Water Immers esting of Metal Protective Paints INFLUENCE OF BARE METAL AREAS Inasmuch as laboratory water-immersion tests are being used extensively to evaluate the water resistance and protective qualities of metal finishing systems, a study of some of the variables involved was considered essential. One of the most important of these, the influence of bare metal on or in contact with the test panel, is discussed and illustrated in this paper.
The New Jersey Zinc Company (of Pa.), Palmerton, Pa.
tain variables upon the over-all result is greatly magnified. A thin spot or break in a metal protective paint film may permit rusting over a n area which, although no larger than it would be under similar conditions on a steel bridge, for example, would so completely alter the rating of this paint with respect to the standard or other paints of the test series that an entirely erroneous idea of its probable protective value might be obtained. To avoid accidents of this sort, it would be possible to apply statistical methods by testing many ETERMINING the servicp quality or durability of a panels; in effect this would be nothing more than inckeasing paint film-that is, the protection it affords under the test area. This procedure, however, would defeat the conditions of actual service-is one of the most immajor purpose of the laboratory test principle. The alternaportant as well as one of the most difficult tasks confronting tive is to control all variables carefully and to study their the paint chemist. I n actual use the majority of painted effects, one a t a time. articles are subjected to a wide variety of external influencss; Paint chemists who are aware of these facts and principles in addition, these intluences vary in intensi y over a relagenerally maintain more or less effective control over such tively wide range so that it becomes difficult to study the recognized variables as paint compokition, film thickness, effect produced by each individual factor or by any combinapanel material, and surface preparation; but frequently imtion or sequence of factors, and to correlate the results obportant variables escape control because they are not recogtained into a reliable picture of the value of the paint film as a nized or even suspected of having any influence on the results. protective medium. In the course of our work on the development and evaluaOne solution of this problem is found in the application of tion of metal protective pigments and primers, we have had statistical methods and procedures. No particular effort is occasion to make rather extensive use of water immersion made a t controlling or studying the effect produced by all tests and to give some consideration to the variables involved variables individually, but a sufficiently large number of in this type of exposure. One variable in particular demands tests are conducted so that the final average values arrived elucidation, not only because it has a marked effect upon the a t will have taken into consideration all kinds, degrees, and results of water immersion exposures, but also because a numcombinations of variable conditions. This would be analogous ber of technologists have not designed their tests to control it. to evaluating an exterior house paint by studying its behavior This variable is the electrochemical effect of an immersed area on hundreds of houses of different construction in many locaof bare metal which is also in metallic contact with the test tions over a period of years. panel. I n laboratory water immersion tests this condition For obvious reasons this procedure cannot be readily folwill most frequently exist when the panels are incompletely lowed, especially under existing conditions. More than ever coated, either by before, the paint design or by accichemist is forced to resort to less dent. Also, the m e t h o d of s u p timc- c o n s unii ng laboratory methp o r t i n g thc test panels may perods which in Inany mit external concases have to be designed specifitact Letu een cally for the parpanels and ivith ticular service t h e m e t a l of a conditions under metal-lined water invest iga t i o n . bath. Since the influIVh e n t h e t e s t ence of bare metal area is reduced to areas of this type t h e s i z e s which can be handled upon the bchavior of painted stecl c o n v e n i e n t 1y in panels may be laboratory tests, Sone 1 sq. mm. 78.5 sq. mm. partially deduced FIGURE1. RELATIVE the effect of cerSIZESOF BAREMETALAREAS 943
D
.
W. W. BITTELBERGER
INDUSTRIAL AND ENGINEERING CHEMISTRY
944
Vol. 34, No. 8
from theoretical considerations, a brief review of some of the pertinent features of the electrochemistry of corrosion may be in order. Electrochemistry of Corrosion
Paint NO.
