Corrosion Control and Prevention
Course Overview
• Why
• Frequency
• No Pressure Washer
• Pressure Washer
• Cleaning Materials
• CPC’s
• The Cost of Corrosion
• Types of Corrosion
• Factors of corrosion
• Stopping & Preventing
Fundamentals1Aircraft Wash Procedures2
Corrosion Control and Prevention
Course Overview
• Inspection Frequency
• Inspection Procedures
• Inspection Methods
• Corrosion Prone Areas
• Preventive Maintenance
• Fastener Sealing
• Sealing Techniques
• Recommended Materials
Preventive Maintenance3
Inspection & Repair4
Corrosion Control and Prevention
Course Objectives
Why is prevention important
What are the different types of corrosion
What are some mechanical factors
How to stop & prevent corrosion
Corrosion Control and Prevention
Corrosion Refresher
Why is corrosion
prevention important
Corrosion Basics
Factors influencing corrosion
Types of corrosion
How to prevent
corrosion
Corrosion Control and Prevention
Aviation Corrosion Preventionand Control Policy
CORROSION PREVENTION AND CONTROL PROCEDURES
are in compliance with TM 1-1500-328-23
(Aeronautical Equipment Maintenance Management
Policies and Procedures)
Corrosion Control and Prevention
Aviation Corrosion Preventionand Control Policy
IF THERE IS NO CPC PROGRAM ESTABLISHED,
AIRCRAFT WILL BE SCHEDULED CPC
INSPECTION ON A 90-CALENDAR-DAY
INTERVAL
Corrosion Control and Prevention
Aviation Corrosion Preventionand Control Policy
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Control and Prevention
0
2
4
6
8
10
12
14
16
1995 1996 1997 1998 1999 2000 2001
Direct cost of corrosion damage– Parts Cost
$M
22
3%
-23
%
40
% -4%
8%
-20
%
Corrosion Control and Prevention
0
.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
1995 1996 1997 1998 1999 2000 2001
$M
Indirect cost of corrosion damage– Man-Hours Cost
-8
%
84
%
18
%
2%
15
1%
-2
2%
Corrosion Control and Prevention
0
2
4
6
8
10
12
14
16
18
20
1995 1996 1997 1998 1999 2000 2001
$M
Corrosion Total Cost due to Corrosion
Corrosion Control and Prevention
Total Corrosion Costs
Corrosion Control and Prevention
Total Corrosion Costs
Corrosion Control and Prevention
Aloha Airlines Flight 243 – April 1988
Corrosion Control and Prevention
Aloha Airlines Flight 243 – April 1988
Corrosion Control and Prevention
Aloha Airlines Flight 243 – April 1988
Corrosion Control and Prevention
Aloha Airlines Flight 243 – April 1988
Explosive decompression @ 24,000 feet
Airplane operated in coastal environment with exposure to salt and humidity
Root cause – Epoxy bond failure which allowed water to enter gap and start corrosion process
Corrosion Control and Prevention
Total Corrosion Costs
WITH PROPER MAINTENANCE BY TRAINED CREWS COST AND DAMAGE CAN BE REDUCED
PREVENT
Corrosion Control and Prevention
Why study corrosion?
Corrosion Control and Prevention
Why study corrosion?
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Description
Corrosion Control and Prevention
What is Corrosion?
Weakening of material due to:
Loss of cross-sectional area
Shattering of metal due to hydrogen embrittlement
Cracking of polymer due to heat or sunlight exposure
Corrosion Control and Prevention
What is Corrosion?
Corrosion Control and Prevention
What is Corrosion?
Corrosion Control and Prevention
What is Corrosion?
