Aggressive Conditions Influence on the Rust Structure in the Chemical Production of the South Kazakhstan

Abzalova D. A., Rizaeva D. S., Koblanova O. N., Abramova G.I.

International Kazakh-Turkish University named after H.A. Yassavi

Shymkent Institute

Metal and metal items corrosion fighting still remains the most serious problem in the national economy, actually, solving this problem will provide for the effective usage of the main funds almost in every field, which consume the metal constructions.

Constructions and equipments in the non-ferrous metallurgy are operating under the influence of different aggressive conditions: acid gases (HCl, SO₂, SO₃, Cl), high air moisture content – more than 75 %, mineral acids solutions of various concentration and so on. In these conditions carbon steel corrosion velocity is more than 0,5 – 1,0 mm/per year, anti-corrosion plating lose their protective characteristics before the fixed period, corrosion is revealed under the plating film, and this provides for the plating destruction.

Rust composition and characteristics in different operating conditions are often not taken into account while fulfilling anticorrosion services, particularly while choosing the type of surface preparation. Metal corrosion products, formed in the different operating conditions possess different phase and chemical composition (1-3, 6).

So, investigation of equipment rust composition performed in the Stock Company ‘Shymkentmai’ and hydro metal and sulphuric acid workshops equipment performed in the Stock Company ‘Yuszhpolymetal’ presented some interest.

Roentgen –structural analysis of the corrosion products performed in different surfaces of the equipment and constructions of the above mentioned workshops proved that corrosion products having colours from light brown to dark brown with different shadings contain in their composition mainly, magnetite, hematite, vustite, and mixed ferric oxides. Corrosion products enhance partially porous structure, and in some places – tight coalescence layer.

The main ferric oxidizing product in the air corrosion of the carbon steels is FeOOH hydroxide, which undergoes crystallization in several modifications. Rust phase composition is almost always represented by α –FeOOH (hetite) and γ – FeOOH (lepidochronite). These substances proved to be good sorbents.

Hetite and lepidochronite in the rust are mixed with the magnetite (Fe₃ O₄), while the latter may be in the form of the thinnest interlayer. In these conditions the magnetite Fe₃ O₄ takes little moisture and is not prone to swelling. Different performance of these three products in the changing air conditions is mainly determined after the rust process.

There are general tendencies and differences in the process of the aggressive gases absorption by the hetite and lepidochronite. The quality of the absorbed sulphuric gas is increasing when the air comparative moisture content is increasing, this may be stated as the general tendency. SO₂ content increasing in the air leads to the increasing of its absorption. The link character of the air moisture with sulphate concentration testifies to the fact of the SO₂ dissolution in the absorbed water, besides absorption. Acid solution easily penetrates through the rust layer to the metal and enhances corrosion.

However, lepidochronite is the more active absorber than hetite at every gas concentration in the air. It is well known from scientific sources that lepidochronite is more easily dissolved in the acids and salt acid solution than hetite, that’s why the acid formed at dissolving is much more wasted in the chemical reactions mainly with lepidochronite.

It has been stated that aggressive condition corrosion products of the Stock Company ‘Yuszhpolymetal’ enterprises contain up to 20% of sulphates and up to 5 % of chlorides.

O_{2 -} content in the air is of great importance for the corrosion. The authors - researchers have supposed that SO₂ should act as an oxidizer like oxygen.

However, it is possible that corrosion was caused by sulphuric acid influence, which is formed in the reaction of SO₂, O₂ and H₂ O; whereas it is catalyzed by the ferric oxide. As a result, Fe SO₄ appears in the rust in the form of the hygroscopic impurity, water solution of which has acid reaction as the result of hydrolyze. Consequently, as the result of such oxidation, ferric sulphate catalyzes rusting, entering the further reactions. According to the experimental data magnetite is of particular importance in the oxygen reduction mechanism (Stock Company ‘Yuszhpolymetal’ sulphuric acid workshop). Ferric oxide as an ordinary rust contacting the metal ferric and the solution of the ferric sulphate is easily reduced to the magnetite, which in its turn is easily oxidized by oxygen. Consequently, Fe₃ SO₄ magnetite thin layer is in the surface of ferric, the pores of which are easily filled with Fe SO₄ solution of (7-9).

The ferric is dissolved in the form of the Fe⁺² ions in this solution, the ions in their turn partially oxidized to Fe⁺³, and solid phase of Fe₃ O₄ is formed. A thick layer of Fe OOH or Fe₂ O₃ is placed over the magnetite.

Cathode conjugate reaction of the corrosion process consists of electrons transition through the semi conducting magnetite to the contact points between Fe₃ O₄ and Fe OOH particles, where the electrons reduce Fe OOH to Fe₃ O_4.

This reaction mechanism is important because of the formation of such elements; during this process a spacious division of corresponding local electrodes may take place. As a result of this Fe SO₄ acidic solution high local concentrations appear, just as in the pitting corrosion. Actually, this may be an explanation of the aggressive conditions influence on rust structure.

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