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Corrosion: Public enemy #1
This unit of work addresses aspects of the following syllabus outcomes:
A student:
H 1.2. differentiates between the properties of materials and justifies the selection of materials, components and processes in engineering.
Extract from Engineering Studies Stage 6 Syllabus © Board of Studies NSW 1999
By studying the following pages and visiting the web links students will learn to identify the types of corrosion attacking aircraft materials, their causes, methods of inspecting aircraft structure for corrosion damage and methods employed to protect aircraft materials from corrosion damage.
Corrosion: the enemy (original picture by author)
Introduction
Timber was one of the first materials used to make a powered, manned flying machine. Timber presented its own problems, borers, dry rot, and changing properties due to moisture content and of course, deterioration of the glues and adhesives of the time. The timber framed aircraft today is solely the domain of home builders. Although it was used in early jet fighters, the fuselage of the DeHavilland Vampire being an example, it is rarely used today.
The Germans pioneered metal aircraft production for the huge “airships” or Zeppelins designed to provide luxury travel in a style similar to that provided by the ocean liners of the day. Timber was quite unsuitable as long lengths of material of consistent quality were required to produce these huge structures.
Pure aluminium with a tensile strength of 30 MPa did not have the physical properties necessary for the requirements of aviation. Dr Alfred Wilm, a German scientist, was testing aluminium alloys as early as 1906 and found that introducing alloying elements in relatively small quantities imparted superior characteristics to the metal. The major alloying element used in those early alloys was copper, and a new term was coined, “duralumin” or dural as it was commonly known, to describe the new material. Duralumin should really be Düralumin, since it was originally the product of the Dürener Metallwerke in Germany[1].
The modern material derived from the original dural is
2024 
an aluminium alloy, with copper as the major alloying
element and a tensile strength of 450 MPa in its heat treated
state. Its strength compares most favourably with steel but it is
30% less dense.
The trade off was that the introduction of an alloying element severely reduced the corrosion resistant properties of the pure aluminium. The problem was largely resolved by producing alclad. A thin slab of pure aluminium was placed on either face of the alloy ingot during the rolling process The final sheet material has a thin coating of corrosion resistant pure aluminium on each face (See figure 1). This thin coating is impervious and provides protection to the aluminium-copper alloy as it excludes moisture (electrolyte) from the metal it covers. An example of this protection is the statue of Eros in Picadilly Circus in London.
Figure 1 Magnified section of alclad material (original picture by author)
Corrosion still attacks all metal structures. Engineers, however, have been able identify the causes of corrosion, and introduce measures to control and prevent corrosion and produce aircraft with long and safe service lives. Statistics confirm that through preventative maintenance and rigorous inspection procedures corrosion damage is not a significant factor in aircraft structural failure.
Open the Statistical Summary of Commercial Jet Airplane Accidents – Worldwide Operations 1959 – 2002 Examine the table Accident Categories by Airplane Generation. Compare and discuss the causes of accidents with particular emphasis on the cause “Aircraft Structure”.
Discuss the lessons learned by earlier aircraft accidents with the De Havilland Comet (Accident Reports).
Look at the causes of the Aloha incident (1988)
What is corrosion
Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action and converts it back to a metallic compound.
Four conditions must exist before electrochemical corrosion can occur. (See figure 2.) They are:
- A metal that is going to be subject to corrosion (Anode) must be present.
- Another dissimilar conductive material (Cathode), which has less tendency to corrode must be present.
- There must be a continuous, conductive liquid path (Electrolyte).
- There must be electrical contact between the anode and the cathode (usually in the form of metal-to-metal contact such as rivets, bolts, wire, etc).
Figure 2 Corrosion cell showing
conditions that must exist for electrochemical corrosion
(original picture by author)
Elimination of any one of these conditions will stop electrochemical corrosion. (See figure 3.)
Figure 3 Eliminating corrosion with a protective film (original picture by author)
NOTE: Paint can mask the initial stages of corrosion. Since corrosion products occupy more volume than the original metal, painted surfaces should be inspected often for irregularities such as blisters, flakes, chips and lumps.
Factors influencing corrosion
Factors which influence metal corrosion and the rate of corrosion are:
- Type of metal
- Heat treatment and grain direction
- Presence of a dissimilar, less corrodible metal
- Anodic and cathodic surface areas (in galvanic corrosion)
- Temperature
- Presence of electrolytes (hard water, salt water, battery fluids, etc.)
