Corrosion 1: Aircraft Corrosion Protection C1

• Aircraft are made of many materials, with corrosion being the most insidious form of damage to metal structures or components.
• From the moment metal is manufactured, it requires protection from adverse environmental effects.

Corrosion Protection

• Protection involves introducing certain elements into the base metal to create a corrosion-resistant alloy.
• The addition of a surface coating like a chemical conversion coating, metal, or primer and paint is a form of protection.
• Also provides additional moisture barriers, such as viscous lubricants and protectants added to the surface, act as another form of protection.
• Corrosion is a natural process where metal returns to a metallic compound like oxide, hydroxide, or sulfate.
• Aluminum needs refining from alumina ore, as it's never found in its pure state.
• Pure aluminum exposed to elements combines with oxygen, reverting to alumina.
• Corrosion is a process where metals return to their natural state through chemical or electrochemical attacks.
• Metal deterioration occurs internally on the surface and may change the smooth surface, weakening the interior or loosening adjacent parts.

Electrochemical Attack

• Electrochemical attack involves an electrical potential difference within the same metal or between different metals in contact with each other and in the presence of an electrolyte.
• Electrochemical attack is a primary cause of corrosion in aircraft structures and components.
• Electrochemical corrosion needs four conditions to occur:
  • A metal subject to corrosion (anode).
  • A dissimilar conductive material (cathode) that is less prone to corrosion.
  • A continuous, conductive liquid path (electrolyte)
  • Electrical contact between the anode and cathode
• Electrochemical corrosion stops can be accomplished by removing, any one of these conditions

Electron Transfer in Electrochemical Corrosion

• Corrosion is based on electron transfer between two metals, where one metal (anode) is converted back to a metallic compound like oxide, hydroxide, or sulfate.
• The anode is often described as "eaten away".
• When two metals contact in the presence of an electrolyte, the less active metal becomes the cathode and attracts electrons from the corroding anode.
• Metals and alloys have specific electrical potentials in a chemical environment.
• A metal's nobility refers to this potential.
• The less noble (more anodic) a metal, the easier it corrodes.
• A less noble metal gives up electrons more easily than a more noble metal.
• The further apart metals are on the nobility chart, the more likely the anode corrodes in a conductive liquid (electrolyte).

Direct Chemical Attack

• Direct chemical attack, or pure chemical corrosion, results from direct exposure of a bare surface to caustic liquid or gaseous agents.
• Caustic Liquid = a corrosive substance, either acidic or alkaline.
• Gaseous Agent = a fume or vapor, either acidic or alkaline.
• Since corrosive liquids and gaseous agents can harm living tissue, PPE is important.
• The changes in direct chemical attack occur simultaneously at a single point, unlike electrochemical attack.
• Common agents causing direct chemical attacks on aircraft:
  • Spilled battery acid or fumes from batteries
  • Residual flux deposits from inadequately cleaned, welded, brazed, or soldered joints
  • Entrapped caustic cleaning solutions
• Corrosion occurs where the spilled liquid or gas exposure has taken place.
• Aircraft commonly experience direct chemical corrosion in and around the battery box.
• An acid-proof paint is used as a preventative measure for corrosion in the battery box area.

Factors Affecting Corrosion

• Metal type
• Heat treatment and grain direction
• Presence of a dissimilar, less corrodible metal
• Anodic and cathodic surface area
• Temperature
• Presence of an electrolyte
• Availability of oxygen
• Presence of biological organisms
• Mechanical stress
• Time of exposure to a corrosive environment
• Lead/graphite pencil marks
• Water or water vapor containing salt combines with oxygen in the atmosphere, which is the main source of aircraft corrosion.
• Aircraft operating in marine or industrial fume environments are susceptible to corrosive attacks.
• Aircraft environments are categorized as mild, moderate, or severe based on corrosion severity.
• Many factors increase corrosion severity, including airborne industrial pollutants, chemicals used on runways, humidity, temperature, and prevailing winds.
• Suggested frequencies for maintenance:
  • Mild zones: every 90 days
  • Moderate zones: every 45 days
  • Severe zones: every 15 days
• Urea, potassium acetate, sodium formate, and potassium formate are used to deice runways and new ice control chemicals are also being developed.

Forms of Corrosion

• Many forms of corrosion can occur that depend on the metal, its size, shape, function, atmospheric conditions, and corrosion-producing agents.
• General surface corrosion, or uniform corrosion, is the most common type.
• Surface corrosion appears as a general roughening, etching, or pitting, with a powdery deposit.
• Surface corrosion may be caused by direct chemical or electrochemical attack.
• Sometimes, corrosion spreads under surface coatings.
• Improperly removed paint stripper/metal prep products can cause uniform corrosion.

Localized Corrosion

• Localized corrosion involves the selective removal of metal in small areas or zones on a metal surface exposed to a corrosive environment.
• Localized corrosion involves intense attacks in confined areas of surface components.
• This can be the result of environmental effects or material properties, like the creation of protective film oxide.

Filiform Corrosion

• Filiform corrosion is a special case of oxygen concentration cell corrosion, occurring on metal surfaces with an organic coating system.
• It's characterized by wormlike traces of corrosion beneath the paint film.
• Filiform corrosion occurs at 78-90% relative humidity and slightly acidic conditions.
• This corrosion attacks steel and aluminum surfaces.
  • On steel, the traces do not cross.
  • On aluminum, traces cross, making the damage deeper.
• Untreated filiform corrosion leads to intergranular corrosion, especially around fasteners and seams.

