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3C100 .U5753N0.1253 1988 V198C.1 NIST- iNanonai insuiuie of Standards and Technology

(formerly National Bureau of Standards)

NIST Technical Note 1253

An Electrochemical Technique for Rapidly Evaluating

Protective Coatings on Metals

Changjian Lin and Tinh Nguyen

ORGANIC COATING

REFERENCE ELECTRODE

COUNTER ELECTRODE

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TEEL SUBSTRATE
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20899
NIST Technical Note 1253

An Electrochemical Technique for Rapidly Evaluating

Protective Coatings on Metals

Changjian Lin

Xiamen University

People's Republic of China

Tinh Nguyen

Building Materials Division

Center for Building Technology

National Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

October 1988

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NOTE: As of 23 August 1988, the National Bureau of

Standards (NBS) became the National Institute of

Standards and Technology (NIST) when President

Reagan signed into law the Omnibus Trade and

Competitiveness Act

U.S. Department of Commerce

C. William Verity, Secretary

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Ernest Ambler, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent

and Technology Washington: 1988 of Documents

Technical Note 1253 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Washington, DC 20402

Tech. Note 1253

23 pages (Oct. 1988)

CODEN: NTNOEF
Contents

Page

1. Introduction 1

2. Multicyclic Potentiodynamic Polarization (MCPDP) Technique 2

2.1 Basics of Electrochemical Measurements. 2

2.2 Multicyclic Potentiodynamic Polarization (MCPDP) Technique 2

3. Experimental Procedures 4

3.1 Specimen Preparation 4

3.2 Electrochemical Instrumentation Set-Up and Measurements 4

4. Results and Discussion 6

4.1 Effects of Coating Integrity on MCPDP Results 6

4.2 Evaluation of the Effect of Coating Type on Corrosion Protection of

Organic Coatings on Steel 8

4.3 Evaluation of the Effect of Coating Thickness on Corrosion Protection

of Organic Coatings on Steel 8

4.4 Evaluations of the Effect of Contaminants on Corrosion Protection of

Organic Coatings on Steel 12

4.5 Evaluation of the Effect of Defects in the Coatings on Corrosion

Protection of Organic Coatings on Steel 12

4.6 Evaluation of the Effect of Curing Time on Corrosion Protection of

Organic Coatings on Steel 18

5. Conclusions 19

6. Acknowledgment 19

7. References 19

m

An Electrochemical Technique for Rapidly

Evaluating Protective Coatings on Metals

Changjian Lin* and Tinh Nguyen

National Institute of Standards and Technology

Gaithersburg, MD 20899

A new electrochemical technique based on multicyclic scanning potentials applied to a specimen in an electrolyte was developed and evaluated as a method for rapidly evaluating the corrosion protection

performance of organic coatings on metal. Opaque alleyd and clear acrylic coated steel samples prepared

at different thicknesses, curing conditions, defect inclusions, and contamination concentrations were

used for the evaluation. The applied potentials were between - 1.8 and + 1.8 V, and the electrolyte was

a 3% NaCl solution. Preliminary results indicate that this technique is useful for evaluating the corrosion protection properties of organic coatings on steel. The technique appears to offer a number of

advantages: fast and in situ measurements, good reproducibility, high sensitivity, and minimal damage

to the coating.

Key words: accelerating test method; coating; contamination; corrosion; electrochemical; evaluation; multicyclic polarization.

1. Introduction

The yearly cost of all forms of metallic corrosion in the United States has been estimated at 4.2% of the

gross national product [1]. The use of organic coatings is the most common method for protecting metal

structures against corrosion. It has been suggested that approximately one-half of corrosion control costs are

related to the use of organic protective coatings [2]. Despite such widespread application of organic coatings

for corrosion control, no acceptable technique exists for rapidly evaluating their performance. One of the

reasons for this is that the failure of a protective coating is a complex process depending on many factors,

including the metal substrate, the coating, the metal /coating interaction, and the service environment [3]. Techniques for evaluating the corrosion protection properties of organic coatings on metals can be

divided into two main types: nonelectrochemical and electrochemical. Applications of these techniques to coated metals have been adequately reviewed [4-10]. Since failures of a coating system, such as blistering,

cathodic delamination, and corrosion, are affected by the conductivity of the coating and the electrochemical

processes occurring at the metal/coating interface [11], evaluation techniques based on electrochemical principles may be useful. Five main electrochemical and electrical measurement techniques have been used for predicting the

corrosion protective ability of organic coatings on metals [5-10]; these are dc electrical resistance, corrosion

potential, polarization resistance, polarization curve, and ac impedance. The last technique has been considered as one of the most promising [7]. Although these techniques have been useful for understanding the

corrosion mechanisms of metals, with and without coatings, and for providing information on the protective

properties of organic coatings, each has shortcomings.

