Sixth International Aerospace Corrosion Control Symposium

Citation

(2003), "Sixth International Aerospace Corrosion Control Symposium", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 2. https://doi.org/10.1108/aeat.2003.12775bac.001

Sixth International Aerospace Corrosion Control Symposium

Sixth International Aerospace Corrosion Control Symposium

Keywords: Aerospace industry, Aircraft, Corrosion

The sixth international Aerospace Corrosion Control Symposium, held recently in Amsterdam, attracted speakers and delegates from industry and academia making it the most influential conference on aerospace corrosion (Plate 1).

The symposium, held over 1½ days, saw 18 papers presented by the industry's leading corrosion experts. The programme attempted to cover the whole spectrum of aerospace corrosion prevention and control.

Plate 1

The first paper in session one, was presented by Hans Cromwijk, Sergem Engineering, The Netherlands. Entitled Corrosion behind the sheets, the paper maintained that corrosion appeared to be an unpredictable maintenance item. Hans Cromwijk stated that corrosion on exterior panels is obvious and can easily be detected as damage to the decoration paint system. This can lead to, particularly with filiform corrosion, the costly task of repainting the aircraft, and these activities can be planned.

However, other types of corrosion occur on the inside and cause unplanned failure, disassembly, repair and replacement. The paper outlined several examples of interior corrosion, including corrosion of the skin and interior structure caused by isolation blankets. Another well-known trouble area highlighted was the structure below toilets and pantry.

The paper also examined corrosion sensitive material combinations which cannot be avoided for functional reasons. The problem of corrosion to seat rails was addressed. Corrosion to cables and cable drums was also discussed. Other areas of potential corrosion examined included ice protection plates, lower antenna assembly and static discharge assembly.

Drs Ruud van Overbeek, Akzo Nobel Aerospace Coatings, The Netherlands, presented a paper entitled, Chromate-free exterior corrosion protection with Aviox System. The paper examined the replacement of harmful products containing pigments from primers, currently in use for aerospace applications.

In his paper, Ruud van Overbeek discusses a research project, started 8 years ago, towards developing a primer aiming at a complete chromate-free corrosion protection for the exterior of civil and military aircraft. The paper contends that Aviox CF primer is an example of a proper design between a binder system and selection of corrosion inhibitor. He states that this product performs at the same level as currently chromate- containing products, both in accelerated corrosion tests and in practice. As an example he quotes the first aeroplane with the Aviox chromate-free system which was recently stripped after 6 years of service life and inspected for corrosion. It was concluded that the performance of the system was at least at the same level as a conventional system comprising Cr-containing washprimer, intermediate primer and topcoat. Ruud van Overbeek also stated that in addition, process time and levels of VOC could be decreased.

Aerospace corrosion control: a developing country's viewpoint, this paper was presented by Tanveer Ahmad Choudbary, of the College of Aeronautical Engineering, National University of Sciences and Technology, Pakistan. In his paper he states that there are three main players in the fight against aerospace corrosion problems namely manufacturers of aerospace equipment, manufacturers of corrosion control equipment/products, and the people involved in maintenance of aerospace equipment. He also suggests that aerospace corrosion control activities in the developing world are generally limited to management of corrosion control programmes. The problem of inefficient management of corrosion activities in developing countries has, he believes, been highlighted and thoroughly investigated.

Organisations owning aerospace equipment, the manufacturers of aerospace equipment, the manufacturers of corrosion control products, and the people employed on corrosion control activities are all jointly responsible for this dismal state of aerospace related corrosion control programmes. The author suggests a system approach to solve the problem to the mutual benefit of all the system elements. He recounts his personal experience of how he successfully led a new corrosion control programme in his organisation. Powered with knowledge, equipped with proper equipment and safety gears, and well supported by the manufacturers from USA, a modern corrosion control centre was set up against heavy odds.

In conclusion he states that few recommendations have been made to encourage the manufacturers from developed world to invest in formal training of aerospace people from developing countries to finally create a win-win situation.

