An electrochemical investigation of the corrosion behavior of Al-Si-Cu hypereutectic alloys in alcoholic environments

Al-Si-Cu hypereutetic alloys produced by spray forming are mostly used in the automotive industry, especially for cylinder liners. They have the advantage of low weight associated with low coefficient of thermal expansion and excellent mechanical properties mainly wear resistance at high temperatures. The corrosion resistance of these alloys in fuels, particularly alcoholic media, however is not yet known. In this investigation, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation have been used to evaluate the corrosion resistance of a hypereutectic Al-Si-Cu alloy in alcoholic environments. The EIS tests were carried out in pure ethanol, and ethanol with small additions (1 mM) of acid and chloride, to investigate the effect of these contaminants on corrosion resistance. The corrosion resistance of a grey cast iron has also been evaluated in pure ethanol for comparison. The AhSi-Cu alloy showed high corrosion resistance in pure ethanol, far superior to that of grey cast iron in the same medium.


INTRODUCTION
In the last few decades there has been significant growth in technological development of new materials, mainly related to improving products and technologies used in the production area in the automobile, aerospace and power sectors^ ' .
The race among automobile manufacturers in turn has created increasing interest in materials with improved properties for new applications.In this sector, the properties most pursued are, reduced weight associated with low coefficient of thermal expansion, along with excellent mechanical properties, mainly abrasion resistance at high temperatures.In this context various new materials have been studied, and the high performance aluminium alloys have drawn a lot of interest, mainly the AhLi and Al-Si alloys.
Among the AhSi alloys, the ones with hypereutectic compositions have been found to be more interesting as they have optimum properties in terms of abrasion resistance.This is because of the large volumetric fraction of the primary silicon phase^^'^ ^" \ However, in spite of the optimum properties, it was only with technological advances enabled by spray forming that they could be optimised^^.Spray forming permits the production of hypereutectic AhSi alloys with a fine and homogeneous distribution of primary silicon.In this manner, the manufacture of industrial components such as cylinder liners has been made possible with various advantages such as weight reduction, decrease in emission of exhaust gases and savings in fuer \ In the last few decades, along with technological advances, concerns about environmental pollution caused by exhaust gases from petroleum fuel driven cars have increased in the automotive industry.This was due mainly to significant increase in the number of automobiles in large cities.
Advances in pollution control technologies have been significant, but not sufficient to reduce the problems of environmental pollution.Research on alternate sources of energy for traction increased rapidly, both in terms of old options such as alcohol's in the form of methanol and fuel alcohol, as well as in other fuels such as natural gas, hydrogen and the use of electric energy.
In Brazil, in the 70s and early 80s, there were significant efforts and attempts to resolve corrosion problems created by fuel alcohol on automobile components^ ^^^ ^'^ .Even though most of the corrosion problems were solved, as a result of this effort, various aspects of corrosion in alcoholic media, especially those related to mechanism and the effects of impurities present in the fuels have not yet been resolved.The behaviour of new materials, subsequently developed in the 90s, in these corrosive media is as yet unknown.
The aim of this investigation was to determine the corrosion behaviour of a hypereutectic AhSi-Cu'Mg alloy obtained by spray forming, in alcoholic media, both pure and containing small amounts of contaminants.This alloy is used in automobile engine cylinder liners.The contaminants of fuel alcohol studied in this investigation were chlorides and acids.Another aim of this study was to compare the behaviour of this alloy with that of grey cast iron, an alloy widely used as cylinder liners in alcoholic media.

Materials
The materials studied were an AhSi hypereutectic alloy, and grey cast iron.Their compositions are given in tables I and II respectively.The test environments used in this investigation were pure ethanol, pure ethanol with addition of sulphuric acid (ImM), and pure ethanol with addition of analytical grade (AG) lithium chloride (ImM).

Methods
The methods used to investigate corrosion behaviour were electrochemical impedance spectroscopy and potentiodynamic polarisation.To carry out the impedance tests, a Solartron frequency response analyser coupled to an EG&G PARC potentiostat were used and controlled with appropriate software.To carry out the potentiodynamic polarisation tests an EG&G PARC potentiostat was used with a proper software that permitted ohmic drop compensation.
The potential amplitude applied in the electrochemical impedance test was 20 mV and the frequency range varied from 10 Hz to 10 Hz.The polarisation tests were carried out from -500 mV, with respect to the corrosion potential, up to + 1500 mV vs the reference electrode (Ag/AgCl/KCl sat).All the tests were carried out An electrochemical investigation of the corrosion behavior of Al-Si-Cu hypereutectic alloys in alcoholic environments SM.TRALDI, J.L. ROSSI AND Í. COSTA after one day of immersion of the specimens in the different media at room temperature and the media were deaerated.

