Electrolytic Resistance in Evaluating
Protective Merit of Coatings on Metals
R. CHARLES BACON, JOSEPH J. SMITH, AND FRANK M. RUGG
Bakelite Corporation, Bloomfield, N . J.
*
against the passage of corrosive material from the environment
to the corrodible metal surface.
Since this combination of barrier properties determines the
protective life of the coating, a thorough study of underwater
coatings for metals should be centered on the complete system
metal-coating-aqueous environment. I n this system the ionic
mobility within the coating was believed to be related to the ease
of passage of corrosive constituents through the coating to the
metal. As a result, the electrolytic resistance due t o the cbating
was thought t o be a measure of protection.
To test the validity of this idea experimentally, the coated
metal and aqueous environment were incorporated in an electrochemical cell aa follows:
In the examination of over 300 test systems the electrolytic resistance of organic coatings on immersed metals has
been found to be reliable for following protective behavior
and for predicting coating life, generally, in less than one
fifth the time required by the usual exposure tests based
on visual observation. These results are consistent with
the assumption that the protection offered by underwater
coatings is determined by the ease of passage of corrosive
materials or ions through the coating to the corrodible
metal surface, and that this diffusibility is in turn related
to the electrolytic resistance of the coating. In measuring
coating resistances the standard method for the determination of the internal resistance of batteries was employed, the voltage being measured by means of an electronic potentiometer. As only exceedingly small currents
are drawn and these only during the seconds of measurement, coating failure is not accelerated and significant
polarization is precluded.
metal/coating/aqueous environment/HgCl/Hg
The standard technique for determining the internal resistance
of a battery waa then employed. This method consists in
measuring the open circuit potential, EO,of this cell and then the
potential, E,, obtained when a n appropriate known resistance,
R,, is momentarily connected across the cell terminals. From
these values the internal resistance, R,, can be calculated from the
following equation:
A
NEED of long standing has been a satisfactory laboratory
test for predicting the corrosion protective merit of organic
coatings on submerged metals. In considering t h e development
of such a test, a study of the possible significance of various
measurable properties indicated the likelihood of a correlation
between the electrolytic resistance of the coating and corrosion
protection. To test the validity of this concept, a suitable apparatus was constructed and a series of experiments carried out
in which both coating resistance and extent of visible substrate
corrosion were followed. The purpose of this publication is to
show b y means of representative data the reliablity of electrolytic resistance values as a measure of coating protection and as a
means of predicting protective life.
Wirth (4) appears to be the only obher investigator to report
specifically on the electrolytic resistance of submerged protective coatings. He measured the galvanic currents which
flowed from steel to zinc and t o silver electrodes through films,
both attached to the steel and unattached to any substrate.
From these current values and the cell potentials obtained by
opening the circuit momentarily, the film resistances were calculated. The results were used as an aid in constructing a theory
on the mechanism of coating protection.
RI
=
R,
(2 -
I)
The resistance due to the presence of the coating can be determined by subtracting from resistance Ri the resistance obtained
using a similar cell in which the coating is absent. This latter
correction is often negligible, since the resistance due t o the
coating is usually of a much higher order of magnitude than that
due t o the remainder of the cell.
Only the small currents which flow while obtaining the closed
circuit potentials E, are passed through the test cell. Since the
time involved in obtaining this value is very short (less than
5 seconds) and the measurements are not made frequently, no
Bignificant cell polarization or galvanic breakdown of the coating
occurs.
INSTRUMENT USED
A detailed wiring diagram of the instrument used in measuring
open and closed circuit potentials is shown in Figure 1. This
instrument, referred t o as the Protectometer, is divided functionally into three sections: the potentiometer, the cell and
switch circuit, and the null indicator. The null indicator registers
zero when the voltage supplied by the potentiometer is equal,
but opposite in sign, t o the voltage across the test cell. This
indicator is a slightly modified Leeds & Northrup thermionic
amplifier (No. 7673) together with a Leeds & Northrup enclosed
lamp and scale galvanometer (No. 2420~). Since the modification of the amplifier consists primarily in changing the values of
certain resistors, Figure 1 includes approximate values of all
resistances.
