AN ALTERNATIVE TO CHROMATES FOR CORROSION PROTECTION FOR ALUMINUM ALLOYS

Aluminum alloys, such as Al 6061-T6 and Al 7075-T6, are widely used in industry due to their high strength to weight ratio and good mechanical properties. The corrosion of these alloys, however, is an expensive and critical problem since the alloys are susceptible to pitting and crevice corrosion in marine environments. The most significant environmental factor, which contributes to the corrosion of these alloys, is the chloride ion found in marine environments or water condensed from humid air contaminated with soluble chloride salts. Traditionally, chromate based conversion coatings have been used for many years for the protection of aluminum alloys. Chromates are efficient corrosion inhibitors for aluminum and its alloys in near neutral marine environments containing aggressive anions such as chlorides. Although the hexavalent chromium ion, Cr, may be a superior corrosion inhibitor and used in numerous industrial systems, it is environmentally unsafe. Over the past several years, federal agencies, such as the Environmental Protection Agency (EPA) and the Department of Defense (DoD), have increasingly limited the use of chromium containing compounds due to their toxic and carcinogenic effects. In addition, there is a direct economic challenge associated with costs for environmental compliance along with increased liability for claims of exposure in the workplace with the continued use of chromates. Therefore, there is a need to identify new corrosion inhibitors for aluminum alloys. As an alternate conversion coating, a new titanate conversion coating was researched and developed for the Al 2024-T3 alloy and was shown to be effective. The objective of this research was to determine if the coating process could be applied to Al 6061-T6 and Al 7075-T6. The coating process involves immersion of the alloy in a titante solution bath, which produces a passive film. The corrosion resistance of coated samples has been evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic electrochemical techniques. Electrochemical testing and energy dispersive X-ray analysis indicated that the titanate ion would retard corrosion in a similar manner to the chromate ion if fluoride ions (F) were not present on the surface. A study was also conducted to determine if Al 6061-T6 and Al 7075-T6 were easily susceptible to crevice corrosion in a marine environment. The study yielded important results regarding protection of the alloy against crevice corrosion by the titanate ion. Corrosion was only seen on samples not exposed to the titanate ion. A conclusion may be made that titanate coatings appear to be viable alternatives to chromate coatings but further investigation will be required in order to determine an optimum conversion coating bath, which will produce impedance magnitudes comparable to those measured for the Al 2024-T3 alloy.


LIST OF TABLES CHAPTER I
Aluminum (Al), when coupled with small amounts of other materials, is a fundamental and beneficial metal used in a wide range of industrial applications due to its high specific strength [1]. These alloys are commonly used in marine applications where low-density materials, good mechanical properties and improved resistance to corrosion are desired [2]. Aluminum alloy 6061-T6 is known for its superior mechanical properties, such as high strength to weight ratio, good ductility and good corrosion resistance [3]. The composition of this alloy is 1.0% Mg and 0.6% Si and lesser amounts of Cu, Mn, Fe and Cr. The balance of the alloy is aluminum.
Aluminum alloy 7075-T6 is extensively used for structural applications due to its high strength/density ratio and reasonable high fracture toughness [4]. The composition of this alloy is 5.5% Zn, 2.6% Mg, 1.55% Cu and lesser amounts of Cr, Si, Mn and Fe.
Once again, the balance of the alloy is aluminum. The "T6" classification indicates that the alloy was solution treated and artificially aged [5].
During solidification and thermomechanical processing, heterogeneous microstructures are developed to produce a desirable mix of mechanical properties.
The dominant feature of alloy microstructures is the distribution of second-phase particles that contain high concentrations of alloying and impurity elements [6]. These particles have electrochemical characteristics that differ from the surrounding alloy matrix, making the alloy more susceptible, in general, to localized corrosion.

