Extract CrashLAB Aviation Corrosion Control Course

AVIATION CORROSION COURSE
Identification, Inspection, Rectification &
Prevention
By
Coenraad J.C. Snyman
EXTRACT – FOR DISPLAY PURPOSES ONLY!

© Failure Analysis & Investigation Laboratory (Pty) Ltd
CORROSION COURSE INTRODUCTION
Coenraad J.C. Snyman (Primary)
 Physical Metallurgist (1991-)
 20 years Aviation Corrosion Control Experience – ALL THE DISCUSSED (CASE STUDY)
INVESTIGATIONS WERE ACCOMPLISHED BY THE PRIMARY FACILITATOR!
 Aircraft Accident Investigator (1998-)
Facilitators & Contributors
Visiting Facilitators and Representatives
 Expert/s in Aircraft Organic Coating Methodologies
 Expert/s in Corrosion Inhibiting & Control Methodologies
 Responsible Operator CPCP Manager/s
 Representatives from the Corrosion Control Supply Industries (Invitation)

CORROSION COURSE INTRODUCTION
Classroom
Printed: Handouts, Checklists & Report Template/s
Electronic: Manuals, Additional Information and Templates
Samples: Physical Samples relating to Aircraft Corrosion
Damages
Support Material
Practical Exercises
Identification: Sample ID and Report
Inspection: Physical Inspection of an Aircraft per Checklist,
Compiling detailed Corrosion Report from Template

CORROSION COURSE LAYOUT
Corrosion Course Layout
Part 1:
IDENTIFICATION
Impacts on
Aviation
Costs:
Maintenance and
Operational
Safety
Corrosion
Fundamentals
What is Corrosion?
Corrosion Theory
Forms of Corrosion
Environments
PRACTICAL:
Corrosion Type
Identification
Part 2: RECTIFICATION
Aircraft Structural
Material Selection
Manufacturing
Methods
Measurement
Methodologies
Rectification
Methodologies
DEMONSTRATION:
Rectification
Part 3: PREVENTION
Corrosion
Prevention
Methodologies
Coatings
Organic
Metallic/Inorganic
Anodic/Cathodic
Protection
Modifying the
Environment
Storage Systems Operational
Exposure
Washing
Procedures
Corrosion
Prevention
Program
What is it?
Layout
Implementation
Data Collection
Maintaining &
Training
Part 4:
Inspection
Inspection
Methods
Inspection Reports
PRACTICAL:
Airframe Inspection

PART 1: IDENTIFICATION

IDENTIFICATION
Safety
Direct Impacts on Aviation Safety:
Structural-, Engine-, Instrumentation- and
Component Failures
Indirect Impacts on Aviation Safety:
Perceptions
Impacts on Aviation

IDENTIFICATION
Impacts on Aviation
Operations
Average Days Downtime (NAD’s) per USAF Aircraft due to
Corrosion ALONE: 15.9 days
Civilian Operators severely limited to generate revenue
Overextended Maintenance capabilities

IDENTIFICATION
The Required Conditions for Corrosion
 Anode THE CORRODING METAL
Cathode THE METAL DRIVING
CORROSION
Conductive Path ELECTRICAL
CONDUCT BETWEEN ANODE AND
CATHODE
Electrolyte WATER, SALT, ACID, etc.
Fundamentals of Corrosion
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte

IDENTIFICATION
Factors Influencing Corrosion
The Nature and Extent of Corrosion are Influenced by the
Following Factors:
 Metal Characteristics – Susceptibility to Corrosion
 Environment
 Concentration and Agitation of the Electrolyte
 Ambient Temperature, or Variations
 Electrode Potential – position on the Galvanic Series
 Surface Hydrogen concentration
 Material Stresses and Defects
Fundamentals of Corrosion

TYPES OF CORROSION:
SURFACE CORROSION

IDENTIFICATION
Controlling Mechanisms: General Surface & Uniform Etch
Stop Movement of Electrons
Isolate Surfaces from Oxygen (Dry)
Isolate from Electrolytes
Use Sacrificial Coatings/Metals
Types of Corrosion

IDENTIFICATION
L29 Fuselage Corrosion Damages
Case Study: Uniform Corrosion
 Sasol Tiger Aerobatic Team
 AAD 2006 Cape Town
 Water Impact – seawater
exposure over 48 hours
 Exposure to Oxygen – increased
Corrosion Rate extensively
 Mg-containing Al-Alloys (skin)
 Mg-engine casing

