aircraft corrosion

1. Cleaning & Corrosion Control
Author: Herbert Alpiger
THIS INTELLECTUAL WORK IS THE PROPERTY OF THE AUTHOR. In reference to KCTCS AP & P 3.3.5 .This work may
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2. ATE 102
Aircraft Cleaning and Corrosion and
Control
Part II
Corrosion Control

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

4. Corrosion
In 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

6. Corrosion
Electrolyte – 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.

7. 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.

8. Corrosion
Corrosion, while impossible to prevent in
most real world applications, can be
controlled by controlling the conditions
which cause corrosion.

9. Corrosion
Controlling 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.

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

11. Corrosion
Direct 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.

12. Corrosion
The 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

13. 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.

14. Corrosion
Alloyed 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.

15. 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.

16. Forms of Corrosion
Oxidation – 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.

17. 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.

18. Forms of Corrosion
Intergranular 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.

19. Forms of Corrosion
Intergranular 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.

20. Forms of Corrosion
Intergranular 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.

21. Forms of Corrosion
Exfoliation – 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.

22. Forms of Corrosion
Stress 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.

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

24. Forms of Corrosion
Galvanic corrosion (a.k.a. dissimilar metal corrosion)
– occurs anytime two dissimilar metals make contact in
the presence of an electrolyte.

26. Forms of Corrosion
Galvanic corrosion(contd.) – the severity of the
corrosion depends upon the difference in activities of
the two metals.

27. Electrochemical
series to
determine the
chemical activity
of various
metals.

28. Forms of Corrosion
Concentration cell corrosion –
1. Low oxygen concentration cell
corrosion
2. High metal ion concentration cell
corrosion

29. Forms of Corrosion
Low 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.

30. 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.

31. Forms of Corrosion
High 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.

32. Forms of Corrosion
Fretting 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.

33. Forms of Corrosion
Fretting 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.

34. Forms of Corrosion
Filliform 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.

35. 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.

36. 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

37. Sea Planes
Amphibians or seaplanes should be
checked daily and critical areas cleaned or
treated, as necessary.

38. Corrosion Prone Areas
1. Exhaust Trail Areas
2. Battery Compartments and Battery Vent Openings
3. 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

39. Corrosion Prone Areas
7. Piano-type hinges – typically found on flaps, ailerons,
elevators and trim tabs.
8. Welded areas of aircraft skins

40. Corrosion Prone Areas
9. 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.

41. Steps in Corrosion Removal & Treatment
1. Clean and strip the corroded area
2. Remove as much of the corrosion products as
practicable
3. Neutralize any residual materials remaining in pits and
crevices
4. Restore protective surface films
5. Apply temporary or permanent coatings or paint
finishes.

42. 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.

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

44. Corrosion of Ferrous Metals
The 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.

45. Corrosion of Ferrous Metals
Removal 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.

46. 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.

47. 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.

48. 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

49. Corrosion Removal
Sandblasting (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

50. Anodizing is the most common surface treatment
of nonclad aluminum alloy surfaces.
Anodizing
The aluminum alloy sheet or casting is placed in an
electrolytic bath in which chromic acid or other
oxidizing agent produces an aluminum oxide film
on the metal surface.
This oxide film helps protect the aluminum alloy
surface from further corrosion.

51. Anodizing
The 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.

52. Alodizing
Alodizing is a chemical treatment for all
aluminum alloys to increase their corrosion
resistance and to improve their paint-bonding
qualities.
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

54. 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.

55. 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.

56. Corrosion Damage
Corrosion 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 #2
4. damage necessitating replacement of parts - when corrosion
damage exceeds the damage limits to the extent that repair is not
possible

57. Mercury
Mercury 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.

58. Mercury
Mercury 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.