One of the main disadvantages of the corrosion potential, polarization resistance and ac impedance

techniques is the long immersion time of the coated sample in the test environment before any change in the

signal is observed. For example, Walter [7] reported that corrosion potentials could not be measured on a 25-u.m vinyl coating on aluminum sample in 5% NaCl solution until the 41st day of immersion. Similarly,

Skerry and Eden [12] indicated that it took about 9 months immersion in a 0.6 M NaCl solution before a 60

u.m (total thickness), commercial-quality alkyd top coat over an alkyd primer on a steel substrate showed any

resistive change by the ac impedance technique. In addition, the interpretation of the results obtained by the ac impedance technique is often difficult, and the equipment required is complex and expensive. On the other

hand, the dc resistance technique does not provide a true result of the corrosion protection ability of a coating

* Guest Scientist from Xiamen University, People's Republic of China.

1
because it only measures the resistance of the coating, and gives no information on the complicated reactions

occurring at the metal/coating interface [13]. Furthermore, much of the dc resistance work has been on free standing films, which do not always provide a reliable guide to the performance of a coating system [14]. The

main objection to the polarization curve techniques is the use of a single dynamic scan, which may produce

the desired currents for bare metals but may not be able to cause current flow in coated metals because of the

high resistivity of the coatings. In order to produce current in a coated specimen with a single scan, the

applied potential must be high. This might cause changes in the properties of the coatings and the metal/coating interface [7]. Since no single technique is adequate, it is necessary to combine various techniques to effectively evaluate the corrosion protection performance of coated metals. This paper reports on a new electrochemical

technique, the multicyclic potentiodynamic polarization (MCPDP) technique.

2. Multicyclic Potentiodynamic Polarization (MCPDP) Technique

2.1 Basics of Electrochemical Measurements

When a piece of metal corrodes, it has cathodic and anodic sites. For example, for steel immersed in water containing oxygen, the primary reaction (oxidation reaction) at the anode is: Fe^Fe++ + 2e~ (1)

This leads to the dissolution of the steel. The primary reactions (reduction reactions) at the cathode are:

2 + 2H2 + 4e" ^40H" (2)

2H+ + 2e--^H2 (3)

Electrochemical techniques provide a means to accelerate these corrosion processes and at the same time

examine them. The two fundamental quantities used to characterize an electrochemical reaction are potential

and current. Potential can be a result of the natural reaction (as from corrosion reactions at equilibrium), or can be imposed by an external source. In the latter case, the external source stimulates either the oxidation or

the reduction reactions.

As an oxidation or reduction reaction occurs at the metal surface, a flow of electrons (either to or from

the metal) gives rise to a current. The current can be related to the rate of the electrochemical reaction

because it is a measure of the number of electrons that flow in a given period of time. The potential at which

a given metal specimen is at equilibrium with its environment is called the corrosion potential, £«,„; there is no net current flow at this potential because the oxidation and reduction currents are equal in magnitudes. An

electrode that is maintained at a potential other than Ecorr by an external instrument is said to be polarized.

When the specimen is polarized in the positive direction (with respect to £<*,„), the anodic current (oxidation

reaction) predominates. Similarly, when a negative potential (relative to Ec„n) is applied, the cathodic current predominates.

2.2 Multicyclic Potentiodynamic Polarization (MCPDP) Technique

In a multicyclic potentiodynamic polarization (MCPDP) experiment, the first applied potential scan

starts at Eco„ (the corrosion potential), and continues in the negative (cathodic) direction. When it reaches a preprogrammed negative value, it is reversed and begins in a positive (anodic) direction, past 'Eco„. When it reaches a preprogrammed positive potential, it is reversed and scans in the negative direction again to the

preprogrammed negative potential to complete a cycle. The cycle may be repeated many times until measureable polarization currents occur. The scanning times and the polarization currents are recorded and presented

in a typical potential /current plot. The currents and scanning times are compared for a set of samples. The

lower the polarization current at the same scanning time, or the longer the scanning time at the same

polarization current, the better the corrosion resistance of the specimen. This procedure is similar to cyclic

voltammetry in electrochemistry.

