Corrosion and Coatings for the Aerospace Maintainer

Mr. Terry GabbertMr. Michael Jones

Housekeeping Issues

• Course Schedule• Breaks• Emergency evacuation

Corrosion is not new concern

• HMS Alarm (1762)• One of first documented cases

of galvanic corrosion• Hull had copper sheathing to

reduce woodworm damage & barnacles

• Iron nails fastening copper to the hull "rotted" after only 2 years

First Flight - World War I

• 1903 Wright flyer recognized as the first practical airplane

• 1909 Army purchased its first airplane: Signal Corps No. 1.

• Aluminum engine block • Spruce & steel wire structure • Fabric skin

World War II• Start of the war all aircraft used

camouflage paint• Hiding characteristics along

with friend/foe recognition

• 1942 RAF noted speed gains with polished surfaces

• 1944 all aircraft coming off production lines were not painted

Post World War II• Military and commercial aerospace

needs led to the introduction of a range of high strength aerospace alloys such as the 7xxx series

• Especially valuable due to its strength to weight ratio and workability

• Corrosion protection

Cold War and Space Race• New alloys along with special

machining and joining methods were developed

• Titanium had a high strength-to-weight ratio, corrosion and heat resistance, and a propensity to become stronger as was heated

Todays Aircraft• New aircraft that fly higher, faster,

and longer require many different structure components:• Steels/Superalloys• Composites• Aluminum• Titanium

Corrosion Challenges

• Operating aircraft beyond their design service life will require new MX paradigms• Aircraft are flying well beyond their design service lives• C-130, C-5, B-52, KC-135 all have aircraft in operation from 1950s & 60s• Increased reliance on all aspects of MX (people, processes, procedures,

equipment, training, etc.)

Corrosion Challenges• (Safety) Weapon system mishaps have been attributed to corrosion)

• C-130 corrosion caused the grounding of an entire wing• F-16 corroded electrical contacts caused uncommanded fuel

valve closures (with subsequent loss of aircraft)• C-130 corroded propeller blade that came off mid-flight

Corrosion Impact• LMI’s FY18 Update of the Estimated Impact of Corrosion on Cost

and Availability of DoD Weapon Systems was $20.6 billion

• Aviation and missiles account for approximately 49 percent of the total corrosion costs ($10.2 billion of $20.6 billion)

• Airframe and engines have the highest total corrosion NAHs for Air Force aviation assets, with the airframe incurring a larger amount of corrosion-related NAHs than any other system.

Corrosion Impact• Top 10 Contributors to Air Force Aviation and Missile Corrosion Cost

Corrosion Impact• Top 10 Contributors to Air Force Aviation and Missile Corrosion NAHs

Corrosion Impact• Top 10 Aviation and Missile Availability Impact by System

Course Objective• The objective of this course is to support corrosion prevention and

control during the sustainment life cycle of DoD systems

• The course familiarizes aerospace maintainers with corrosion mitigation and coating application practices

• The program is designed to enlighten the aircraft maintainers to the critical nature of the core processes they perform daily, and help prevent the costly mistakes that result in increased corrosion cost and decreased system availability

Course Content• Corrosion Basics and Control

• Safety

• Surface Preparation

• Coating Types

Corrosion and Coatings for the Aerospace Maintainer

Corrosion Basics and Control

Scope and Topics

• This section acquaints students with the basic principles of corrosion, especially those that occur most frequently on aircraft

• Topics covered:

• Fundamentals of corrosion

• Galvanic series and galvanic corrosion• Common types of corrosion• Prevention measures• Corrosion prone areas on aircraft• Environmental factors that contribute to corrosion

Corrosion Basics and Control

Corrosion Defined• Corrosion is:

• The deterioration of a material• A natural process

• Metals “give up energy” and return to their natural state

• Process must be stopped or slowed down

Fundamentals of Corrosion

• Metals “give up” energy and return to their natural state

• Metal oxides forms on a metal structure

• Metal is lost from surface• Reduces cross section and strength• Can cause contamination of the

environment and industrial products

• Can foster and accelerate other forms of material degradation

Fundamentals of Corrosion• Elements required for corrosion to occur (oxygen assumed):

• Anode• Cathode • Metallic Path• Electrolyte

• Analogy: Dry Cell Battery

Fundamentals of Corrosion• The Anode

• Negative side on a substrate• Deteriorates in corrosion process

• The Cathode

• Positive side on a substrate• Remains intact or protected

Fundamentals of Corrosion• The Electrolyte

• External conductive media

• Source of chemicals for reactions at the cathode

• Reservoir for the metal ions and corrosion product at the anode

Rate of Reaction• Rate at which corrosion occurs is limited by the rate

of reaction at the least active component of the electrochemical cell

• Rate of the corrosion reaction is effected by:

