Institute of Inorganic Chemistry of Riga Technical University, 34 Miera Str., Salaspils

Inhibition efficiencies of some polyolborates on the corrosion of steel and non-ferrous metals in water media

I. Zarina, R. Ignash, Z. Rubess

Institute of Inorganic Chemistry of Riga Technical University, 34 Miera Str., Salaspils, Latvia

Dr.chem. Inara Zarina, Head of the Laboratory of Protective Coatings

Dr.Chem. Roza Ignach, Sen. researcher of the Laboratory of the Protective Coatings

Zigurds Rubess, Leading eng. of the Laboratory of Protective Coatings

Institute of Inorganic Chemistry of Riga Technical University, 34 Miera str., Salaspils, LV-2169, Latvia

Abstract.

The present paper reports in detail the interaction of D-sorbitol and D-mannitol with sodium tetraborate by the method of isomolar series using conductometry and polarimetry. From the specific electroconductivity and optical rotation angle data in the polyol- sodium tetraborate -water systems the stability constants of sodium sorbitolborate and mannitolborate were calculated using the classic laws of electrochemistry. Deviations from additivity of the specific electroconductivity and the rotation angle of light polarization plane were investigated the formation of two complex anions [BPolyol]־ and [B2Polyol]²־. Based on the results of chemical analysis, thermoanalytical curves, IR absorption spectra of the resulting product prove the individuality. Provided is the use of sodium sorbitolborate and sodium mannitolborate with moral ratio of components 1:1 as a corrosion inhibitor in combination with additives for steel and non-ferrous metals (aluminium, copper, brass, solder) in a water media. The results of corrosion tests show that the corrosion rate is dependent of initial concentration and character of inhibitors and additives. Corrosion tests were carried out by gravimetric method under static and dynamic conditions at 200 and 700C.

Keywords: corrosion inhibitors, sorbitolborate, mannitolborate, compositions, water media

1.Introduction

Boron compounds, especially borax, are widely used as part of several compositions for inhibiting corrosion in water, but their efficiency is low and great concentrations must be used – till 7% for midl steel [1,2]. Role of borax is only to maintain the pH of the solution constant – that is to attach the buffer properties to the solutions, in which the protected metal (midl steel) constructions act. The attained inhibition effectiveness of borax in artesian water at 700C is 10, 30, 65 % at corresponding concentrations, g/l : 1, 3, 5. We propose the boron compounds of other type – the salts of boron coordination compounds, where ligands are polyhidroxy organic acids or polyols. In the coordination compounds of tetracoordinated boron are not toxic, as sorbitol and mannitol are food products or remedies as calcium gluconate.

The main object of discussing are boron-coordinated compounds with polyols in particular, their sodium salts – Na sorbitolborate and Na mannitolborate, which have been first synthesized in RTU Institute of Inorganic Chemistry [3] and investigated as corrosion inhibitors of steel and non-ferrous metals in water media in heating and cooling systems. Since these compounds are well soluble in water, syrup-like concentrated solutions in water were used to investigate their inhibitory properties. It prevents formation of the boiler scale; it is effective in static as well as in dynamic regime.

Sodium sorbitolborate and sodium mannitolborate were separated as solid phase by evaporating the concentrate at room temperature. With reference to the data of chemical and thermal analyses and the data of IR absorption spectra study, the uniqueness of Na sorbitolborate and Na mannitolborate compounds NaC6H14BO8 has been proved. D-sorbitol and D-mannitol are isomers of six-atom ethanol. Na sorbitolborate and Na mannitolborate are white glassy substances, well soluble in water, ethylene and propylene glycols.

2. Experimental

Thermal decomposition of the complexes was investigated using a derivatograph Q-1500 at a heating rate of 10 0C per minute. IR absorption spectra were taken by a spectrometer SPECORD using tablets with KBr (2).

The interaction of D-sorbitol and D-mannitol with Na tetraborate was investigated in detail by a method of isomolar series using conductometry and polarimetry. Electroconductivity and rotation angle of light polarization plane deviations from additivity were investigated at the total concentration of components being 0.125 mol/l. Electroconductivity was measured by a Conductolyze LKB-5300 B device at 25  0.1 0C. The angle of rotation was measured by a circular polarimeter CM-3 at 18  2 0C.

