protective coatings: life prediction and performance evaluation

Protective Coatings: Performance Evaluation and Life Prediction

Dr. Jianhai Qiu School of Materials Engineering

Nanyang Technological University Nanyang Avenue, Singapore 639797

ABSTRACT Methods used to evaluate the performance of protective coatings and their capability to predict the service life in real environments are discussed in the context of the degradation process of a coating system. The accelerated tests based on existing standards (ASTM) are compared with some non-standardized methods such electrochemical impedance and electrochemical noise methods. Fairly good qualitative correlation of accelerated test results with service performance has been reported. It is noted that these "accelerated" tests may not really yield results in an "accelerated" way as most methods require several thousands of hours of exposure and the they are often destructive in nature. On the other hand, electrochemical impedance and and electrochemical noise methods are non-destructive and non-accelerating in nature yet they can produce quantitative or semi-quantitative results within a few days or even hours. When real-life exposure tests are used in conjunction with the non-destructive electrochemical impedance/noise methods, realistic models for life prediction of protective coatings may be developed.

Introduction Protective coatings is probably the most widely used method for combating corrosion. Steel structures exposed to atmospheres, buried in the soil or immersed in the sea water are commonly protected with coatings either alone or in combination with cathodic protection. This broad term - "protective coatings" encompasses metallic coatings, inorganic coatings and organic coatings as shown in Table 1.

Table 1 Types of Protective Coatings Type of Coatings Examples

Metallic coatings

hot-dip galvanizing, electroplating, electroless plating, anodizing, thermal spraying or metallizing, cladding, diffusion coating

Inorganic coatings porcelain coating, glass-lining

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No matter how complicated the design and formulation of a coating system may seem to be, its protective properties all boil down to two basic functions:

Physical Barrier Function - to separate the metal from coming into contact with a corrosive environment Chemical Barrier Function - to control the micro-environment at the metal/coating interface

Any factors that may influence how the above two functions work will have an influence on the protective property of a coating system (for better or for worse). Some of these factors are coating selection, surface preparation, application condition, inspection and routine maintenance. When a coating system failed prematurely, the coating's supplier, the contractor and the facility owner often do not agree with each other over the causes of the coatings failure and who should pay for cost of rectifying the problem. Sometimes lawsuits do arise from dispute of this nature. It was reported that a coating system expected to last for 25 years failed only after 12 months in service [1]! While it is possible to compare the relative salt spray resistance of different coating systems, there is no straightforward answer to the simple question of how long the coating will last. It is hoped that this paper will bring the coatings designers/specifiers, suppliers, contractors and facility owners a step closer in the understanding of the pros and cons associated with various methods of coatings evaluation and life prediction.

The Degradation Process of A Coating System A brief discussion on the degradation process of a coating system is helpful in understanding the nature of various methods for evaluation and life prediction. The durability of a coating system is ultimately determined by its capability to resist the ingress of moisture/water, oxygen and other ionic species such sodium and chloride (Fig.1). In addition to this barrier function, metal coatings such zinc or aluminum on steel substrate (galvanized or aluminized steel) can provide sacrificial cathodic protection to the steel substrate. Anodic coating material such as zinc and aluminum applied on cathodic steel substrate represents a "safe" coating system in that at coating breaks, corrosion of the steel substrate would not be accelerated. In contrast, cathodic coating material such nickel and chromium platings applied on anodic steel substrate ensues the risk of accelerated attack on the steel substrate at coating breaks. There is a general consensus that the service life of zinc coatings is directly proportional to the coating thickness/zinc mass. However, no such generalization can be extended to polymer paints. The service life of a polymeric coating system can be roughly said to be proportional to the diffusion time (for example, the time it takes for a critical amount of moisture, sodium ion or chloride ion to reach the Steel/Coating interface). Once the reactants reach the Steel/Coating interface, corrosion of steel substrate can take place readily. Accumulation of voluminous corrosion product (rust) causes paint film to blister and this in turn accelerated the ingress of moisture/water and other species into the steel/coating interface. Performance evaluation methods can be based on the detection of the early changes in the dielectric property of a dry

Organic coatings

paints, vanishes, lacquers and numerous other polymeric materials that readily form durable dry films


polymer paint film before any visible signs of staining or rusting occurs (stages 1 and 2 in Fig.1). The desirable attributes for the method of coating evaluation and life prediction is listed in Table 2.