A battery is an engineered form of controlled corrosion
By releasing electrical energy
Corroding the internal metal
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Description
Result of a material with its
Process are
Occurs because of the tendency of metals to to their natural states
Corrosion Control and Prevention
Corrosion Description
Corrosion Control and Prevention
Basic Chemical Definitions
Basic Elements Involved in the Corrosion Process
Corrosion Control and Prevention
Conditions for Corrosion
A corroding metal
A metal driving corrosion
Electrical contact between the anode and cathode
Water, salt water, battery acid, etc
Corrosion Control and Prevention
Conditions for Corrosion
Electrons flow to cathode
Negative ions from the electrolyte
combine with metal ions from the anode
Ion flow
Electron flow
Corrosion Control and Prevention
Diagram of Corrosion in a Car Battery
A car battery contains three of the four required elements for corrosion:
The Anode, Cathode, and the Electrolyte
Anode + Cathode -
Electrolyte
Corrosion Control and Prevention
Diagram of Corrosion in a Car Battery
Powering the light, or any other appliance, adds the fourth element - the Conductive Path - thus starting
the corrosion process
Conductive Path
Anode + Cathode -
Electrolyte
Corrosion Control and Prevention
Diagram of Corrosion in a Car Battery
PittingPlating
Now, electrons flow from cathode to anode and ions will flow from the anode to the cathode resulting in a
pitting of the anode and a plating of the cathode
Electron flow
Ion Flow
Anode + Cathode -
Electrolyte
Conductive Path
Corrosion Control and Prevention
Diagram of Corrosion in a Car Battery
Electrolyte
Re-plated
Deposits
removed
A car battery is designed to reverse this corrosive action once a reverse voltage is
applied from the car alternator
Alternator
Anode + Cathode -
Conductive Path
Electron flow
Corrosion Control and Prevention
Corrosion Description
Acidity or alkalinity of the medium
Stability of the corrosion product
Biological organisms
Variation in composition of the corrosive medium
High Temperatures
OTHER FACTORS WHICH AFFECT A METAL’S TENDENCY TO CORRODE:
Corrosion Control and Prevention
Corrosion Description
Elimination of any of the four conditions stops corrosion
Corrosion Control and Prevention
Corrosion Description
Elimination of any of the four conditions stops corrosion
Corrosion Control and Prevention
Electrochemical Process
Corrosion Control and Prevention
Elimination
Corrosion Control and Prevention
REVIEW
Q. What are the four requirements for electrochemical corrosion?
A. Anode, cathode, electrolyte, conductive path
Q. Why is a separator commonly used between the anodic and cathodic cells of a battery?
A. Eliminates the conductive path & prevents the corrosion process. It interrupts the electron flow between the anode and cathode.
Corrosion Control and Prevention
Excellent References
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Control and Prevention
Six Common Types Of Corrosion
1 • Uniform Etch / General Surface
2• Pitting Corrosion
3 Galvanic / Dissimilar Metal
4 Crevice Corrosion
5 • Filiform Corrosion
6 Intergranular / Exfoliation
Corrosion Control and Prevention
Seven Common Types Of Corrosion
1 • Uniform Etch / General Surface
2 • Pitting Corrosion
3 Galvanic / Dissimilar Metal
4 Crevice Corrosion
5 • Filiform Corrosion
6 Intergranular / Exfoliation
7 • Concentration Cell
Corrosion Control and Prevention
Types Of Corrosion
Corrosion Control and Prevention
Corrosion Control and Prevention
Uniform Etch Corrosion
Corrosion Control and Prevention
General Surface Corrosion
A bare metal surface is exposed to an electrolyte
Parts of the surface become anodes and others cathodes
Metal ions combine with air to form corrosion products
Corrosion Control and Prevention
General Surface Corrosion
Cathode AreaAnode Area
ElectrolyteConductive Path
Corrosion Control and Prevention
Uniform Etch & General Surface Corrosion
Corrosion Control and Prevention
REVIEW
Q. When would general surface (uniform etch) corrosion be a concern?
A. Thinning of structure to the point of failure.
Q. What is a type of protection for general surface corrosion?
A. Application of protective coating (platting, painting, oxide coating such as cladding)
Corrosion Control and Prevention
Corrosion Control and Prevention
Pitting Corrosion
Corrosion Control and Prevention
Pitting Corrosion
Corrosion Control and Prevention
Pitting Corrosion
Corrosion Control and Prevention
Pitting Corrosion
Corrosion Control and Prevention
Pitting Corrosion
ANODE CATHODE
ELECTROLYTE CONDUCTIVE
PATH
Corrosion Control and Prevention
Pitting Corrosion
Corrosion Control and Prevention
Pitting Corrosion
Corrosion Control and Prevention
REVIEW
Q. Why would pitting corrosion be much more prone to provoke a catastrophic failure than uniform corrosion generally does?
