50
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17,No. 1, 1978
I *Ot
B \
Acknowledgment The authors are grateful to many companies for their cooperation in providing samples and instructions. We are particularly grateful to BASF, AG Ludwigshafen, for their continuous supply of raw materials of their own and other companies' products, and for useful discussions and technical information, particularly with Dr. Neubert, Dr. Brussmann and Dr. Morcos of ANETA-LARO department. The work represents part of the activities under contract No. 0001475-C-1112 between the National Research Centre of Egypt and the Office of Naval Research of the Department of the Navy of the U.S.A.
t
60-
.
v
a b
17
I
VI 0-0
- e
30
63
90
120 '50 TIME (DAYS)
183
210
210
270
303
Figure 9.
Comparison between leaching rates o f antifoulings w i t h different plasticizers: X, w i t h nonhydrolyzable AF no. 6 ; 0 ,w i t h hydrolyzable AF no. 17; 0 , w i t h swellable AF no. 21.
Literature Cited Abd El-Maiek, M. M., Ph.D. Thesis, Faculty of Science, Cairo University,
1972. Abd El-Malek, M. M., Ghanem, N. A., J. Paint Techno/., 47 (608),75 (1975). Abd El-Malek, M. M., Ghanem, N. A,, "Proceedings of the Fourth International Congress on Marine Corrosion and Fouling, Juan-les-Pins, France, June
1976", 1977. Abou-Khalil, M. A., Abd El-Malek, M. M., Ghanem, N. A,, Paint Manuf., 40 (lo),
32 (1970).
rather severe climatic conditions with nontoxic pollution provides valuable information as to the efficiency of underwater protective systems. In anticorrosive paints, incorporation of lamellar aluminum in formulations based on a vinyl copolymer as binder provides better protection than when inhibiting pigments of the anodic passivation class are used without aluminum. In antifouling paints, a swellable but nonhydrolyzable plasticizer is superior to both nonhydrolyzable and hydrolyzable counterparts in regulating toxin release and allowing retention of surface copper in the form of basic copper carbonate.
Abou-Khalii, M. A., Ghanem, N. A., "Proceedings of the Fourth International Congress on Marine Corrosion and Fouling, Juan-ies-Pins, France, June
1976",1977. Egyptian Standard Specifications No. 197,1962,and 765,1966.The Egyptian General Authority for Standard Specifications, Cairo, Egypt. Ghanem, N. A., Abd El-Maiek, M. M., J. Chem. Egypt Arab Rep., 9, (3),377
(1966). Ghanem, N. A,, Abou-Khalii, M. A,, Farbe & Lack, 79 (3),201 (1973a). Ghanem, N. A., Abou-Khalii, M. A., Farbe & Lack, 79 (ll),1041 (1973b). Ghanem, N. A.. Moustafa, A. B., Abou-Khaiii, M. A,, Farbe& Lack, 77 (lo),961
(1971). Hippe, Z., Jedlenski, Z., Korat, J., Uhacz, K., J. Oil Colour Chem. Assoc., 45,
653 (1962). Marson, F., J. Oil Colour Chem. Assoc., 47, 323 (1964). Megally, MSc. Thesis, Faculty of Science, Alexandria University, 1970. Woods Hole Oceanographic institution, "Marine Fouling and Its Prevention", U S . Naval Institute, Annapolis, 1952.
Empirical or Scientific Approach to Evaluate the Corrosion Protective Performance of Organic Coatings W. Funke* and H. Haagen Forschungsinstitut fur Pigmente und Lacks, 0-7000Stuftgart 1, West Germany
Water and oxygen permeability of organic coatings and their adhesion when exposed to high humidity are proposed as basic properties to estimate the corrosion protective performance. Contrary to the visual criteria commonly used in conventional corrosion testing, these film properties provide information on how to improve the corrosion protective performance of organic coatings. A method of estimating adhesion of the humid film on a steel surface is described and the influence of some experimental variables on oxygen permeability is discussed.