Corrosion of Al 6061-T6 and Al 7075-T6
Corrosion, in general, can be defined as the "degradation of engineering materials by exposure to a wet surface [7]." For corrosion of a metal to take place, four conditions need to be satisfied. The first process is an oxidation or anodic reaction.
The second is a reduction or cathodic reaction and the third is ionic transport for which a conductive electrolyte is required, such as water, seawater, or an acidic or basic solution. Finally, the fourth process is electron transport between the anode and cathode. If one of these processes is not present, corrosion will not occur.
If these alloys are left untreated, they will corrode at a rate depending on the alloys composition and local environment. Generally, aluminum is resistant to most environments due to a layer of oxide film, which forms on the surface and reforms rapidly if damaged. However, this film is an insufficient barrier for relatively longterm corrosion protection. This is because aluminum is able to react both as a base or an acid, which means its oxide film is stable in neutral conditions but soluble in acidic and alkaline environments. This relationship is expressed by the Pourbaix diagram, which shows the relationship between potential and the solution pH. Figure 1-1 is the Pourbaix diagram for aluminum, which indicates the circumstances in which aluminum should show corrosion [8].
The resistance of aluminum to corrosion depends significantly on its purity and microstructure. Pure aluminum is more resistant than any of its alloys. The 6xxx series alloys are susceptible to corrosion but resistance decreases as the copper and iron content increase. At copper levels higher than 0.5%, intergranular corrosion can occur. Also, when the magnesium and silicon contents in Al 6061 are balanced to form only Mg 2 Si, corrosion is slight, but if the alloy contains silicon in excess of that needed to form Mg 2 Si, susceptibility to corrosion increases. Al 7075, which contains a significant amount of copper, is less resistant to corrosion than those of the same series that do not contain copper, as well as the 6xxx series.
The most common form of aluminum corrosion is pitting, which is a localized corrosion form. It has been attributed to the breakdown of the natural passive film on the metal. The resistance to pitting corrosion is then determined by the electrochemical stability of the protective passive film. The tendency for pitting for a given metalelectrolyte system is defined by the pitting potential (E p ), which is the potential above which pits will initiate and below which they will not [9]. For aluminum, pitting corrosion is most commonly produced by halide ions, of which the chloride (Cl -) is the most frequently found. The presence of chlorides can create local corrosion potential drops between the metal surface and the obstructed region at which the chloride is accumulated. Chlorides facilitate the breakdown of the oxide film by forming AlCl 3 .
When aluminum ions migrate away from the pits, alumina precipitates as a membrane, which further isolates local acidity and pitting of the metal results [8].
Pitting can be separated into two different stages, namely pit initiation and pit growth. While the growth mechanism is well understood, the initiation mechanism is not very clear. However, pitting has been shown to initiate at constituent particles, which are either anodic or cathodic relative to the matrix. Local interactions between the particles and the matrix enhance the rate of pit growth. In Al 7075-T6 samples, the constituent particles show significant pitting after being exposed to sodium chloride (NaCl) solution. It can also be seen that pits developed around neighboring constituent particles tend to coalesce to form larger pits. Research has been done, which presents standard electrode potentials of the strengthening precipitates as well as constituent particles. The Mg 2 Si particles found in Al 6061-T6 and the MgZn 2 particles found in Al 7075-T6 are both significantly anodic. The presence of anodic particles implies that they contribute to the overall pitting process after long exposures to NaCl [10].
Once initiation takes place, pits begin to increase in size. The exposed surface outside the growing pit is cathodically protected by the reduction of oxygen to hydroxyl ion (OH -) reaction: As this cathodically protects the region outside the pit, the metal dissolution region cannot spread laterally across the surface. In addition, the large cathodic surface can maintain this reaction and form a large cathode to small anode ratio, which accelerates the anodic reaction. Within the pit, the metal dissolution reaction is taking place. This is the anodic reaction of: Al ! Al 3+ + 3e -(oxidation half cell reaction) Since it is the only reaction within the pit, an electrical imbalance results again, thereby attracting negatively charged ions, usually chloride ions. The autocatalytic reaction to form hydrochloric acid in the pit is initiated and continues: Since pitting is an autocatalytic reaction, once it is started, the pH decreases while the chloride ion concentration increases inside the pit. The pitting mechanism can be seen in Figure 1-2 for both Al 6061 and Al 7075 alloys.
One other type of possible corrosion that may be seen on aluminum alloys is crevice corrosion. The general conditions include a stagnant solution and a gap between two surfaces, one of which is metal. Initially, the usual cathodic (Eqn 1) and anodic (Eqn 2) reactions occur over the surface of the metal. However, a restriction occurs in the crevice region in which the dissolved oxygen in the crevice cannot easily be replaced. Therefore, the region inside the crevice cannot support a cathodic reaction but can still support an anodic reaction. Outside the crevice region the cathodic reaction proceeds but the anodic reaction ceases.
An electrical charge imbalance exists between the high positive charge from the metal ions within the crevice and the negative charge outside the crevice. As a result, negative ions, such as chloride ions, are attracted into the crevice. Associated with the negative chloride ion is the positive hydrogen ion. Both the chloride ion concentration and the hydrogen ion concentration increase within the crevice, decreasing the pH to acidic conditions, which allows the corrosion rate inside the crevice to increase. This mechanism can be seen in Figure 1-3 [11].

Corrosion Protection of Al 6061-T6 and Al 7075-T6
The most significant environmental factor, which contributes to the corrosion of these alloys, is the chloride ion found in marine environments or water condensed from humid air contaminated with soluble chloride salts [12]. Since long-term corrosion resistance is unlikely due to the thin natural oxide film, a finishing process is required to reduce corrosion susceptibility. To prevent rapid deterioration, various methods, which usually involve several layers of protection on top of the Al substrate, have been developed. For example, an artificially thick aluminum oxide (Al 2 O 3 ) layer can be grown, either chemically or electrochemically, directly above the bare alloy.
This allows for various paints and coatings to be applied to the oxide film.
Corrosion resistant coatings prevent corrosion on aluminum alloys by various methods, including barrier protection and active corrosion protection as well as conversion coatings. Barrier coatings prevent contact of the underlying aluminum substrate with the environment. They are either organic or inorganic and work to suppress the cathodic reaction and limit the transport of electrons to the metal surface.
In the active corrosion protection strategy, corrosion inhibitors are used to slow the corrosion cell process on aluminum by undergoing reduction at the active corrosion sites to form insoluble oxides. This provides a barrier against corrosion by limiting the permeability of electrolytes, such as chloride ions.
Conversion coatings are applied to aluminum and aluminum alloys to improve corrosion resistance or to improve adhesion. It is a term that describes the removal of the native oxide on a metal and its replacement with an oxide coating that provides a barrier to corrosion. Conversion coatings are adherent surface layers of low-solubility oxide phosphate or chromate compounds produced by the reaction of suitable reagents with the metallic surface. These coatings affect the appearance, electrochemical potential, electrical resistivity, surface hardness, absorption, and other surface properties of the material. They are formed by a chemical oxidation-reduction reaction at the surface of the aluminum [13]. Currently, the most effective and widely used way to inhibit corrosion of aluminum alloys is a chromate-based conversion coating.

