1-Hydroxy Ethylidene 1,1-Diphosphonic Acid (HEDP) as a Corrosion Inhibitor for Aluminium

The present study is to use 1-hydroxy ethylidene 1, 1-diphosphonic acid (HEDP) as corrosion inhibitor for aluminium in aqueous solution containing 60 ppm Cl ion was chosen as the inhibitor. Zn 2+ is selected as the synergist, as Zn 2+ ions in association with HEDP inhibitor is considered environmental friendly inhibitor for aluminium corrosion in alkaline (pH=11) media. The environment chosen is aqueous solution containing 60 ppm Cl ions. To investigate the nature of the protective film, surface analytical techniques such as, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) have been put to use in the present study. The formation of protective film has been confirmed using the electrochemical techniques such as polarization study and AC impedance spectral study. The above studies support the weight loss study.


Introduction
At the start of Second World War, Al 0.05% Copper alloys were used in marine and aircraft [1]. Al -Copper alloys are widely used because of their high strength to weight ratio; but they have relatively low corrosion resistance. Corrosion behaviour of B26S Al in lemon juice containing sweeteners such as glucose, sugar, saccharine and sorbitol was studied by Pandaya and Joshi [2]. Al has high resistance to corrosion in many environments due to the presence of protective surface film formed rapidly in air or in neutral aqueous solution [3]. However, in environments containing aggressive anions, primarily chlorides, the protective film gets locally damaged, and a corrosive attack takes place. Significant contributions have been made by Trabanelli [4] and Trasatti [5]. Despic et al., [6] and Radosevic et al., [7] have investigated the inhibition of corrosion of Al.
Al and its alloys containing metals such as In, Mg, Zn, and Ga have been found to be useful as anodes in alkaline batteries [8,9]. Corrosion of Al in NaOH solution has been prevented by addition of catechol and calcium oxide [10]. Due to environment-protection awareness, there is a move to make use of environmental friendly natural products as corrosion inhibitors.
Rajendran et al., have used an aqueous extract of onion, in controlling corrosion of Al in an aqueous solution containing 60 ppm of Cl at pH 11 and 12 NaOH. Influence of N-cetyl-N, N, Ntrimethyl ammonium bromide, (CTAB), sodium sulphite, Zn 2+ and immersion period on the inhibiting nature of onion extract has been investigated. The protective film has been analyzed by FTIR spectra [11]. Corrosion behaviour of Al in rain water containing garlic extract has studied at pH 11 and 12. The influence of CTAB, sodium sulphite, Zn 2+ and duration of immersion has also been investigated [12].
The present study is to use HEDP as a environmental friendly corrosion inhibitor for aluminium in aqueous solution containing 60 ppm Cl ion and Zn 2+ is selected as the synergist.

Experimental Preparation of the aluminium metal specimens
Aluminium metal specimen of the dimensions 1.0 x 4.0 x 0.2 cm containing 95% pure aluminium was polished to mirror finish, degreased with trichloroethylene and used for weight -loss and surface examination studies. Aluminium metal rod encapsulated in Teflon with an exposed cross section of 1 cm 2 area was used as the working electrode in potentiostatic polarization studies. The surface of the electrode was polished to mirror finish and degreased with trichloroethylene.

Preparation of the stock solution
1 gm of HEDP was dissolved in double distilled water, neutralized and then made upto 100 ml in a standard measuring flask. One ml of this solution was diluted to 100 ml, which yields exactly 100 ppm of HEDP.

Determination of surface area of the specimens
The length, breadth and the thickness of aluminium specimens and the radius of the holes were determined with the help of vernier calipers of high precision and the surface areas of the specimens were calculated.

Weighing the specimens before and after corrosion
The weights of the specimens before and after immersion were determined using a balance, Shimadzu AY62 model.

Determination of corrosion rate
The weighed specimen, in triplicate were suspended by means of glass hooks in 100 ml beakers containing 100 ml of aqueous solution containing 60 ppm Cl of various concentrations of the inhibitor in the presence and absence of Zn 2+ for 1 day of immersion. After 1 day of immersion the specimens were taken out, washed in running water, dried and weighed. From the change in weights of the specimen, corrosion rates were calculated using the following relationship.

Weight loss method
The percentage of inhibition efficiency was calculated using the following equation.
ρ-density of the metal in g/cm 2 (7.86)

Potentiostatic polarization study
This study was carried out using EG and G electrochemical impedance analyzer model 6310. A three electrode cell assembly was used. The working electrode was used as a rectangular specimen of aluminium with one face of the electrode of constant 1 cm 2 area exposed. A saturated calomel electrode (SCE) was used as reference electrode. A rectangular platinum foil was used as the counter electrodes. A time interval of 5 to 10 min was given for the system to attain a steady state open circuit potential. The results such as Tafel slopes, Icorr and Ecorr values were calculated.
The working electrode and platinum electrode were immersed in aqueous solution containing 60 ppm of Cl in the absence and presence of inhibitor. Saturated calomel electrode was connected with the test solution through a salt bridge. Potential (E) vs log current (I) plots were then recorded. Corrosion potential (Ecorr) and Tafel slopes ba and bc were determined from E vs log I plots. Tangents were drawn on the cathodic and anodic polarization curves .From the point of intersection of the two tangents Icorr and Ecorr were calculated.