• Temperature• Potential difference between the metals

Corrosion Elements -Video

Types of Corrosion

• Uniform (General) Corrosion

• Dissimilar Metal Corrosion (Galvanic)

• Pitting Corrosion

• Crevice Corrosion

• Stress-Corrosion Cracking

• Hydrogen Embrittlement

• Corrosion-Fatigue

• Intergranular Corrosion

• Exfoliation

• Fretting Corrosion

• Filiform Corrosion

Uniform Corrosion

• Corrosion is uniformly distributed over a metal surface

• Proceeds at about the same rate over the surface

• Also known as general corrosion

Uniform Corrosion

• Uniform corrosion may be prevented by:• Stopping oxygen from reaching the substrate• Slowing down or stopping the movement of electrons• Use protective coatings and/or cathodic protection• Prevent metal from giving up electrons• Use sacrificial coating and/or inhibitors

Dissimilar Materials (Galvanic) Corrosion

• Occurs when two dissimilar materials are in contact with each other in an electrolyte

• Galvanic Series lists materials in order of their electrical potential

• Determines which material will act as the anode or cathode

Dissimilar Materials (Galvanic) Corrosion

• Dissimilar metal corrosion may be prevented by:• Selecting metals close together on the galvanic

series• Break the electrical contact using insulators or

coatings between the metals

Dissimilar Material (Galvanic) Corrosion - Video

Pitting Corrosion• Severe form of localized corrosion

• Caused by physical or chemical variation on a metal surface in the presence of an electrolyte

• Physical variation• Impurities in the metal itself

• Chemical variation• Salt deposits or local acidic areas in an electrolyte

Pitting Corrosion

• Pitting may be avoided by:• Proper selection of materials with resistance to service

environment• Control pH and chloride concentration• Cathodic protection• Use of higher alloys for increased resistance to pitting

corrosion• Use of protective coatings or metallic coatings such as

plating or anodizing

Crevice Corrosion• Occurs within or adjacent to a fissure formed by

contact with either:• Two pieces of the same metals• Another metal• A nonmetallic material

• Special type of pitting• Occurs in crevices and other shielded areas on

metal surfaces• Associated with a stagnant solution caused by:• Gasket surfaces• Lap joints• Crevices under bolt and rivet heads

Crevice Corrosion

• Prevent crevice corrosion by:• Avoiding crevices in design stage• Eliminate crevices by use of continuous welds• Use higher alloys for increased resistance to crevice corrosion• Crevices under bolt and rivet heads

Stress-Corrosion Cracking (SCC)• Occurs in statically loaded components

• Brittle fracture of an alloy, exposed to a specific corroding medium, at low tensile stress levels

• Cracking caused by corrosion together with stresses in a metal

• Time of failure in each environment depends on:• Total stress• Temperature• Effective concentration of the electrolyte

solution

Stress-Corrosion Cracking (SCC)

• Prevention can be achieved by:• Reducing the overall stress level and designing out

stress concentrations• Selection of suitable materials• Design to minimize thermal and residual stresses

Hydrogen Embrittlement • Hydrogen embrittlement is a

problem with high-strength steels, titanium, and some other metals

• Process results in a decrease of the toughness or ductility of a metal due to the presence of atomic hydrogen

• Accumulation of hydrogen in high-strength alloys often leads to cracking

Hydrogen Embrittlement

• Prevent hydrogen embrittlement by:• Using a resistant or hydrogen free material• Avoid sources of hydrogen such as cathodic

protection and certain welding processes• Removal of hydrogen in the metal by baking

Corrosion-Fatigue• Result of alternating or cyclic stresses of metals in a

corrosive environment

Corrosion-Fatigue

• Corrosion may be reduced or prevented by:• Use of protective coatings• Proper design that reduces stress concentration• Removing or isolating sources of cyclic stress

Intergranular Corrosion• Selective corrosion attack occurs along grain boundaries of

some alloys

• Welding and heat-treating causes alloying elements to form extremely small particles at the grain boundary called precipitates

• Precipitates may be anodic or cathodic

• Selective dissimilar metal corrosion occurs at grain boundaries

• Detection usually requires a microscope

Intergranular Corrosion

Intergranular Corrosion

• Fatigue crack exposes untreated metal, this then allows a place for corrosion to start

• IGC also frequently stems from initial pitting, many IGC findings are IGC and something else

• Avoid intergranular corrosion by:• Selection of stabilized materials• Good inspection critera• Control of heat treatments and processing to avoid

susceptible temperature range

Exfoliation Corrosion• Advanced stage of intergranular corrosion

• Delamination of metal along grain boundaries

• Rolled products are particularly susceptible

• Aluminum alloy plates

Exfoliation Corrosion

• Exfoliation corrosion can be prevented through:• Selection of a more exfoliation resistant aluminum alloy• Using heat treatment to control precipitate distribution• Use of protective coatings

