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A11103110155 I V198C.1NIST- luiuyy

NIST Technical Note 1266

Development of a Method to Measure In Situ

Chloride at the Coating IMetal Interface

Tinh Nguyen and Changjian Lin

Microelectrode

Electrolyte

Scribe

QC

100

.U5753

#1266

1989

C.2
rhe National Institute of Standards and Technology^ was established by an act of Congress on March 3, 1901. The Institute's overall goal is to strengthen and advance the Nation's science and technology and

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' Located at Boulder, CO, with some elements at Gaithersburg, MD.
QjL/oo

NIST Technical Note 1266

Development of a Method to Measure In Situ

Chloride at the CoatingIMetal Interface

Tinh Nguyen

Building Materials Division

Center for Building Technology

National Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

Changjian Liu

Xiamen University

People's Republic of China

July 1989

^^<°>^.

•^4TES O^

'^

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

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

Raymond G. Kammer, Acting Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent

and Technology Washington: 1989 of Documents

Technical Note 1266 U.S. Government Printing Office

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

Tech. Note 1266

17 pages (July 1989)

CODEN: NTNOEF
Contents

page

1. Introduction 1

2. Experimental Procedures 2

2.1 Materials and Preparation of Coated Panels 2

2.2 Preparation of Chloride-Containing Specimens 2

2.3 Preparation of Microelectrodes 2

2.3.1 Reference Microelectrodes 3

2.3.2 Double-Barreled Chloride Ion Selective Microelectrodes 4

2.4 Chloride Ion Measurements 5

2.4.1 Inverted Electrode Microsampling Method 5

2.4.2 In Situ Chloride Ion Concentration 6

2.5 Corrosion Potential Measurements 7

3. Results and Discussion 7

3.1 Calibration 7

3.2 Measurements of Chloride Ion at the Coating /Steel Interface. 8

3.2.1 Inverted Electrode Microsampling Method 8

3.2.2 In Situ Chloride Ion Measurements 9

3.3 Corrosion Potential Measurements 11

4. Conclusions 13

5. Acknowledgments 13

6. References 13

m

Development of a Method to Measure In Situ

Chloride at the Coating/ Metal Interface

Tinh Nguyen and Changjian Lin

Building Materials Division Center for Building Technology National Institute of Standards and Technology

GaUhersburg, MD 20899

One of the main reasons for the lack of a complete imderstanding of corrosion and adhesion failures of a coated metal is the lack of analytical instrumentation to probe the behaviors of corrosive agents at the coating/steel interface. We have developed a procedure based on microelectrodes for studying in

situ the behaviors of potential and chloride ions in blister and at a coating/metal interface. The proce- dure requires an attachment of a double-barreled CI" selective microelectrode at the coating/metal

interface, thus allowing direct measurements of Cl~ concentration and corrosion potential changes at localized areas under a coating. Although it is intricate to prepare these microelectrodes, the procedure

provided very useful information for mechanistic studies of corrosion under coatings, as well as for transport studies of Cl~ ions through a coating on metal. The procedure should also be very useful for studying the roles of Cl~ in localized corrosion. The utility of an inverted electrode microsampling

method for studies of CV in very small voliunes such as blisters was also demonstrated.

Key words: blister; chloride; coating; coating/metal interface; corrosion; in-situ; microelectrode; microsampling; potential.

1. Introduction

Organic coatings are widely used to prolong the service life of a metal substrate. One important function

of a protective coating is to act as a barrier between the environment and the substrate. Although the

technology of protective coatings has progressed greatly in recent years, problems continue to exist in providing protection to metals from exposiure to the environment. One of the key barriers to the effective

selection, use, and maintenance of protective coatings for metals is the lack of measurement methods to characterize coatings and coating systems before and during service. A close examination of 50 questions

raised by leading coating researchers [1] reveals that more than half of the questions relate to measurement

and characterization. For example, inadequate understanding of the role of such corrosive factors as oxygen,

water, and ions in failures of a coated metal system is due, in part, to the inability to characterize the

microstructure of the coating and to quantify these elements in the coating system and at the coating /metal

interface. Without this information, reliable degradation and transport models cannot be developed.

Chloride is one of the most detrimental of the ions contributing to the corrosion of bare steel, organiccoated steel, and steel in reinforced concrete. Most of the present knowledge on the movement of the chloride

ion in a coated metal system is derived from studies of the movement of this ion through a detached film. However, barrier properties of a detached film may not be the same as those of an attached film; thus, the

controlling steps and the reaction processes may not be correctly interpreted. Surface analytical techniques can provide chemical information at the atomic level but the techniques must be xised in a high vacuiun

environment. Therefore, they are not suitable for in situ measurements. A method needs to be developed to monitor, in situ, the appearance of the chloride ion at the coating /metal interface and the way its concentration changes with time.

