J. Porcayo-Calderon

Corrosion Evaluation of Alloy 800 in Sulfate/Vanadate Molten Salts

Abstract This paper is devoted to the evaluation of the corrosion behavior of Alloy 800 (UNS N08800) when exposed to 80 mol% Na2SO4-20V2O5 (80 mol% sodium sulfate-20vanadium pentoxide; high sulfate...

Oxidation behaviour of Fe–Si thermal spray coatings

Abstract The oxidation resistance of Fe x Si metallic coatings, with x =17, 25, 35, 45 and 75% in weight, have been evaluated in pure O 2 , in the temperature range of 600–900°C after exposure for up to 24 h. The substrate was a 304 type stainless steel, and between the alloy and the coating a Ni20Cr coating was used to improve the adhesion of the Si-base. The coatings were applied with a powder flame spraying gun. The Ni–20%Cr thermal spray coating had the lowest oxidation resistance, followed by the Fe x Si coating. The uncoated 304 type stainless steel had the highest oxidation resistance. The Fe x Si coatings had a higher oxidation rate than the 304 type stainless steel due to the fact that the coatings had a very high porosity regardless of the content of Si and also to the fact that the diffusion coefficients for Si in Fe–Cr alloys is at least two orders of magnitude lower than that of Cr, so the time required to form a protective SiO 2 layer is much longer than the one for Cr 2 O 3 . There was no major effect of the Si content in the metallic coatings on their oxidation resistance in the range of temperatures tested here. The Ni–20%Cr has a higher oxidation rate than the uncoated 304 type stainless steel, although the Cr content is very similar in both of them, due to the fact that the Cr diffuses much faster in Fe–Cr than in Ni–Cr alloys, and the solubility and diffusivity of O 2 is higher in Ni–Cr than in Fe–Cr alloys.

Hot corrosion of atomized iron aluminides doped with boron and reinforced with alumina

Abstract The hot corrosion resistance of sprayed and atomized Fe–40 at.% Al, Fe40Al+0.1B and Fe40Al+0.1B+10Al 2 O 3 intermetallic materials have been evaluated in NaVO 3 at 625 and 700°C using polarization curves and linear polarization resistance measurements. Also, the results were supported by X-ray diffractometry and electron microscopy studies. The tests lasted 10 days. At 625°C, the Fe40Al+0.1B+10Al 2 O 3 material exhibited the lowest corrosion rate, whereas the Fe40Al had the highest corrosion rate. At 700°C the three materials exhibited erratic behavior during the first 100 h, and after this all the intermetallics had the same corrosion rate. However, the corrosion rate was higher at 625 than at 700°C. The results are discussed in terms of an electrochemical mechanism, the establishment of an Al 2 O 3 layer, which is more protective in the Al 2 O 3 -containing aluminide and seems to increase its protectiveness as the temperature increases from 625 to 700°C.

High-Temperature Degradation and Protection of Ferritic and Austenitic Steels in Steam Generators

AbstractThe useful life of superheaters and reheaters of power stations which use heavy fuel oil is shortened and their continuous service is inhibited by corrosion (fireside) and creep-type problems. The increase of corrosion attack on boilers is caused by the presence of fuel ash deposits containing mainly vanadium, sodium, and sulfur which form low-melting-point compounds. The tubes are exposed to the action of high stresses and high temperatures, producing the so-called “creep damage.” In this work, two kinds of results are reported: lab and field studies using a 2.25Cr-1Mo steel. The laboratory work was in turn divided into two parts. In the first, the steel was exposed to the action of natural ash deposits in oxidant atmospheres at 600 °C for 24 h. In the second part, tensile specimens were creep tested in Na2SO4, V2O5, and their mixture over a temperature range of 580 to 620 °C. In the field work, components of a power station were coated with different types of nickel- and iron-base coatings containing chromium, Fe-Cr, and Fe-Si using the powder flame spraying technique. After testing, the coated tubes were analyzed using electron microscopy. The results showed that all the coating systems had good corrosion resistance, especially those containing silicon or chromium.

Phase Stability Diagrams for High Temperature Corrosion Processes

Corrosion phenomena of metals by fused salts depend on chemical composition of the melt and environmental conditions of the system. Detail knowledge of chemistry and thermodynamic of aggressive species formed during the corrosion process is essential for a better understanding of materials degradation exposed to high temperature. When there is a lack of kinetic data for the corrosion processes, an alternative to understand the thermodynamic behavior of chemical species is to utilize phase stability diagrams. Nowadays, there are several specialized software programs to calculate phase stability diagrams. These programs are based on thermodynamics of chemical reactions. Using a thermodynamic data base allows the calculation of different types of phase diagrams. However, sometimes it is difficult to have access to such data bases. In this work, an alternative way to calculate phase stability diagrams is presented. The work is exemplified in the Na-V-S-O and Al-Na-V-S-O systems. This system was chosen because vanadium salts is one of the more aggressive system for all engineering alloys, especially in those processes where fossil fuels are used.