1
When a piece of iron is placed in ordinary water, it immediately develops anodic and cathodic areas a t which the two parts of the corrosion reaction occur. At the anodes, which vdl have a potential of about -0.5 volt (hydrogen scale), iron goes into solution; that is, it is changed from the atomic to the ionic state, losing electrons in the process. At the cathode areas which will Paint have a potential of around +0.3 NO. volt, hydrogen ions acquire 2 electrons and are plated out as atomic hydrogen. This leaves the solution in the vicinity of the cathodes with an excess of hydroxyl ions and, therefore, alkaline. Theoretically there are three methods of controlling the rate of this corrosion reaction. The flow of current between the electrodes, and therefore also the rate of corrosion, will be a function of the electrical resistance in the circuit (e. m. f. Paint No. being constant). Hence, the 3 higher the resistance, the lower the rate of corrosion will be. Since the anodic and cathodic reactions are not independent of each other but must proceed simultaneously and a t the same rate, changing the rate of either the anodic or the cathodic reaction will effect a corresponding change in corrosion rate. Polarization of either e l e c t r o d e will a c c o m p l i s h this. Cathode areas become polarPaint NO. ized when there is an inade4 quate supply of oxygen (or other oxidizing agent) to remove the hydrogen as fast as it is plated out. The absence of any oxidizing agent will cause complete cathodic polarization; the cathode potential will change from about $0.3 volt to that No bare area 1 sq. mm. hare 78.5 sq. mm. bare of the anode (about -0.5 volt) FIGURE 2. RESULTS OF SALTWATERIVMERSION TESTS and current flow and corrosion will cease. Anodic polarization generally results from the formation of a protective layer of oxide, oxygen, corrosion product, etc. dinary steel in a chromate solution are protected from corroFor example, stainless steel in many corrosive media and orsion by anodic polarization. In this!jcase the completely
August, 1942
I N D U S T g I A L A N D E N G I N E E R I N G CHEMISTRY
945
Panel Tests After a few preliminary tests had shown that a definite effect could be obtained, the following more comprehensive studies were undertaken: Paint No. Panels, 4 X 6 inches (10.16 X 5 15.24 cm.), of S. A. E. specification 1010 steel with rounded edges and corners, were thoroughly scrubbed with water and a mildly abrasive soap powder, rinsed in cold running water and then in boiling water, and dried as rapidly as possible with clean paper toweling. After a final cleaning in boiling toluene vapor, the panels were stored in dry redistilled toluene until time for painting. Seven raw linseed oil paints were prepared with seven widely recPaint No. 6 ognized metal protective paint pigments. Single coats of each of the seven paints were sprayed on both sides of six steel panels, making a total of forty-two test specimens. The paint films were allowed to dry for 3 weeks under room conditions before the tests were started. Finally, holes 1.17 mm. (0.046 inch) in diameter were drilled into the backs of two panels of each paint. They were located at the center of the area which would subsequently be immersed, and their depth was made slightly greater than the length of the taPaint NO. pered end of the bit (about half 7 the panel thickness). The area of bare metal thus exposed was slightly over 1square mm. (0.0017 square inch). Circular areas (78.5 square mm. or 0.0122 square inch) approximately 1 cm. (0.394 inch) in diameter were laid bare on one No bare area 1 sq. mm. bare 78.5 sq. m y . bare side of two other panels of each paint. This was done by cleaning FIGURE 2 (Contznued) off a larger area, cementing a small ring of glass (1 cm. inside diameter) to the center of thisareawithparaffin, and then carefully coating the bare area outside the ring polarized anode has the same potential as an unpolarized with paint and paraffin. Thus two sets of test panels were cathode of the same metal. completely coated on all sides, having no bare metal exposed; Experience has shown that with most metal protective two sets had bare metal areas of about 1square mm., and two paints an area of bare metal on a painted steel panel immersed in ordinary water will normally be anodic to the coated had bare metal areas of about 78.5 square mm. on each panel. The ratios of bare metal to painted test area were approxiarea, except perhaps in the case of paints which actually incite corrosion. However, the coated area of the panel might mately 1/12,000and 1/135 (Figure 1). The edges of the specimens not always be capable of serving as a cathode. For example, were given the additional protection of a coat of quick-dryif the electrical resistance of the coating were high, or if the ing paint and two coats of paraffin, all applied by dipping. The panels were strung on glass rods by holes punched becoating prevented oxygen from reaching the metal underfore cleaning and painting, and were separated from one anneath to act as a depolarizer, a portion of the bare metal area might then serve as a cathode. If the coated portion of the other with 1-om. lengths of rubber tubing. All panels were panel becomes an active cathodic area, the reactions which immersed to a depth of 4 inclhes in water a t 25' C. (77' F.) result in a certain degree of alkalinity in the neighborhood for 32 days. A 3 per cent solution of sea salt was used for one set of the panels and running tap water for the other. The sea of this electrode should not be without influence upon the coating itself. And it was to learn more about the behavior water was circulated by a motor-driven propeller and was of painted steel panels under such conditions that the followchanged once a week to minimize the effect of soluble maing experiments were carried out. terial extracted from the test films upon the blistering and
046
I N Du
s TR I A L
corrosion results. Blistering and rusting gradings were made a t intervals during exposure; finally, on completion of the test, the paint films were removed and the underlying metal was graded for corrosion. The final condition of the test panels is shbwn in Figures 2 and 3, and the numerical failure gradings are presented in Table I. The blistering values are averages for both sides of the panels and for four exposure intervals. The rusting values, on the other hand, represent only the final condition of the test panels but are averages of independent gradings by two operators.
