An unusual question? Of course we wash aircraft to make them clean again after service. Why do we wish to have them clean? Is it purely for aesthetic reasons or is there a more important agenda behind this expensive and time consuming regime? Experts conclude the real answer is: the washing of airframes is, or should be, an integral part of the operators CPCP (Corrosion Prevention and Control Programme).
The dynamics of aviation are continually changing, however one aspect that is becoming more relevant for aircraft owners and operators, is that increasing economic pressure has encouraged extension of the service lives of many civil and military aircraft fleets far beyond their original design goals.
Naturally, the incidence of corrosion increases with aircraft age. Therefore the effect of corrosion as a life limiting key element increases significantly as the fleet ages. Thankfully, only a relatively small proportion of aircraft accidents and incidents are directly attributed to airframe corrosion. However, the reality of corrosion damage causing or accelerating structural failure in aircraft and massively increasing maintenance and its associated costs is very real. Even newer airframes with significant amounts of composite in their make up still have many integral members and sub frames of alloy composition.
A prime example of corrosion induced catastrophic failure is the 1988 incident involving an Aloha Airlines Boeing 737-200. The 18 foot long top half of fuselage section 43 was torn off the aircraft in flight. Examination of the remaining structure eventually revealed a disbonding problem, extensive corrosion damage and fatigue cracking along the lap joints. Moisture had entered the lap joint in the area of the disbond and then corrosion had commenced. Considerable evidence of corrosion such as swelling and bulging of the skin, scaling and flaking paint were obvious at many sites along the lap joints.
The use of approved aircraft cleaners does not necessarily have a significant effect on the reduction of corrosion. Tests such as: BOEING D6-17487 REVISION P, Boeing D6-7127 REVISION M, AMS 1526B, AMS 1550B and even Mil-Spec evaluate the cleaning product as to the suitability of use on an aircraft, which does include some short term testing as to the corrosiveness of the product. However, bear in mind that even the testing for conformance to; MIL-C-87936 Type 1, MIL-C-87937 Type 11, and DEF (AUST) 5570A, only determines whether the product “does not corrode a range of metals and dissimilar metal couples at a rate greater than specified in the documentation”. By example polarisation testing is conducted over a thirty minute time frame so the result does not accurately reflect the long term “in field” performance of the cleaning product.
The most significant testing that has possibly ever been undertaken any where in the world on this topic was carried out in Australia in 1993 by the DSTO (Defence Scientific and Technology Organisation) on behalf of the Australian Defence Forces. Whilst the testing and documentation is classified, there are academic papers in the public domain that reference the data. A regime of 6 tests were undertaken over a 3 month period on a number of cleaning products. Tests included: constant and alternate immersion, crevice, underfilm, washing, electrochemical and galvanic. The testing showed that not only did the Zi-400 have a major effect on reducing corrosion, it has the ability to arrest existing corrosion. Something that no other product can do. On the strength of this exhaustive testing Zi-400 was chosen as the preferred cleaning product for Australian Military Aircraft.
The positive results for Zi-400 in this testing has had a flow on effect, and in addition to the ADF, other users including large commercial and RPT operators, international military forces, contractors and even the United Space Alliance for the recently retired NASA Space Shuttle fleet have chosen to make the use of Zi-400 an integral part of their CPCP.
The success of Zi-400 in corrosion prevention is NOT because it has corrosion inhibitors added to it. The ability to fight corrosion is unique to the Zi-400 due to its formulation. The product is not a conventional surfactant based cleaner, it is based on colloidal chemistry. The word colloid is a chemistry term for describing a state of matter. The colloidal state is extremely tiny, only just above the ionic state, which is individual atoms. The particles, or micelles as they are correctly called, are typically between 10 to 50 Angstrom’s in diameter, an Angstrom being one 10-billionth of a metre! The micelles are able to interact with the substrate at the molecular level and interrupt the galvanic action that causes the corrosion.
It has also been found that continual usage of Zi-400 on airframes can have a positive, long term effect on painted surfaces as well as the metal substrate. Most cleaning agents subtly attack the paint surface and lead to long term degradation evidenced as fading and chalking. This leads to film loss which is accelerated further by surface restoration procedures. Cleaning agents can also be responsible for paint loss from surfaces by allowing or encouraging exfoliation and/or underfilm and even filiform corrosion in and around stone chips, nicks and scratches. Regular aircraft washing with Zi-400 not only will reduce or alleviate the issues mentioned above, it can also be of benefit by reducing oxidisation of the paint surface.
Whilst the aspects raised are more likely seen from an engineering view point, they also have relevance to the financial aspect of aircraft operations. Any reduction in airframe corrosion is a significant financial gain, both from engineering costs and down time that the aircraft is not in the air. Improving the life span of a paint job is also significant from a financial perspective. To increase the time frame between repaints, or possibly reduce the number of repaints during the service life of the aircraft is a major financial gain. Not to mention cost saving in touch ups during regular servicing that often occurs.
However, the biggest short term financial gain that operators realise by using Zi-400 is simply the dilution ratio that the product is used at. Predominantly aircraft cleaners dilute at ratios of somewhere between 1 part cleaner and 8 to 12 parts water for general usage. The recommended dilution ratio for Zi-400 is 1 part product to 32 parts water. Obviously, from this we can see that an aircraft wash can be done with Zi-400 using only 25% of the amount of other products. The ability of Zi-400 to clean both exterior and interior surfaces also increases cleaning cost efficiencies.
How can Zi-400 manage to do all of these things? Simply, all other aircraft cleaning compounds rely on conventional alkaline, surfactant technology. They are detergents, not really very different to truck wash or industrial cleaners. Zi-400 has a completely different chemical structure. It is formulated with colloidal technology. Whilst the technology behind the Zi-400 is proprietary, it can be said that the main difference between colloidal cleaners and surfactants is that colloidal cleaners bump the contaminants off the surface rather than burning them off chemically as alkaline based surfactants tend to do.