Influence of Potential Scan Rate on Corrosion Behaviour of Heat Treated AA 7075 Alloy in Sulphuric Acid Solution

In this paper, the effect of scan rate on the corrosion parameters and electrochemical behaviour of Retrogression and Re-aging (RRA)heat treatment of 7075 aluminium alloy has been investigated by Potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS) methods. The Potentiodynamic polarization (PD) curves were obtained at various scan rates for aged aluminium alloys in 0.5 M H2SO4 solution. The results show that at high scan rates shows unpredicted fluctuation of charging current, whereas at lower scan rates fewer disturbances are observed. The aged samples were chemically and mechanically characterized. It was observed that the RRA at 0.7 h shown improved corrosion property as well as mechanical properties compared to T6 temper. The corrosion properties were confirmed by EIS and polarization experiments. Article History Received: 28 February 18 Accepted: 16 March 18


Introduction
Aluminium alloys of 7075 kind containing Zn, Mg and Cu as major alloying elements have shown potential applications in automobile and aerospace applications 1 .They possess good corrosion and mechanical properties than other Al alloys 2 .These alloys show specific behaviour in Retrogression and Re-aging (RRA) heat treatments 3 .The most important feature of the aluminium alloy is to form oxide on surface which act as a barrier 4 .This barrier oxide breaks due to aggressive environments 5 and thus study of the corrosion parameters of any alloy is required to understand to estimate the current and its reaction with a metal surface 6 .The electrochemical processes consist of anodic and cathodic reactions in addition have current to voltage relationships that follow Butler-Volmer kinetics [7][8] , which is applicable for a corroding system and is shown in Eq. 1.
... (1)  Where (E −E corr ) is the over potential; E and I corr are the applied corrosion potential and corrosion current density; β a is the anodic Tafel slope and βcis the cathodic Tafel slope.Eq.1 indicates that unless the over potentials are small compared to the Tafel slopes, a large potentially damaging currents can be generated on the material.When E is distant from corrosion potential, there can be a similar relationship between the corrosion current and corrosion potential as per the tafel equation 9 ... (2)   Eq. 2 shows that the logarithm current is directly proportional to the applied potential at the high value of over potential and helpful for determining the corrosion current.The electrode in contact with aqueous solution tends to develop a potential difference across the interface.When metal dissolve to form ions, the metal becomes positively charged with respect to the solution.The positive ions in the solution form a layer of negative charged on the metallic surface.It means that this concept is analogues to the electrical behaviour of a capacitor.The charging current across the capacitor is directly proportional to the product of time derivative of the potential field (Scan rate) and interfacial capacitance.Thus, the effect of scan rate in the potentiodynamic experiment can be visualized.At very low scan rates, the charging current through the capacitor becomes small and at high scan rates, the fraction of charging current flows through the capacitor will be increased 10 .This effect will cause an underestimation of corrosion rate and may be incorrect.In this case, it is very important to know the correct polarization data without any kind of disturbance of charging current.When studying the corrosion behaviour the effect of potential scan rates are ones concern as it is crucial to opt a potential scan rate value.Various studies have performed to check the effect of the scan rate on the corrosion behaviour of alloys 11 .But not much work is carried out on the Al 7075 alloy.This paper discusses the effect of scan rates on the corrosion and electrochemical behaviour of RRA treated AA 7075 alloys in acidic environment.

materials and experimental methods
Five specimens each of AA 7075 alloy in cylindrical shape of 14 mm (exposed surface area of 0.785cm 2 ) in diameter have been taken for study.All the samples were given suitable heat treatment to produce desired microstructures.Details of the heat treatment are mentioned in Table 1.All the samples were prepared using conventional metallography up to 0.25 micron diamond polishing.Etching was carried to reveal the microstructures using Keller's reagent (95 vol.%H 2 O, 2.5vol.%HNO 3 ,1.5 vol.%HCl and 1.0 vol.% HF).
The Potentiodynamic Polarization (PD) and Electrochemical Impedance Spectroscopy (EIS) are measured using electrochemical characterization techniques (PGSTAT203N AUTOLAB).The three electrodes are immersed into electrolyte for electrochemical measurements.The saturated calomel electrode (SCE) as reference electrode, platinum electrode used as the counter electrode, and the aged sample served as the working in 0.5 M H 2 SO 4 solution.An optical microscope (ZESIS inbuilt AXIO VISION Rel.4.8 software) used to capture the microstructure of aged samples.Hardness is measured by Vickers microhardness tester (Shimadzu-HMV-2E) with the applied load of 1kgf.

Results and Discussions metallography examination
The optical micrographs of the various aged samples are shown in Fig. 1.Microstructure of the aged aluminium alloys shows, elongated grains in a particular direction, which is because of rolling condition and shows wide and tiny grains.The wide grains may be un-recrystallized and small grains related to recrystallized, which look like pancake structure 12 .The microstructure shows dark and light regions, which is attributed to the presence of intermetallic particles (dark) uniformly distributed in the aluminiummatrix (bright) 13 .

