Prediction of coating durability
Early detection using electrochemical methods
Prediction of coating durability
Early detection using electrochemical methods
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus, prof.dr.ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 11 maart 2008 om 12:30 uur
door
Wilhelmus Maria BOS
Bachelor of Science in Environmental Technology geboren te Den Helder
Prof. dr. J.H.W. de Wit
Samenstelling promotiecommissie: Rector Magnificus voorzitter
Prof. dr. J.H.W. de Wit Technische Universiteit Delft, promotor Prof. G.E. Thompson The University of Manchester
Prof. dr. R. Boom Technische Universiteit Delft Prof. Dr.-Ing. G. Grundmeier Universität Paderborn
Prof. dr. ir. H. Terryn Vrije Universiteit Brussel Dr. ir. J.M.C. Mol Technische Universiteit Delft Ing. L.G.J. van der Ven Akzo Nobel Car Refinishes
The research described in this thesis was financially supported by the ‘IOP Oppervlaktetechnologie’ research programme of SenterNovem (project number IOT99001) and TNO Science and Industry.
ISBN 978-90-9022815-0
Copyright © 2008 by W.M. Bos
Printed by: Gildeprint drukkerijen B.V., Enschede, The Netherlands
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the author.
1 Introduction 1
1.1 Corrosion 1
1.2 Economic impact of corrosion 1
1.3 Mitigation of corrosion damage 2
1.4 Protective coatings 3
1.5 Research objective and topological breakdown 7
1.6 Thesis outline 8
1.7 References 10
2 Performance testing of protective coatings 13
2.1 Introduction 13
2.2 Outdoor Exposure 13
2.3 Accelerated tests 14
2.3.1 Salt spray test 14
2.3.2 Cyclic testing 16
2.3.3 Advanced cyclic testing 17
2.3.4 Overview accelerated tests 20
2.4 Evaluation methods 22
2.4.1 Visual evaluation 22
2.4.2 Electrochemical evaluation 23
2.5 Acceleration factor 23
2.6 Correlation 24
2.6.1 Calculation correlation coefficients 24
2.6.2 Brief overview of correlation studies 26
2.7 Conclusions 28
2.8 References 30
3 Electrochemical techniques for coating characterisation 37
3.1 Introduction 37
3.2 Electrochemical impedance spectroscopy 37
3.2.1 History 37
3.2.2 Measurement principle 38
3.2.3 Data analysis 41
3.2.4 Impedance expressions of electrical elements 43
3.2.6 Typical impedance spectra of protective coatings 45
3.2.7 Physical meaning elements 48
3.2.8 Constant phase element 51
3.2.9 Kramers-Kronig transforms 56
3.3 Electrochemical noise measurements 58
3.3.1 History 58
3.3.2 Measurement principle 59
3.3.3 Data analysis 59
3.3.4 Electrochemical cell 60
3.3.5 Comparison of noise parameters with impedance spectra 61
3.4 References 63
4 Outdoor exposure of coating systems monitored with EIS 67
4.1 Introduction 67
4.2 Experimental 67
4.2.1 Coating material 67
4.2.2 Measurement equipment 67
4.2.3 Measurement procedure 68
4.3 Results and discussion 70
4.4 Conclusions 83
4.5 References 84
5 Pre-qualification of protective coatings using EIS 91
5.1 Introduction 91
5.2 Measurement procedure 92
5.3 Results 95
5.3.1 Overall results 95
5.3.2 Mixing ratio epoxy 97
5.3.3 Zinc-rich paint 99
5.3.4 Zinc-rich silicate primer 102
5.3.5 Corrosion-inhibitive pigments 105
5.3.6 Chemical resistance moisture-curing silicone rubber 109
5.4 General conclusions 110
5.5 References 113
6 Application of ENM to the study of protective coatings 117
6.1 Introduction 117
6.2.1 Measurement equipment 117
6.3 Drift 118
6.4 Asymmetric electrodes 123
6.5 EIS versus ENM 125
6.6 General conclusions 128
6.7 References 129
7 General conclusions 131
Summary 135
Samenvatting 139
Publications related to this work 143
Dankwoord 145
1.1 Corrosion
Although many definitions of corrosion exist, for this thesis, corrosion is defined as electrochemical degradation, occurring at the metal-solution interface where the metal is oxidised (anodic reaction) and species from the solution such as oxygen (cathodic reaction) are reduced [1].
Corrosion occurs because most metals are inherently unstable. Metals are produced from ores that are rich sources of the requisite elements. Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. These processes are non-spontaneous and it will therefore require a large amount of energy to obtain the pure metal from the ore. On the other hand, once the metal is refined, it spontaneously tries to revert to its original stable state [2]. These counteracting processes are schematically shown in Figure 1.
Ore thermodynamically stable naturally occurring Refined metal thermodynamically unstable man-made Refining process non-spontaneous requires energy Corrosion spontaneous Ore thermodynamically stable naturally occurring Refined metal thermodynamically unstable man-made Refining process non-spontaneous requires energy Corrosion spontaneous
Figure 1 Loop of the counteracting refining and corrosion processes.
1.2 Economic impact of corrosion
Corrosion of metals has an enormous economic impact. Cost-of-corrosion studies have been undertaken by several countries. Although the methodology varies between studies, all estimates of the total annual cost of corrosion range from 1.5 to 5.2 percent of each country’s gross national product (GNP, see Table 1) [3].
In the period from 1999 to 2001, a more systematic study was performed in the U.S. In this study, the total direct cost of corrosion was determined by analyzing 26 industrial sectors in which corrosion is known to exist and extrapolating the results for a nationwide estimate. The total direct cost of corrosion was determined to be $ 276 billion per year, which equates to 3.1 percent of the U.S. gross domestic product (GDP) [3].
The U.S. study defined the total direct annual corrosion costs as those incurred by owners and operators of structures, manufacturers of products and suppliers of services. Indirect costs include factors such as plant downtime (e.g. shutdown of a nuclear power plant), loss of product (e.g. oil spill), loss off efficiency (e.g. reduced heat transfer of heat exchangers), contamination (e.g. soluble corrosion products in pharmaceutical production facilities), etc. [4]. The U.S. study conservatively estimated the indirect cost to be equal to the direct cost. This suggests that the combined direct and indirect cost of corrosion probably exceeds $552 billion, representing 6% of the GDP [5].
Table 1 Overview of cost-of-corrosion studies by country [3].
Country Total annual cost of corrosion Percent of GNP Year
United States $ 5.5 billion 2.1 1949
India $ 320 million – 1960
Finland $ 54 million – 1965
West Germany $ 6 billion 3.0 1967
United Kingdom £1.365 billion* 3.5 1970
Japan $ 9.2 billion 1.8 1974
United States $ 70 billion 4.2 1975
Australia $ 2 billion 1.5 1982
Kuwait $ 1 billion 5.2 1987
United States $ 276 billion 3.1 (GDP) 2002
*Not reported in U.S. dollars.