AND EN G INEERIN G
c H E‘M I s T R Y
Val. 34, No. 8
Pniirt So. 1
Discussion of Results Considering all tests in the aggregate, the presence of bare metal areas Paint s0. on the exposure panels resulted in an 2 increase in both the rate and degree of blistering of the paint coatings. The effect was marked in the case of the salt water tests, but appreciably less so in the fresh water exposures. It is probable that this difference results from the different conductivities and osmotic pressure effects of the two solutions, but it will not be discussed in detail until further work on the subject has been completed. The presence of bare metal areas caused a decrease in the amount of rusting under the paint films, even though this accompanied P:tiiir so. an increase in blistering. 3 The results for the individual paints show a number of reversals which may be due in part to the characteristics of the different coatings and to variables still not under perfect control. These reversals are clearly shown in Figure 4, in which the failure gradings are plotted against bare metal areas. However, in eleven of the fourteen tests (fresh and salt water) the completely coated panel was blistered less than either panel with bare metal areas. I n the other three cases the differences in 1’:iiiit blistering were slight. Similarly, in A-4. ten of the fourteen tests the completely 4 coated panel showed most rusting under the fdm. The effect of the bare metal areas upon the order of quality of the seven test paints was essentially negligible in the case of blistering and rusting in fresh water and rusting in salt water 78.5 89. mm. bare 1 sq. mm. bare No bare area (Table I and Figure 4). I n the salt water tests there was no essential RESULTS OF FRESH WATER IMMERSION TESTS FIGURE 3. difference in the order of blistering of the two sets of panels bearing. the small and the large bare areas, reand salt mater results and between blistering and rusting in spectively. However, the order of blistering of these two either exposure, it is far from perfect. sets differed somewhat from that shown by the completely The marked increase in the rate of blistering of most of coated panels, possibly because the latter as a group still these seven paints in salt water, which resulted from the presshowed only slight failure. ence of only a small area of bare metal, indicates that erroneAlthough there is some degree of correlation between fresh
N G CHEMISTRjY
INDUSTRIA
August, 1942
Paint NO.
5
Paint
No. 0
941
avoided. If, however, in the preparation of a test panel with paint 3 an edge were poorly protected or the back of the panel had been marred so that some of the metal came in contact with the salt solution upon immersing the panel while paint 5 was applied with the proper care, the former paint would be rated slightly poorer than the latter in blistering and definitely poorer in rusting. Similar reversals are readily found upon examination of the results of the fresh water tests. The fundamental aspects of the influence of bare metal areas in water immersion tests have not yet been fully studied. However, some of the practical implications are of considerable importance and widespread interest, particularly since the need for speed in testing fosters a tendency to place greater reliance upon laboratory tests.