Hardness measurements
Fig. 2 shows the micro hardness measurements of Aged 7075 Aluminium alloy as a function of the various tempering process.It can be depicted from the figure, that the RRA at 0.7 h shown highest hardness 14 than other samples.The reason behind the increase of hardness is due to formation of η (coherent) and η'(semi-coherent intermediate) phases in the aluminium matrix 15 .However, the rapid decrease of hardness at RRA 0.9h is due to the partially transformation of η' Phases to stable η phases in the alloys.It can be understood that the decrease of hardness further RRA0.7 h is due to over aging 16 .

electrochemical measurements
Fig. 3 shows a plot against applied potential (mV) and logarithm plot of corrosion current density (μ A/cm 2 ) obtained for 7075-T6 aluminium alloy.The polarization experiments were carried out at varied potential scan rates from30mV/m into 240mV/min.The charging current across the capacitor is directly proportional to the product of time derivative of the potential field (Scan rate) and interfacial capacitance.Thus, the effect of scan rate in the Potentiodynamic experiment can be visualized.
From the potentio dynamic curve, it can be clearly seen that, T6-60, T6-120, and T6 -180 specimens showed a rapid decrease in corrosion current with respect to potential.It is attributed to the formation of protective oxide film on the metal surface.On the other side, T6-30 follows the lowest corrosion rate due to the formation of uniform passive layer and show a sudden decrease in the current density.The cathodic branches for T6 -120, T6 -180 and T6 -240 exhibits a linear trend in polarization curve.Where all the electrode parameters have their usual significance, R p (ohm-cm 2 ) is the polarization resistance, B is proportionality constant, corrosion potential (E corr ) and I corr is the corrosion current (A), EW is the equivalent weight of metal specimen (g), F is the Faraday constant (96500 C), d is the density of the metal (g /cm 3 ).The polarization resistance values of various scan rates are determined from Stern-Geary equation [17][18] and the various parameters of corrosion for aged 7075 aluminium alloys are summarized in the below Table 2.
The data show that corrosion potential (E corr ) shifts to slightly higher negative values for low scan rate (30 mV/minutes) to high scan rate (240 mV/minutes).
The change in the potential values could be possibly due to the crack of double layer on the interface11.The difference between corrosion current and open circuit potential provides slight variation during the scan.The more difference between E corr and E ocp is due to the enrichment of disturbance of charging current [6][7][8][9][10][11][12][13][14][15][16][17][18][19] which was observed at high scan rates.This charging current may affect both positive and negative direction.In the positive scan rate, The Potentiodynamic curve does not provide the appropriate measurement at rapid scan rate, which are representative of the system in the steady state 20 .With increasing scan rate, current density slightly deviates from the steady state 11 .The value of current density at 60 mV/ minute gives good agreement as compared to other results obtained at other scan rates.It can be understood by Table 2, that the electrochemical system does not achieve steady state at a high scan rate and also with very low scan rate.
The data of T6 temper has been summarized from Potentiodynamic curve shown in Fig. 3.The corrosion potential, E corr is almost the same for T6 -60, T6-120, T6-180, and T6-240.Comparatively, T6-240 is observed to have higher potential value than other temper samples.Thus exhibiting a bit of nobility nature same as T6-30.Corrosion rates for all the samples at different scan rates (SR) follow the order: T6-30 <T6-180 <T6-240 <T6-120 <T6 -60.The lowest corrosion rate is observed for AA T6-30(0.01 mm/year).It means that T6-30 corrodes at slower rate at minimum scan rates and this may be because promotion of oxide film on the surface.
Fig. 4 shows the corrosion potential variation with different scan rates for a solution heat treated sample.Corrosion potential of the SHT decreases from -0.58 mV to -0.62V, as scan rate increases.
Similarly, the corrosion rate of SHT-60 has lower tendency to corrode at low scan rate.It is indicated from corresponding corrosion data which clearly shows that corrosion current of film formation over the surface has significantly improved by varying scan rate.The observed sequence of corrosion rate (CR) according to the above discussion is as follows:SHT-30 >SHT-180 >SHT-240 >SHT-120 >SHT-60.In this regard, the reaction mechanism would be helpful for the formation of oxide film in presence of the sulphuric acid.