1.3 Mitigation of corrosion damage
Fortunately, the rate at which corrosion processes take place can be reduced, often to a point where, at least temporarily, corrosion is virtually non-existent.
Major corrosion-control methods include barrier protection, cathodic protection and the use of corrosion inhibitors. In many cases, these methods are used in conjunction. The concept of barrier protection is to separate the metal substrate from the corrosive environment. These barriers are frequently applied in the form of organic coatings. Alternatively, corrosion resistant alloys such as stainless steel spontaneously form a thin barrier layer that protects the underlying alloy.
Cathodic protection prevents corrosion by making the structure to be protected the cathode of the corrosion cell. This is achieved by either using a sacrificial anode (of zinc, aluminium or magnesium) or by impressing a current using an external power supply and an inert electrode.
Corrosion inhibitors are chemical compounds which, when added in small quantities to an aggressive environment, are able to reduce the corrosion rate of the exposed metal. Some inhibitors are adsorbed at the metal surface and thereby produce barrier films, while other inhibitors reduce the corrosivity of the environment by e.g. pH-buffering or oxygen scavenging [4,6].
It has been estimated that 25 to 30% of annual corrosion costs in the U.S. could be saved if optimum corrosion management practices were employed [5].
Many ways have been developed to protect metals from the environment, but most prominent is the use of organic coatings [1,2,7]. The worldwide production of coatings in 1996 was about 230 million metric tons and the global coating market is estimated at about $ 60 billion [in:8].
For economic reasons, the lifetime of these coatings should be as long as possible. The corrosion protective performance is determined by the quality of the total system, consisting of the metal substrate, surface pre-treatment, coating type, application method, curing procedure and the environment [9].
Obviously, the development, selection and qualification of new coatings for a specific environment, requires a thorough characterisation of the most important coating properties [10].
1.4 Protective coatings
In this thesis, various properties of protective coatings will be discussed. Therefore, some general coating characteristics will be addressed first.
Coatings are mainly a dispersion of pigments and fillers, additives and solvents in a binder matrix [8-12]. These constituents will be introduced in the following paragraphs.
Binder
The binder forms the matrix of the coating, the continuous polymeric phase in which all other components may be incorporated. With a few exceptions (e.g. silicate-based binders in inorganic zinc-rich primers), binders are a mixture of one or more organic polymers. Binders form the bulk of the physical barrier a coating usually provides in the protection of its substrate. Its cross-link density and composition largely determine such factors as the permeability, chemical resistance and UV-resistance of the coating [10,12].
Binders can be categorized according to the way the coating transforms from the liquid phase to a solid state, viz. by physical drying, chemical network formation (curing), coalescence, or a combination [9].
Physical drying takes place by the evaporation of the solvents from the liquid coating after application. The solid (non-volatile) parts of the coating remain. The coating can be re-dissolved in the appropriate solvent. Examples of physically drying coatings are classic nitrocellulose lacquers and chlorinated rubbers [13].
Coatings that cure by chemical reactions are generally two-component coatings that polymerize through a chemical reaction initiated by mixing resin and hardener. In contrast with physical drying, cured coatings cannot be re-dissolved since the reactions are irreversible. Examples of chemically curing coatings are epoxies that cure with amines [14] or polyurethanes that are formed by reaction of polyisocyanate with polyol (polyalcohol) [15].
Film formation by coalescence usually starts with a dispersion of small particles in water. When the ‘solvent’ evaporates the polymer, particles blend together and build an insoluble film. This is true for water-borne acrylic coatings. These one-component materials cure by ‘solvent’ evaporation followed by coalescence of the resin particles [16]. Another example of this type of film formation is the sintering of polymer particles in a powder coating by heating the coated specimen. The powder coating particles will melt by the temperature increase and fuse together [9].
Pigments
Pigments contribute to several properties of organic coatings. In fact, a number of different pigments may be used within the same coating, all with their specific contribution to the coatings characteristics.
Obviously, pigments may provide the coating with almost any colour required, but pigments are also frequently added for corrosion protection. The mechanical properties of the coating are also affected by pigments.
Three types of corrosion-protective pigments can be distinguished: barrier, sacrificial and inhibitive types. Some pigments are involved in more than one protection mechanism.
Barrier pigments are added to a coating to lower the permeability for water, oxygen and other corrosive species. Besides this, they protect the binder against UV radiation. Maximum effectiveness can be reached if the following requirements are met [9,12]:
− The pigments are impermeable.
− The pigments are chemically inert in the barrier coating.
− The pigments have a flake- or plate-like shape that is aligned parallel to the substrate’s surface. This way the length of the diffusive pathway is increased. − Wetting of the pigment by the binder must be excellent, and the binder-pigment
adhesion must be resistant to wet conditions.
Examples of barrier pigments are mineral-based materials such as micaceous iron oxide (MIO) and metallic flakes of, for example, aluminium.
Unlike barrier pigments, sacrificial and inhibitive pigments can become chemically active in the cured coating.
Metallic zinc, when present in high enough concentration, provides sacrificial (cathodic) protection. When in electrical contact with the steel surface, zinc acts as the anode and protects the steel cathode, until metallic zinc becomes depleted [12].
Coatings utilizing inhibitive pigments release soluble species, such as phosphates, into any water that penetrates the coating. These species migrate to the metal substrate, where they inhibit corrosion by facilitating the growth of protective surface layers [9,17,18].
Fillers
Since many of the newer pigments are mostly rather expensive, only a minimal amount is used. Fillers can further increase the volume of the coating through the incorporation of low cost materials (chalk, mica, clay, etc.). They also may be used to improve coating properties such as impact and abrasion resistance and water permeability. For polyester coatings, fillers are used to minimize the internal stresses
in the coating. This is particularly important since polyester coatings shrink considerably during the curing process and the cured film has a high thermal expansion coefficient [9,10,12].
Additives
With respect to the corrosion protective properties, the most important components of a coating are the binder and the corrosion-protective pigments. Additives are primarily related to the manufacturing, application and curing process of the coating.
Additives refer to a large group of components with very specific properties, which typically are added to paint in small quantities. The functions of the different additives are very diverse.
Thixotropic agents, for example, are used to control the viscosity of the coating. Thixotropic fluids are shear-thinning fluids. Coating application by brushing, rolling or spraying all produce different amounts of shear stress to the coating. For thixotropic coatings, this temporarily lowers the viscosity, which eases its application. Once applied, the coating film is subjected to the low shear conditions of gravity. Consequently, the viscosity of the coating increases back towards its low-shear value. This process is also desirable as it ensures that the film remains in place during the drying process.
