CLEANING & CORROSION CONTROL

CLEANING & CORROSION CONTROL

Aviation Maintenance Tech Handbook Ch.8

APPEARANCE OF CORROSION ON DIFFERENT METALS

Steel

Aluminum Alloys & Magnesium

Copper & Copper Alloys

CORROSION CHARACTERISTICS BY METAL TYPE

CORROSION CHARACTERISTICS BY METAL TYPE CONT.

CORROSION CHARACTERISTICS BY METAL TYPE CONT.

FACTORS AFFECTING CORROSION

Metal Properties

Enviromental Conditions

Electro- chemical Factors

FACTORS AFFECTING CORROSION

Climate

Manufacturing Processes

Foreign Material

Geo- graphical Location

Micro- organisms

NORTH AMERICA CORROSION SEVERITY CHART

This map illustrates the varying corrosion risk levels across North America. Coastal regions, particularly in the southeast and Gulf Coast, show the highest corrosion severity due to salt air exposure and high humidity.​

TYPES OF CORROSION ATTACK

Electrochemical Attack

Direct Chemical Attack

Direct chemical attack, or pure chemical corrosion, is an attack resulting from direct exposure of a bare surface to caustic liquid or gaseous agents.​ This type of damage can occur when aircraft surfaces are exposed to:

• Battery acids
• Hydraulic fluids
• Engine exhaust gases
• Industrial pollutants

FORMS OF CORROSION

COMMON CORROSIVE AGENTS

Acids

Alkalines

Salts

Water

Atmosphere

PREVENTATIVE MAINTENANCE

Weather Protection

Thorough Lubrication

Regular Cleaning

Detailed Inspections

Critical Area Wipe Down

Drainage & Ventilation

Prompt Treatment

Record Keeping

Proper Materials & Training

Finish System Maintenance

Daily Fuel Sump Draining

Gasket & Sealant Maintenance

CORROSION PRONE AREAS

Battery Compartments

Exhaust Trail Areas

Bilge Areas

Lavatories & Galleys

Wheel Wells & Landing Gear

Water Entrapment Areas

CORROSION PRONE AREAS

Wing Flap & Spoiler Recesses

Engine Areas

External Skin Areas

Electronic Compartments

Miscellaneous Trouble Areas

CORROSION INSPECTION TECHNIQUES

Non-Destructive Inspection (NDI)

Visual Inspection

The most widely used and effective method for the detection and evaluation of corrosion. Enhanced by:​

• Proper lighting (including UV for some applications
• Magnifying glasses and borescopes
• Mirrors for accessing hidden areas
• Tactile inspection (sense of touch)

Advanced methods for detecting hidden corrosion:

• Liquid penetrant testing
• Magnetic particle inspection
• Eddy current testing
• X-ray inspection
• Ultrasonic testing
• Acoustical emission testing

CORROSION REMOVAL

Titanium & Alloys

Surface Preparation

Ferrous Metals

Protection of Dissimilar Metal Contacts

Mechanical Methods

Aluminum & Alloys

Magnesium Alloys

CORROSION LIMITS AND CLASSIFICATION

Negligible Damage

Damage Repairable by Patching

Light corrosion that can be removed by light hand sanding or a change in preventative maintenance practices. No structural implications

Corrosion damage exceeding the limits of negligible damage but not severe enough to require replacement of parts. It can be repaired by reinforcing the area.

Damage Necessitating Replacement

Damage Repairable by Insersion

Corrosion damage requires the removal of the affected area and replacement with a new section. Original strength much be restored.

Corrosion damage is so severe that repair is impractical or uneconomical. The entire part must be replaced.

Summary: Key TakeAways

Corrosion Fundamentals

Corrosion Types & Identification

Corrosion is the deterioration of metal by chemical or electrochemical attack. If left unchecked, it can cause structural failure. Different metals exhibit characteristic corrosion patterns and products.​

Understanding the various forms of corrosion—from surface and pitting to intergranular and stress corrosion—is essential for proper identification and treatment.​

Ongoing Vigilance

Prevention & Treatment

Effective corrosion control combines preventive maintenance, proper inspection techniques, and appropriate treatment methods specific to each metal type.​

Corrosion control is not a one-time task but requires continuous attention throughout the aircraft's service life. Regular inspections and prompt treatment of any findings are essential.​

Exfoliation Corrosion

Exfoliation corrosion is an advanced form of intergranular corrosion that occurs in wrought aluminum alloys with highly directional grain structures.​ Characteristics:

• Corrosion products force metal layers apart
• creates a leaf-like or layered appearance
• Most common in high-strength aluminum alloys (2000 and 7000 series)
• Often occurs at edges of sheet, extrusions, or around fasteners

Corrosion on Steel

Creates a reddish corrosion byproduct commonly referred to as rust.

Corrosion on Copper & Copper Alloys

Due to water chemistry, particularly acidity, copper corrodes with a blue-green stain.

Manufacturing Processes

The manner in which metals are formed, joined, and finished affects their corrosion susceptibility.

