Effect of V addition to Al and Al grain refined by Ti on chemical corrosion rates in NaOH solution with and without inhibitor at different temperatures

The paper presents the effect of vanadium addition to aluminum grain refined by titanium, in the range from 0.005 wt % to 0.236 wt %, on corrosion rate of these micro alloys in caustic soda (NaOH) solution at different temperatures. The corrosion rate decreased by the addition of any percent of vanadium at 25°C compared to pure Al. The corrosion rate increased with increase of solution temperature from 25 to 40 and 60 °C at any percent of vanadium addition. However, addition of vanadium reduced the corrosion rate at these temperatures at most wt.% of V addition. The maximum achieved reduction in corrosion rate due to vanadium addition was 40 % at 0.148 wt % and 60°C. The addition of 3 % potassium dichromate as corrosion inhibitor to NaOH solution reduced the average corrosion rates at any wt.% of vanadium. The inhibition efficiency ranged from 44- 58% at 25 °C and from 97-99% at 40 and 60°C.


Introduction
Aluminum (Al), as well as its micro alloys, has been extensively applied in industry due to their particular properties such as high strength-to-weight ratio, good appearance and corrosion resistance [1][2]. Aluminum and its alloys solidify in a coarse columnar structure of large grain size [3], which tends to reduce their mechanical strength and surface quality, therefore they are grain refined by different rare earth elements such as titanium (Ti) or Ti plus boron (B) to produce fine and equiaxed homogeneous structure, to enhance their mechanical properties and surface quality [4]. The literature on the effect of addition of different grain refiners to Al and its alloys to improve their metallurgical structure and their mechanical properties, e.g. hardness, fracture toughness and fatigue strength is voluminous. However, little research work is reported on the effect of these grain refiners on their chemical corrosion resistance [5]. Vanadium (V) enhances the grain refining efficiency of the Al micro alloys when added at a weight percentage of 0.1% or more, i.e., above the peritectic limit on the Al-V phase diagram [4]. Vanadium addition was also found to improves hardness, mechanical strength and surface quality of Al and its alloys [6]. A wide spectrum of corrosion problems are encountered in industry as a result of combination of materials, environments and service conditions. Corrosion may not have a deleterious effect on a material immediately but it affects its mechanical strength, physical appearance and it may lead to serious operational problems in future [7]. Corrosion attack on Al surfaces is usually quite obvious since the products of corrosion are white and generally more voluminous than the original base metal taking the forms of general etching, pitting or roughness of metal surfaces [8,9]. In highly alkaline solutions, e.g. sodium hydroxide (NaOH), the dissolution process occurs easily and fast resulting in hydrogen gas (H2) evolution [10]. First the reaction begins with the dissolution of the protective oxide film (hydration process at the film) and as a result hydroxide ions (OH -) are formed: Then aluminum reacts with hydroxide ions in alkaline solution as follows: The overall main reaction taking place can be represented by the following equation: Potassium dichromate (K2Cr2O7) is a common inorganic chemical reagent; it is a crystalline ionic solid with a very bright, red-orange color most commonly used as an oxidizing agent in various laboratory and industrial applications, [11]. As with all hexavalent chromium (Cr) compounds (chemical compounds that contain this element in the +6 oxidation state), dissolve as stable complexes in water, transport easily, and adsorb on oxide surfaces. The octahedral, d3, trivalent compounds of Cr form very stable inert oxides; K2Cr2O7 is a mild oxidizer, the reaction between aluminum and K2Cr2O7 in alkaline media is given in the following equation:

Materials
Commercially pure Al of 99.8% having the chemical composition shown in Table1, high purity titanium and vanadium were used. Graphite crucibles were used for melting and graphite rods were used for stirring.

Preparation of the maser and binary alloys
The binary Al-Ti and Al-V master alloys were laboratory prepared and used for preparing the five different micro alloys which are used throughout this work are shown in table2. The grain refinement process started by manufacturing the binary Al-Ti and Al-V master alloys by adding the calculated amount of pure Ti or V to the molten Al in the graphite crucible at 1100 C under cryolite flux to avoid oxidation. The temperature was kept constant for 30 minutes, then the crucible was brought out from the furnace, and stirred for two minutes before casting the alloy into a plate form of less than 10mm thickness by spreading it over a thick cast iron plate. The preparation of the different microalloys by melting the predetermined quantity of Al and the calculated amount of the binary master alloy was added to the molten Al bath, stirred and its temperature was raised to 820 C, then lowered to 750 C for 10 minutes and finally the crucible was brought out of the furnace, stirred for two minutes and left to solidify and cool inside the crucible, outside the furnace. (2)