Fretting Corrosion• Gradual wear• Rubbing between two surfaces• Protective film on the metallic surface is removed by rubbing

action• Conditions must be present for fretting corrosion:

• Interface must be loaded• Relative motion should be sufficient enough to produce

deformation• Common in:

• Riveted joints/structures• Bolted joints/flanges

Fretting Corrosion

• Mechanical design is the most important role

• Can be prevented by:

• Reducing relative movement• Using the correct materials• Increasing the hardness of the material• Using contact lubricants• Using seals to absorb vibrations

Filiform Corrosion• Corrosion exhibiting a thread-like structure

• Directional growth under coatings on metal surfaces including aluminum

Filiform Corrosion

• May be prevented by:• Ensuring the correct coating is used• Controlling the relative humidity

Aircraft Corrosion-Prone Areas

• Galley Locations• Exhaust Trail• Battery Components• Battery Vent Openings• Bilge Areas• Wheel Well and Landing Gear

• Water Entrapment Areas• Engine Frontal Areas and Cooling Air Vents• Wing Flap and Spoiler Recesses• External Skin Areas• Miscellaneous Trouble Areas

Galley Locations• Liquid spills are a prime cause

• Coffee, fruit juices, and other liquids are particularly severe on light-alloy structures

Exhaust Trail Areas• Jet and reciprocating exhaust deposits are

very corrosive

• Trouble areas downstream of exhaust pipes• Gaps• Seams• Hinges• Fairings

• Exhaust deposit must be removed to avoid corrosion

Battery Components and Battery Vent Openings

• Moderately strong acids will severely corrode most alloys used in airframes

• Fumes from overheated electrolyte cause a rapid corrosive attack on all unprotected metal surfaces

• Cleaning and neutralization of acid deposits will minimize corrosion

Bilge and Latrine Areas• Natural sumps for waste hydraulic fluids,

water, and dirt

• Interior components are painted to protect these areas

• Human waste and chemicals used in lavatories are very corrosive

• Under galleys, lavatories, and human waste disposal openings should be inspected

Wheel Well and Landing Gear• Area is prone to damage mud, water,

salt, gravel, etc.

• Tough to attain and maintain complete area paint film

• Inspection areas include:• Magnesium wheels• Exposed rigid tubing• Exposed position indicator

switches• Electrical equipment• Crevices

Water Entrapment Areas• Drains must be installed where water may collect

• Debris, grease, or sealants cause drains to be ineffective

• Low point drains must be inspected daily

Engine Frontal Areas and Cooling Air Vents

• Airborne dirt and dust, gravel, and rain erosion remove the protective finish

• Inspection• All sections in the cooling air path • Salt deposit areas

Wing Flap and Spoiler Recesses• Flap and spoiler recesses collect dirt and water

• Inspect spoilers and/or flaps in fully deployed position

External Skin Areas• Configurations and combinations of

materials become troublesome under certain conditions

• Trimming, drilling, and riveting destroy surface treatment

• Inspect magnesium skins, edges, fasteners, and cracked, chipped, or missing paint

External Skin Areas, cont’d• Piano-type hinges corrode due to

dissimilar metal contact• Steel pin• Aluminum hinge

• Natural traps for dirt, salt, and moisture

• Inspection of hinges include several cycles of water-displacing lubricants

Internal Aircraft Concerns• Cargo areas has several moisture traps

• Cargo tie downs• Behind seats• Cargo wall installation• Cargo ramp

Miscellaneous Trouble Areas• Helicopter Rotor Heads and Gearboxes

• Bare steel surface exposed to the elements

• Many external parts• Dissimilar metal contacts

• Corrosion can be prevented by• Proper maintenance• Lubrication• The use of preservative coatings

Miscellaneous Trouble Areas• Control Cables

• Should be inspected to determine their condition at each inspection period

• Inspect cables for corrosion by cleaning with solvent soaked cloths

Environmental Factors• Plays a large role in corrosion• Easily can start corrosion on a

substrate• Coating system to failure can

start • Common Corrosive Agents

• Acids• Alkalis• Salts• Water • Microorganisms

Environmental Factors• Marine Environment

• Contains chlorides in the form of salt particles or droplets of salt-saturated water

• Salt solutions are electrolytes and corrosively attack aluminum and magnesium alloys

Environmental Factors• The Atmosphere

• Corrosive agents are oxygen and airborne moisture

• May also contain other corrosive gases and contaminants from industrial and marine environments

Environmental Factors• Industrial Atmosphere

• Most common contaminant is oxidized sulfur compounds

• Sulfur compounds combine with moisture and form sulfur-based acids that are highly corrosive to metals

Ozone Environment Effects

Summary• Corrosion is a continuous process that occurs

everywhere

• Different alloys have different tendencies to corrode

• Prevention starts with knowing corrosion prone areas

• Environmental factors play a significant role in corrosion