Article ID: 1003-6326(2005)03-0626-05

Inhibition effects of PMA/SbBr₃ complex inhibitor on copper and copper-nickel alloy in LiBr solutions  

HU Xian-qi(扈显琦), LIANG Cheng-hao(梁成浩), HUANG Nai-bao(黄乃宝)

(School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China)

Abstract: The effects of PMA/SbBr₃ inhibitor on copper and copper-nickel alloy in 55%LiBr solution were investigated by chemical immersion and electrochemical measurements. The results indicate that in boiling 55%LiBr solution containing PMA/SbBr₃ inhibitor, corrosion rates of copper and copper-nickel alloy are 67.48μm/a and 38.14μm/a, respectively. Since both anodic and cathodic electrochemical reactions can be inhibited, PMA/SbBr₃ belongs to complex inhibitor. PMA has the effect of inhibiting hydrogen evolution and [PMo₁₂O₄₀]^3-, the anion of PMA, has a strong oxidizing effect. Sb³⁺ also shows an oxidizing effect. It may exist in LiBr solutions stably with PMA. Because of the synergistic effect of PMA and Sb³⁺, a protective film, comprising CuO, Cu₂O and Sb, formed on copper and copper-nickel alloy surface may prevent Br^- from diffusing to the surface of metals. As a result, the anticorrosion performance of copper and copper-nickel alloy may be improved.

Key words: copper; copper-nickel alloy; lithium bromide; corrosion; inhibitor CLC

number: TG174.42              Document code: A

1 INTRODUCTION

To substitute for Freon, LiBr-H₂O has been used as working fluid pairs of lithium bromide absorption chiller. Because of the excellent heat transfer performance, copper and copper alloy are often used for making heat exchanger. But lithium bromide solution is a highly corrosive medium for copper, copper alloy, carbon steel, even stainless steel. Some researchers^[1] tried to make nickel alloys as lithium bromide absorption chiller. The performances of heat transfer and anticorrosion of nickel alloys are excellent, but they are very expensive. Thus the emphasis of research should be focused on adding inhibitors into lithium bromide solution to decrease corrosion rate by using traditional materials, such as carbon steel, copper and copper alloy^{[2, 3]}, as lithium bromide absorption chiller. Usually, the currently used inhibitors have some weaknesses in lithium bromide solution. Li₂CrO₄ may pollute the environment. Moreover the oxidation value and concentration of chromium are difficult to control. Low concentration of chromium may lead to pitting corrosion^[4]. To substitute for Li₂CrO₄, the application of Li₂MoO₄ or Na₂MoO₄ has been studied deeply^[5-7]. Na₂MoO₄ can form protective film on metal surface and avoid pitting corrosion. But researches show that the solubility of Li₂MoO₄ in LiBr solution is limited, and it is difficult to maintain at an effective concentration. For these reasons, it is very important to look for highly effective inhibitors for lithium bromide absorption chiller. It was reported that heteropoly acid is a polynuclear coordination compound with excellent oxidation-reduction and film-forming characteristics^[8]. But as an inhibitor in LiBr solution, it has been scarcely reported. In this paper, the inhibition performance and mechanism of a new type mixed inhibitor composing phosohomolybdic acid(PMA) and SbBr₃ on copper and copper alloy in boiling 55% lithium bromide(LiBr) solution are discussed.

2 EXPERIMENTAL

The materials used for measurements were copper and copper-nickel alloy and their chemical compositions are listed in Table 1.Test solution was 55% LiBr solution containing 700mg/L inhibitor composing PMA and SbBr₃. Its pH value was controlled at 11.

Rectangular sheet specimens (30mm×20mm×1mm) and square specimens (10mm×10mm×1mm) were used for mass-loss tests and electrochemical measurements, respectively. All specimens were abraded with SiC paper, and rinsed by deionized water and ethanol. 1cm² working area was exposed to the solution in electrochemical measurements and the other part was covered by Shin-Etsu Silicone.

With a polytetrafluoroethylene(PTFE) cylinder bush(50mm in inner diameter, 65mm in length), stainless steel autoclaves were used in mass-loss tests. The 100cm³ of 55% LiBr solu-  

Table 1 Chemical compositions of specimens(mass fraction, %)  

tion, containing 700mg/L mixed inhibitor, was deoxygenated for 1h before the experiments. The autoclave was then held at predetermined temperature for 200h in a thermostat(Kosumosu AT-S13). The specimens were cleaned by 5% HCl for 3min at room temperature and then rinsed by deionized water and ethanol. Corrosion rates were calculated from the mass loss of the specimens. Surface state of the copper specimen immersed for 200h was analyzed by electron probe microanalysis(EPMA).

