THE INFLUENCE OF INHIBITORS ON MAGNESIUM ANODES EFFICIENCY
OstaninaT. N., OvsyannikovaA. N., RudoiV. M., ZaikovY.P.
Magnesium and its alloys have a wide application as sacrificial anodes in protecting of steel constructions against corrosion and as anodes, used in manufacturing of chemical sources of a current. However, magnesium is largely liable to spontaneous dissolution which is observed even in the conditions of anode polarization [1]. The unproductive losses caused by spontaneous dissolution reduce useful life of magnesium anode materials. Structure of environment in which anodes are used hasa significant influenceon the self-dissolution process. So the efficiency of magnesium protectors in sulfate solutionsis 45-50%, and it decreases to 24-25 %in more aggressive electrolytes, which contain chloride-ions. One of the ways of cutting the metals losses is the use of corrosion inhibitor. When chasing the inhibitors, it is necessary to take into accountthe mechanism of the processes proceeding during anode dissolution of magnesium.
In water solutions in addition to magnesium oxidation process
(1)
there proceeds the self-dissolution on the mechanism of electrochemical corrosion under anodic current
,(2)
which can be accompanied by formation of intermediate compound – magnesium hydride (II) [2]
. (3)
Magnesium hydride is an unstable compound. It is capable to be oxidized under the influence of anode current (4), to be exposed to hydrolysis (5), or to participate in the corrosion process (6)
,(4)
,(5)
.(6)
Accumulation of magnesium hydride on a metal surface takes placewhenthe speed of the formation reaction (3) exceeds the speeds of oxidation reactions (4-6). Hydrogen implantating intothe structure of magnesium with formation of MgH2 on grains borders of metal causes the decreasingof mechanical strength and promotes decompositionof small particlesof magnesium into solution [3].
Recently we have shown [4], that with the growth of anode current and raising of intensity of anode dissolution there is a development of an active surface of metal that leads to increasing of magnesium self-dissolution speed on mechanism of electrochemical corrosion and by means of decomposition of small particles. Small particles of magnesium, getting into solution, are exposed to dissolution with hydrogen evolution (2). All these processes result in increasingof volume of escaping hydrogen with growth of an anode current, so-calledthe negative differential effect.
The purpose of the present work was to research the influenceof BTA and EDTAinhibitors in solutions with various composition at anode polarization of magnesium.
Experimental part
As an object of research, the alloy of magnesium containing 0,45 mass %of calcium and 0,01 mass %of manganese has been chosen.
Investigations were carried out in solutions0,5mol/lNaCl (A) and 0,17mol/lNa2SO4 + 0,017mol/lNaCl (B) with theрН being equal to8. The A solution is a standard electrolyte for carrying outthe corrosion tests. The choice of B solution composition is caused by the fact that an activator layer, into which the protector is placedfor increasingthe environment electroconductivity, contains sodium sulfate. Adding a small amountof chlorine anions intothe solution promotes increasing of inhibitor protective action owing to synergistic action. The solutions tested differ inaggressiveness, but they have an equal ionic force. The contentof BTA and EDTAinhibitors varied from 0,0025 to 0,05 mol/l.
Researches of corrosion behavior of magnesium at anode polarization were carried out within 2 hours under galvanostaticconditions with current density iа= 20A/m2. Cylindrical samples (S= 10cm2) oftested alloywere subjected to these tests. The graphite core served as a counter electrode. Measurements of the potentialwere made in relation to silver chloridereference electrode, and then values of potentials were recalculated to a normal hydrogen electrodescale. Before carrying out the experiments the working surface was subjected to smoothing and degreasing in a soda solution. The total velocity of dissolution (, g/(m2∙h)) was determined by changing the samples weight. The amount of the metal dissolved under the current, was the same in all experiences, and its value was calculated according to the Faraday law, was equal to =9,06 g/(m2∙h). During the experiment the volume of escaped hydrogen was measured and the speed of self-dissolution (, g/(m2∙h)) was found. Weight of the metal which has passed into the solution as a result of decomposition, was found as a difference:
(7)
By a technique developed by Nazarov A.P. and YurasovaT.A. [2], the quantity of magnesium hydride which was on a samples surface after carrying out the tests at a constant anode current was determined. For this purpose the sample was subjected to anode polarization in a solution 1mol/lNaOH at the constant potential (Е =-1,2 V н.в.э.). During the experiment current changeswith time and the volume of escapedhydrogen were fixed. In these conditions magnesium was covered by a passive film, and the observed anode current (fig. 1) was caused by ionization of magnesium hydride:
(8)
. (9)
The charge passed and the volume of the hydrogen escaped allowed to estimate an yield of reactions 8 and 9 and to calculate the amount of magnesium hydride on metal samples surfaces (, g/(m2∙h)).