It should be mentioned here that the MCPDP technique differs from the cyclic polarization technique

commonly used for measuring the pitting tendency of a specimen in a given bare metal-solution system. In the

cyclic polarization technique, a potential is applied until a large increase in current occurs (instead of to a predetermined potential value as in MCPDP); when the current reaches a certain value, the potential reverses
and scans in the opposite direction. In the cyclic polarization technique, only one cycle of potential is used,

and it does not provide adequate driving force to produce a detectable change in the current in a coated metal

specimen. Another difference is that, in the cyclic polarization technique, the shape of the loop between the

forward and backward scans, and the critical potentials are the two primary concerns. In the MCPDP

technique, the resulting current levels and the time required to produce a specific current value are the most

important measurement results. The parameters of measurement used in an MCPDP experiment can be changed depending upon the

requirements of the samples. For steel coated specimens, the maximum positive potentials can be selected in the range between 0.5 and 1.8 V and the negative potentials can be selected in the range between —0.5 and — 1.8 V depending on the thickness, type, and physical condition of the coating, and the type and concentration of the electrolyte. The scanning rates are typically between 0.01 and 0.1 V per second, which are about

60 to 600 times faster than the scanning rates recommended by the ASTM G5-87 method [15]. The fast scanning rate used in the MCPDP technique should force the specimen to follow its corrosion reaction

pattern before it has the opportunity to passivate or return to equilibrium with the environment [16]. Although the actual physical process involved is not completely understood, the MCPDP technique seems to perform the functions of a number of testing methods that have been very useful for studying the

corrosion of bare and organic coated metals. When the applied potential is scanned in the positive direction,

the MCPDP technique utilizes the potentiodynamic anodic polarization technique, i.e., the specimen is forced

to act as an anode such that it corrodes. This technique can be used to determine the corrosion behaviors of

a metal in an environment of interest. When the applied potential is scanned in the negative direction, the

specimen acts as a cathode thus accelerating the reduction reactions at the surface of the substrate. When the scan is in the negative direction, it utilizes the potentiodynamic cathodic polarization technique, which is useful for studying the reduction characteristics of a metal in an environment. By using the negative applied

potential, the MCPDP technique also utilizes the principle used in the ASTM G8-85 standard test method for cathodic disbonding of pipeline coatings, which evaluates the protective performance of a pipeline coating on

steel substrates under the influence of an electrical stress in a conductive alkaline electrolyte [17]. The

electrical stress, which accelerates the disbonding of the coated panel near the perforated area, is provided by means of a sacrificial magnesium anode or from a cathodic potential. The disbonding of the coating in this case is due to the attack of the coating /steel bonds by the highly alkaline environment caused by the OH~

ions formed on the surface of the steel as a result of the reduction reaction (Reaction 2) [3,18]. The applied

cathodic potential can increase the size of the existing defects, such as holidays, and in some cases, can induce

defects in a coated specimen.

Besides accelerating the processes at the metal/coating interface, the MCPDP technique may also accel- erate the migration of corrosive elements from the environment through the coatings to the metal /coating

interface. For example, Leidheiser [3] reported that, in the presence of an applied cathodic potential of — 0.8 V (vs. SCE), the diffusion coefficients of Na+ and water were increased by one order of magnitude as compared to those in the absence of the applied potential. The applied anodic potential may also provide the

driving force to accelerate the movement of anions through certain coatings [19]. Another important advantage of the MCPDP technique is that the procedure is simple and takes a short time, sometimes only a few

minutes, to complete the measurements.

Under certain experimental conditions, the potential /current curves obtained by the MCPDP technique

may provide information on the polarization resistance and corrosion rates of coated metals using the same

principle described for the polarization resistance technique. This technique has been useful to obtain quantitative information on the corrosion of metals in solutions. The main disadvantage of the MCPDP technique may be the relatively large excursions of the applied

potential relative to the corrosion potential. This may disturb the coating barrier properties and nature of

substrate surface during testing as discussed earlier for the polarization curve techniques. However, in the

MCPDP technique, although there are somewhat larger excursions from E^ than in the polarization resis- tance technique, the applied potentials are lower than those employed in the polarization curve technique that has been used for evaluating coating systems [7]. Further, in an MCPDP experiment, the ion movement in the

anodic direction is quickly balanced by the ion movement in the opposite direction; the net effect is probably

minimal. The fast scanning rate used in an MCPDP experiment may also reduce the possibility of introducing

physical change in the coating and metal/coating interface during testing. From the above discussion, it appears that the MCPDP technique may potentially become a valuable technique for (1) evaluating the

corrosion resistance performance of protective coatings and coating systems, (2) predicting the performance

of coating systems in service, (3) screening coatings and corrosion inhibitors, and (4) optimizing parameters in coating formulations and development.
3. Experimental Procedures

3.1 Specimen Preparation

Rough, AISI 1010, 75 x 125 mm, cold-rolled steel panels with various coatings were used for the evaluation. One white alkyd and one clear acrylic coatings were used. Prior to applying the coating, steel surface was cleaned repeatedly with acetone and methanol and blown dry with dry air. The coating applications were carried out using appropriate blade applicators. To test the usefulness of the MCPDP technique, the

following experimental parameters were varied in the samples: film thickness, coating curing conditions,

defects in coatings, and substrate surface contamination concentration. A thickness gage was used for the

coating thickness measurements. No attempt was made to optimize the curing conditions of the coated panels

used in this work. The characteristics of the tested specimens are summarized in table 1. Except for the

specimens used in figures 3 and 4, where the defect-free areas were selected for the test, the testing areas of

other specimens were selected randomly; that is, the selected test areas may contain minor defects such as

particle inclusion, air bubbles, etc.