Another important group of additives are surfactants. Surfactants are used to control the surface energy of the complete coating or of one of its constituents.
Wetting agents that are used to lower the surface tension of the complete coating ensure that the coating spreads out and adequately wets the surface of the metallic substrate, forming a continuous film.
The incorporation of pigments in the polymer matrix is a complicated process due to the intrinsic incompatibility with the binder. For this reason additives like wetting or pigment dispersion agents are used.
Other examples of additives are biocides, UV-absorbers, antioxidants, antifoam agents and corrosion inhibitors. The latter example should not be confused with inhibitive pigments. The additives are completely soluble in order to provide additional corrosion protection upon application of the coating [9,10,12].
Solvents
Solvents are used to (temporarily) reduce the viscosity of the coating. Reduction of the viscosity is necessary to enable homogeneous mixing of the binder and other components. Furthermore, the reduced viscosity makes it possible to apply the coating in a thin, smooth and continuous film on a specific surface.
properties and to control the drying or curing process. The choice of the solvent mixture is very important. In the liquid state, prior to application, paint should form a stable dispersion of binder, pigments and additives in the solvent. All solid components should be homogeneously distributed in the liquid phase. This requires a high compatibility between solvent and components. An improper formulation can diminish the barrier properties of the coating, or can even cause phase separation of the dispersion [9,10].
Due to tougher environmental and health measures, such as VOC (volatile organic compounds) regulations, new coating formulations have been developed to reduce the use of volatile organic solvents. This led to the development of ‘solvent-free’ epoxy coatings and waterborne coatings [9,14].
1.5 Research objective and topological breakdown
The objectives of this Ph.D. research are two-fold.
From an industrial perspective, it is economically attractive to develop methods for the mitigation of corrosion damage of coated structures.
From a scientific point of view, it is interesting to acquire a fundamental understanding of how protective coatings function and how these coatings deteriorate with time. Broadly speaking, there are two ways to determine the suitability of a coating for a specific environment. The first approach involves the use of so-called accelerated tests. Since the in-service degradation of modern protective coating systems is extremely slow, accelerated testing aims to reproduce, in a much shorter time than in the field, natural degradation processes without changing the degradation mechanism. The second approach utilizes sensitive electrochemical techniques such as Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise Measurement (ENM) to allow early detection of relevant degradation processes under ‘non-accelerated’ conditions.
Both approaches will be addressed in this thesis, as is pointed out in the topological breakdown shown in Figure 2.
Mitigation of corrosion damage of coated metal structures
Review of ‘accelerated’ test methods Early detection of relevant degradation processes Electrochemical Impedance Spectroscopy Electrochemical Noise Measurements General conclusions
Mitigation of corrosion damage of coated metal structures
Review of ‘accelerated’ test methods Early detection of relevant degradation processes Electrochemical Impedance Spectroscopy Electrochemical Noise Measurements General conclusions
Figure 2 Topological breakdown of this thesis.
1.6 Thesis outline
In this thesis, various properties of protective coatings will be discussed in detail. Therefore, general coating characteristics are introduced in chapter 1.
Chapter 2 reviews the most widely used (‘accelerated’) tests for protective coatings. The associated evaluation methods are also discussed. Based on this the review, conclusions will be drawn upon the usability of these tests.
As corrosion of (coated) metals is an electrochemical process, it is sensible to characterise the performance of protective coatings with electrochemical measurement techniques. In this thesis, Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise Measurements (ENM) are used to this purpose. Chapter 3 provides a theoretical background of these techniques.
Chapter 4 discusses the results of outdoor exposure of model coating systems, periodically measured with EIS. The results were analysed and related to a unified model for coating degradation presented by Nguyen and Hubbard [19].
Though a large amount of EIS data are available on protective coatings, measurement and analysis procedures vary between research groups. This obstructs direct comparison of measurement results obtained by different groups. Therefore, in chapter 5, a benchmark measurement protocol is introduced. More than 100 industrial coatings have been measured in compliance with this protocol. After a general overview of the results, various examples are discussed in more detail.
Currently, EIS is the most commonly used electrochemical method for evaluation of the protective properties of coatings. Recently, the application of ENM for the same purpose has become of interest. There are number of advantages of this technique over EIS, such as the low cost of the required equipment and minimal interference with the measured system.
Despite these advantages, the application of ENM for corrosion studies is only starting to mature. In chapter 6, results of ENM will be related to those of EIS measurements. From this, conclusions will be drawn upon the usability of ENM for the study of protective coatings.
Finally, in chapter 7 general conclusions will be drawn from the work described in this thesis.
Acknowledgements
The research described in this thesis was financially supported by the ‘IOP Oppervlaktetechnologie’ research programme of SenterNovem (project number IOT99001) and TNO Science and Industry.
1.7 References
1 Lenderink, H.J.W. Filiform Corrosion of coated Aluminium Alloy - “a study of
mechanisms”, Ph.D. Thesis, Delft University of Technology, 1995.
2 Hamer, W.J., Polypyrrole Electrochemistry - Environmentally friendly corrosion
protection of steel: (im)possibilities, Ph.D. Thesis, Delft University of Technology, 2005.
3 Koch, G.H., M.P.H. Brongers, N.G. Thompson, Y. P. Virmani, J.H. Payer, Corrosion
Cost and Preventive Strategies in the United States, FHWA-RD-01-156, Federal
Highway Administration, U.S. Department of Transportation, Washington, D.C., 2001. 4 Jones, D.A., Principles and Prevention of Corrosion, Second Edition, Prentice Hall, New
Jersey, U.S.A., 1995, ISBN 0133599930.
5 Corrosion - A Natural but Controllable Process, Supplement to Materials Performance,
July 2002, 3.
6 Moavenzadeh, F. (Ed.), Concise Encyclopedia of Building and Construction Materials, ,
MIT Press, U.S.A., 1990, ISBN 0262132486.
7 Dobbelaar, J.A.L., The use of impedance measurements in coating research – The
corrosion behaviour of chromium and iron-chromium alloys, Ph.D. Thesis, Delft
University of Technology, 1990.
8 Wicks, Z.W., F.N. Jones, S.P. Pappas, Organic Coatings: Science and Technology -
Second Edition; Wiley, New York, 1999, 630, ISBN 0471245070.
9 Westing, E.P.M. van, Determination of coating performance with impedance
measurements, Ph.D. Thesis, Delft University of Technology, 1992.
10 Geenen, F.M., Characterisation of Organic Coatings with Impedance Measurements, Thesis Delft University of Technology, Delft, The Netherlands, Ph.D. Thesis, Delft University of Technology, 1991.