IDENTIFICATION
Surface Corrosion: Pitting
Localized Attack – Isolated or in Clusters
Detection Difficult due to Powder
Coverage/ Size
Alloy Steels/Aluminum Alloys - Macroscopic
Stainless Steels/Super-Alloys – Microscopic
Progress into Metal Matrix from Surface
Very Dangerous Type due to Growth Rate
Unpredictability
Types of Corrosion

IDENTIFICATION
Controlling Mechanisms: Pitting Corrosion
Material Selection – Resistance to Pitting
Frequent Washing – Electrolyte Removal
Use Inhibitors – Additives (fuel)
Apply Protective Coating – Paint/Plating
Maintain Protective Film – Limit Fretting/Mechanical
Damages
Types of Corrosion

IDENTIFICATION
Spitfire MK IXe Supercharger Clutch Failure
Case Study: Pitting Corrosion
 SAAF Museum Spitfire No 5553 –
1520hp Rolls Royce Merlin 70 Engine
 In-flight Clutch Failure controlling P1
and P2 Supercharger Pressures
 Investigation reveals Severe Pitting
Corrosion on Clutch Plate at the
Plate/Disc Interface
 Resulted in +-30% Loss of Contact
Surface Area = Slippage = Failed
Mixture Control = Engine
Malfunction

IDENTIFICATION
Surface Corrosion: Crevice
 Overlapping Metal Surfaces most Susceptible
 Electrolyte enters the Gap (crevice) between the
Metals
 Corrosion occurs on one of the Metals – Anode
 Difficult to Detect by Visual Inspection alone
 More Pronounced in the presence of Dissimilar
Metals (galvanic)
 Very Common underneath Inspection Panel Covers,
Skin, Ribs/Stringer interface, Window Frames, Wheel
Hubs, etc.
Types of Corrosion

IDENTIFICATION
Controlling Mechanisms: Crevice Corrosion
Design – Limit Stagnant areas/Unprotected Edges
Utilize Approved Sealants (‘wet assembly’)
Welding vs. Riveting & Bolting
Design – Select most Resistant Materials
Apply Protective Coating - eliminate Electrolytes
Types of Corrosion

IDENTIFICATION
Windlass Trike Main Spar Failure
Case Study: Crevice Corrosion
 Left Hand Wing Main Spar failed
at Strut Cable Attachment point
 Outer Pipe with two inner Pipes
of decreasing Diameter
 Crevice Corrosion Set in at the
Pipe Interfaces – seepage of
Corrosive Electrolytes from the
Strut bolt hole/s
 Multiple Fractures = weakened
Structure = Failure

IDENTIFICATION
Surface Corrosion: Fretting
Types of Corrosion
 Mechanical- and Corrosion formed Surface Pits =
surface Stress Raisers = Fatigue Fracture Initiation
 Enhanced by the presence of Vibration (Rotary Wing
aircraft, Turbine Engines, etc)
 Transport conditions may contribute
 Common in Bearings = surface spalling = increased
Rate of Wear = temperature Increase = Lubricant
breakdown = Seizure
 Prevailing in Corrosive Environments
 Recognition by Surface Damages combined with
Corrosion Products and Pitting

IDENTIFICATION
Robinson R44 Flex Plate Failure
Case Study: Fretting Corrosion
 Forward Flex Plate failure
in flight
 Pilot opted for dry land
 Main Rotor rpm decrease
 Wings level – high RoD
impact
 Severe injuries
 Operating in Severe Level
Corrosive environment

IDENTIFICATION
Excessive rotational movement
resulted in Mechanical
Interaction at the Support
Washer/Plate interface = Fretting
Combined with the Corrosive
Operating Environment +
Surface Film breakdown due to
Fretting = Pitting Corrosion

IDENTIFICATION
Pitting Corrosion Pits =
Surface Stress Raisers =
Fatigue Fracture Initiation
Difficult to detect
Time dependent progression

IDENTIFICATION
Surface Corrosion: Sulfidation Attack (Hot Corrosion)
 Requires Three Components: Heat (combustion), Sulphur (combustion
by-product) and Corrosive Electrolytes/Catalysts (operating
environment – Oceanic, Industrial, etc)
 Attacks Power/Compressor Turbine Blades
Types of Corrosion

IDENTIFICATION
Surface Corrosion: Sulfidation Attack (Hot Corrosion)
 Reveals ‘Banded’ Blade Surface
damages
 Depleting CT/PT Blade Protective Layer
 Corrosive attack on Base Metal = Pitting =
Surface Stress Raisers = Fatigue Fracture
Initiation
 Not Repairable – Replacement at High
Cost
 Detection by Visual Inspection – Boro-
scope or during Hot-Section Inspections
Types of Corrosion