Table 1. Preparation conditions of organic coated steel specimens

Fig. No. Coating Type Thickness Cured Condition Contaminant / Defects

3 opaque alkyd 130 ttm 20 days, ambient -

4 clear acrylic 70 (Am 6 days, ambient -

5a5b opaque alkyd

clear acrylic

60 (jldi 70fim

2 h, 90 °C air-circ. 2 h, 90 °C air-circ. -

6a6b

6c

clear acrylic clear acrylic clear acrylic95 u.m 70 Jim 30 jxm5 days, ambient

5 days, ambient

5 days, ambient :

8a

8b opaque alkyd opaque alkyd

130 u.m 75 u.m

8 days, ambient

8 days, ambient

-

9a

9b

9c

opaque alkyd opaque alkyd opaque alkyd60 |±m 60 jj-m 60 jjim5 days, ambient

5 days, ambient

5 days, ambient

without cont. 0.00001 M, KHSO4

0.001 M, KHSO4

10a 10b

10c

clear acrylic clear acrylic clear acrylic30 (im

30 |j.m 30 n-m

2 h, 90 °C

2h, 90 °C

2 h, 90 °C

without chip

chip inclusions chip inclusions

12 clear acrylic 60 jim 6 days, ambient air bubbles

14a

14b opaque alkyd opaque alkyd

50 u.m 50 |xm2 h, 90 °C air-circ. 24 h, 90 °C air-circ.

-

3.2 Electrochemical Instrumentation Set-Up and Measurements

All electrochemical measurements were carried out in a 50-mm diameter by 25-mm height polymethylmethacrylate) cylinder cell, which was mounted on the specimen by a silicone rubber adhesive. The area of

the specimen under test was 1960 mm2

. The reference electrode was a Ag/AgCl wire in IN KCL solution

and the counter electrode was a platinum plate. The configuration of the test cell and the electrodes are

illustrated in figure 1. All measurements presented in this report were taken at room temperature (24 °C) using

3% NaCl (by mass) as the electrolyte.

To provide the dynamic applied potentials, we use a potentiostat system equipped with a microprocessor.

The system allowed automatic control of the scanning rate and potential magnitudes. An X-Y recorder was

employed to record the potential /current signal. Figure 2 shows the schematic diagram of the measurement

system. Before each measurement, the specimen was allowed to stand in the electrolyte for several minutes to

attain a relatively stable Eon. The experiment began with a scan in the negative direction, starting from E^

(about -0.5 V vs. Ag/AgCl for most of the specimens). When reaching the preset —1.8 V potential, it reversed the direction and scanned toward the positive potential, going through £„>„. When reaching the
preset + 1.8 V potential, it reversed the direction and scanned toward the negative potential again. This cycle

between — 1.8 and 4- 1.8 V potentials was repeated until a preselected value of the polarization current was

attained or a desired testing time was reached. (The current was read directly from the potential /current

chart.) A scanning rate of 0.1 V/s was used throughout the study. High stable and current interruption modes

were used to stabilize the small signal generated and to compensate for the ohmic potential drop, respectively.

The polarization current values presented in this report were derived from the corresponding maximum

applied potentials, which are - 1.8 V for the negative potential and + 1.8 V for positive potential.

3% NaCI SOLUTION

SILICON RUBBE

ADHESIVE

ORGANIC COATING

REFERENCE ELECTRODE

(Ag/AgCI)

COUNTER ELECTRODE

(Platinum Plate)

CYLINDER CELL

(PMMA)

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$ * X"STEEL SUBSTRATE

Figure 1. Electrochemical test cell.

RECORDER

POTENTIOSTAT

RE

CE

WE

ELECTROCHEMICAL

CELL

Figure 2. Schematic diagram of the multicyclic potentiodynamic polarization measurement system.
4. Results and Discussion

4.1 Effects of Coating Integrity on MCPDP Results

As indicated earlier, the primary concern about the MCPDP technique is that the applied potentials may

damage the coatings. To address this question, two apparently defect-free (as observed by an optical microscope) coated specimens, one opaque and one clear, were subjected to the MCPDP test for an extended

period in 3% NaCl solution. The polarization currents were constantly monitored during this period. At the

end of the period, the sample was gently pricked with a fine needle, and the MCPDP test resumed.

