INTRODUCTION

1. Atmospheric corrosion is one of the leading causes of structural damage to aircrafts. This structural damage is a result of intergranular corrosion (IGC), which develops into fatigue cracks, stress corrosion cracks, or exfoliation. The aircraft industry uses aluminum alloy because of its high mechanical performance and low density, however it is highly susceptible to intergranular corrosion. The foreign precipitate injection into the alloy results in a solute depleted zone which is adjacent to the grain boundaries because of the particle growth at the grain boundaries. These solute depleted zones act as the anodic region that forms pathways for IGC. Corrosion initiates at the surface as pitting and then develops into sub-surface network of IGC. Relative humidity (RH)  is defined as the ratio of the quantity of water vapor present in the atmosphere to the saturation quantity (what air can hold) at a given temperature. The relative humidity affects the ability to provide sufficient moisture on the metallic surface for corrosion to proceed. Corrosion becomes more severe when the aircraft is operating above sea and oceans where high humidity content is present. Dusty surfaces and polluted environments are more prone to corrosion than clean surfaces. Dust keeps the area humid, and humidity encourages corrosion. Furthermore, it lowers the critical level of relative humidity and hence it increases condensation. It can also act as a catalyst between reactions of pollutants found in air such as nitrogen oxides and sulfur dioxides and the moisture film. The rate of corrosion of aluminum is high in both highly acidic and basic environments. In normal conditions, aluminum reacts readily with atmospheric oxygen and forms a thin layer of aluminum oxide which protects it from corrosion. This layer is highly soluble in strong acidic and basic solutions, therefore aluminum has high rate of corrosion in highly acidic environments. INTRODUCTION PHENOMENON SELECTION OF MATERIALS - Anodizing The goal of anodizing aluminum alloys is to thicken the naturally occurring, corrosion-resistant aluminum oxide produced when pure aluminum comes in contact with oxygen. This process is done by emerging the metal into an electrolytic solution and allowing a current to pass through the solution where the aluminum alloy is at the anode The electrolysis of water produces oxygen: Anode (oxidation): 2 H2O(l) → O2(g) + 4 H+(aq) + 4e− Cathode (reduction): 2 H+(aq) + 2e− → H2(g) With the O2(g) produced from the oxidation reaction Al2O3(s) can be produced: 2Al(s) + 3/2O2(g) = Al2O3(s) CORROSION CONTROLS Aluminum Anodizers Council (n,d). Anodizing...the Finishing of Choice. [ONLINE] Available at: http://www.anodizing.org/Anodizing/what_is_anodizing.html. [Last Accessed 22 November 2013]. Frankel, G.S. McCreery, R.L, (2001). Inhibition of Al Alloy Corrosion by Chromates. The Electrochemical Society Interface Frankel, G.S., (2001). Mechanism of Al Alloy Corrosion and the Role of Chromate Inhibitors . . (), pp.83 Federation Aviation Administration (2013). Aviation Maintenance Technician Handbook - General. [ONLINE] Available at: http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_handbook/. [Last Accessed 27 November 2013]. Hassan, R. M., & Zaafarany, I. A. (2013). Kinetics of Corrosion Inhibition of Aluminum in Acidic Media by Water-Soluble Natural Polymeric Pectates as Anionic Polyelectrolyte Inhibitors. Materials, 6(6), 2436-2451. Sherif, E. M. (2011). Corrosion and corrosion inhibition of aluminum in Arabian Gulf seawater and sodium chloride solutions by 3-amino-5-mercapto-1, 2, 4-triazole. Inter J ElectrochemSci, 6, 1479-1492. Vargel, C. (2004). Corrosion of aluminium. Access Online via Elsevier. ENVIRONMENTAL FACTORS Intergranular corrosion is an electrochemical process primarily involving electrochemical reactions with the environment. In electrochemical reactions, a metal anode is oxidized by a cathode which acts like a voltaic cell. