NBSiR 80

Measuring the Rate of

Corrosion of Reinforcing Steel in

Concrete

E. Escalante, S. !tc, and M. Cohen

Corrosion and Electrodeposition Group

Chemical Stability and Corrosion Division

and

Ceramics, Glass, and Soiid State Science Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

March 1S30

Annua' Report

Prepared for Federal Highway Administration

Paving and Structural iVSaterials Group

HRS-22

Washington, DC 20590

• Q0— * 100

,1156

io.80-201z

1980

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NBSIR 80-2012

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MEASURING THE RATE OF

CORROSION OF REINFORCING STEEL IN

CONCRETE

NATIONAL BUREAU OF STANDARDS

LIBRARY JUN ' 5 1980

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80-2012

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5.2.

E. Escalante, S. Ito, and M. Cohen

Corrosion and Electrodeposition Group

Chemical Stability and Corrosion Division

and

Ceramics, Glass, and Solid State Science Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

March 1980

Annual Report

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Prepared for Federal Highway Administration

Paving and Structural Materials Group

HRS-22

Washington, DC 20590

U.S. DEPARTMENT OF COMMERCE, Philip M. Kiutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

Abstract

The progress on a research program directed at developing a nondestructive

method for measuring the corrosion of steel in concrete as related to bridge

deck deterioration is reported. Several polarization techniques for measuring

the corrosion rate of steel in concrete are correlated to actual weight loss

measured gravimetrically. The design of a prototype automated minicomputer

system for measuring the corrosion of steel in concrete is also described.

Included are the results of a laboratory basic study on the effect of pH,

Cl”, On, and moisture on initiating and maintaining corrosion in mortar.

INTRODUCTION

New technology is being used at an increasing rate in efforts to

control corrosion of reinforcing steel in concrete bridge decks. These

approaches range from modification of the concrete environment by barrier

type road surfacing materials that exclude chemical components necessary

for the corrosion to proceed, to protective coatings applied directly to

the steel reinforcing bar during its manufacture. Other techniques

include impressed current systems which electrochemically modify the

reactions at the surface of the steel in concrete thereby reducing its

corrosion.

Many of these techniques appear promising. However, there is a

need for some method of evaluating the effectiveness of these approaches.

It is the purpose of this project to develop a nondestructive field

instrument and associated methodology to accurately measure the rate of

corrosion of steel in concrete slabs and evaluate its applicability to

bridge structures. This capability will allow one to monitor the progress

of corrosion as a function of time and bridge deck modifications.

This program is divided into six tasks briefly described in the

following.

Task A: This task involves making a literature search to identify

the techniques that have been developed for measuring the rate of corrosion

of metals, and, in addition, identify commercial equipment available

for measuring the corrosion of steel in concrete.

Task B: The two most promising techniques identified in task A are

then to be adapted to measuring the corrosion of steel in concrete and

evaluated on their applicability.
- 2 -

Task C: On the basis of the information found in task B, one

technique is chosen and further tested and developed for application to

measuring the corrosion of steel in concrete.

Task D: Define the measuring procedures, and design and build an

automated prototype system to permit routine use. Test the prototype on

steel in concrete specimens.

Task E: Update the drawings and specifications to permit fabrication of the instrument by a commercial firm.

Task F: Evaluate the applicability of the instrument to measuring

the corrosion of reinforcing steel in a bridge deck.

Summary of the Previous Year, Task A: The literature search conducted

for Task A resulted in a National Bureau of Standards Special Publication

Number 550 titled "A bibliography on the Corrosion and Protection of

Steel in Concrete." This document is a bibliographic list of references

of published papers, reports, and talks regarding the corrosion of steel in

concrete and related subjects. The references are presented in two

forms: 1) A subject index divided into six major subheadings including

reviews, factors affecting corrosion, measurement techniques, protection

techniques, concrete design and related fields, and 2) An author index

in alphbetical form. A total of 394 references are listed covering the

period from January 1964 to November 1978.

A second document, titled "A Review of Techniques for Measuring the

Corrosion of Metals in Concrete" described the methods presently being

used to measure the corrosion of metals in an electrolyte. The possible

problems and limitations of these techniques when applied to corrosion

in concrete were discussed. The use of these techniques in commercially

available corrosion measuring devices is also described.
- 3

-

Results During the Past Year

This portion of the report will be divided into three sections.

The first section describes the application of several techniques for

measuring the corrosion of steel in concrete. The second section

discusses the procedure being followed in the development of an automated system for making the corrosion measurement. The third section

describes the results of a basic study on the effect of four parameters

(pH, O^, Cl”, moisture) on the corrosion of steel in three environments,

including mortar.

