coatings life prediction and performance evaluation

8/8/2019 Coatings Life Prediction and Performance Evaluation

http://slidepdf.com/reader/full/coatings-life-prediction-and-performance-evaluation 1/9

Protective Coatings: Performance Evaluation and LifePrediction

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 predictthe 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 protectivecoatings may be developed.

Introduction

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

Table 1 Types of Protective Coatings

Type of Coatings

Examples

Metalliccoatings

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

Inorganiccoatings

porcelain coating, glass-lining

www.corrosionclinic.com [email protected]



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 whichcarries the standard designations of ASTM B117, BS3900-Part12 and ISO 7253. Salt spray was firstused 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 aqualifying/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 beenmany cases where the coated steel lasted thousands of hours in the salt spray tests but failedprematurely in outdoor service (Fig.2) [8]. There are also cases where a coating system performedwell in outdoor exposure but failed quickly in the salt spray cabinet. It has long been recognizedthat the coating's resistance to the salt spray environment can not be directly translated into theresistance to other environmental conditions. In fact, it states very clearly in the ASTM standardB117-97 that "prediction of performance in natural environments has seldom been correlated withsalt 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 temperaturevariation 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 possiblecorrosion 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 bettercorrelation 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 industrialatmospheres. The test cycle alternates between 1 hour of fog and 1 hour of dry-off. During eachdrying-off cycle, the salt concentration would progressively increase thus exposing the samples to awider 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 thefollowing:

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 coatingperformance, there does not exist a magic conversion factor where X hours of laboratory exposure isequivalent to Y years of actual weathering in service condition. If one considers the fact that theduration for a full scale test requires over 4000 hours (about 6 months) and two sets of salt sprayequipment, not many companies may be able to justify the high cost and the long wait for runningsuch an "accelerated test". This is particularly true for in-house quality control/assuranceapplications 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 coatingsis related to the changes in the dielectric properties of the polymer paint film. The response of acoated metal to a small AC signal at certain frequency can be described by an equivalent circuitmodel 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 thecharge transfer resistance, Cdl is the double layer capacitance at the metal/coating interface and Ccis the coating capacitance.

Corrosion underneath a coating system can be described by the charge transfer resistance of Rt andthe double layer capacitance Cdl, while the coating performance can be described by the coatingcapacitance and the pore resistance. With an increase in the permeability of a coating, theconductive paths in the coating will increase and this will lead to the reduction in the poreresistance. 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 steelsurface. 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 inthe 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. Thecorrosion 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 Tafelpolarisation.

In conjunction with the conventional salt spray test (ASTM B117), Kendig and co-workers came upwith 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-uptakein 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 afterimmersion in sea water, electrochemical impedance spectroscopy was used to study the correlationbetween 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 forthe chemical species such as oxygen, sodium ions and chloride ions to reach the metal/coatinginterface (Fig.1) and to initiate corrosion. The breakpoint frequency measurements after theinitiation 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 timerequired for the diffusion/permeation/migration of sodium ions to the metal/coating interface provedto be less successful in correlation with the long-term coating performance.

The electrochemical impedance spectroscopy is not a standardized method yet. It is non-destructiveand 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 thetechnique make it also a powerful tool for quality control/assurance in steel coating lines and othermanufacturing processes involving coatings.

Electrochemical Noise

Another electrochemical technique that has shown promise in coating's evaluation and lifeprediction is the electrochemical noise. The fluctuations of current and potential for a given systemcan 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 theactive/negative direction, eventually approaching the potential value of a bare steel. Mirroring thepotential 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 impedanceand electrochemical noise techniques were also successfully used to rank the performance of severalcoatings systems (polyurethane, epoxy-polyamide and alkyds) after 12 month exposure to anindustrial atmospheric environment [7]. It was observed that the electrochemical measurements

after exterior exposures showed the same general trend as those generated by laboratory immersiontests.

Concluding Remarks

The availability of various standardized and non-standardized test methods means that the search forthe magic conversion factor where X hours of test (accelerated or non-accelerated, in laboratory orin field) can be extrapolated to Y years of service life is continuing. In many consulting projects, theauthor is often asked to prove how long a particular coating system will last. It takes a lot of effortto explain to the clients that no such test methods exist yet. The standardized ASTM methods areaccelerated 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 natureand the test results are qualitative. In contrast, the electrochemical impedance and electrochemicalnoise methods, though not standardized, can provide rapid and quantitative measurements of theprotective 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 monitorsthe long-term electrochemical responses of coating system under exterior exposure condition. Thecoatings 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. 20012. D. M. Brasher and A. H. Kingsbury, J. Appl. Chem., Feb., 1954, p623. J. Wolstenhole, Corrosion Science, Vol. 13, p521, 1973.4. J. H. Qiu, Corrosion resistance of rubberised fibre coating, to be published5. M. Kendig, S. Jeanjaquet, R. Brown and F. Thomas, J. Coatings Tech., Vol.68, p39, 19966. J. R. Scully, J. Electrochem. Soc., Vol.136, No.4, p979, 19897. C-T. Chen and B. S. Skerry, Corrosion, Vol.47, p598, 19918. J. H. Qiu, to be published

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


coatings life prediction and performance evaluation

Documents