Cleaning & Corrosion Control

Author: Herbert AlpigerTHIS INTELLECTUAL WORK IS THE PROPERTY OF THE AUTHOR. In reference to KCTCS AP & P 3.3.5 .This work may be distributed for educational purposes, provided that credit is given to the author (by this page). All uses must be approved on a yearly basis by the author in writing. No changes can be made to this material without written consent of the author.

ATE 102Aircraft Cleaning and Corrosion

and Control

Part IICorrosion Control

Corrosion

• Corrosion – the deterioration of the metal by chemical or electrochemical attack.

CorrosionIn order for corrosion to form, three conditions

must exist;1. There must be an electrical potential

difference. (a voltage difference) One metal acts as the anode & is more likely to corrode. A second metal must serve as the cathode which will be less likely to corrode.

2. There must be a conductive path between the areas of potential difference. Typically metal to metal contact or connection made by a conductive fastener.

3. There must be some sort of electrolyte, or electrically conductive liquid or gas

CorrosionElectrolyte – A chemical, either a liquid or

a gas, which conducts electrical current by releasing ions that unite w/ ions on the electrodes. An ion can be electrically positive or negative, depending upon the ratio of protons to electrons.

Water or water vapor containing salt combines with oxygen in the atmosphere to produce the main source of

corrosion in aircraft.

If left unchecked, corrosion can cause eventual structural failure.

CorrosionCorrosion, while impossible to prevent in

most real world applications, can be controlled by controlling the conditions which cause corrosion.

CorrosionControlling corrosion:• Reduce the electrical potential

difference between the metals.• Eliminate the conductive path between

areas of electrical potential difference.• Protect the surface of the metal by

applying a coating that shields it from contact w/ the electrolyte.

Types of Corrosion There are two general types of corrosion:1. Direct chemical attack2. Electrochemical attack

CorrosionDirect chemical attack, or pure chemical corrosion, is

an attack resulting from a direct exposure of a bare metal surface to caustic liquid or gaseous agents.

CorrosionThe most common agents causing direct chemical

attack on aircraft are;• Spilled battery acid or fumes from batteries• Residual flux deposits resulting from inadequately

cleaned, welded, brazed, or soldered joints• Entrapped caustic cleaning solutions

Corrosion

The electrical chemical attack is responsible for most forms of corrosion on aircraft.

An electrical chemical attack is similar to the electrolytic reaction which takes place in a dry cell battery. When electrons leave the zinc sides of a dry cell battery, chloride ions from the electrolyte replace them and change some of the zinc into zinc chloride, a salt of corrosion.

CorrosionAlloyed metals contain within themselves the

different metals that serve as the anode & cathode necessary for an electrical chemical attack to begin. Pure aluminum will does not corrode easily however in its pure state it is not strong enough to serve as structural components of an aircraft. To increase its strength it is commonly alloyed w/ copper.

Corrosion

Aluminum is more negative than copper and serves as the anode in the electrochemical reaction. When an electrolyte such as water contaminated w/ acids, salts, or other pollutants completes the circuit between the two alloying elements, electrical chemical corrosion occurs.

Forms of CorrosionOxidation – a.k.a. ‘dry corrosion’ or ‘rust’.

Occurs when oxygen from the air combines w/ molecules of metal to form an oxide ie; aluminum oxide or iron oxide. Aluminum oxide appears as a dull white powdery deposit on the surface of the metal. Unlike iron oxide, aluminum oxide will seal the aluminum surface from further contact w/ the elements to prevent further corrosion.

Forms of Corrosion

Surface corrosion – Occurs anytime unprotected metal is exposed to an atmosphere containing industrial contaminants, exhaust fumes, or battery fumes. Can result in pitting that in extreme cases can eat completely through the metal.

Forms of CorrosionIntergranular corrosion – aluminum alloys are made of extremely

fine grains of aluminum & its alloying elements. When heated to a high temperature these alloying elements form a solid w/ the aluminum. Immediately after heating the alloy is quenched in cold water to lock all of the elements into tiny grains. If quenching is delayed the grains will grow to a size where they create anodic & cathodic areas where corrosion can form.

Forms of CorrosionIntergranular corrosion (contd.)Spot welding & seam welding can also cause the grain

structure inside the metal to grow in the same manner. Intergranular corrosion is difficult to detect because it

forms beneath the surface, sometimes appearing as a blister on the surface which, when opened, reveals a cavity filled w/ white powder.

Forms of CorrosionIntergranular corrosion (contd.)Ultrasonic & x-ray inspection are the two ways to

inspect for intergranular corrosion.Once intergranular corrosion is found the part must be

replaced.

Forms of CorrosionExfoliation – an extreme case of intergranular corrosion

in an extruded metal part. It occurs along the grain boundaries & causes the metal to delaminate.

By the time it is observed on the surface the strength of the metal has been destroyed.

Forms of CorrosionStress corrosion – a form of intergranular corrosion

that forms in metals that are subject to continuous tensile stress.

