The standard “Airbus AIMS 09-00-002 Maintenance materials – Assessment of external and general cleaners”, developed by Airbus, a multinational aerospace company headquartered in Europe, describes a procedure for assessing external and general cleaners used as maintenance materials on aircraft and airbuses. Regulations require those working in aircraft maintenance to ensure the acceptability of products used in aviation products.
Regulations from the Federal Aviation Administration (FAA), an agency of the United States Department of Transportation, require aircraft maintenance personnel to use only approved chemicals. When failure to meet this legal requirement can have serious consequences for aircraft and personnel safety, how does a maintenance technician ensure compliance? This can only be determined through testing.
It is known from incidents that using unapproved or inappropriate cleaning fluids in aviation products can cause hydrogen embrittlement, corrosion, and other serious defects. Chemical damage to aircraft is a safety-of-flight issue and can seriously affect airframe and engine integrity. Accident files and structural damage reports show cases of airframe corrosion, fatigue, and failure. While environmental influences or substandard inspection and maintenance also contribute to such incidents, using inappropriate cleaning agents on aircraft is a critical risk that can lead to serious and costly consequences.
White corrosion is readily apparent and requires extensive repair to restore structural integrity. Corrosion aside, independent testing by the Department of Defense has found that the cleaner is a catalyst for hydrogen embrittlement in stressed metals and aluminum alloys. Routine maintenance will determine the type of failure, but airframe mating joints and stressed structural components may have come into contact with the cleaner and may be the first and only indication of failure.
Two classifications of aircraft corrosion are of interest when considering the effect of cleaning products: direct chemical attack and electrochemical attack. Both forms of corrosion convert a metal back to a metallic compound, which may be an oxide, sulfate, or hydroxide.
Direct chemical attack refers to exposure of a bare surface to a caustic gas or liquid. Battery acid spills in an avionics bay, uric acid attacking structures around aircraft toilets, or hot engine exhaust hitting wing structures can also cause aggressive corrosion, as can caustic cleaning agents when not used as specified by the manufacturer.
In the presence of an electrolyte, an electrochemical attack occurs on two dissimilar metals. The effect is similar to a battery, where one metal acts as the cathode and the other as the anode. The presence of the electrolyte allows current to flow from the anode to the cathode, and the anode turns into a metallic compound and corrodes. Salt water or unapproved cleaning products can be ideal electrolytes to facilitate this process, especially when trapped in lap joints or cracks in aircraft fuselages.
A third and more specific classification of chemical attack is more insidious than chemical and electrochemical corrosion, because it only becomes apparent when there is component failure, but cleaning chemicals can initiate it. This is called hydrogen embrittlement.
Hydrogen embrittlement is a complex phenomenon defined as the permanent loss of ductility caused by diffusible hydrogen atoms or ions present in a metal or alloy when under stress. The focus is on chemical or electrochemical processes that can produce hydrogen ions on the surface of a metal before they can diffuse into the structure of the metal.
Failure due to hydrogen embrittlement occurs when atomic hydrogen within a metal or alloy diffuses or migrates to the area of highest stress. As hydrogen concentrations increase, the metal or alloy loses ductility and becomes brittle. Eventually, the hydrogen concentration reaches a point where the stress causes microcracks. The hydrogen migrates to the tip of the crack, which continues to widen. Eventually, the metal or alloy becomes overloaded and fractures at stresses significantly lower than the designed strength of the component.
The effectiveness of some cleaning products depends on weak acid solutions. However, acids in solution release hydrogen ions that are small enough to penetrate protective surface films. These ions are absorbed into the base metal and reduced to hydrogen before diffusing into the metal as described earlier.
The Federal Aviation Administration (FAA) provides test criteria to determine the adequacy of general cleaning agents for application to aviation products. The criteria are designed to ensure that these agents do not degrade airworthiness. In general, six tests are applied to chemicals used for general cleaning of painted and unpainted surfaces using various international standards.
Airbus AIMS 09-00-002 standard, uses all six tests above. This standard also applies to airbuses.
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