This paper describes a procedure to measure in situ a) the presence, as well as the change in concentration, of the chloride ion at the coating /steel interface, and b) the concentration of Q~ ion in a blister formed

at the coatiog /steel interface during environmental exposure. In addition, the application of a microelectrode

method using \sJL sample size to measure CI" ion concentration in blister solutions will be demonstrated. The

latter method will be useful for determining the concentration of Q~ ion in an area in which the liquid

volimie is small.
2. Experimental Procedures

2.1 Materials and Preparation of Coated Panels

Unless otherwise stated, all coated panels used in this experiment consist of a Ti02"pigniented, alkyd top

coat applied on SAE 1010, low-carbon, cold-rolled steel substrates. The 100 x 150 mm substrates had a

surface roughness of 0.9 to 1.3 fim and had been vapor-degreased at the factory. These substrates were

cleaned further by rinsing repeatedly and alternately with acetone and methanol (reagent grades) to remove

any dirt deposits and contaminants just prior to coating application. Except for one case where the coating was applied by a brush, all coatings were applied on the substrate using a draw down blade. The coated

panels were then cured at ambient conditions for at least five days before use. The coating thicknesses of the

panels were in the 50-100 \im range. Sodium chloride (reagent grade) standard solutions at various concentrations, as expressed in mole/liter (M), in triply-distilled water were prepared for calibration purposes. For

exposure environments, 0.5 M NaCl in deionized water was used.

2.2 Preparation of CMoride-Containing Specimens

Two types of specimen configuration were used in obtaining in situ measurements of Cl~ ion at the

coating /steel interface; these configurations are shown in figures la and lb. The first type (type I), shown in

figure la, is labeled the "cathodic blistering configuration" because the blisters at the coating /steel interface are generally known to result from cathodic reactions induced by the anodic reactions at the exposed metal

surface [2]. A cylinder, 25 mm in diameter, 15 mm in height, and 2 mm in thickness, was mounted on the

coated panel using a silicone adhesive. A continuous scribe was made on the panel section around the inside

of the cylinder. A second cylinder 10 mm in height and twice the diameter of the first one was mounted on

the coated panel to enclose the first cylinder. A 0.5 M NaCl solution was added to fill the first cylinder, and

deionized water was placed in the space between the two cylinders. Blisters were observed to form on the

coated panel section aroimd the outside of the NaCl-containing cylinder.

The second specimen configuration (type II) prepared for Cl~ ion in situ measurements was an "artifi- cially delaminated defect configuration." A schematic example of this configuration is shown in figure lb. A

small area of about 5 mm in diameter on a coated panel was soaked with distilled water (using a rubber

gasket) for a few hours to soften the coating. The thickness of the coating was about 95 ji-m. A chisel was used

to scribe three sides of a square area adjacent to the soft spot and then a 5 x 5 mm section of the coating was

lifted as depicted in figure lb. After the cylinder was mounted on the coated panel, the microelectrode was

inserted into the artificially delaminated defect and the loose coating was pressed down and sealed with a

silicone adhesive. The cylinder was filled with 0.5 M NaCl prior to making in situ measurements.

In addition to the in situ measurements of CI" ion at the coating /steel interface, measurements (in situ) of

the corrosion potential at the coating /metal interface were also made. For this purpose, a microelectrodeimbedded specimen, as depicted in figure Ic (type III specimen), was employed. The tip of the microelectrode was coated first with a water soluble adhesive to prevent direct contact between the Cl~ ion sensing element

and the coating. The microelectrode was placed on a cleaned, bare steel panel at 30" angle from the horizontal

plane in such a way that the tip just touched the substrate. The latter was accomplished by the use of a stereo microscope and a micromanipulator similar to that shown in figure 5. The coating then was applied by brush

and cured for 5 days before exposure to 0.5 M NaCl solution. The adhesive was dissolved by the electrolyte

present during measurement enabling the electrode to be in direct contact with the liquid layer between the

electrode and the coating. It was determined that this test configuration would provide more stable readings

than would be obtained if the coating was placed in direct contact with the electrode. In addition, blisters that

normally occur as a result of exposure for extended periods in NaCl solution were prepared to demonstrate

the use of the inverted electrode microsampling method, to be described later.

23 Preparation of Microelectrodes

Two types of microelectrodes were prepared for use in this study. The first was the single-barreled

reference electrode to be used in the inverted electrode microsampling method for Cl~ ion measurements and

for making measurements of corrosion potentials at the coating /metal interface. The second was the doublebarreled CI" ion selective microelectrode, used for Cl~ ion in situ measurements.
Reference

electrode crselective Nv electrode

::^ Blister

I r 0.5 M NaCI ^ _ y 'Cylinder

Double-barreled

microelectrode — AS^ — —

i^

/ Adhesive ^ Coating

i^^^^m m

(a)

Scribe

~~^

Steel

Reference

electrode crselective

electrode

Double-barreled

microelectrode

Cylinder

"Delaminated" defect Scribe

Coating

Steel

(b)

Water soluble adhesive

Reference

microelectrode

Cylinder

(C)

Scribe

Figure 1. Three types of specimen configurations for in situ measurements of chloride ion and corrosion potential at the coating/metal interface: a) and b) type I and type n for Cl~ ion measurement; c) type IH for potential measurement