- Availability of oxygen
- Presence of biological organisms
- Mechanical stress on the corroding metal
- Time of exposure to a corrosive environment.
Most pure metals are not suitable for aircraft construction and are used only in combination with other metals to form alloys. Most alloys are made up entirely of small crystalline regions, called grains. Corrosion can occur on surfaces of grains which are less resistant and also at boundaries between grains, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most active metals, (those which lose electrons easily), such as magnesium and zinc, corrode easily. The most noble metals (those which do not lose electrons easily), such as gold and silver, do not corrode easily.
Metals can be listed in a Galvanic Series. Metals at the “Noble” end of the series (most cathodic), in contact with another metal lower on the scale (more anodic) will cause the more anodic metal to corrode.
Aluminium, which is an active metal, does not corrode easily due to the formation of a natural oxide coating. This coating is hard and impermeable and chemically non-active. Therefore it acts to reduce the action of other corrosive elements on the aluminium itself.
Visit http://www.rfcafe.com/references/general/galvanic_series.htm, print out the galvanic series table on the page. From your study of materials used in aircraft construction highlight those materials in the galvanic series and provide examples of areas where dissimilar metals may need to be joined, and possible methods of reducing galvanic action between those metals.
Fasteners used in aircraft production are made from a variety of materials. Take a look at http://www.hi-shear.com/fastener_hl.htm , a page on the HI-Lok® Fastening Systems web site. List what coatings are placed on their fasteners to reduce galvanic action?
2. Heat treatment and grain direction
The solution heat treatment of aluminium alloys is a critical process. If an alloy is heated to an incorrect temperature, or allowed to cool before quenching, alloying elements come out of the solid solution and crystallise in the grain boundaries providing a galvanic couple for corrosion to take place.
Figure 4 Critical heating temperature from which quenching must take place during solution heat treatment (Source: Gibson, J (1977) Equilibrium diagrams. Clarendon: Kensington)
Typically, aluminium alloy heat treatment furnaces are placed in an elevated position above the quenching tank. Parts are loaded from the underside so that when opened the hot air remains in the furnace. Hot components are then dropped from the furnace into the quenching tank with a minimum of heat loss during transfer from furnace to tank.
Figure 5 Critical heating temperature
obtained by rapid quenching solution heat treatment .
(original picture by author)
Rolled aluminium alloy sheet has long grains due to the elongation of the grains during the rolling process (figure 6). A similar elongation occurs during the extrusion process. The potential for large areas containing dissimilar metals is evident.
Figure 6 Grains are elongated in the
rolling process.
(original picture by author)
3. Presence of a dissimilar, less corrodible metal
There is a need to use dissimilar metals where specific properties are required. Typical examples would be the leading edges of aircraft flying at speeds greater than the speed of sound, landing gear components, structural fasteners and balance weights where lead, tungsten and even depleted uranium have been used.
4. Anodic and cathodic surface areas (in galvanic corrosion)
Surface areas on aircraft may contain quantities of the same material from a different manufactured batch or indeed from a different supplier. Even minor differences will produce the anodic and cathodic areas necessary to initiate corrosion in the right circumstances.
Corrosion is accelerated by high temperature environments that increase the rate of chemical reactions and increase the concentration of water vapour in the air.
6. Presence of electrolytes (hard water, salt water, battery fluids, etc.)
Electrolytes (electrically-conducting solutions) form on surfaces when condensation, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic gases and engine exhaust gases can dissolve on wetsurfaces, increasing the electrical conductivity of the electrolyte, thereby increasing the rate of corrosion. Fluid spills are a particular concern in bilge areas of commercial jet transport aircraft.
When some of the electrolyte on a metal surface is partially confined, (such as between flying surfaces or in a deep crevice) the metal around this area corrodes more rapidly. This type of corrosion is caused by the formation of an oxygen concentration cell. Corrosion occurs more rapidly because the reduced oxygen content of the confined electrolyte causes the adjacent metal to become anodic to other metal surfaces on the same part that are immersed in oxygen rich electrolyte or exposed to air.
8. Presence of biological organisms
Slime, moulds,
fungi and other living organisms (some microscopic) can grow on
damp surfaces. Once they are established, the area usually
remains damp, increasing the possibility of corrosion.
Biological organisms are a particular problem inside integral
fuel cells where tanks have been filled or topped up with
contaminated fuel. Water contamination in fuels adds to the
problem as the organisms absorb the water and form an electrolyte
against the surfaces on which they grow.
