Corrosion in aerospace materials

KMEB 4346 : AEROSPACE MATERIALS

Corrosion in Aerospace Materials

Nur Fatihah Saadiqin Binti Ahmad Zauti

KEB100022

Outline1. Introduction

a) Definition corrosionb) Factor of corrosion in aerospace materials

2. Causes of corrosion

3. Types of corrosion4. Method of

corrosion control / protection

5. Case study – corrosion in the

Aloha Airlines flight 243

Introduction• Corrosion of metals used in aircraft structures

and engines is a large and expensive problem for the aviation industry.

• It accounts for about 25% of all metal components failures on aircraft.

• The risk and cost of corrosion damage increases with the age of the aircraft, with the hours spent on corrosion maintenance often higher than the actual flight hours for many old aircraft

Introduction

A) Definition of corrosion• Chemical attack of metals that

results in deterioration and loss of material.

• A corrosive fluid is usually involved with the most common being water containing reactive chemicals (such as chloride ions).

Introduction

B) Factors that determined the type of corrosion and rate of corrosion

Composition, metallurgical

properties and heat

treatment of metal alloy

Type of surface films

and protective systems on

metal

Presence of stresses, voids and

other defects in the metal

Composition and

concentration of the

corrosive liquid or gas

Temperature and humidity

of the environment

Causes of Corrosion

Three conditions must exist simultaneously in order for the corrosion to occur• The presence of an anode and a cathode. This occur when two dissimilar metals or two regions of

differential electrolyte concentration create a difference in electrical potential• A metallic connector between the anode and the cathode• An electrolyte such as water

Types of corrosionGeneral (or

uniform) surface

corrosion

Intergranular corrosion

Fretting corrosion

Exfoliation corrosion

Stress corrosion

Galvanic corrosion

Pitting corrosion

Crevice corrosion

a) Surface corrosion , b) corrosion cracking , c)exfoliation cracking

Types of corrosion• Stress Corrosion

Cracking (SCC)- Known as environmental

assisted stress corrosion.- Occurs rapidly and

follows the grain boundaries in aluminum alloys

- Use materials that is not susceptible to SCC at design stress levels

• Exfoliation corrosion- Also follows grain

boundaries in aluminum alloys

- Occurs in multiple planes, causing a leaf like separation of the metal grain structure.

- Ensure that grain structure that is not susceptible to exfoliation

Methods of corrosion controlMaterial selection• Selecting the proper material is essential for long-term corrosion control. • Aluminum is the most widely used airplane material.• Clad aluminum sheet and plate are used where weight and function permit, such

as for fuselage skins. • Corrosion-resistant aluminum alloys and tempers are used to increase resistance

to exfoliation corrosion and SCC.

Finish selection• For aluminum alloys, the coating system usually consists of a surface to which a

corrosion-inhibiting primer is applied. • In recent years it has become common practice not to seal the anodized layer.

Although this reduces the corrosion resistance of the anodized layer, the primer adheres better to the unsealed surface.

• The corrosion-inhibiting primers used are Skydrol-resistant epoxies formulated for general use, for resistance to fuel, or for use on exterior aerodynamic surfaces

Methods of corrosion controlDrainage• Effective drainage of all structure is vital to prevent fluids from becoming

trapped in crevices.• The entire lower pressurized fuselage is drained by a system of valved drain

holes. • Fluids are directed to these drain holes by a system of longitudinal and cross-

drain paths through the stringers and frame shear clips.

Sealants• Remove the potential for joint crevice corrosion• The polysulfide sealant is typically applied to such areas as the skin-to-stringer

and skin-to-shear tie joints in the lower lobe of the fuselage, longitudinal and circumferential skin splices, skin doublers, the spar web-to-chord and chord-to-skin joints of the wing and empennage, wheel well structure, and pressure bulkheads

Methods of corrosion controlGalvanic coupling of materials• graphite fibers, which are used to reinforce some plastic structure, present a

particularly challenging galvanic corrosion combination. • The fibers are good electrical conductors and they produce a large galvanic

potential with the aluminum alloys used in airplane structure.

Applications of corrosion inhibitions materials• CIC offer additional protection, especially when periodically reapplied in service.• CICs are petroleum-based compounds dispersed in a solvent and are either water

displacing or heavy duty. • Water-displacing CICs are sprayed on structure to penetrate faying surfaces and to

keep water from entering crevices. • These CICs must be reapplied every few years, depending on the environment in

which the airplane has been operated. Heavy-duty CICs are sprayed on as well, but they form a much thicker film and have much less penetrating ability.

• They are used on parts of the airplane most prone to corrosion.

Case study – corrosion in the Aloha Airlines flight 243

On April 28 1988, a 19 year old Boeing 737 operated by Aloha

Airlines lost a large piece of the upper fuselage as a result of

stress-corrosion cracking.

A 4 to 6 m section from the aluminum upper fuselage

suddenly broke away when the aircraft was cruising at an altitude

of 24000 feet.

The flight crew had no warning before a large piece of the

fuselage was torn off the aircraft.

A flight attendant was killed and many passenger were injured by

flying debris, but the pilot managed to land the aircraft

without further incident on the island Maui, Hawaii.

Inspection revealed – presence of multiple cracks in the fuselage, with many growing from rivet

holes in skin lap joints.

The cracks were caused by stress corrosion and corrosion fatigue.

Aircraft was operated for many years by flying between hawaiian Islands – aircraft was exposed to

sea mist & salty air.

The combination of fatigue stressing and seawater caused

corrosion-fatigue cracks to develop at a rapid rate in the

fuselage

The Aloha incident dramatically shows the danger of stress

corrosion cracking.

Case study – corrosion in the Aloha Airlines flight 243