Corrosion Inhibition by Chromates
Cr(VI) compounds, mainly chromates, are widely used as corrosion inhibitors in aqueous media. A wide range of metals and alloys, such as iron, steel, aluminum alloys, zinc, copper, and others, can be protected using chromates. Their high efficiency to cost ratio has made them the standard inhibitors [14].
There are many ways to inhibit corrosion through the use of Cr(VI) compounds. Two of the most prominent are chromic acid anodization and chromate conversion coatings. Chromic acid anodization involves the electrochemical growth of an aluminum oxide surface film in an aqueous solution where a non-porous oxide layer is formed with a thicker porous layer above it. Coatings on aluminum alloys are on the order of 2-50 !m in thickness. Anodization is carried out in an acidic bath, which contains ingredients that promote formation of an adherent oxide film [15].
Chromates seal the porous layer with chromic acid (H 2 CrO 4 ), producing a thicker oxide layer, which provides barrier protection for the bare metal as well as providing active passivation. Although anodization offers superior corrosion protection, chromate conversion coatings are more preferable due to economic benefits and practicality. Anodization can be expensive and therefore not affordable when dealing with large aluminum structures.
Chromate conversion coatings are generally used to increase the corrosion resistance of aluminum. The high corrosion resistance provided by chromate coatings is due to the presence of hexavalent and trivalent chromium ions. The trivalent chromium, Cr 3+ or Cr(III), is present as an insoluble hydrated oxide, while the hexavalent chromium, Cr 6+ or Cr(VI), adds a self-healing nature to the film during corrosive attack by species such as chloride ions. During corrosion, the hexavalent chromium is reduced to form trivalent chromium, which terminates the corrosive attack [13].
Chromate ions increase the pitting potential of aluminum alloys in chloride media and inhibit pit initiation and dissolution of active intermetallic phases.
Chromate conversion coatings (CCC) form on aluminum through reduction of Cr 6+ (dichromate) in solution and are usually acidic with a pH between the range of 1.6 and 3.0. Coating formation is assisted by the addition of sodium fluoride (NaF), which helps to activate the aluminum surface. A CCC is a chemically grown oxide layer on the alloy substrate that provides an active barrier layer, which reduces the rate of the cathodic oxygen reaction. The chemical and electronic variety found in Cr chemistry leads to the ability of Cr 6+ oxoanions to inhibit corrosion [16]. The electrochemical reactions for the chromate conversion coating process are well known [17].
Understanding the mechanism for chromate inhibition of aluminum alloy dissolution is important. Chromate is a very soluble and a high-valent oxidizing ion with a low-valent form that is insoluble. The oxidation of Al in the presence of competing fluoride ions produces electrons to reduce the hexavalent Cr 6+ of the dichromate ion, Cr 2 O 7 2and form a protective hydrated 3-valent Cr(OH 3) . The final result is a film thickness of at least several hundred nanometers on matrix regions, with thinner coatings at second phase particles [18]. This film, which provides the barrier protection against corrosion, is one mechanism of corrosion protection offered by CCCs.
Another very important mechanism is the self-healing feature of chromate conversion coatings. The coating layer consists of an amorphous and insoluble chromium oxide, where the formation of Cr(III)-O-Cr(VI) bonds takes place. These bonds act as adsorption sites for chromate ions from the coating bath. Therefore, the coating is a mixture of hydrated amorphous Cr(III)-Cr(VI) oxide. Where Cr(VI) is in contact with the electrolyte, it migrates to the defects of the coating layer, where it is more susceptible to corrosion attack [19]. In other words, the easily broken down hexavalent chromium in the coating is released into a solution contacting the surface. It has been proposed that three factors contribute to the performance of chromate conversion coatings: (1) barrier protection, (2) hydrophobicity and (3) active species that protect weak spots or emerging pits. The oxide layer itself is inert and acts as a barrier layer, which provides protection to the underlying bare metal. Although it is a clear fact that Cr(VI) is the active species in chromate conversion coatings, where corrosion protection is provided by the reduction of Cr 6+ to Cr 3+ , precisely how chromate works to forestall corrosion remains unclear. In addition, in spite of its good performance as an anti-corrosion treatment, the Cr(VI) species are well known to be environmentally unfriendly.