11 Munger, C.G., Corrosion Prevention by Protective Coatings, National Association of Corrosion Engineers, Houston Texas, 1984.
12 Forsgren, A., Corrosion control trough organic coatngs, 2006, CRC Press ISBN 9780849372780.
13 Hare, C.H., Vinyl and Chlorinated Rubber, Journal of Protective Coatings and Linings, December 1995, 41-58.
14 Salem, L.S., Epoxies for Steel, Journal of Protective Coatings and Linings, September 1996, 77-98.
15 O’Donoghue, M., R. Garrett, V.J. Datta, Straining at a Gnat and Swallowing a Camel:
Health and Performance Issues with Two-Part Polyurethane Finish Coats, Journal of
Protective Coatings and Linings, December 2006, 28-27.
16 Smith, L.M., Introduction to Generic Coating Types, Journal of Protective Coatings & Linings, July 1995, 73-82.
17 Hernández, M., F. Galliano, D. Landolt, Mechanism of cathodic delamination control of
zinc–aluminum phosphate pigment in waterborne coatings, Corrosion Science, 46 (2004),
2281-2300.
18 Hare, C.H., Inhibitive Primers for Metal: Fundamental Considerations, Journal of Protective Coatings and Linings, May 1998, 48-62.
19 Nguyen, T., J.B. Hubbard and J.M. Pommersheim, Unified model for the degradation of
organic coatings on steel in a neutral electrolyte, Journal of Coatings Technology, 68
2.1 Introduction
Protective coatings of many different types and formulations are extensively employed to protect metal structures against corrosion. Obviously, the development, selection and qualification of new coatings for a specific environment, requires a thorough characterisation of the most important coating properties [1]. Therefore, coatings have been evaluated using a variety of outdoor and laboratory tests for more than 100 years [2,3]. The ability to accurately predict the service life of organic coatings would be extremely important to coating manufacturers, suppliers and end-users [4-8].
In this chapter, the most widely used tests will be reviewed, together with associated evaluation methods. Based on this review, conclusions will be drawn upon the usability of these tests.
2.2 Outdoor Exposure
The most reliable way of studying the suitability of a coating for a specific substrate and environment is to actually expose coated substrates to the environment in which the coating will ultimately be applied [1,7,9-16]. This is generally done by mounting coated panels to exposure racks as shown in Figure 1.
Though long-term exposure to the actual environment offers a good representation of the actual service life, ‘natural’ degradation of modern protective coating systems is extremely slow. Outdoor exposure tests frequently require 10 to 20 years, before reliable conclusions can be drawn. Consequently, results are not provided in a commercially acceptable time period [11,14,15,17,18].
A second drawback to this approach is the variability of the “natural” environment. No time-period or location is the same as any other and thus exposed materials are degrading in constantly changing rates and fashions. This greatly influences the accuracy of the service life prediction [11,19-21].
Figure 1 Typical setup for outdoor exposure of coated panels.
2.3 Accelerated tests
Many researchers have attempted to speed up the natural coating degradation process by increasing the physical and chemical stresses (e.g. by altering temperature, humidity, pH, salt concentrations and the intensity of UV radiation) in so-called accelerated tests. Ideally, the stresses only cause the system to fail faster than it normally would, while the mechanism of failure remains the same as in the non-accelerated conditions [12,22-25].
The ability to relate the performance of a coating exposed in such tests to the field performance is a universal need. If such a linkage can be made, then it should be possible to greatly reduce the time-to-market for new products by substituting short-term laboratory exposure results for results from long-short-term field exposures [3,5,6,11,26].
2.3.1 Salt spray test
The most widely used accelerated test for coating evaluation is the salt spray test [11,27-31]. This has been true despite severe criticisms this type of testing has received [e.g. 6,11,24,26-37].
The salt spray test has its origins in the early 1900s and the procedure was standardized in 1939 under the designation ASTM B117 [31,38]. Since that time, it has found its way into all spheres of industry.
Salt spray tests are cabinet tests where a salt solution is pumped into a nozzle where it meets a jet of humidified compressed air, forming a droplet spray. The standardized pH-neutral salt spray tests ASTM B117 and ISO 9227* continuously subject test specimens to a fog of salt particles (5 wt% NaCl) at an elevated temperature of 35 °C. The justification for these extreme conditions has always been that a coating system that can resist these test conditions should also perform well in aggressive service environments. The assumption was that the mechanisms of corrosion and degradation in service would be similar to those in the test cabinet [27].
However, many examples can be found in literature suggesting that little, if any, correlation exists between the results from salt spray tests and in-service performance. Most remarkably, negative correlations have been observed quite regularly [13,30,34-37,39,40]. This means that certain coating formulations, which performed well in salt spray tests, actually perform worse in the field and conversely, systems that failed in the salt spray test show a good durability in field exposure.
The constant stress imposed during the test is frequently mentioned as a cause of the poor correlation with field performance. More specifically, the test does not allow relaxation of the stress, such as a drying out period. Because the level of stress normally changes periodically under natural conditions, constant stress produces degradation that is not observed in the field [11,24,27,29,31]. For example, in zinc-rich coatings or at galvanized substrates, zinc is not likely to form a passive film as it does in the field [41].
The prescribed spray electrolyte and the high temperature are also held accountable for the poor correlation. For barrier coatings, the osmotic forces are much less than in the field. In fact, they may be reversed completely from which is seen in reality [41]. Except for the harshest of heavy marine exposure sites, the use of 5 wt% NaCl produces unrealistic results. The exclusion of chemical species, present under normal conditions, result in unnatural chemistries of the corrosion products [6,33,42]. The
*
Current standards:
− ASTM B117-03, “Standard Practice for Operating Salt Spray (Fog) Apparatus”, ASTM International.
− ISO 9227:2006, “Corrosion tests in artificial atmospheres -- Salt spray tests”, ISO.
ASTM B117 and ISO 9227 are essentially similar. ASTM B 117 is specific to neutral salt spray (NSS), whereas ISO 9227 also covers acetic acid (AASS) and copper acetic acid (CASS) tests.
high temperature in the test cabinet takes many coatings above their Tg’s†, rendering
them non-protective and poor in film properties [24,41,43-45].