The utility of this amplifier depends largely upon its stability
and the low grid current (less than
ampere) of the Westinghouse RH 507 tube. This low grid current permits voltages to
be measured to the closest millivolt in systems having an internal
resistance up to 10,000 megohms. A detailed discussion of the
thermionic amplifier and the behavior of this tube has been
THEORETICAL BASIS
Metallic corrosion, whether an electrochemical or a direct
chemical process, is a surface attack on the metal resulting from
contact with certain constituents of its environment. It is
generally accepted that organic underwater coatings, such as
paints, retard corrosion primarily by reducing the amount of
corrosive material which gains access to the metal surface. Accordingly, a coating should be continuous and should remain,
throughout extended periods of immersion, reasonably impermeable, coherent, and adherent t o the metal. Furthermore, many
underwater primers contain inhibitive pigments which further
retard corrosion, presumably by interposing an impervious passivating film over the metal surface. The combination of these
properties in the coating, then, enables it t o serve as a barrier
161
162
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 1
curve designated. “good” in Figure 2 is
typical of such variations.
The cause of the initial rise and subsequent variations in coating resistance
designated as “repairing tendency” is
not known with certainty. However,
since measurements of the resistance of
immersed detached resin films do not
as a general rule show this repairing
tendency, this behavior appears to be
due, a t least in part, to processes occuralv
ring a t the metal-coating interface. The
following mechanism appears to explain
this phenorrenon. The initial decrease
in resistance is due to the permeation
of water and the tiansport of conducting constituents within the coating. As
a result, corrosion product barriers form
Switch Circuit
a t the metal-coating interface and perFigure 1. Schematic Wiring Diagram for Protectonieter
haps in the pores and interstices new
this interface. These barriers-for exK~ (ohmli) = 10,000,Rz = 5 , & = 10-20 depending o n the floating grid bias for the particular
RH 5017 tube used, Ra = 10,000, Rs = 5000, Rs = 10,000, R7 = 15, RE = 20, Rs = 10,000, RID= 150,
ample, gases, oxides, passive filmR I I = 50. Rn = 3500.
El = t w o EverReady air cells, A2600; Ez = one EverReady air cell, A2600.
block the passage of conducting particles
Care must be taken to Pnsure adequate insulation in the selection of switches Sc, s2) Sa, Sd,
to the metal surface, and this leads to the
and S o .
increase in resistance. As the corrosion
proceeds at a retarded rate beyond this
presented by Cherry (1) and Hayes (3). I n order to determine
point, the pressure due to increased amounts of corrosion products
the cell resistance (E,) from Equation 1 it must be possible t o
produces localized ruptures in the film structure. These permit
a less hindered transport of conducting constituents and henre
connect a known resistance (Re)across the cell when desired.
This is accomplished by switches 8 2 , 8 3 , and 8,. The purpose
lead t o a decrease in resistance. The cycle may repeat itself many
of the auxiliary unit i,s to permit the connection of a low resistance
times before the complete breakdown of the film. In conformity
with this mechanism, microscopic examination of unpigmented
source of voltage (+1.5 t o -1.5 volt) in series with the test cell.
coatings having high resistancrs during immersion revealed
This arrangement allows good accuracy ( * 5%) in resistance
black spots under the coating which are presumably manifestadeterminations when the voltage supplied by the test cell is
tions of one type of corrosion-product bariier formation.
less than 0.1 volt. The resistance contributed by this potentiometric auxiliary unit is negligible in comparison to the resistances
normally determined.
