M. A. Espinosa-Medina

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.

Corrosion protection of 1008 carbon steel by hybrid coatings

Abstract Potentiodynamic polarisation curves have been used to evaluate the corrosion performance of 1008 carbon steel coated with hybrid (inorganic-organic) and mixed oxide coating systems deposited by dip coating. Several coatings have been prepared by the sol-gel method, using metallic alkox ides, such as tetraethylorthosilicate, aluminium isopropoxide, and zirconium propoxide, and polymers like allyl methacrylate (AMA) and polymethylmethacrylate (PMMA), together with zirconium dioxide (ZrO2) and silicon and aluminium oxides (SiO3-Al2O3). The aqueous test solutions included hydrochloric acid (HCl), sodium chloride (NaCl), and sulphuric acid (H 2SO4) at various concentrations (0·1, 0·5, and 1M). Scanning electron microscopy was used to examine the coatings following testing. The results have shown that SiO2-Al2O3 based coatings are not beneficial for corrosion protection in HCl, but are of value in the other aqueous media. In 1·0M HCl, the best protection was offered by a hybrid SiO2-ZrO2-poly(methylmethacrylate-allyl methacrylate) (P(MMA-AMA)) coating. In NaCl, the best results, were generally obtained with SiO2-Al2O3 and SiO2-ZrO2-P(MMA-AMA) coatings. Similarly, in H2SO4, the best results were generally displayed by the SiO2-Al2O3 and SiO2-ZrO2-P(MMA-AMA) coatings. The corrosion behaviour of the coatings is discussed in terms of the integrity and stability of the film.

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.

Characterisation of soil/pipe interface at a pipeline failure after 36 years of service under impressed current cathodic protection

Abstract Study on corrosion behaviour of buried pipeline in saline soil is presented, the pipeline is protected by coating and impressed current cathodic protection system. In this work, spectrometric and spectroscopic techniques were used to characterise the pipe/coating/soil interfaces. The results show the coating/pipe interface forms corrosion products including the following phases: magnetite, goethite, akaganeite and hematite. However, the main phases found at coating/soil interface are magnetite and ferric ion. The behavior is consistent with the corrosion of carbon steel in soil with oxygen and chloride ion. The reaction paths are proposed under the conditions of deteriorated coating, cathodic protection, saline soil and seasonal fluctuations.

Evaluation of SCC susceptibility of supermartensitic stainless steel using slow strain rate tests

Abstract The susceptibility to chloride stress corrosion cracking (Cl-SCC) of supermartensitic stainless steel (SMSS) was evaluated at different temperatures through slow strain rate tests. In order to evaluate the Cl-SCC susceptibility the reduction in area and the time to failure ratio were considered. It is clear that all specimens tested in the chloride solution exhibited a generally high resistance to chloride SCC. However, according to the slow strain rate tests results, at lower temperature (5°C) the susceptibility to Cl-SCC increases compared to the other temperatures studied; also an increase in the corrosion rate was observed as the temperature decreased according to electrochemical impedance spectroscopy measurement. The corrosion mechanism observed on the material under dynamic stress was a combination of high activation resistivity with a diffusive process on the metallic interface given by a thin film from a protector layer of chromium oxides. This mechanism generates a high corrosion resistance and therefore good performance to chloride induced SCC.

Stress Corrosion Cracking Behavior of X60 Pipe Steel in Soil Environment

Stress corrosion cracking (SCC) susceptibility of API X60 pipeline steel in a soil solution by slow strain rate tests (SSRT), and surface fracture analysis was investigated. The SSRT were performed at strain rate of 25.4 × 10 -6 mm/sec in a glass autoclave containing the soil solution called NS4 with pH of 3 and 10 at room temperature and 50°C. Both anodic and cathodic polarization potentials of 200 mV referred to E corr was applied. The results of ratio reduction area (RRA), time to failure ratio (TFR) and elongation plastic ratio (EPR) indicate that X60 pipeline steel was susceptible to SCC at pH 3 and cathodic polarization of -200 mV at room temperature and 50°C. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The SCC process and mechanism of X60 steel into NS4 solution was hydrogen based mechanism. With the different applied potentials the dominance of SCC process changes. At low pH the temperature effect on SCC susceptibility is more noticeable at 20°C. However at high pH this effects changes, being the steel more susceptible to SCC at 50°C.