Corrosion Testing Procedure
Cylindrical specimens having 10 mm diameter and 25 mm 2 working surface area were prepared from Al and its five micro alloy, immersed in 1 % HCl solution at room temperature for two minutes to remove any protective or oxide layer, rinsed in distilled water, kept in acetone for one minute, dried in hot air and then fixed in position in the PVC teflon holder, shown in figure1. The specimens were then immersed in 0.2 M NaOH solution of pH 13.3 at 25, 40 or 60 o C in a stand still condition, (without stirring). After a specified period of time (taken as 24 hr), the specimens were removed from the teflon holder, rinsed with acetone, dried and kept in the desiccator for two hours and weighted again from which the weight loss per mm 2 is calculated in each specimen. Each test was repeated twice to guarantee the repeatability of the results within an accepted difference of 3%. The average corrosion rate was then obtained. The weight loss technique involves exposing a specimen of material to a process environment for a given duration, then removing the specimen for analysis. The weight loss taking place over the period of exposure in the corrosive medium [2]. is being expressed as a corrosion rate according to the following expression: where CR is the corrosion rate, ΔW is weight loss (mg), t is the time (yr), A is the surface area exposed in the corrosive medium (mm 2 ).

Surface Examination Procedure
Microstructural examination of the surface of each specimen was examined by light microscope before and after corrosion test. Photographs of the etched specimens were taken using optical microscope. Detailed procedure of etching and surface examination is described elsewhere [2].  Figures. 2, 3 and 4 show the effect of vanadium addition on the corrosion rate of commercially pure aluminum grain refined by Ti, (Al-0.15 % Ti) at solution temperatures of 25, 40 and 60 o C, respectively. It can be seen from Fig.2 that, except at infinitesimally small percent of V addition (less than 0.05%), vanadium addition has generally resulted in a decrease in corrosion rate comparable with the commercially pure Al specimen. The maximum decrease in corrosion rate was 38 % and the minimum was 11 % at 0.053 and 0.148 wt % vanadium addition respectively.This is attributed to the small grain size caused by the grain refinement caused by the addition of Ti or V either alone or together, which results in increasing the number of grain boundaries and redcing the surface roughness, both of which reduces corrosion rate, references, [3,6]. Increasing the temperature of the NaOH solution to 40 and then to 60 o C resulted in higher rates of corrosion for a given micro alloy (fixed wt.% V). However, as the wt. % of vanadium is increased in the microalloy, the corrosion rate decreased significantly at the three solution temperatures . At the higher temperatures, the trend remained the same but addition of V became more effective as the temperature of the NaOH solution is increased. As can be deduced from Fig.3, the maximum reduction in the corrosion rate was approximately 28 % at 0.112 wt % V, and the minimum reduction was approximately 5% at 0.053 wt % V at 40 o C. Figure 4 indicates that the maximum reduction in the corrosion rate was approximately 40 % at 0.112 wt % V, compared to the minimum reduction of approximately 6 % at 0.148 wt % V at 60 o C. All reduction percentages are obtained from comparison with the performance of pure aluminum with zero wt % V.

Effect of Solution Temperature
In Figure Figure 6a shows the original microstructure of the commercially pure Al at magnification of X400 while figure6B is a photomicrograph showing the microstructure of commercially pure Al after an exposure of 24 hrs to 0.2 M NaOH solution at 60 o C at the same magnification. The difference between the two graphs indicates clearly the corrosion action in the forms of pitting as well as wiping off the grains and their boundaries.   Figure 7 shows the effect of V addition at different wt.% to commercially pure Al grain refined by Ti on the corrosion rate in NaOH solution at different temperatures in the presence of 3 wt% potassium dichromate (K2Cr2O7) as inhibitor. It can be seen from this Figure that the corrosion rate increases as the V percent increases, but remained much less than the corrosion rate in the absence of the K2Cr2O7 inhibitor. A comparison between Fig.7 and Fig.5 reveals that the highest corrosion rate in presence of inhibitor is only 8 mg/mm 2 .yr whereas that in absence of inhibitor is 330 mg/mm 2 .yr. In making this comparison, the corrosion rate data at 0.05 wt. % V is excluded as indicated earlier.

Effect of Inhibitor and Inhibition Efficiency
Where CRb is the corrosion rate in blank (uninhibited) solution and CRwi is the corrosion rate in solution with inhibitor, both in (mg/mm 2 .yr) units. The effect of V addition on the inhibition efficiency at different temperatures is shown in Table3 It can be seen from the result in this table that the presence of K2Cr2O7 as an inhibitor in NaOH solution improved the inhibition efficiency of the solution by 90-99.5% for commercially pure Al and 97-99 % in case of addition of Al-vanadium micro alloys. IE is obviously a function of temperature.

Conclusions
The addition of vanadium in small weight percentages (0.053-0.236) to micro alloys of aluminum and Al grain refined by Ti is beneficial in improving their corrosion resistance in highly alkaline (pH=13.3) NaOH solution, both blank (uninhibited) and in presence of potassium dichromate at 3%. The corrosion rates of these micro alloys increased with temperature increase in the range 25-60•C in