Chapter 7
Quantification of Coating Aging Using Impedance Measurements
1
2
1
1
Ε. P. M. van Westing , D. H. van der Weijde , M. P. W. Vreijling , G. M. Ferrari , and J. H. W. de Wit Downloaded by IOWA STATE UNIV on February 17, 2017 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch007
2
1
Department for Corrosion Prevention, TNO Centre for Coatings Research, P.O. Box 57, 1780 AB Den Helder, Netherlands Laboratory for Materials Science, Division for Corrosion Technology and Electrochemistry, Delft University of Technology, P.O. Box 5025, 2600 GA Delft, Netherlands 2
This chapter shows the application results of a novel approach to quantify the ageing of organic coatings using impedance measurements. The ageing quantification is based on the typical impedance behaviour of barrier coatings in immersion. This immersion behaviour is used to determine the limiting case for the interpretation of results on coatings exposed to artificial and outdoor weathering. Impedance studies on organic coatings, performed for many years already (1-9) are mostly aimed to estimate the quality and performance of the coating and anti corrosion pigments and the remaining service life. The majority of these studies are based on the determination of the DC-resistance (8) or on the amount of blistering (9). The results of these studies are often poor as both a low DC resistance and the growth rate of blisters only yield information on a coating that has already failed. The time before failure is more important than the corrosion rate after failure as the first period will and should be significantly longer. It could be argued that the study of corrosion rate after failure is important, for instance when the performance of anti corrosion agents on the occurrence of blistering is evaluated. However, this still requires a detailed study of the coating impedance (10), as results presented in the preceding chapter by van der Weijde (11) show the pitfalls in the estimate of the corroding surface area underneath a coating. It has been shown in former studies that coatings can retain a very high DCresistance while water and ions (12,13) are taken up followed by loss of adhesion and the start of the corrosion process (14,15). This high resistance makes the DC-signal unreliable as a detection method for this event. However, detection of delamination is possiblefromthe changing dielectrical properties of the coating polymer, leading to the onset of corrosion. Even so, these changes are not reflected by the "ordinary" coating capacitance. Only analysis of the coating impedance using a Constant Phase ©1998 American Chemical Society
Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
69
70 Element (denoted by Q f, pf = paintfilm)enables the accurate detection of the onset of the coating delamination, leading up to the start of the corrosion process. The changes in the dielectric properties of the coating polymer that indicate the onset of corrosion are the result of the swelling (13-15) that occurs with the delamination of the coating. The previous chapter (11) showed that the life cycle of barrier coatings (without any sign of defect) in immersion can be divided in three stages: Stage 1) The pre saturation stage: in the beginning of the immersion where water and ions are taken up; Stage 2) Stationary phase: where values of Yo and η (Q f) are determined by the type of coating and the thickness of the individual coatings layers; Stage 3) Degradation stage: In this stage the degradation of the coating starts by the formation of blisters and the start of the corrosion process. During this period the coating maintains its barrier properties -high DC-resistance- and there are no visual signs of the presence of defects. P
Downloaded by IOWA STATE UNIV on February 17, 2017 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch007
P
Experimental Method Materials. Two types of coatings for maritime applications were used in the investigations: one epoxy and one alkyd based. The primers of both coating systems contain zinc phosphate. The coatings were applied to the steel panel by air spraying. The steel substrate for these coatings consisted of standardised test panels. The mill surface on one side of these panels was completely removed by abrasive grinding. The coatings were applied to this side of the panels. Part of the experiment was carried out on polyester coil coated material. This is a pre-coated material consisting of a hot dip galvanised cold rolled steel substrate with a chromate surface pre-treatment, a polyester primer and a polyester topcoat. This coated material was selected for its uniform properties. Different sections of the same sheet only have small differences in layer thickness and dielectric properties, so effects of a cyclic immersion test can be accurately monitored with this material. 1