EXPERIMENTAL PRQCEDLRE
BEHAVIOR OF COATING RESISTANCE
The resistance technique was developed as an aid in studying
snd evaluating coating compositions being considered for use
on the hulls of seagoing vessels. As a result, the experimental
work has been concentrated on the test system mild steel/coating/
sea water. In order t o avoid contamination of, and junction
potentials with, these test half-cells, a special reference electrode
Hg/HgCl/sea water, was used.
When coating resistance determinations were first undertaken
the values for 1 square centimeter of submerged surface were
f m n d t o be as liigh as 1OLl ohms shortly after the beginning of
the immersion, and as low as lo3ohms after a number of months
of exposure. As a result, it has been more convenient to represent
the data graphically in the form of logarithm of resistance us.
time instead of resistance us. time.
Figure 2 gives representative resistance behaviors for three
classes of coatings, those giving poor, fair, and good protection,
respectively. Most coatings, if continuous, have resistances in
the neighborhood of log R = 9 during the first 5 t o 10 minutes of
immersion, followed by a decrease in resistance which may
vary considerably in steepness and duration. The resistance
of poor coatings continues to decrease and leads to failure during
the first thirty to sixty days of immersion. Fair coatings may
level off or even rise slightly in resistance after the initial decrease.
However, a subsequent decided decrease in resistance heralds
coating failure in approximately six months of exposure. With
good coatings an initial decrease in resistance is followed by an
abrupt rise to approximately the original value. Thereafter
the log R value may remain substantially unchanged or increase
and decrease irregularly in the high resistance region. The
After having observed in preliminary experiments that coatings
which maintained higher electrolytic resistances during immerIO
r;
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TIME IN DAYS
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schematic~~~i~~~~~~ ~
Immersed Metals
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1948
Hat i.up. Thc rudn u C I C ihcn sanii-bli
mirig IL dopiin (:hat of 10 to 60 niesh in gril
nud a i l nir prcssurc of 72 pounds per square
Xicroscopic measuieinents (450 x ) revealed
t,Iicsc smdhlasted srirfacc.; had maxixiwn
i p s i o n u uf appruximstely 1.5 mils in di
Immcdiately afl,iv sandblasting, t h e rods
sorubbed with 8% dry hand brush t o I C U I O Y ~
nrntrll pmticlt:~of sand from lhc sirface. I3
r:uating, cach d w i wdt. ivas washed in BCC
atxi w c i g h d
I';xcq?t for 1 inch at tlic Aat top end, eacl
IVHS unifwmly cowled by 3 t o 6
applir:at,io:is using IL p ~ s e l <
c l r m d TBLC of fmni 0.021 t o 0.075 inrlr
sceonrl. I)rying t,ook ~ I B C
with
B the ~ o ~ l . t
suspended, hemisphcrieal t.ip down, in
controlled :it. 25" ('. and 40": relative liiinii
t.ha drying time WAS 1 to 2 hours betwen
mat.ing and 48 hours betweeii tho aypliestb
the first mitt and tiic tmt, irnnicision. Tnenty-fuur hours,
t,lie npplicat.iun of ttie la1boai thc rods were given B find W I
ing. T h e average coating t h i
was calculated from
density of the dry film, tho inw
weight due t o the prcr
of tha Iwotecbive film, arid t2ic R ~ coalcd
L
(50.7sq. an,), aa
ing asrnootb s u b t r a t c surface.
At the beginning of tho work it wss found that a e a k spots
o f t e r r obt,ainrd at, the hemispkwrieal t,ip of thc cloctrodes.
led tu prernat,ure failure at t h e x aicm itrid mid;ieading resiit,
values. To eliminaie this difficulty, the hemispherical ti
each clt.ctroiio was given a thick reirrfuorcing dip coating 01
test mnterinl being used, irnmodiafcly after thr final wig1
This reinfomd mea w a s always the litiit t o sliow m y sigr
eodting failure and thedore \*BY not believed to enter si6
cantly into the resistance behavior of the submerged I
Cortsequently, the &ED of the hcmispherieal tip w m elimin
from thc calculatioii of ttrc resiutsnre value8 (ohms for 0110 sq
ceetimeber o f submerged surfme) used in giving results.