IDENTIFICATION
Controlling Mechanisms: Sulfidation Attack
Types of Corrosion
 Adhere to OEM Washing Procedures and Schedules
 Determine your Operating Environment Effects –
Natural Severity Level, Industrial Contaminants, Natural
Contaminants
 Combine Environment Exposure and OEM Schedules
to your unique Operating Scenario
 Consult the OEM/Field Engineers/Other Operators
 Regular Inspections (Boro-scope and/or Hot Section)
 Limit Environmental Exposure during Operations,
where possible

IDENTIFICATION
Pilatus PC7 P&W PT6 Engine Damages
 Pilatus PC7 MK II aircraft @ AFB Langebaanweg
 Pratt & Whitney PT6-25A Engines – TT 1100-1400 hours
 Noted increase in Corrosion Related Damages during Maintenance
 Investigation revealed Incorrect Washing Procedures and Schedules
 Due to Incorrect Environment Classification (CSIR vs. Global) and
Training
 Desalination- vs. Compressor Washing Procedures
 Effects of High Pressure Washing Equipment
 Changes to Storage Conditions
 Changes to Operating Limits and Washing Methodology/Schedules
 Resulted in Extensive Losses
Case Study: Sulfidation Attack

TYPES OF CORROSION:
ENVIRONMENTALLY INDUCED &
INTERGRANULAR CORROSION

IDENTIFICATION
EIC: Stress Corrosion Cracking (SCC)
 Brittle Failure mode
 3x Prerequisites: Susceptible Alloy, Corrosive
Environment, Applied Stress (mild)
 Not Equally Applicable to all Materials/Alloys
 Crack Propagation direction normal to
Applied Stress – TG/IG combinations – TG less
common
Types of Corrosion
SCC
Applied
Stress
Susceptible
Material
Corrosive
Environment

IDENTIFICATION
EIC: Stress Corrosion Cracking (SCC)
 Difficult to Detect by Visual Inspection
 Requires Non-Destructive Testing to detect
 Requires Expert Analysis to determine Root Cause/s
 Can be a Symptom of another Corrosion Type –
Sequence of Events
 Prominent in High Strength Materials
 Aircraft Components i.e. Undercarriage (Gear),
Wing Spars, Structural Components, Fasteners, etc.
 Beware of Components with an attached non-
Operational Time-Limitation
Types of Corrosion
SCC
Applied
Stress
Susceptible
Material
Corrosive
Environment

IDENTIFICATION
Controlling Mechanisms: SCC
 Adhere to OEM Storage Requirements, Operational
Limits, Inspections, Overhaul Procedures and Fitment
Instruction/s
 Exclude one or more of the Contributors – Stress
(design, fitment)/Material (design)/Corrosive
Environment (exclude environment, alter operating-
and storage conditions)
 Regular Washing of Aircraft
 Consider De-humidifying Systems
Types of Corrosion
SCC
Applied
Stress
Susceptible
Material
Corrosive
Environment

IDENTIFICATION
Boeing 707 Main Gear Failure
Case Study: SCC
 Boeing 707 – Close to MAUW – Rear
Main Gear Bogey Axle failed during
Towing Procedure
 Assembly was in Storage for 10 Years
following an Overhaul Procedure
 Overhaul included the Re-plating of
the Hard Chrome Bearing Support
Layer
 Main Axle Material AISI 4340 -
susceptible to SCC

IDENTIFICATION
Boeing 707 Main Gear Failure
Case Study: SCC
 Investigation revealed ‘Chicken Wire/Mud’
cracking of the plated Cr Layer
 Fractograpahy analysis revealed an Inter-
Granular fracture surface geometry
Perpendicular to the Surface – SCC
 Fracture Cr-plating allowed exposure of
the 4340 Base Metal to a Corrosive
Environment over long period of Storage
 Mild Stress applied to axle due to Storage
Conditions
 SCC growth over long period

IDENTIFICATION
EIC: Hydrogen Induced Cracking (HIC)
Types of Corrosion
 Atomic Hydrogen (H+) combines to
form Hydrogen Molecules (H2) – Applied
Molecular Forces > Yield Stress
 Sub-Surface Fracture parallel to the
Surface (blisters)
 Inter- and/or Trans-Granular Geometries
 Difficult to Detect
 Requires Expert Analysis to determine
Root Cause/s