Cathodic Protection Effect on the Assessment of SCC Susceptibility of X52 Pipeline Steel

Assessment of anodic and cathodic potentials on stress corrosion cracking (SCC) of API X52 pipeline steel through slow strain rate tests (SSRT) was studied. The SSRT were carried out in a NS4 solution to simulated dilute ground water that has been found to be associated with SCC of pipelines. SSRT were performed and evaluated in air and in the NS4 solution at room temperature at an extension rate of 1×10 -6 in/sec. Tests were performed at controlled electrochemical polarization potentials, both anodic and cathodic (100, 200, 400 mV) versus the open circuit corrosion potential. The results of reduction in area ratio (RAR), time to failure ratio (TFR) and plastic elongation ratio (PER) of the specimens tested in the soil solution indicate that X52 pipeline steel was susceptible to SCC at cathodic potentials. These specimens showed a brittle type of fracture with transgranular appearance. The SCC proceess and mechanism of X52 steel in the NS4 solution is mixed-controlled by both anodic dissolution and the hydrogen involvement. At positive potentials the SCC is based mainly on the anodic dissolution mechanism. When the applied potentials shifted negatively, the SCC on the steel follows mainly hydrogen embrittlement mechanism. This mechanism was confirmed through the internal cracks observed in the specimens.

Oxidation behaviour of Fe–40 at.-% Al intermetallics with Li or Cu additions at high temperature

ABSTRACT Fe–40 at.-% Al intermetallics with the addition of Li (1 and 3 at.-%) and Cu (3 and 5 at.-%) were produced using standard casting techniques. The oxidation behaviour was obtained using thermogravimetric analyses in an atmosphere of 99.99% oxygen at 800, 900, 1000, and 1100°C. The oxidation product layers on the top and cross-sectional views were characterised using SEM and energy-dispersive X-ray spectroscopy mapping. The different alumina phases can produce a double layer with different mechanical and chemical properties. The results showed that the addition of 1% Li assisted the activation of the oxide production at 1000 and 1100°C, while the rest of the third element additions slightly modified the oxidation resistance.

Assessment of pH and temperature effects on stress corrosion cracking of 1018 low carbon steel

Abstract Stress corrosion cracking tests (SCC) were performed on 1018 low carbon steel into 0·5M NaCl solution at different pH values and temperatures of 25, 50 and 70°C using the slow strain rate testing technique. Evaluations were complemented with hydrogen permeation measurements and electrochemical impedance spectroscopy. Studies show hydrogen diffusion effects depending on temperature and pH. The maximum hydrogen diffusion was observed at 70°C (pH<1). The SCC susceptibility was measured as the percentage reduction in area and the time to failure and was maximal at pH<1 at the temperatures studied. The most likely mechanism for the cracking susceptibility of 1018 steel in the chloride solutions seems to be hydrogen assisted anodic dissolution assisted by hydrogen embrittlement. After failure, the fracture surfaces were observed by scanning electron microscopy and chemical analysis was obtained by X‐ray energy dispersive spectroscopy. The specimens tested in air exhibited a ductile type of failure, whereas, in the corrosive solution the specimens showed a brittle fracture.

Structural characteristics and thermal stability of bimetallic Cu-Ru nanocluster

Classical molecular dynamics simulations were used to study the structural characteristics and thermodynamic stability of Cu-Ru nanoclusters of most common structural variants like decahedra, icosahedra and cuboctahedra. As a result, it is found that the three types of structures have consistent stability orders from the point of view of bimetallic elemental distributions. The Ru-core/Cu-shell structures and the eutectic-like structures are more stable than solid solution and Cu-core/Ru-shell. The formation energies of bimetallic clusters are consistently positive, which shows that they are not thermodynamically stable. All clusters including the unstable Cu-core/Ru-shell and solid solution clusters, and the relatively stable Ru-core/Cu-shell transform into eutectic-like cluster during heating process. Additionally, the large difference in melting point between pure Cu and pure Ru cause the Cu part in the formed eutectic-like clusters melt first and escape forming separate cluster in the further heating process.