A. Pitting corrosion can form from many different corrosions. Once corrosion pits are started, they can be hidden and difficult to detect. Add stress for one reason or another, sharp pits led to the generation of cracks, which can lead to Stress Corrosion Cracking
Corrosion Control and Prevention
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
Galvanic corrosion occurs when:
– Two different metals are in contact and are in electrolyte
ELECTROLYTE CONDUCTIVE PATH
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
Anode is the metal that gives up electrons easily and corrodes fastest
Cathode accepts electrons and shows less corrosion
ANODE CATHODE
ELECTROLYTE CONDUCTIVE PATH
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
Area of dissimilar metal
contact
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
Area of dissimilar
metal contact
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
Cadmium plated steel
washer
Stainless Steel Screw
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
– Electrochemically dissimilar metals
– These metals must be in electrical contact
– The metals must be exposed to an electrolyte
For Galvanic Corrosion to occur,three conditions must be present:
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
Galvanic SeriesMetals grouped together have little tendency to produce galvanic corrosion
Coupling metals from different groups will result in accelerated corrosion
There are exceptions and the chart is a learning aid
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
GALVANIC CHART
Corrosion Control and Prevention
Galvanic / Dissimilar Metal Corrosion
GALVANIC GROUPING OF METALS
Corrosion Control and Prevention
REVIEW
Q.Where is galvanic corrosion most likely to occur on an aircraft?
A.Anywhere different types of metal come in contact with each other, especially where moisture is present
Q. Is it a good practice to clean corrosion on a magnesium gearbox housing with a stainless steel brush? Why or why not?
A. No. This would introduce dissimilar metals to the component and set-up a very aggressive corrosion situation
Corrosion Control and Prevention
Corrosion Control and Prevention
Crevice Corrosion
CREVICE CORROSION OCCURS WHEN:
- Electrolyte enters gap between metals
Corrosion occurs on one metal
Corrosion is generally not easily seen
ANODECATHODE
ELECTROLYTE
CONDUCTIVE
PATH
Corrosion Control and Prevention
Crevice Corrosion
MOISTURE HAS PENETRATED THE HINGE’S FAYING SURFACES
Corrosion Control and Prevention
Crevice Corrosion
THE CIRCLED RIVET HAS BROKEN THE PAINT COATING AND IS ABOUT TO FAIL
NOTICE THE BULGING OR SEPARATION OF FAYING SURFACES
Corrosion Control and Prevention
Crevice Corrosion
Crevice corrosion combined with dissimilar metal corrosion at this wing attachment point resulted in an
in-flight break-up of a Cessna 310
Corrosion Control and Prevention
Crevice Corrosion
Full blown crevice corrosion in an otherwise very seawater resistant material
Corrosion Control and Prevention
Crevice Corrosion Prevention
CREVICE CORROSION CAN BE REDUCED BY:
Corrosion Control and Prevention
REVIEW
Q.What is a ‘faying surface’?
A.The metal that is covered in a lap joint or a metal-to-metal joint
Q. Where are some lap joints that are susceptible to corrosion on the aircraft you work on?
Corrosion Control and Prevention
Corrosion Control and Prevention
Occurs on metal surfaces which have organic coatings
Caused by the diffusing of oxygen and water through coating
Filiform Corrosion
Corrosion Control and Prevention
Filiform Corrosion
Pigments and resins block surface to dissipate moisture
Moisture beneath paint film allows corrosion to form
Corrosion Control and Prevention
Filiform Corrosion
Corrosion Control and Prevention
Filiform Corrosion
FILIFORM CORROSION OCCURS WHEN:
- There is a break in the coating
- Corrosion grows under the coating
- Coating lifts as corrosion products form
CONDUCTIVE PATH
ANODE
CATHODE
ELECTROLYTE
Corrosion Control and Prevention
Typical Filiform Corrosion
Corrosion Control and Prevention
Typical Filiform Corrosion
Corrosionby-products cause paint or plating to lift off surface and bubble
Bubbled paint or plating
Because of bubbled appearance this can be confused for improper painting
Not coating failure
Corrosion Control and Prevention
Filiform Corrosion Prevention
Store aircraft in low humidity environment (below 65% relative humidity)
Low humidity
Decrease corrosion by using coatings that have a high resistance to diffusion by water