Electrochemical measurements were conducted in a three-electrode system and HA-501 potentiostat/galvanostat. The specimen was immersed in a glass vessel with a platinum counter electrode and a SCE reference electrode. The solution was deoxygenated by nitrogen during the whole process of measurement. When the solution was maintained at boiling state and the blend potential became stable, the polarization curves were measured at a sweep rate of 20mV/min.

3 RESULTS AND DISCUSSION

Corrosion potentials of copper and copper-nickel alloy in boiling 55%LiBr, 55%LiBr+0.10mol/L LiOH +200mg/L Na₂MoO₄ and 55%LiBr+ PMA/SbBr₃ solutions are shown in Fig.1. According to Fig.1, the corrosion potential of copper in 55% LiBr solution is -617mV, which is close to that of copper-nickel alloy, -613mV. While various inhibitors are added into 55%LiBr solution, the corrosion potential moves to positive direction. In 55% LiBr solution with 0.10mol/L LiOH+200mg/L Na₂MoO₄ , the corrosion potentials of copper and copper-nickel alloy are -579mV and -494mV, respectively. In the solution containing 700mg/L PMA/SbBr₃ inhibitor, their corrosion potentials rise to -540mV and -485mV, respectively. The rising value caused by adding PMA/SbBr₃ inhibitor is higher than that by adding 0.10mol/L LiOH+200mg/L Na₂MoO₄ inhibitor. Obviously, the electrochemical performance of copper and copper-nickel alloy may be improved by PMA/SbBr₃ inhibitor in 55% LiBr solution.

Fig.1  Corrosion potentials of copper and copper-nickel alloy in different solutions

Fig.2  Polarization curves of copper in different boiling solutions

  Fig.2 displays the polarization curves of copper in boiling solutions of 55%LiBr, 55%LiBr+PMA/SbBr₃ and 55%LiBr+0.10mol/L LiOH+200mg/L Na₂MoO₄. As seen in Fig.2, adding PMA/SbBr₃ inhibitor into 55%LiBr solution causes an obvious left shift of both anodic and cathodic polarization curves and an increase of corrosion potential of copper. Compared with LiOH/Na₂MoO₄ inhibitor, the left shift range of polarization curves caused by PMA/SbBr₃ inhibitor is wider. Both anodic and cathodic electrochemical reaction processes of copper may be inhibited by PMA/SbBr₃ additive. It is reported that the electrode reactions of metals in LiBr solution are as follows^[9]:

Anode:

M+nH₂O→M^z+·nH₂O+ze

M^z+·nH₂O→M(OH)z+zH⁺+(n-z)H₂O

Cathode:

2H₂O+2e→2OH^-+H₂

Thus PMA/SbBr₃ inhibitor may prevent copper from dissolving in 55% LiBr solution and inhibit hydrogen evolution.

   Polarization curves of copper and copper-nickel alloy in boiling 55%LiBr+ PMA/SbBr₃ solution are exhibited in Fig.3. Compared with copper, both anodic and cathodic polarization curves of copper-nickel alloy shift left. The anodic polarization curve of copper-nickel alloy shows an obvious passive region near -300mV, and the width of it is about 150mV. Whereas the polarization curve of copper shows that copper keeps an active dissolution state in the same situation. With the potentials moving towards positive direction, its anodic polarization current density increases.

Fig.3  Polarization curves of copper and copper-nickel alloy in boiling 55%LiBr+PMA/SbBr₃ solution

    Curves in Fig.4 present the corrosion rates of copper and copper-nickel alloy in boiling 55%LiBr+PMA/SbBr₃ solution. The corrosion rates of copper and copper-nickel alloy increase with the rise of solution temperature. At the same temperature, the corrosion rate of copper-nickel alloy is lower than that of copper, which indicates that the anti-corrosion performance of copper-nickel alloy is better than that of copper. This result coincides with the results displayed in Fig.3. According to Refs.[5, 10], with the increase of nickel content in copper-nickel alloy, impedance of alloy increases and corrosion potential moves towards positive direction in LiBr solution. It is because that adding nickel into copper can improve the thermodynamic stability of metal. It is the main factor that causes the different corrosion behaviors of copper and copper-nickel alloy in LiBr solution. Corrosion rates of copper and copper-nickel alloy in boiling 55%LiBr solution are 114.35μm/a and 96.29μm/a, respectively. When PMA/SbBr₃ inhibitor is added into 55%LiBr solution, their corrosion rates reduce to 67.48μm/a and 38.14μm/a, respectively. It is obvious that the corrosion of copper and copper-nickel alloy may be inhibited by this additive to a great extent in 55%LiBr solution.