Practical Implications The observation that bare metal areas of the type described in this paper exert a greater influence upon the behavior of painted metal panels when immersed in salt water than when immersed in fresh water is significant in view of the fact that many of the paint systems a t present receiving the attention of paint Paint chemists and engineers are intended No. 7 for the protection of marine structures. It has been demonstrated that, in evaluating the protective properties of organic coatings by salt water immersion tests, a lack of control over areas of bare metal on or in contact with the test panel may lead to seriNo bare area. 1 sq. mm. bare 78.5 sq. mm. bare FIGURE 3 (Continued) ous errors. To produce this effect the bare area need not be large. I n our tests an area 1 / 1 2 , ~ . ~of the painted test area had a marked influence, and it appears that an accidental bare spot on the edge or back of ous results would be obtained in a series of tests in which some a test panel may be sufficient to make the results unreliable. panels were completely coated and others contained inadvertIn view of these observcitions it becomes necessary to exerent small bare areas. For example, in the salt water tests, cise effective control over this factor, either by eliminating all paint 3 would be rated definitely superior to paint 5, either in bare areas by careful paint application with double or triple blister or in rust resistance, if the tests were conducted under protection for the edges, or by intentionally exposing bare carefully controlled conditions and bare areas were carefully
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TABLEI. EXPERIMENTAL DATA
7
Paint No.
Blistering-
Salt Water Immersiona c -
78 sq. mm.b
--
,--
Rusting-
78 sq. Noneb 1 sq. mm.b None 1 sq. mm. mm. 1.3 0.5 1.7 3.3 17.0 7.7 2.7 1.8 3.6 8.1 0.5 0.7 6.4 5.7 Av. 1.7 6.3 7.4 6.2 5.3 3.4 slight; 10 = moderate; 15 = bad: 20 very bad Gradings: 0 = no failure; 5 b Refers to bare steel area..
-
None
t6
0.3 16.5 8.2 13.4 2.1
6.6 failure.
Fresh Water Immersiona --Rusting78 sq. 1 eq. mm. mm. None 1 sq. mm. 5.8 1.2 5.1 0.4 1.4 1.4 0.5 0.4 3.5 1.1 4.2 0.6 3.0 17.6 1.6 16.6 16.8 4.0 3.4 15.0 11.4 13.1 0.9 0.4 6.0 0.6 0.1 4.0 7.8 8.6 2.1 1.5 Blistering-
78 sq. mm. 0.1 0.2 4.0
0.8 0.6 0.4 0.3 0.9
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 8
may be altered materially by the character of the backing paint. If the latter should possess appreciably poorer water resistance than the test paint, it may break down early during the immersion test, lay bare some metal on the back of the panel, and in this manner produce a more rapid and greater degree of blistering of the test paint. On the other hand, should the hacking paint be superior in water resistance to the test paint, it might afford some sort of protection to the paint under test. It is clear, then, that in a test series consisting of several paints varying appreciably in water resistance, different conclusions regarding the relative quality of the various members of the test series might be obtained, depending on whether the backing paint possesses relatively good, intermediate, or poor water resistance. Similarly, Salt Water the behavior of a test paint may be affected A - N o bare areas B 1 8q. mm. bare metal by the water resistance of another test C 78 sq. w. bare metal paint applied beside it on the face of the same test panel. The same effect would be obtained if all of the panels in a test series were in electrical contact with one another-for example, by being suspended from the same metal rod. To avoid possible errors from these sources it would appear necessary: ( 0 ) t o apply the test paint t o the back as well as to the face of the panel, (b) to avoid placing more than one paint on any one panel, (c) to avoid electrical contact between panels and any other metal in contact with the water in which the panels are immersed, (d) to protect carefully all edges and coriiers of the test panels, and ( e ) to standardize the size of any bare metal areas either on the front or the back FIGURE 4. EFFECTOF BAREAREASO N BLISTERING A ~ RUSTING D of the test panels. If these precautions are observed, results may be expected which. when interpreted on the basis of extended experience, will afford a reliable indication of the water resistance and the metal protective value of a metal areas of the same size on all panels. Which one of these paint system under these particular exposure conditions. two schemes is followed is of little importance a t the moment and will probably depend upon a number of considerations Acknowledgment varying from case to case. This opportunity is taken to acknowledge the assistance Another aspect to this phenomenon deserves some conof C. L. Blose and L. H. Farber of the Research Division staff sideration. I n the preparation of test panels for the evaluation of The New Jersey Zinc Company (of Pa.) with the experiof metal protective paints, it has been common practice to mental phases of this investigation. upe a standard paint for coating the backs of the panels and to apply the test paint to the face of the panel only. It is PRESENTED before the Division of P a n t , Varnish, and Plastics Chemistry at conceivable that the behavior of the paint film under test the 103rd Meeting of the AMERICAN CHEMICAL SOCIETY, Memphis, Tenn.
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