Better quality coatings
Corrosion Control and Prevention
REVIEW
Q. Where is filiform corrosion most likely to occur on an aircraft?
A. Under a dense coating of topcoat enamel such as polyurethane.
Q. What is the likely cause for filiform corrosion?
A. Poor preparation or improperly cured primers.
Q. Where are some areas that are susceptible to filiform corrosion on the aircraft you work on?
Corrosion Control and Prevention
Corrosion Control and Prevention
Intergranular Corrosion
Corrosion Control and Prevention
Intergranular Corrosion
Intergranular or exfoliation corrosion occurs when:
- Electrolyte attacks grain boundaries
- Corrosion products expand
- Layers of metal separate
ANODE
CATHODE
ELECTROLYTE
Corrosion Control and Prevention
Intergranular Corrosion
Corrosion Control and Prevention
Intergranular Corrosion
A highly magnified cross section of any alloy shows the granular structure of the metal
The alloy has a large number of individual grains
Each tiny grain has a clearly defined boundary
Corrosion Control and Prevention
Intergranular Corrosion
The boundaries of the grains, chemically differs from the metal within the grain center
Frequently, the grain boundaries are anodic to the main body of the grain
When in contact with an electrolyte, rapid selective corrosion of the grain boundaries occurs
Corrosion Control and Prevention
Intergranular Corrosion
Corrosion Control and Prevention
Exfoliation Corrosion
Corrosion Control and Prevention
Exfoliation Corrosion
Exfoliation is an extreme form of
intergranular corrosion
Corrosion Control and Prevention
Exfoliation Corrosion
The lifting up, or swelling is visible evidence of
intergranular corrosion and is most often seen on
extruded sections where grain thicknesses are usually
less than in rolled forms
Corrosion Control and Prevention
Intergranular / Exfoliation Corrosion
Q.Where is intergranular corrosion most likely to occur on an aircraft?
A. Along the grain boundaries of aluminum alloys that have been improperly heat-treated. Extruded aluminum alloy is susceptible to intergranular corrosion.
Corrosion Control and Prevention
Corrosion Control and Prevention
Concentration Cell Corrosion
•Metal ion concentration cells1
• Oxygen concentration cells2
• Active / Passive cells3
TYPES OF CONCENTRATION CELL CORROSION
Corrosion Control and Prevention
Concentration Cell Corrosion
Corrosion Control and Prevention
Metal Ion Concentration Cells Corrosion
The area between the sheets contains a high concentration of positive metal ions, and it is
the cathodic area of the corrosion cell
Corrosion Control and Prevention
Metal Ion Concentration Cells Corrosion
The area between the sheets contains a high concentration of positive metal ions, and it is the
cathodic area of the corrosion cell
Corrosion Control and Prevention
Oxygen Concentration Cells Corrosion
The positive aluminum ions attract negative hydroxide ions from the water
and form corrosion
Corrosion Control and Prevention
Oxygen Concentration Cells Corrosion
The positive aluminum ions attract negative hydroxide ions from the water and form corrosion
Corrosion Control and Prevention
Active/Passive Cell Corrosion
Appears on metals that depend on a tightly adhering passive film for corrosion
protection, such as stainless steel
Corrosion Control and Prevention
Active/Passive Cell Corrosion
Starts when the passive film is broken by dirt or other foreign substances
deposited on the surface
Corrosion Control and Prevention
Active/Passive Cell Corrosion
Once the film is broken, the metal beneath exposed to attack and the result is rapid
pitting of the active metal
Corrosion Control and Prevention
REVIEW
Q. What type of concentration cell corrosion forms between a lap joint in aircraft skin?
A. Oxygen Concentration Cell
Q. Where will you most likely see the formation of Metal Ion Concentration Cell corrosion?
A. At the edge of aircraft skin.
Q. Where are some components that are susceptible to Concentration Cell Corrosion on the aircraft you work on?
Corrosion Control and Prevention
REVIEW
Q. What are the six common types of corrosion found on PGAG 18 aircraft?
A. Uniform etch/general surface, pitting, galvanic/dissimilar metal, crevice, filiform & intergranular/exfoliation.
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Control and Prevention
Mechanical Factors to Corrosion
Corrosion Control and Prevention
Stress Corrosion Cracking
Stress can be residual as a result of the production
process
The combined action of a
static tensile stress and corrosion
forms cracks
Stress may be a result of externally
applied cyclic loading
Corrosion Control and Prevention
Stress Corrosion Cracking
Stress corrosion cracking is caused by the
simultaneous effects of constant tensile stress
and corrosion
Corrosion Control and Prevention
Stress Corrosion Cracking
Internal stresses are produced by;
– Non-uniform deformation during cold working
– By unequal cooling from high temperatures
– By internal structural rearrangement involving volume changes
Corrosion Control and Prevention
Stress Corrosion Cracking
Stressed Corrosion Cracking (SCC) is the most subtle forms of corrosion because environmental cracks are microscopic in
their early stages of development
Corrosion Control and Prevention
Stress Corrosion Cracking
Pitting will greatly increase SCC
Crack will follow the grain structure
Crack driven by sustained and intermittent tensile stresses
Corrosion Control and Prevention
Stress Corrosion Cracking Prevention
Prevention;