Fig.4  Effects of temperature on corrosion rate of copper and copper-nickel alloy in 55%LiBr+PMA/SbBr₃ solution

   As observed from the EPMA surface microphotograph of copper immersed in boiling 55%LiBr+PMA/SbBr₃ solution for 200h(Fig.5), copper surface displays a general corrosion state and some protective films exist on it. Proved by Fig.5(b) and Fig.5(c), the surface layer contains elements Sb, O and Br. This indicates that Sb and O take part in the process of the films forming. PMA/SbBr₃ inhibitor is a mixture of complex transitional metal anion and compound SbBr₃. PMA is a heteropoly complex anion and its chemical formula is [PMo₁₂O₄₀]^3-. PMA has a high solubility in LiBr solution, which may be helpful to maintain an effective concentration in solution. It has the effect of inhibiting hydrogen evolution. The chemical valence of element Mo in anion [PMo₁₂O₄₀]^3- is +6, so PMA shows a strong oxidizing ability. Sb³⁺ also behaves an oxidizing ability. It may exist in LiBr solutions stably with PMA^[11-14]. A protective layer comprising CuO, Cu₂O and Sb may form on copper surface by PMA/SbBr₃ inhibitor^[15]. Electrochemical reactions of copper in 55% LiBr+PMA/SbBr₃ solution are shown as follows:

Anode:

Cu+2OH^-→CuO+H₂O+2e(1)

2Cu+2OH^-→Cu₂O+H₂O+2e(2)

Cathode^{[14, 16]}:

[PMo(Ⅵ)₁₂O₄₀]^3-+2H₂O+ne→

2OH^-+[H₂PMo(Ⅵ)_12-nMo(Ⅴ)nO₄₀]^{(1+n) -}(3)  

Fig.5  EPMA micrographs of copper immersed in boiling 55%LiBr+PMA/SbBr₃ for 200h

   Sb³⁺+3e→Sb(4)

Following cathodic reaction may occur too:

2H₂O+2e→2OH^-+H₂(5)

For the reactions above, corrosion rates of copper and copper-nickel alloy may be reduced by this film in LiBr solution. Cathodic reaction(3) can produce OH^- so that the occurrence of cathodic reaction(5) will be inhibited. This proves that PMA has the effect of inhibiting hydrogen evolution.

Since [H₂PMo(Ⅵ)_12-nMo(Ⅴ)nO₄₀]^(1+n)-, the product of cathodic reaction, can dissolve into LiBr solution easily and its concentration is low, element Mo cannot be detected on the metal surface. It is found that elements Sb and O are absorbed on copper surface(Fig.5). This evidence proves that the discussion of inhibition mechanism above is reasonable. Therefore, because of the synergistic effect of PMA and Sb³⁺, the protective layer comprising CuO, Cu₂O and Sb will be formed on the surface of copper and copper-nickel alloy, which can prevent Br^- from diffusing to the surface of metals. As a result, the anticorrosion performance of copper and copper-nickel alloy may be improved by PMA/SbBr₃ inhibitor in 55% LiBr solution.

4 CONCLUSIONS

1) In boiling 55% LiBr+ PMA/SbBr₃ solution, corrosion rates of copper and copper-nickel alloy are 67.48μm/a and 38.14μm/a, respectively. Corrosion of copper and copper-nickel alloy may be inhibited by PMA/SbBr₃ inhibitor to a great extent in 55% LiBr solution.

2) PMA/SbBr₃ inhibitor can inhibit both anodic and cathodic electrochemical reactions of copper and copper-nickel alloy in 55%LiBr solution, so it belongs to complex inhibitor.

3)The corrosion inhibition mechanism of PMA/SbBr₃ in 55%LiBr solution is suggested as follows: PMA has the effect of inhibiting hydrogen evolution. For [PMo₁₂O₄₀]^3-, the anion of PMA, has a strong oxidizing effect, Sb³⁺ also behaves an oxidizing effect. Sb³⁺ may exist in LiBr solutions stably with PMA. Because of the synergistic effect of PMA and Sb³⁺, the protective film comprising CuO, Cu₂O and Sb formed on copper and copper-nickel alloy surface may prevent Br^- from diffusing to the surface of metals. As a result, the anticorrosion performance of copper and copper-nickel alloy may be improved by PMA/SbBr₃ inhibitor in 55% LiBr solution.

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(Edited by YANG Bing)

Received date: 2004-07-20; Accepted date: 2005-03-22

Correspondence: LIANG Cheng-hao, Professor; Tel: +86-411-88993926; E-mail: liangch@chem.dlut.edu.cn