Pitting Corrosion

• Pitting corrosion is a destructive and intense form of corrosion.
• Pitting occurs in any metal, but is most common in metals that form protective oxide films, such as aluminum and magnesium alloys.
• Noticeable as a white or gray powdery deposit.
• When cleaned away, tiny holes or pits are seen.
• Can penetrate deeply into structural members, causing disproportionate damage to the structure's appearance.

Galvanic Corrosion

• Galvanic corrosion is caused when two dissimilar metals make electrical contact in the presence of an electrolyte.
• The more different the electrical potential, the faster the corrosion.
• Rate of galvanic corrosion depends on the size of the parts in contact.
• If the corroding metal's surface area is smaller than that of the less active metal, corrosion is sped up.
• Extensive pitting damage results from contact between dissimilar metal parts with a conductor.
• Galvanic action occurs at contact points where insulation has failed/been omitted.
• Electrochemical attack can be severe since it often occurs out of sight.
• The only detection method is by disassembly and inspection.
• Contamination of a metal surface by mechanical means can cause dissimilar metal corrosion.
• The improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminum or magnesium, can embed steel particles into the metal, causing corrosion.
• Abrasive pads used on one type of metal are not to be used on a different metal surface.

Concentration Cell Corrosion

• Concentration cell corrosion means parts of a metal surface deteriorate at different rates.
• Different deterioration rates are because of the surface connecting with different concentrations of the same electrolyte.
• Differing concentrations create a difference in electrical potentials across a metal surface.
• Concentration cell corrosion, or crevice corrosion is due to one of these general corrosion types:
  • Corrosion of metals in a metal-to-metal joint
  • Corrosion that happens at the edge of a joint even though the joined metals are identical
  • Corrosion of a spot on the metal surface covered by a foreign material

Metal Ion Concentration Cell

• A potential difference can occur if an electrolyte having a nonuniform concentration of metal ions covers the surface.
• A high concentration of metal ions exists under faying surfaces where the solution is stagnant.
• A low concentration of metal ions exists adjacent to the crevice, created by the faying surface.
• The area of the metal in contact with the low concentration of metal ions is anodic and corrodes.
• The area in contact with the high metal ion concentration is cathodic and does not show signs of corrosion.

Oxygen Concentration Cell

• The electrolyte in contact with the metal surface generally contains dissolved oxygen.
• Concentration cell can develop where the oxygen in the air is not reaching the surface, causing differences in oxygen concentration.
• Typical locations of oxygen concentration cells are under gaskets, wood, rubber, and other materials in contact with the metal surface.
• Corrosion occurs where the anode is, that is area of low oxygen concentration.
• Alloys like stainless steel tend to have this type of crevice corrosion.

Active-Passive Cell

• Metals depending on an oxide for corrosion protection are prone to rapid corrosive attack by active-passive cells.
• starts as an oxygen concentration cell.
• The passive film is broken by a dirt particle exposing the active metal so corrosion.
• Electrical potential develops between the large, passive area and the small, active area, leading to pitting.

Intergranular Corrosion

• Intergranular corrosion is an attack along the grain boundaries of an alloy.
• Commonly from lack of uniformity in the alloy structure.
• Aluminum alloys and some stainless steels are susceptible to it.
• Lack of uniformity is due to alloy changes from heating and cooling during manufacturing.

Exfoliation Corrosion

• Exfoliation corrosion is an advanced form of intergranular corrosion.
• It is seen by lifting surface grains of metal by the force of expanding corrosion products at grain boundaries.
• Often seen on extruded materials, like spars, channels, or angles, where the grain structure is more laminar.
• This type of corrosion is hard to detect early on.
• Ultrasonic and eddy current inspection methods are used with excellent results.

Stress Corrosion

• Stress corrosion or cracking involves the combination of constant or cyclic stress with a damaging chemical environment.
• Stress can be internal or external.

Internal Stress

• Internal stress can be trapped during manufacturing.
• Processes such as cold working or unequal cooling from high temperatures.
• Stress relief operations are followed to avoid this.

External Stress

• External stress can also occur by riveting, welding, bolting, clamping, press fit, etc.
• Over-torquing is also a cause.
• Internal stress is more important than design stress.
• This is because stress corrosion is hard to recognize before it overcomes the design safety factor.

Fatigue Corrosion

• Fatigue corrosion combines cyclic stress with a corrosive environment.
• Metals can undergo cyclic stress for an infinite amount when the stress is below the endurance limit.
• When the limit is exceeded, the metal shows cracks and failure from metal fatigue.
• When cyclic stress happens within a corrosive environment, the stress level may get reduced.
• Failures generally occur at dangerously low stress, based on the number of cycles assigned to the life limited part.
• Fracture of metal from fatigue corrosion mostly occurs at a stress level lower than the fatigue limit of metal that is not corroded.

Fretting Corrosion

• Fretting corrosion happens when two surfaces, normally at rest in relation to each other, show slight relative movement.
• It is categorized by pitting and the creation of a fine debris, which increases deterioration in the presence of water vapor.
• Movements between the two surfaces prevent debris from escaping.
• The result is an extremely localized abrasion.
• The most common example is a smoking rivet found on the engine cowling/wing skins.
• A smoking rivet is identified by a black ring around the rivet.

Oxidation

• Oxidation is the simplest form of corrosion, that is, dry corrosion.
• Aluminum exposed to gases containing oxygen forms aluminum oxide.
• Oxide film appears as a white powder substance and can slow oxygen reactions.
• This is not the case for steel or iron with iron oxide (rust).

Microbiological Growth

• Micro-organisms (slimes, molds, and fungi) can grow on damp surfaces, increasing corrosion.
• Micro-organism growth inside jet fuel tanks can cause scum on the bottom and concentration cell-type corrosion.