J. G. Gonzalez-Rodriguez

Effect of heat treatment on the stress corrosion cracking of alloy 690

Abstract The susceptibility to stress corrosion cracking (SCC) in 0.01 M Na2S2O3 of alloy 690 under different heat treatments has been investigated using slow strain rate tests. Double-loop electrochemical potentiokinetic reactivating tests in 0.5 M sulfuric acid+0.0001 M potassium thiocyanate was used to evaluate the resistance to intergranular attack (IGA). Alloy 690 was solution annealed at 1100xa0°C for 30 min and water quenched (WQ), followed by sensitization at 700xa0°C for 5, 12, 24, 36, 48 and 72 h and then WQ. The susceptibility to IGA and intergranular stress corrosion cracking (IGSCC) in 0.01 M Na2S2O3 at 90xa0°C increased with increasing the aging time, reaching a maximum value at the aging time of 48 h, and then, it decreased again for the specimen sensitized during 72 h. The chromium-depletion profile was deeper also for the specimen aged for 48 h. These results are discussed in terms of consumption of carbon during sensitization, and a continuous diffusion of chromium, ‘back-filling’ the prior depletion profile, recovering the IGSCC resistance. Also, it is suggested that the presence of a continuous network of grain boundary carbides in the specimen sensitized during 72 h, rather than discrete carbides, as in the one aged during 48 h, is another reason for the increased SCC resistance.

Oxidation performance of a Fe–13Cr alloy with additions of rare earth elements

Abstract The influence of rare earth elements (REEs) i.e. Neodymium (Nd) and Praseodymium (Pr) on the oxidation behavior of a Fe–13Cr alloy has been studied, and its role on the oxidation rate and oxide morphology and formation is discussed. Specimens were isothermally oxidized in oxygen at 800xa0°C for 24xa0h. It was found that a small addition (≤0.03xa0wt.%) of either Nd or Pr, reduced the oxidation rate of the Fe–13Cr base alloy. Moreover, the simultaneous addition of both elements to the alloy produced a dramatic reduction in the oxidation kinetics. Analysis by scanning electronic microscope (SEM) revealed that the morphology of oxides formed on Fe–13Cr specimens with and without REEs specimens was very different. In fact, a fine-grained oxide morphology was observed for alloys with REEs addition. For these alloys only, chromium enrichment at the metal/scale interface was observed. From transmission electronic microscope (TEM) analysis, it was found the following: at the early stages of oxide formation, after 0.25xa0h, Cr 2 O 3 , Fe 3 O 4 , α-Fe 2 O 3 and γ-Fe 2 O 3 were formed; at 6 h, Cr 2 O 3 , FeCr 2 O 4 and α-Fe 2 O 3 were identified and, for exposure times greater than 6xa0h, Cr 2 O 3 , α-Fe 2 O 3 and a spinel which was presumably transformed into a solid solution (Fe 2 O 3 ·Cr 2 O 3 ) were found.

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 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.

SCC of X-65 Weldment Assessment in Diluted NaHCO3 Solutions with Chloride and Sulfate Ions

In the petroleum Mexican industry, most transportation of hydrocarbons is carried through underground pipelines, which are protected by coating and cathodic protection; however, pipelines could be susceptible to the stress corrosion cracking (SCC) and hydrogen embrittlement damage. In this sense, SCC assessment of API X-65 pipeline weldments in sodium bicarbonate solutions at 50 °C using slow strain rate tests (SSRT) was realized. SSR tests were complemented with hydrogen permeation and electrochemical polarization evaluations. Results showed that corrosion rate, i corr, was highest in both 0.01 M NaHCO3 and 0.01 M NaHCO3 with additions of SO4 2− ions solutions. The SCC susceptibility, in accord with the reduction in area percentage (RA%), was higher within the 0.01 M NaHCO3 and NaHCO3 with Cl− + SO4 2− ions addition solutions. The amount of hydrogen uptake for the weldment was also higher in the 0.01 M NaHCO3 solution with and without Cl− + SO4 2− ions. The SCC susceptibility of X-65 weldments in diluted NaHCO3 solutions was associated with the anodic dissolution, assisted by hydrogen entrapping within the weldment microstructure defects, as hydrogen embrittlement damage. Results were complemented by Scanning Electronic Microscopy and Energy of Dispersion of X-Ray Spectroscopy characterization.