Corrosion Measurement Techniques, Task B: The essence of this

portion of the study was reported at the FHWA Research Conference in

Williamsburg, VA, December, 1979. That report is attached as Appendix A.

Parts of that report will be elaborated upon here along with an update

of some of the latest work.

The design of the concrete slab was changed several times through

the course of the initial parts of the study. Two considerations were

deemed important in the final design. First, it was desirable to simulate

the geometric arrangement and the chemical environment of a bridge deck.

This resulted in the use of parallel steel bars in a typical bridge deck

concrete mix as reported. Second, it was necessary that we be able to

weigh our rods for weight loss determinations; the weight and, thus, the

size of the rods was determined by the maximum capacity of our analytical

balances. The first trial castings were made using standard rebar.

After curing, the castings were broken and the rebar removed. It was

quickly apparent that complete removal of the concrete would be difficult

without damaging the steel bar. It was decided to use a smooth finish

bar of low carbon steel instead. This was a benefit for the study
- 4 -

because not only did it make it easier to remove the concrete from the

steel, but now the surface area of the steel was clearly defined. In

addition, the chemical composition of low carbon steel rod is consistent,

unlike the composition of rebar which varies from batch to batch. Three

bars were arranged in the slab so that the center rod could serve as a

counter electrode. The other two were positioned so that one was close

to two outer surfaces of the slab, and the other located so that it was

close to only one surface (Appendix Figure 1). In this way more oxygen

was available to one than the other.

All of the data reported in Appendix A were obtained on concrete

slabs immersed for various periods of time in an aqueous NaCl solution

after curing. More recent studies include data taken on slabs to which

the chloride is added to the concrete before casting. The measurements

of corrosion by polarization methods on these new slabs indicate that the

corrosion of the steel is initiated almost immediately after casting as

shown in Figure 1. At the same time, the resistivity of the concrete is

measured as described in Appendix A. As noted in the earlier work, the

corrosion appears to have a strong dependence on the resistivity of the

concrete.

On two occasions corrosion measurements were also made on concrete

slabs belonging to the Federal Highway Administration (FHWA) and located

at their Fairbank Highway Research Station in McLean, VA. These measurements

consisted of using Stern-Geary ' s , Barnartt's, and Mansfeld's techniques.

The following table displays the results obtained.
- 5 -

7 September 1979 20 September 1979

Block Stern-Geary Barnartt Stern-Geary Mansfel

283 72.3 yA 16.6 yA 14.5 yA poor least square

fit to equation.

Not acceptable.

302 325 yA solution

indeterminate

181 yA Same

Thus, it was found that of the two slabs, block 283 on slab 67-33 exhibited

the lowest corrosion current as compared to block 302 on slab 95-17.

In summary of Task B, four polarization techniques have been applied

to measuring the corrosion of steel in concrete. These techniques were

Stern-Geary ' s , Schwerdtfeger ' s , Mansfeld's, and Barnartt's. Compensation

of IR was accomplished through use of a modified Holler's bridge circuit.

Comparison of weight loss calculated from polarization data to weight

loss gravimetrical ly determined indicated that electrochemical measurements

tend to overestimate weight loss by 17% to 86% with the Stern-Geary

technique giving the best agreement at 17%.

Refinement of Technique, Task C: Refinement of the polarization

technique to improve its suitability to the measurement of corrosion of

steel in concrete is the goal of this segment of the study.

The biggest concern remains the determination of current distribution on a bridge deck during polarization measurements. This is of

Importance because this distribution affects the corrosion current

density calculation. Tn the case of small slabs, this determination is

not a problem since the experiment is designed so that the entire

surface of the steel bar being measured is polarized. With a bridge

deck, however, the structure is so large and complex that it is difficult
- 6 -

and impractical to attempt to polarize all of the steel in the concrete.

In fact, if one could limit the area being polarized, then local high

corrosion areas could be found if they exist.

Thus, in addition to characterizing the current distribution from a

single counter electrode, we are also attempting to develop a counter

electrode system that would confine the distribution of current to a

known region of the bridge deck. In principle this is done by using a

counter electrode having a second counter electrode concentrically

located around it. Both would be maintained at the same electric potential

with respect to the steel in the concrete, but the driving current to

the two electrodes would not come from the same source. The electric

field of the outer concentric electrode would confine the current

distribution of the inner electrode to a known area. Since the current

sources are separate, the polarizing current of the inner electrode,

whose current distribution is now known, can be measured.

Up to this point in the study, Cu/CuSO^ reference electrodes at the

concrete surface have been used to measure the potential of the steel.