2.3.2 Cyclic testing
To overcome the deficiencies of continuous salt spray tests, cyclic weathering tests were developed. An early example is the Prohesion test. The development of this test started in the 1960s when Harrison and his co-workers argued that salt spray tests based on sodium chloride spray alone could never simulate corrosion in industrial atmospheres. Instead, they used a mixture of 0.25 wt% NaCl and 3.25 wt% (NH4)2SO4
dubbed “Harrison’s mix” to replace the 5 wt% NaCl solution used in the salt spray test. When Harrison used this new mixture in the salt spray test, he observed an improved correlation with coatings exposed for 14 years in an industrial environment. Timmins developed Harrison’s ideas further. He used a diluted Harrison’s mix (0.05 wt% NaCl and 0.35 wt% (NH4)2SO4) for his tests and introduced wet and dry cycling
(one hour of fog at ambient temperature and one hour of drying at 35 °C). He assumed that the addition of wet and dry cycles and lowering the solution temperature to ambient would correspond more closely to natural weathering. Timmins called this procedure the Prohesion test, which is an acronym for “Protection is Adhesion” [14,27,46,47].
In the 1980s, Lyon further refined this method as did Skerry, to the point when it was recommended to ASTM for adoption in 1994. It is currently issued as ASTM G85 - Annex 5‡ [46].
Incorporation of cycling steps seems intuitively justified, considering that coatings exposed to the outdoor environment undergo similar effects on a frequent basis [32]. Indeed, many have claimed the superiority of cyclic tests over conventional salt spray tests, as these tests produce failures more representative of field results, with better correlation to actual environments [7,11,29,35,37,48]. In fact, one of the most important distinctions among exposure tests is whether they are constant stress or cyclic stress [11]. There are a number of reasons for this:
†
The glass transition temperature (Tg) of a non-crystalline material is the temperature at which a
material's characteristics change from that of a glass (below Tg: hard and brittle) to that of rubber
(above Tg: elastic and flexible). ‡
Current standard:
− ASTM G85-02e1, “Standard Practice for Modified Salt Spray (Fog) Testing - Annex A5, dilute electrolyte cyclic fog dry test”, ASTM International.
− Cyclic variation of temperature allows for some thermally induced expansion and contraction of the materials causing stresses between the coating and the substrate [27,49].
− Absorption of water into a coating, from humid air or surface deposition causes a volume expansion resulting in stresses within the material. A wet period followed by a dry period then causes a volume contraction of the surface layers setting up more stresses within the coating. Cycling of such stresses can eventually result in fatigue, setting the stage for further chemical and mechanical change or degradation [20].
− Corrosion and wet-dry cycling causes the successive formation and drying of corrosion products. This results in a mechanical load at the coating substrate interface which affects the (wet) adhesion of the coating [33,42].
− Corrosion on coated steel substrates (e.g. near scribes) is faster during wet-dry transitions. On wetting, the corrosion rate rises rapidly as accumulated surface salts dissolve. The rate then decreases as the surface electrolyte dilutes with continued wetting. The corrosion rate also rises noticeably during drying because of both the increasing ionic activity as the surface electrolyte concentrates and the diffusion layer thickness for oxygen as the condensed phase becomes thinner. Finally, when the ionic concentration of the electrolyte layer becomes very high and salts begin to crystallize, the corrosion rate decreases again. These effects may be even more pronounced on zinc substrates [41].
This all adds up to impart a more realistic stress onto the coating system in an accelerated manner. Recognizing this, there has been a trend toward cyclic testing and several manufacturers have developed their own cyclic test methods [33,39,48,50].
2.3.3 Advanced cyclic testing
Test chambers in which specimens are exposed to ultraviolet radiation (UV) are widely used to obtain weathering data for a wide range of polymer products, including coatings. Commercially available UV chambers already began to appear circa 1920 and since then numerous modifications have been made [51].
While cyclic testing was seen as an important advance, it was also suspected that ultraviolet radiation (UV) plays an important role in natural weathering of coated metals. This led Skerry and his co-workers to investigate the influence of an added UV weathering cycle. The results indicated that the corrosion performance characteristics of organic coatings were markedly affected by the UV-weathering factors in the test [32].
Furthermore, the combined cyclic salt fog/UV test showed an improved reproduction of coating performance ranking and failure modes observed in practice [6,12,24,27,31,32]. In this respect, this cyclic salt fog/UV test is superior to continuous salt fog, or even cyclic salt fog alone.
The upshot of these and subsequent studies was the publication of ASTM D5894 in 1996§ [27,31]. This test standardized procedure, exposes panels to alternating UV/condensation cycles and wet/dry salt-spray cycles.
The UV/condensation cycle begins with 4 hours of exposure to a UVA-340 fluorescent lamp at 60 °C followed by 4 hours of condensation exposure at 50 °C. Condensation takes place by heating a water reservoir. The produced hot water vapour condensates on the test panels. The duration of this cycle is one week.
The salt-spray cycle conditions are similar to that of the Prohesion test: one hour of fog [0.05 wt% NaCl and 0.35 wt% (NH4)2SO4] at ambient temperature and one hour of
drying at 35 °C. This cycle is also repeated for one week.
This test protocol is usually repeated three to six times, resulting in a total test time of six to twelve weeks [27,29,31].
Although ASTM D5894 is highly recommended [27,29,37,40], it is not yet the pinnacle for accelerated test methods.
For example, too-rapid corrosion of zinc has been cited in literature. The standard spray solution is inappropriate due to the inherent solubility of zinc sulphate corrosion products. In addition, the pH of the standard solution is between 5.0 and 5.4. At this pH, zinc reacts at a significantly higher rate than at neutral pH levels. If the performance of zinc-based and non-zinc coatings must be compared, an alternate (non-sulphate) electrolyte can be substituted under de guidelines of the standard. More generally, the electrolyte could be modified for composition or concentration according to the prevailing atmospheric conditions [27,41].
While ASTM D5894 has been found useful for industrial (onshore) applications, the test described in NORSOK standard M-501** has been developed particularly for the offshore industry. The NORSOK M 501 test is a standardised Norwegian weathering test designed for testing materials intended to bear the tough environmental conditions
§
Current standard:
− ASTM D5894-05, “Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal, (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet)”, ASTM International.
**
Current standard:
present in the North Sea. Even though it is a national standard, it is today probably the most recognized standard within the field of offshore coatings [12,28,52].
Common for ASTM D5894 and the NORSOK M-501 test is the cyclic exposure to salt spray, drying and UV/condensation. They differ in the prescribed spray electrolyte, number of hours exposed in each chamber and the total number of cycles/weeks for the whole test.
The former NORSOK M-501 test cycle (revision 4) takes 168 hours. Each cycle consists of 72 hours of salt spray (with synthetic sea water) at 35 °C, followed by 16 hours of drying in air and 80 hours of UV/condensation (4 hours of exposure to UV at 60 °C followed by 4 hours of condensation exposure at 50 °C). The test runs for 25 cycles, resulting in a total test time of 25 weeks [29,41,48,53].
In the fifth revision of NORSOK M-501, published in June 2004, performance testing of coating systems was brought in accordance with ISO 20340††[54].
