aircraft corrosion for off & tech july 2010

8/12/2019 Aircraft Corrosion for Off & Tech July 2010

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Aircraft Corrosion Control &

Prevention

For Aero Officers & Technicians

July 2010


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What is Corrosion?

It is a electro chemical deteriorationofmetal because of its chemical reaction

with the surrounding environment


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Is it a Problem?

Corrosion is most often a slow processof metaldeteriorationtakes considerable time period

Eg Pitting, Exfoliation Also sometimes occurs very quickly(In days or

hours) - Can leads to catastrophic failures

Eg Stress Corrosion cracks, Corrosion fatigue

THE RESULTSSTRUCTURAL FAILURE WITHOUT WARNING


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Why is it Important?

Aging SLAF aircrafts fleetover 25 -30 years Kafir Year of Mfc 19791984

Mi 24/35 19851996

Bell 412 19841997

Bell 212 1979 -1984

Mig 27 1981 -1985

Corrosion is unavoidableone of the most

critical maintenance problem (Aircraft structures& Ground Equipment)

Because metals are unstable in their refined forms(Tends to revert back to its natural forming state)


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Then What?

It is important as technical officers &Technicians To inspectcorrosion frequently

To identifyCorrosion types To Treat Corrosion

These are the responsibilities of the operator Should be accomplished based on

Manufacturers recommendation Common advisory circular Operators own program


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Corrosion Control & PreventionProgram (CCPP)

Is a Preventive Maintenance and control Is required to prevent In-flight mishap or

excessive down time for repairs Effectiveness Depends directly on

operating Environment Exposed to salt

Atmospheric Industrial Pollutions

Over water operations


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Corrosion Control & PreventionProgram (CCPP)contd

Requirements to prevent corrosion Constant Cycle of cleaning

Inspection Operational preservation

Lubrication


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Corrosion Control & PreventionProgram (CCPP)contd

The basic philosophyof CCPP Should adequately trained staff

Knowledge of corrosion identification techniques

Emphasis on the responsibility of program Inspectionson Schedule basis

Aircraft washingat regular scheduled intervals

Routine cleaningof all exposed unpainted surfaces

Keep drain holes free

Inspection removal & re application of preservation compounds Prompt Corrosion treatmentafter detection

Accurate record keeping

Use appropriate materials, equipmentetc


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Corrosion Theory


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Chemical Definitions Element: Any substance that cannot be separatedinto different substances by

ordinary chemical methods.

Atom: The smallest unit of an element. Consists of three parts, the proton (positivecharge), electron (negative charge), and neutron (no charge).

Electron: A negatively charged particle. An electric current charge occurs whenelectrons are forced to move through metal conductors. Electrons flow through

water solutions only in the presence of ions.

Ions: Atoms or groups of atoms bound together, which are positively or negativelycharged. An electric current occurs when ions are forced to move through watersolutions. Ions cannot move through metal conductors.

Electrolyte: A liquid (usually water) solution containing ions. Salt water is an

electrolyte

Ionization: The formation of one or more ions by the addition of electrons to or theremoval of electrons from an electrically neutral atomic or molecular configuration


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COMPONENTS OF ANELECTROCHEMICAL CORROSION CELL

Fourcomponents ANODE: A metal, which has a tendency to

corrode.

CATHODE: A dissimilar material, or aprotected part of the same metal. Its size

(large or small) determines rate of corrosiveaction.

METAL PATH: An electrical contact, orconductor, that allows transfer of electronsfrom anode to cathode.

ELECTROLYTE: Any form of liquid thatcontains ions and conducts electricity.

electrolyte is the only factor we can control


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Corrosion Theory

It is a natural Phenomenon Attacks metal by chemical or Electro chemical

action Convert metalinto metallic compoundOxide,

Hydroxide, Sulfate.

Erosionis not a corrosion. It is a metaldestruction by mechanical action

Noble Metals(Gold, Platinum) do no corrode


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Important

Elimination of ANY of these conditions willstop Corrosion

ExamplesApplication of a paint film on the metal

surface

Passive filming (invisible Oxide film)Corrosion products are tightly bound to thecorroding metale.g. Stainless Steel,Titanium


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Important

ALL corrosive attacks begins on theSurface of metal. Then go inside


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Important

Paint coating can MASK the initial stagesof corrosionInspect paint for Blisters,

Flakes, Chips or Lumps Corrosive products generally X5 higher

than the original volume of material.


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Factors influencing Corrosion

Type of metal Most active metals (Which tend to lose electrons

easily) corrode easily eg; Mg, Al

Most noble metals (do not lose electrons easily) donot corrode easily eg; Gold, Silver

Heat treatment & Grain direction Leaves stresses inside the material

Allows moisture at saturation in air

Presence of electrolyte Hard water, dirt, Salt, battery fluid, engine exhaust

gases will increase the rate of corrosion


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Factors influencing CorrosionContd

Temperatureaccelerate chemical reaction Anode & Cathode surface area(in galvanic

corrosion)

Mechanical stressesleft on the material- duringmanufacturing processes (machining, forming,welding or heat treatments)

Presence of biological organisms Molds, Fungi & other living organism grow on dampsurfaces