AC impedance measurements
EG and G electrochemical impedance analyzer model 6310 was used to record AC impedance measurements. The cell set up was the same as that used for polarization measurements. A time interval of 5 to 10 min was given for the system to attain a steady state open circuit potential. Then over this steady state potential, an A.C. potential of 10 mV was superimposed. The AC frequency was varied from 100 kHz to 100 mHz. The real part (Z') and imaginary part (Z") of the cell impedance were measured in ohms for various frequencies. The Rt (charge transfer resistance) and Cdl (double layer capacitance) values were calculated. Cdl values were calculated using the following relationship.

Surface characterization studies
The aluminium metal specimens were immersed in aqueous solution containing 60 ppm Cl -, as well as inhibitor solutions, for a period of one day. After 1 day, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimens was analyzed by various surface analysis techniques.

Scanning Electron Microscopic studies (SEM)
The aluminium metal specimen immersed in aqueous solution containing 60 ppm Cl and in the inhibitor solution for a period of one day was removed, rinsed with double distilled water, dried and observed in a scanning electron microscope to examine the surface morphology. The surface morphology measurements of the aluminium were examined using Quanta 200 FEI, Netherland, GENESIS computer controlled scanning electron microscope.

Atomic Force Microscopy characterization (AFM)
The aluminium metal specimen immersed in aqueous solution containing 60 ppm Cl and in the inhibitor solution for a period of one day was removed, rinsed with double distilled water, dried and subjected to the surface examination. The surface morphology measurements of the aluminium surface were carried by atomic force microscopy (AFM) using PicoSPM 2100 with the software version of Picoscan version 5.4.

Analysis of weight loss method
Corrosion rate (CR) of aluminium immersed in aqueous solution containing 60 ppm of Cl ions at pH=11 for one day in the absence and presence of inhibitor 1-hydroxy ethylidene 1,1-diphosphonic acid (HEDP) has been evaluated and values are given in the Table 1(a) and Table 1(b). The inhibition efficiencies of HEDP are also given in these Tables. The inhibition efficiencies of the HEDP-Zn 2+ systems as a function of concentration of HEDP are shown in Fig.1.

Fig.1: Corrosion rates (CR) of aluminium immersed in various test solutions
It is observed from Table 1 that HEDP shows some inhibition efficiencies and inhibition efficiency (IE) increases when the concentration increases. For example 50 ppm HEDP has 9 percent IE while 250 ppm HEDP has 20 per cent. This is due to the fact that as the concentration of HEDP increases, the protective film (probably aluminium HEDP complex) formed on the metal surface goes on increasing. That is the system passes from active region to passive region [13][14][15].

Table 1(a): Corrosion rates (CR) of aluminium immersed in aqueous solution containing 60 ppm Cl ions in the absence and presence of inhibitor system at various concentration and the inhibition efficiencies (IE) obtained by weight loss method
Inhibitor system: 1-hydroxy ethylidene 1,1-diphosphonic acid + Zn 2+ (0 ppm)

Analysis of potentio dynamic polarization study (pH=11)
Polarization study has been used to confirm the formation of protective film formed on the metal surface during corrosion inhibition process [16][17][18][19]. If a protective film is formed on the metal surface, the linear polarization resistance value (LPR) increases and the corrosion current value (Icorr) decreases.
The potentiodynamic polarization curves of aluminium immersed in aqueous solution containing 60 ppm Cl ions in the absence and presence of inhibitors are shown in Fig.2. The corrosion parameters are given in Table 2. When aluminium was immersed in aqueous solution containing 60 ppm Cl ions, the corrosion potential was -1305 mV vs SCE. When HEDP (250 ppm) and Zn 2+ (50 ppm) were added to the above system, the corrosion potential shifted to the noble side -1236 mV vs SCE. This indicates that a film is formed on the anodic sites of the metal surface. This film controls the anodic reaction of metal dissolution by forming Al 3+ -HEDP complex on the anodic sites of the metal surface. The formation of protective film on the metal surface is further supported by the fact that the anodic Tafel slope (ba) increases from 300 mV/decade to 349 mV/decade. Further, the LPR value increases from 1.2463 x 10 5 ohm cm 2 to 1.6487 x 10 5 ohm cm 2 , the corrosion current decreases from 0.3402 µA/cm 2 to 0.2563 µA/cm 2 . Thus, polarization study confirms the formation of a protective film on the metal surface.
For blank system, the value of current remains constant from potential -1.2 volt to 0.425 volt. At this volt, the film is broken. Hence the corrosion current increases suddenly. The break down potential is -0.425 volt. Similar observation is made for the inhibitor system also. Here the break down potential is -0.38. The reason for the current remaining constant is due to the formation of passive film on the metal surface. Similar observation has been made [20,21] by R.D. Klassen and P.R.Roberge.

