SRM Corrosion

STRUCTURAL REPAIR MANUAL

Page 1 51-22-00

Printed in GermanyFeb 01/02

CORROSION PREVENTION

1. General

A. Corrosion is the destruction of metals by chemical or electrochemical

effects. These effects change the metal into different chemical comÈ

pounds, and thus the strength of the aircraft structure may be effected

by corrosion.

B. Corrosion prevention is one of the most important tasks to keep the airÈ

craft in a safe and serviceable condition. The steps which follow are

those which are necessary for a satisfactory control of corrosion:

- regular maintenance,

- clean structure,

- the initial identification of corrosion,

- the complete removal of corrosion when it occurs,

- to regularly examine the applied corrosion protection on the structure

and to rectify any damage immediately.

C. Corrosion resistant materials and protective treatments are used in the

construction. This is done to reduce the possibility of corrosion occurÈ

ing. Some areas, for example under the toilets and galleys, are given

special anticorrosion treatments.

D. This topic is divided into:

- Causes of Corrosion,

- Types of Corrosion,

- Locations of Corrosion,

- Inspection for Corrosion,

- Removal of Corrosion,

- Types and Functions of Corrosion Protection Procedures.

2. Causes of Corrosion

A. Corrosion is an electrochemical reaction of a metal with its environment.

The necessary condition for this reaction is an electrical potential

difference and an electrolyte. Corrosive agents such as:

- Acids,

- Alkalies,

- Salts,

- The atmosphere,

- Water,

- Microorganisms, give the best conditions for corrosion to start when

they come in contact with the metal surface.

B. Corrosive Agents

(1) Acids

Most acids will cause corrosion on most of the alloys which are used

in the construction of an aircraft. But in the list which follows

you are given those acids which can quickly cause corrosion:


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- sulphuric acid (battery acid),

- halogen acids (hydrochloric, hydrofluoric and hydrobromic),

- organic acids such as human and animal waste.

(2) Alkalies

Alkalies do not usually cause corrosion as much as acids. But alumiÈ

num is very sensitive to alkaline solutions which do not contain a

corrosion inhibitor. The list which follows gives you some of the

alkaline solutions to which aluminum alloys are specially sensitive:

- wash soda,

- potash,

- lime.

Not given is alkali-ammonia, to which aluminum alloys are very reÈ

sistant.

(3) Salts

Most salt solutions are good electrolytes and can cause corrosion. On

some stainless steel alloys corrosion does not occur by a salt soluÈ

tion. Aluminum alloys and steels are very sensitive to some solutions

which contain salt.

(4) The Atmosphere

The general atmosphere contains moisture and oxygen, which are the

primary causes of corrosion. Corrosion of ferrous alloys will occur

easily in the general atmosphere, if they have no protection. There

are other gases and contaminants in the atmosphere which also cause

corrosion, for example the atmosphere in industrial and marine areas

can give you special problems.

(a) Industrial Atmosphere

The atmosphere in industrial areas will frequently contain oxiÈ

dized sulphur and nitrogen compounds. When these compounds mix

with moisture they make an acid which easily causes corrosion.

(b) Marine Atmosphere

The atmosphere in marine areas contains predominently sodium

chloride, or droplets of salt-saturated water. Corrosion on aluÈ

minum and magnesium alloys will occur very easily in this type

of atmosphere. This is because a saline moisture is a good elecÈ

trolyte.

(5) Water

Water contains mineral and organic impurities, and dissolved gases,

for example oxygen. The amount of these impurities will alter the

ability of water to cause corrosion. Sea water contains a lot of

salts and will very quickly cause corrosion on an aircraft structure.

Hard water usually contains a lot of alkalines and thus it does not


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easily cause corrosion on most metals. But this sort of water easily

causes corrosion on aluminum and aluminum alloys (Refer to Paragraph

2.B.(2)).

(6) Microorganisms

It is usual to get water in aircraft fuel tanks, this water can

contain iron oxides and mineral salts. This water can support bacteÈ

rial and fungal growth, both of which can increase corrosive attack

by physical, chemical and enzymatic means.