This type of reference is very satisfactory where corrosion measurements

are made at infrequent intervals of time. At some future time it may be

desirable to monitor the corrosion of a bridge deck continuously. For

this purpose, a permanently imbedded reference electrode is best.

Several solid reference electrodes are under consideration. These

metallic zinc, thallium,

Cu/CuS0

4

• CaS0

4

• 1/2 H

2 0

Zn/ZnCl

2

• CaS0

4

• 1/2 H

2 0

are:
- 7 -

The electrodes have been made and will be included in the next set of

concrete slab specimens.

To summarize Task C, the problem of current distribution is being

addressed in two ways. First by characterizing the current distribution

from a single counter electrode, or second, by confining a concentric

field electrode. The suitability of solid permanent reference electrodes

is being investigated. Finally, the polarization measurements are being

continued with slabs under various conditions of exposure.

Automated System, Tasks D and E: The ultimate result of this

program is to be the design and development of a laboratory system

capable of serving as a prototype for an instrument to be used in the

field for the measurement of corrosion rates by electrochemical technique

Much of the work performed on this project will therefore be ultimately

utilized in the design philosophy and implementation of the measurement

system. By implication, this could be considered to be a valid reason

for delay in the development of the instrument. However, there already

exists an array of information sufficient to establish basic criteria

for the initial design.

At this stage, it is worth noting that all existing methods for the

measurement of corrosion rates by electrochemical techniques, ultimately

devolve to a measurement of voltages [AC or DC) proportional to either

corrosion current or potential. Thus, it is apparent that any technique

used in the final system requires a means of determining potential in a

precise manner. A flexible means of choosing the source to be measured

is also required.
a

Any system to be used for the routine measurement of a physical

quantity must of necessity be capable of operation in a non-complicated

fashion. Operator skills vary and complexities in the experiment should

be limited to the setup of the cell, in order to minimize the necessary

training. In principle, once connected, the measurement should involve

only a "Start" signal, with all other aspects of the experiment

controlled by the equipment itself. However, provision must be made

for changes in the experimental technique used, in order to suit either

varying conditions or as a result of the ongoing work in the other

portions of this project. As already discussed, easy access to a

variety of voltage sources and a precision voltage measurement method

must also be part of the instrument.

Data output must be reliable, relatively noise-free, and capable

of permanent storage. Provision must be made for data acquisition

over extended periods of time which may imply large quantities of data.

The capability of, at least, initial data reduction should be included.

Of potential interest would be the capability of transmitting the

accumulated data, at fixed intervals, to a remote control site.

Another important consideration, is that the ultimate prototype

must be capable of redesign for field use, with a miniumum of effort.

This implies that all semiconductors used (discrete devices or integrated circuit) should have militarized (wide temperature range)

equivalents, and also that the circuit boards shall be capable of

reconstruction in a minimum volume and weight configuration. In

addition, circuitry should be low power in order to extend battery life

in the field version.
- 9

-

Excluding consideration of the cell, the above discussion provides

enough guidelines to design an instrument. This system will be capable

of handling almost any type of cell that might be used for measurement.

The first aspect of the problem that must be addressed is the

decision as to whether the system should be digital or analog in

design. Analog systems have great flexibility, but in general, are not

capable of changes without intervention by a skilled operator. Analog

data outputs are more susceptible to noise, and as regards permanent

storage, are more difficult to save without some kind of analog to

digital conversion. Data reduction to a more usable form is generally

more difficult. On the other hand, digital systems overcome the data

output and reduction difficulties and are essentially as flexible as an

all analog system. Where analog systems have a distinct advantage is

the greater sensitivity possible and the fact that they can take the

data continuously. Even these factors can be handled by a properly

designed digital system. An analog "front end" can be used for sen- -

sitivity when required, and data can be taken at rates fast enough to

emulate the continuous characteristic of analog detection. For these

reasons, we have decided to utilize a digital system.

Although the detailed design has only been started, we can outline

the basic system (Figure 2). Overall control of the system resides in

the CPU (Central processing Unit which will be a unit based on an

MC6800 type microprocessor. The CPU will contain an ROM (read only

memory) and all the utility programs needed to operate each of the other

components of the system as well as to perform data manipulations. Also

included on the CPU printed circuit board will be generally needed
- 10 -

peripherals such as serial communication interfaces, a "Real-Time Clock,"

power failure detection and handling, and I/O interfacing to the control -

Display panel

.