23.1 Reference Microelectrodes

Reference microelectrodes, shown in figure 2, were made following several careful steps. The first step was the pulling (using a puller xmder heat) of the heavy-wall (1.0 mm o.d. by 0.58 mm Ld.), single-barreled,

borosilicate glass capillary into a micropipette having the shape shown in figure 2. The tip of the micropipette was about 10 fjtm in diameter. The micropipettes were filled with a KCl-saturated gel. The gel was prepared

by mixing 3 g of agar in 100 mL of boiling 0.5 M KCl solution. After the agar was completely dissolved, the

micropipettes were totally immersed in the boiling solution for 5 minutes. After withdrawing the micropipettes from the solution and allowing them to cool, each tip of the micropipettes was checked visually

under a microscope to verify the completeness of the filling and to insure the absence of air bubbles in the

filled micropipettes. The agar gel prevented leakage of the KCl filling solution into the sample environment

and also prevented possible contamination of the electrode from the sample solution. A section (opposite the

tip) of the agar gel-filled micropipette was then cut off to obtain a micropipette of about 40 mm in length.

Each micropipette was inserted into a pyrex glass tube (i.d. 3 mm) and sealed with molten wax at the

joint (fig. 2). The glass tube was then filled with the reference electrolyte (0.5 M KCl). Care was taken to insure that there were no air bubbles in the glass tube. Finally, a silver /silver chloride electrode (Ag wire

coated with AgCl) of 250 ixm in diameter was placed in the glass tube and sealed with wax to prevent the
To ion analyser

Wax

Ag/AgCI

KCI (0.5 M)

Glass tube

Wax

KCI+Agar (0.5 M)

Micropipette

Tip

Figure 2. Single-barreled reference microelectrode.

evaporation of the KCI filling solution. A voltmeter or reference input of an ion analyzer was connected to the end of the Ag/AgQ wire. The Ag/AgCl wire was made by electrodeposition of AgCl on Ag wire.

Because Ag/AgCl electrodes are sensitive to light, care was taken to minimize exposing the electrodes to

light during the preparation and measurements.

23.2 Double-Barreled Qiloride Ion Selective Microelectrodes

While the construction of the double-barreled Cl~ ion selective microelectrodes is difficult, the doublebarreled microelectrodes offer many advantages for use in the coating /metal interfacial studies over that of a

single-barreled CI" ion selective microelectrode in conjunction with a reference microelectrode. The main

advantage is the relative ease of inserting or attaching one, rather than two, electrodes to the specimen.

Another advantage is that the double-barreled microelectrodes allow direct readings of Q~ ion concentration

at almost the same point of the sample, thus reducing the effects of the environment between the two

electrodes.

Double-barreled Q~ ion selective microelectrodes were constructed from double-barreled capillaries.

These microelectrodes, which are illustrated in figure 3, contain two compartments: one serves as the refer- ence electrode; the other serves as the Cl~ ion selective electrode. As for the reference electrode, many steps are involved in their construction. A double-barreled, 1.5/0.8 mm (o.d./i.d.) borosilicate glass capillary was

first pulled into a pointed shape and one of the two barrels was trxmcated at the tip (fig. 3). In this way, the

sensing parts at the tips of the two compartments could be electrically separated more easily. The longer of

the two barrels was used as the reference electrode and the shorter as the Cl~ ion selective electrode. The

compartment that serves the reference microelectrode was filled with KCl-sattjrated agar gel. The two

barrels of the double-barreled micropipette were cut to the desired lengths, about 40 mm for the longer barrel

and 32 mm for the shorter barrel. An Ag/AgCl wire (made by the same electrodeposition method as described earlier), 100 \x.m in diameter, was inserted into the shorter barrel (the Cl~ ion selective electrode) and

secured with an epoxy adhesive. In addition to securing the wire, the adhesive also serves as a seal to prevent
Reference

barrel

Reference electrode

Wax

Ag/AgCI

KCI (0.5 M)

Glass tube

'— CI selective electrode

Wax

Ag/AgCI

KCI+Agar (0.5 M)

CI selective barrel

Epoxy sealant

Figure 3. Double-barreled chloride ion selective microelectrode.

possible contamination of the Ag/AgQ wire by the sample environment. Precaution was taken to insure that

the Ag/AgQ wire did not touch the tip and wall of the longer barrel (the reference electrode compartment).

The gel-filled, reference electrode compartment was inserted into a glass tube and sealed, and the same

procedures were followed as those described for the preparation of the single-barreled, reference microelectrodes. Wax was also used to seal the top of the Q" ion selective electrode compartment.