Analysis of AC impedance spectra
AC impedance spectra (electrochemical impedance spectra) have been used to confirm the formation of protective film on the metal surface [22][23][24][25][26]. If a protective film is formed on the metal surface, charge transfer resistance (Rt) increases, double layer capacitance value (Cdl) decreases and the impedance log(z/ohm) value increases. The AC impedance spectra of aluminium immersed in aqueous solution containing 60 ppm Cl ions in the absence and the presence of inhibitors (HEDP-Zn 2+ ) are shown in Fig.3 (Nyquist plots). The AC impedance parameters namely charge transfer resistance (Rt) and double layer capacitance (Cdl) derived from Nyquist plots are given in Table 3. The impedance log(z/ohm) values derived from Bode plots are also given in Table 3. These results lead to the conclusion that a protective film is formed on the metal surface. Equivalent circuit diagram of Aluminium immersed in various test solutions is shown in Scheme 1. In presence of inhibitor, the phase angle value increases from 48° to 52°. This is due to formation of protective film.

SEM Analysis of Metal Surface
SEM provides a pictorial representation of the surface. To understand the nature of the surface film in the absence and presence of inhibitors and the extent of corrosion of aluminium, the SEM micrographs of the surface are examined [27][28][29][30].
The SEM images of magnification (X 1000) of aluminium specimen immersed in aqueous solution containing 60 ppm of Cl ions for 1 day in the absence and presence of inhibitor system are shown in Fig.4 (a,b).
The SEM micrographs of aluminium metal surface immersed in aqueous solution containing 60 ppm of Cl ions ( Fig.4(a)) shows the roughness of the metal surface which indicates the highly corroded area of aluminium metal in aqueous solution containing 60 ppm of Cl ions. However Fig.4 (b) indicates that in the presence of inhibitor (250 ppm HEDP and 50 ppm Zn 2+ ) the rate of corrosion is suppressed, as can be seen from the decrease of corroded areas. The decrease in corrosion is due to the formation of insoluble complex on the surface of the metal [31][32][33]. In the presence of HEDP and Zn 2+ , the surface is covered by a thin layer of inhibitors which effectively controls the dissolution of aluminium metal. ; Magnification X 1000 Mar c h 08, 2 0 1 4

Atomic Force Microscopy Characterization
Atomic force microscopy is a powerful technique for gathering of roughness statistics from a variety of surfaces [34]. AFM is becoming an accepted method of roughness investigation [35][36][37][38][39].
All atomic force microscopy images were obtained in a VECCO Lab incorporation. AFM instrument operating in contact mode in air. The scan size of all the AFM images are 05 µm x 05 µm areas at a scan rate of 6.68 µm/second. Root-mean-square roughness, average roughness and peak-to-valley value AFM image analysis was performed to obtain the average roughness, Ra (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, Rq (the average of the measured height deviations taken within the evaluation length and measured from the mean line) and the maximum peakto-valley (P-V) height values (largest single peak-to-valley height in five adjoining sampling heights). Rq is much more sensitive than Ra to large and small height deviations from the mean. Table 4 is the summary of the average roughness (Ra), rms roughness (Rq) maximum peak-to-valley height (P-V) value for aluminum metal surface immersed in different environments.
The value of RRMS, Ra and P-V height for the polished aluminium metal surface (reference sample) are 4 nm, 3 nm and 50 nm respectively, which shows a more homogeneous surface, with some places in which the height is lower than the average depth. The presence of 250 ppm of HEDP and 50 ppm of Zn 2+ in aqueous solution containing 60 ppm of Cl ions reduces the Rq value decrease from 37 nm to 20 nm and the average roughness value is significantly reduced to 15 nm when compared with 29 nm of aluminium metal surface immersed in aqueous solution containing 60 ppm of Cl ions. The maximum peak-to-valley height also was reduced to 78 nm from 115 nm. These parameters confirm that the surface appears smoother. The smoothness of the surface is due to the formation of a compact protective film of Al 3+ -HEDP complex and Zn(OH)2 on the metal surface thereby inhibiting the corrosion of aluminium metal. Fig.5 (c,f,i) displays smooth surface of the aluminium metal due to the formation of protective film.
Also the above parameters observed are somewhat greater than the AFM data of polished metal surface which confirms the formation of the film on the metal surface, which is protective in nature.