The specific program to be performed [which utilizes the utility

programs on the CPU board, as subroutines} will be contained on a

PROM (Programmable Read Only Memory} which will be installed by the

operator when the unit is to be activated. This PROM will be programmed

to direct the instrument through the experiment, according to the

technique determined to be useful under the prevailing conditions.

Thus, operation of the system would be simplified since the operator

would only have to insert the PROM, turn the unit on, enter any needed

parameters-, as requested by the program, and then push a start button.

(This assumes connection of the cell has been performed prior to system

activation. [ The PROM could be replaced by a prerecorded tape. In

this case, the operator would load the tape into the system and then

perform the other operations. This procedure, however, has a major

disadvantage. With this technique, the program would reside in RAM

(/'Random Access" memory = read-write memory!, Power failure, or a burst

of excessive noise could ruin the program, whereas a PROM program would

be undisturbed except for catastrophic failure of the system.

The "Display-control" panel would be used by the operator on start-up.

The display portion would request needed parameters during start-up,

whicn would be entered via the controls. When the start procedure is

completed, the CPU would disable the panel except for the power switch.

It is worth noting here that these parameters are to be saved in a small

RAM that would be saved, in the event of power failure, due to a

backup power supply.
- 11 -

The RAM Board would contain a sufficient read-write memory for

all temporary data storage. We expect to transfer the results of each

reading onto tape as soon as the data reduction is completed. Thus in the

event of failure, minimum data loss would occur. Since this also reduces

the memory requirement, it also reduces needed power.

The tape recorder and the modem are the data output ports for the

system. Each can be used individually, or they can be used in tandem.

Upon completion of data reduction, the quantities computed (and/or the

raw data) can be recorded on an incremental tape recorder. If a

telephone hookup is available, the data could be transmitted from the

remote site via the modem to a central facility.

The Converter Board will consist of at least one Digital to Analog

converter (DAC) and one Analog to Digital Converter (ADC) with associated

controls, amplifiers, etc. The desired resolution and speed will be

determined in the course of this study. Inputs to and outputs from the

converter board will go to or come from the "Front End" board (analog),

or the data bus [digital 1.

The "Front End" board is not capable of full definition at the

present time. Design of this board is technique dependent. If DC

methods are used, the front end would essentially be a small potentiostat CLO noise, capable of galvanostatic operation, small signal!. If

the AC Impedance techniques are used, the front end will be different.

All circuitry will be designed utilizing circuits that can be

replaced by either Low Power devices or CMOS circuits. Every attempt

will be made to ensure that wide- temperature range devices are available

as replacements for the prototype semiconductors. In addition, the
possibility of entering a power down mode between measurements will be

examined. Either or both of these adaptations would be needed when

field models of the prototype are built.

Basic Studies: The literature search revealed that the information

on the corrosion of steel in concrete was heavily concentrated on the

effects of chloride salts. However, there was little information on the

effects of oxygen, pH, and moisture. In fact, no studies on the corrosion

of steel in an alkaline environment were found. The first stages of

this work sought to develop an understanding of the effects of oxygen,

pH, and chloride concentrations on the corrosion of steel. On a more

practical level, this information was needed to enable us to control the

corrosion process during the development of the corrosion measuring

techniques

Three environments have been used in this phase of the study. The

first was an alkaline solution of Ca(0H)

2

. The second was a wet sandsalt mixture, and the last was mortar. Steel specimens were cut from

1 mm sheet to approximately 2 x 10 cm.

Flat steel specimens were placed in a CaCOH^ solution in which the

oxygen content was controlled by bubbling an oxygen-nitrogen mixture

through the solution in which the pH and chloride content were also

controlled. The results indicated that pH controlled the corrosion

initiation time as shown in Figure 3. Notice that at a pH of 10.5,

corrosion started immediately on immersion. As the pH was raised to

11.5 and 12, about 200 hrs. passed before corrosion started. At a pH of

12.5, in the saturated CaCOH)^ solution, corrosion of the steel did not

occur during the experiment. These results developed in an oxygen

saturated environment (3 ppm) in the absence of chloride. With chloride,
- 13 -

the results are essentially the same except that the initiation time to

corrosion was reduced as shown in Figure 4, but again, no corrosion

occurred in the high pH saturated solution of CaCOHjg. It is of interest

to note that in these oxygen saturated solutions, once initiated, the

corrosion rate (slope of the curves) was constant irregardless of pH or

chloride content. Not surprisingly, however, the oxygen content of the

solution had a large effect on the corrosion rate of steel as shown in

Figure 5. As before, corrosion did not develop in the high pH of a

Ca(0H)

2

saturated solution at any level of oxygen. The results clearly

show that oxygen controls the corrosion rate, pH controls the initiation

time which is also somewhat affected by chloride content.

