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Volume 2 Extended Abstract 15

Submitted 26th August 1999

Short Term Testing and Real Time Exposure

James Maxted 
Akzo Nobel Nippon Paint , North Woolwich Road, Silvertown, London E16 2AP
 

Keywords: Coil coating , Cut edge corrosion , Field Performance , Chromate free Primers, Cyclic tests , Continuous Salt Fog , Service life prediction methodology.

Introduction

Control of corrosion on pre-coated, coil applied, galvanised steel continues to be an area of much academic and industrial effort [1]. Since the beginnings of the industry in the 1960s, the corrosion performance of these materials have been controlled by the combination of at least three coating elements: chromic acid based pretreatments , chromate containing primers of thicknesses between 5 and 50 microns and topcoats of builds ranging from 20 and to 200 microns. Together these have provided the requisite adhesion, barrier and inhibitive properties to facilitate the growth of the product in the construction industry. Such is their success that failures today of such materials, within their product lifetimes probably amount to no more than 0.25 – 0.5% of the total installed area for U.K. However, over 75% of reported failures were linked to corrosion and, more particularly, most of this corrosion occurred in critical areas such as edges of cut sheets [2]. This type of failure mode could not be generally predicted in standard, laboratory, accelerated tests. Examples show how these long standing accelerated tests, and in particular continuous ones like the salt fog test, can fail to predict service lifetime or anticipate early failures in the field. Corroded cut edge appearance and its related corrosion product morphology from long term field exposures are compared to that generated in salt fog, wet/dry cycling and a scab corrosion tests. The lack of red rust and presence of discrete blisters rather than edge peel and flake on the continuous salt fog test suggests quite a different mode and mechanism of failure. Only the severe marine site generated patterns of corrosion that approached those in the salt fog test. Cyclic wet and dry tests generally show a closer approximation to the field results as shown in Figure 1 below.
  

Figure 1

Since most coil-coated products are used in non-marine environments it is clear that traditional methodologies should not solely be relied on as an assessment tool for coating systems. Further examples from natural exposure tests provide evidence that the nature of electrolytes, the influence of wet/dry cycling stresses, coating photodegradation and coating physicomechanical changes can all participate in corrosion failure modes and mechanisms. The importance of understanding the application and design variables for the coated product is also discussed and consideration given to how to include these in the laboratory testing protocols. It is crucial both to identify and test the role of each of these parameters in establishing the overall corrosion performance of a system. This necessitates adopting accelerated tests, which focus on one or another of the specific degradation mechanisms. It is felt unlikely that any single accelerated test will be able to provide a complete and realistic prediction of service life despite the moves within some current and proposed international standards to try to do just that. Today, new demands in the industry mean the development of more environmentally friendlier coatings which, in turn mean a move away from traditional pretreatments, and both hexavalent chromium primers and thick film polyvinylchloride systems which have for so long been the mainstay for corrosion protection. Indeed one of the features of hexavalent chromium is its excellent performance in the salt fog test and one of the drawbacks of the newer chromate-free primers, is their often, poorer performance on the same test thus condemning a new technology. To avoid erroneous and untimely conclusions being drawn a new approach to performance assessment has had to be derived alongside the development of these new technologies.   The inclusion of cyclic testing techniques in addition to the continuous tests is shown to be an important feature of this new approach. Techniques such as Prohesion[3], PUCAT testing [4], CCT testing [5] and a combined Salt Fog/UV Exposure test [6] help to broaden the means by which to simulate exterior performance. However, like a jigsaw puzzle, it is the combination of the results from all tests (including salt fog !), that enable the formulator to gain the overall picture of performance. Each piece is vital to complete the picture, but on its own, it can not only be uninstructive, but potentially misleading. Finally a methodology for corrosion performance prediction is proposed based on one suggested for the automotive industry [7]. This entails the integration of results from three sources ; first that from standard, natural exposure sites, second, from a broad spectrum of laboratory accelerated performance tests and third, that from fundamental property measurements. The latter may involve measurement of the electrochemical, adhesive, or film porosity/barrier characteristics to name but a few. By integrating the responses from all three sources into a central model, it then becomes possible to both predict service life in untested environments and to understand and rectify early failures. This approach represents a significant change from the traditional one which regarded a limited number of severe outside tests as the final arbiter of performance and in which the accelerated test was often seen as a means of representing the real world in all its many guises. The objective of this paper is not to promote any single new test to replace the salt fog procedure, but rather to argue for this more coherent approach. Once the behaviour and responses to the various outdoor corrosion mechanisms have been understood and simulated, a model can be constructed which then has the potential to provide a rapid, realistic and reliable prediction of performance.

References

  1. Howard,R.L., Lyon,S.B., Scantlebury,J.D., Proc.13th Int, Corrosion Congress (1996) , Vol 1, Paper 22
  2. Ryan,P.A., Wolstenholme,R.P., Howell,D.M , Durability of Cladding ; A State of the Art Report, WS/Atkins . Thomas Telford 1994. ISBN 0 7277 2012 0
  3. Timmins,F.D., J. Oil & Colour Chemists’ Assoc.,62 ,131 (1979)
  4. Walter,G.W., Corrosion Science. 35, Nos. 5-8. pp 1391 – 1404. 1993
  5. Brennan,P.J., Introduction to Cyclic Corrosion Testing .Finish. pp 13 -18 Jan –March 1995.
  6. ASTM D 5894-96, "Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal", Annual Book of ASTM Standards, Vol. 6.02, Paint - Tests for Formulated Products and Applied Coatings (Philadelphia , PA: ASTM,1996).
  7. Dickie,R.A., J. Coatings Technol., 64 , 809 , pp 61-64 , 1992

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