Introduction There are not many subjects of paint research that have been more extensively studied over many years than the evaluation of corrosion protection by paint films. Despite these manifold efforts and the fact that the fundamentals governing corrosion under a paint film have been known for a long time the testing of the corrosion protective property still remains on a remarkably primitive level. Salt spray tests, exposure to sulfurous dioxide, water, or water vapor a t elevated temperatures, or water condensation tests are com0019-7890/78/1217-0050$01.00/0
monly used and results obtained by them often differ significantly from practical behavior. It is not surprising therefore, that practical exposure tests are still considered to be the most reliable means to recommend a paint or coating to a customer for special applications. Irrespective of whether a laboratory or practical corrosion test is considered, the testing results are mainly based on the appearance of the samples after exposure. This means that the evaluation is based upon the result and not on the properties which are responsible for good or poor protective be0 1978 American Chemical Society
Ind. Eng.
'
Chem. Prod. Res. Dev., Vol.
17, No. 1, 1978
51
.'!51202
-~ 1
7
Figure 1. Dependence of oxygen permeation rate on film thickness at 20 O C and 0% relative humidity (Po, = 150 mmHg).
Table I. Influence of Relative Humidity at 23 "C on the Oxygen Permeation Rate
Z x 105 le cm-2 d-ll Binder Cellulose acetate Cellulose nitrate PMMA PVCIPVA Chlorinated rubber Alkyd melamine resin Epoxide resin Alk. mel. resin, water-soluble pigment Acrylic resin, water-soluble pigment
0% R.H.
95% R.H.
1.4
2.6
9.4 1.9 0.96
11.5 1.3
0.13 1.1
1.19 0.16
Figure 2. Dependence of oxygen permeation rate on the oxygen pressure difference at 20 "C and 0% relative humidity.
1.1
0.76 0.91
0.64 1.8
0.89
2.1
havior. The situation is aggravated still more by a lack of differentiation between applicational causes, such as locally insufficient film thickness, pores or poor film formation, and paint-specific causes. However, it is just this differentiation which is a prerequisite for eliminating existing paint defects. More recently, it has been increasingly tried to substitute the common phenomenological evaluation by measurement of film properties which are intrinsically involved in the protection mechanism. For normal atmospheric corrosion of steel in neutral to weakly alkaline media, water and oxygen are needed. Some ions, frequently present as impurities such as chloride and sulfate, exhibit a catalytic effect on corrosion. If no water and oxygen are present, the rate of corrosion is zero or negligibly low. Therefore it seems logical to suppose that painted steel only corrodes with a substantial rate if H20 and 0 2 may permeate to the film/metal interface sufficiently rapidly. In this connection the action of anticorrosive pigments is not considered. If some limitations are observed, it is therefore feasible to measure permeation rates of both HzO and 0 2 and to compare these data with the consumption of HzO and 0 2 within the practical range of corrosion rates of unprotected steel (Baumann, 1972a,b). Permeability of Organic Coatings Permeability of paint films to water has been thoroughly to studied. Most data are within the range of g cm-2 d-l. On the contrary, reliable data on oxygen permeability are only recently available for a series of paint films (Baumann, 1972b; Funke and Machunsky, 1976). Other than water permeability, the permeation rate of oxygen may be reasonably well described by the diffusion laws of Fick in most organic coatings. Thus oxygen permeation rate
relates linearly to the reciprocal film thickness (Figure l), provided the films are dried completely throughout the layer and have a homogeneous composition. Accordingly, the dependence of the oxygen permeation rate from the oxygen pressure difference between the upstream and downstream side of the film was found to be linear (Figure 2). It should be mentioned, however, that this result is contradictory to that of Baumann (1972b), who found some deviation from linearity. Whereas pigmentation influences water permeability in a very specific way, depending on the participation of different diffusional pathway mechanisms, the rate of oxygen diffusion was always found to decrease markedly with increasing pigment concentration. Obviously the amount of oxygen carried by water permeating via internal interfaces between pigment particles and binder or via capillaries is negligibly small. Accordingly, oxygen permeability of dry films differed very little from that of films exposed to high humidity (Table I). The efficient way to reduce oxygen permeability by pigments explains why some pigments like iron oxide or mica, probably due to their shape, have proved to be most useful in formulation of corrosion protective paints. Increase of temperature very significantly enhances oxygen permeation (Table 11). By this result the fact is still more stressed that for the reliability of corrosion tests, it has to be considered