9. Mechanical stress on the corroding metal
Manufacturingprocesses such as machining, forming,
welding or heat treatment can leave residual stress in aircraft
parts and can cause cracking in a corrosive environment.
It is important to note that corrosion of a component reduces the
strength of that component, and while the stress remains
constant, the component becomes weaker thus increasing the
likelihood of failure.
10. Time of exposure to a corrosive environment
Time
plays an important role, particularly in the treatment of
corrosion. A light aircraft making a forced landing on a beach
would become contaminated with salt water, and a major inspection
to remove all traces of the salt would be necessary.
Light aircraft complete mandatory 100 hourly inspections and the
engineer completing the inspections thoroughly inspects the
aircraft for signs of corrosion, right down to inspecting control
cables for signs of corrosion between the strands.
A system of reporting is in place throughout
the world where damage detected is reported, and those reports
are circulated by the aircraft manufacturer and the airworthiness
authorities to owners and operators of similar aircraft. Owners
are then required to carry out inspections on those similar
aircraft for similar types of damage.
Common corrosive agents
Any substance that causes corrosion is called a corrosive agent. Common corrosive agents are acids, alkalies, salts and organic acids. All will severely corrode metals used in aircraft. The atmosphere, and water, may carry these as dissolved minerals.
The most destructive acid is sulfuric acid (used in lead-acid batteries).
Organic acids are found in the wastes of humans and animals and these may be present in certain parts of an aircraft.
Aluminum and magnesium alloys are prone to corrosive attack by many alkaline solutions such as washing soda, potash (wood ashes) and lime (cement dust).
Corrosion often results from the direct action of atmospheric oxygen and moisture on metal. Atmospheric moisture often accelerates corrosive attack, particularly on ferrous alloys, and often contains other corrosive contaminants, particularly industrial pollution and marine salt spray.
Micro-organisms such as bacteria and fungi are serious corrosion agents. They become a particular problem inside aircraft fuel tanks.
Identifying corrosion
Corrosion can be categorised into groups based on the ease of identification.
- Corrosion that is readily identifiable by visual
inspection
We should all be familiar with uniform
corrosion (direct chemical attack). A typical example is
surface rust on steel left exposed to the atmosphere. Aluminium
is highly resistant to most forms of corrosion. The metal's
natural coating of aluminium oxide provides a highly effective
barrier to the ravages of air, temperature, moisture and chemical
attack.
Additional information on uniform corrosion
Localised damage to the protective coating on a surface,
combined with water chemistry factors cause pitting
corrosion, a form of corrosion where holes or cavities are
eroded into the material.
Poor application of a protective coating or contamination of the coating product may also cause pitting corrosion.
Pitting is harder to detect than uniform corrosion, and may remain in a metal beneath an area from which uniform corrosion has been removed. Subsequent growth of the corrosion beneath the metal surface reduces the strength of the component forming stress risers. Fatigue and stress corrosion cracking often begins at the base of corrosion pits.
Pitting corrosion is one of the most common forms of corrosion seen on stainless steel. Iron particles deposited on the surface of stainless steel through mechanical contact with, usually carbon steels, quickly corrode and form a rust stain. If chloride ions are present this can increase an environment likely to cause pitting. Tools used when working on carbon steels should not be used on stainless steels. A common mistake, particularly in the home, is to use steel wool to clean the kitchen sink, or on light aircraft to clean exhaust fittings. Inclusion of fine steel particles of steel in the stainless steel can cause it to rust.
Additional information on pitting
Crevice corrosion is a localised form of
corrosion and tend to occur in crevices,
typically lap joints, under gaskets, washers, fastener
heads etc. It is usually associated with a stagnant solution
on the micro-environmental level and changes in local chemistry
within the crevice. The stages of crevice corrosion are shown on
http://www.corrosion-doctors.org/Forms-crevice/Crevice.htm An advanced form of crevice corrosion is called pillowing. Notice how the rivet heads appear to be lower than the surrounding skin surface.
Additional information on crevice corrosion
Filiform corrosion occurs under surface layers
such as paint. The corrosion product leaves a bubbled
trail that looks like a worm has burrowed under the paint
layer. The mode of attack is similar to pitting corrosion in that
the front of the attack is supported by moisture which penetrates
the surface layer and becomes depleted of oxygen making the area
anodic. It depends on the relative moisture of the air and the
quality of the surface treatment preparation prior to
coating.