Discussion of results
Potentiometric measurements have shown, steady-state potentials of magnesium in sodium sulfate and sodium chloride solutions to be close invalue. That is the anion nature does not exert an appreciable influence on potential valueat equal ionic force of a solution.
Fig. 1. Dependence of anode current density on time at potentiostatic polarization () of magnesium in a solution 1mol/lNaOH. Samples were previously subjected to anode polarization (iа= 20A/m2) within 2 hours in a solution 0,5mol/lNaCl. BTA concentration, mol/l: 1 – 0; 2 – 0,005; 3 – 0,0075; 4 – 0,01.
With addingBTA and EDTA inhibitors into solutions the steady-state potential was shifted to the negative area. And, the higher concentration of the addition is, the greater the shift of potentialis (tab. 1).
Table 1.Steady-state potentials of magnesium alloy in the tasted solutions at differentinhibitor concentrations
Solution / Values of steady-state potential of magnesium alloy, Vwithout
inhibitors / BТА concentration, mol/l / EDTA concentration, mol/l
0,005 / 0,01 / 0,05 / 0,005 / 0,01 / 0,05
A / -1,42 / -1,49 / -1,59 / -1,67 / -1,59 / -1,62 / -1,67
B / -1,47 / -1,53 / -1,63 / -1,69 / -1,60 / -1,64 / -1,73
The steady-state potential reduction may be caused by increasing of hydrogen overvoltage. The polarization measurements made showed, that in presence of inhibitors with contents 0,005 mol/l and 0,01mol/l resistance of cathode reaction increased in both solutions. However, the process of hydrogen reductionbecame easier at 0,05mol/l concentration.As there are passive films on a magnesium surface, steady-state potential reduction may mean that surface activation takes place inthe presenceof inhibitors.As a result dissolution becomes more uniform, and the probability of pits formation decreases. Indeed, as it will be shown below, more uniform magnesium dissolution is observedat the presence of inhibitors.
In the absence of inhibitors, rate of magnesium dissolution, calculated on a weight decrease (), was considerably higher in A solution (fig. 2). This can be explained by the fact that being depassivator chlorine anion promotes increase in asegment of an active surface of magnesium where self-dissolution processes take place. Indeed, intensity of hydrogenevolution, which characterizes the rate ofcorrosion processes at anode polarization, was higher in a solution of sodium chloride in comparison with a sulfate solution (fig. 3).
Fig. .2 Influence of BTA (1,3) and EDTA (2,4) concentration on magnesium dissolution rate at anode polarization (iа= 20А/m2)in A (1,2) and B (3,4) solutions.
Fig. 3. Influence of concentration БТА (1,3) and ЭДТА (2,4) on intensity of magnesium corrosion selfdissolution at anode polarization in solutions A (1,2) and B (3,4).
Withincreasing of BТА and EDTAinhibitors concentration in a solution the intensity of dissolution(fig.2) and corrosion selfdissolution (fig.3) changedon a curve with a minimum. Such kind of dependence of self-dissolution rateoninhibitor concentration was observed in both tested solutions. At thatthe area of inhibitor concentration, where magnesium losses decreased, was wider (from 0,025 to 0,3 mol/l) in a solution of sodium chloride in comparison with inhibitors concentrations in a sodium sulfate solution (from 0,005 to 0,15 mol/l). Besides, the maximum degree of protection in a solution of sodium chloride was 54% in the presence of 0,01 mol/l BТА and it was 50% in the presence of 0,0075 mol/l EDTA, whereas in sodium sulfatesolution it was 46 % and 43 % in the presence of 0,02 mol/l BТА and EDTA, correspondingly. In calculatingthe protection degree extent in presence of inhibitorsthe total losses of magnesium in process of anode dissolution were taken into account (fig.2).
It should be mentioned, that inhibitorstested reduceselfdissolution only in the limited range of concentration (fig. 3). Under high concentrations of inhibitorsthe intensity raising of hydrogenescapingwas observed which testifies the increasingof corrosion selfdissolution rateas compared to solutions without inhibitors.
The range of concentrations, at which the reducing of the corrosion process rate was observed, and extent of protection degree were the same for both tested inhibitors in solution of sodium chloride (fig. 3). In a solution of sodium sulfate,the action of EDTA inhibitor began at a smaller concentration, than that of BTA inhibitor. However, in both solutions the maximum effect ofBТАadding was a little higher, than from EDTA introduction.
Comparison of the total losses of magnesium at anode polarization () with rate of dissolution under the influence of a current () and intensity of self-dissolution by the mechanism of electrochemical corrosion () has shown, that there takes place the decomposition of magnesiumsmall particles into a solution (decomposition process). The estimation of decomposition intensity () according to the equation (7) has shown, that magnesium anodes are subject to the greatest mechanical destructionin a solution of sodium chloride without inhibitors (fig. 4).