3. Types of Corrosion

Corrosion is an electrochemical effect which changes the metal into oxide,

hydroxide or sulphate compounds. There are two electrochemical procedures:

- the metal oxidizes and changes anodically,

- the corrosive agent reduces and changes cathodically.

Corrosion usually starts on the surface of the material but the initial efÈ

fects cannot always be found visually. The subsequent paragraphs give examÈ

ples of the different types of corrosion, which can occur on the aircraft

structure.

A. Pitting Corrosion (Refer to Figure 1)

Pitting corrosion starts on the surface of a material, and then extends

vertically into the material but can then extend radially from the iniÈ

tiation site. This type of corrosion is dangerous because of the vertiÈ

cal extension which decreases the material strength. You do not always

get a large indication of this defect on the material surface. This type

of corrosion can be the starting point for intergranular corrosion (Refer

to Paragraph 3.C.).

NOTE: Pitting corrosion should not be confused with shot peening, for exÈ

ample, as applied to bottom wing skins in some cases. Shot peening

produces an 'orange peel' effect on the surface, and is not always

of a uniform appearance.


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Typical Pitting Corrosion

Figure 1

B. Filiform Corrosion (Refer to Figure 2)

Filiform corrosion occurs in clad aluminum alloy sheets. The corrosion

effects continue thread-like sideways as filiform corrosion and do not go

into the core. This type of corrosion can also start at rivets and exÈ

tends along the surface of the painted sheet below the paint.

C. Intergranular Corrosion (Refer to Figure 3)

Intergranular corrosion usually goes into the core of the material along

the grain boundaries with little or no indication on the surface. This

type of corrosion can also be seen as a network of corrosion or cracks

on the metal surface. This very dangerous type of corrosion can make the

structure very weak before any visual indication is given. Some high-

strength aluminum alloys are specially sensitive to intergranular corroÈ

sion when the material is stressed.


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Typical Filiform Corrosion

Figure 2

Typical Intergranular Corrosion

Figure 3


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Electrical Potential Serie of Metals

Figure 4

D. Galvanic Corrosion (Refer to Figure 5)

(1) This type of corrosion can occur between two different metals or

some metals and carbon fiber (Refer to Figure 5), when they touch

each other and have moisture between them. The potential difference

of the materials and the dielectric moisture causes the galvanic efÈ

fects. The intensity of these corrosion effects is not always in

proportion to this difference. Galvanic corrosion can be seen as

white or grey powderlike particles on fittings, overlapping and rivÈ

eted joints, and other structural parts.

(2) Figure 4 shows the electrical potential differences of materials in

relation to Aluminum 2024 Alclad. The greater the difference of the

electrical potential between the materials the higher the risk of

galvanic corrosion. If different materials touch each other the most

anodic one will corrode.

E. Stress Corrosion (Refer to Figure 6)

The simultaneous application of a tensile load and a corrosive environÈ

ment can cause stress corrosion. This metal defect is known as stress

corrosion cracking. Externally applied operational or structural loads

usually cause these stresses. Pitting corrosion (Refer to Paragraph 3.A.)

can give a local increase to the effect of stress corrosion, and can

cause any cracks which have occured, to extend.

F. Biological Corrosion (Refer to Figure 7)


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A large quantity of microorganisms such as bacteria, fungi and algae

cause this type of corrosion. These microorganisms can usually occur:

- in areas with contaminated fluids,

- in hot and humid climates.

The microorganisms, or the fungi, together with humidity, increase the

electrochemical effects. This will give a polarisation of the cathode

(Refer to Paragraph 2.) which can cause unlimited corrosion.

G. Fretting Corrosion (Refer to Figure 8)

Fretting corrosion is the result of micromovent between two heavily

loaded surfaces, at least one of which must be metallic. With this type

of corrosion the protective surface of the material is damaged and parÈ

ticles of metal are removed from the material surface. These particles

oxidize and increase the abrasive effect between the two surfaces. It is

also possible for this type of corrosion to cause fatigue cracking.