This ISO standard combines two well-developed cyclic tests: the Norwegian NORSOK M-501 and the French Standard, NFT 34-600 [55]. A useful feature to come out of the French standard was to include the freeze cycle at -20ºC [56]. Since this freeze cycle is not obligatory, ISO 20340 makes allowance for cyclic testing under differing conditions as indicated in Table 1.
Table 1 ISO 20340 Cyclic testing options [57].
Option 1
72 hours Salt spray 5% NaCl at 35 °C 24 hours Dry out at -20 °C
72 hours Condensation/UV - 4 hours UV at 60 °C
- 4 hours condensation at 50 °C
Option 2
72 hours Salt spray 5% NaCl at 35 °C 24 hours Dry out at +23 °C
72 hours Condensation/UV - 4 hours UV at 60 °C
- 4 hours condensation at 50 °C
For both options, one cycle takes 1 week (168 hours). The ISO 20340 test also runs for 25 cycles, resulting in a total test time of 25 weeks [57].
The different dry-out temperatures of -20 °C and +23 °C can have a significant effect on the results obtained [56,57].
Mitchell, for example, mentioned the effect of the freeze cycle on barrier coating systems as compared to zinc-rich primed systems. The freeze cycle basically doubled the under-film creep of the high solids hydrocarbon modified epoxy, whereas on the
††
zinc-rich primed systems the freeze cycle had virtually no effect on performance. In both cases, the incorporation of a freeze cycle gives results much more similar to those seen in practice [56].
Unfortunately, ISO 20340 has become problematic because of the need to compromise between various national interests [56,58]. One important difficulty was to obtain agreement on the scribe dimensions. The French delegation and members of the ISO committee wanted to sustain the use of a narrow scribe of 0.05 mm, while the Norwegian delegation -supported by most other members- wanted a 2 mm scribe. As a result, two different scribes are now prescribed for each panel. The problem is that these scribes can cause cathodic disbondment on immersion panels, with the large scribe becoming an anode, showing severe metal loss while protecting the narrow scribe, the cathode [58].
The latest NORSOK standard allows for the omission of the 0.05 mm scribe specified in ISO 20340.
2.3.4 Overview accelerated tests
In sections 2.3.1 to 2.3.3 a sequential overview has been given of the most widely used accelerated tests. Parallel to these developments, many other standardized tests have emerged and many manufacturers have developed their own tests. Table 2 lists some available test methods.
Table 2 Overview of some (standardized) accelerated tests.
Test Method Conditions
Continuous salt spray tests
ASTM B117a
ISO 9227 (NSS)b
DIN 50021 SSc
JIS Z 2371 (NSS)d
Continuous, pH-neutral salt spray (5 % NaCl at 35 °C).
Immersion test
ASTM D870a Coated specimens are partially or completely immersed in distilled or
de-mineralized water at ambient or elevated temperatures. Humidity Tests
ASTM D2247a
ISO 6270b
Coated specimens are exposed to atmosphere maintained at approximately 100 % relative humidity with the intention that condensation forms on the test specimens.
− ISO 20340:2003, “Paints and varnishes -- Performance requirements for protective paint
Table 2 (continued) Overview of some (standardized) accelerated tests.
Test Method Conditions
Cyclic tests
ASTM G85 - Annex 5a Wet and dry cycling (1 h of fog (0.05 wt% NaCl + 0.35 wt% (NH4)2SO4) at
ambient temperature and 1 h of drying at 35 °C).
ASTM G 154a
ISO 4892-3b
Coated specimens are exposed alternating UV/condensation cycles. Advanced Cyclic Tests
ASTM D5894a
ISO 20340b
Alternating UV/condensation cycles and wet/dry salt-spray cycles. ISO 20340 includes an optional freeze cycle.
Automotive
GM 9540P/B (General
Motors)e
Wet/dry and humidity cycling. Electrolytic solution: 0.9% NaCl, 0.1% CaCl2,
0.25% NaHCO3, pH: 6-8.
Total cycle time: 24 h. The typical duration of the test is 80 cycles (1,920 h) [60].
CCT-I, IV (Nissan)e Wet/dry and humidity cycling. Electrolytic solution: 5 % NaCl.
The typical duration of the test is 200 cycles (1,600 h) for CCT-I and 50 cycles (1,200 h) for CCT-IV [60].
VDA 621-415e Wet/dry and humidity cycling. High time-of-wetness, poor correlation for zinc
pigments and galvanized steel. Also used for testing heavy infrastructure paints [41].
HCT (Hoogovens
Cyclic Test)e
Based on actual weathering conditions in The Netherlands. The test simulates two years of exposure including daily and seasonal variations in 1,680 hours. The temperature an relative humidity are varied from
respectively 25 to 50 °C and 50 to 98 %. Test specimens are periodically dipped in an electrolyte. The composition of the electrolyte differs during winter and summer simulations [62].
VICT (Volvo Indoor
Corrosion Test)e
Different variants exist. Stresses used are temperature, humidity and salt solution (spray or submerged). Tends to produce filiform corrosion at scribes. The typical duration of the test is 12 weeks [41].
SAE J2334f 24 h cycle consisting of: 6 h 100% relative humidity at 50 °C, 15 min. salt
application (0.5% NaCl + 0.1% CaCl2 + 0.075% NaHCO3) and 17 h 45 min. of drying at 60 °C and 50% relative humidity. The typical duration of the test is 60 days [41]. Other tests Kesternich: ASTM G87a ISO 3231b DIN 50018c
8 h exposure to water vapour and sulphur dioxide at elevated temperature and humidity levels, followed by 16 h of ambient laboratory conditions. Test originally designed for bare metals exposed to a polluted industrial
environment. The relevance for organic coatings is highly questionable [41,59].
SCAB (Simulated Corrosion Atmospheric Breakdown) test:
ISO 11474b
Accelerated outdoor corrosion test. Test specimens exposed outdoors are periodically sprayed with a salt solution.
a ASTM International - American Society for Testing and Materials [http://www.astm.org] b ISO - International Organization for Standardization [http://www.iso.org]
c DIN - Deutsches Institut für Normung e. V. [http://www2.din.de] d JIS - Japanese Standards Association [http://www.jsa.or.jp]
e Corporate standard
f SAE International - Society of Automotive Engineers [http://www.sae.org]
2.4 Evaluation methods
2.4.1 Visual evaluation
Coating degradation is usually evaluated using standardized methods [e.g. in:6,7,10,29,32,34,61]. The most common evaluation methods are summarized in Table 3. Unfortunately, the majority of these methods are based on visual observation [63], rendering them rather subjective.
Table 3 Common standardized evaluation methods of specimens subjected to accelerated tests.