Figure 3 shows the MCPDP curves of a 20-day, air-cured, 130-|xm, white alkyd coating on steel that was

subjected to the MCPDP test for 3 hours and then pricked with a needle. The dark line on the horizontal axis

is from the repeated scans during the 3-hour test, and the curves away from the horizontal axis are for scans

at various times after the specimen was pricked. Figure 3 indicates that no detectable current flowed during

the 3 hours when there were no conductive paths in the coating, and that polarization currents (anodic and

cathodic) occurred as soon as the coating was damaged.

Figure 4 presents similar curves for a 70-urn, clear acrylic coating on steel subjected to the MCPDP test for 90 minutes (dark line on the horizontal axis) before being pricked with a needle (curves away from the

horizontal axis). Similarly to the alkyd coated specimen, the currents increased sharply due to the presence of

a breakthrough defect in the coating, and no current flowed when there was no continuous conductive path

between the substrate and the solution. The immediate occurrence of currents in these samples after pricking

indicated that corrosion had occurred on metal at the exposed area (pricked area). This was seen clearly by

a microscopic examination after withdrawing the electrolyte and exposing the specimens to the air for a few

hours. No corrosion was observed if the specimen was not pricked with a needle. The results shown in figures

3 and 4 suggest that the MCPDP technique caused very minirrmm, if any, physical damage to the coatings

during testing. It should be noted that the polarization resistance at any time of the pricked, coated samples

may be estimated from figures 3 and 4 by using the narrow range in the vicinity of £«,„ on the MCPDP

curves.

— 6 min

(after pricked)

Figure 3. MCPDP curves for a 130-ji.m, opaque alkyd coating on steel in 3% NaCl solution before (dark line on the horizontal axis) and

after pricking with a needle (curves away from the horizontal axis).
-0.5

- +40

l(uA) — 36 sec

(after pricked)

0.0+0.5 +1.0 +1.5 E(V)

-—10

---20

-—30

-40

--50

— 1.5 h

Figure 4. MCPDP curves for a 70-p.m, clear acrylic coating on steel in 3% NaCl solution before (dark line on the horizontal axis) and after pricking with a needle (curves away from the horizontal axis).
4.2 Evaluation of the Effect of Coating Type on Corrosion Protection of Organic Coatings on Steel

Figure 5a is the MCPDP curve of a 60-um, opaque alkyd coating on steel that was cured for 2 hours at 90 °C in an air-circulated oven, and figure 5b illustrates the MCPDP curves of a 70-u.m, clear acrylic coating on steel that went through the same curing schedule. It took 40 minutes under the MCPDP test to generate an anodic current of 0.003 uA for the opaque alkyd coating. On the other hand, it took only 4 minutes to produce an anodic current of 0.014 uA for the clear acrylic coating. Obviously, the tested alkyd coating,

despite being thinner, provided better corrosion protection than the tested acrylic coating.

(b)

— 4 min

(a)

+ .01

-1.5 -1.0 -0.5

l(uA)

-- 40 min

+0.5 +1.0 +1.5 E(V)

-.01

Figure 5. MCPDP curves for steel coated with different coatings in 3% NaCl solution: (a) 60-u.m, opaque alkyd coating, (b) 70-nm,

clear acrylic coating.

43 Evaluation of the Effect of Coating Thickness on Corrosion Protection of Organic Coatings on Steel

The thickness of a coating on a metal surface is an important factor in determining the protective

properties of the coating. Figure 6 presents the MCPDP curves for a clear acrylic coating having thicknesses

of 95, 70 and 30 \im on steel. These coatings were cured for 5 days at ambient environment (24 °C and 50%

RH). The 95-p.m coated specimen gave rise to an anodic current of 0.003 uA after 3 hours of testing (fig. 6a),

while the 70-u.m coated specimen produced an anodic current of 0.004 uA after 90 minutes of testing (fig. 6b).

The 30-u.m coated specimen produced a 0.015 uA anodic current and 0.034 uA cathodic current after only

4 minutes of MCPDP testing. Even after polarization currents had occurred in these clear coating specimens,

the corrosion products could not be seen, even under a microscope at 60X. The corrosion products were
observed only after further immersion in the electrolyte. This can be seen from figure 7. These results indicated that the MCPDP technique is very sensitive to corrosion reactions at the metal/coating interface. Not only does it accelerate the corrosion reactions but it also detects the resulting currents as soon as there

is continuous pathway through the coating to conduct the currents from the metal to the solution. This

technique may, therefore, provide early information rapidly about the corrosion protection of a coating in the

medium of interest.