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Types/Forms of Corrosion

Uniform Etch Corrosion Direct chemical Attackon the metal surface

Involves ONLY the metal surface

Initially a dull surface later become rough

Discolouration due to temperature is not a UE

This type of corrosion appears to be uniform

because of the microscopic anodes andcathodes are constantly shiftingfrom onearea to another


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Types/Forms of CorrosionContd

Pitting Corrosion Most common in Al & Mg Alloys

First noticeable as a white or gray powderdeposits

Once removed tiny pits or holescan be seen

This localized corrosion is rapid and severe

because its molecular construction iscomprised of large passive cathodes and smallactive anodes


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Galvanic Corrosion Occurs when two dissimilar metals contact with

electrolyte

Recognizable by the presence of build up of corrosion

at thejoint between metals(mainly anode)- verycommon around fasteners

Surface area of the anode is smaller than the cathodeRapid & Severe

Greater the difference in activity table, the faster thecorrosion occurs

Aluminum & GoldfasterAluminum & Cadmiumslower


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Concentration cell/ Crevice Corrosion Most commonly found in faying (overlapping) areas

The electrolyte seeps into crevicesdue to capillaryaction

It becomes stagnated and create potential differenceof Oxygen and iron

It has Three categories Metal Iron Concentration cellcorrosion just outsidethe

crevice, usually occurs in similar meals

Oxygen Concentration Cellcorrosion occurs within thecrevice, usually formed in metal to non metal couples

Active Passive Celloccurs on metals with oxide films(passive films). Deposit of dirt of contaminants willdeteriorate the oxide film


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Filiform corrosion It is a special form of oxygen concentration cell corrosion

occurs on metal surfaceshaving an organic coatingsystem(paints)

corrosion actually takes place between the metal surface and the

coating worm like traces

occurs on aluminum, magnesium, and steel and may lead tomore serious corrosion

Occurs due to paint deteriorationdue to the Sun or aging

more prominent when the relative humidity is between 78-90%and the surface is slightly acidic

It starts at breaks in the coating system, such as scratches, andcracks around fasteners and seams


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IntergranularCorrosion Occurs AROUND the grains

High strength Al Alloys (2014,7075) are more susceptible

Grain boundary & Grain centercan react with each other asAnode & Cathode

MOST Dangerous types ofcorrosion

Unnoticed during Visual

inspection Detected ONLY through NDT

7075 T-6 Aluminum Alloy


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Forms of Corrosion

Exfoliation Corrosion-Advance form of intergranular corrosion

Surface grains of metal are lifted by the forcesof expanding corrosion products just bellowthe surface

Most prone in wrought products asExtrusions, Thick sheets, thin plates

Occurs ALONG the grain boundaries


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Corrosion and Mechanical Factors

Two major Mechanical factors which aggravate corrosion Residual stresses (with in the part)

Cyclic loads applied (Outside the part)

Three types Stress Corrosion cracksintergranular cracks

Corrosion fatiguecombine effect of cyclic loadsandcorrosion

Fretting Corrosion cracksoccurs at two highlyloaded surfaces

Erosion by sand or rain remove surface protective filmsand leads to corrosion


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Stress Corrosion & FatigueCorrosion Cracks identification

O O O O O

O O O O O

O O O O OFatigue CracksStress Cracks

along the grainboundaries


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Common Corrosive Agents

AcidsAlkaline

Salt SubstancesAtmosphere Water


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Acids Most destructive is Sulfuric (Battery Acid)

Halogen Acids (Hydrofluoric, Hydrochloric) Nitrous Oxide compounds

Organic Acids (Urine)


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Alkaline Not corrosive as Acids

Al & Mg Alloys are prone to corrosive attack Washing soda, Potash (wood ashes), Lime(cement dust)

Salts

Good electrolyte Some stainless steels are resistance to attack

Mg & Al alloys are highly Vulnerable to attack


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Atmosphere Oxygen

High humidity Marine Precipitation

Acid Rains

Water Contaminated with industrial wastes

Salt in natural water


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Inspection Requirements

Should be a part of a routine maintenance job(i.e. daily, pre flight)

Inspection is a continuous need

Arrange inspection in such a way to minimizedown time Always Refer manufactures recommendations If trouble areas observed-pay more attention

Concern of the area of operation (salty, dry etc) In addition to routine inspections introduce

special inspection frequencies


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Inspection RequirementsContd

If aircraft operate in sever conditionsmore frequent inspections (Eg: 15 days)

Aircraft should be washed prior to ANYinspection Instruct technicians with specific

objectives and areas to look for corrosion

If utilization is low, corrosion inspectionshould be based on calendar (not hrs)


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Corrosion Inspection

Primary approach Should be regular & Schedule basis

All inspection should starts with thoroughcleaning

Visual inspection

Detail inspection Done through NDT


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General Safety Procedure

Handling solvents, special cleaners, surface activationmaterials (alodine ) Avoid prolong breathing of vapours (use respirators, adequate

ventilation)

Never add water to acid, always add acid to water

Mixing chemicals as per manufacturers advice

Make access to emergency water source

Avoid prolong usage or contact of solvent

Wash immediately if solvents are body contacted

Do not eat or keep food in close areas (will absorb)

Wash hands before eating or smoke

Ensure proper disposal of waste material

aircraft corrosion for off & tech july 2010

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