Evaluation of the pitting corrosion for aluminum alloys ... · , Hani Aziz Ameen 2 and Sami Ibrahim Jafer Al-Rubaiey 3 1 University of Baghdad-Col. of Science-Chemistry Dept - Baghdad,

AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH © 2011, Science Huβ, http://www.scihub.org/AJSIR ISSN: 2153-649X doi:10.5251/ajsir.2011.2.2.283.296

Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5% NaCl solution with range of temperature (100-500)°C

Abdulkareem Mohammed Ali Alsamuraee1, Hani Aziz Ameen

2 and Sami Ibrahim Jafer

Al-Rubaiey3

1 University of Baghdad-Col. of Science-Chemistry Dept - Baghdad, Iraq,

E-mail:[email protected] 2 Technical College – Baghdad – Iraq, E-mail:[email protected]

3 University of Technology – Baghdad-Iraq, E-mail: [email protected] ABSTRACT

The effect of homogenous heating variables (temperature and time holding) on the tendency of high strength Al-alloys type 7020 is investigated. The experiments were carried out by heating the specimens at different temperatures (100, 150, 200, 250, 300, 350, 400, 450 and 500) °C for two periods of holding time (one hour and two hours). Microstructure of an alloy was investigated by the optical microscope before and after heating at each temperature and precipitation phase and clearing revealed MgZn2. An immersion test was applied on specimens after heating to show the reduction in weight. Polarization technique was used to study the effects of microstructures on their electrochemical behavior in 3.5% NaCl solution. Microscopic and X-ray examination were used to reveal pitting corrosion on the microstructure of non-heated and heated 7020 Al-alloys. The results show that increasing the temperatures of heating from 100°C to 500°C caused the appearance of pitting corrosion on the precipitated phases and increase of pitting corrosion tendency with increase of temperature of heating; increasing time holding of heating from 1 hr. to 2 hrs. increases the pitting corrosion.

Keywords : Pitting corrosion , Heat treatment, Aluminum alloys INTRODUCTION

Many metals suffer from corrosion; aluminum alloys are one of them. They are extensively employed as structural components in industrial practice. Some surface treatments (e.g. anodizing, chromating, painting, coating and cathodic production) are used to improve their corrosion resistance. Very few studies have been devoted to the case of pitting corrosion. Zhao et al, 1998 study of the morphology of pits formed by corrosion of thin films of aluminum using an in-plane light scattering technique. It can be shown that the corrosion front of the thin aluminum film can be treated as a quasi-two-level random rough surface. Zuhair and Amro, 2002. The electrochemical behavior of aluminum alloy 6061/Al2O3 metal matrix composite was investigated in a 3.5% NaCl aqueous solution; cyclic polarization

tests were carried out to determine pitting potentials and repassivation potentials in 3.5% NaCl solution. This work included an investigation of pitting corrosion of aluminum alloy 7020 is predicted, thus the prediction of the pitting corrosion is a challenging task. This paper has a novel discussion of the pitting corrosion behavior.

Experimental Work

Materials and Equipments : A plate of aluminum alloy (7020) having dimension (300x80)mm of 10mm a thickness was prepared. Table 1 illustrates the comparison of the chemical composition of the alloy with the standard condition of 7020 aluminum alloys.

An analysis of the chemical composition was carried out by using Arun Technology Metal Scan Desktop Metals Analysis (2500 series – England, 2004).

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Table 1: The Chemical Composition of 7020 Alloy Elements Standard value %

(Aluminum manual,2007) Measured value%

Si < = 0.35 0.121 Fe < = 0.40 0.290 Cu < = 0.20 0.200 Mn 0.05 - 0.5 0.0764 Mg 1 -1.5 1.25 Cr 0.10 – 0.35 0.228 Zn 4 – 5 4.56 Ti 0.08 0.0319 Al Balance Balance

The Preparation of the Specimen: Specimens of (25x30) cm were cut from the metal plate. The dimension of the surface to be tested is 25cm x 25cm, a 0.5cm upper edge was left in order to suspend the specimen in the solution from a 0.2cm diameter hole located in the center upper edge of the 0.5cm.