For the test irnmemion each coated rod WN-' suhinerged
tieally to a dept,h of 5 cm. (erpmd area = 17.4 sq. cm.
cluding the hemispherical tip) in a I-pi,int Mason jar mntai
..
:
I
I
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IO
20
30
.
..-
.
TIME IN OATS
...
...
I
40
50
I
0
0
I
I
I
i
I
IO
20
30
40
50
T l M F IN "A"?
INDUSTRIAL AND ENGINEERING CHEMISTRY
164
300 ml. of natural sea water. This sea water had a pH of 8
and was filtered before use. A No. 30 cork stopper with a hole
for the coated rod and another for the reference electrode bridge
w w used in each jar. The reference half-cell contained Hg,
HgCI, and sea water in t h e flask, and an agar-sea water gel in
the glass tube bridge. This reference half-cell has a constant
potential of +0.299 volt, assuming the volta'ge of the normal
hydrogen electrode t o be zero.
The coated panels used in the test cells of type B were prepared
by members of the New Jersey Zinc Company Research Division
in Palmerton, Pa. This was done so that the results in the
resistance studies on this paint system could be correlated with
osmotic studies performed a t the New Jersey Zinc Company
laboratories with the same coating combination on identically
prepared panels (3). The primer (U. S. Navy specification
52 P 18) was allowed one week of air drying before application
of the low gloss, alkyd hull paint top coat (made from 52 P 25
base). The test exposure was begun two weeks after the application of the top coat.
Two days before the test exposure was started, a glass cylinder
(5 X 1 3 / 4 inches) open a t both ends was attached to the central
portion of each coated panel by use of a rosin-wax mixture applied in the molten state. Each cylinder contained 100 ml. of
test aqueous environment during the exposure period. The
test environments employed were distilled water and aqueous
solutions of 0.780, 2.31, and 3.77% sodium chloride by weight.
Measurements were obtained by using the specimen steel substrate as one electrode and a platinized platinum electrode as the
other.
At the time of measurement, and during the intervals between
measurements, all the test cells were kept in a constant tcmperature room at 25" C. and 40y0 relative humidity. The results
given for any particular test system were obtained in duplicate.
VARIATIONS IN IMETAL SURFACE PREPARATION
Maintenance of good adhesion between the substrate surface
and the applied coating is obviously desirable in obtaining a
durable protective barrier. I n order to effect good adhesion of
the primer coat, the metal substrate is often given an inorganic
surface conversion treatment-for
example, Parkerizing and
Bonderizing-prior t o primer application. The idea was developed in this laboratory that a conversion-type coating which
12 I
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1
I
Vol. 40, No. 1
contained an organic binder could provide an improved base for
adhesion of the primer. In order to test the practicality of this
idea, a phosphoric acid-zinc chromate-vinyl resin conversion
coating system was formulated. By use of this wash primer
significant increases in adhesion and protective performance were
effected with a number of vinyl resin marine primers for mild
steel in test exposures. As a result, experiments were undertaken to determine whether resistance values could be used to
detect the improvement in protective performance due to the
presence of this wash primer. A representative set of resistance
results is given in Figure 4.
After the first thirty days of sea water immersion this vinyl
resin-red lead primer, coated on untreated sand-blasted mild
steel, displayed a gradual but steady decrease in resistance.
This represented a more progressive decrease in barrier effectiveness than was indicated by the higher and almost constant resistance values obtained during this period when the wash primer
was used. Even though no visible signs of coating failure could
be detected in either case after fifty-six days of immersion, the
differences in resistance became quite pronounced-that is, by a
factor of 1000-as the system without wash primer approached
the resistance region (log R = 6) indicative of the onset of failure.
The validity of this resistance evaluation was confirmed by visual
observation of the test specimens after six months of exposure.
