Inhibition of Copper Corrosion by Flavonoids in Nitric Acid

A study has been made to investigate the effect of some substituted flavonoids on copper dissolution in 2.0 M HNO3 for 4.0 hours at different temperatures by the weight loss method. Percentage of inhibition increases as concentration of the flavonoids increases and reaches a maximum value, due to the formation of a monolayer film on the surface of the metal. 92% Inhibition was observed in some of these flavonoids. As temperature increases, percentage of inhibition decreases. Energy of activations were deduced and discussed. Keyword: Copper; Corrosion, Flavonoids, Inhibition, Quercetin-3,3′-dimethylether.


Introduction
Copper and its alloys are widely used materials for their excellent electrical and thermal conductivities in many applications such as electronics 1 and recently in the manufacture of integrated circuits [2][3][4] .Copper is relatively noble metal, requiring strong oxidants for its corrosion or dissolution.The chemical dissolution and electrolytic plating are the main processes used in the fabrication of electronic devices.The most widely used corrosive solution contains nitric acid, so this medium has induced a great deal of research on copper corrosion [5][6][7][8][9][10] .In order to study the corrosion of metals, several techniques have been applied.The use of chemical inhibitors is one of the most practical methods for the protection against corrosion in acidic media.Most of the excellent acid inhibitors are organic compounds containing nitrogen [11][12][13] , oxygen [14][15][16][17] , phosphorus 18 and sulphur [19][20][21][22] .Studies of the relation between adsorption and corrosion inhibition are of considerable importance.
Flavonoids are water soluble polyphenolic molecules containing 15 carbon atoms (Scheme 1).Flavonoids are widely disbursed throughout plants and give the flowers and fruits of many plants their vibrant colors.Flavonoids have antioxidant activity, anti-allergic, anti-cancer, anti-inflammatory and anti-viral 23 .

Chemicals
Nitric acid solutions were prepared from the RP Normapur grade HNO 3 (65% Merck) in distilled water.Copper sheet (99.9% Prolabo Chemicals or 99.98% Aldrich Chemicals) was cut as coupons of total surface area of 2.21 cm 2 .Compounds 1-4 were isolated from Varthemia iphionoides.The compounds exhibited the following characteristics.

Experimental
The weight loss measurements were carried out in a test tube placed in a thermostat water bath.The solution volume was 5 mL.The used copper coupons had a rectangular form (length = 1 cm, width = 1 cm, thickness = 0.03 cm).Prior to all measurements, the coupons were first polished successively with metallographic emery paper of increasing fineness up to 1200 grits.The electrode was then washed with doubly distilled water, degreased with acetone, washed using doubly distilled water again and finally, dried with tissue paper at room temperature.The coupons were weighed and suspended in 5 mL of 2.0 M HNO 3 solution containing flavonoids derivatives at the desired concentrations for 4 h at 25 °C.At the end of the tests, the coupons were taken out, washed with distilled water, degreased with ethanol, washed again with distilled water, dried and then weighed using an analytic balance (precision: ±0.1 mg).

Results and Discussion
The variation of copper weight loss (g) in the presence of varying concentration (1×10 -4 -5×10 -7 M) of the different organic compounds (1-4) (Scheme 1) in 2.0 M HNO 3 for 4.0 h at 25 °C were collected in Table 1 and were plotted in Figure 1.

Inhibition of Copper Corrosion by Flavonoids in Nitric Acid 329
According to Figure 2 the following observations could be deduced for the dissolution of copper metal in 2.0 M HNO 3 for 4.0 h.-Drastic decrease in the weight loss of copper metal as the concentration of the organic compounds 1 and 2 increases, especially at low concentration.-A slight decrease in the weight loss of copper metal as the concentration of the organic compounds 3 and 4 increases.-Weight loss of copper metal reaches limiting value for all organic compounds 1-4.
All additives appeared to acts as inhibitors over the concentration range studied.The percentage inhibitions (I) calculated from the weight loss (equation 1) 24 for all organic compounds (Table 2) are plotted in Figure 2.