The Preparation of the Surface Specimen: The surface specimen is grounded with abrasive paper of grades 400, 800, 1000 and 1200 respectively .The grinding process was continued until the clay layer was removed. This process was followed by two stages of polishing. In the first stage alumina paste having particles of 5µ diameter was used. In the second stage 0.5µ diameter of alumina paste was used in order to obtain good polished surface. The specimen was then cleaned with acetone to remove any dirt or grease from the polished surface.

Heating: Nine heat treatments were performed in the electrical furnace type GARBOLITE – SHEFFIELD-ENGLAND +15°C with maximum of 1200°C in the following manner:

1. Heating No.1: homogenization treatment at 100°C, for 1hr and 2hrs; each follow by cooling in furnace.


3. Heating No. 3: homogenization treatment at 200°C, for 1hr and 2hrs; each follow by cooling in furnace.






9. Heating No.9: homogenization treatment at 500°C , for 1hr and 2hrs; each follow by cooling in furnace.

Classification of specimens: The specimens were classified into three groups, as shown in Table 2.

Program Procedures: Program procedures are summarized in Figure 1 .

Fig 1: Flowchart of the Program Procedure

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Environment “Electrolyte Preparation”: In order to study the pitting corrosion of 7020 aluminum alloy; a solution of a 3.5%NaCl and de-ammonized distilled water for the solution was prepared and measured by a pH meter. It was found to be 6.9 .

Corrosion test: The pitting corrosion behavior of the specimen in Table 2 was carried out by two examination methods:

Table 2: Classification of Specimens

Symbol State A As received B Heat (100 – 150 – 200 – 250 – 300 – 350 – 400 – 450– 500°C) for 1 hr and furnace

cooling C Heat (100 – 150 – 200 – 250 – 300 – 350 – 400 – 450 – 500°C) for 2 hr and furnace

cooling. Immersion tests: Immersion test was applied on the specimens of Table 2 as follows: Sea water was prepared containing a 3.5%NaCl solution and one liter of distilled water. Specimens were measured before immersion for seven days in cups containing sea water.

After surface preparation, the surface areas of the specimens of Table 2 were carefully measured. Since area enters into the formula for calculating the corrosion rate, the original areas 25mm x 25mm were used to calculate the corrosion rate. Specimens are electrically insulated and isolated from contact with another metal to avoid any galvanic effects. All original the areas of 25mm x 25mm are completely immersed in the tested solution.

Electrochemical tests: The prepared specimen of Table 2 was fixed in the holder shown in Figure 2. The reference electrode was fixed about 1mm away from the surface of the specimen to be tested. The reference electrode used in this study was Saturated Calomel Electrode (SCE). The auxiliary electrode used in the electrochemical cell was platinum type. The specimen holder (working electrode), together with the reference and auxiliary electrodes were inserted in their respective positions in the electrochemical cell used for this purpose so that they fit all these electrodes as shown in Figure 3. The cell used was made of glass. Constant potentials (anodic or cathodic) can be impressed on the specimen, by using the potentiostat (Mlab200 of Bank Eleck Germany). This potentiostat is able to induce a constant potential ranging from –1V to +1V. They are the potentials of the standard reference electrode used in this study. The potential difference between the working electrode (WE) and the reference electrode (RE) and any current passing in the circuit of the working electrode where the auxiliary electrode can be measured by using the SCI

Computer Software. Any potential difference between the working and reference electrodes as well as any current in the working electrode circuit were be automatically recorded. The results and plots were recorded using Window XP. The scan rate can be selected also. Polarization resistance tests were used to obtain the micro-cell corrosion rates. In the tests, cell current reading was taken during a short, slow sweep of the potential. The sweep was taken from –100mV to +100mV relative to OPC. The scan rate defines the speed of potential sweep in mV/sec. In this range, the curve of current density versus voltage is almost nearly linear. A linear data fitting of the standard model gives an estimate of the polarization resistance in order to calculate the corrosion current density (Icorr) and corrosion rate. The tests were performed by using a WENKING MLab multi-channels and SCI-Mlab corrosion measuring system from Bank Electronics- Intelligent controls GmbH, Germany 2007, as shown in Figure 4.