The interaction of oxygen, pH, and chloride is more complex and in

combination can develop conditions that lead to corrosion of steel even

in saturated CaCOH^ solutions. Figure 6 is a summary of these data

showing these three parameters. The surface developed by these parameters

is a boundary within which corrosion does not occur. Outside of this

boundary, corrosion of steel has been observed. For example, where it

was shown earlier that chloride or oxygen acting alone could not cause

corrosion of steel at a pH of 12.5, this figure shows that at an oxygen

concentration as low as 2 ppm and at a Cl” concentration of 8 mol/1,

corrosion does occur, and can also develop at lower concentrations of

chloride at higher oxygen contents. This figure will be referred to again.

To observe the effect of moisture on the corrosion of steel,

specimens were imbedded in a mixture of sand, Ca(0H)

2

, and NaCl. The

mixture was held in small perforated crucibles in an atmosphere of

flowing air saturated with moisture. This configuration allowed the specimens
to be removed for weight loss determination during the course of the

experiment. These specimens were initially exposed to two conditions of

moisture. In the first condition, specimens were imbedded in a dry sand

mixture, and the second condition consisted of specimens imbedded in a

moisture saturated sand mixture. Figure 7 illustrates the response of

the corrosion as a function of time. Specimens placed in the saturated

sand mixture did not corrode, but during the same period, the specimens

in the dry mixture began to corrode almost immediately as moisture was

introduced. The corrosion rate of these specimens decreased steadily as

the moisture content of the sand mixture increased; and at saturation,

the corrosion rate ceased. When these same specimens were transferred

to a dry air environment, the corrosion rate began to increase again as

shown in the figure. Thus, the degree of moisture plays a very important

role as expected; however, the cessation of corrosion at saturation was

not anticipated.

After the initial studies in sand mixtures and solutions, the work

was extended to include steel in mortar. Steel specimens were cast with

a 5 mm cover of mortar containing chloride. After 24 hours, half of the

specimens were immersed in a Ca(0H)

2 -NaCl solution, and the other half

were allowed to dry for one week before immersion. The corrosion current

of each specimen was evaluated by polarizing the specimen from -10 to 10 mv

with a potentiostat and plotting the overvoltage as a function of the

polarizing current. Figure 8 illustrates the type of data that resulted.

The slope of this curve is inversely proportional to the corrosion rate

of the steel in mortar. As in the case of the sand mixture, the corrosion

of the steel in mortar that was kept continuously wet was near zero as

illustrated in Figure 9. The specimens that were allowed to dry for one
- 15 -

week and then were immersed, initially developed a h_i gh corrosion rate

that decreased with time to a very low value.

On the basis of the 0

2

, pH, Cl” diagram shown in Figure 10, the

corrosion observed in mortar can be explained. The pH of the mortar

after casting is very high and of the order of 12.5, and the concentrations of Cl” and 0

2 are well within the region of no corrosion.

However, as the concrete cures and dries, the chloride becomes more

concentrated. At the same time this moisture loss allows an increase of

oxygen diffusion through the mortar leading to the development of conditions

that move the concrete into the corrosion region of Figure 10 (dashed

line).. Once corrosion is initiated by this process, the pH is forced to

decrease at the anode by the reaction:

Fe ++

+ 2 H

2

0 -> Fe(0H)

2 + 2 H +

These concentration cells are very small and act over a short distance

because of the increasing resistivity of the concrete. Once this initial

phase takes place and the pH is reduced locally, then corrosion proceeds

more easily.

In summary, the basic study has shown that at low concentrations of

0

2

and Cl”, pH controls the initiation of corrosion. But as 0

2 and Cl”

increase, they cause a reduction of pH at the anodic areas. In concrete,

it is hypothesised that Cl” and 0

2 can increase at the steel surface

during drying of the concrete to sufficiently high values to initiate

corrosion. TFiis corrosion process reduces the pH of the concrete at the

anodic areas of the steel and makes it easier to initiate corrosion in

future times when moisture and 0

2 are available.
- 16 -

Plans for this Coming Year

The techniques of applying a polarizing potential to a steel

specimen and the measurement of the resulting signals will continue to

be improved.

This will include increasing our knowledge of the distribution of

current from a single counter electrode, or developing a system of

concentric electrodes to confine the current to a known area.

An evaluation of various permanent reference electrodes to determine

their stability over several months of use in different conditions of

concrete will also be carried out.