2.4 Chloride Ion Measurements

2.4.1 Inverted Electrode Microsampling Method

The method used in this experiment follows a microanalytical procedure reported for measuring nanoliter volumes of F~ and Q" ions containing solutions [3]. The method utilizes a reference microelectrode

brought in contact with a microdrop of the measured solution deposited on the surface of an inverted

selective macroelectrode. This method is referred to in this paper as the inverted electrode microsampling

method. Figure 4 presents the Q" ion measurement set-up using the inverted electrode microsampling

method. An Orion^ half-cell, flat surfaced, Q" ion selective macroelectrode was adapted for use in the

inverted position (as opposed to the upright position conventionally used). The electrode surface, which was

clamped in the inverted position, was first rinsed with triply-distilled water and then dried by wiping to remove any dirt or contamination deposits on the electrode surface that could interfere with the sensitivity of

the electrode. A cylinder (about 2 mm in height) above the electrode surface was made by placing pressure

sensitive tape aroimd the top of the electrode to hold the samples (fig. 4). Mineral oil was first poured into the

cylinder, and then a sample of about 1-2 (iL was deposited on the surface of the Q" ion sensing element of

the electrode using a S-m,L chromatographic syringe. Mineral oil prevented the evaporation of a sample

during the measurement.

The reference microelectrode and the inverted Q" ion selective macroelectrode were connected to the

reference and Cl~ ion sensing inputs, respectively, of a high impedance (10** ohm) Orion EA 940* ion

analyzer. The tip of the reference microelectrode was brought into the center of a sample with the help of a

' Certain commercial equipment is identified in this paper in order to adequately specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply necessarily the best available for the purpose.
Reference microelectrode

Mineral oil Microsample

Cup

Chloride

macroelectrode

in inverted

position

Figure 4. Experimental set-up for chloride ion measurement using inverted elec- trode microsampling method.

Stereo microscope, a micromanipulator attached to the reference microelectrode, and a manipulator attached

to the CI" ion selective macroelectrode. The concentration was read directly from the analyzer. (Multiple

samples can be present on the electrode surface at the same time.) To insure the reliability of the data,

microdrops of the CI" ion standard solutions were also deposited on the electrode surface, and their concentrations measured just prior to sample measurements. If space on the electrode surface were available, additional samples were added and measurements made; otherwise, samples and mineral oil were removed by

suctioning or wiping. It is to be noted here that the ion selective electrode responds to activity, not to concentration. The activity is related to the concentration by the activity coefficient. However, because the

measurement system was calibrated in terms of concentration, we report our results in terms of concentration.

2.4.2 In Situ CMoride Ion Concentration

In situ measurements of Q~ ion concentration in type I and type II specimens were carried out using

double-barreled Cl~ ion selective microelectrodes following the arrangement shown in figure 5. For a type I specimen (fig. la), the insertion of the microelectrode and the actual measurements started only after the

blisters had grown to a measurable size, about 1 mm in diameter. The formation and growth of the blisters were quite difficult to observe because the coating was constrained by the two attached cylinders. However,

with the help of a blunt toothpick, the location and the size of the blisters could be estimated. After a blister of measurable size had formed, the water in the area between the two cylinders was removed. The excess water was wiped up and the blister was punched with a sharp needle. The double-barreled microelectrode was then inserted into the blister at the pimched hole. Care was taken so that the electrode tip just touched

the substrate surface without damaging it. A fast-curing silicon adhesive was used to seal around the hole (see

fig. la). The reference and CI" ion selective electrodes of the double-barreled microelectrode were connected

to the ion analyzer and measurements were begim. It should be noted that water was not replaced in the

volume between the cylinders during the measurement for this type of specimen.

For a type II specimen (fig. lb), the measurements started as soon as 0.5 M NaCl solution was placed into

the cylinder. The analyzer was programmed to print Q" ion concentration at desired time intervals; that is, 10 minutes at the beginning of exposure and one hour at later exposures.
Double-barreled microelectrode

(Reference & chloride sensor

Micromanipulator

(Probe positioner) Sealant

- NaCI solution

Plastic

cellCoating

Steel

Figure 5. Experimental set-up for in situ chloride ion and potential measurements at the coating/metal interface.

2^ Corrosion Potential Measurements

Measurements of corrosion potential at the coating /steel interface were carried out using a reference

microelectrode and the type III specimen configuration (fig. Ic). After the coating was cured, a cylinder was

mounted on the coated panel in such a way that the electrode leaned on the cylinder (fig. Ic). An area of the

coating outside the cylinder was removed to expose the metal. The bare metal served as the working elec- trode. The reference microelectrode and the exposed metal of the coated panel were connected to a volt- meter, and corrosion potentials as a function of exposure time were measured as soon as 0.5 M NaQ solution was placed into the cylinder.