When the potential applied to the electrochemical cell, the following sequence of reaction is supposed to occur on the cathode.Sulphuric acid initiate to decompose and the hydrogen ions move to the cathode, where hydrogen gas is reduced: Similarly, the negatively charged anions such as hydroxide, sulphate, and some other oxide ions goes to the anode side, which acts as working electrode in the cell.The dissolution of aluminium in form of positively charged aluminium ions (Al3 + ) moves toward the cathode.On the anode surface, they react with the oxide or hydroxide ions to form a high oxide barrier of Al 2 O 3 .The reaction on anode is mentioned below: ...Then, the aluminium hydroxide converts into aluminium oxide: Apart from the Aluminium oxide, the sulphate ions also play a vital role by forming an oxide coating that contains very few percentages of sulphate ions.It is believed that the sulphate ions assist the movement of hydrogen ions and thus reducing the corrosion potential.Secondly, in the presence of acid solution, a dense even layer of oxide film is formed on the metal surface.It is the passivity that break downs when kept in the aggressive medium [5][6][7][8][9][10][11][12][13][14][15][16][17][18] .2), the corrosion of RRA-0.5h-180 is more than the RRA-0.5h-30 and RRA-0.5h-240 alloys in acidic environment.This can be confirmed by the increase of the corrosion parameters with scan rates.The values of polarization resistance obtained for aged aluminium alloys have shown an increasing trend from RRA 0.5-60 to RRA0.5h-180 with respect to scan rate.Further, the less negative shift of corrosion potential as can be seen from Fig. 5 and also the corrosion rate (CR) increased in the specific scan rate.It is important to mention that the corrosion current increases nonlinearly during scan rate from 60 mV/min.to 240 mV/min.The reason behind increasing current is due to the breakdown of protective oxide film.
Fig. 6 depicts that the cathodic and anodic current density show a decreasing trend within finite values of scan rate.This can be attributed to the formation of film on the metal surface and is observed to have noeffect on the higher side of scan rate.The Corrosion potential (Ecorr) shifts to the less negative direction.The slightly variation in corrosion rate of the aged aluminium samples with increasing the potential scan rate are confirmed by the polarization parameters from Table 2.
Fig. 6 depicts that the cathodic and anodic current density show a decreasing trend within finite values of scan rate.This can be attributed to the formation at room temperature of film on the metal surface and is observed to have noeffect on the higher side of scan rate.The Corrosion potential (E corr ) shifts to the less negative direction.The slightly variation in corrosion rate of the aged aluminium samples with increasing the potential scan rate are confirmed by the polarization parameters from Table 2.
The Nyquist plots of aluminium alloy in 0.5 Morality of H 2 SO 4 solution with different ageing treatments are depicted in Fig. 8.It is clearly seen in Fig. 8 that only one semicircle for aged RRA 0.7h in the sulphuric acid solutions is observed.The variation from ideal semi-circular shape are due to frequency distribution of interfacial impedance which probably came into existence by formation of a porous oxide layer on metal surface.The diameter of the semicircle plotted with their centre under real axis for RRA 0.7h is recorded as the highest value which indicates that the corrosion resistance for aged aluminium at 0.7 h is more than the AA 7075-T6 temper treated sample.This reveals that with increase of aging time on the material, not only the corrosion resistance is increased, but also helps to identify the favourable heat treatment required for enhancement of the corrosion properties under acid environments 21 .It also discourses that there is a possibility of enhancing the corrosion properties due to distribution of precipitates in the matrix.... (10)   where j2 = -1 and n range 0 -1, when n = 0, Qp will behave as resistor, when n=1, as purely capacitance with homogeneous surface 22 .The fitting values of aged alloys are shown in Table 3.The obtained values of n from Eq.10 are observed to be less than 1 that means it is behaving like a resistor.It can also be understood that there can be a possibility of corroding surface and the same has been reflected in the R ct values.
hand, if scan rate is slow, the system may deviate from correct value during the measurement and can indicate unsteady state condition.The RRA 0.7 h displays the characteristic age hardening process and attains maximum hardness value compared to the T6 temper sample.EIS measurements show that the corrosion resistance of RRA 0.7 h is more compare to T6 heat treatment process.With increase in aging time, the corrosion resistance is observed to be increased.This study clearly demonstrates that potentiodynamic studies can be used to disclose the electrochemical interaction of an alloy in an acidic environment.

Fig. 2 :
Fig. 2: Variation in the micro hardness of aged 7075 aluminium alloy under different temper process.

Fig. 3 :
Fig. 3: Potentiodynamic curve of 7075-T6 temper on different potential scan rate in 0.5 m H 2 So 4 at room temperature.Stern and Geary17 have simplified the kinetic expression to provide the charge transfer controlled reaction kinetics by Eq. 1 for the case of small over potentials with respect to Ecorr.The Polarization Resistance (Rp) can be calculated from slopes at E corr .

Fig. 5 :
Fig. 5: Potentiodynamic curve of RRA 0.5 h on different potential scan rate in 0.5 m H 2 So 4 at room temperature

Fig. 6 :Fig. 7 :
Fig. 6: Potentiodynamic curve of RRA 0.7 h on different potential scan rate in 0.5 m H 2 So 4 at room temperature

Fig. 8 (
Fig.8(c) shows the Bode phase angle plots of various Aged 7075 alloy.The equivalent circuit is shown in below Fig. 9.The electrochemical parameters are defined as the Rct charge transfer resistance, Rsol represents solution resistance, and Q p (CPE p ) is the constant phase elements of double oxide layer.The CPE p is the function of frequency (Hz) and capacitance(C) which defined as the following equation.