A prototype automated system for measuring the corrosion of steel

in concrete will be assembled and tested in the laboratory. On the

basis of this, the design will be updated for fabrication by an outside

firm. Task F will evaluate the applicability of the instrument for

measuring the corrosion of steel in a bridge deck.

The basic study will be continued with an emphasis at applying what

has been learned to the development of a new technique for measuring the

corrosion of steel in concrete.
1000

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FIGURE

1.

Corrosion

Current

and

Resistivity

Versus

Time
FIGURE

2.

Block

Diagram

Illustrating

Design

of

Automated

System
Figure 3. The Effect of pH on the Corrosion of Steel

in the Absence of Chloride
FIGURE 4. The Effect of pH on the Corrosion of Steel

in the Presence of Chloride
FIGURE 5. The Effect of Oxygen on the Corrosion of Steel

( 6ui ) SS01 1H9IUM

FIGURE

7.

The

Effect

of

Moisture

on

the

Corrosion

of

Steel

in

a

Sand,

Ca(0H)

9

,

NaC 1

mixture
CORROSION

INITIATION

AFFECTED

BY

DRYING

FIGURE

8.

The

Effect

of

Degree

of

Corrosion

of

Steel

on

the

Slope

of

the

Polarization

Curve
EFFECT

OF

IMMERSION

TIME

ON

CORROSION ( e -0TX A/V) ’’adO'IS/T

cogDOKH

EHZOH

cn

PhwH i

FIGURE

9.

The

effect

of

Oxygen

Depletion

Resulting

From

Immersion

on

the

Corrosion

of

Steel

asa

Function

of

Time
6a
APPENDIX A

MEASURING THE CORROSION RATE OF STEEL IN CONCRETE

E. Escalante and S. Ito*

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D.C. 20234

The corrosion of steel in concrete where deicing salts are

used has become a serious problem in the United States. The

corrosion of steel is particularly insidious in that generally

the extent of' corrosion is not evident until damage to the

structure is sufficiently severe that remedial measures are

necessary. There is a need for the development of some process

whereby the progress of corrosion deterioration can be evaluated

at its initial stages before external damage appears.

An earlier study of the literature revealed that much of the

work reported has been concentrated on the effect of chloride

while considerably less work has been done on the effect of pH,

oxygen, and moisture (1). The study also revealed that the most

widely accepted method for detecting corrosion in concrete is

5tratfull's technique of mapping the half cell potential of

steel from the surface of the concrete (2). This approach,

however, does not reveal information on the rate of deterioration.

Dawson has shown that the a.c. impedence technique is useful in

measuring the rate of corrosion of steel in alkaline solutions

and concrete slurries in a laboratory setting, but evaluation

of the data becomes difficult when applied to steel in concrete (3)

It is the purpose of this paper to set forth one approach to the

solution of this question. Presently, studies are being carried

out at the National Bureau of Standards where the factors

affecting the corrosion of steel in concrete are being investigated and an electrochemical technique for measuring the

corrosion rate of steel in this environment is being developed,

Research Associate, Nippon Steel Co., Japan
2

Materials ard Procedure

Steel specimens for this study were made from 1.27 cm diameter

AISI 1017 steel rod 55 cm long. Pretreatment of the rods consisted of immersion in a 5C% solution of HNO^, rinsing in water,

followed by immersion in a saturated solution of CatOH)^ for

2 hours. Specimens were rinsed again and dried in preparation

for weighing with an analytical 2 Kg capacity beam balance having

a sensitivity of -0.5 mg.

Figure 1 illustrates the concrete slab and the placement of the

three specimens within the casting. The composition ratio of

the concrete was 1 part cement, 0.5 parts water, 1.9 parts sand,

and 3.4 parts pea gravel. After casting^ the concrete was

vibrated and covered with a plastic sheet during initial curing.

After several days of curing, the concrete surface from which

the steel specimens exit from the concrete was coated with a silicone base rubber to reduce the corrosion effects at that

interface. More recent slabs are being coated with beeswax.

The resistivity probes also shown in the illustration are two

3 mm diameter type 304 stainless steel rods each 6 cm long.

The Drobes are placed in the concrete during casting, and are

2 cm apart center to center and imbedded 4 cm. Calibration of

this conf iguration of rods using a 120 Hz signal indicates

that the resistance reading between the two rods multiplied by

a factor of approximately 5.1 is the resistivity of the system

in .TLcm. The resistivity of all recent slabe is determined

each time a corrosion rate measurement is made. Three conditions

of treatment of the 3labs will be described here. The first

condition of exposure was to simply allow the slab to remain in

laboratory air having a relative humidity of approximately 30%

with an air temperature of 22°C. The second condition of exposure was to cure the concrete for 29 days, then immerse the

slab into a saturated solution of NaCl for six days, and then

allow it to remain in laboratory air. The third condition of

exposure was to allow curing to proceed for 50 days, then immerse the 3lab in a 3.5% solution of NaCl for 24 hours, and
3

follow this with daily ponding (wetting) of the slab during

week days with a 3.5/o NaCl solution. This ponding was stopped

after 40 days.