IDENTIFICATION
Controlling Mechanisms: HIC
Types of Corrosion
 Adhere to OEM and/or Engineering Overhaul
Procedures and Instruction/s
 Exclude one or more of the Contributors – Material
(design)/High Concentrated Hydrogen
Environment (Correct Welding techniques, Plating-
and Cleaning Methodologies)
 Utilize Reputable and OEM Certified Overhaul
Vendors

IDENTIFICATION
C130B Main Gear Strut Failure
Case Study: HIC
 C130B Hercules RH Main Strut
Catastrophic Failure during Ground Run
 Soft-strut Configuration
 Strut was exposed to an Overhaul
Procedure prior to Fitment
 Procedure included Cadmium Surface
Plating

IDENTIFICATION
C130B Main Gear Strut Failure
Case Study: HIC
 Fracture initiated at Drag Link Pin
Lug position – progressed at High
Rate – Exploded
 Fractographic SEM Analysis
revealed Inter Granular attack =
Hydrogen Induced Cracking
 Investigation revealed Incorrect
Re-plating Process – No post
Plating Heat Treatment
completed

TYPES OF CORROSION:
AVIATION SPECIFIC

IDENTIFICATION
Manufacturing Methodologies: Rolling
 Hot and Cold Rolling processes
 Aircraft Skin
 L- and U- Shaped Structural Components
 Al-Clad Process – Pure Al rolled unto Alloy
 Uni-Directional Grain Structure
 Directional Mechanical Properties
 Prone to Exfoliation Corrosion Attack
 Repairable & Weldable (Alloy dependent)
Aircraft Structural

IDENTIFICATION
Aircraft Turbine Engines
 Consult relevant OEM Inspection Methodology &
Schedules
 Refer to latest Service Bulletins, Service Letters,
etc. involving Corrosion Repair/Prevention
 Damages to Turbine Engines = High Cost + Safety
Issue
 Requires Expert Analysis due to presence of
Titanium-, Nickel-, Aluminum-, High Strength Steel-
and Super Alloys
 Prevailing High Operating Temperatures, FOD,
Corrosive Environment/s and the possibility of
Contamination
Aviation Specific Corrosion

IDENTIFICATION
Aircraft Reciprocal Engines
 OEM Specifications towards Inspection/s
 Storage – Short- and Long Term requirements
 Cylinder Barrels and Rocker Covers prone to Uniform/Pitting Corrosion
Attack
 Cylinder Barrel damages = Blow-by and Increased Oil Consumption
 External Uniform Corrosion on Magnesium Based Castings – Engine- and
Gearbox Casings
 Crevice Corrosion between the Crankshaft Drive End and Propeller
Mount
 Crankshaft, Journals and Bearings prone to Corrosion Attack due to
Contaminated Oil (water content)
 Turbocharger Casing Corrosion – exhaust Gases extremely Corrosive
https://youtu.be/PSx2_vdozMg

IDENTIFICATION
Composites in Aviation
Utilized extensively in Modern
Aircraft Construction
Superior Weight/Strength
Ratio
Basic Construction:
Fiber/Filament (Particulates,
Fibrous, Laminate) + Matrix
(Ceramic, Metallic, Polymer)

PART 2: RECTIFICATION

RECTIFICATION
General
The Type of Action required depends on the Form- and
Classification of the Corrosion Damage as well as the
Base Material Composition (Alloy)
Corrosion Damage can be Classified as follows:
A. Negligible Damage (Nuisance Corrosion)
 No Clear evidence of Corrosion Products
 Physical Properties not affected (Filiform)
 Treatment: Paint removal + Chemical Neutralization +
Surface Conversion Coating + Paint Reapplication
Corrosion Damage Classification

RECTIFICATION
General
B. Significant Damage
 Clear evidence of Corrosion Products
 Rough Surface evident
 Remaining Thickness > Limits as per OEM
Structural Repair Manual/Other
 Treatment: Paint removal + Mechanical
Removal + Chemical Neutralization + Surface
Conversion Coating + Paint Reapplication

RECTIFICATION
General
C. Repairable Damage
 Remaining Thickness < Limits as per OEM Structural Repair
Manual/Other
 Treatment: Damaged Section Removal + Repair as per
Authorized Repair Scheme (Engineering)
D. Damage Requiring Replacement
 Remaining Thickness < Limits as per OEM Structural Repair
Manual/Other over Large Area/Section
 Structural Integrity compromised
 Treatment: Damaged Component Removal + Replace as
per Authorized Repair Scheme (Engineering)