Cracks caused by stress corrosion grow rapidly because the corrosion concentrates at the end of the crack rather than along its sides.

Stress corrosion is a form of intergranular corrosion that progresses along the grain boundaries in a material that is under a constant tensile stress.

Forms of CorrosionGalvanic corrosion (a.k.a. dissimilar metal

corrosion) – occurs anytime two dissimilar metals make contact in the presence of an electrolyte.

Forms of CorrosionGalvanic corrosion(contd.) – the severity of the

corrosion depends upon the difference in activities of the two metals.

Electrochemical series to

determine the chemical activity

of various metals.

Forms of CorrosionConcentration cell corrosion – 1. Low oxygen concentration cell

corrosion2. High metal ion concentration cell

corrosion

Forms of CorrosionLow oxygen concentration cell

corrosion occurs when electrons leave areas where oxygen molecules do not have ready access such as between lap joints, under labeling tape & decals, & under ferrules on aluminum tubing. As the electrons leave in the presence of water they leave behind positive aluminum ions which become the anode in a corrosion cell.

Low oxygen concentration cell corrosion forms in areas where there is not enough oxygen for the formation of hydroxide ions. Electrons migrate from this area and leave it anodic. The positive aluminum ions attract negative hydroxide ions from the water and form corrosion.

Forms of CorrosionHigh metal ion concentration cell corrosion results

from a high concentration of metal ions in the electrolyte covering exposed metal. Water on the surface removes electrons from the metal by forming hydroxide ions leaving behind positive aluminum ions which are continually washed away by the water except in those areas covered by a lap joint, etc. where they concentrate & form a cathodic area.

Forms of CorrosionFretting corrosion – forms between

close fitting parts that have a slight relative movement between them.

The protective oxide films that form between the two surfaces are continually eroded away by the movement between the two close fitting parts which keeps the bare metal exposed to the corrosive elements.

Forms of CorrosionFretting corrosion (contd.) – Fretting

corrosion is often observed as dark streaks streaming out behind rivet heads referred to as rivet smoking.

By the time fretting corrosion is observed the part must be replaced.

Forms of CorrosionFilliform corrosion – occurs beneath paint films, etc.

when an improperly cured wash primer has left some acid on the surface beneath the paint.

Filliform corrosion does not require light, electrochemical differences within the metal, or bacteria, only high humidity in the presence of an acid.

Factors Affecting Corrosion Many factors affect the type, speed, cause, and seriousness of metal corrosion. Climate: moisture-laden air is considerably more

detrimental to an aircraft than it would be if all operations were conducted in a dry climate.

Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate.

Corrosion Preventive Maintenance 1. Adequate cleaning

2. Thorough periodic lubrication

3. Detailed inspection for corrosion and failure of protective systems

4. Prompt treatment of corrosion and touchup of damaged paint areas

5. Keeping drain holes free of obstructions

6. Daily draining of fuel cell sumps

7. Daily wipe down of exposed critical areas

8. Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days

9. Maximum use of protective covers on parked aircraft

Sea PlanesAmphibians or seaplanes should be

checked daily and critical areas cleaned or treated, as necessary.

Corrosion Prone Areas

1. Exhaust Trail Areas 2. Battery Compartments and Battery Vent Openings3. Bilge Areas …located under galleys and lavatories and

human waste disposal openings on the aircraft exteriors.4. Wheel Well and Landing Gear – subject to mud, water, salt,

gravel, and other flying debris.5. Water Entrapment Drains

6. Wing Flap and Spoiler Recesses

Corrosion Prone Areas7. Piano-type hinges – typically found on flaps, ailerons,

elevators and trim tabs.8. Welded areas of aircraft skins

Corrosion Prone Areas9. Control Cables - should be inspected to determine their

condition at each inspection period. Inspect cables for corrosion by random cleaning of short sections with solvent soaked cloths.If external corrosion is evident, relieve tension and check the cable for internal corrosion. Replace cables that have internal corrosion.Remove light external corrosion with a nonwoven abrasive pad lightly soaked in oil.Recoat the cable with preservative.

Steps in Corrosion Removal & Treatment1. Clean and strip the corroded area2. Remove as much of the corrosion products as

practicable3. Neutralize any residual materials remaining in pits and

crevices4. Restore protective surface films5. Apply temporary or permanent coatings or paint

finishes.

Corrosion of Ferrous Metals

Unlike aluminum, the oxidation (rusting) of ferrous metals does not protect the surface beneath the layer of rust from further oxidation. Instead its presence actually promotes additional corrosive attack by attracting moisture from the air and acting as a catalyst for additional corrosion.

Corrosion of Ferrous Metals

For complete control of the corrosive attack is to be realized, all rust must be removed from steel surfaces.