Toxicity of Chromates
Studies over the past 10-15 years indicate that chromates are both highly toxic and carcinogenic. The oral ingestion of chromates is known to cause gastrointestinal damage, kidney failure, liver damage, blood disorders and eventually death. Prolonged exposure to skin may cause rashes, blisters, and ulcers and has also been associated with lung cancer and intestinal tumors. Chromates can also penetrate the body by inhalation, which may eventually cause lung cancer.
Although Cr 6+ may be a superior corrosion inhibitor and used in numerous industrial systems, the same properties that make it so are also the same that make it environmentally unsafe. Earlier studies document Cr(VI) as a human carcinogenic associated with lung cancer. However, it is not the static presence of Cr 3+ or Cr 6+ that contributes directly to the DNA damage that leads to cancer. Rather, the molecular debris associated with the process of reducing Cr 6+ to Cr 3+ induces the critical changes in DNA. Chromate alone does not damage DNA in the absence of reducing agents.
Instead, it is the biological antioxidants that lead to DNA damage.
The intracellular reaction of Cr 6+ in the presence of reducing agents produces Cr 5+ , Cr 4+ , Cr 3+ , free radicals and reactive oxygen, which are all potentially genotoxic.
Although there is no general agreement on the details for Cr 6+ -induced damage to DNA, it is clear that Cr 6+ is highly soluble in water and passes through cell membranes. In addition, small molecule antioxidants appear to form highly reactive intermediates such as Cr 5+ and Cr 4+ , which in turn react either directly or through free radical intermediates to damage DNA [16].

Alternatives to Chromate Conversion Coatings
Due to the highly toxic and carcinogenic nature of Cr(VI), and it being far from environmentally friendly, research studies have begun to focus greater attention on non-Cr(VI) conversion processes. Low toxicity conversion coatings prepared in non-Cr(VI) solutions, such as titanium, zirconium, molybdenum and cerium salt baths, have been widely researched and developed. Although they have the potential to replace existing Cr(VI) conversion coatings, their anticorrosive performance remains inferior.
In recent years, studies have been done to find more ecological alternatives to protecting aluminum alloy surfaces in order to replace chromates in their different fields of application. Efforts have been focused on the search for new corrosion inhibitors and new formulations of both anodizing baths and conversion coatings.
However, many of the new systems are still in the beginning stages and many alternative technologies are being investigated.
In the last five to six years, researchers have begun to look at trivalent chromium conversion coatings as a promising alternative because their treatment solutions are less toxic than hexavalent compounds but seem to produce similar results.
However, the Cr(III) conversion process is a novel study for aluminum alloys [21]. A new approach is to replace the chromate ion with the titanate ion since titanium is an element that has many similarities to chromium. It is one of the elements whose Pourbaix diagram closely resembles that of chromium. Pourbaix diagrams show the relationship between potential and the solution pH to predict whether an electrode will be immune, active or passive in the environment [25].  2. To investigate the basic mechanisms of coating formation, such as the coating composition and deposition rates along with mechanisms of corrosion protection.

Introduction
Aluminum (Al) and its alloys are fundamental and beneficial metals used in a wide range of industrial applications, due to their high specific strength, low density and good mechanical properties. Other than Al 2024, little to no research on electrochemical behavior has been conducted on other aluminum alloys of industrial interest, such as Al 6061 and Al 7075. Al 6061-T6 is an Al-Mg-Si alloy, which is known for its superior mechanical properties, such as high strength to weight ratio, good ductility and good corrosion resistance [1]. Al 7076 is an Al-Zn-Mg alloy, which is widely used for structural applications due to its high strength/density ratio and reasonable high fracture toughness [2].
However, the electrochemical behavior of these alloys is beginning to attract the attention of many researchers. The natural passivating oxide film on aluminum is an insufficient barrier for relatively long-term corrosion in a marine environment.
Therefore, inhibitors are being used to improve protection on the surface. Traditionally, chromates have been applied in anticorrosive pre-treatments of aluminum alloys as conversion coatings [3][4][5]. A chromate conversion coating is a chemically grown oxide layer on the alloy substrate that provides an active barrier layer, which decreases the rate of the cathodic reaction, therefore inhibiting corrosion. However, these chromate coatings contain the hexavalent chromate ion, (Cr 6+ ) which is toxic and carcinogenic and the consequent health hazards associated with them have led to restrictions imposed on the use of these conversion coatings as well as an initiative to find alternative methods of corrosion protection [6][7][8][9][10][11]. At present, a suitable candidate for chromate replacement has not yet been developed for Al 6061 and Al 7075, which are used for the most demanding applications.
There are several ways to inhibit corrosion including a coating that decreases the reaction rate of the substrate in which the anodic oxidation reaction is suppressed.
A second method, which is of interest here, is to suppress the cathodic reduction reaction. As a result, there are no electrons available to support the anodic reaction.
A chromate-free conversion coating has successfully been developed for Al 2024-T3 using the titanate ion, which has many similarities to the chromate ion. This study mainly focuses on determining if the titanate coating process can be applied to aluminum alloys 6061-T6 and 7075-T6.

Titanate Coating Techniques
The material used throughout the research investigations was commercially produced Al 6061-T6 and Al 7075-T6, cut into 1.5-inch squares with a thickness of 0.6 inches. Typical compositions of each alloy are shown in Table 2 hours. This process can be seen in Figure 2-1.
One aim of any coating process is to reduce the number of steps in the process.
In this research, the alkaline cleanser was changed and the use of a proprietary acid cleaner was removed. The alkaline cleanser was enough to be sufficient pretreatment for these particular alloys for successful conversion coating.
The alternative coating technique, therefore, includes a different cleaning step.