RECTIFICATION
Measurement
Methodologies
Measurement
of Corrosion
Technology Pro's Con's Type of Defects
Visual - Low Power Light
Microscope/Magnifying Glass
Inexpensive Subjective General Surface
Cover Large Area Not Precise Exfoliation
Portable Surface only Pitting
Labour Intensive Filiform
Crevice
Galvanic
Fretting
Erosion
Sulfidation
Bacterial
Enhanced Visual - High Power
Light Microscope
Sensitive to Lap Joint
Corrosion (Crevice)
Quantification difficult Same as Visual
Inexpensive Subjective
Cover Large Area Surface Preparation Required
Portable
Eddy Current
Inexpensive Interpretation requires
Experience
Surface Damage
High Resolution Slow Process Sub-surface Cracks
Multi-Layer Capable Exfoliation at Rivets/Fasteners
Portable
Ultrasonic
Good Resolution Requires Couplant and
Specific Transducer
Material Thickness
Can Measure Material
Loss/Thickness
Single layer only Delamination (Exfoliation)
Slow process Sub-surface Voids/Cracks
(orientation)
Interpretation requires
Experience
Radiography
Best Resolution Expensive (non-digital) Surface Corrosion Damages
Digital Systems allow Image
Manipulation
Safety Precautionary
measures required
Sub-surface Corrosion
Damages (orientation)
Expensive Equipment

RECTIFICATION
Measuring Corrosion
Measurement Methodologies
Ultrasonic Inspection
 Generates and Detects Vibrations/Waves within solid objects
 UT waves are reflected at boundaries where material properties
change – Corrosion Pits, Cracks, etc.
 Used for thickness measurements – Material Loss
 Portable instruments
 Requires Trained and Certified Operator
 Should be applied while conforming to an OEM and/or ASTM or other
applicable Standard
 Most Common NDT Instrument utilized for Corrosion Damage
measurement

RECTIFICATION
Measuring Corrosion
Thermographic Inspection
 Measuring of Thermal outputs from Input Stimulus
 Common Method with Increasing popularity as equipment
cost drops
 Detects hot-sports, heat dissipation as a function of
Material Properties – discontinuities, flaws, voids, etc.
 Flash Thermology used to detects delamination other flaws
in Composite Materials
 Sub-surface discontinuity/crack detection capability
 Relative Expensive Equipment (Quality)
 Require Trained Operator to interpret results

RECTIFICATION
Measuring Corrosion
Radiographic Inspection
 Detects discontinuities in castings, forgings
 Used extensively for Integrity Inspection of Welds
 Advances in Digital imagery
 Require extensive Safety Precautions and
Preparation
 Trained and Certified Operator to interpret results
 Expensive equipment

RECTIFICATION
Measuring Corrosion
Reliability of NDT
It is imperative to know what the probability of finding is in
relation to discontinuities of interest for determined type of NDT
inspection. This is usually called the Probability of Detection
(POD). The process of POD estimation requires several
inspections to be performed:
 Using the complete, OEM pre-defined inspection system that is
being assessed including representative equipment, procedures,
inspectors, and target parts,
 Using parts with discontinuities that represent the discontinuities of
interest or a means to assess the difference between the two,
and
 Using an inspection procedure and environment typical of the
deployed environment.

RECTIFICATION
Rectification Methodologies
General
Complete Corrosion Removal involves the
following Steps:
 Cleaning and Stripping of the corroded area,
 Removing as much of the Corrosion Products as
practicable,
 Neutralizing any Residual Materials remaining in pits
and crevices,
 Restoring Protective Surface films, and
 Applying temporary/permanent Coatings or Paint
Finishes.

RECTIFICATION
Corrosion Removal Techniques
The following Standard Methodologies apply :
 Mechanical and Chemical
 Mechanical: Hand Sanding - Abrasive Mat/Paper
and/or Metal Wool
 Composite surfaces, paint removal should be done
by mechanical removal techniques (scuff sanding)
only
 Mechanical: Powered Tools – Sanding, Grinding
and Buffing using Abrasive Mat/Paper, Grinding
Wheels, Sanding Discs
 The Removal Method will depend on the Extent of
Damage/s, Base Material and OEM Specification/s

RECTIFICATION
Corrosion Removal Techniques
The Tools of the Trade:
NOTE: Only qualified and certified personnel are allowed to
apply approved Mechanical Corrosion Removal techniques
while utilizing equipment for which a Competency Certificate
are held.
 Abrasive Papers, Pads and Compounds
 Hand Wire Brush and Steel Wool
 Powered Buffing Machine and compounds
 High-Pressure Bead Blasting Machine
 Anti-Static Mini Spot Blaster
 Stiff Fibre Brush