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path through which electrons may flow, corrosion begins as the metal decays by oxidation. During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal and thus becoming neutralized. Al(s) =>  Al3+ (aq) + 3e- (1) O2(g) + 2H2O(l) + 4e-=> 4OH-(aq) (2) Al3+(aq) + 3OH-(aq) => Al(OH)3(s) (3) 2 Al(s) + 6 H2O(aq) => 2 Al(OH)3(aq) + 3H2(aq) (4) Unlike other metals such as iron, pure aluminum is not extremely reactive in the presence of air. It forms a coating of oxide that protects it from further reaction with oxygen. 4Al(s)+ 3O2(g)=> 2Al2O3(s) (5) Aluminum metal will react under certain  circumstances such as in the presence of acids and bases.   An example of an acidic corrosion reaction is:     2Al + 6HCl => 2AlCl3 + 3H2 (6) An example of a basic corrosion reaction is:     Al + NaOH + 4H2O => NaAl(OH)4 + 2H2 (7) All of these reactions serve to deteriorate the aluminum which has many negative industrial impacts. Group 19: Jennifer Li, Razwan Arshad, Danny Wong, SarabjitKaler, Saad Khan Corrosion on Aircrafts The fuselage of the aeroplane was blown away midair. The pilot was able to land the plane safely, but an air hostess was blown away and lost her life. The National Transportation Safety Board in its investigation concluded that the incident occurred because operator’s maintenance program failed to detect corrosion damage. The fuselage came apart because skin panels came apart at lap joints. Corrosion accelerated the fatigue in the fuselage skin panels. Aloha airline flight 243 operated in an environment where presence of chloride ions is high. This aggressive environment accelerated the corrosion process in lap joints which resulted in corrosion fatigue. CASE STUDY The airframe may not be subjected to corrosion, but the metal components and accessories within the aircraft most certainly will. For example, rust (ferrous oxide) is one of the most well-known types of corrosion. In aircrafts, rust will appear on hold-down nuts and bolt heads. This usually leads to the contamination (via corrosion) of other major components. In turn, the structural strength and stability of the aircraft may be compromised. With aluminum, considerable attacks can be sustained before a serious loss of structural strength occurs because of the protective oxide barriers. The aircraft industry uses aluminum alloy because of its high mechanical performance and low density, however it is highly susceptible to intergranular corrosion The optimal mechanical properties of the aluminum used are achieved by introducing strengthening precipitates within the grains under high pressure and high temperature conditions. Some methods of measuring extent of intergranular corrosion: • Cross-sectioning: view and measure the depth of attack • X-ray computed tomography: assess the depth and volume of corrosion as a function of time in 97% humidity and as a function of relative humidity. Polarize alloy in anodic direction then transfer to 97% relative humidity. Depth of IGC determined by X-ray images. • Potentiodynamic test: photokinetic polarization of samples from catholic potential REFERENCES - Painting Painting the surface of the aircraft is one of the easiest ways to prevent corrosion.  By using a non-water based paint, the metal is no longer in contact with elements in the air, such as water vapour and oxygen, that cause corrosion. The degradation of paint over time is one major concern.  As soon as the barrier between the metal and air is lost, the metal will react with the air causing it to corrode. - Inhibitors Chromate based inhibitors are commonly referred to as chromate conversion coating (CCC) and are an effective method to control corrosion. It is a coating that is applied to the exterior of a metal surface to prevent corrosion. Metals that are exposed at the base can be protected for years after a CCC coating is applied.  A very useful and important property of CCC coating is that it has the ability to store the chromate and release it to areas of the metal that have damage. This means that intergranular corrosion, the most destructive and troublesome type of corrosion, can be prevented with a CCC because the grain boundaries of the aluminium alloys are protected. They are now looking into the use of titanium in the wings and the fuselage. This allows for the plane to fly at higher altitudes and higher speeds. Titanium offers more strength and stiffness than aluminum. SR-71 Blackbird