Figure 2. Effect of organic compounds concentration on inhibition efficiency (%I)
for corrosion of copper in 2.0 M HNO 3 after 4.0 h immersion at 25 °C Where Wt (wo) and Wt (w) are weight loss of copper metal without and with the presence of the organic compounds in 2.0M HNO 3 for 4.0 h.4.0 20.5 61.9 70.5 Weight loss, g The inhibition effect of all organic compounds (1-4) increases with increasing the additive concentration and reach a limiting value.This limiting value is believed to be related to the formation of a monolayer film of the adsorbent on the active sites of the metal.Evidence to support this conclusion was obtained by plotting Langmuir adsorption isotherm 25 .Figure 3 illustrates the plotting of log (θ/1-θ) versus log concentration for all compounds (equation 3).The resulting straight line relationships support the fact that the inhibition is due to adsorption of the organic compounds.By comparing the value of I at the limiting value and the corresponding inhibitor concentration, one can conclude that the relative strength of these organic compounds as inhibitors decreases in the following order 4>3>2>1.All substrates in Scheme 1 are substituted flavonoids.Thus one can correlate the inhibitory effect of these compounds with the substituents on the flavonoids.It is believed that compounds 1 adsorbed on the active sites of copper metal via the carbonyl and hydroxyl groups at position 4 and 5, respectively.Addition of methoxy group (OCH 3 ) at position 3, which is an electron donating group (compound 2), enhances to some extent the adsorption process.Addition of another OCH 3 group at position 3 (compound 3) and another OCH 3 group at position 6 enhances remarkably the adsorption process through increasing the %I to 91.6% for compound 4 when the concentration of compound 4 was 1×10 -4 .
All flavonoids 1-4 have the same stem with different substituents of OCH 3 groups at different positions.The %I of compounds 3 and 4 are very high even at low concentrations, indicating that both compounds adsorbed strongly at the surface of copper metal due to a decrease in sterric effect.
Upon increasing temperature from 25 °C to 55 °C the %I decreases for all compounds, with the same order obtained at 25 °C in 2.0 M HNO 3 for 4 hour at 1×10 -4 M of the different organic compounds (Table 3) as shown in Figure 4, indicating that the corrosion inhibition takes place by adsorption of the inhibitors at the electrode solution interface 26 .4) and shown in Figure 5. Ea was found to be 114.1,123.8, 127.3 and 132.9 kJ/mol for compounds 1 to 4, respectively.And for copper in 2.0 M HNO 3 without the presence of the inhibitor is equal 73.4 kJ/mol, which in accordance with literature value 27 .This result is in accordance with the strength of adsorption on the surface of copper metal 28 .
Where, R corr is the corrosion rate (mg/cm 2 .s),W 1 is the initial weight of the specimen (mg), W 2 the specimen weight (mg) after the immersion period, 't' is the immersion time (s) and A is area of the specimen (cm 2 ).

Scheme 1 .
Scheme 1.Chemical structures of the tested flavonoids used for inhibition of copper corrosion in nitric acid.

Figure 1 .
Figure 1.Weight loss curves for copper in 2.0 M HNO 3 in the presence of different concentrations (M) of compounds (1-4) at 25 °C for 4.0 h.

Figure 3 .
Figure 3. log (θ/1-θ) vs. log C curves for corrosion of copper in the presence of different concentrations of all flavonoids derivatives at 25 °C.All substrates in Scheme 1 are substituted flavonoids.Thus one can correlate the inhibitory effect of these compounds with the substituents on the flavonoids.It is believed that compounds 1 adsorbed on the active sites of copper metal via the carbonyl and hydroxyl groups at position 4 and 5, respectively.Addition of methoxy group (OCH 3 ) at position 3, which is an electron donating group (compound 2), enhances to some extent the adsorption process.Addition of another OCH 3 group at position 3 (compound 3) and another OCH 3 group at position 6 enhances remarkably the adsorption process through increasing the %I to 91.6% for compound 4 when the concentration of compound 4 was 1×10 -4 .All flavonoids 1-4 have the same stem with different substituents of OCH 3 groups at different positions.The %I of compounds 3 and 4 are very high even at low concentrations, indicating that both compounds adsorbed strongly at the surface of copper metal due to a decrease in sterric effect.Upon increasing temperature from 25 °C to 55 °C the %I decreases for all compounds, with the same order obtained at 25 °C in 2.0 M HNO 3 for 4 hour at 1×10 -4 M of the different organic compounds (Table3) as shown in Figure4, indicating that the corrosion inhibition takes place by adsorption of the inhibitors at the electrode solution interface26 .

Figure 4 .
Figure 4. Inhibition efficiency (%I) at 1×10 -4 M of compounds (1-4) for corrosion of copper in 2.0 M HNO 3 after 4.0 hour immersion at 25, 35, 45 and 55 °CEnergy of activation (Ea) was obtained by plotting log rate of corrosion (mg/cm 2 .s)versus 1/T (Table4) and shown in Figure5.Ea was found to be 114.1,123.8, 127.3 and 132.9 kJ/mol for compounds 1 to 4, respectively.And for copper in 2.0 M HNO 3 without the presence of the inhibitor is equal 73.4 kJ/mol, which in accordance with literature value27 .This result is in accordance with the strength of adsorption on the surface of copper metal28 .

Table 2 .
Inhibition efficiency (%I) of the different concentrations of compounds (1-4) for the corrosion of copper in 2.0 M HNO 3 after 4.0 h immersion at 25 °C

Table 4 .
Variation of log rate without and with the presence of inhibitors verses reciprocal of temperature