(c)

-1.0 -0.5

l(uA) - 4 min

+0.5 +1.0 +1.5 E(V)

- 3 .5 h

+1.5 E(V)

-1.0 -0.5

- 3 h

+0.5 +1.0 + 1.5 E(V)

-»--.01

Figure 6. MCPDP curves for steel coated with a clear acrylic coating at different film thicknesses in 3% NaCl solution: (a) 95 jim,

(b) 70 jim, (c) 30 ji.m
(c)

(b)

(a)

Figure 7. Photographs of corrosion of the samples corresponding to the results presented in figure 6.

10
Figure 8 presents similar results for 130- and 70-um. films of an opaque alkyd coating on steel. These

coated specimens had been cured for 8 days at ambient conditions. The 130-u,m coated specimen produced

only 0.002-uA anodic current after 30 minutes of testing (fig. 8a), while the 75-u.m coated specimen gave rise to 0.022-pA anodic current after the same period. (The difference in corrosion protection between the

specimen whose result is illustrated in fig. 8a and that shown in fig. 3 may be due to the shorter curing time

or the inclusion of minor defects in the former.) The results in figures 6 and 8 indicate that the MCPDP

method can provide very useful information on the variation of one or more properties of a coating that affect

its corrosion protection performance.

(b)

— 30 min

+1.5 E(V)

(a)

-+.01

-1.5 -1.0 -0.5 —30 min

+0.5 +1.0 +1.5 E(V)

•I- -0.1

Figure 8. MCPDP curves for steel coated with an opaque alkyd coating at different film thicknesses in 3% NaCl solution: (a) 130 jim,

(b) 75 n,m.

11
4.4 Evaluations of the Effect of Contaminants on Corrosion Protection of Organic Coatings on Steel

Contaminants present on a metal surface before or after coating application are one of the main factors

contributing to the early failure of a protective coating system [20]. Contaminants can affect the performance

of a coating by accelerating the corrosion of the substrate, and promoting the blistering and adhesion failure of the coating. Development of a method that can provide information on the effects of contaminants on the

performance of a protective coating should have many uses, such as for evaluating surface preparation and

cleaning methods, and investigating the effects of acid rain and marine deposits. Figure 9 provides one

example that demonstrates the use of the MCPDP technique for rapidly evaluating the effects of contaminants on the corrosion protection property of an opaque alkyd coating on steel. Coating thicknesses of these

samples were about 60 u.m. The contamination was introduced by depositing two ca. 5-mm diameter drops of

KHS04 solution on the cleaned substrate surface, then drying in a desiccator for 1 day before the coating

application. The electrochemical test cells were mounted so that the contaminant spots were within the

testing area. Figure 9a presents the MCPDP curves of a coated specimen in the absence of a contaminant on the

substrate surface. This figure shows an anodic current of only about 0.007 uA after 10 minutes of the MCPDP

test. When the steel surface was contaminated with 0.00001 M KHSO4 solution, the anodic current increased

to 0.017 u.A after only 5 minutes under the MCPDP test (fig. 9b); and when the KHSO4 contamination

concentration increased from 0.00001 to 0.001 M, the anodic current increased nearly three times at + 1.2 V

applied potential after about 4 minutes of testing (fig. 9c). Figure 9 indicates that, within the concentration

range studied, the higher the KHSO4 concentration, the shorter the time required for corrosion of steel under

the coating to occur. The results also indicate that the higher the KHSO4 concentration the higher the rate of

corrosion. It is interesting to note that, at higher contamination concentrations, the anodic processes of steel under a coating in NaCl electrolyte became more complicated than the cathodic processes under the same

conditions, as evidenced by the erratic behaviors of the currents in the anodic region (fig. 9c).

4.5 Evaluation of the Effect of Defects in the Coatings on Corrosion Protection of Organic Coatings on Steel

The performance of an organic protective coating depends on its barrier properties. The presence of

defects, such as pinholes, air bubbles, and particle inclusions, will provide pathways for corrosive agents to reach the metal surface. This will result in premature failure of a coating system. Figures 10 and 12 are presented to demonstrate the ability of the MCPDP technique to detect the effects of air bubbles and chip

inclusions on the performance of a clear acrylic protective coating. Figures 11 and 13 are the corresponding

photographs of the defect-included samples, which show the sites where corrosion starts and the direction of

its propagation. The clear coating was selected so that the defects could be observed with either the naked

eye or an optical microscope. For obtaining a bubble-free coating on steel, the coated specimen was cured for

1 day at ambient conditions then 2 hours at 90 °C in an air-circulated oven. The chip inclusion specimens were

also prepared in the same manner, except that the chips (from the partially cured, gummy resins around the

opening of the can containing the coating) were mixed with the coating before the coating application. These

chips were not obvious to the naked eye before the testing because they were small and not very different in appearance from the coating. The air bubbles were introduced in the specimens by mixing the viscous coating

vigorously just before the coating application. The distribution of the bubbles on the panel was not uniform.