At this latter time the primer applied over the wash primer
showed no signs of failure, whereas in the case of the primer
applied directly over the sahd-blasted mild steel localized rusting
and some film peeling was readily observable.
A consideration of the resistance-time curves given in Figure
4 also suggested that the difference in protective performance
between these vinyl resin-red lead primers, with and without
wash primer, was traceable t o differences in adhesion well before
this could be confirmed in the subsequent visual examination
of the test specimens. A relatively small difference in barrier
effectiveness was indicated from the very high resistance values
obtained with both of these coatings during the f i s t twentyfive days of immersion. This indicated very little difference
during this period in the low rates of transmission of the corrosive materials through the protective systems because of the
presence of the wash primer. In conformity with this indication
a coating of the wash primer alone, like conventional conversion
coatings, v a s seen to offer very little protection (Figure 4).
Therefore, the significant differences in resistance behavior observed after the f i s t thirty days of immersion appeared to depend
upon differences in the ability of the coatings to prevent the
spread of corrosion along the coating-substrate interface. Such
a spreading of corrosion would bring about a gradual loss in
film integrity and lower the observed resistance. For these
reasons this difference in resistance behavior was interpreted
as being due to the difference in adhesion obtained at the
substrate-caating interface by use of the wash primer.
Differences in protective merit due to an improved adhesion
produced by the wash primer treatment were detected by use of
resistance determinations with other vinyl resin marine primer
formulations on sand-blasted mild steel. I n addition, other
experimental results indicated that differences in protective
performance due to other variations in rr-eta1 surface preparation or in the nature of the substrate metal can be predicted
and followed by resistance determinations.
VARIATIONS IN COATING
TIME IN DAYS
Figure 6. Effect of Variations in Pigment
Composition on Protection
3-AMilpolyvinyl butyral primers, sand-blasted mild steel aubstrates, sea water immersion
It is well recognized that, for a given metal surface and a
given aqueous corrosive environment, differences in the binder
composition and in the pigmentation of the coating can lead to
large differences in the protection obtained. I n this section
results are presented which show how the effects on protection
by these and other variations in the coating can be studied and
rapidly determined from resistance values.
After the first twenty days of immersion the gradual decrease in the resistance of the polyvinyl acetate-chloride
k
s
during a six-month exposure.
The results with polyvinyl butyral coatings presented in Figure
6 illustrate how resistance values may be used t o predict differences in protection due to differences in the nature of the
pigment used. The rapid drop in the resistance of the coating
pigmented with titanium dioxide revealed that this coating was
poorly protective. Considerable substrate rusting and coating
blistering could be observed on this specimen after two weeks
of immersion. The unpigmented and red lead-pigmented polyvinyl butyral coatings maintained approximately the same high
resistance values during the fist month of testing. Thereafter,
the higher resistance values obtained with the red lead coating
revealed the improved protection attained through the incorporation of this pikment into the polyvinyl butyral vehicle.
This improvement stands in contrast t o the detrimental effect
obtained when titanium dioxide was used, After four months
of immersion some localized rusting was observed on the specimen containing the unpigmented coating, but no significant signs
of failure could be observed with the red lead coating after a sixmonth exposure.
Another coating variable which is probably of more importance
than is often realized is film thickness. Interest in the use of
thick marine coatings of the hot plastic type prompted resistance
determinations with different thicknesses of a rosin-amorphous
wax coating composition. The results obtained are presented in
Figure 7 as an illustration of how the effect of coating thickness on
protection may be determined from resistance values.
It is seen that the resistance values for this rosin-wax formulation are consistently higher with increasing coating thicknesses.
The visible protective ratings on these specimens after immersion
periods up t o fifteen months have been found to conform with the
relative order of merit which was predicted from the resistance
values obtained during the first two months of immersion.
