Long-term effects of a chemical decontamination procedure on the corrosion state of the heat exchanger tubes of steam generators

Our previous studies have revealed that a ”hybrid” structure of the amorphous and crystalline phases is formed in the outermost surface region of the austenitic stainless steel tubes of steam generators (SGs) as an undesired consequence of the industrial application of the AP-CITROX (AP: alkaline permanganate; CITROX: citric and oxalic acid) decontamination technology. The formation of this mobile oxide-layer increased the amount of the corrosion products in the primary circuit significantly, resulting in magnetite deposition on fuel assemblies. Owing to the fact that there is no investigation method available for the in-situ monitoring of the inner surfaces of heat exchanger tubes, a research project based on sampling as well as on ex-situ electrochemical and surface analytical measurements was elaborated. Within the frame of this project, comprehensive investigation of the general corrosion state and metallographic features of 36 stainless steel specimens, cut out from various locations of the 21 steam generators of the Paks NPP in the time period of 2000-2007 has been performed. The present work gives a brief overview on the general corrosion state of the heat exchanger tubes of SGs, concerning the long-term effects of the AP-CITROX procedure on the chemical composition and structure of the protective oxide-layer.

Kinetic Study of Oxide Growth at High Temperature in Low Carbon Steel

High-temperature surface oxidation kinetics were determined for low-carbon steel using a Joule heating device on hollow cylindrical specimens. The growth of the oxide layer was measured in situ between 800 and 1050 ∘C under isothermal oxidation conditions and in an air laboratory atmosphere (O2 = 20.3% and humidity = 42%). Through a laser and infrared measuring system, the expansion and temperature were measured continuously. From the data acquired, the oxidation kinetic parameters were obtained at different temperatures with a parabolic-type growth model to estimate the rate of oxide layer generation. The convergence degree of the data fitted with the oxidation model was acceptable and appropriately correlated with the experimental data. Finally, comparisons were made between the estimated kinetic parameters and those reported in the literature, observing that the activation energy values obtained are in the range of the reported values.

Effect of kerosene combustion atmosphere on the mild steel oxide layer

In arson cases, accelerants were usually used by criminals to achieve the purpose of rapid arson. Therefore, fire investigators aim to determine whether accelerants was used in the fire scene. Metallic material has to react with corrosive gas around it at high temperature and the oxidation products may store the information of reactants. Accelerants present in fire scenes impart some oxidative characteristics on metallic materials. The aim of this work is to figure out the possibility to identify the presence of accelerant in a fire according to the oxidation patterns of metallic material. This paper researched the oxidation behavior of mild steel at high temperature in a simulated flame environment. The surface morphological and cross-sectional microstructural features of the samples were characterized by X-ray diffractions, X-ray photoelectron spectroscopy and scanning electron microscopy with energy-dispersive spectroscopy analysis after oxidation. The carbon in the combustion atmosphere had a carburizing effect on the metal oxide layer. It was mostly C–C, C–O and C=O of organic matter could be used as in fire investigation. Various oxidizing atmosphere composite systems promote the formation of metal oxide layers. And bidirectional oxidation mode in the oxide layer further accelerates the oxidation rate. The (wustite) FeO phase was not found in the oxide layer because of the strong oxidation of the combustion atmosphere. These results offer complementary information in fire characteristics, which combining the characterization of surface scale with traditional chemical analysis of recovering ignitable liquid residues from fire debris are expected to offer crucial information for determining the presence of combustion accelerants at a fire scene.

Wear Resistance of Aluminum Matrix Composites’ Coatings Added on AA6082 Aluminum Alloy by Laser Cladding

Ceramic-reinforced metal matrix composites are known for their high wear resistance. A coating based on these materials would be helpful to improve the wear behavior of aluminum alloys. Laser cladding has been used to deposit a coating consisting of an aluminum alloy reinforced with SiC particles on an AA6082 aluminum alloy. Laser cladding is a very energetic technique that causes the SiC particles to react with the molten aluminum to form Al4C3, which degrades the particles and reduces the properties of the coating. The formation of this detrimental compound was successfully achieved with the addition of Silicon and Titanium to the composite matrix. The microstructures of the newly developed material were characterized and the wear behavior was studied under dry sliding conditions on a pin-on-disc tribometer. The relationship between the microstructure and wear behavior was identified. The absence of Al4C3 in the Al40Si/SiC and Al12Si20Ti/SiC coatings’ microstructures resulted in an abrasion mechanism instead of a delamination mechanism. The wear behavior changed along the sliding distances. During the first 200 m of sliding distances, the wear rate of all coatings was lower than the uncoated one due to their higher microhardness. For longer sliding distances, the wear resistance of the uncoated AA6082 was higher than the coated ones due to the formation of a lubricant oxide layer on the AA6082 worn surface. For 1000 m of wear distances, the wear behavior was different for each coating. The wear rate of the Al12Si/SiC coating continued growing due to the delamination mechanism and the presence of Al4C3 that acted as starting crack points. The wear rate of the Al40Si/SiC coating decreased due to the formation of a thin, superficial oxide layer. The wear rate of the Al12SiTi/SiC progressively decreased along the sliding distance to below the substrate wear rate.