carefully whether the glass transition temperature of a film is below or above the experimental exposure temperature. In Table I11 water and oxygen permeation rates are compared with the supply of both needed in atmospheric corrosion of bare steel within the practical range of corrosion rates. In accordance to results of Guruviah (1970) and Baumann (1972a),the water permeation rate is sufficiently high to allow corrosion to proceed on a painted steel surface as fast as if there were no film above. On the other hand, the rate of oxygen permeation is well within or even lower than the range of oxygen consumption of steel corroding under various exposure conditions. As may be seen from the ratio of permeation rates ZO~/ZH~O,and provided corrosion beneath a film may occur at
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
Table 11. Influence of Temperature on the Oxygen Permeabilitv Coefficient
Table IV. Time after Which Water Absorption of Supported Films Exceeds That of Free Films (F < S) (Cross-over Time) Film thickness
Paint system Cellulose acetate Cellulose nitrate PBMA Alkyd melamine resin Epoxide resin Alkyd melamine resin, water-soluble PVCIPVA Chlorinated rubber Polyisocyanate Alk. mel. resin, water-soluble, pigment Acrylic resin, water-soluble, pigment Polvester resin. water-soluble. Diement
10 "C
40 O
0.56 3.60 2.00 0.30 0.24 0.62 0.21 0.03 0.10 0.30 0.60 0.02
1.25 112.20
C
8.00
1.61 0.95 2.35 0.58 0.25 0.39 1.30 3.35 0.21
Table 111. Water and Oxygen Permeabilities of Paint Films, Diffusion Rate of Oxygen Dissolved in Water, and Water and Oxygen Required in Usual Corrosion Rates of Steel Permeation rate (HzO) I H ~ [g O cm-2 d-11 X 105 0 2 Solubility in HzO S = [g cm-33 x 105 Permeation rate 0 2 via HzO diffusion [g ern+ d-11 X 105
Permeation rate Zo2exp[g cm-2 d-l] X lo5
HzO and 02 required for corrosion I H ~ [g O cm-2 d-l] X lo5 Io2 [g cm-2 d-'1 X lo5
96-1150 (25-100 gm) 33-653 (70-150 pm)" 0.903 0.0087-0.00104 (25-100 fim) 0.0030-0.00059 (70-1 50 pm) 0.22-6.3 (20-35 gm) 0.13-9.4 (100 pm)C 0.0014-0.6 (100 pm)d 0.43-1.7 (100 gm)e 0.35-5.7" 0.87-15.13a
a Baumann (1972a). Funke, unpublished results. chunsky (1975). d Baumann (1972b). e Guruviah (1970).
Ma-
all, the rate of oxygen permeation may well be the rate-determining step for underrusting of paint films. Up to this point, the question of why corrosion below a paint film starts at all is still open. It was found that some films exhibit both high water permeability and very good corrosion protection. On the other hand, there are also examples of low permeation rates but poor protective property. Obviously there is some other important factor which determines whether under certain experimental conditions corrosion beneath paint films may begin. This factor was found to be the adhesion to the substrate when the painted sample is exposed to water or high humidity. Adhesion of Coatings Exposed to High Humidity Measurement of adhesion of organic coatings to metal substrates is possible by the pull-off test. Though some difficulties may be encountered even in dry films, measurement of adhesion is still more difficult and controversial when the film is exposed to high humidity or liquid water. These are the critical conditions by which adhesion is challenged most in practical performance. In previous work on water absorption of paint films when immersed in distilled water, it could be shown that in most cases pigmented films took up more water (based on the
Paint types Electrocoating no. 1 Electrocoating no. 2 Electrocoating [cathodic] no. 3 Electrocoating no. 4 Electrocoating no. 5 Electrocoating no. 6
Free (F) Supported (S) F
< S, h
35 32 20
34 29 20
>24 18 2
35.5 30.5 28.5
32 28
0.23 0.23 >24
26
binder fraction of the film) than unpigmented ones (Funke et al., 1969).This surplus of water accumulates a t the internal pigmenthinder interfaces. The fraction of the interfacially absorbea water could be easily determined by comparing absorption of pigmented films with that of unpigmented ones. Accordingly, in order to find out whether and how much water may accumulate at the film/substrate interface, we have compared water absorption of free and supported films. Due to the much smaller area of this interface, the fraction of water absorbed there is correspondingly small. It is therefore necessary to apply more sensitive weighing devices, such as an electrical balance, and to avoid the blotting-off technique by exposing the samples to high humidity instead of immersing them in liquid water. Preliminary experiments proved that under these conditions water absorption may occur a t the film/substrate interface. If the water absorption of the supported film remains below that of the free film, it may be assumed that water has not interfered at the filmjsubstrate interface up to that time. If the water absorption of the supported film surpasses that of the free film, there is no reason to suspect that the surplus water could be located elsewhere than at the film/support interface. Consequently a decrease of adhesion must be involved. The water absorption