Microstructure Inspection: The microstructure evolution was investigated by using computerized optical microscopy as shown in Figure 2. The specimens were prepared for this purpose, the figure in Table 2 were taken for all specimens .

1. By using electro-etching in solution [2.5% NaCl + 8.7% NaSo3 + 11.92% NaCo3] with distilled water .

2. The photographs were investigated as shown in Figure 5.

RESULTS AND DISCUSSIONS

This investigation includes the results and discussions of the effects of temperatures (100,150 ,200, 250, 300, 350, 400, 450 and 500 ) °C, and holding time (1 hr. and 2 hrs.) of heat treatment on a microstructure and pitting corrosion of Al-alloys type

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7020 in 3.5% NaCl. The results were divided into three categories:-

1. Effects of heating temperature. 2. Effect of holding time of heating. 3. Study the Morphologhy of pits.

Effect of Heating Temperature: Figure 6 shows the effect of heating temperature at 1 hr. holding time on the electrochemical behavior of 7020 Al-alloys in 3.5% NaCl solution. They indicate that increasing the heating temperature (100, 150, 200, 250, 300, 400, 450 and 500) C° led to a decrease in the passive region of the alloy and shift the Ecorr to more negative direction (-745,-830,-805,-790,-760,-595,-621,-705,-695,-680) respectively. From this figure, it can be concluded that the pitting resistance of 7020 Al-alloys decreases with the increase of the heating temperatures. The microstructure of an alloy consists of solid solution and inter-metallic particles (IMCs) remain non-dissolved, when heating at low temperatures. Inter metallic particles can be grouped into coarse inter metallic particles and fine precipitates in aluminum alloys (Rollason,1973). Coarse inter metallic particles form during the solidification process, while fine precipitates (including hardening precipitates in the matrix and grain boundary precipitates) form during ageing process (Rollason ,1973). As mentioned earlier, inter-metallic particles play a crucial role in localized corrosion of aluminum alloys. The coarse inter-metallic particles can be divided into two groups. Active particles and Noble particles relative to the Al-matrix. Therefore, the inter-metallic particles exhibit electrochemical properties different from the Matrix. Mg containing phases are active to the matrix and act as anode. They are susceptible to active dissolution or (Mg) de-alloying when immersed in chloride solution. However, the higher is the level of precipitation hardening which occurs at high heating temperatures the higher is the susceptibility to corrosion. Active components of the matrix alloy and the inter-metallic phases may corrode selectively, resulting in changed corrosion properties. Pitting is a highly localized type of corrosion as shown in Figure

6 in the aggressive chloride ions. Pits are initiated at weak sites in the oxide film by chloride attack. Pits propagate according to the following anodic reactions:-

Al - Al+3 + 3e َ (1)

Al+3 + 3H2O Al(OH)3 + 3H+ (2)

Hydrogen evolution and oxygen reduction are the important reduction processes at the inter metallic cathodes as

2H+ +2e َ H2 (3)

O2 + 2H2O + 4e َ 4OH َ (4)

According to reaction 2, the pH will decrease as pitting propagates. Therefore, the environment inside the pit (anode) changes. To balance the positive charge produced by reactions 1and 2, chloride ions will migrate into the pit. The resulting HCl formation inside the pit causes accelerated pit propagation. It is postulated that, at the critical pitting potential, breakdown occurs by a process of field assisted Cl adsorption on the hydrated oxide surface and formation of a soluble basic chloride salt which readily goes into solution. This salt was considered to be AlCl3 or Al(OH)2Cl. Figure 6. Table 3 shows also the pitting current density. The pitting current density (as shown in this Figure and Table) decreases sharply when the oxide film of Al2O3 gives good protection against environment attack. The pitting current density have 50.1 μA/cm2 when the alloys 7020 have heated to 100 °C and remain 1hr then cooling in furnaces. This value indicates low pitting corrosion. The oxide Al2O3 can be used as a protective film. When the heating temperatures increases, the current density of pitting corrosion increases. This means that the oxide film fails to protect the surface alloy from corrosion attack. From Figure 6 the maximum value of current pitting corrosion take place at heating temperatures as follows: 100> 150> 300> 400> 200> 350> 450> 250> 500°C. These results are not only attributed to the change in the oxide film nature but also in the characteristics of the inter-metallic particles and Al- matrix.