The initially low resistance values obtained with the 2-mil
coating suggested the presence of discontinuities, or only very
thin protective films, at the peaks of the sand-blasted substrate
surface. This indication and the subsequent rapid decrease in
resistance during the first day heralded early failure. Considerable localized rusting occurred with this specimen during
the second week of testing. Appreciable localized rusting of the
steel could be observed shortly after three months of exposure
exposure. No significant signs of failure could be observed with
the &mil coating of this rosin-wax composition during fifteen
months of immersion.
The specimens used in this latter experiment were carefully
prepared and were not subjected t o any impacts or abrasions
during the test immersion. The coatings were applied by
successive dippings of preheated sand-blasted mild steel rods in
the molten water-white rosin (25y0)-amber amorphous wax
(75%) mixture at 105" C. Immediately after application of the
coatings the specimens were placed in an oven and allowed t o
cool gradually from 90" C. t o 35" C. during a two- t o three-hour
period. Such a technique had to be used t o avoid the formation
of a very thick (50-100 mils), cracked, porous, and poorly adherent coating which is obtained when an ordinary hot dip procedure is used. Therefore, although good protection is indicated
for the 8-mil coating under the experimental conditions used, the
brittleness of the coating and the difficulty of obtaining continuous adherent films of tnis formulation under service conditions
militate against its use in practice.
The results on .the effect of coating thickness indicate that a
coating formulation which gives at least reasonably good protection at the usual test film thicknesses may give a markedly
improved protection at greater film thioknesses. This brings
out the value of applying thicker primer coatings (greater than
3 mils) than are often used in service and the importance of
considering film thickness as a significant variable in obtaining
comparable results in laboratory and field tests. Good protection cannot be expected with a formulation which is known t o be
poorly protective with thin but continuous coatings-for example,
the polyvinyl acetate-zinc tetraoxychromate system (Figure 5)simply by using thicker coatings.
I n Figure 8 the drop in the resistance of a 2-mil copper oxide
antifouling coating, applied directly t o the steel surface, to below
a log R value of 6 during the first ten days is indicative of poor
protection. Since the resistance values for this coating were as
high as log R = 8 during the first two days, a poor water impermeability waa not indicated. The continued decrease of the
resistance indicated the rapid spread of corrosion along the metal
surface, which was probably accelerated b y the presence of
galvanic cells formed by copper being transported in solution
from the pigment t o the metal surface. It ivould be expected,
166
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No, 1
surface of an organic coating may then
be expected to lead to a decrease in the
protective merit. In Figure 9 resistance
values are seen to reveal this effect with
a polyvinyl butyral-zinc tetraoxychromate primer. The curves show conc MIL^ ur UN I iruuLi NU ~ U RI I u
OVER 4 MILS OF PRIMER
sistently lower resistance values for the
!
system in which oxygen was bubbled into
sea water. Localized substrate rusting
became visible shortly after two weeks
of immersion on this latter specimen,
whereas similar signs of failure did not
appear until after four months of exposure with the specimen immersed in
sea R-ater under static atmospheric cona
L
ditions.
These results not only illus84
v)
trate that resistance determinations are
P
reliable in coating evaluations under
these two different environmental cons 2.
ditions but also show how great may
0
20
40
GO
80
100
120
140
160
be the effect of such variations in oxygen concentration on the protection obTIME IN DAYS
tained.
