Materials Science, Vol. 45, No. 1, 2009

BRIEF COMMUNICATIONS

EVALUATION OF THE EFFICIENCY OF CORROSION INHIBITORS FOR PIPE STEELS IN HYDROGEN-SULFIDE-CONTAINING MEDIA

R. M. Yurkevych UDC 620.194

About one third of newly explored gas fields in the Ukraine contain at most 1 vol.% of hydrogen sulfide.For gas fields with relatively low contents of hydrogen sulfide, the application of materials with elevated resis-tance to hydrogen-sulfide stress-corrosion cracking (HSSC) and hydrogen-induced cracking leads to unreasonab-ly high costs spent for the equipment of gas fields. At the same time, the application of insufficiently resistantmaterials may lead to failures of the equipment with unpredictable consequences [1–3]. Thus, it is most reason-able to use steels with relatively low resistance to HSSC in combination with inhibitors.

For the protection against hydrogen-sulfide corrosion, it is proposed to use various domestic and foreign in-hibitors [4–6]. They decrease the rate of general corrosion but not always serve as reliable protectors of steelagainst stress-corrosion cracking. Until recently, the gas-processing equipment of the Lokachyn Gas Field hasbeen protected against possible corrosion failures by the Naftokhim-3 corrosion inhibitor produced by the “Galy-chyna” Enterprise (Drohobych) or by the DODICOR inhibitor (Germany) with similar properties [7]. At thattime, the Naftokhim-8 inhibitor was not commercially produced. Its testing was originated in June 2005, parallelwith testing of the Naftokhim-3 and DODICOR inhibitors.

In view of these facts, the development of highly efficient commercially available domestic inhibitors andthe analysis of their protective action under the conditions close to operating is a quite urgent problem.

Materials and Methods of Investigations

The influence of the Naftokhim-8, Naftokhim-3, and DODICOR inhibitors on the retardation of the generalcorrosion of 20 pipe steel and steel of the hardness group K (Table 1) were studied gravimetrically according to

to the degree of protection Z and the coefficient of corrosion inhibition γ (principal criteria of the efficiency ofinhibitors) in a NACE solution (5% aqueous solution of NaCl + 0.5% CH3 COOH saturated with H2 S; pH 3–4;

23 ± 3°C) [8]. The mechanism of protective action of the Naftokhim-8 and DODICOR inhibitors was studied by the elec-

trochemical method based on measuring the stationary potential (for 24 h) and recording polarization curves onspecimens of low-alloy K steel in the NACE solution. The working area of each specimen was 1 cm2. Theelectrochemical characteristics were measured with a P-5827M potentiostat in the potentiodynamic mode. Therate of changes in the potential was equal to 1.67 mV / sec. As a reference electrode, we used a saturated

silver–silver-chloride electrode (ϕ = 0.2 V) connected with the investigated electrode by using a bridge and aLugin capillary.

Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine; e-mail: iurkevych@ipm.lviv.ua.

Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 45, No. 1, pp. 117–119, January–February, 2009. Original article sub-mitted August 18, 2008.

132 1068–820X/09/4501–0132 © 2009 Springer Science+Business Media, Inc.

EVALUATION OF THE EFFICIENCY OF CORROSION INHIBITORS FOR PIPE STEELS IN HYDROGEN-SULFIDE MEDIA 133

(a) (b)

Fig. 1. Degree of protection (a) and the coefficient of corrosion inhibition (b) of the hydrogen-sulfide corrosion of K steel by the Naf-

tokhim-8 (1), Naftokhim-3 (2), and DODICOR (3) inhibitors.

Table 1. Chemical Compositions and Mechanical Characteristics of K Steel and 20 Steel

SteelContents of elements, % σ0.2 σu

C Mn Si Cr Ni S P other MPa

K 0.3 0.8 0.17 — — 0.02 0.013 — 542 987

20 0.17 0.35 0.17 ≤ 0.25 ≤ 0.25 ≤ 0.02 ≤ 0.04 0.25 Cu 175 350

Results and Their Analysis

It was established that the degrees of protection ( Z ) of the Naftokhim-8, Naftokhim-3, and DODICOR in-hibitors are equal to 92, 86, and 80%, respectively. At the same time, the coefficients of inhibition of hydro-

gen-sulfide corrosion in the NACE medium ( γ ) are equal to 13, 7, and 5, respectively (Fig. 1). The visual inspection of the samples of 20 and K steels after testing in the hydrogen-sulfide-containing me-

dium with Naftokhim-8 inhibitor proves its efficiency even after three months of holding. The surface of thesamples was clean and without pittings (Fig. 2).

The high efficiency of the Naftokhim-8 inhibitor is also confirmed by the data of electrochemical measure-ments (Fig.3). As a result of adding 50 mg / liter of the Naftokhim-8 and DODICOR inhibitors in the NACEmedium, the corrosion potential shifts in the negative direction by 20 and 10 mV, respectively (Table 2). Thecorrosion current density for K steel becomes 8 and 4 times lower and the degrees of corrosion protection Zcomputed in the electrochemical investigations are equal to 89 and 76% for the Naftokhim-8 and DODICOR in-hibitors, respectively, which agrees with the data obtained by the gravimetric method (Fig. 1). As a result of ad-sorption of the inhibitors on the surface of K steel, the Tafel slopes of the cathodic and anodic polarizationcurves increase. Under the action of the inhibitors, the rate of cathodic reaction becomes two times lower andthe rate of the anodic reaction decreases by a factor of 1.7, which means that the analyzed inhibitors are ofmixed action.

134 R. M. YURKEVYCH

Fig. 2. Surfaces of 20 steel (1, 1′ ) and K steel (2, 2′ ) after testing for three months in NACE solutions: (1, 2) without inhibitors, (1′,2′ ) in the presence of the Naftokhim-8 inhibitor.

Table 2. Electrochemical Characteristics of K steel in NACE Hydrogen Sulfide Solution without and withthe Naftokhim-8 and DODICOR Inhibitors

Inhibitors C, mg / liter – Ecor , V icor , A / m2 bc , V ba , V

— — 0.63 0.97 0.038 0.030

DODICOR 50 0.62 0.23 0.055 0.050

Naftokhim-8 50 0.61 0.11 0.078 0.060

Thus, the best protective properties among three investigated inhibitors are exhibited by the Naftokhim-8inhibitor produced from domestic raw materials and, thus, characterized by serious cost advantages.

EVALUATION OF THE EFFICIENCY OF CORROSION INHIBITORS FOR PIPE STEELS IN HYDROGEN-SULFIDE MEDIA 135

Fig. 3. Polarization curves of K steel: (1) NACE solution, (2) Naftokhim-8 inhibitor, (3) DODICOR inhibitor.

CONCLUSIONS

On the basis of the analysis of polarization curves, it is shown that the Naftokhim-8 and DODICOR are in-hibitors of mixed action. It is also demonstrated that the Naftokhim-8 inhibitor exhibits the best protective pro-perties in hydrogen-sulfide-containing media.

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