Removal of the specimens from the concrete after exposure consists of breaking away the concrete from the specimen using a

1 Kg hammer. Practically all of the concrete is removed in

this way. The final small residue remaining is removed by

lightly tapping with the side of 1 cm diameter steel rod approximately 10 cm long. Final cleaning is done with a plastic

scouring pad. Thin stubborn films of oxide are not removed.

However, where active corrosion is taking place and pitting has

occured, the surface is easily cleaned in the manner described.

A satisfactory chemical cleaning procedure has not been found.

The^ specimens are weighed for weight loss determination.

Polarization Measurements

The d e te rmination of corrosion rat e from polarizati on data is

based on the observat ion that the current necessary to cha nge

the c orrosion potenti al o f a metal is directly rela ted to the

rate of corrosion of the metal in the ele c troly te

.

The te chnique s d iffer only in how this cha nge in potential is achi eved

.

Four tec hniques of polarization ha ve been used in this stu dy

»

howev er, most of the data has been obtain ed through Stern and

Geary ' s method

•

Stern an d Geary consi der the s lope of the overpoten tial , A E,

of th e s pecimen versu s th e app lied curren t, I, for small v alues

of ov erp otential (4 ) . Th is slope is rela ted to the corros ion

current,* I , by their relationship: corr’ J ^

dE

dl

b b a c 2. 3 (b +b ) I a c corr

where b and b are the anodic and cathodic Tafel slopes

a c

r respectively.J For calculation of the corrosion current, I , 'corr*

values for the Tafel slopes must be obtained from other sources.
4

In this case the overpotential used has been 6 mv and the

assumption made that b

g = b

c = 100 mv.

Thus

I corr

21.7 _I_

A E

mv

The second technique used is Schwerdtfeger ' s empirical determination cf the corrosion current, I

, from the chanqe in

slope of the polarization curve (5). The cathodic corrosion

current, Ip, and the anodic corrosion current, Iq, are found

at this "break" in the curve, and the corrosion current is

then calculated from:

I =1-1

corr

I + I P q

The technique requires that the polarization curve cover an

overpotential of - 100 mv so that the "breaks" in the pclari ization curve may be observed.

The three point method of Barnartt is the third technique that

has been used (6). Three overvoltage/current data points are

measured along the polarization curve, and from this b , b , and I are calculated. If the first overpotential point corr

is at AE, then the second and third are at 2 AE and -2 AE

rsspec tively . The resulting currents at each potential are

measured and substituted into the ratios

r. =

X 2 AE /X -2 AE

and

r

2 = l 2 AE AE

which in turn are related to each other through the quadratic

equation

+ v/"r^ - 0.

The solution of this equation yields the relations that allow

one to solve for the Tafel slopes b^ and b . Using the relationship of I/I versus A E, the corrosion current for

corr

the process is found.

The fourth technique is a computer analysis of polarization
5

data developed by Mansfeld (7). The computer makes a best fit

analysis to theoretical curves generated for different values

of Tafel slopes and in this way b , b , and I are found.

a c corr

The polarization techniques described employ a three electrode

system having one electrode as the steel specimen under study,

a second electrode serving as a voltage reference, and a third

electrode from which current is impressed on the specimen. The

type of reference electrode used in this study is the Cu-CuSO^

electrode. Contact is made to the concrete slab through a small wetted sponge located at the concrete surface directly

above the steel specimen being polarized. Where the concrete

is chloride free, the sponge is wet with a saturated solution

of CadDH^. If chloride is present in the concrete then a 3.5% NaCL solution is used to moisten the sponge. In all cases

the sponge is firmly squeezed to remove excess solution which

may affect the moisture content of the concrete. Generally,

the counter electrode, the electrode from which current is impressed on the specimen, is the center rod of the three rods in

the slab. Corrosion measurements are made on the outer two

rods, but not simultaneously.

The circuit used for polarization measurements is Holler's (8)

which incorporates a Wheatstone bridge for IR compensation as

illustrated in Figure 2. The IR compensating design takes

advantage of the fact that when an external polarizing current

is applied to a specimen, its rate of change in potential due

to polarization is very small as compared to the potential

change due to IR. By repeated applications of current of short

duration, and by increasing the Wheatstone balancing resistance,

the IR component can be compensated as illustrated in Figure 3.