Equipment and data analysis. The electrochemical impedance measurements were carried out using the following Solartron instruments: 1286 Electrochemical Interface and 1250/1255 Frequency Response Analysers. This equipment was controlled by a PC. Further information on the instrumental set-up and the electrochemical cell can be found elsewhere (16). Impedance measurements were performed in the range between 100 kHz and 100 Hz when coatings are immersed only for short periods (< 60h) and the coating impedance changes rapidly. This set-up allows for a highly accurate impedance measurement every 7 minutes. For longer immersion periods coating impedance is measured between 500 kHz and 0.1 Hz. The results were normally analysed between 10 kHz and 1 kHz as it is impossible to measure the DC-resistance at low frequencies with the barrier coatings used here. Low frequencies were measured only to be sure of the possibility to detect failure of the barrier properties. The results of the impedance measurements were analysed using Boukamp's EQUIVCRT (17,18). For the analysis of the impedance of barrier coatings an Q-panel Company, Cleveland, Ohio USA, type S Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
71
Downloaded by IOWA STATE UNIV on February 17, 2017 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch007
equivalent circuit consisting of a resistor (electrolyte resistance) in series with a Constant Phase Element (CPE, denoted by a Q) (17,18) is used. The CPE represents the response of the coating. The instrumental set-up (19) allows for very accurate results of the impedance measurements. The variation in the results can be derived from the scatter in the plots of Yo and n, as no data reduction was carried out. Electrolytes. All coatings tested in the laboratory only were immersed in a 3% NaCl solution. The coatingsfromthe accelerated weathering and the outdoor exposure were immersed in artificial rainwater based on coastal rainwater composition in The Netherlands. The concentrations were increased 50 times. This electrolyte was chosen to measure the impedance of coatings of coatings in outdoor exposure from time to time. An electrolyte for this purpose should not be more aggressive than rainwater. The composition of this electrolyte is listed in table 1. Both the saltsfromwhich the electrolyte is composed and the resulting ionic composition are givea Table I. The composition of the artificial rainwater. The salts from which the electrolyte is composed as well as the ionic composition are given. (pH = 4.06) Salts
CuS0 .5H 0 FeCl .4H 0 NiCl .6H 0 MgCl .6H 0 4
2
2
2
2
2
2
2
NH4CI
Na S0 NaCl (NH4)S0 2
4
4
NH4NO3
Ca(N0 ).4H 0 3
2
KNO3
NaF HNO3
NaHC0
3
Contents (μιηοΐ/ΐ)
Contents (umg/l)
0.2 5.0 0.1 62.5 135.2 135.2 479.6 114.6 10.6 75.0 30.0 7.5 59.4 2.0
0.0499 0.9941 0.0238 12.7069 7.2318 19.2038 28.0278 15.1432 0.8484 17.7113 3.0333 0.3149 3.7428 0.1680
Ions
+
NH4 Na K Ca Mg F cr +
+
2+
2+
NO32
so Cu Fe Ni 4
2+
2+
2+
Ί Contents (umol/1) 375 760 30 6.5 62.5 7.5 750 250 250 0.2 5.0 0.1
Experimental Results and Discussion Results of experiments on the dielectrical behaviour of organic coatings during longterm immersion are presented in the preceding chapter (11). This behaviour can be used as a basis for the interpretation of impedance results for the determination of coating performance in alternating environments as outdoor exposure and artificial weathering. When a dried coating that had already reached Stage 2 once, the stationary situation, is reimmersed, the impedance of the stationary situation will be re established rapidly (20). This is illustrated in Figures la, b, and c, by impedance Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
Downloaded by IOWA STATE UNIV on February 17, 2017 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch007
1.00
•δ
ε
Ο
1.4Θ-010
200
400
600
800
1000
Exposure time (h)
Figure 1 :
Curves of Y and η (Q ) of an epoxy coating system (105 :m) as function of time during immersion in a 3% NaCl solution, a) first immersion, b) second immersion after 30 days of drying and c) third immersion after 53 days of drying. 0
pf
Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
73 curves of an intact barrier epoxy coating system («105μπι) during the first, second and third immersion. The stationary levels of the coating impedance parameters Y and η (Q f) are represented by dashed lines. In Figure la, it can be observed that the time to reach the stationary situation (Stage 2) for thefirsttime was about 2000 h. In the second (Figure lb) and third (Figure lc) immersion this level was reached in much less time. The physical differences between these situations indicate the irreversible changes resultingfromageing of the coating polymer. From a similar experiment on a barrier epoxy coating that had reached Stage 3 (with a defect, but high DC-resistance, probably loss of adhesion due to localised excessive swelling, no visual sign of any defect) similar behaviour can be observed. This is illustrated with the impedance curves in Figure 2a and b. From thisfigure,the values of Yo and η (Q f) at the end of thefirstand the second immersion are the same. However, these values are reached in a much shorter time in the second immersion. From experience it is known that a stationary level for η (Q f) for this type of coating would be around 0.92. Due to the poor performance of the coating, it does not show any stationary behaviour (Stage 2). From these results one can deduce that the curves of the coating impedance parameters contain relevant information for the determination of the ageing of coatings in alternating exposure. This is further shown by the behaviour of a polyester coil coating system in an immersion cycle in the laboratory. This cycle consisted of 24h immersion in a 3% NaCl solution and 72 h drying at 35