Chris J. E. Smith and Harriet J. Kimpton of QinetiQ Future Systems Technology, in their paper Alternatives to cadmium plating for aerospace electrical connectors, stated that cadmium plating is currently employed in the corrosion protection of electrical connectors used on civil and military aircraft. They also point to the fact that it has been the preferred protective treatment for this application since it provides an effective level of corrosion protection, whilst offering galvanic compatibility with structural aluminium alloys.

However, the authors go on to state that on environmental grounds, the replacement of cadmium plating has been identified as a major priority within the aerospace industry. Cadmium compounds are highly toxic and the use of cadmium plating for many general engineering applications is now prohibited within the European Union.

This paper reviews the coating requirements for electrical connectors in terms of corrosion resistance, galvanic compatibility and physical properties. A range of possible alternatives to cadmium plating is considered and data obtained for zinc alloy and aluminium based coatings are presented. Recent research into aluminium alloy and zinc-nickel-manganese electrodeposited coatings is also outlined. It is concluded that several protective schemes may eventually provide suitable alternatives to cadmium plating for connector applications.

In their paper, Aircraft corrosion prevention and control technology: TRICORR sensor, R. Fletcher, R. Cook and R. V. Thomas of Avonwood Developments Ltd, explain that the monitoring of corrosion relies upon the regular and costly inspection of the complete airframe. The use of a sentry device within the airframe to monitor the conditions associated with corrosion would enable better and quicker corrosion management. The initial experimental work has shown that the use of a plasma sprayed track for the electrical resistance monitoring of atmospheric corrosion provides a sensitive and stable system.

Furthermore the simultaneous monitoring of other environmental conditions associated with corrosion provides greater opportunity to detect the earliest onset of corrosion. These combined within a unit that is both small and lightweight provides engineers with a valuable tool to monitor the corrosion within an aircraft and especially the areas that have poor access.

The TRICORR unit discussed in this paper is a unit containing three independent sensors that monitors the parameters associated with atmospheric corrosion. The unit is described as being small, light and can be fitted to any surface within an aircraft, especially the inaccessible areas that are difficult to inspect. The sensor takes the three independent measurements, and incorporates correction, calibration, integration and output conditioning through the use of dedicated software.

Roger Blackford of AECMA-STAN in his paper, European standards for aerospace surface finishes, examines the need to rationalise European standards for aerospace surface finishes.

He outlines the fact that within the European aerospace industry there are many standards used to specify and to control the quality of paint finishes. These are mainly either European national standards, European Company standards or US specifications. He also explained that for some time there has been an initiative by the industry to try to rationalise the complex specification situation within Europe and to try to utilise European EN standards to help achieve this. But progress in publishing EN paint material specifications has been very slow.

The paper then discussed a new working system, using mainly electronic communication, that has been set up by AECMA-STAN to try to speed up the process of publication of EN standards. This system is explained together with a review of the current needs of the industry and an update on latest progress. Also the specific area of standardisation of corrosion testing is examined within this system.

In an interesting paper, Fundamental studies to improve the performance of aerospace alloys, G.E. Thompson, P. Skeldon, K. Shimizu and X. Zhou, Corrosion and Protection Centre, UMIST, illustrated the role of alloying elements in aluminium and alloy fabrication on performance during surface treatment and surface finishing.

For example, it is known that aluminium supports an air-formed film over the macroscopic surface; the largely insulating film contains a myriad of flaws of differing physical, chemical and electrical behaviours, which can be activated under appropriate conditions. Generally, alloying increases the flaw or defect population density with a deterioration in material performance. Surface treatment, for example acid pickling or alkaline pickling, is used to remove mill finishes, with surface finishing, including anodising, employed to reduce the deleterious effects of flaws on material performance. Alternative or additional protection schemes include application of organic coatings with inhibitive pigments to provide acceptable performance.

The paper examines the influence of alloying on aluminium, including impurity element effects. In addition to trace impurities, the effects of individual alloying elements within commonly used aerospace alloys are considered. In particular, their influence on film formation is explored since this may, in the opinion of the authors, provide insight into the generation of functional coatings by further control of additions or their strategic location. Additionally, information gained from different alloy systems is introduced since this may prove relevant to improving the performance of the widely used aerospace alloys. The paper also states that in consideration of metal/film interfaces at the nanometre scale and of processes proceeding within pores of diameters less than a few tens of nanometres, the introduction of a nanotechnological approach to engineering of aerospace alloys is possible. The paper concludes that such an approach may then allow informed tailoring of surfaces for the improved application of high strength aluminium alloys.