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Table 3: Values of; Ecorr , Icorr , Erp , and Eb Estimated from Tafel and Cyclic Polarization Curves of Al-7020 at Different Heat Treatment Temperatures with Time.

Specimens Al 7020

Heat treatment temperature ºC

Ecorr / mV vs SCE

Icorr / µA/cm2

Erp / mV vs SCE

Eb / mV vs SCE

1 With out(R.T.) -745 14.3 -306 55 2 100/1h -830 50.1 -305 48 3 100/2h -870 53.5 -300 55 4 150/1h -805 42.7 -330 50 5 150/2h -819 47.0 -338 53 6 200/1h -790 22.5 -350 0 7 200/2h -822 25.9 -367 11 8 250/1h -760 15.5 -358 14 9 250/2h -810 18.6 -364 16 10 300/1h -595 31.9 -197 23 11 300/2h -630 33.8 -207 26 12 350/1h -621 22.2 -211 24 13 350/2h -609 17.9 -246 28 14 400/1h -705 24.3 -198 82 15 400/2h -700 19.9 -200 89 16 450/1h -695 18.1 -166 110 17 450/2h -691 15.8 -176 118 18 500/1h -680 12.4 -198 145 19 500/2h -663 11.6 -220 153

Effect of Holding Time: Figures 7, 8, 9, 10 and 11 show the effect of holding time on the electrochemical behavior of 7020 Al- alloys in 3.5% NaCl solution at various heating temperatures (100, 150, 200, 250, 300, 350, 400, 450 and 500) °C. They indicate that the increase of the holding time from one hour to two hours led to an increase of the potential of the pitting corrosion Epit to a more negative value. This indicates that the pitting resistance of 7020 Al-alloys decreases with an increase in the holding time of the heating. This is due to precipitation of phases and intermetallic compounds. It was observed that when the holding time of the heating temperature increases, the precipitation process would have sufficient time to occur. The kinetics of phase precipitations are usually affected by the variations in the holding time of the heating temperature. This behavior is attributed to the difference in the microstructure of the alloy. The study of Wojcirch,1991, evidenced that time and temperature of heating must be chosen to give the proper precipitate size; if temperature is too low, the critical dispersion was never be achieved, although the hardness grows with time as the precipitate size increases. If the temperature is too high, the precipitate is too coarse. Thus, the system’s free energy is lowered in steps by passing through a series of metastable state on the road to equilibrium.

As there are differences in the composition of the alloy, the first step in the precipitation is the segregation of the atoms of the alloy element into clusters or platelets with a few angstroms in thickness and a hundred angstroms in diameter; it is still part of the parent lattice. These platelets are often called Gainure- Preston Zones or GP1 Zones. In the next step, the atoms of the alloy element diffuse to the GP1 Zone to form a larger one called GP2 Zone. A new transition phase which is coherent with the matrix is precipitated. With increasing time and temperature of heating, the coherent transition phases convert into the equilibrium precipitate without coherency. The tendency to pitting corrosion is associated with these intermediate forms of precipitates. Wojcirch,1991 had observed that the optimum strength was obtained by heat treatment at 165°C for ten hours, after which, if the treatment time is prolonged, a rapid precipitation of non coherent particles would take place. The solid solution (Matrix) rejected the non- coherent phase. This process in this structure due to faulty heating is generally termed “reversion”. It is seen from Figure 7, that corrosion current density decreases with increasing heating temperature from 100, 150, 200 and 250 °C. This can be attributed to intermediate forms of the precipitate. It is seen also that, at 2 hrs. holding time the current density is always higher than at 1 hr. holding time. At 300°C temperature the specimens were protected by

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very adhesive oxide layer which acted as a barrier between the environment and the surface specimen. Therefore, the current corrosion increases. The tendency of different heated specimens to corrosion is illustrated in Figure 8. It is seen that when heating temperature increases, the Ecorr shifts to a more negative value. This is because of the particles precipitation. Figure 9 illustrates that the breakdown potentials are constant at heating temperature of 100 and 150°C, at temperature of 400, 450 and 500 °C, the Eb increases due to particle precipitation. The behavior of 2 hrs. holding time is the same at that of the 1 hr. holding time. Figure 10 illustrates the dependence of Erp on the heating temperature. At temperatures of 100, 150 and 200 °C the Erp decreases; at temperature higher than 250°C, the Erp increases and the mean value of Erp = -300 ≈320 mV.