Instead of NaOH, an industrial alkaline cleanser was procured from Henkel
International. Using this cleanser, a 500 mL solution and 15% by volume to water was made using 75 mL of alkaline cleanser at pH 10.6 and 425 mL of de-ionized water.
The coating process includes the following: (1) solvent clean with acetone, rinse in deionized water, (2) chemical cleaning with alkaline cleanser at pH of 10.6 for 10 minutes at 60ºC, rinse in de-ionized water, (3) conversion coating in titanate bath for 3 minutes at pH 4.0 and 60ºC, rinse in de-ionized water and finally (4) air dried for 24 hours.

Electrochemical Impedance Testing
Once alloys had gone through the coating process, their corrosion resistance was monitored by electrochemical impedance spectroscopy (EIS). EIS measurement is a non-destructive method able to provide time dependent data on the surface properties of materials in marine environments. The test was conducted using a Gamry Instrument PC4 potentiostat connected to a computer. The test cell, which can be seen in Figure 2-2, has a glass cylinder clamped with an O-ring seal in the middle of the specimen surface to provide an exposed surface area of approximately 0.785 in 2 . The cell contained about 50 mL of 0.5N sodium chloride (NaCl) electrolyte and the counter electrode was platinum foil, while the reference electrode was a saturated calomel electrode (SCE). Open circuit potential was measured for 100 seconds prior to the experiment and the impedance spectra was measured with a frequency range from 100,000 Hz to 0.01 Hz in logarithmic decrement. EIS measurements were taken over a period of 42 days (1,000 hours).

Potentiodynamic Tests
Potentiodynamic scans (PDS) were also conducted to determine the anodic and cathodic behavior of the alloys when titanate is in solution, but not a conversion coating. The aim is to determine if the titanate is an inhibitor to these alloys and what type of inhibitor it is. These tests were carried out in a flat sample cell, seen in Figure   2

Surface Characterization
Surface characterization was performed on immersed and conversion coated samples. To analyze the surface of conversion coated samples, a scanning electron microscope (SEM) was used with X-ray EDS capability, in which the local compositions were studied. Photographs of the immersed samples used to test for crevice corrosion were taken at various intervals over 1,000 hours. SEM imaging was done to analyze these surfaces as well. The spectra were obtained at an acceleration voltage of 20 keV.

Introduction
Several different methods of measuring corrosion on aluminum alloys 6061-T6 and 7075-T6 were employed in this study. Electrochemical impedance spectroscopy year. The methods of measuring corrosion employed in this study were visual observation recorded by a digital camera and surface characterization, which was performed on immersed and conversion coated samples and conducted using a scanning electron microscope with an energy dispersed X-ray system.

Electrochemical Impedance Spectroscopy
The results of the impedance measurements varied. Al 6061-T6 samples were conversion coated using two different methods. In the first method, samples were chemically cleaned with NaOH at pH 12.5 for 10 minutes at 40ºC and then submerged in a proprietary solution of Smut-Go for 10 minutes. Once cleaned, they were coated in the conversion bath at pH 5.5 for 3 minutes at 60ºC. Impedance measurements exhibited varied results. Most samples had resistances well below 10,000 ohms"cm 2 within the first week of testing. However, one sample challenged these results. On day 1, the impedance was only 45,107.8 ohms"cm 2 but over a 42-day (1,000 hours) period, the impedance increased to 138,441 ohms"cm 2 . Bode plots for the sample that tested well and for a sample that tested poorly can be seen in Figure  Since the cleaning process seemed to be damaging the Al 6061-T6, a second conversion coating process involving a new cleaning step was implemented. Instead of using NaOH and Smut-Go, samples were cleaned in an industrial alkaline cleanser at pH 10.5 for 10 minutes at 60ºC. Preliminary EIS measurements resulted in poor impedance so the pH of the titanate coating bath was decreased to 4.0. This resulted in very high impedance results. On day 1, the impedance was 270,824 ohms"cm 2 .
However, over the next 30 days, sodium chloride was observed to be leaking through the O-ring (Figure 2-2) and corrosion was apparent. The impedance, however, fluctuated and after 30 days, it was 265,579 ohms"cm 2 . Bode plots, which compare a plain sample to the coated sample, can be seen in Figure 3 Repeated results were desired but could not be achieved.

Potentiodynamic Scans
Potentiodynamic curves can be used to gain a better understanding of the When 3 g/L K 2 TiO 3 was added to the system, two different reactions occurred.
There was a titanium reaction and an aluminum reaction and each had their own corresponding open circuit potentials. When purged with oxygen, the OCP of the titanium reaction was approximately -1.16 V SCE, while the OCP was -1.18 V SCE when purged with nitrogen. For the aluminum reaction, the OCP was -760 mV SCE when the system was purged with either oxygen or nitrogen. The limiting current density of the oxygen and nitrogen purged systems with titanate additions was 0.94 !A/cm 2 and 1.7 !A/cm 2 , respectively.