In a case study, Electrochemical evaluation of the defect growth in primers applied on aluminium alloy 2024T3, C. Wrubl*, M. Fassin*, A. Buffoli** and A. Mollica*, *CNR-ISMAR, **AGUSTA, LTS, outline the protective properties of two epoxy-primers, the first one containing SrCrO₄ and the second Zn₃(PO₄)₂ as inhibiting pigments against corrosion of the aluminium alloy 2024T in marine atmosphere, were investigated. The study explains that potentiostatic polarisation and impedance measurements were utilised to evaluate, during 24 months of exposure to marine atmosphere, both the spontaneous onset of defects on coated samples and the propagation of a small artificial defect of known dimension applied since the beginning of the test on each sample.

The authors explain that these techniques allowed to quantitatively evaluate the protective efficiency of the two primers and the effect of the surface pre-treatments of the metallic substrate.

The corrosion protection against atmospheric corrosion of aluminium alloys, widely employed in aeronautical field, is at present realised by covering their anodised surface by means of two distinct layers, the first one made up of a primer, normally containing a corrosion inhibiting pigment, and the second one of a top coat which acts as a barrier against corrosive agents present in the atmosphere. At present the most frequently used pigments contain chromate Cr⁶⁺ ion, generally in the form of its sparingly soluble salts of strontium or zinc.

The paper points out that chromate-based inhibitors will be fazed out in the future because of their negative environmental impact and their high toxicity and therefore the search of other less dangerous and sufficiently efficient pigments is very urgent.

The case study explains that in the present work electrochemical methods were utilised as relatively simple tools to evaluate, and then to compare, the performance of several combinations of metal substrate-surface treatments-primers in marine atmosphere.

The following combinations are examined:

• •

  aluminium alloy 2024T3, clad or not with AA1230, as base material;

• •

  surface pre-treatment with anodising chromic acid or by chemical conversion with Alodine;

• •

  epoxy primers containing SrCrO₄ or Zn₃(PO₄)₂.

For each combination the onset of spontaneous defects on the treated samples during 24 months of exposure to marine atmosphere and the propagation of small defects artificially applied on the coat since the beginning of the tests were evaluated.

Olga Guseva*, Samuel Brunner, Peter Richner, Swiss Federal Laboratory for Materials Testing and Research, presented a paper entitled Analysis of the environmental parameters for aircraft coatings.

In the opinion of the authors, aircraft coatings have both protective and aesthetic functions. The paint has to shield the aircraft against corrosion and environmental effects thus making them safer and more durable. The aesthetical appearance is also said to be important because the aircrafts carry the company logos and colours to the airports throughout the world.

The fact that aircraft coatings are subjected to rather severe environmental conditions is explored in this paper. The temperature, UV radiation and pollutants are known to play a major role in degradation of the airspace coatings. The air temperature differences exceed 100°C, ranging from 60°C ( 76°F) at a flying altitude of 10km to over 50°C (104°F) in airports located in tropical desert regions. Moreover, it is known that the coating surface temperature is much higher than the air temperature, especially for dark colours. Apart from the temperature and UV, wetness is also a known factor influencing the life of coatings.

This paper gives an understanding to the phenomena of the damages of the aircraft coatings in the years from 1992 to 1995. During this period of time, the majority of aircraft coatings had only half of the expected service life: instead of usual 4-5 years in service. The coatings became aesthetically unacceptable and lost some of their protective function much faster. It was found that the cause of these damages was the eruption of volcano Pinatubo in 1991, which had injected more than 20,000,000 tons of SO₂ into the stratosphere, the atmospheric layer above the clouds, in which lowest level of the commercial air traffic takes place.

The paper evaluates the environmental parameters and their ranges for estimation of the service life for aircraft coating, focusing on UV radiation and pollutants.

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