However, the test area on the panel was selected, by moving the test cell around, so as to include the desired

air bubbles. The dry coating thicknesses of these samples were about 30 u,m. The coated specimen without chip inclusion (fig. 11a) produced an anodic current of 0.002 uA after 10

minutes of the MCPDP test as presented in figure 10a. The chip-included specimen illustrated in figure lib

gave rise to an anodic current of 0.010 uA (fig. 10b) after 8 minutes of testing. Another chip-included

specimen (fig. lie) produced an anodic current of 0.025 uA after 5 minutes of testing (fig. 10c). These results suggested that the chip inclusion in the specimen shown in figure lie caused this specimen to be less corrosion resistant than the chip-included specimen presented in figure lib. Figures lib and lie show the greenish

corrosion products developed around the chips; the anodes are at the chip sites and the cathodes are at the

periphery. Apparently, the coating and the partially cured chip particles did not form a continuous phase,

thus allowing the corrosive elements to reach the metal surface quickly at the chip sites. The photographs

presented in figures lib and lie were taken after 3 hours of testing; and the greenish corrosion products

could be seen by the naked eye about 1 hour before that. Figures 10b and 10c, however, showed the

polarization currents only after very short testing periods, suggesting again that this technique is very sensi- tive to electrochemical changes resulting from corrosion reactions, and could be used for detecting corrosion

long before it can be seen by the microscope or visually.

12
— 4 mxn

— 5 min

— 10 mm

-•--.01

Figure 9. MCPDP curves for steel contaminated with different concentrations of KHSO4 solution then coated with a 60-jim, opaque

alkyd coating: (a) no contamination, (b) 0.00001 M KHSO4, (c) 0.001 M KHSO4.

13
— 5 min

3= ~ 1 \ -1.0 _o.5

— 8 min

_ — 10 mins

i r +0.5 +1.0 +1.5 E(V)

Figure 10. MCPDP curves for steel coated with a 30-jim, clear acrylic coating with and without chip inclusions in the samples:

(a) without chip inclusion, (b and c) with chip inclusions.

14
(c)

(b)

(a)

mu

Figure 11. Photographs of corrosion of the samples corresponding to the results presented in figure 10.

15
— 5 min

Figure 12. MCPDP curves for steel coated with a 60-jLm, clear acrylic coating containing air bubbles.

Figure 12 presents the MCPDP curves of an air-bubble-inclusion specimen. The corrosion started in the

air bubble and expanded to areas around the bubble as seen in figure 13. However, corrosion did not spread

symmetrically around the bubble but seemed to prefer one side. Figure 12 shows that the polarization currents rose sharply after only 5 minutes of testing suggesting that there may be a breakthrough hole at the bubble.

The curves of figure 12 indicate that the cathodic currents were much higher than the anodic currents for the

duration of testing, suggesting that the reduction reactions were the more favorable reactions for corrosion in the air bubble. The availability of large amount of oxygen in the bubble may be the contributing factor.

16
Figure 13. Photograph of corrosion of the sample corresponding to the results shown in figure 12.

17
4.6 Evaluation of the Effect of Curing Time on Corrosion Protection of Organic Coatings on Steel

The state of cure in a coating also has a great influence on the performance of a protective coating on

metal. Under-cured coatings tend to allow faster permeation of corrosive elements, such as water, oxygen,

and ions, through the coatings. Figure 14 illustrates an example of the use of the MCPDP technique for studying the effect of curing time on corrosion resistance properties of an opaque alkyd coating on steel. Figure 14a shows the MCPDP curves of a coated steel specimen that was cured for 2 hours at 90 °C in an

air-circulated oven. Figure 14b is from the coated specimen that had the same film thickness (50 urn) and was

cured at the same temperature but for a longer time, 24 hours. As evident in figure 14, both specimens

produced almost the same polarization currents at the beginning of the test. However, the polarization currents of the 2-hour-cured specimen increased at a faster rate, to 0.008 uA, as compared to 0.005 uA for the

24-hour-cured specimen, after 30 minutes of testing. These results suggest that the 24-hour-cured, coated

specimen has better corrosion resistance properties than the 2-hour-cured, coated specimen. This technique is thus potentially helpful for optimizing the curing conditions and other experimental variables in the development and formulation of protective coatings.

(b)

I(UA)

- + .01 — 30 min

+0.5 +1.0 +1.5 E(V)

-.01

(a)

~£ -- 3 min

* -.01

Figure 14. MCPDP curves for steel coated with a 50-ji.m, opaque alkyd coating cured for different times at 90 °C: (a) 2 hours, (b) 24

hours.