Morphology of pits : The microstructure for aluminum alloy 7020 which presented the specimen classification A,B and C for Table 2 were shown in Figure 11. It is observed that variation in grain size of the alloy 7020 depend mainly on the proportion of the zinc and magnesium elements, their values and the presence of other alloying elements causing the formation of different soluble or insoluble compounds in the solid state (Rao ,2004). The morphology of pits depends mainly on the microstructure of 7020 Al-alloys. Heating at low temperatures results in a fine distribution of (G-P) zones. Therefore, these zones suffered pitting corrosion. The pit growth usually also causes a fine distribution of shapes. As heating temperature increases, (G.P) zones grow into indirect phases of (MgxZny) or (Al2Mg3Zn3). Therefore, the alloy hardness increases with increasing temperature. In this condition, uniform pitting attack is promoted and coherent precipitates are most desirable for pitting corrosion. This is particularly remarkable for the shape of precipitates as shown in Figures 11 and 12. At high heating temperatures, dark portions are formed. The portions consist of a mixture of α-Al and precipitates. These results are

good evidences that the heating temperature and holding time have an important and similar effect on the microstructure of the alloy and its electrochemical properties. It is evidenced from the figures that with increasing heating temperature the corrosion resistance decreases. This is an important case related to the amount of the precipitated phases. However, the higher is the level of precipitation, the higher is the susceptibility to corrosion. The degree of both precipitation and corrosion susceptibility is a function of heated temperature and holding time. Pits have the shape of the active phase, which may corrode preferentially. Some phases may serve as a critical anode and provide cathodic protection to the surrounding matrix. Moreover, pit nucleation and propagation occur in the zones where MgZn2 is accumulated which take place at high heating temperature (400-500)°C. At these temperatures, a double pitting behavior is observed; the first one is in the Mg and Zn enriched region and the second is in the matrix. Tables 4 and 5 show the XRD results after and before immersion. \

The intermetallic precipitate phase were formed after heating at different temperatures. The diffusion of element alloy (zinc, magnesium) was high but the solubility at normal temperature was low. Figure 11 shows an increase of grain size if the grain size is compared with specimen classification A in Table 2 and specimen classification B in the same Table. After heating at different temperatures for one hour; it can be seen that the grain size for specimen in classification B was large; this shows the effect of heating on grain size. The grain size of specimen classification C heated at different temperatures for two hours shows increasing the solubility of element.

Results of Immersion Test : The results of the weight of specimens at different temperature before immersion and after immersion are shown in Table 6.

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Table 4: Analysis of X – Ray Diffraction Sample Temperature

°C 2θ D(A°) measured D(A°) standard Phases

A Without heating 38.4 44.4 44.8 41.5

2.34 2.03 2.01

2.174

2.35 2.03 2.01 2.18

AlMg4Zn11 Al Al

MgZn2 B 100 40.5

41.4 45.4 38.6 44.8 37.2

2.225 2.179 2.330 2.02

2.41

2.23 2.182

0.02

2.40

MgZn MgZn2

Al Al

Al3Mg2 B 450 45.0807

40.55 38.06 41.5

42.01 44.85

2.009 2.222 2.362 2.174 2.251 2.018

2.02 2.22 2.36 2.18 2.25 2.01

Al Mg4Zn7 MgZn MgZn2 MgZn2 MgZn2

C 400 41 44.5 40.5

2.199 2.03

2.225

2.182 2.02 2.22

MgZn2 Al

Mg4Zn7C 350 37.5

41.4 40.5

2.39 2.179 2.225

2.40 2.18 2.22

Al2Mg3 MgZn2 Mg4Zn7

Table 5: X – Ray Diffraction Analysis of 7020 at 400°C after Immersion

2 θ D(A) D(A) standard Phases

400 °C 1 hr.

