, and reheaters; and (2) use corrosion rates as process variables. In the former, corrosion data could be used to schedule maintenance periods and in the later, processes can be altered to decrease corrosion rates. The research approach involves laboratory research in simulated environments that will lead to field tests of corrosion probes in coal-fired boilers. Laboratory research has already shown that electrochemically-measured corrosion rates for ash-covered metals are similar to actual mass loss corrosion rates. Electrochemical tests conducted using a potentiostat show the corrosion reaction of ash-covered probes at 500?C to be electrochemical in nature. Corrosion rates measured are similar to those from an automated corrosion monitoring system. Tests of corrosion probes made with mild steel, 304L stainless steel (SS), and 316L SS sensors showed that corrosion of the sensors in a very aggressive incinerator ash was controlled by the ash and not by the alloy content. Corrosion rates in nitrogen atmospheres tended to decrease slowly with time. The addition of oxygen-containing gases, oxygen and carbon dioxide to nitrogen caused a more rapid decrease in corrosion rate, while the addition of water vapor increased the corrosion rate

Simulated fireside corrosion of T91 in oxy-combustion systems with an emphasis on coal/biomass environments

Oxy-combustion is the burning of a fuel in oxygen rather than air for the ease of capture of CO2 for reuse or sequestration. Corrosion issues associated with the change in heat exchanger tube operating environment (replacement of most of the N2 with CO2 and potentially higher SOx levels) from air- to oxy-combustion were examined. The ferritic-martensitic alloy T91 was used in accelerated fireside corrosion tests using several different gas compositions and ash deposit overcoats to simulate air-fired, oxy-fired coal, and oxy-fired co-fired coal/biomass conditions. Initial corrosion was observed after 240 h of exposure by examining cross-sections with retained ash. Metal section losses were determined after exposures of up to 1440 h at 600–700°C. Severe corrosion was observed, and a corrosion response with respect to ash deposit chemistry was observed. Corrosion response differences with respect to gas phase chemistry were minimal. Alloy-oxide scale-ash morphologies were consistent with oxide fluxing mechanisms

Thermal-Spray Coatings for Coastal Infrastructure

ABSTRACT Several protection strategies for coastal infrastructure using thermal-spray technology are presented from research at the Albany Research Center. Thermal-sprayed zinc coatings for anodes in impressed current cathodic protection systems are used to extend the service lives of reinforced concrete bridges along the Oregon coast. Thermal-sprayed Ti is examined as an alternative to the consumable zinc anode. Sealed thermal-sprayed Al is examined as an alternative coating to zinc dust filled polyurethane paint for steel structures

Oxidation of alloys for advanced steam turbines

Ultra supercritical (USC) power plants offer the promise of higher efficiencies and lower emissions. Current goals of the U.S. Department of Energy’s Advanced Power Systems Initiatives include coal generation at 60% efficiency, which would require steam temperatures of up to 760°C. This research examines the steamside oxidation of advanced alloys for use in USC systems, with emphasis placed on alloys for high- and intermediate-pressure turbine sections