Recent studies by Kittelberger and
Figure 8. Effect of Copper Oxide Antifouling Coating on Protection
Obtained with ZTO Chromate-Polyvinyl Butyral Primer
Elm (3)showed that the water absorption and blistering of a coating combiSand-blasted mild steel substrates, sea water immersion
nation made up of a zinc chromate primer
(Kavy specification 52 P 18) and a
low-gloss gray alkyd top coat (made from 52 P 25 base) applied to
then, that the presence of a suitable thickness of primer would
give an improved bond a t the metal surface and prevent dissolved
mild steel decreased with increased sodium chloride concentracopper from reaching this surface. Under these conditions the
tion in the aqueous environment. In order t o obtain some idea
of the correlation between the results of such studies and coating
antifouling top coat .could serve as an additional barrier in proresistance determinations, identical test panels and envirgntecting the steel. This appears to be the case, since a high resistments were used in resistance studies (Figure 3 B ) . These
ance is maintained longer when 2 mils of the antifouling top
results afforded informatidn regarding the reliability of coating
coat are applied over 4 mils of a zinc tetraoxychromate-polyresistance determinations for studying and rapidly evaluating
vinyl butyral primer than when either the antifouling coat or the
differences in protective behavior due to differences in environpiimer is used alone (Figure 8).
mental salt concentration.
The coating resistance values presented in Figure 8 are in
The curves obtained in these studies during four and one
conformity with the visually observed protective merits of the
coatings after prolonged sea water immersion. With the 2-mil
half months of immersion are given in Figure 10. During this
test period the resistance values are consistently lower, the lower
antifouling coating alone, rusting of the substrate became
the sodium chloride concentration in the aqueous environment.
readily visible after seven weeks of exposure. After seven months
The marked decrease in the resistance values during the first
the 4 m i l primer coating showed some slight localized failure,
five t o eight days corresponds to the rapid diffusion of water into
whereas the 2-mil antifouling coat-4-mil primer coat system has
the films before coating saturation is approached. Thereafter,
given perfect protection for more than two years.
In addition t o the variables discussed in this section, the
effects on protection of other variations in the coating may be
studied by use of resistance values. Experimental results show
that film discontinuity and the effects of variations in coating
drying schedule can be rapidly detected by resistance determinations. Resistance values have also been used in the laboratory
determinations of the highest permissible and optimum pigment
concentration for marine primers. Obviously this technique
STATIC ATMOSPHEFqlC CONDITIONS
10
lends itself to the prediction of the relative merit of commercial
formulations provided the proper service conditions can be approximated in the laboratory testing.
e
VARIATIONS IN AQUEOUS ENVIRONMENT
The third major source of variables which may affect the protective behavior of an underwater coating on a metal is the nature of
the environment. Two of the more important factors in this regard-namely, the concentrations of dissolved oxygen and of salt in
the aqueous environment-have been selected to demonstrate
the validity of resistance values in studies involving differences
in the corrosive environment.
That oxygen is ordinarily an accelerating constituent in the
corrosion of mild steel immersed in sea water is well recognized.
An increase in the concentration of dissolved oxygen a t the
v)
2
K
Figure 9.
4
6
8
IO
TIME IN DAYS
Effect of Environmental Oxygen Concentration on Protection
2.5-Mil coatings of v i n y l resin-ZTO chromate primer, sandblasted mild steel substrates, sea water immersion
January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY-
167
m
0
20
40
60
80
100
I20
140
160
TIME OF EXPOSURE IN DAYS
Figure 10. Three-Mi1 Coatings Composed of Navy 52 P 18 Primer and Low
Gloss Alkyd Hull Paint
Exposure in aqueous solutions varying i n NaCl concentration, smooth mild steel panel substrates
the almost constant resistance vilues are due to the maintenance
of film integrity. Since the water absorption of this coating has
been shown to increase with decreasing environmental salt concentration (S),$he results given in Figure 10 indicate that during
this test period the dissolution of water by the coating is a more
important factor in determining protective behavior than the
imbibition of environmental salt by the coating.
The prediction of protective merit from the resistance values
obtained during this test period are in agreement with the visual
protective rating of the specimens after extended exposure.
After ten months of immersion the coating exposed to the distilled
water was appreciably blistered, although no substrate rusting
was observed. At this time slight blistering of the coating and a
small amount of substrate rusting were noted with the specimen
exposed to the 0.78% sodium chloride solution. However,*no
signs of coating failure were observed after the ten-month immersion with the coatings exposed to the 2.31% and 3.7’7y0
sodium chloride solutions.