PART 3: PREVENTION

PREVENTION
Most Common Methods
 Organic barrier coatings (Paint).
 Galvanic isolation.
 Corrosion Preventative Compounds (CPC).
 Anodic & Cathodic protection.
 Sacrificial coatings (e.g., galvanizing).
 Sealants (e.g., wet installed fasteners, crevice fillers,
etc.).
 Storage systems.
 Aircraft Washing & Cleaning.
Corrosion Control Methods

PREVENTION
Organic Coatings: Paint
Corrosion Prevention Methods
Paint is more than Aesthetics, it shields the Exposed Surfaces from Corrosion
and Deterioration, makes it Easier to Clean and protects the Structural
Integrity
 Consists of Three Components: Resin (coating material), Pigment (color)
and Solvents (viscosity)
Types of Paints:
 Dope: Used on Fabric Covered aircraft
 Synthetic Enamel: Oil-based Single-stage, mix with Hardener, Cost
Effective
 Lacquers: Not environmentally friendly, mostly discontinued
 Polyurethane: Best stain/abrasive/chemical/UV Resistance, Popular
choice
 Urethane: Two-part Coating – Base & Catalyst produced High Gloss and
Durability
TO BE APPLIED BY QUALIFIED AIRCRAFT PAINTERS ONLY!
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte

PREVENTION
Wet Assembly & Corrosion Inhibiting
Compounds (CIC)
 The application of OEM Approved
Sealants (PPG PRC, Primers, etc.) will
Enhance Structural Integrity, provide
Pressure/Environmental Sealing and
increase Corrosion Resistance (galvanic,
crevice, exfoliation)
 Sealants are Formulated for Specific use
i.e. Fuel tanks, High Temperature,
Chemical attack, etc..
 Sealants should be applied by Trained
Personnel (Sheet Metal
Technicians/Aircraft Painters/OEM
Applicators) only
Product Characteristic Application
AV-8 Excellent Penetration
Light/Average Corrosion &
pre-Treatment
AV-30 Good Penetration
Average Corrosion & Pre-
Treatment
AV-40
Heat Resistant up to
210°C, sprayed over
paint, high pressure
treatment
In Areas exposed to High
Temperatures
AF-100 D Heavy Duty
Serious Corrosive
Conditions
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte

PREVENTION
Corrosion Inhibiting Compounds (CIC)
Before Applying ANY CIC, Know the
Following:
Product Description and Use
Approvals – both OEM and SAAF
Safety Data Sheet Information

PREVENTION
Surface Protection: Allodizing of Aluminium
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte
What is Allodizing or Chromate Conversion Coatings?
 It is a simple chemical treatment (Alochrome ® /Anochrome ® /Alodine ®)
for all aluminium alloys to increase their corrosion resistance (Passivation)
and to improve their paint bonding qualities.
 Because of its simplicity, it is rapidly replacing anodizing in aircraft work.
 The process consists of precleaning with an acidic or alkaline metal
cleaner that is applied by either dipping or spraying. The parts are then
rinsed with fresh water under pressure for 10 to 15 seconds. After
thorough rinsing, Alodine® is applied by dipping, spraying, or brushing.
 A thin, hard coating results which ranges in colour from light, bluish green
with a slight iridescence on copper free alloys to an olive green on
copper bearing alloys.
 Can be damaged during handling and Corrosion Removal!

PREVENTION
Sacrificial Coatings
A Sacrificial Coating is a form of corrosion control done through the application of
thin metal layers resulting in a barrier on the surface being protected.
 The Sacrificial Coating will oxidize (corrode) more readily than the metal surface
that it protects.
 Example: Zinc-coated steel or galvanized steel. The more active sacrificial
coating release electrons and flow to that part of metal that is being protected,
turning it into a cathode - preventing corrosion of the metal.
 Coatings from more active noble metals, such as steel plated with nickel and tin,
can provide protection from corrosion so long as the coating remains intact. All
forms of coating defects in the localized area can lead to intensive corrosion,
particularly on the part of the steel as it acts similarly to the anode in a galvanic
cell. The presence of tin, which is cathodic, hastens steel corrosion.
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte

PREVENTION
An aircraft is considered stored when it is removed from active
operational status for any reason while the aircraft remains on the
certificate holder’s operations specifications (Op-Specs) or is
removed from the Op Specs. The level of preservation depends on
the length of storage, the aircraft design features, and the storage
environment (inside/outside, etc.).
 Short-Term Storage. An aircraft is subject to short-term preservation
procedures when it is removed from operational status for less than 60
days
 Intermediate-Term Storage. An aircraft is subject to intermediate-term
preservation procedures when it is removed from operational status for
more than 60 days but less than 120 days.
 Long-Term Storage. An aircraft is subject to long-term preservation
procedures when it is removed from operational status for 120 days or
more.
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte
Modifying the Environment: Storage