Thermal decomposition of the complexes was investigated using a derivatograph Q-1500 at a heating rate of 10 0C per minute. IR absorption spectra were taken by a spectrometer SPECORD using tablets with KBr.

Corrosion tests were carried out by gravimetric method under static and dynamic conditions at 200 and 700C within 336 hours, using samples of steel, copper, aluminium and brass. Samples of the following materials were underwent corrosion tests: steel 08PC (% mass: C 0.07; Si < 0.03; Mn 0.27; P 0.0022; S 0.03; Ni < 0.2; aluminium (% mass Cu 3,8; Mg 1.2; Mn0.3; Ni< 0.1; Fe< 0.5; Si < 0.5; Zn<0.3); copper M-1; brass Л63; dimensions 40.0 x 20.0 mm. Prior to tests, the sample plates were sanded (polished by emery paper, washed with water, ethanol or acetone and dried to a constant mass. The samples were placed in special vessels for corrosion tests, which were filled with 300 ml of the tested liquid, plugged hermetically with a cover, heated at 200 C and 700 C . In 336 hours of exposing to the tested solutions, the samples were taken out, cleaned a soft brush, washed in sequence with distilled water and ethanol, dried and weighted. By measuring the change in sample mass, the corrosive action of the tested liquid was determined.

The protecting action of the inhibitor was determined from the formula: E= [(v – v1)/v].100%, where v is the metal corrosion rate without inhibitor (g/m2.h); v1 is the metal corrosion rate with inhibitor (g/m2.h). The efficiency of protection was evaluated from the coefficient of corrosion retardation: γ = v/v1, where v is the metal corrosion rate without inhibitor (g/m2.h); v1 is the metal corrosion rate with inhibitor (g/m2.h).

Compositions based on Na sorbitolborate, Na mannitolborate and containing Na silicate, hexametaphosphate (HMF), borax (Na2B4O710H2O), which inhibit the corrosion, were also investigated.

3. Results and Discussion

The results of investigations the systems D-sorbitol – (D-mannitol )– Na tetraborate – H2O there is observed a decrease of electroconductivity if compared to that of Na tetraborate. The composition-property diagram shows ( Fig.1, line 1) that deviations from additivity have minima at the molar ratio of polyol:Na tetraborate 1:1 and 1:2. For D-mannitol (Fig.1, line 2) these minimum are less pronounced and shift to the right. The above said testifies that under these conditions two complex anions exist in the systems: [B2L]2- and [BL]-, where L denotes sorbitol or mannitol.

The measurements of the rotation angle prove the formation of the two complex anions (Fig.2). It is confirmed by the deviation of the rotation angle value from additivity. Maxima are observed at molar ratios of polyol to borate-ion 1:1 and 1:2, i.e., the presence of two different complex ions [B2L]2- and [BL]- in the system is acknowledged Fig.2., 1-line sorbitolborate, 2-line mannitolborate).

The present paper reports the investigation results on the complexes of Na sorbitolborate and Na mannitolborate, with the component ratio being 1:1, as steel and non-ferrous metal corrosion inhibitors.

For steel in the presence of Na sorbitolborate and Na mannitolborate (Fig.3,4) the corrosion rate decreases with the increase of the inhibitor concentration, and at the concentration 1.5 g/l an abrupt retardation of corrosion is observed.

With the concentration of Na sorbitolborate and Na mannitolborate 2 g/l, the protection of steel against corrosion achieves 90-94% (Table 1). For copper the corrosion rate does not strongly depend on the inhibitor concentration (Fig.3, 4). In the presence of Na sorbitolborate at the increasing of the inhibitor concentration from 0.25 to 2.0 g/l the protection efficiency enhances from 61 to 72% and Na mannitolborate protects copper almost similarly at all investigated concentrations from 0.5 to 3.0 g/l – 80-86% (Tables 1, 2).

Table 2.