Water

Acts as an electrolyte, enabling electrochemical corrosion. Impurities in water increase its conductivity and corrosiveness.

Metal Properties

• Type of metal
• Heat Treatment and grain direction
• Pure metals vs. alloys

Climate

Temperature and humidity levels have a direct impact on corrosion rates. High-humidity environments accelerate the corrosion process.

Dissimilar Metal Corrosion

Dissimilar metal corrosion occurs when two different metals are in electrical contact with each other in the presence of an electrolyte. The less noble metal (anode) corrodes at an accelerated rate while the more noble metal (cathode) is protected. Factors affecting severity:

• Relative position of metals in the galvanic series
• Size ratio between cathode and anode
• Conductivity of the electrolyte

Salts

Sodium chloride (sea salt) is the most common and damaging. Salt deposits absorb moisture and form highly conductive electrolytes.

Filiform Corrosion

Filiform corrosion occurs under painted or plated surfaces when moisture penetrates the coating. It appears as thread-like filaments spreading out from the original corrosion site.​ Characteristics include:

• Typically occurs in high humidity environments (above 70%)
• Common in aluminum and magnesium alloys
• Filaments do not cross each other
• More cosmetic than structurally damaging​

Alkalines

Caustic compounds that can attack aluminum and magnesium alloys. Found in some cleaning compounds and battery electrolytes.

Concentration Cell Corrosion: Metal Ion Cells

This type of corrosion occurs when one part of a metal is exposed to a different concentration of its own ions than another part. Common in:

• Crevices where stagnant solution can develop
• Areas with limited oxygen access
• Under deposits or dirt accumulation

The area with the lower metal ion concentration becomes anodic and corrodes more rapidly.

Enviornmental Conditions

• Temperature
• Availability of oxygen
• Presence of biological organisims
• Mechanical Stress

Atmosphere

Industrial pollutants, volcanic ash, and salt air create corrosive environments. Humidity accelerates the corrosion process.

Magnesium Alloys

Treatment of Installed Magnesium Castings

Treatment of Wrought Magnesium Sheet & Forgings

Magnesium alloys are highly susceptible to corrosion, but are used in aircraft for their light weight. Treatment procedures include:

1. Clean the area thoroughly to remove all contamination.
2. Remove corrosion products using mild mechanical methods
3. Apply chemical treatments specific to magnesium.
4. Apply protective coatings immediately after treatment.

Magnesium corrosion appears as white, powdery deposits and can progress rapidly if not treated. Chromic acid solutions are commonly used for treating magnesium corrosion, but environmental regulations are limiting their use.

Aluminum & Aluminum Alloys

Treatment of Unpainted Aluminum Surfaces

Treatment of Intergranular Corrosion

For heat-treated aluminum showing intergranular corrosion:

1. Remove all visible corrosion products.
2. Check for intergranular corrosion using eye penetrant.
3. Remove all intergranular corrosion by mechanical means.
4. Verify remaining material meets thickness requirements
5. Apply protective treatments and finishes.

Severe cases may require replacement of the affected part if the minimum thickness cannot be maintained.

For unpainted surfaces showing light corrosion: Clean the area with a mild abrasive (Scotch-Brite or fine aluminum wool) Apply chemical conversion coating (aldine) Apply protective oil or wax coating if surface will remain unpainted.

Treatement of Anodized Surfaces

Anodized surfaces require special care:

1. Clean with mild abrasives to avoid removing the anodic coating
2. Use chemical conversion coating to restore protection.
3. Re-anodize if extensive damage has occurred.

Intergranular Corrosion

Intergranular corrosion is an attack along the grain boundaries of a metal. It occurs when the grain boundaries are more reactive than the grains themselves. Particularly problematic in:

• heat-treated aluminum alloys (2000 and 7000 series)
• Improperly heat-treated stainless steels
• Areas affected by welding heat

This type of corrosion can progress undetected and cause significant structural damage with little surface evidence.

Galvanic Corrosion

Galvanic corrosion occurs when dissimilar metals are in electrical contact in the presence of an electrolyte. The more active metal becomes the anode and corrodes at an accelerated rate. Factors affecting severity:

• Distance between metals in the galvanic series
• Area ratio (cathode to anode)
• Electrolyte conductivity
• Temperature

Electrochemical Factors

• Presence of dissimilar metals
• Anodic and cathodic surface areas
• Presence of electrolytes

Surface Corrosion

Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products.​It is caused by direct chemical attack on a metal surface or by electrochemical attack, which is the most common.​ Surface corrosion may spread under the surface if not treated, leadingto more serious forms of damage.​

Surface Preparation

Surface Cleaning and Paint Removal Before corrosion can be treated, surfaces must be properly prepared:

• Remove all contaminants (oil, grease, dirt)
• Strip paint from the affected area
• Ensure the area is completely dry
• Identify the full extent of corrosion damage

The size of the area to be stripped depends on the extent of corrosion and the type of metal involved.

Geographical Location

Coastal areas with salt spray create highly corrosive environments. Industrial areas with air pollution also increase the risk of corrosion.