of a series of electrodeposited paint films at 90% relative humidity at 23 "C was measured continuously over exposure times up to 20 h and compared with the absorption characteristics of the corresponding free films. Two examples are shown in Figure 3. If no interfacial failure occurs within the exposure time, the water absorption of the supported film remains below that of the free one. It is also seen that absorption equilibrium of the free film is attained after a very short exposure time. If interfacial failure occurs due to the influence of water, the absorption curve of the supported film crosses over the curve of the unsupported film after some characteristic exposure time. As may be seen (Table IV), the cross-over time may be taken as a measure of the resistance to adhesion failure of paint films after prolonged exposure to high humidity. As far as the adhesion is concerned, no principal difference in the interfacial behavior of coating seems to exist between exposure to high humidity and to liquid water. Moreover, the "cross-over time" corresponded very closely to the paint film behavior in salt spray in our outdoor exposure tests. The measured amount of absorbed water has been based on the film weight. The exposed area and the film thickness of free and supported films were roughly the same. However, the steel panels were coated on both sides. As the weight increase by water absorption was based on the film weight, the initial absorption rate of the free films would be expected to be about twice as high as that of the supported films. The relatively high initial rate of the supported film indicates that obviously water may already enter the film/substrate interface before equilibrium absorption of the film is attained.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
53
a 15t
;i
A
Figure 4. Cross-over time of water absorption of free and supported films, water absorption of free films, and water permeability of free
films.
L io 23 o i
10
50
iMl6NO
Figure 3. Water absorption/time relation rate of free (F)and supported (S) electrodeposited films at p / p o = 0.9, 23 “C.
If water absorption of free films and their water permeability are compared with the cross-over time, no correlation may be recognized (Figure 4).The reason for this is the special relationship between these parameters, which must be considered when their role in the protective mechanism is regarded. This will be explained in the following conclusion.
Conclusion From all the film properties basically involved in the corrosion protective mechanism of organic coatings, the adhesion of the film in humid atmosphere or liquid water seems to be the most important one. As long as adhesion does not fail under these conditions, no corrosion will occur beneath a paint film, irrespective of anticorrosive pigments being present or not. However, if water interferes at the film/substrate interface-and this is indicated by the cross-over time of the water absorption curves of free and supported films-water permeability becomes the rate-determining factor for the loss of adhesion. On short exposure times a low water permeability may therefore delay the onset of adhesion failure. Such systems apparently exhibit good protective properties a t the beginning but are not to be considered as “safe”. This “weakness” will only be revealed after longer exposure times. On the other hand, a paint system may exhibit good protective properties, even if water permeability is not remarkably low, if its wet-state adhesion is superior. This special
relationship between wet-state adhesion of organic coatings and their water permeability may also explain why discussions of the latter in the literature in connection with the protective property have been so controversial. Finally, the rate of oxygen permeation, which is much lower than that of water, is to be considered as the rate-determining factor for the corrosion reaction beneath a paint film. A low oxygen permeability then explains the experimental result that in some cases no traces of corrosion products may be detected initially, despite the adhesion having been lost on exposure to water already shortly after the test has been started. Wet-state adhesion and water and oxygen permeability are inherent film properties which determine the corrosion protective capability of paint systems. The knowledge of them and, as indicated above, the proper correlation between them, is the key for understanding how paint films protect against corrosion and what measures have to be taken to obtain optimal results.
Acknowledgment The authors gratefully acknowledge the support given by the Bundesministerium fur Forschung und Technologie and the DECHEMA. Literature Cited Baumann, K., Plaste Kautsch., 19, 455 (1972a). Baumann, K., Plaste Kautsch., 19, 694 (1972b). Funke, W., Machunsky, E., Xlll FATIPEC Congress, Congress Book, p 215, 1976. Funke, W.. Zorll, U., Murthy. B. G. K., J. Paint Techno,., 41, 211 (1969) Guruviah, S., J. Oil Colour Chem. Assoc., 53, 669 (1970). Machunsky, E., Dissertation, University of Stuttgart, 1975.