Steel and non-ferrous metals corrosion inhibition by sodium mannitolborate and with other additions in water media

For brass, with Na sorbitolborate as an inhibitor, the corrosion rate varies insignificantly with the increase of inhibitor concentration from 0.25 to 1.0 g/l, but at an inhibitor concentration of 1.5 to 2.0 g/l the corrosion rate greatly decreases and the protection degree is 83-85% (Fig.3, Table 1).

At inhibiting with Na mannitolborate, the optimum concentration of the inhibitor is 2.0 g/l (Fig.4, Table 2). Yet, even at the inhibitor (Na mannitolborate) concentration 0.5 g/l the protection is 67% and the corrosion rate decreases 3 times.

With the investigated inhibitor concentrations neither Na sorbitolborate nor Na mannitolborate protects aluminium against corrosion (Fig.1, 2) only the composition C on the base of Na sorbitolborate with an addition of Na silicate and the compositions A, C on the base of Na mannitolborate strongly retard the corrosion of aluminium and protect it by 84-86% (Tables 1, 2).

The data of Fig.5, 6 and Tab. 1, 2 show a good effectiveness of the proposed compositions. The composition A and B, containing Na sorbitolborate, practically completely protect copper and brass against corrosion, but they are not that effective for steel and do not protect aluminium. The composition C retards the corrosion of aluminium, but is not effective for copper; it protects steel and brass by 94.1 and 90.1%, accordingly.

Fig.5.Corrosion of steel and non-ferrous metals. Medium water, inhibitor –sodium sorbitolborate (BS)

Compositions: A – BS-1+HMF(2+2) g/l

B – BS-1 +HMF(2+6) g/l

C – BS-1 +SiO2(2+0,2) g/l

The results of corrosion tests of the compositions on the base of Na mannitolborate are the following: (Figs.5, 6; Tables 1, 2). The composition A is effective for steel – the corrosion rate decreases 32,7 times; for aluminium the corrosion rate decreases 6.5 times and the protection effect is 84.5%. For copper and brass the corrosion rate insignificantly increases.

The composition B is ineffective for steel (protection effect 47.3%), copper (protection effect 15.5%). It does not protect brass. The protection for aluminium is 58.8%.

The composition C is very effective for steel (protection effect 98.1%), for aluminium (E = 84.1%). This composition is less protecting for copper and brass than Na mannitolborate with no additives (protection effect 82.9 – 74.1% and 81.9-78.2%, accordingly).

Fig.6. Corrosion of steel and non-ferrous metals. Medium water, inhibitor –sodium mannitolborate (MB)

Compositions: A- MB+SiO2 (1,0+0,2) g/l

B- MB+HMF (1,0+0,5) g/l

C- MB+SiO2+Na2B4O7 (1+0,2+3,0) g/l

Conclusions

1. The interaction of D-sorbitol and D-mannitol with odium tetraborate in water solution waas studied by the method of isomolar series.
2. The rotation angle and electroconductivity mearsurments have show the formation of to complex anions: [BL]־ and B2L]²־ with moral ratio of components 1:1 and 2:1 (L-ligand: D-sorbitol, D-mannitol, B-borateion).
3. Sodium sorbitolborate and sodium mannitolborate with molar ration 1:1 and its combination with other additives have been investigated for their inhibition properties on steel, copper, brass and aluminium in water media.
4. Sodium sorbitolborate is effective inhibitor of the corrosion for steel, copper, brass and under optimal conditions the protection efficiency 94%, 72%, 85% accordingly, but does not protect aluminium. The compositions on the base of sodium sorbitolborate, containing hexametaphospate (composition A) are very effective for copper, brass (protection effect 99,9%). The best results for aluminium were obtained with composition (C), containing silicate of alkali metals (protection degree is 85,7%). These compositions (A,B,C) are less protesting for steel than sodium sorbitolborate with no additives.
5. Sodium mannirolborate suppress corrosion of steel, copper, brass ans under optimal conditions inhibition effenciency are 92,5%, 82,9%, 88,7% accordingly, but does not protect aluminium. The best results for aluminium were obtained with compositions containing silicates of alkali metals (composition (A). The protection degree reaches 84,5%. For steel composition C the protection degree reaches 98,15. these compositions (A,B,C) ara less protecting for copper and brass, than sodium mannitilborate with no sdditives.

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