Foreign Material

Dirt, oil, grease, and exhaust residues can trap moisture against metal surfaces, promoting corrosion.

Aluminum

Aluminum corrosion is a chemical process in which the metal degrades due to reactions with oxygen and moisture, forming a white, chalky layer of aluminum oxide.

Magnesium

Magnesium corrosion is a chemical reaction between the highly reactive metal and its environment, leading to the formation of dull, flaky white or grey corrosion products like magnesium oxide and hydroxide.

Pitting Corrosion

Pitting corrosion is a localized form of corrosion that creates small holes or pits in the metal surface. It is one of the most dangerous forms of corrosion because:

• it can be difficult to detect due to small surface openings
• pits can penetrate deeply into the material
• it can lead to stress concentration and fatigue cracking
• the actual extent of damage is often greater than visible surface indications

Concentration Cell Corrosion: Oxygen Cells

This occurs when a difference in oxygen concentration exists between two areas of the same metal. The area with less oxygen becomes anodic and corrodes, while the area with more oxygen becomes the cathodic and is protected. Common locations:

• under gaskets or seals
• under accumulated debris or dirt
• inside lap joints and spot welds
• under paint films and washers

Common Corrosive Agents

Strong and weak acids attack most metal systems. Aircraft battery acid (sulfuric acid) is particularly damaging to structures and components.

Micro-organisims

Certain bacteria and fungi can accelerate corrosion through their metabolic processes.

Titanium & Titanium Alloys

Titanium alloys are highly corrosion-resistant but require special consideration.

1. Normally resistant to corrosion and require only degreasing and cleaning.
2. Can develop surface oxidation at high temperatures above 700 F.
3. Must not be cleaned with chlorinated solvents at elevated temperatures.
4. Avoid contact with cadmium-plated toos or mercury.

Stress Corrosion/Cracking

Stress-corrosion cracking (SCC) results from the combined effect of sustained tensile stress and a corrosive environment on a susceptible material.​ Key Characteristics:

• cracks propagate perpendicular to the applied stress
• can occur at stress levels well below the material’s yield strength
• often shows little evidence of corrosion
• particularly dangerous in high-strength alloys

Common in wing spars, engine mounts, and landing gear components.

Corrosion of Ferrous Metals

Mechanical Removal of Iron Rust

Chemical Removal of Iron Rust

Use wire brush, abrasive paper, or abrasive wheels to remove surface rust. For heavy rust, use chipping hammers and scrapers with care to avoid damaging the base metal.​

Phosphoric acid-based rust removers convert iron oxide to phosphate compounds. These must be thoroughly rinsed after use to prevent residual acid from causing further corrosion.​

Chemical Surface Treatment

Highly Stressed Steel Parts

After rust removal, apply chemical surface treatments like phosphate conversion coatings to improve corrosion resistance and paint adhesion.​

Special care is required for critical components. Avoid using acidic rust removers on highly stressed parts, springs, or parts with case-hardened surfaces.​

Fretting Corrosion

Fretting corrosion occurs when two mating surfaces experience light relative motion under load. The protective oxide films are mechanically removed, exposing fresh, reactive metal surfaces.​ Common locations:

• press-fit joints
• bearing surfaces
• bolted or riveted joints
• splined connections

Often appears as a reddish-brown or black powder at the interface.

Concentration Cell Corrosion: Oxygen Cells

This type of corrosion occurs on metals that can exist in either an active or a passive state, such as stainless steels and aluminum alloys.​ The passive film (usually an oxide) protects the metal; however, if it’s damaged in a small area, that area becomes anodic to the larger passive area, resulting in rapid, localized corrosion. Common causes of passive film damage:

• mechanical scratches
• chemical attack (especially chlorides)
• high temperature

Mechanical Methods

Fairing or Blending Reworked Areas

After corrosion removal, affected areas often need to be blendedto maintain aerodynamic properties and structural integrity:​

• Remove all traces of corrosion and corrosion products
• Blend the area to eliminate sharp edges and stress concentrations
• Ensure the remaining material meets the minimum thickness requirements
• Apply appropriate protective treatments and finishes

Fatigue Corrosion

Fatigue corrosion is the combined effect of cyclic stress and corrosion. The corrosion process accelerates fatigue crack initiation and growth.​ Key points:

• reduces fatigue life significantly compared to either factor alone
• corrosion pits often serve as stress concentration points
• particularly dangerous in primary structural components
• can occur at stress levels well below normal fatigue limits

Protection of Dissimilar Metal Contacts

Contacts NOT Involving Magnesium

When dissimilar metals must be in contact:

1. Apply protective coatings to both metals.
2. Use sealants or tapes to prevent moisture access.
3. Install nonabsorbing washers or gaskets between metals.
4. Apply primer and paint to the assembled structure.

Contacts Involving Magnesium

Magnesium requires additional protection.

1. Apply a chromate treatment to the magnesium.
2. Apply two coats of primer to the magnesium.
3. Use stainless fasteners with nonabsorbing washers.
4. Seal all fraying surfaces with sealant.
5. Apply exterior protective finish.