18
5. Conclusions

A new electrochemical technique based on repeated cycling of the applied scanning potentials for evaluating the corrosion resistance of a coating system was developed. The technique not only accelerates the

corrosion processes and measures the magnitudes of the corresponding currents, but also provides the driving

force to accelerate the movements of corrosive elements through the coatings. Preliminary results indicated

that the technique is very sensitive to electrochemical processes at the metal surface. The technique also produces quick, reproducible and in situ results with very little damage to the coatings. The testing procedure

is simple and uses commonly available instrumentation. This technique seems to possess most of the elements

of a good accelerated method, as well as a good measuring technique for studying the corrosion protection

and degradation mechanisms of a coating system in a corrosive environment. Further exploratory experiments, and work on the relationship between the results obtained by this technique and those from actual

exposures are needed before the full potential of the technique can be determined.

6. Acknowledgment

We would like to express our thanks to Mr. Larry Masters, Dr. Mary McKnight and Dr. Geoffrey

Frohnsdorff of the Building Materials Division, National Institute of Standards and Technology (NIST), for

their many helpful discussions and suggestions. We also thank Dr. Anna Fraker, Metallurgy Division, NIST,

and EG&G Princeton Applied Research for kindly making the instruments available during the course of this study, and Mr. Jim Seiler and Mr. Ned Embree, Building Materials Division, NIST, for their technical

assistance.

7. References

[1] L. H Bennett, et al., Natl. Bur. Stand. (U.S.), Spec. Publ. 511-1, 1978.

[2] G. Wranglen, An Introduction to Corrosion and Protection of Metals, Inst, for Metallskydd, Stockholm, 1972, p. 191.

[3] H. Leidheiser, Jr., Corrosion 39, 189 (1983).

[4] M. E. McKnight, T. Nguyen and J. W. Martin, Natl. Bur. Stand. (U.S.), NBSER 85-3293, 1986.

[5] J. Wolstenholme, Corrosion Science 13, 521 (1973).

[6] H. Leidheiser, Jr., Progr. Organic Coatings 7, 79 (1979).

[7] G. W. Walter, Corrosion Science 26, 39 (1986).

[8] G. W. Walter, Corrosion Science 26, 681 (1986).

[9] T. Szauer, Progr. Organic Coatings 10, 157 (1982).

[10] T. Szauer, Progr. Organic Coatings 10, 171 (1982).

[11] J. E. O. Mayne, J. Oil Colour Chemist Assoc. 40, 183 (1957).

[12] B. S. Skerry and D. A. Eden, Progr. Organic Coatings 15, 269 (1987).

[13] J. D. Scantlebury, K. N. Ho and D. A. Eden, Electrochemical Corrosion Testing, ASTM STP 727, F. Mansfeld and

U. Bertocci, Eds., ASTM, Philadelphia, 1981, p. 187.

[14] D. C. Mills and J. E. O. Mayne, in "Corrosion Control by Organic Coatings," H. Leidheiser, Jr., Ed., NACE, Houston, 1981, p. 12.

[15] ASTM G5-87, Annual Book of ASTM, 1987, vol. 03.02, p. 124.

[16] O. W. Siebert, Laboratory Corrosion Tests and Standards, ASTM STP 866, G. S. Haynes and R. Baboian, Eds., ASTM,

Philadelphia, 1985, p. 65.

[17] ASTM G8-85, Annual Book of ASTM, 1988, vol. 06.01, p. 895.

[18] H Leidheiser, Jr., W. Wang and L. Igetoft, Progr. Organic Coatings 11, 19 (1983).

[19] YL Leidheiser, Jr., private communication.

[20] A. R. Appleman, J. Protective Coatings and Linings 4, 68 (1987).

19

NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NIST/TN-1253

2. Performing Organ. Report No. 3. Publication Date

October 1988

4. TITLE AND SUBTITLE

An Electrochemical Technique for Rapidly Evaluating Protective Coatings on Metals

5. AUTHOR(S)

Changjian Lin and Tinh Nguyen

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURQ, MD 20699

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

SAME AS ITEM #6 ABOVE

10. SUPPLEMENTARY NOTES

! |] Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

A new electrochemical technique based on multicyclic scanning potentials applied to a specimen in an electrolyte was developed

and evaluated as a method for rapidly evaluating the corrosion protection performance of organic coatings on metal. Opaque alkyd and

clear acrylic coated steel samples prepared at different thicknesses, curing conditions, defect inclusions, and contamination concentrations were used for the evaluation. The applied potentials were between - 1.8 and + 1.8 V, and the electrolyte was a 3% NaCl solution. Preliminary results indicate that this technique is useful for evaluating the corrosion protection properties of organic coatings on steel. The technique appears to offer a number of advantages: fast and in situ measurements, good reproducibility, high sensitivity, and

minimal damage to the coating.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

accelerating test method; coating; contamination; corrosion; electrochemical;

evaluation; multicyclic polarization

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