H. Exfoliation Corrosion (Refer to Figure 9)

Exfoliation corrosion is a type of the intergranular corrosion (Refer to

Paragraph 3.C.). It occurs when the corrosion at the grain boundaries

below the surface extends and pushes up the metallic grains on the surÈ

face. Exfoliation corrosion frequently occurs on extruded sections beÈ

cause the grain density is usually less than in rolled forms.

Typical Galvanic Corrosion

Figure 5


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Typical Stress Corrosion

Figure 6

Typical Biological Corrosion

Figure 7


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Typical Fretting Corrosion

Figure 8

Typical Exfoliation Corrosion

Figure 9


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4. Locations for Corrosion

A. The conditions, which follow, have an influence on where and when corroÈ

sion can occur:

- density of corrosive agents,

- properties of the corrosion protections,

- corrosion resistance of the material.

These conditions are not the same at every location of the aircraft

structure. There are areas where corrosion can be expected earlier than

in others.

B. Chapter 51-21-00, Figures 1 and 2, show you the location of the strucÈ

ture where corrosion may occur sooner than otherwise expected e.g. in

and below galleys, toilets or battery stowage compartments. These areas

are indicated as Category C.

5. Inspection for Corrosion

A. The early identification and removal of corrosion will help to maintain

the serviceability, safety and function of the aircraft. This is only

possible if the inspection for corrosion is done regularly and with a

maximum of precision.

B. Corrosion can be found, for example, by the following methods:

- visual inspection,

- dye penetrant inspection,

- ultrasonic inspection,

- eddy current inspection,

- X-ray inspection.

The applicable procedures are given in the Nondestructive Testing Manual

NTM.

C. The list which follows gives you some of the equipment you can use to

help you to do a visual inspection for corrosion:

- magnifying glass,

- mirrors,

- borescope, fiber optics,

- other equivalent equipment.

D. When you do the inspection on a surface which is painted, you will usuÈ

ally see corrosion as:

- a scaly or blistered surface,

- a change of the color,

- blisters in the paint.

When you have corrosion on a metallic surface you will usually see a

dulled or darkened area and a pitted surface. Usually you will also see

white, grey or red dust or particles.


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Printed in GermanyNov 01/02

6. Removal of Corrosion

A. All corrosion which you find must be immediately and fully removed. This

is necessary because corrosion which remains will cause new corrosion and

further decrease the strength of the structure.

The applicable procedures for the verification of corrosion removal are

given in the Non-destructive Testing Manual (NTM) Chapter 51-10-02 Page

Block 601 or 901 and Chapter 51-10-04 Page Block 401 or 901.

B. The applicable procedures and the necessary equipment are given in ChapÈ

ter 51-74-00.

C. After the corrosion is fully removed the extent of the damage must be

examined and compared with the allowable damage limits. Refer to Chapter

51-11-00.

D. Make sure that the repair area is given the correct surface protection

to prevent further corrosion. Refer to Chapter 51-21-00.

7. Types and Functions of Corrosion Protection Procedures

A. General

(1) The maximum possible resistance to corrosion is given to the airÈ

craft, before it is delivered. The good corrosion resistance of the

aircraft structure is the result of the interaction of different

types of corrosion protections. Paragraph 7.B. gives you information

about the different types of protection and is divided into:

- Pretreatments,

- Paint Coatings,

- Special Coatings,

- Sealants.

(2) The subsequent paragraphs give information about:

Paragraph 7.C. - Fastener Installation, Paragraph 7.D. - Special

Conditions of Mating Surfaces, Paragraph 7.E. - Airframe Drainage,

Paragraph 7.F. - Preventive Maintenance.

B. Types of Corrosion Protection

(1) Pretreatments

(a) The pretreatment is the initial treatment of the metal and has

the subsequent functions:

1 To increase the corrosion resistant properties of the metal by

chemical or electrolytical procedures.