PREVENTION
Important Considerations:
 OEM Requirements: The OEM might have prescriptions
relating to Storage – airworthiness/support/warranties.
 CPCP Requirements: The prescribed CPCP Storage
methodology to be followed.
 Pre-Storage & In-Storage Inspections: Scheduled
inspections as per OEM/CPCP requirements.
 Pre-release Inspection: Aircraft/Equipment to be
inspected prior to Release to Service.
Corrosion
Cathode
Anode
Conductive
Path
Electrolyte
Modifying the Environment: Storage

THE CORROSION PREVENTION AND
CONTROL PROGRAM (CPCP)

PREVENTION
What is the CPCP??
 The Corrosion Protection and Control Program is a systematic approach for controlling
corrosion in the airplane’s primary structure. The objective of a CPCP is to limit the
material loss due to corrosion to a level necessary to maintain airworthiness.
 The CPCP is part of a Service Life Extension program for the Aircraft System.
 The other part of this Service Life Extension program is the Supplemental Structural
Inspection Program (SSIP), which extends the life of the aircraft past a pre-determined
number of flight hours. The CPCP supplements the SSIP but is independent of the SSIP.
 The main purpose of the CPCP is to detect corrosion in the primary structure and correct
the problem before serious damage has taken place. The objective is to limit the loss of
material before having to perform costly repairs to the aircraft. Ultimately, this will lead to
improved safety, reduced downtime, reduced operating costs, and provide the owner
with a sellable asset when the aircraft is sold.
Corrosion Prevention & Control Program (CPCP)

PREVENTION
Advantages of an Effective CPCP?
Flight Safety
Lower Maintenance and Repair Costs
Limit Operational Downtime
Adhering to Insurance Requirements
Enhancing Perceptions/Image

PREVENTION
CPCP designated Levels of Corrosion:
Level 1:
 Corrosion, occurring between successive corrosion
inspection tasks, that is local and can be reworked or
blended out within the allowable limit; or
 Operator experience has demonstrated only light
corrosion between each successive corrosion
inspection task.
Level 2:
 Corrosion occurring between any two successive
corrosion inspection tasks, that requires a single
rework or blend out which exceeds the allowable
limit. The detection of Level 2 corrosion requires repair,
reinforcement, or complete or partial replacement of
the applicable structure.

PREVENTION
CPCP designated Levels of Corrosion (cont.):
Level 3:
 Corrosion occurring during the first or subsequent
accomplishment of a corrosion inspection task that the
operator determines to be an urgent airworthiness concern.
The CPCP requires the operator to maintain
the aircraft to Level 1 Corrosion or better.

PART 4: INSPECTION

INSPECTION
General
Corrosion Control and the required Inspections
are Maintenance Prerequisites as stipulated by
the following:
OEM Maintenance Manuals
Operator’s CPP & CPCP documentation
Authorities (Civilian)
Insurers
Corrosion Inspection Methods

INSPECTION
General
Although the Required Inspection Methodologies may
differ for each type of Aircraft or Equipment the
following commonalities are applicable:
 Must be performed by a Trained and Qualified
Inspector.
 Inspection Schedules to adhered to.
 The Corrosion Inspector must be clearly mandated.
 Corrosion Reports must be issued.
 Logbooks to be updated, where applicable.
 Do not rush it!

INSPECTION

INSPECTION
Corrosion Inspection
Corrosion Prone Areas
Although Corrosion Prone Areas may differ between Aircraft
Types and Models, the following are addressed by the majority of
OEMs:
Fuselage (External)
 External Skin – Faying surfaces (corrosion products,
delamination)
 Rivets & Bolts (corrosion products, bulging)
 Inspection panels (corrosion products – remove for inspection)
 External fitments (remove if possible)
 Windshield & Door frames (and locking mechanisms)
 Piano Hinges
 Locking devices (doors, cowlings, fuel caps, etc..)

INSPECTION
Corrosion Inspection
Corrosion Prone Areas
Although Corrosion Prone Areas may differ between Aircraft Types
and Models, the following are addressed by the majority of OEMs:
Fuselage (Internal)
 Battery Compartments & Vents (spillage, corrosion products)
 Bilge Areas (fluid spillage)
 Lavatories and Gulley's (Lower areas, spillage, corrosion
products)
 Cockpit (Lower areas, instruments, switchgear, seat guides, etc..)
 Control Cables (corrosion products, use correct method)
 Cabin (lower areas, storage, seat guides, etc..)
 Electrical connectors (corrosion products, arcing, etc..)