Fig. 1 Ingress of reactants into the steel/coating interface

Table 2. Desirable Attributes for Methods of Coating Evaluation and Life Prediction

Accelerated Tests for Performance Evaluation ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus

The first and most widely used accelerated test is the conventional salt spray (Fog) test which carries the standard designations of ASTM B117, BS3900-Part12 and ISO 7253. Salt spray was first used in 1914 for corrosion testing and was standardized by ASTM as test method B117 in 1939. There have been many revisions with the latest designation being B117-97. As a qualifying/acceptance test, it does provide RELATIVE corrosion resistance information for coated/plated metals exposed to constant static condition of 5% NaCl at 35oC. There have been many cases where the coated steel lasted thousands of hours in the salt spray tests but failed prematurely in outdoor service (Fig.2) [8]. There are also cases where a coating system performed well in outdoor exposure but failed quickly in the salt spray cabinet. It has long been recognized that the coating's resistance to the salt spray environment can not be directly translated into the resistance to other environmental conditions. In fact, it states very clearly in the ASTM standard B117-97 that "prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand alone data". The natural environment is a dynamic and ever-changing one. The cyclic wetting and drying when the rain comes and goes, the temperature variation from day to night and the UV radiation from sunlight are all missing links in correlating

Responsive (rapid measurements from a few hours to a few days) Quantitative (quantitative parameter to describe the possible corrosion state of a coating system under real life exposure) Non-destructive (evaluation of real life structures in the field). Predictive (correlation with real life exposure performance)


the salt spray test results with real life performance.

Fig.2 Corrosion at the Metal/Coating Interface

ASTM G85 Standard Practice for Modified Salt Spray (Fog) Testing

As an improvement to the original salt spray test, the modified salt spray (fog) testing (ASTM G85) has introduced 5 modifications to the standard salt fog test:

continuous acetic acid-salt spray test cyclic acidified salt spray test cyclic sea water acidified test cyclic SO2 salt spray test dilute electrolyte cyclic fog dry test

Among these modifications, the dilute electrolyte cyclic fog dry test was reported to give better correlation with outdoor exposure test. This procedure uses a much diluted electrolyte (0.05% sodium chloride) with small amount (0.35%) of ammonium sulphate to represent industrial atmospheres. The test cycle alternates between 1 hour of fog and 1 hour of dry-off. During each drying-off cycle, the salt concentration would progressively increase thus exposing the samples to a wider range of salt concentrations.

ASTM D5894-96 Practice for Cyclic Salt Fog/UV Exposure of Painted Metal

Extensive research work on the effect of condensation and UV radiation led to the incorporation of UV radiation and condensation cycles into the cyclic salt spray test. The improved simulation of natural atmospheric conditions is found in the ASTM D5894. Basically, the procedure involves the following:

1 weeks (168 hours) fluorescent UV-condensation cycle as per ASTM G53 with 4 hours of UV exposure at 60oC using UVA-340 fluorescent lamps and another 4 hours of condensation (pure water) at 50oC After 1 week, transfer the samples to a cyclic salt fog chamber and expose for another week as


per ASTM G85 Annex 5. After 1 week, transfer the samples back to the fluorescent UV chamber and repeat for a total of 4 or 12 2-week cycles, giving a total test duration of 1344 to 4030 hours.

Although this test is currently the most "realistic" laboratory test available for evaluating the coating performance, there does not exist a magic conversion factor where X hours of laboratory exposure is equivalent to Y years of actual weathering in service condition. If one considers the fact that the duration for a full scale test requires over 4000 hours (about 6 months) and two sets of salt spray equipment, not many companies may be able to justify the high cost and the long wait for running such an "accelerated test". This is particularly true for in-house quality control/assurance applications where rapid response to a process change is required.

Electrochemical Impedance Spectroscopy (EIS) It has been recognized for many decades that the corrosion protection by organic polymeric coatings is related to the changes in the dielectric properties of the polymer paint film. The response of a coated metal to a small AC signal at certain frequency can be described by an equivalent circuit model as shown in Fig.3 below:

Fig.3 Equivalent Circuit Model for a Metal Coated with Organic Polymeric Materials

where Rs is the electrolyte resistance, Rpore is the pore resistance in a coating system, Rt is the charge transfer resistance, Cdl is the double layer capacitance at the metal/coating interface and Cc is the coating capacitance.

Corrosion underneath a coating system can be described by the charge transfer resistance of Rt and the double layer capacitance Cdl, while the coating performance can be described by the coating capacitance and the pore resistance. With an increase in the permeability of a coating, the conductive paths in the coating will increase and this will lead to the reduction in the pore resistance. By measuring the changes in coating's capacitance, one can calculate the water-uptake (by volume) in a coating system [2]:


where Ct is the coating capacitance at time t and Co is the initial coating capacitance.

Fig.4 [4] is the impedance spectra for a rubberized fibre coating applied on a properly prepared steel surface. The pore resistance remained high even after 14 weeks immersion in 3.5% NaCl solution, whereas for the same coating applied on the steel substrate with mill scales, a noticeable decrease in the pore resistance is observed after 1 week immersion in NaCl solution (Fig.5). .