Standard Aspect Description
ISO 4628-2, ASTM D714
Blistering These standards describe a method for assessing the degree of
blistering of coatings by comparison with pictorial standards. The ISO standard has adopted the pictorial standards from ASTM and includes the correlation between the ISO and ASTM rating systems.
ISO 4628-3, ASTM D610
Rusting These standards describe a method for assessing the degree of
rusting of coated steel surfaces by comparison with pictorial standards.
The ISO standard includes the correlation between the ISO and ASTM rating systems.
ISO 4628-4, ASTM D661
Cracking These standards describe a method for assessing the degree of
cracking of coatings by comparison with pictorial standards. ISO 4628-5,
ASTM D772
Flaking These standards describe a method for assessing the degree of
flaking (scaling) of coatings by comparison with pictorial standards.
ISO 4628-6, ISO 4628-7, ASTM D42148
Chalking These standards describe methods for assessing the degree of
chalking of coatings by comparison with pictorial standards.
ISO 4628-8, ASTM D1654
Delamination and corrosion
These standards specify methods for assessing delamination and corrosion around a scribe in a coating on a test panel or other test specimens. The ISO standard describes one method which involves the use of pictorial standards.
Both standards include numerical rating of failure.
The results heavily depend on the ability of the operator to translate a visual observation into a performance rating [12,64]. For high-performance coatings that
show almost no visual signs of deterioration after weathering, discrimination between systems is even impossible [42].
2.4.2 Electrochemical evaluation
There has been considerable effort to develop evaluation methods for organic coatings that are numerical, reproducible and accurate. The use of electrochemical techniques, in particular electrochemical impedance spectroscopy (EIS), has been shown to be very useful. EIS not only provides results in a short time but the obtained data can give indications on the actual corrosion mechanisms. In addition, corrosion and coating damage may be determined prior to its visual manifestation [1,7,17,25,36,42,65-71]. This coincides with the present trend towards the development of methods which enable early prediction of coating performance, even before the occurrence of any substantial changes in its appearance [7,17,42,67,68,69,71-73].
2.5 Acceleration factor
The amount of “acceleration” provided by a laboratory test can be determined by relating the test results to those of an outdoor exposure test. With equation 1, the acceleration factor of the laboratory test can be calculated [41,74,75]:
test field field test t t x x A= ⋅ (1) where: A = acceleration factor [-]
xtest = response from accelerated test, e.g. creep [mm]
xfield = response from field exposure, e.g. creep [mm]
tfield = duration of field exposure [h]
ttest = duration of accelerated test [h]
Table 4 Acceleration factors of accelerated tests compared to outdoor exposure. Acceleration factors have been calculated for average scribe creep.
Accelerated test Outdoor Environment Acceleration factor (A)
ASTM B 117 [61] marine exposure site, Sea Isle City,
New Jersey, USA
12.5 Cyclic Salt Fog
(modified version of ASTM G85) [61]
marine exposure site, Sea Isle City, New Jersey, USA
10.6 NORDTEST NT BUILD 228
(cyclic salt spray, ASTM G85) [75]
offshore field test site, Snorre, Norwegian sector of the North Sea
55 NORSOK M501
(Rev. 1, 1994) [75]
offshore field test site, Snorre, Norwegian sector of the North Sea
14 Freeze/UV-condensation/Cyclic Salt Fog
(non standardized) [61]
marine exposure site, Sea Isle City, New Jersey, USA
4.16
Tiemens et al. report different ‘acceleration’ factors ranging from less than 1 to more than 8. The amount of acceleration was found to depend on the exposed materials. In addition, similar materials reacted differently to two different accelerated tests [76]. As mentioned in section 2.2, the outdoor environment is highly variable and exposed materials are degrading in constantly changing rates. Consequently, acceleration factors are also found to vary over time [74].
Attempts to increase the acceleration factor by intensifying the laboratory testing conditions, will result in unnatural changes in degradation mechanism and accounts for a less realistic comparison with natural weathering [15,20,30,77]. Results of Chong [61] and Knudsen et al. [75] imply that an inverse relation between the acceleration factor and the correlation of accelerated tests with natural conditions exists. The correlation to field exposure was found to decrease with increasing test acceleration.
2.6 Correlation
2.6.1 Calculation correlation coefficients
The significance of an accelerated test is measured against how well it correlates with field performance [27,48]. Correlation coefficients can be considered as indicators of the uniformity of acceleration within a batch of samples [41]. Correlations coefficients are often calculated by using the following equation [61,78]:
∑
∑
∑
− − − − = 2 2 ) ( ) ( ) )( ( y y x x y y x x r i i i i (2)where:
r = correlation coefficient [-]
xi = single response value from test A, e.g. creep [mm]
x= average response value from test A, e.g. creep [mm]
yi = single response value from test B, e.g. creep [mm] y = average response value from test B, e.g. creep [mm]
Equation 2 assumes a linear relationship between the two tests and is calculated for data from samples run in an accelerated test versus the response of identical samples in a field exposure [41].
A value of r = 0 indicates no linear relationship, whereas a value of r = 1.0 or r = -1.0 suggests a strong linear relationship between the two tests.
Many variations exist in the calculation of the linear correlation coefficient, including the use of overall performance indices (sum of the individual ratings, e.g. of blistering, rusting and delamination) [40] and weight factors that reflect their importance in practical situations [76].
Alternatively, the Spearman rank correlation coefficient is also used for correlation analysis [6,40,79]. This use of this coefficient has some advantages. The method does not assume a linear relationship and is based on the ranks of the performance rather than on values of physical properties, even when the actual values of the measured systems are unknown. The Spearman rank correlation coefficient is calculated with [80]: ) 1 ( 6 1 2 2 − − =
∑
n n d r i s (3) where:rs = Spearman rank correlation coefficient [-]
di = difference in statistical rank of two corresponding systems [-]
n = number of pairs of values [-]
Table 5 shows a fictitious example of the calculation of rs. Raw data of an accelerated
test and an outdoor exposure test are converted into a performance ranking. Note that the tied score of 2.2 mm scribe creep is assigned the average of the ranks 2 and 3. The
differences di of the corresponding ranks are calculated and are squared ( 2 i d ). The sum of 2 i
d is used to calculated rs with equation 3.
Table 5 Example of the calculation of the Spearman rank correlation coefficient.
Coating Accelerated test Outdoor Exposure
i
d 2
i d
Scribe Creep (mm) Rank Scribe creep (mm) Rank
A 3.6 1 2.2 2.5 1.5 2.25 B 4.9 3 4.3 5 2 4 C 5.1 4 3.6 4 0 0 D 5.3 5 2.2 2.5 -2.5 6.25 E 3.7 2 1.3 1 -1 1 F 14.2 6 6.7 6 0 0
∑
2 i d = 13.5 rs = 0.922.6.2 Brief overview of correlation studies
In literature, many investigations into the correlation of accelerated test with outdoor exposure have been described. The number of extensive, long-term studies is much smaller. A few studies deserve to be mentioned here.