3. Results and Discussion

3.1 Calibration

Microelectrode methods are widely used today in biological systems. However, appropriate microelectrodes are not commercially available at present, and their use is not standardized, nor routine. Researchers

must fabricate the microelectrodes themselves to suit their own needs following many careful steps. The first steps in any measurement using the microelectrode method are to insure that the electrodes, both reference

and ion selective types, are working and to calibrate them. This is done by using standard solutions in the

concentration range of interest to establish a potential-concentration calibration curve. Figure 6 presents four calibration curves for the three chloride electrode systems under study in NaQ

solutions having concentrations ranging from 1 to 0.00015 M; the concentration is expressed in the logarithmic form. Curve 1 was obtained using a commercial combination Q" ion selective macroelectrode (C\~ ion

selective and reference electrodes are built in one body) in 10-mL standard solutions following the normal

procedures (upright position). Curve 2 was from Q~ ion measurements by the inverted electrode microsampling method using 2 ^lL of the same standard solutions. Curves 3 and 4 were obtained by using two different
200

160

120

>

E

80

40

-40

-80-

Conventional

Inverted

> Double-barreled

% CI

0.00035 0.0035 0.035

0.00001 0.0001 0.001 0.01

MCI"

Figure 6. Calibration curves of the three chloride ion measurement methods. Curve 1: conventional; curve 2: inverted electrode microsampling; and curves 3 and 4: double-barreled

microelectrode.

double-barreled CI" ion selective microelectrodes constructed at two different times and 10-mL volumes of

sample solution, an indication of the excellent reproducibility of the construction of these electrodes. The

parallelism of the four curves in figure 6 indicates that the three measurement methods are equivalent in providing Cl~ ion concentration. All four curves show no evidence of a deviation from the Nernstian law

(potential is inversely linear with logarithm of concentration) in the concentration range studied. Previous

works [4] also showed a straight line relationship between the potential and the logarithm of concentration of an Ag/AgCl electrode in KCl solutions between 5 and 0.001 M concentration.

3.2 Measurements of CMoride Ion at the Coating/ Steel Interface

3.2.1 Inverted Electrode Microsanq>ling Method

As indicated earlier, the inverted electrode microsampling method is useful for measuring Cl~ ion concentrations in small volumes of electrolyte, such as that in blisters formed between coatings, in blisters formed

between a coating and a metal surface, and in other areas where the liquid sample size is small, such as in the

corrosion of crevices and nits. Since the tip of a reference microelectrode can be made very small (as small as 0.045 (Jim has been reported [5]), the limiting factor of this method is the ability to obtain and transfer very

small samples to the macroelectrode svu-face. For coated metal, this factor restricts the use of the inverted

electrode microsampling method for measuriQg Cl~ ion concentrations to blisters that are visible and perforable so that the liquid can be withdrawn. This means that the Q~ ions found in the blister may not be

present at the early stage of the blister formation. The following example demonstrates the usefulness of the

inverted electrode microsampling method for studies of corrosion of coated metals.
A Ti02-pigmented alkyd coated panel (top coat only) of about 100 \im thick was allowed to stand

(without constant stirring but with distilled water added occasionally to maintain the solution level in the

cylinder) under a 0.5 M NaCl solution. Two large blisters formed after six months of exposure; one was

formed prior to the other. Chloride ion concentrations of the two blisters and of the bulk solution were

measured using the inverted electrode microsampling method. It was interesting to find that the Cl~ ion

concentration of the larger of the two blisters was 5.2 M and that of the smaller blister was 1.2 M. These were more than 10 and 2 times higher, respectively, than that of the bulk solution, which changed very little after 6 months exposure. Early workers [6], by drawing the liquid from a blister with a syringe and then analyzing

it potentiometrically, also reported a three to foiu" times higher Q" ion concentration at the coating /steel

interface than that in the bulk. Apparently, the migration of CI" ions was more favorable to one blister than the other. When the coating was peeled off the substrate, it was observed that the substrate under the blister having the higher CI" ion

concentration was heavily corroded over the entire blister area, while the area under the other blister appears

to be free of corrosion products. Thus, it appears that the metal area imder the blister having the higher Cl~

ion concentration was in an anodic state, while the other was in a cathodic state. The latter was probably

formed as the result of the extended anodic region of the former. This phenomenon seems to be simil ar to the

cathodic blistering observed for a scribed coated panel subjected to an electrolyte [2,7]: corrosion occurs at the scribe which serves as the anodic site; and cathodic blisters, resulting from the cathodic reactions, were

formed around and away from the scribe. The higher level of CI" ion concentration in the cathodic blister (the lower Q" ion concentration blister) as compared with the bulk solution suggests that some Q~ ions from

the blister having the higher CI" ion concentration migrated to the blister having lower Q" ion concentration, probably through a water-thickened layer at the coating /steel interface or a breakthrough between the

two blisters. Heavy chloride deposits on the corrosion products at the scribe of a scribed coated panel immersed in a NaCl solution analyzed by an electron microanalysis technique was also reported [8]. The preferentially

higher concentration of Cl~ ions at the anodic sites (i.e., the higher Cl~ ion concentration blister and the

scribe) was similar to that observed in crevice and pitting corrosion [9]. For example, the fluid within crevices

exposed to neutral NaQ was reported to be 3 to 10 times greater in Cl~ ion concentration than that of the

bulk solution. The acidic environment created by the anodic reactions at the corroded sites in the blister having the higher Cl~ ion concentration was probably responsible for the higher concentration of Cl~ ion of

this blister. At anodic sites, iron is dissolved and subsequently hydrolysed to form iron hydroxides and H^

ions. The depletion of oxygen at the corroded areas, because of the barrier property of corrosion products,

also enhances the positive properties of this region. Chloride ions, which probably migrate from the bulk

solution to the blister through the defects or expanded pores of the coatings, are needed to counterbalance the

positive charge of the H^ ions.