Galvanic corrosion occurs when dissimilar
metals are in contact in the presence of an electrolyte. One of
the metals becomes the anode, and corrodes faster than it would
all by itself, the other becomes the cathode. Either metal may
not readily corrode on its own, and galvanic corrosion will not
take place without an electrolyte.The difference (and similarity) between the metals is shown on a galvanic series and metals close to one another on the scale such as aluminium and cadmium present less of a problem than would aluminium and low alloy steel. Steel fasteners used in aircraft are usually cadmium plated.
Larger problems occur in avionic equipment where gold, copper, silver and a variety of other metals are in contact.
-
Corrosion that may require a supplementary means of identification
Erosion corrosion is an acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. This type of corrosion is usually associated with fluid carrying pipe lines but can take place anywhere a fluid flows over a surface.
The type and purity of a fluid influences this type of corrosion and it may also be influenced by faulty workmanship such as burrs left at cut tube ends, or on the surfaces of sea plane floats. Spray from the undercarriage of aircraft taking off and landing contains such impurities as concrete dust, salt, rubber compounds and other pollutants and is particularly corrosive and erosive in wheel wells.
The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also cause localised turbulence and high flow velocities, resulting in erosion corrosion. A combination of erosion and corrosion can lead to extremely high pitting rates.
Additional information on erosion corrosion
Ever had trouble with your car door closing?
You’ve probably been the victim of fretting
corrosion.Where two surfaces are joined and a small amount of oxide is locked in the joint, the oxide compound forms an abrasive which, with even slight movement, perhaps caused by vibration, abrades the surface protection from the components of the joint, and corrosion escalates. With aluminium components fretting corrosion is typified by white powder in the vicinity of the joint.
Additional information on fretting corrosion
Intergranular corrosion is corrosion at
microscopic levels. When aluminium alloys are heat treated by
solution heat treatment, the alloying
elements go into a solid solution with the aluminium. Over time
with natural ageing, or by further artificial ageing by
precipitation heat treatment some of the alloying elements
precipitate from the solid solution and form a compound in the
grain boundaries, reducing corrosion resistance at that
point.Incorrect heat treatment, or quenching from below the critical area shown in figure 4 increases the likelihood of the formation of intergranular corrosion.
Once corrosion starts, the corrosion product further dislocates or distorts the adjacent grains and initiates cracking as indicated in the diagram. For a larger view and complete information click on the diagram.
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Corrosion where verification is usually required by microscopy (optical, electron microscopy etc.)
Today’s aircraft are mostly built using semi monocoque principles. Commercial aircraft are monstrous structures, the Boeing 747-400 for example has six million parts, half of which are fasteners, 274 km of wiring and 8 km of tubing.
Lift is generated for flight from its 524.9 m2 of wing area (an area large enough to hold 45 medium-sized cars) to lift a fuselage structure 6.1 metres wide which when fully loaded, weights 394 tonnes, 173 tonnes of which is fuel (216,319 Ltr). The wing tips move up and down through 5 metres with the varying loads applied to the aircraft during flight. While jacked for maintenance, one can stand on the scaffolding and easily move them up and down through 300 mm. But in testing they are really bent. Photograph by Craig Murray of wings bending on take off
It is powered by four Rolls-Royce RB211-524H2-T engines rated at 264 Km/h which accelerate it to 250 Km/h to get it off the ground then to a cruise speed of 912 km/h (Mach 0.855)
When flying at its cruise altitude 10 600 m the pressure differential between the cabin and the ambient pressure is 51.4 kPa, causing the fuselage to expand approximately 300 mm in circumference.
Departing from a tropical port where the temperature could be as high as 48.50 C it ascends to an environment where the ambient temperature is –51.3420 C. What sort of thermal expansion and contraction occurs?
Concorde flew at almost twice the altitude of the 747, 18 200 m at a speed of Mach 2.04 (2200 km/h). The differential pressure between the cabin pressure and ambient would have been greater. Flying in the troposphere, ambient air temperature is constant at –570 C. Heat generated by flight speeds heats the fuselage from between 910 and 970C causing it to expand in length by almost 300 mm. Wing temperatures range from 1050C at the leading edges down to the fuselage temperature.
Concorde’s 358.25 square metres of wing area lifts 186 880 kgs (including 9 5680 kg of fuel). While undergoing the transition between subsonic and supersonic flight, the aircraft’s centre of lift moves backward 1.8m. To maintain balance about 19.7 tonnes is transferred from forward tanks to tanks in the tail.