38.8 40.44

45

2.319 2.179 2.01

2.30 2.182 2.02

Mg7Zn3 MgZn2

Al

400 °C 2 hrs.

40.6 38.45

45

2.220 2.319 2.01

2.22 2.30 2.02

Mg4Zn7 Mg7Zn3

Al

Table 6: Results of Weight of Specimens during the Immersion Test

Temperature Weight of specimen before immersion (mg)

Weight of specimen after immersion (mg)

Corrosion Rate C.R=(534*Δ w)/ (ρ *A*t)

(mpy) 100 2.8941 2.8928 0.0003 150 4.7258 4.7248 0.00015 250 4.0490 4.0487 0.000058 300 4.1954 4.1946 0.00014 350 5.2502 5.2498 0.000068 400 4.1816 4.1805 0.00018 450 5.7875 5.7856 0.00026 500 5.5740 5.5710 0.0004

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Figure: 2 Figure: 3 Figure: 4 Figure:5

Without Heat Treating (R.T.) Heat Treated At 100 °C /1hr. Heat Treated At 150 °C /1hr. Heat Treated At 200 °C /1hr.

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Heat Treated At 250 °C /1hr. Heat Treated At 300 °C /1hr. Heat Treated At 350 °C /1hr. Heat Treated At 400 °C /1hr.

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Heat Treated At 450 °C /1hr. Heat Treated At 500 °C /1hr.

Fig 6: Tafel and Cyclic Polarization Curve of Aluminum alloy 7020

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Figure 7: Icorr of Different Specimen’s conditions Figure 8: Ecorr at Different Specimen’s Conditions

Fig 9: Eb at Different Specimen’s Conditions Fig 10: Erp at Different Specimen’s Conditions

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Fig 11: Comparison Pit Shape with the Predicated Particles Shape at Different Heating Temperatures for 1 Hour Holding Time

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Fig 12: Comparison Pit Shape with the Predicated Particles Shape at Different Heating Temperatures for 2 Hours Holding Time

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CONCLUSIONS:

The results of the study evidenced the following conclusions:

1- Heating of the alloys has a great significant on the corrosion resistance of 7020 Al-alloy.

2- As temperatures of heating increase from 100-500°C, pitting corrosion appears on the precipitated phases.

3- Pitting corrosion tendency increases with increasing the temperature of heating.

4- Increasing the time holding of heating from 1 hr. to 2 hrs increases the pitting corrosion.

REFERENCES [1] Y.P. Zhao, C.F. Cheng, G.C. Wang, and T.M. Lu

"Characterization of Pitting Corrosion in Aluminum

Films by Light Scattering", Applied Physics Letters, Volume 73, Number 17, October 1998.

[2] M. G. Zuhair and M. Amro "Corrosion Behavior of Powder Metallurgy Aluminum Alloy 6061/Al2O3 Metal Matrix Composites", The 6th Saudi Engineering Conference, KFUPM, Dhahran, December 2002.

[3] Mat web, The Online Materials Database, “Aluminum 7020, 7000 series Aluminum Alloy, Aluminum Association, 2007.

[4] E.C. Rollason "Metallurgy for Engineering”, Edward Arnold Publishers, 4th Edition, 1973.

[5] J. Wojcirch " Faculty of Mechanism and Electrics Influence of the Heat Treatment in the Electrochemical Corrosion of Al-Zn-Mg alloys", revised 2, pp(81-103), Glynia Poland, October 1991.

[6] K. S. Rao "Pitting Corrosion of Heat Treatable Aluminum Alloys and Weld", Trans. India in St. Met, Vol. 57, No. 6, pp (503-610), December 2004.

Documents

Evaluation of the pitting corrosion for aluminum alloys ... · , Hani Aziz Ameen 2 and Sami Ibrahim Jafer Al-Rubaiey 3 1 University of Baghdad-Col. of Science-Chemistry Dept - Baghdad,