3.2.2 In Situ Chloride Ion Measurements

Figure 7 presents the Cl~ ion concentration as a function of exposure time for a type I specimen configuration (as shown in fig. la). The coated panel was immersed in a 0.5 M NaCl solution and a double-barreled

Cl~ ion selective microelectrode was used. As mentioned earlier, the insertion of an electrode into this type

of specimen was only possible after the specimen had been immersed for an extended period of time to cause formation of a sizable blister; the first measurement was made after 12 days of immersion as indicated in figure

7. The Cl~ ion concentration in the blister increased rapidly after inserting the electrode. A value four times

that of the bulk solution was reached after 3 days of insertion. After that the Cl~ ion concentration remained

almost constant for the next 4 days. It is noted here that the blister, in which the measurements were made,

formed on the coated panel section immersed in water external to the NaQ-containing cylinder. There was

no liquid medium in contact with the outside of the blister during the measurement. Thus, the C\~ ions

measured were the result of their migration laterally along the coating /metal interface extending from the

scribe inside the NaQ-containing cylinder to the blister area. At the start of the measurement, the Q" ion concentration in the blister was almost equal to that of the

bulk solution. This indicates that the coating /metal interfacial region extending from the blister to the scribe

has been delaminated and /or that the water layer in this region was sufficient to allow C[~ ions to pass through. This may be understandable in terms of the mechanisms of adhesion loss and the cathodic delamination processes of a defective coated panel exposed to an electrolyte [2,10]. The loss of adhesion as the result of water buildup at the coating /metal interface before blistering begin facilitates lateral movement of ion
[CI " ] in Blister vs. Time Immersed in 0.5 M NaCI

12 14 16 18

EXPOSURE TIME (days)

Figqic 7. Chloride ion concentratioa as a function of time obtained by using double-barreled chloride

ion microelectrode for type I specimen immersed in 0.5 M NaCl solution.

between the anodic and cathodic areas [7]. A breakthrough between the hollow cone of the anodic corrosion

products near the scribe and cathodic blisters was also proposed as a mechanism to account for the higher Q"

ion concentration in the blisters than in the bulk [6]. The rapid rise of Q" ion concentration at the beginning of the measurement may be attributed to the

increase of the corrosion activity resulting from the resupplying of oxygen into the blister during the insertion

of the microelectrode. The reason for the leveling off is not certain at this time. One of the possibilities may

be a cessation of corrosion reactions due to the depletion of O2 in the blister. The low O2 concentration in aqueous solutions containing greater than 3 percent (0.52 M) NaCl has been attributed to the decrease of

corrosion of iron immersed in these solutions [11]. A decrease between the 19th and 20th day of exposure may

be a result of complexing or association of Cl~ ions with corrosion products. The ion selective electrodes

respond only to free Cl~ ion. If Q~ ions in an electrolyte are not free, that is, if they are somehow are

associated or bound, the ions will not be measured by the ion selective electrode method.

Figure 8 is the curve representing Cl~ ion concentration change as a function of exposure time in a blister immersed in 0.5 NaCl solution for a type II specimen configuration. The measurement started immediately

after the solution was added to the cylinder. It should be noted that there was a scribe through the coating to the steel substrate about 25 mm from the microelectrode. We started to detect the presence of Q" ions at the

"artificially delaminated" defect about 3 hours after immersion and the Q" ion concentration rose almost

steadily up to 65 hours. Except for a possible decrease between 75 and 85 hours, the concentration of Q" ions

ia the defect continued to rise. The concentration increased more sharply after 85 hours and reached almost

two times the concentration of the bulk solution after 120 hours of exposure.

Assuming the delamination was 1 mm deep and using the Q" ion concentration value given in figure 8,

it was estimated that 1.8x 10~* g of CI" ions had entered the 5x5 mm artificially delaminated defect area

after 65 hours of exposure. Using Pick's first law of diffusion and assuming that the rate of diffusion of CI"

ions through the coating was constant and that no Q~ ion entered the defect through the scribe, we obtained a diffusion coefficient of 4.0 x 10"^** cm^s"^. This is somewhat higher than the "migration" coefficient values

of 0.6 X 10" ^"^ and 0.83 x 10"" cm^s"^ reported [12] for scribed and imscribed, respectively, alkyd top coat

panels immersed in 0.5 M NaCl solution without an applied potential. It is not known how the silicone

adhesive and the sealed areas around the defect affected the calcinated value reported here.

10
[Cr] in "Delaminated" Defect vs. Time Immersed in 0.5 M NaCI

0.8 1 1 1 1 1 1 1

1 1 1 1 1^

0.7 - h^-

0.6 - J -

5^ 0.5 1

o

- bulk solution 1

^0.4 - L

^0.3 - 1

0.2 — 1

0.1 —

o-i

Ot BrV\ 1 1 1 1 1 1 1 1 1 1 20 40 60 80 100

EXPOSURE TIME (hours)

120

Fignre 8. Chloride ion concentration as a function of time obtained by using double-barreled chloride ion microelectrode for type II specimen immersed in a 0.5 M NaCl solution.