2 To give a good surface for the adhesion of the subsequent

paint coatings.

3 One procedure that is used to prevent corrosion is to apply a

thin layer of different metal. This layer has a lower electroÈ


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lytical or electrochemical potential than the main metal. If

corrosion occurs it will remove the thin layer first. This is

referred to as sacrificial corrosion prevention.

(b) Table 1 gives you the pretreatments which are usually used to

give the maximum resistance to corrosion. For details of each

treatment and its use refer to Chapter 51-21-11.

MATERIAL PRETREATMENT REMARKS

ALUMINUM

ALLOYS

CHROMIC OR SULPHUÈ

RIC

ANODIZING

ELECTROLYTICAL TREATMENT, THE SURFACE

GETS AN OXIDE COATING

CHEMICAL CONVERSION

COATING

CHEMICAL TREATMENT, SAME FUNCTION AS

ANODIZING

WASHPRIMER USUALLY USED IN FIELD REPAIRS

STEEL

ALLOYS

CADMIUM PLATING ELECTROLYTIC APPLICATION OF CADMIUM

SACRIFICIAL PROTECTION

PHOSPHATIZATION CHEMICAL TREATMENT, APPLICATION OF ZINC

OR MANGANESE PHOSPHATES


HARD CHROMIUM OR

NICKEL PLATING

ELECTROLYTICAL TREATMENT, PREVENT A

CONTACT OF MOISTURE AND OXYGEN WITH

THE

STEEL ALLOY, HIGHLY RESISTANT TO WEAR,

LOW COEFFICIENT OF FRICTION

SILVER PLATING ELECTROLYTICAL TREATMENT, GOOD

RESISTANCE AGAINST FRETTING CORROSION

UNDER HOT CONDITIONS

Pretreatments

Table 1


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MATERIAL REMARKSPRETREATMENT

CORROSION

RESISTANT

STEEL

CADMIUM PLATING USED WHEN IN CONTACT WITH ALUMINUM

ALLOYS, DECREASES GALVANIC EFFECTS,


ZINC SPRAYING THIN LAYER OF PURE ZINC; USED WHEN IN

CONTACT WITH ALUMINUM ALLOY, DECREASES

GALVANIC EFFECTS, SACRIFICIAL PROTECÈ

TION

TITANIUM ANODIZING ELECTROLYTICAL TREATMENT, DECREASES

GALVANIC EFFECTS

Pretreatments

Table 1

(2) Paint Coatings

(a) Paint coatings can be divided as follows and have the subsequent

functions:

1 Primer: The primer increases the corrosion resistant properties

because it contains corrosion inhibitors. The primer also proÈ

tects the surface against corrosive agents and gives a good

surface for the adhesion of the subseqent paint coatings.

2 Top Coat: The top coat or finish paint has the function to

protect the layers of the primer and gives the aircraft the

necessary appearance.

(b) The subsequent chapters give you information about paint coatings

and their use:

CHAPTER CONTENTS

Chapter 51-23-00 PAINT COATING GENERAL

Chapter 51-23-11 PAINT BUILDUP AND AREAS, INFORMATION ABOUT THE APPLICABLE

PAINT COATINGS

Chapter 51-75-00 PAINT COATING REPAIR, GENERAL

Chapter 51-75-12 REPAIR OF PAINT COATINGS

Cross References

Table 2

(3) Special Coatings

(a) Special coatings are applied to those areas which require a speÈ

cial corrosion protection. Two types of special coatings are used

as follows:


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Type 1 - Water repellent coating: Generally made from silicone

free materials organically bound with a mineral oil base to repel

moisture.

Type 2 - Heavy duty corrosion preventive compound: Grease-like

coatings containing corrosion inhibitors which protect against

corrosive agents.

NOTE: Each type is available in various grades. These can be eiÈ

ther soft film forming or hard film forming.

(b) For the information on these coatings and their application areas

refer to Chapter 51-23-11 and Chapter 51-23-12.