Fig. 4. Influence of BТА (a) and EDТА (b) concentration on decomposition intensity (1, 2) and rate of accumulationof magnesium hydride on a samples surface (3, 4) in A (1,3) and B(2,4) solutions.
Due to this, after anode dissolution the samples of magnesium had on their surface deep ulcers, pits and furrows (fig.5 a). As you can see, after the electrolysis in sodium sulfatesolution, there are numeroussmalllesions on the surface of samples, which are uniformly distributed on metal surface(fig. 5 b).
Adding the BTA and EDTA inhibitors intotested solutions decreased the intensity of decomposition process (fig. 4), that led to a more uniform dissolution of magnesium and to preservation of a smooth surface of samples after electrolysis (fig. 5 c,d,e, f). Magnesium losses,caused by particles decomposition into a solution, sharply decreased with the growth of BTA and EDTA inhibitors concentration in both solutions. After reaching the inhibitor concentration, at which the maximum reducing of corrosion process rate was observed, (fig.3), the decomposition, actually, stopped. With the further increasingof inhibitor concentration, metal decomposition did not take place.
The analysis of chronoamperograms(fig. 1) obtainedunderpotentiostatic polarization of samples in sodium hydroxide solution has allowed to determine MgH2 amount on a magnesium surface (). The greatest amount of magnesium hydride was found on the surface of samples which were subjected to dissolution by an external current in solutions without inhibitors (fig. 4). The increase of BTA and EDTA inhibitors concentration in both solutions led to decrease in the content of magnesium hydride on a surface (fig. 4). Comparison of curves course both andallowedto make a conclusion, that magnesium losses caused by decomposition, depend on maintenance of MgH2on a surface.
Hydrogen saturation of magnesium in sodium chloride and sodium sulfate solutions was identical; however decomposition intensity in the A solution was almost 2 times higher than this index in the B solution. Obviously, the process of mechanical decomposition depends on solution aggressiveness. In presence of chlorine anions there is an intensive and non-uniform dissolution of magnesium, with the formation of deep ulcers and pits which leads to mechanical destruction of metal.
Thus, the researches carried out allowed to state, that optimum concentration of BTA and EDTA inhibitors in sodium chloride solution is 0,2 mol/l, and in a solution of sodium sulphate is 0,0075 mol/l for EDTA and 0,01mol/l - for BТА. Onpictures (fig. 5 е, f) it is clearly seen, that samples which were exposed to anode dissolution in solutions at optimum concentration of inhibitors, have an even, smooth surface. When inhibitor concentration becomes above optimum, braking degree of corrosion process of anodes destruction decreases, but character of dissolution remains uniform as decomposition of small particles does not take place.
Conclusions
1. It is stated, that BTA and EDTA inhibitors slow down the process of magnesium selfdissolutionat anodic polarization both in a solution 0,5mol/l NaCl, and in a solution 0,17 mol/l Na2SO4 + 0,017mol/l NaCl.
2. Therangeofconcentrations, whereitisobservedthe decreasing of magnesium corrosion selfsolution rate of in anodic polarization is wider in sodium chloride solution.
3. With addition of investigated inhibitors into solution magnesium losses are noticeably reduced, as a result of decomposition of small particles, it promotes the uniform dissolution of samples and the preservation of smooth surface.
4. Decomposition intensity in solution of sodium chloride is noticeably higher than the one in solution of sodium sulfate, also it depends on hydrogen saturation (presence of magnesium hydride on the surface).
Fig. 5. Appearance of samples after anodic dissolution (iа=20А/м2) within 2 hours in A (a,c,e) and B (b, d, f)solutions. The concentration of inhibitors: a, b – is not present; c – 0,02mol/lBTA; d –0,075mol/lBTA; e – 0,01mol/l EDTA; f – 0,02 mol/lEDTA.
The present work has been donewiththe financial support of innovative educational programof the USTU-UPI.
References
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2. Nazarov A.P., Jurasova T.A.: ‘Hydrogen evolution, hydrideformation and magnesium dissolution at the presence of complexing agent’. Zaschita metallov1995 31 (2) 139-144.
3. NazarovА.P., LisovskyА.P., MihailovskyYu.N.:‘Formation of MgH2 in electrochemical dissolution of magnesium in aqueous electrolytes’. Zaschitametallov198925 (5) 760-765.
4. Kozlova A.N., Ostanina T.N., Rudoi V.M., and others:‘Corrosion and anode dissolution of magnesium alloys in presence of inhibitor’. Zaschitametallov200844 (6) (Paper is being in publication)
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