One of the reasons for the rapid rise after 85 houxs of exposure may be a breakthrough delamination

between the scribe and the artificially delaminated defect allowing "flooding" of the delaminated area with

NaQ from the bulk solution. A similar phenomenon has been reported to account for the difference in the

migration coefficients of Cl~ ions of xmscribed and scribed coated panels immersed in Q" ion containing

solutions [12]. Another possible reason may be an increase in the corrosion activity in the delaminated area, which would cause an increase in Cl~ ion concentration as discussed earlier. Judging from the rapid rise after the 85th hour and the continuation of the increase, it is suggested that both events probably occurred with

flooding preceding the accelerated corrosion. The latter event was probably induced by the former because

of the accelerating effect of Cl~ ions on corrosion. Chloride ions can accelerate the corrosion of metals

through one or a number of mechanisms: by adsorbing and penetrating the oxide layer, by complexing with

the metal and by catalyzing the corrosion reactions [13].

33 Corrosion Potential Measurements

The value of the corrosion potential of a metal substrate (acquired when a corroding metal is in contact with an electrolyte) by itself is of little value, but its change as a function of immersion time may provide

valuable information about the mechanism of the corrosion reactions and the rate controlling factors [14]. In

general, a rising potential indicates passivity and a falling potential denotes activity. In the case of coated

metals it is a function of the permeability of the coating to water and ions and the integrity of the coating.

Figure 9 depicts the results of measurements of the corrosion potential at a coating /steel interface using

the microelectrode method and a type III specimen. The corrosion potential moved in the active direction at

first, then moved to the noble direction and finally moved in the active direction, reaching a corrosion

potential of 0.675 V vs Ag/AgCl at the end of the test. The potential decreased sharply after 100 hours of

exposure indicating that the coated panel had severely corroded with a great increase in the anodic /cathodic

surface area ratio. Although the time span is quite different, the trends shown in figure 9 are similar to the

general potential-time curve obtained by the conventional method [14], in which the potential is measured by

immersing a reference macroelectrode in the electrolyte solution, not at the coating /metal interface. The time

required for the onset of corrosion (the beginning of the decline after the peak) to occur took only a few days

in this experiment compared to weeks using the conventional method.

11
Ecorrat Coating / Metal Interface

of a Scribed Coated Steel Panel

cfNb^

J

100 200 300

EXPOSURE TIME (hours)

400

Figure 9. Corrosion potential at the coating/metal interface as a function of time obtained by using

reference microelectrodes for type m specimen immersed in a 0.S M NaCl solution.

The main advantage of measuring corrosion potential using the microelectrode at the coating /metal

interface is the ability to obtain the corrosion potential at microareas, such as in blisters, cracks, and pits; this cannot be measured by a conventional reference electrode. Another advantage is the ability to obtain the

potential as soon as the electrode is in contact with electrolyte at the coating /metal interface rather than

waiting imtil a continuous conductive pathway through the coating is established. Not many coated metals

provide a continuous pathway from the moment of immersion. For example, it has been reported that a

corrosion potential could not be measured on a 2S-M.m vinyl coated aluminum sample in S percent (0.86 M)

NaCl solution until the 41st day [15]. An electrolyte at the interface may be formed, as a result of corrosion

reactions, the presence of surface contaminations, or migration of ions from the coatings, before the arrival of

electrolyte from the bulk solution.

One of the biggest concerns in measuring Q~ ions and corrosion potentials using the microelecfrode

method is the stability of the electrode. For that reason, it is always recommended that the electrodes be

reconditioned before use. Our experiences indicate that the microelectrodes produced and used in this study were very stable in the pure solutions. However, their long term stability in solutions containing complex

corrosion products during the in situ measurements is not known. Our limited data indicate that these microelectrodes are still stable after one week of testing in a corrosion-products solution; this test is still on

going and data on long term stability will be reported when further results become available.

Another concern is the relatively large tip, ~200 p-m, of the solid state, double-barreled microelecfrode

used in this study. This tip may reduce the optimal spatial proximity provided by the double-barreled method.

We have made and tested the double-barreled microelectrodes having tips of about 10 ^.m using a liquid ion

exchanger in place of the Ag/AgQ wire at the tip. However, their stability was poor (about 3 days) and not

suitable for the ia situ measurements. The current limitation of the tip size of a solid state, double-barreled

microelectrode is the size of the Ag/AgC3 electrode. Thus, we see no reason why a smaller tip double-barreled microelectrode using a solid state elecfrode could not be made if smaller size Ag wire were available.