1) Reduce the overall stress in the initial design
2) Use suitable materials
3) Design the part to minimize residual stresses
4) Use suitable protective coatings
Corrosion Control and Prevention
REVIEW
Q. Where is stress corrosion most likely to occur on an aircraft?
A. In a metal component that is continually under a tensile stress.
Q. Does the application of a part directly effect the susceptibility of Stress Corrosion Cracking?
A. Yes, high stress in corrosive environment will cause cracking sooner than high stress alone.
Corrosion Control and Prevention
Corrosion Control and Prevention
Fretting Corrosion
Caused by motion between two surfaces
Corrosion Control and Prevention
Fretting Corrosion
Rapid corrosion occurs at the point of contact between the highly loaded metal surfaces when subjected to small vibrations
Corrosion Control and Prevention
Fretting Corrosion
This may be an indicator
of motion between two tight
fitting parts
Corrosion Control and Prevention
Fretting Corrosion Prevention
1) Design out vibration
2) Lubricate metal surfaces
3) Increase the load between the surfaces to stop the motion
4) Surface treatments to reduce wear and increase friction coefficient
Corrosion Control and Prevention
Fretting Corrosion
Q. Where is fretting corrosion most likely to occur on an aircraft?
A. In a location where there is a slight amount of relative movement between two components and no way for the corrosive residue to be removed.
Q. What are some visual indications of fretting corrosion?
A. Smoking hinges or rivets, residue seeping from fayed surfaces after cleaning.
Q. Where are some location on the UH-60 or CH-47 where fretting is common?
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Fatigue
• Caused by combined effects of cyclic stress and corrosion
• No metal is immune
• Rate of decay is greatly increased by corrosive environments
• Damage is greater than the sum of the damage from both cyclic stresses and corrosion
Corrosion Control and Prevention
Corrosion Fatigue
Corrosion Control and Prevention
Corrosion Fatigue
Corrosion Control and Prevention
Corrosion Fatigue
The fatigue limit in both stress and cycles of a metal is greatly reduced in a corrosive environment
Corrosion Control and Prevention
Corrosion Fatigue
Fatigue fractures are brittle and cracks will follow metal grain structure
Corrosion Control and Prevention
Corrosion Fatigue – Aloha Airlines Flt 243
Corrosion Control and Prevention
Corrosion Fatigue – Aloha Airlines Flt 243
Suffered an explosive
decompression
Lost a large part of it’s
upper fuselage while climbing through 24,000
feet
One flight attendant was sucked out of
the aircraft and the cockpit
sagged down 3 feet
Corrosion Control and Prevention
Corrosion Fatigue – Aloha Airlines Flt 243
Corrosion Control and Prevention
Corrosion Fatigue – Aloha Airlines Flt 243
The aircraft had 35,496 hours TT and 89,680 cycles
History of cracks and corrosion
This aircraft had 24 fuselage skin repairs before the accident
Operated in a severe corrosion environment
Corrosion Control and Prevention
Corrosion Fatigue – Aloha Airlines Flt 243
The 1300 rivets to be inspected were painted
Night shift, boredom, fatigue and length of the inspection task
Inadequate work stands, lighting
Insufficient training to recognize corrosion fatigue
Corrosion Control and Prevention
REVIEW
Q. What are the two stages in which corrosion fatigue occurs?
A. Material damage stage (due to corrosion and cyclic stress) and material fatigue stage (cracks spread).
True or False; once the corrosive environment is removed, the metal can be considered to meet original strength limits.
False, stage one damage has already happened. Material damage has occurred.