(4) Sealants

(a) Sealants have many functions on the aircraft. Those which are

used for corrosion prevention have the subsquent functions:

1 Sealing the external joints of the aircraft structure to make

sure that water does not go into the structure.

2 Sealing the riveted, bolted or bonded joints to make sure that

liquids do not get into the joints.

3 To prevent corrosion (galvanic action) between different metals.

4 To prevent fatigue, stress or vibration between parts of the

structure which can cause fretting corrosion.

5 To level the drain paths to the drain holes (Refer to ParaÈ

graph 7.E.).

(b) In specified areas of the aircraft, for example the lower shell,

a protective layer is put on the sealant. This layer makes sure

that other materials (for example, fuel, hydraulic oil, engine

oil and waste fluids from the toilets and galleys) do not cause

a deterioration of the sealant. Refer to Chapter 51-23-12 for

information about this protective layer.

(c) For information about sealants and sealant repairs refer to ChapÈ

ter 51-76-00 and Chapter 51-76-11.

C. Protective Treatment of Fasteners during Installation

(1) If different metals touch each other, galvanic corrosion occurs. The

fasteners used are not always made of the same metal as the strucÈ

tural parts because of the necessary strength. This fact makes a

protection against galvanic corrosion necessary.

(2) Fasteners are usually pretreated by the manufacturer. Make sure that

the fasteners are protected and lubricated before the installation as

given in Chapter 51-42-00.


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(3) Where galvanic corrosion has to be prevented, the fasteners are

installed wet with sealant.

(4) Information about applicable sealants, their use and the application

procedure during the installation is given in Chapter 51-24-00 and

Chapter 51-76-11.

D. Mating Surfaces

(1) Mating surfaces are very sensitive to corrosion because moisture and

contamination can go between the surfaces and cause corrosion. If the

attached parts are made of different metals, galvanic corrosion ocÈ

curs with the conditions which are described in Paragraph 3.D..

(2) All static mating surfaces, seams and joints must be sealed with a

sealant which is applied to each surface before the installation. For

the use of the correct sealant and its application refer to Chapter

51-24-00 and Chapter 51-76-11.

(3) Figure 10 shows a typical fuselage-stringer protection in the lower

shell, where you can see the complete protection of mating surfaces.


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Typical Fuselage-Stringer Protection, Lower Shell

Figure 10


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E. Airframe Drainage (Refer to Figures Figure 11 and Figure 12)

(1) During normal flights, liquids can collect in the lower part of the

fuselage shell. These liquids can be present as a result of conÈ

densation or leakage from the aircraft systems. It is very important

that these liquids do not remain in the fuselage shell because they

can cause corrosion.

(2) To make sure that the liquids which collect in the fuselage are

drained from the fuselage, the subseqent procedures are used:

- Drain holes are constructed in those parts of the fuselage which

are not pressurized in flight.

- Special drain valves (Refer to Figure 12) are installed in those

parts of the fuselage which are pressurized in flight.

The drain holes and the drain valves are usually at the lowest part

of the fuselage.

(3) It is important that any unwanted liquids get to the drain holes or

valves. The structure of the lower fuselage is constructed so that

a path is given for these liquids. When you do a repair make sure

that you keep this path, and that unwanted materials do not block

this path. Use the correct sealant where it is necessary to prevent

liquids remaining in the structure.

(4) Figure 11 shows you examples of drain paths in the structure. For

more information about drainage, locations and maintenance refer to

the relevant chapters in the Aircraft Maintenance Manual AMM.

F. Preventive Maintenance

(1) A satisfactory and complete corrosion prevention requires, in addiÈ

tion to the existing corrosion protection, a preventive maintenance.

For example a regular washing and waxing of the outer skin increase

the corrosion resistance, and maintains the given corrosion protecÈ

tion.

(2) For information about preventive maintenance refer to the corrosion

prevention recommendations in the Maintenance Planning Document MPD.


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Fuselage Draining Examples

Figure 11


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Typical Pressure Drain Valve

Figure 12

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SRM Corrosion