12
4. Conclusions

Despite considerable research on the corrosion and failiire of coated metals in the last decade, much is

still unknown. One of the main reasons for that is the lack of analytical instrumentation to probe the behavior

of corrosive agents at the coating /steel interface. We have developed a procedure based on using a microelectrode for studying in situ the behaviors of chloride ions and corrosion potentials at the coating /metal inter- face. This procedure will aid greatly in imderstanding the mechanisms of corrosion of steel under a coating,

the blister formation of a coating system, and the corrosion of steel in reinforced concrete exposed to salt water. In addition, we have also demonstrated a useful microsampling electrode method for measuring Q"

ion concentrations in small volumes, such as in blisters. The major drawbacks of the method are the diMcult procedures reqxiired for the preparation of the

microelectrodes and microelectrode-imbedded specimens. The former is intricate and time-consuming while

the latter is delicate. There should be as little disturbance as possible to the blisters and coating /metal

interface while probing them. At the same time, the sensitivity and integrity of the microelectrode must be

maintained.

5. Acknowledgments

"We thank Dr. G. L. Vogel and Mr. C. M. Carey of the Paffenbarger Research Center, NIST, for making some of their instruments available and their helpful suggestions in the measurements of Q~ ion concentration

using inverted electrode microsampling method. We also thank Dr. M. McKnight, Building Materials Division, NIST, for her helpful discussions during the course of this study.

6. References

[1] W. Funke, H. Leidheiser, Jr., R. A. Dickie, et al., J. Coatings Technology 58, 80 (1986).

[2] H. Leidheiser, W. Wang, and L. Igetoft, Prog. Org. Coat 11, 19 (1983).

[3] G. L. Vogel, L. C. Chow, and W. E. Brown, Anal. Chem. 52, 375 (1980).

[4] A. J. de Bethune, in "The Encyclopedia of Electrochemistry," C. A. Hampel, Ed., Reinhold Publishing, NY, 1964, p. 1206.

[5] D. Ammann, "Ion-Selective Microelectrodes, Principles, Design, and Application," Springer-Verlag, NY, 1986, p. 6.

[6] J. J. Ritter and M. J. Rodriguez, Corrosion 38, 223 (1982).

[7] W. Funke, Ind. Eng. Chem. Res. Dev. 24, 343 (1985).

[8] T. Nguyen, A. Sharpiro, M. McKnight, J. Martin, and C. Lin, Application of Electron Probe Microanalysis and Imaging in Corrosion Studies of Coated Steel, in preparation.

[9] M. G. Fontana, Corrosion Engineering, McGraw-Hill Book Company, 1986, pp. 51-73.

[10] R A. Dickie, in "Polymeric Materials for Corrosion Control," R. A. Dickie, Ed., Am. Chem. Soc., Washington, DC, 1986, p. 136.

[11] H. H. Uhlig, "Corrosion and Corrosion Control," John WUey and Sons, NY, 1971, p. 113.

[12] J. Parks and H. Leidheiser, Ind. Eng. Chem. Prod. Res. Dev. 25, 1 (1986).

[13] R. T. Foley, Corrosion 26, 50 (1970).

[14] H. Leidheiser, Prog. Org. Coat 7. 79 (1979).

[15] G. W. Walter, Corrosion ScL 26, 681 (1986).

13

NIST-114A

(REV. 3-89) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

PUBUCATION OR REPORT NUMBER

NIST/TN-1266

PERFORMING ORQANIZATION REPORT NUMBER

PUBUCATION DATE July 1989

4. TITLE AND SUBTITLE

Development of a Method to Measure In Situ Chloride at the Coating/Metal Interface

5. AUTHOR(S)

Tinh Nguyen and Changjian Lin

6. PERFORMING ORQANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURQ, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Same as item #6

10. SUPPLEMENTARY NOTES

DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-185, FIPS SOFTWARE SUMMARY, IS ATTACHED.

11. ABSTRACT (A 20G-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. LITERATURE SURVEY, MENTION IT HERE.)

IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOQRAPHY OR

One of the main reasons for the lack of a complete understanding of corrosion and adhesion failures of a coated metal is the lack of analytical instrumentation to probe the behaviors of corrosive agents at the coating/steel interface. We have developed a procedure based on microelectrodes for studying in

situ the behaviors of potential and chloride ions in blister and at a coating/metal interface. The proce- dure requires an attachment of a double-barreled Cl~ selective microelectrode at the coating/metal

interface, thus allowing direct measurements of Cl~ concentration and corrosion potential changes at localized areas under a coating. Although it is intricate to prepare these microelectrodes, the procedure

provided very useful information for mechanistic studies of corrosion under coatings, as well as for transport studies of Cl~ ions through a coating on metal. The procedure should also be very useful for studying the roles of CI" in localized corrosion. The utility of an inverted electrode microsampling

method for studies of Cl~ in very small volumes such as blisters was also demonstrated.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

blister; chloride; coating; coating/metal interface; corrosion; in -situ;

microelectrode; microsampling; potential

13. AVAILABIUTY

X~| UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

17

15. PRICE

lELECTRONIC FORM

i^U . S . GOVERNMENT PRINTING F F I C E i 1 9 8 9 - 2 4 2 - 2 / 4





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