Corrosion of Ferrous MetalsThe most practicable means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means and restoring corrosion preventive coatings. Except on highly stressed steel surfaces, the use of abrasive papers and compounds, small power buffers and buffing compounds, hand wire brushing, or steel wool are all acceptable cleanup procedures. It is practically impossible to remove all corrosion products by abrasive or polishing methods alone. As a result, once a part cleaned in such a manner has rusted, it usually corrodes again more easily than it did the first time.

Corrosion of Ferrous MetalsRemoval of Corrosion from Highly Stressed Steel Parts Any corrosion on the surface of a highly stressed steel part is potentially dangerous, and the careful removal of corrosion products is required. Surface scratches or change in surface structure from overheating can also cause sudden failure of these parts. Use mild abrasive papers such as rouge or fine grit aluminum oxide, or fine buffing compounds on cloth buffing wheels. Nonwoven abrasive pads can also be used. It is essential that steel surfaces not be overheated during buffing.

Corrosion of Aluminum and Aluminum Alloys Corrosion products (oxidation) of aluminum and its alloys can be recognized by their white powdery appearance and their increased volume as compared to the un-oxidized metal.Unlike ferrous metals, an oxidized layer on the aluminum surface can help seal and protect the metal beneath.

Corrosion of Aluminum and Aluminum Alloys Pure aluminum lacks the strength to be utilized in structural components of an aircraft and is therefore alloyed with other metals to increase its strength and improve other desirable properties. However because aluminum alloys are less corrosion resistant than pure aluminum a thin sheet of relatively pure aluminum is applied on either side of a sheet of aluminum alloy to improve its corrosion resistance. This process is commonly referred to by the trade name Alclad.

Corrosion of Aluminum and Aluminum Alloys To prepare aluminum surfaces for painting:

1.Thoroughly clean the affected surfaces of all soil and grease residues

2.If residual paint films remain, strip the area to be treated.3.Treat superficially corroded areas with a 10 percent solution of

chromic acid and sulfuric acid.4.Dry the treated surface and restore recommended permanent

protective coatings

Corrosion RemovalSandblasting (aka bead blasting) is a process

by which corrosion products can be removed from a surface. The basic system utilizes air pressure combined with an abrasive compound to accomplish this. The abrasive compound is typically sand or other abrasive material such as aluminum oxide or glass beads. Air pressure forces the abrasive through a hardened nozzle in a jet stream which allows it to remove contaminants from the surface of the target

Anodizing is the most common surface treatmentof nonclad aluminum alloy surfaces.

Anodizing

The aluminum alloy sheet or casting is placed in anelectrolytic bath in which chromic acid or otheroxidizing agent produces an aluminum oxide filmon the metal surface.

This oxide film helps protect the aluminum alloy surface from further corrosion.

AnodizingThe anodized coating provides excellent resistance to corrosion. However the coating is soft and easily scratched, making it necessary to use extreme caution when handling it prior to coating it with primer.When this coating is damaged in service, it can only be partially restored by chemical surface treatment. Therefore, any corrosion correction of anodized surfaces should avoid destruction of the oxide film in the unaffected area. Do not use steel wool or steel wire brushes. Do not use severe abrasive materials. Nonwoven abrasive pads have generally replaced aluminum wool, aluminum wire brushes, or fiber bristle brushes as the tools used for cleaning corroded anodized surfaces.

AlodizingAlodizing is a chemical treatment for allaluminum alloys to increase their corrosionresistance and to improve their paint-bondingqualities. Because of its simplicity, it is rapidly replacing anodizing in aircraft work.

The alodizing process consists of • Precleaning with an acidic or alkaline metal cleaner. • Rinsing with fresh water• Applying the alodine by dipping, spraying, or brushing. • A thin, hard protective coating results

Water Break Test - The process of pouring water onto a surface to determine whether oil remains on the surface. When water is poured onto an oily surface, the water forms beads.

Magnesium and Titanium

Magnesium and Titanium are two metals commonly used in aircraft construction. The methods of detecting, treating and protecting them from corrosive attack can be found in the recommend resource materials for this course.

Corrosion DamageCorrosion damage is classified in four varying levels:1. negligible damage - is corrosion that has scarred or eaten away

the surface protective coats and begun to etch the metal. The corroded surface should be cleaned, treated, and painted as appropriate.

2. damage repairable by patching - should be repaired in accordance with the applicable structural repair manual.

3. damage repairable by insertion – same as #24. damage necessitating replacement of parts - when corrosion

damage exceeds the damage limits to the extent that repair is not possible

MercuryMercury is a highly corrosive metal that is in a liquid state at room temperatures. Because of its liquid state it will flow through any available crack or crevice to the lowest point in the aircraft.Not only is it highly corrosive, but it is also hazardous to humans so care must be taken to reduce exposure by either physical contact or by breathing mercury vapors.

MercuryMercury contaminates should be removed w/a vacuum tube w/a glass trap installed in the vacuum line or with a rubber suction bulb or medicine dropper.Never attempt to dislodge or move mercury with compressed air. It may disperse the mercury making removal more difficult & more hazardous.