Crevice Corrosion
The simple test, which was set up to determine if Al 6061-T6 is prone to crevice corrosion, yielded important results regarding protection of the alloy against crevice corrosion by the titanate ion. The sample exposed to 0.5N NaCl solution, without the addition of titanate, is slightly corroded in the region where the commercial modeling compound was present and formed a crevice. The alloy was only 0.120 inches thick in this region against a starting thickness of 0.125 inches.
When varying concentrations of K 2 TiO 3 were added to the NaCl solution, corrosion was not apparent anywhere, including the area where crevice corrosion was found without titanate addition. This can be seen in Figure 3-8.

Surface Characterization
Surface characterization and morphology of the conversion coating on Al 6061-T6 was studied using a scanning electron microscope and energy dispersive Xray analysis. One sample cleaned with NaOH and Smut-Go had a pitted surface as well as precipitates at higher magnifications. Other samples showed no signs of pitting but still contained precipitates. Energy dispersive (EDS) X-ray analysis showed these precipitates to be sodium fluoride (NaF) crystals. A SEM image of a precipitate along with its corresponding EDS spectrum can be seen in Figure 3-9. The sample cleaned with the industrial alkaline cleanser had no signs of pitting at high or low magnifications. However, at high magnifications, precipitates could be seen. Energy dispersive X-ray analysis showed these precipitates to be potassium fluoride (KF) crystals. The EDS spectrum can be seen in Figure 3-10 along with its corresponding SEM image.

Electrochemical Impedance Spectroscopy
When cleaned with a combination of NaOH and Smut-Go and coated with a titanate bath at pH 5.5, the results were poor. On day 1, the impedance was 571.828 ohms"cm 2 . Samples were only tested for 31 days due to apparent corrosion and on day 30, the impedance was 394.415 ohms"cm 2 . Bode plots comparing a plain, uncoated sample and a coated sample can be seen in Figure 3-11. Similar to Al 6061-T6, SEM imaging showed pitting on the surface, although not as extreme. This can be seen in Introducing the industrial alkaline cleanser and decreasing the pH of the coating to 4.0 also had a positive effect on Al 7075-T6 samples. On day 1, the resistance was 30,478.6 ohms"cm 2 . However, similar to the Al 6061-T6 samples, sodium chloride was observed to be leaking through the O-ring and after 31 days, the final resistance was 18,532.3 ohms"cm 2 , which was still exceedingly high above a plain, uncoated sample. Bode plots comparing the two can be seen in Figure 3-13.
Also similar to Al 6061-T6 was the surface characterization. As seen in Figure 3-14, the surface was smooth with no pitting. Repeated results were again unable to be obtained.

Potentiodynamic Scans
The potentiodynamic curves for Al 7075-T6 in 0.5N NaCl solutions with and without titanate, in both oxygen-purged and nitrogen-purged systems can be seen in which also happened in the case of the system purged with nitrogen. For the aluminum reaction, the OCP was -760 mV SCE for the oxygen-purged system. Removing oxygen by purging the system with nitrogen increased the OCP slightly to -740 mV SCE. The limiting current density of the oxygen and nitrogen purged systems with titanate additions was 2.25 !A/cm 2 and 1.9 !A/cm 2 , respectively.

Crevice Corrosion
The simple test also provided useful information regarding the titanate ion decreasing crevice corrosion for Al 7075-T6. The sample exposed to 0.5N NaCl solution, without the addition of titanate, was severely corroded where the commercial modeling compound created a crevice. The measured thickness of the crevice was 0.049 inches against a starting thickness of 0.0625 inches. Adding varying concentrations of titanate to the NaCl solution did not seem to corrode the samples anywhere, including the region where crevice corrosion was found without the addition of titanate. Digital images of each sample can be seen in Figure 3-17.

Surface Characterization
Surface characterization and morphology of the conversion coating on Al 7075-T6 were studied using energy dispersive X-ray analysis and scanning electron microscope imaging. Similar to the Al 6061-T6 alloy, SEM imaging showed precipitates on the surface whether the alloy was cleaned with NaOH and Smut-Go or the alkaline cleanser. Energy dispersive X-ray analysis indicated that the precipitates on the surfaces cleaned with NaOH and Smut-Go were NaF crystals. A SEM image of a precipitate along with its corresponding EDS spectrum can be seen in Figure 3-18.
On surfaces cleaned with the industrial alkaline cleanser, KF crystals were seen, which are shown in Figure 3-19.