Corrosion Control and Prevention
Corrosion Control and Prevention
Hydrogen Embrittlement
Affects high strength steels
Caused by action due to plating
Hydrogen enters metal during plating process
Corrosion Control and Prevention
Hydrogen Embrittlement
Corrosion Control and Prevention
Hydrogen Embrittlement
Corrosion Control and Prevention
Hydrogen Embrittlement
Electroplating and/or
replating
Corrosion on high strength
fasteners
Use of chemical
cleaners and strippers
MAJOR CAUSES OF HYDROGEN EMBRITTLEMENT:
Corrosion Control and Prevention
Hydrogen Embrittlement
British Columbia – June 2000
Manufacture improperly heat-treated screws on FCU
Once cad-plated, hydrogen was introduced into the steel
Screw head separated from FCU cover
Corrosion Control and Prevention
Hydrogen Embrittlement
10 October 1992, near Fort Richardson, Alaska
Cause was failure of upper boost actuator fastener
Improper cleaning agent used (Simple Green)
Aircraft destroyed. Crew received minor injuries
Corrosion Control and Prevention
Hydrogen Embrittlement
How many pieces of aircraft hardware
are zinc or cadmium plated?
Corrosion Control and Prevention
Hydrogen Embrittlement
Q. What are some factors that cause hydrogen embrittlement?
A. High strength fasteners, plating processes, improper cleaning agents & chemical strippers,
Q. What are some areas/components that are prone to hydrogen embrittlement on Group 18 aircraft?
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Control and Prevention
Factors Influencing Corrosion
Corrosion Control and Prevention
Factors Influencing Corrosion
Corrosion Control and Prevention
Factors Influencing Corrosion
Sea water is an aggressive corrosive
agent
Corrosion Control and Prevention
Factors Influencing Corrosion
Corrosion Control and Prevention
Factors Influencing Corrosion
Electrolyte Concentration is one of the most important
factors influencing the rate of corrosion on Group 18
Aircraft
Corrosion Control and Prevention
Factors Influencing Corrosion
Contaminants like salt, atmospheric pollution, dirt, soda, increases the strength of the water electrolyte
The more concentrated the electrolyte the faster corrosion
occurs
Corrosion Control and Prevention
Distilled Water Sea Water
Corrosion Control and Prevention
Factors Influencing Corrosion
Corrosion Control and Prevention
Factors Influencing Corrosion
Corrosion Control and Prevention
Factors Influencing Corrosion
Small anode toLarge cathode
FAST CORROSION
Large anode toSmall cathode
SLOW CORROSION
Corrosion Control and Prevention
Factors Influencing Corrosion
Steel Rivets (anodic) in Copper Bar (cathodic) at Start of Experiment
Copper Rivets in Steel Bar at Start
of Experiment
Corrosion Control and Prevention
Factors Influencing Corrosion
Steel Rivets in Copper Bar
Copper Rivets in Steel Bar
Corrosion Control and Prevention
Factors Influencing Corrosion
Steel Rivets in Copper Bar
Copper Rivets in Steel Bar
Corrosion Control and Prevention
Determines speed of corrosion:•Most anodic metal corrodes
first• The more dissimilar the metal,
the faster the corrosion
Galvanic Chart:• Shows corrosive potential of
dissimilar metals• Show reactivity of metals
Factors Influencing Corrosion
Corrosion Control and Prevention
Factors Influencing Corrosion
GALVANIC CHART
Corrosion Control and Prevention
REVIEW
Q. What are some main factors that influence the rate of corrosion?
A. Electrolyte concentration, humidity, ambient temperature, anode to cathode ratio, galvanic difference between metals
Q. What would corrode fastest in identical atmospheric conditions: steel rivets in a copper bar or copper rivets in a steel bar?
Corrosion Control and Prevention
Corrosion Control and Prevention
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Electrons flow to cathode
Ion Flow
Electron Flow
Negative ions from the electrolyte combine with
metal ions from the anode
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Controlled by the DesignersThe materials used in the aircraft were selected by engineers
Some of these materials react with other materials
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Do not accelerate corrosion by introducing a galvanic difference
Don’t use unapproved hardware
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Eliminate the conductive path by:
Using primer, sealant, & dry film lube
Wet fastener installation
Sealing faying/fretting surfaces
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Considerations:
Flexing of joints
Electrical bonding
Installation & removal requirements;
• Inspection
• Replacement
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Removal may be accomplished by:
Clearing drain holes
Regular washing of the aircraft
Drying the aircraft
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Primers, sealants, greases and dry film lubricants that contain corrosion inhibitors should always be used
Inhibitor acts like small ‘anode’ embedded in the CPM
Corrosion Preventative Materials (CPM’s)
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
CPM is sacrificial and will “corrode” first
Effect is only temporary and lasts for short period of time
Repaint, reseal, reapply when CPM is damaged or deteriorated
Corrosion Preventative Materials (CPM’s)
Corrosion Control and Prevention
Inhibitors in the Primer provide sacrificial protection to the metal substrate when coating becomes cracked or damaged
Primer
Top Coat
Sacrificial elements
suspended in the coating
Stopping and Preventing Corrosion Basics
Breaks or defects in coating
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Removing any one of the four elements of the battery model, will stop the advancement of corrosion and prevent further
corrosion from starting
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Of the four parts of the battery model, the easiest to remove is
the electrolyte.