Structures
Consult relevant OEM Inspection
Methodology & Schedules
Refer to latest Service Bulletins,
Service Letters, etc.. involving
Corrosion
Repair/Prevention/Washing
Damages to Aircraft Structural
Components = High Cost +
Safety Implications
INSPECTION

Fixed Wing Structures: External Inspections
Engine Cowling:
Paint Damages
(Hydraulic
Fluid/Fuel/Oil)
Exhaust Outlet Areas:
Paint damages, Crevice
Corrosion (skin/rivets)
Main Wings: Paint Damages
– Leading Edge (FOD),
Crevice Corrosion/Filiform
(Inspection Panels/Covers)
Cockpit Doors &
Windshield Frames:
Crevice Corrosion
(Hinges/Locks)
Fuselage: Exfoliation/Crevice/Filiform
Corrosion (Skin/Aerials/Attachments)
Empennage:
Exfoliation/Crevice/Filiform
Corrosion (Skin/Flight
Controls/Attachments )
Gear:
Exfoliation/Crevice/Pitting
Corrosion
(Castings/Hubs/Links/Fasten
ers)
INSPECTION

Fixed Wing Structures: Internal Inspections
Gulley/WC:
Crevice/Uniform/Exfol
iation Corrosion
(Hinges/Locks/Structu
ral Components)
Engine Bay/Cowling:
Crevice/Pitting/Exfoli
ation Corrosion
(Attachments/Engine
Cradle/Fasteners/Fue
l/Oil Chemical
Attack)
Primary Fuselage
Structural Components:
Crevice/Exfoliation
Corrosion
(Skin/Longerons/Frames/
Stringers/Bulkheads)
Primary Wing Structural
Components:
Crevice/Exfoliation
Corrosion
(Skin/Spars/Stringers/Tanks/
Pipelines/Flight Control
Linkages & Fasteners
INSPECTION

Rotary Wing Structures: External Inspections
Cockpit/Rear Sliding
Doors & Window
Frames: Crevice
Corrosion
(Hinges/Locks/Sliders)
Gear/Skids:
Crevice/Pitting
Corrosion
(Brackets/Paint
damages/FOD)
Main Rotor assy.: Pitting/Crevice/Uniform Corrosion
(Casings/Linkages/Attachments/Cowlings/Exhaust
Tail Rotor assy.: Pitting/Crevice
Corrosion
(Casings/Attachments/Linkages)
Tail Boom: Filiform/Crevice
Corrosion (Inspection
panels/Covers/Attachments)
Internal: Pitting/Crevice/Exfoliation
Corrosion (Main
Structural/Floors/Tanks/Pipelines/Fli
ght Control linkages & Fasteners)
INSPECTION

Aircraft Turbine Engines
Sulfidation Attack
(Ni Alloy CT
Blades)
Erosion Corrosion
(FOD at Intake
Casing)
Pitting Corrosion
(Al/Mg Casting)
Pitting Corrosion
(Stainless Steel +
Exhaust Gas)
Pitting Corrosion
(Contaminated Oil
+ Bearing/Shaft
Surfaces)
Crevice Corrosion
(Accessory
Mounts)
Pitting Corrosion
(Al/Mg Casting)
Pitting/Uniform
Corrosion (St/Steel
Casing Inner)
INSPECTION

Aircraft Reciprocal Engines
Pitting/Erosion
Corrosion (Steel
Rockers/Rollers)
Crevice Corrosion
(Casing Interface)
Uniform/Localised
Attack (Sump Inner –
Contaminated Oil)
Crevice Corrosion
(Accessories
Mount Surfaces)
Pitting Corrosion
(Crankshaft/Journal
Surfaces –
Contaminated Oil)
Pitting Corrosion
(Cylinder Sleeve Inner –
Contaminated
Oil/Moisture)
Crevice Corrosion
(Rocker Covers)
Uniform/Pitting Corrosion
(Outer Surfaces - Al/Mg
Castings)
INSPECTION

END OF EXTRACT
DID YOU ENJOY THIS PRESENTATION?
FOR MORE INFORMATION, FEEL FREE TO CONTACT
COENRAAD SNYMAN AT
+27(0)829211091 (MOBILE),
COENRAAD@CRASHLAB.CO.ZA (E-MAIL)
OR VISIT WWW.CRASHLAB.CO.ZA

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