Considerable work has been undertaken by both the SSPC/ASTM (Steel Structures Painting Council / American Society for Testing and Materials) and CSCT (Cleveland Society for Coatings Technology).
SSPC/ASTM began a round robin program to evaluate protocols for testing industrial maintenance coatings [34]. However, some coatings contained ingredients that are questioned for their environmental impact or are legislatively banned.
In order to broaden the usefulness, CSCT decided to undertake a comparable study, but would investigate newer compliant coatings [5,6]. In the study, the results of common accelerated tests were compared to 9 diverse exposure sites throughout the USA. Table 6 shows the averaged correlation of the tests with those of the outdoor exposure, using different standardized evaluation methods.
Table 6 Overview of Spearman rank correlation coefficients of various tests with 12 months of outdoor exposure. Adapted from Carlozzo et al. [6].
Test Delamination, corrosion ASTM D1654 Rusting ASTM D610a Blistering ASTM D714b Salt fog -0.173 0.045 0.058
Cyclic Salt Fog -0.050 0.315 0.769
Prohesion -0.122 0.541 0.688
Prohesion/QUV 0.519 0.481 0.782
Outdoor exposure
(intercorrelation exposure sites)
0.693 - -
a Correlation based on 5 exposure sites. Remaining sites not differentiated enough to report. b Correlation based on 4 exposure sites. Remaining sites not differentiated enough to report.
The highest correlation for rust creepage (ASTM D1654) was found for the intercorrelation of the 9 exposure sites (rs = 0.69). Of the accelerated tests,
Prohesion/QUV showed the highest correlation (rs = 0.52) for rust creepage [6].
The performance of coated panels is frequently determined by rust creeping from an artificially made scribe, as this is presumed the only relatively quick way of differentiating between the performances of high-quality coating systems. Unfortunately, this is not the way a coating system usually breaks down in practice. When a coating system ages and eventually breaks down - it is due to general rusting, blistering, cracking etc. “Acceleration” by scribing panels prior to the test, facilitates assessment of the damage-protective properties of the coating/substrate system and not the barrier properties of the system [22,52].
Indeed, results of quite some exposure sites of the CSCT study did not show sufficient rusting and blistering.
Although substantial correlations for surface rusting (ASTM D610) and blistering (ASTM D714) appear to be present for some sites, some results are actually an indictment of the unnatural failure modes found in high salt concentration electrolytes [6].
Knudsen et al. performed tests to identify which accelerated test could be used to assess the corrosion performance of coatings in marine atmospheres [75]. Scribed test panels were exposed at an offshore field test site for about 5 years. Analogous test specimens were subjected to 4 different accelerated tests.
The performance was assessed by measuring the maximum and average scribe creep. The overall correlation factor (r) for average scribe creep was 0.34 for both the standard salt spray test and the cyclic salt spray test. The correlation of the Volvo test (Volvo Corporate Standard 1027, 1375 procedure 2A) was 0.76. The correlation of the NORSOK test was determined for two test series and was respectively 0.62 and 0.76.
A few coatings developed large scribe creep during the field test while this behaviour was not predicted by any of the accelerated tests. It appears that the high correlations for the Norsok and Volvo test are mainly determined by outliers with a relatively strong scribe creep. When these outliers are not taken into consideration the correlation coefficients decrease considerably.
In 1999 ECCA (European Coil Coating Association) and TNO (Netherlands Organisation for Applied Scientific Research) executed an extensive research programme to assess the reliability of different artificial corrosion tests and their correlation with 10 years of outdoor exposure. The complete program included 49 different systems, 6 European exposure sites and 9 laboratory tests, including Prohesion/QUV [76].
Correlations were based on the amount of attack in mm2 per m2 (or in mm2 per m along edges and scribes) and the use of weight factors that reflect their importance in practical situations.
Although for specific combinations of substrates, tests and exposure sites correlation coefficients may exceed 0.90, the variation of coefficients is rather large.
From this exhaustive study it was concluded that none of the accelerated tests assessed in this study could reliably predict the medium and long-term durability of coil-coated materials.
Besides the correlation between natural and accelerated weathering, standardized tests also leave some degrees of freedom regarding the operating conditions of the test. This can cause a large scatter in test results. Hubrecht et al. detail a similar test with similar specimens that was executed by 4 different laboratories. The amount of scribe creep was substantially different on many occasions and even affected the performance ranking of the systems [66].
2.7 Conclusions
Based upon this review of accelerated tests and evaluation methods, the following conclusions can be drawn:
− The most reliable way of studying the suitability of a coating for a specific substrate and environment is to actually expose coated substrates to the environment in which the coating will ultimately be applied. Evidently, this approach is very time-consuming.
− The standard salt spray test should not be used for service life predictions or even performance ranking of coated systems.
− Cyclic testing imparts a more realistic stress onto the coating system in an accelerated manner.
− The combined cyclic salt fog/UV tests show a still improved reproduction of coating performance ranking and failure modes observed in practice.
− All (accelerated) tests, including outdoor exposures, are relative tests. They do not give absolute predictions of how many years a material will last in actual service. They merely provide an indication of relative performance.
− Different outdoor service conditions will lead to different coating durabilities. Some materials may perform particularly well in one environment but may produce relatively poor results in other environments. It is therefore not to be expected that a single test method will be representative for all outdoor exposures.
− Attempts to increase the acceleration factor by intensifying the laboratory testing conditions will result in unnatural changes in degradation mechanism and accounts for a less realistic comparison with natural weathering.
− Standardized visual evaluation techniques are subjective. The results heavily depend on the ability of the operator to translate a visual observation into a performance rating. In case of high-performance coatings that almost show no visual signs of deterioration after weathering, discrimination between systems is even impossible.
− The performance of coated panels is frequently determined by rust creeping from an artificially made scribe. Scribing panels prior to the test facilitates assessment of the damage-protective properties of the coating/substrate system and not the important barrier properties of the system.
− The use of electrochemical evaluation techniques, in particular EIS, has been shown to be very useful. EIS not only provides more precise results in a short time but the obtained data can give indications on the actual corrosion mechanisms. Consequently, the sensitivity of this technique allows the total test time to be shortened.
2.8 References
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9 Munger, C.G., Corrosion Prevention by Protective Coatings, National Association of
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10 Morcillo, M., J. Simancas, J.M. Bastidas, S. Feliu, Comparison between laboratory and