Crevice Corrosion
General conditions for crevice corrosion include a stagnant halide ion containing solution and a narrow gap between two surfaces, one of which is metal [1].
In this study, a test was conducted in order to determine if both Al 6061-T6 and Al 7075-T6 are prone to crevice corrosion when exposed to NaCl solution as well as the effect of the titanate ion on crevice corrosion. A sample was placed in a stagnant solution (NaCl) and fixed to the bottom of a beaker using a commercial modeling compound. Therefore, the surfaces, the metal alloy and the modeling compound, formed a crevice. Inside the crevice region, dissolved oxygen could not easily be replaced. The region inside the crevice could not support a cathodic reaction but could still support an anodic reaction, while outside the crevice region, the cathodic reaction proceeded but the anodic reaction ceased. Consequently, an electrical charge imbalance took place between the high positive charge from the metal ions within the crevice and the negative charge outside the crevice, allowing chloride ions into the crevice. Therefore, the corrosion rate inside the crevice increased.
The final thickness of the Al 6061-T6 sample was 0.120 inches against a starting thickness of 0.125 inches. The initial thickness of the Al 7075-T6 sample was 0.0625 inches. After being exposed to NaCl solution, the crevice region was reduced to 0.049 inches. Crevice corrosion on both Al 6061-T6 and Al 7075-T6 can be seen in This test also suggests that the titanate ion would be a good candidate for a conversion coating solution if it can be applied effectively. In a test such as this one, the crevice corrosion can start anywhere where the crevice and the solution conditions become acidic. Clearly the titanate ion is a good inhibitor of crevice corrosion.

Potentiodynamic Scans
To investigate the effect of the titanate ion in electrochemical behavior, When the potential is more negative than -1.15 V, the cathodic reaction takes place as titanate ions are reduced to form titanium on the surface of the aluminum alloy.
When the potential is more positive than -1.15 V, the titanium anodic reaction is initiated and titanium is oxidized to create titanium ions, which react with oxygen to form an oxide passivating layer. When the potential is more positive than -700 mV, the aluminum anodic oxidation reaction is taking place.
Al ! Al 3+ + 3e - The TiO 2 film cannot resist this reaction and is broken down, exposing aluminum.
Removing oxygen by purging the system with nitrogen results in a larger cathodic current density, which may be related to the electrochemical reduction of titanate ions. Comparing both oxygen and nitrogen purged cathodic polarization curves, it can be hypothesized that the presence of oxygen in the solution produces the oxide based film faster than lower oxygen levels can, which results in lower cathodic currents with higher oxygen levels in solution.
The potentiodynamic data indicate one possible mechanism for the titanate conversion coating. The low primary passivation potential of -1.18 V SCE combined with very low critical current densities indicate that the titanate ion is a cathodic inhibitor. Cathodic inhibitions reduce one of the necessary components for corrosion, namely the cathodic reaction rate. As this is lowered, the anodic reaction cannot be supported. Chromates are suspected to work in a similar manner [2] by acting as a cathodic inhibitor. Another requirement is 'self-healing', the ability to repair defects [3]. This mechanism can be seen in Figure 4-1.

Electrochemical Impedance Spectroscopy
When choosing aluminum alloys for industrial use, the 6xxx series is highly suitable in various applications due to its good resistance to corrosion [4].
Electrochemical impedance data indicates that a plain, uncoated Al 6061-T6 sample has a relatively high impedance magnitude of 22,625.4 ohms"cm 2 on day 1. Therefore, samples that have undergone a conversion coating process to enhance corrosion resistance should have impedance measurements well above that of a plain, uncoated sample.
Electrochemical impedance data displayed as Bode phase plots in 0.5N NaCl solution for Al 6061-T6 alloys, which were chemically cleaned with NaOH at pH 12.5 and titanate conversion coated at 5.5 pH for 3 minutes is presented in Figure 3-1.
These plots show time dependent data for two samples, one exhibiting high impedances and a second sample, which showed very poor impedances as a function of time. When examined in a scanning electron microscope, severe pitting was observed on the sample that produced high impedance magnitudes (Figure 3-2a). Only minor pitting was seen, however, on most samples throughout the study. However, their impedance and corrosion resistance was low. The presence of pitting alone was not sufficient to produce a conversion coating that did not increase corrosion resistance. One possible explanation is that the severity of the local surface changes may harm the corrosion resistance. Consequently, few pits with severe surface profile changes may disrupt the coating and impede resistance by initiating flaws. This indicates a non-pitting cleaning stage has to be required.
Unlike the 6xxx series, alloys in the 7xxx series are more susceptible to corrosion especially those containing copper, such as Al 7075. This decrease in corrosion resistance is indicated by an electrochemical impedance for a plain, uncoated sample of a 354.876 ohms"cm 2 on day 1, in comparison to Al 6061-T6, which was 22,625 ohms"cm 2 on day 1. Electrochemical impedance data for Al 7075-T6 cleaned in NaOH and titanate coated with pH 5.5 is presented in Figure 3-10. EIS measurements after the titanate coating bath indicated very low impedance magnitudes.
Electrochemical impedance data displayed as Bode phase plots in 0.5N NaCl solution for Al 6061-T6 and Al 7075-T6 alloys, which were cleaned in an alkaline cleanser with pH 10.5 and titanate conversion coated at 4.0 pH for 3 minutes is presented in Figure 3-3 and Figure 3-12, respectively. Reducing the steps of the coating process to using just an alkaline cleanser instead of both NaOH and Smut-Go produced high impedance magnitudes on only one sample of Al 6061-T6 throughout the study. Relatively high resistances were seen on an Al 7075-T6 sample but were still low enough to be considered poor data.

