DETERMINATION OF WORKABILITY OF ZINC COATINGS IN CONDITIONS OF MECHANICAL STRESSES AND CORROSIVE ENVIRONMENT

2018;
113-117
Authors:
1
KARPENKO PHYSICO-MECHANICAL INSTITUTE OF THE NAS OF UKRAINE

For defense the constructions elements from corrosion on the Black Sea shelf are often used organic and inorganic coatings, and as well corrosion-resistant alloys, which makes it possible to use equipment in more aggressive environments, expanding the conditions for their applying. However, the protective properties of anti-corrosion coatings in media in the presence of hydrogen sulfide and the simultaneous action of mechanical stress is not sufficiently studied. The purpose of the work was to determine the effect of the concentration of hydrogen sulfide in the model seawater environment at the mechanical stress on the protective properties of zinc coating of the obtained from the melted  on the 0.2 % С steel. The determination of the stresses under those the coating begins to destruction in various solutions, samples with a diameter of 6.4 mm slowly stretched (10-6 mm/s) with simultaneously control of the nature of the change in the electrode potential and stress and plotted graph the dependencies of the electrode potential on the magnitude of the stresses.

The testing was conducted in the model of seawater – 3% the sea salt solution (mass. %: NaCl ≤ 86,5; Ca2+ ≤ 1,5; Mg2+ ≤ 2,0; K+≤ 1,11; ≤ 7,68; HCO ≤ 0,41; others ≤ 0,8; pH ~ 6,4), model sea water with different content of hydrogen sulfide mg/l: 15 (pH ~ 5,2); 75 (pH ~ 4,8); 150 (pH ~ 4,7) and saturated (pH ~ 4), standard solution NACE (5% solution NaCl + 0,5% CH3COOH, saturated H2S, pH 3…4, 22±3оС) and 5%-solution NaCl + 0,5% CH3COOH (pH ~ 2,7).

It has been established that the rate of corrosion increases with an increase in the content of hydrogen sulfide from 15 mg / l to saturation of the solution of model of seawater for steel 20 in ~ 1,5 ... 4,0 times, and for zinc coating from the melt in ~ 1,2 ... 3,5 times. In the standard solution of NACE the zinc coatings are intensively corroded, since in acid solutions insoluble zinc sulfides do not affect the electrode processes, because they are formed in an electrolyte and are not bound to the surface of the samples. In the model sea water the zinc coatings reduce the rate of corrosion of steel 20 in ~ 1.7 times, and for the elevated content of hydrogen sulfide from 15 mg / l and the saturation reduce by ~ 2.0 times.

The destruction of the zinc coating obtained from melted, passes under the stresses ~ 275 MPa equal to the fluidity boundary of steel 20 is determined. Тhе damaging zinc coating, obtained from melted, in the environments of the model sea water electrochemically protects steel, while in the model sea water saturated with hydrogen sulfide it has a limited time protective effect. It has been shown that in sea water with saturated hydrogen sulfide hot zinc coating increases the resistance to corrosion cracking of steel 20 by ~ 20% threshold stresses of fracture increase from 188 to 232 MPa.

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