EXTENSION TO OTHER SYSTEMS
A satisfactory cell for determining resistances of coatings on
metal panels which have been immersed in laboratory tanks or in
the field can be set up as follows: With the panel substrate serving ae one electrode, electrolytic connection is made between
the calomel half-cell and the coated surface by placing four to
eight layers of filter paper (for example, 5.5 cm. in diameter),
which have been saturated with sea water, on the coated panel.
The end of the salt bridge is then brought in contact with this
wetted paper, and the method previously described is used to
determine the resistance of the coating under the filter paper.
A modification of this latter technique should also be applicable
to systems exposed t o the afmosphere or in laboratory weathering
units. In this manner a quantitative meashre of protection at
the time of measurement as well as a prediction of relative merits
from the trends of resistance values with time should be possible.
The choice of substrates is limited only to sufficiently conducting materials. Satisfactory work has been carried out in this
laboratory on aluminum (anodized and unanodized), Alclad,
magnesium, and galvanized steel.
resistance behaviors during immersion have always conformed
with the visual protective ratings assigned to the corresponding
test specimens after prolonged laboratory exposure. These
results indicate that the resistance due t o the presence of the
coating is a measure of protection, and that resistance determinations evidently have considerable value in the laboratory evaluation and fundamental study of underwater coatings on metals.
There are variables other than those already studied which,
under practical conditions, may affect protective behavior-for
example, abrasion, erosion, galvanic coupling, radiation, effect of
inhibitors, and chemical deterioration of the coating. However,
there is no reason to believe that these impose any limitation on
resistance values as valid measures of protection. I n so far as
these factors influence the barrier properties of the coating, they
should be reflected in the resistance values obtained. I n fact,
all results show that the resistance values are sufficiently sensitive in this regard to allow a determination of the relative
importance of such variables for a given set of practical conditions.
Therefore, it appears that laboratory resistance values can be
used in the practical rating of underwater coating formulations
for metal surfaces. However, since the protection obtaineq
may differ appreciably with variations in the substrate, the
coating, and the environment, care must be taken to duplicate
closely the proper service conditions in the laboratory testing if
results are to be applicable.
ACKNOWLEDGMENT
The assistance of V. H: Turkington, L. R. Whiting, and L.
A. Micco (at present with Ansco Laboratories) in this work is
gratefully acknowledged. This work was initiated under a contract between Bakelite Corporation and the Office of Research
and Development, and completed under a contract between
Bakelite Corporation and the Bureau of Ships, Navy Department, which has approved this publication. The opinions presented here are those of the authors and do not necessarily
reflect the official opinion of the Navy Department or the naval
service a t large.
LITERATURE CITED
CONCLUSIONS
e
This resistance technique has been used, to date, in the investigation of over 300 test systems involving coated metals immersed
in aqueous liquids. As is illustrated in this paper, these tests
covered a wide range of the variables which influence the’protection afforded the metal substrate. I n every case good protection was obtained at resistances greater than log R = 8, and
poor protection was obtained a t resistances lower than log R = 6.
In addition, the predictions of coating protectite merit from the
(1) Cherry, R. H.,
Trans. Electrochem. SOC.,72,33 (1937); Sutherlin,
L.,and Cherry, R. H., Ibid., 78,11-20 (1940).
(2) Hayes, W.A.,Proc. I.B.E., t o be published; paper presented
before Inst. of Radio Engrs.,Jan. 26,1944.
ENG.CHEM.,38,695(3) Kittelberger, W.W.,and Elm, A. C., IND.
9 (1946).
(4) Wirth, J. K.,Chem. Fabrik, 11, 455-7 (1938); Korrosion u .
Mstalschutz, 16,69-76, 331-8 (1940); Angewandte Chemie, 54,
369-73 (1941); Korrosion u. Metalschutz, 18,203-9 (1942).
RBCEXVED
May 8 , 1947.
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