Mechanical
There are several possible reasons for poor resistance measurements, which can be attributed to the titanium coating on the surface of the aluminum alloy. Possible mechanical explanations include pitting on the surface. In this study, sodium hydroxide (NaOH) attacks the surface and this attack results in pits. Earlier studies did not indicate this severity of attack but this was on Al 2024-T3, a copper rich alloy [5].
As stated in the previous section, if the severity of the local surface changes, the corrosion resistance may be harmed.
SEM imaging shows different types of pitting on the surface, single pits as well as concentrations of multiple pits. Single pits have a hemispherical shape where the majority is underneath the surface. There is an extreme angle change and a bad surface profile, which tends to be sharper. Where there are multiple pits, more material is removed, which leads to the removal of the surface, allowing for a smoother coating over the pitted surface. SEM imaging and EIS measurements indicated that the sample with multiple pits had good corrosion resistance, while samples with single pits throughout lead to low impedance magnitudes. When a second cleaning system is employed to remove the NaOH, no pitting was found yet the EIS indicated that the conversion coating was still not reliable. This suggests another mechanism may be involved with poor coating performance. These mechanisms can be seen in Figure 4-2.

Microstructure
One other possible reason for coating failure is the presence of second phase particles on the surface. If there are Mg 2 Si (Al 6061) or MgZn 2 (Al 7075) particles on the surface, there will be a break in the coating and corrosion will occur. As the titanate ion inhibited crevice corrosion on these alloys, it appears that it can protect against these microstructural features, which will always be present on the surface.
This mechanism can be seen in Figure 4-3.

Chemical Precipitates
Low resistance measurements on Al 6061-T6 and Al 7075-T6 alloys in this study can be explained by the surface characterization of each alloy. SEM imaging showed large clusters of precipitates on the surfaces. A simple mechanism depicting a damaging precipitate on the surface can be seen in Figure 4-4. Energy dispersive Xray analysis indicated that these precipitates were fluoride (F -) crystals. Fluoride ions are known to be extremely aggressive towards titanium [6,7]. High concentrations of fluoride ions will destroy the oxide film that was chemically grown by the titanium bath. If there are damaging precipitates on the surface of the alloy, such as fluoride, coating failure will always occur, which will always lead to corrosion, which can be confirmed by EIS measurements in this study.
For Al 6061-T6 and Al 7075-T6 samples cleaned with NaOH and Smut-Go and then titanate coated with pH 5.5, energy dispersive X-ray analysis indicated that the precipitates on the surface were sodium fluoride (NaF), which was used as an activator in the conversion coating bath.
On samples cleaned with the alkaline cleanser and then titanate coated with pH 4.0, energy dispersive X-ray analysis indicated that the precipitates were potassium fluoride (KF), which seemed to have formed during the coating step when the fluoride ion attached itself to the potassium from the potassium titanate (K 2 TiO 3 ). On the Al 6061-T6 sample that produced high impedance magnitudes, SEM imaging showed a small concentration of KF precipitates. On all other samples with poor coating performance, higher concentrations were seen. It seems that as a result of high concentrations of NaF or KF precipitates, impedance magnitudes were very low.

Conclusions
A test, which was conducted to determine if Al 6061-T6 and Al 7075-T6 are prone to crevice corrosion, yielded critical results regarding the protection of Al 6061-T6 and Al 7075-T6 against crevice corrosion by the titanate ion. It has been determined that the titanate ion protects these alloys against crevice corrosion in a marine environment.
Titanate based conversion coatings hold promise as a replacement for chromates on Al 6061-T6 and Al 7075-T6 alloys as they show passive film that inhibits the surface from corrosion. The potentiodynamic study revealed that the film protects the alloy surface. However, electrochemical impedance spectroscopy exhibited varied results and impedance magnitudes were typically low. It seems that pitting on the surface did not have an effect on corrosion resistance, which is confirmed by the varied EIS measurements.
It can be concluded, however, that low impedance magnitudes can be attributed to the precipitation of fluoride ions during the coating process, which significantly inhibit corrosion protection. Any surface with sodium fluoride precipitates present will corrode. Further investigation will be required in order to determine an optimum conversion bath, which will produce impedance magnitudes comparable to those measured for the Al 2024-T3 alloy.
In conclusion, once an optimum coating process for these alloys is determined, further work will be needed to turn the process into an industrially accepted system, in which an optimized and consistent coating process will meet all the necessary requirements.    Al ! Al 3+ + 3e -  Chapter V

Recommendations and Future Research
A detailed study should be conducted to determine the optimum concentration of sodium fluoride (NaF) in the conversion coating bath. Titanate coating baths should be made with NaF concentrations of 0 g/L, 1 g/L and 2 g/L in order to determine the optimum concentration, in which fluoride precipitates will not be present on the surface.
The solubility limit of NaF in solution needs to be determined under conversion coating conditions.
In addition, research should be conducted to quantify the surface roughness of the alloys. This can be done by stereo imaging in scanning electron microscope images.
Thorough research also should be done to determine the concentration of Ti 2+ and Ti 4+ ions in the conversion coating bath. Also the solubility limit of titanate ions in the solution should be measured so the optimum amount of potassium titanium oxide is in the conversion coating solution, thus addition of excess amounts can be minimized to optimize the cost.
Further investigation is required to verify that the titanate ion protects aluminum alloys Al 6061-T6 and Al 7075-T6 against crevice corrosion.