Study of hot corrosion of alloy 800 using linear polarisation resistance and weight loss measurement

Abstract A study of the hot corrosion of alloy 800 at 700 and 900°C in 80Na2SO4+20V2O5 (wt-%) has been carried out. The techniques included linear polarisation resistance (LPR) and weight loss measurement during 10 days of experiments, supported by X-ray measurements and electron microscopy. The degradation of the alloy was discussed in terms of the dissolution of the external Cr2O3 layer with subsequent oxidation and sulfidation of the alloy. Both LPR and weight loss results showed that the corrosion rate is slightly higher at 900 than at 700°C. At the beginning of the experiments, the corrosion rates obtained using the two techniques increase at both temperatures, but decrease later on until they reach steady values. The measured corrosion rates were within the same order of magnitude for both techniques, but the rates obtained with the electrochemical technique were lower than those obtained using weight loss at 700°C, while at 900°C, the opposite was observed. This is because at 700°C the step that controls the corrosion process is the diffusion of species in the alloy or in the melt, whereas at 900°C, the step that controls the reaction is either charge transfer or there is a mixed control. It is evident that electrochemical techniques are powerful tools for monitoring on line hot corrosion processes when the corrosion rate is controlled by an electrochemical reaction, but may induce some errors when the molten salt changes from an ionic to an electronic conductor.

Oxidation behavior of atomized Fe40Al intermetallics doped with boron and reinforced with alumina fibers

Isothermal oxidation resistance of Fe40 (at.%) Al-based atomized and deposited intermetallic alloys has been evaluated. The alloys included Fe40Al, Fe40Al + 0.1B, and Fe40Al + 0.1B + 10Al2O3 at 800, 900, 1000, and 1100 °C. The tests lasted approximately 100 h, although in most cases there was scale spalling. At 800 and 900 °C, the Fe40Al + 0.1B alloy had the lowest weight gain, whereas the Fe40Al alloy had the highest weight gain at 800 °C (0.10 mg/cm2) and the Fe40Al + 0.1B + 10Al2O3 alloy was the least oxidation resistant at 900 °C with 0.20 mg/cm2. At 1000 °C, the Fe40Al + 0.1B alloy showed the highest weight gain with 0.12 mg/cm2 and the Fe40Al alloy the lowest. At 1100 °C, again, as at 900 °C, the Fe40Al alloy was the least resistant, whereas the Fe40Al + 0.1B alloy performed the best, but the three alloys exhibited a paralinear bahavior on the weight-gain curves, indicating the spalling, breaking down, and rehealing of the oxides. This spalling was related to voids formed at the metal-oxide interface.

Corrosion Performance of Fe-Cr-Ni Alloys in Artificial Saliva and Mouthwash Solution

Several austenitic stainless steels suitable for high temperature applications because of their high corrosion resistance and excellent mechanical properties were investigated as biomaterials for dental use. The steels were evaluated by electrochemical techniques such as potentiodynamic polarization curves, cyclic polarization curves, measurements of open circuit potential, and linear polarization resistance. The performance of steels was evaluated in two types of environments: artificial saliva and mouthwash solution at 37°C for 48 hours. In order to compare the behavior of steels, titanium a material commonly used in dental applications was also tested in the same conditions. Results show that tested steels have characteristics that may make them attractive as biomaterials for dental applications. Contents of Cr, Ni, and other minor alloying elements (Mo, Ti, and Nb) determine the performance of stainless steels. In artificial saliva steels show a corrosion rate of the same order of magnitude as titanium and in mouthwash have greater corrosion resistance than titanium.

Corrosion monitoring using electrochemical noise and linear polarization resistance in fuel oil combustion gas environment

Corrosion monitoring of different steels is carried out online in a combustion rig firing 32 kg/h of fuel oil. Two temperature-controlled probes are designed to allow control of the specimens temperature and the use of electrochemical noise (EN) and linear polarization resistance (LPR) techniques for corrosion monitoring. Two probes are placed where the combustion gas reached a temperature of 850–900°C, and another one at the combustion gas exit where the rig was at 200–240°C. Corrosion rates of an austenitic and a ferritic steel are obtained where the temperature of the combustion gas is 850–900°C, firing fuel oils with different content of Na-V-S. Corrosion monitoring of mild steel is carried out in the test burning a fuel oil with the higher content of Na-V-S by placing a corrosion probe in the low combustion gas temperature zone. The EN results show that this technique is able to assess the corrosion rate in an environment at high temperature where fuel oil ashes deposited and at a temperature high enough where they start to melt and a corrosion process proceeds. Results show that this technique is able to assess the corrosivity of fuel oil ashes originated from fuel oil containing different amounts of sodium, vanadium, and sulfur as corrosion causing impurities. Results of the low-temperature probe show that EN and LPR are able to detect the onset of corrosion on mild steel as a result of sulfuric acid condensation on the probe. However, the corrosion rates are not the same, because localized corrosion is taking place as detected by the EN technique. It is demonstrated that the use of two techniques for corrosion monitoring can give a better understanding of the corrosion process. Electrochemical techniques used to assess the corrosion resistance of alloys at high and low temperatures prove to be a valuable tool for the purposes of materials selection or controlling the main process variables that affect the corrosion resistance of materials in industrial equipment.

Effect of Boron on the Hot Corrosion Resistance of Sprayed Fe40Al Intermetallics

The corrosion resistance of three Fe40Al+10Al203based sprayed intermetallic alloys with different boron contents in molten sodium sulfate (Na2S04) and sodium metavanadate (NaV03) has been evaluated using the weight loss technique. Boron content was 0.1 (at.%), 0.2 and 0.4 and the temperature varied from 873 Κ to 973 Κ for NaV03 , and from 1173 Κ to 1273 Κ for Na2S04 at 25 Κ intervals. The tests were also supported by electronic scanning microscopy and microanalysis. In all cases, less corrosion resistance was obtained in the alloy with 0.4 13, whereas good corrosion resistance was found in the alloy with 0.1 B. The results were discussed in terms of dissolution of the external A1203 layer by the salt and internal sulfidation.