Stress corrosion is classified as a catastrophic form of corrosion, as it can be very difficult to detect and the damage not easily predicted. Stress corrosion or stress corrosion cracking (SCC) is cracking induced from the combined influence of tensile stress and a corrosive environment.
An example of stress corrosion cracking.
Stresses are not only those applied through flight, as we have just illustrated, but include residual stresses from manufacturing processes such as cold deformation and forming, welding, heat treatment, machining and grinding. They are further compounded by the build up of corrosion products in confined spaces.
With a stress corrosion attack, fine cracks penetrate the material, while most of the surface remains apparently free from corrosion. There is then a further build up of corrosion, weakening of the material and an escalation of the cracking.
Additional information on stress corrosion
Stress corrosion cracking may be reduced by redesigning the part with a bigger radius in the cracked corner. The larger radius distributes the stress rather than concentrating it at the corner thus reducing this type of cracking.
Fatigue corrosion is a result of the combined action of a cyclic stress and a corrosive environment. A cyclic stress could be likened to bending a coat hanger repeatedly in alternating directions. The wire will eventually fail. The theory of fatigue corrosion is that if we bend the coat hanger in a corrosive environment failure will occur earlier.
Of course with aircraft materials, the number of cycles is measured in the millions, and a safety factor, usually 2:1 is factored into component design. Components susceptible to fatigue are given a “service life” and are changed after a predetermined number of cycles.
In most cases, the fatigue process is thought to cause a break down of the protective passive film allowing corrosion to accelerate.
Fatigue is a concern in airframe repair as the tendency is to repair a cracked member with a “stronger” repair. This however results in stress, formerly being evenly distributed along the member, being concentrated at the boundaries of the repair and actually weakening the structure. Manufacturers dictate repair schemes in the Aircraft Maintenance Manual to minimise such possibilities.
Detecting corrosion
Corrosion of the first group is detected by visual inspection. Armed with the knowledge of the tell tale signs of corrosion and of the aircraft, necessary cleaning equipment, an inspection mirror and flashlight, the aircraft engineer can readily identify any corrosion damage from this group. The engineer also uses Airworthiness Directives (AD) previously identified example and Manufacturer’s Service Bulletins[3] issued to alert aircraft operators of any maintenance requirements to their aircraft.
Supplementary equipment used in the detection of the other two groups similarly requires an intimate knowledge of the aircraft maintenance documents, and that knowledge is tested before a license to certify that appropriate maintenance has been performed is issued by CASA.
A variety of different methods for the Non-destructive Testing (NDT) of components to detect corrosion are used by the aeronautical industry. Some of these may be found by visiting the HSC On-Line website at: http://www.hsc.csu.edu.au/engineering_studies/transport/non_destructive/Non_Destructive_Testing.html
Other reference material
Helicopter Rotor
Blade corrosion damage
Introduction to corrosion, presentation by Carl Locke, Professor
of Chemical and Petroleum Engineering, University of Kansas.
Boeing design for corrosion control
Stainless steel corrosion
Key to
metals corrosion of metals and their alloys
Heat
treatable aluminium alloys
http://www.engineersedge.com/galvanic_capatability.htm
http://www.corrosion-doctors.org
[1] In 1906 Dr Alfred Wilma, a German metallurgist, discovered that aluminium alloyed with copper and heat treated correctly could be made far stronger. The alloy of aluminium with 4% copper is called Duralumin and the heat treatment process is called precipitation hardening. These alloys have typically low specific gravity (around 2.7) and high strength (450 MPa). They are limited by a maximum service temperature of about 660°C. Since then, other heat treatable aluminium alloys have been developed for aircraft use. These include a range of complex aluminium-zinc alloys which develop the highest strength of any aluminium alloy.
[2] A similar corrosion defect was discovered by the author of this document and reported to the Airworthiness Authority. The aircraft with the defect had been removed from the Register and was being used for maintenance training only. As a result of the report a similar Airworthiness Directive to that used in the text was issued by CASA.
[3] Aircraft operating in Australia require a “Certificate of Airworthiness” (CofA) issued by CASA. A CofA will not be issued unless the owner of the aircraft subscribes to the aircraft Manufacturer’s Maintenance Manual, regularly updated, and receives Service Bulletins. The aircraft MUST be maintained in accordance with those documents and any AD issued by CASA.
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