Applied Engineering Letters

89-98

Authors: G. Kostadinov, P. Danailov, R. Dimitrova, M. Kandeva, T. Penyashki, V. Kamburov, A. Nikolov, B. Elenov

This paper examines the topography and roughness of coatings of hard alloyed materials deposited by electrospark deposition (ESD) on titanium and nickel alloys with different initial roughness. The influence of the electrode materials and the energy parameters of the ESD process on the roughness parameters Ra, Rt, Rq, Rz, Rmax, on the depth of waviness Wt, the asymmetry Rsk, the excess Rku, on the depth of core roughness Rk and the depth of the reduced picks roughness Rpk was established. The conditions for ESD are indicated, under which the traces of the previous treatment are erased and the characteristics of the initial substrate surface are improved. The numerical values of the roughness parameters for electrospark coatings from five types of hard alloys and one type of TiB2 based electrode with nanosized additives have been established.

105-110

Authors: A. Miteva, A. Petrova, G. Stefanov

Characteristic of the samples based on Al-Si alloys, that we have studied, is their propensity for oxidation at high temperatures. The few number of papers on the oxidation kinetics of aluminium alloys associated with Planar Flow Casting technology (PFC) can be found in the literature. The oxide processes and the properties of the oxide layer formed on the aluminium alloy are mainly considered. The aim of the present work is the kinetics of oxide layer formation on aluminium alloys to clarify whether alloys are suitable for use in space and under specific conditions. The formal kinetics of Nikitin was applied for data processing. The influence of the alloying elements on the formed surface oxide coating is shown.

124-131

Authors: I. Ramadan, M. Tănase

The study on the tubes (made of CuZn36 material) of a condenser used in petrochemical industry revealed that thickness reduction by corrosion is the main factor causing the decrease of the equipment service life. The present paper aims to study, from the experimental point of view, the behavior of the heat exchanger tubes, in the presence of different environments. The experimental program conducted in the article included two directions: evaluation of technical condition of in-service brass tubes after a certain period of operation for a condenser used in petrochemical industry in order to see how the working conditions affect the brass tubes and an experimental study based on simulation of exposure to extreme corrosive environments for samples taken from new CuZn36 tubes. The goal is to establish their influence on the tubes degradations by determining the corrosion rate. The analyzed samples were immersed in different corrosive solutions for 21 days. Another sample was boiled in water and cooled in air in order to simulate the working environment of the tubes in the condenser. All the samples were weighted before and after the exposure in the aggressive environment, thus establishing the corrosion rate for each of them. In order to analyze from mechanical point of view the influence of the environment on the brass tubes, the flattening test was made, comparing the behavior of the exposed samples with that of the unexposed sample.