Fig.4 The pore resistance remains high after 15 weeks immersion in 3.5% NaCl solution


Fig.5 The pore resistance decreased to below 108 after 5 weeks immersion in NaCl solution

In general, when a coating system can maintain the impedance value above 109 Ohm.cm2 , corrosion at the metal/coating interface should not be an issue. When the impedance value drops below 107 Ohms.cm2 , one should be concerned about the corrosion activity at the metal/coating interface. The corrosion rate underneath the coating can be determined from Rt:

CorrRate (mm/y) = k* (B/Rt)

where k is the conversion factor and B is the proportionality constants determined by Tafel polarisation.

In conjunction with the conventional salt spray test (ASTM B117), Kendig and co-workers came up with a model to predict the time-to-failure (TTF) for a coating system [5]:

where dx/dt is the disbond rate measured using a tape pullback method and %v is the water-uptake in volume. When there is no appreciable pullback observed, Kendig used a default value of 10-4 mm/hr.

In conjunction with ASTM D610 and D714 for visual evaluation of coating's performance after immersion in sea water, electrochemical impedance spectroscopy was used to study the correlation between the breakpoint frequency of a coating system and its performance in immersed condition for


up to a period of 550 days [6]. For a typical coating thickness of 100 um, it took about 2 ~ 9 days for the chemical species such as oxygen, sodium ions and chloride ions to reach the metal/coating interface (Fig.1) and to initiate corrosion. The breakpoint frequency measurements after the initiation corrosion at the metal/coating interface can be used to predict the coating performance for up to 2.5 years into the future. Impedance measurements taken at exposure times less than the time required for the diffusion/permeation/migration of sodium ions to the metal/coating interface proved to be less successful in correlation with the long-term coating performance.

The electrochemical impedance spectroscopy is not a standardized method yet. It is non-destructive and non-accelerating in nature. The time required to take one measurement on a 100 um thick coating can be as quick as 1 hour. The quick response and the non-destructive nature of the technique make it also a powerful tool for quality control/assurance in steel coating lines and other manufacturing processes involving coatings.

Electrochemical Noise Another electrochemical technique that has shown promise in coating's evaluation and life prediction is the electrochemical noise. The fluctuations of current and potential for a given system can be monitored simultaneously, leading to the potential or current or resistance noise methods. When a coating start to deteriorate, the potential of the system tends to shift towards the active/negative direction, eventually approaching the potential value of a bare steel. Mirroring the potential changes, the current tends to increase with time for the low performance coating systems. The noise resistance (Rn=Vn/In) for a good coating system was found to be above 1010 Ohms.cm2 upon immersion and decreases gradually to 109 Ohms.cm2 after 2000 hours testing. AC impedance and electrochemical noise techniques were also successfully used to rank the performance of several coatings systems (polyurethane, epoxy-polyamide and alkyds) after 12 month exposure to an industrial atmospheric environment [7]. It was observed that the electrochemical measurements after exterior exposures showed the same general trend as those generated by laboratory immersion tests.

Concluding Remarks The availability of various standardized and non-standardized test methods means that the search for the magic conversion factor where X hours of test (accelerated or non-accelerated, in laboratory or in field) can be extrapolated to Y years of service life is continuing. In many consulting projects, the author is often asked to prove how long a particular coating system will last. It takes a lot of effort to explain to the clients that no such test methods exist yet. The standardized ASTM methods are accelerated tests which may require several thousand hours of continuous operation for a typical coating system (ASTM D5894 requires more than 4000 hrs). These tests are all destructive in nature and the test results are qualitative. In contrast, the electrochemical impedance and electrochemical noise methods, though not standardized, can provide rapid and quantitative measurements of the protective properties of a coating system. These measurements are non-destructive and non-accelerating. It may be possible to find a magic conversion factor for life prediction if one monitors the long-term electrochemical responses of coating system under exterior exposure condition. The coatings designers/specifiers, suppliers, contractors and facility owners should consider the


available options and agree on the specific method for performance evaluation of a coating system. They must also realize the risks involved in extrapolating the accelerated test results.

REFERENCES:

1. E. D. Thomas and A. A. Webb, Journal of Protective Coatings and Linings, Feb. 2001 2. D. M. Brasher and A. H. Kingsbury, J. Appl. Chem., Feb., 1954, p62 3. J. Wolstenhole, Corrosion Science, Vol. 13, p521, 1973. 4. J. H. Qiu, Corrosion resistance of rubberised fibre coating, to be published 5. M. Kendig, S. Jeanjaquet, R. Brown and F. Thomas, J. Coatings Tech., Vol.68, p39, 1996 6. J. R. Scully, J. Electrochem. Soc., Vol.136, No.4, p979, 1989 7. C-T. Chen and B. S. Skerry, Corrosion, Vol.47, p598, 1991 8. J. H. Qiu, to be published

This paper is also available at http://www.corrosionclinic.com/corrosion_technical_papers.htm


protective coatings: life prediction and performance evaluation

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