RESULTS
Figure 1 shows the results of the electrochemical impedance tests on AhSi-Cu-Mg in pure ethanol.The data obtained were modelled and the equivalent circuit that best fitted to the results is shown in figure 2.
In figure 2, Rs^ represents the ohmic resistance of the electrolyte.R^^ and C^^ are the resistance and capacitance components of the oxide on the alloy surface, which was porous, permitting access of the corrosive medium to the metallic surface.Rcti and Cdl^ are the charge transfer resistance and electrical double layer capacitance respectively, which correspond to the Faradaic process at the alloy/alcoholic medium interface.
Figure 3 shows the Nyquist diagrams of the alloy AhSi'Cu'Mg in the two media, ethanol with addition of acid and ethanol with addition of chloride.The impedance results shown in figure 3 were fitted to give the equivalent circuits shown in figure 4. In these circuits, Rs2 and RS3 represent the ohmic resistance in the respective media.Rct2 and Cdl2 are the charge transfer resistance and electrical double layer capacitance respectively, related to the Faradaic process at the alloy/media interface.Rct^at is the charge transfer resistance of the cathodic reaction and Rw2 represents the     Warburg impedance related to mass transport of the reactive species to the surface.The results indicate that the cathodic reaction was diffusion controlled.
The addition of acid to pure alcohol apparently resulted in dissolution of oxide on the metallic surface, besides impeding the formation of oxide in this test environment.This was revealed in the electrochemical impedance results, where there was only the response corresponding to the metal/environment interface.
In the alcohol environment with chloride additions, the impedance response is typical of that which occurs in localized corrosion, where Rp^ss and Cp,^ss ^re the passive surface resistance and capacitance respectively, whereas R^ct) C^ct' ^^^d W3, are related to reactions in the active region.
The data obtained indicate that the corrosion process was diffusion controlled, due probably to oxygen transport in the direction of the metal/oxide interface and aluminium ions away from the active metal interface.The potentiodynamic polarisation curves of the AhSi'Cu'Mg alloy and the grey cast iron in pure ethanol and ethanol with acid additions (ImM) are shown in figure 5.These curves permit comparison of the behaviour of these two materials.In figure 5, in ethanol and ethanol with acid additions, the aluminium alloy shows much lower corrosion rates, compared to the grey cast iron.This occurred mainly in pure ethanol, where the corrosion rate of cast iron was around 10^ times higher than that of the aluminium alloy.The corrosion rate of the aluminium alloy in pure ethanol is very low, indicating passivity.However, a behaviour typical of an active metal during anodic polarisation can be observed.The anodic   Tafel slope was however very high compared to that of typical slopes in aqueous media.This apparently is a characteristic of alcoholic media.The large slopes indicate the presence of a film on the surface of the specimen, partially permeable and obstructing the metal dissolution reaction, but permitting an electrochemical reaction to occur.According to Fernandes and Ferreira^^ % the surface oxide on aluminium permits the movement of ionic species and electron or vacancy flux across it.
Figure 6 shows the effect of acid addition on the corrosion behaviour of the two materials.The corrosion rate of the alloy increased ten times after addition of acid.This was probably due to dissolution of the surface oxide.This result supports the electrochemical impedance data, which also suggested dissolution of the surface oxide.
The addition of acid to the alcohol also seemed to have caused depolarisation of the cathodic reaction by favouring the cathodic hydrogen evolution reaction.

CONCLUSIONS
The electrochemical impedance technique has been found to be appropriate to investigate the corrosion behaviour of AhSi-Cu alloy in alcoholic media, enabling the effect of addition of small amounts of contaminants to ethanol to be observed.In pure alcohol the impedance results  indicate the presence of a surface oxide and the polarisation results indicate this oxide to be at least partially permeable.The results also indicated that the addition of acid to alcohol, even in small quantities, causes dissolution of the initial oxide present on the surface and impedes its formation when immersed in the environment.The impedance data of the alloy immersed in environments containing chlorides were characteristic of localised corrosion.The AhSi-Cu'Mg alloy showed higher corrosion resistance than grey cast iron in pure ethanol and acid containing ethanol.

Figure 1 .
Figure 1.Nyquist diagram of alloy Al-Si-Cu-Mg in AG ethanol showing data in: (a) whole frequency range and (b) high frequency region.

Figure 5 .
Figure 5. Potentiodynamic polarization curves of grey cast ¡ron and Al-Si-Cu-Mg alloy in: (a) AG ethanol media; (b) Ethanol media with addition of 1 mM acid.

Figure 6 .
Figure 6.Potentiodynamic polarization curves of Al-Si-Cu-Mg alloy in pure ethanol and in ethanol with addition of acid.

Table I .
Composition of hypereutectic Al-Si-Cu-Mg alloy