It is also the most difficult to keep removed because of
weather
Corrosion Control and Prevention
Stopping and Preventing Corrosion Basics
Understanding what corrosion is and how it works is the first step in combating corrosion
Corrosion Control and Prevention
REVIEW
Q. What are some ways to remove the conductive path on a gearbox installation?
A. Install fasteners ‘wet’, use approved corrosion inhibitors & seal the faying surfaces between casing and mount.
Q. What are some precautions to be aware of when eliminating the conductive path?
A. Flexibility, electrical bonding, frequency of removal/installation.
Corrosion Control and Prevention
REVIEW
Q. How can a technician minimize the galvanic corrosion caused by anodes and cathodes in an aircraft?
A. Use correct hardware during all phases of maintenance and prevent accumulation of debris inside the aircraft.
Q. What are some ways to remove the electrolyte from your current aircraft?
A. Clean out all drain holes then wash & dry the aircraft, that is the best defense.
Corrosion Control and Prevention
Corrosion Refresher
Corrosion Control and Prevention
Corrosion Refresher
Q. Why is it important to understand the different types of aircraft corrosion?
Q. What types of corrosion have been found on your current fleet of aircraft?
Q. What are some of the ways general surface corrosion can be reduced?
Corrosion Control and Prevention
Corrosion Refresher
Q. What type of corrosion forms in riveted joints where there is a slight relative movement?
Q. What is an extreme case of intergranular corrosion that forms in extruded metal?
Q. What is another name for ‘dry corrosion’?
Corrosion Control and Prevention
Corrosion Refresher
Q. Why is it important to understand the different types of aircraft corrosion?
A. To minimize corrosion damage so that maintenance costs are reduced and incidents are eliminated.
Q. What types of corrosion have been found on your current fleet of aircraft?
A. Almost all kinds: Surface, galvanic and pitting are most common.
Corrosion Control and Prevention
Corrosion Refresher
Q. What are some of the ways general surface corrosion can be reduced?
A. Stop movement of electrons, stop oxygen from reaching surface, use sacrificial coating, use sacrificial metal.
Q. What type of corrosion forms in riveted joints where there is a slight relative movement?
A. Fretting.
Corrosion Control and Prevention
Corrosion Refresher
Q. What is an extreme case of intergranular corrosion that forms in extruded metal?
A. Exfoliation, it separates the metal in layers.
Q. What is another name for ‘dry corrosion’?
A. Oxidation or uniform etch corrosion.
Corrosion Control and Prevention
Corrosion Refresher
Q. What type of corrosion is prevented by maintaining intact sealants?
A. Concentration cell corrosion.
Q. What is Stress Corrosion Cracking?
A. A crack formed by the combined action of stress and a corrosive environment. The crack would not have developed by the action of the stress or environment alone.
Corrosion Control and Prevention
Corrosion Refresher
Q. Why is Stress Corrosion Cracking a serious safety problem?
A. Because it can happen ‘unexpectedly’ and rapidly after a period of satisfactory service, leading to catastrophic structural failure.
Q. What are the two stages of corrosion fatigue called?
A. Material damage stage (due to corrosion and cyclic stress) and material fatigue stage (cracks spread).
Corrosion Control and Prevention
Publications Used for Information
TECHNICAL MANUAL TITLE
TM 1-1500-344-23-1Cleaning and Corrosion ControlVolume 1Corrosion Program and Corrosion Theory
TM 1-1500-344-23-2Cleaning and Corrosion ControlVolume IIAircraft (Cleaning, Inspection, Repair, Sealants, Preservation)
TM 1-1500-344-23-3Cleaning and Corrosion ControlVolume IIIAvionics and Electronics
TM 1-1500-344-23-4Cleaning and Corrosion ControlVolume VIConsumable Materials
TM 1-1500-344-23-5Cleaning and Corrosion ControlVolume VConsumable Materials for Avionics
TM 1-1500-204-23 General Aircraft Maintenance and Practices
Corrosion Control and Prevention