Cnee this balance is achieved, then the polarizing potential

free of IR can be measured for all values of applied current.

Since the resistance of the concrete changes from day to day,

this process of IR compensation must be repeated each time that

polarization measurements are made.
6

Results and Discussions

Before a measurement technique for corrosion could be considered, it was necessary that we be able to cause the corrosion of steel in concrete at will. This proved to be a temporary obstacle until a better understanding of the corrosion process developed. This portion of the investigation

will not be discussed here. However, it was determined that

for corrosion of steel to be initiated in chloride contaminated

concrete, it is necessary that the system go through a drying

cycle. It is hypothesised that during this period of moisture

loss the chloride is locally concentrated to high levels

perhaps approaching saturation. At the same time oxygen becomes more easily available at the steel surface as the porous

concrete loses moisture. The micro cells that develop at the

concrete/steel interface lead to an increased rate of passive

film break down, a local reduction in pH, and subsequently, a sustained corrosion process. The increasing resistivity of the

concrete limits the action of the micro cells to short distances of probably less than 1 mm. Eventually, the rising

resistivity stifles the corrosion process. Thereafter, any

action that reduces the concrete resistivity and improves

aeration causes an increase in corrosion of the steel.

The first condition of exposure was a .chloride free concrete in

laboratory air. The data on two slabs indicates that the corrosion current of the imbedded steel ranged from 2 to 0.25yUA

over a period of 210 days, and is equivalent to a total corrosion loss of less than 3 mg over the exposure time. Since

2

the surface area of steel exposed to the concrete is 200 c.m‘,

the calculated corrosion rate is very low at less than

0.007 mg/dm .d. Visual inspection of steel specimens removed

from one of the slabs verifies the excellent condition of the

steel surface and the absence of any visible corrosion. Weight

1 o

s

3 measurements were considered unnecessary on these specimens.

In the case where the concrete slab was immersed for six days

in a saturated solution of NaCL after curing, the second condition of exposure, the corrosion current increased by over an
7

order of magnitude as shewn in Figure 4. After three days

immersion , the data indicates that the corrosion rate continued to increase. By the forth and fifth days, the corrosion

rate reached a maximum and began to decrease. Cn removal from

the solution, the corrosion rate of the steel decreased

gradually. Shown on the same figure is the resistivity of the

concrete as measured with the permanently imbedded probes. In

a manner similar to the corrosion rate, the resistivity decreased to a minimum value after four to five days immersion

and then gradually increased. After removal from the solution

the resistivity continued to increase as shown. Clearly the

effect of immersion on the corrosion rate was much greater

than its effect on the resistivity of the concrete. During

drying, however, the resistivity appears to be having a direct

influence on the degree of corrosion observed. The corrosion

measured on this chloride contaminated slab is over an order

of magnitude greater than the corrosion observed in the

chloride free environment. Weight loss measurements will not

be made on this slab until the data indicates that a measurable

amount of steel has been lost.

The third condition of exposure was a 24 hour immersion of a slab in 3.5$ NaCL solution, after curing, followed by daily

wetting of the slab on weekdays for a period of approximately

40 days. Figure 5 illustrates the response of the corrosion

of steel to this treatment. Measurements were not made during

the first stages after casting, but based on the corrosion

observed on other slabs, it can be assumed that the corrosion

current prior to immersion was of the order of lyU-A. After

the wetting cycles were stopped, the corrosion of the steel

decreased slowly as shown. Resistivity measurements were not

made an this slab. The calculation of weight loss from the

electrochemical data is based on the corrosion current versus

time data of Figure 5 and Faraday's equation (9). Total metal

loss was calculated to be 172 mg for specimen B in slab

number 1. The actual weight loss measured gravime trically

was 147 mg. The average corrosion rate is calculated to be

2 0.33 g/dm ,d. Visual examination of the steel rod after
8

removal revealed that the corrosion was confined to three

areas on the steel surface, and was in the form of broad shallow

pitting. The good agreement between the elec trochemically

measured and the gravime trically determined weight loss of steel

indicates that electrochemical polarization techniques can be

used to measure the corrosion of steel in concrete.

A comparison of the data from the four electrochemical techniques described indicates that all the methods overestimated

the weight loss of metal. The Stern-Geary method overestimated

the weight loss by 17% while Schwerd t f eger ’ s , Mansfeld’s, and

Barnartt's techniques overestimated the corrosion by 40%, 56%,

and 86% respectively. Since the differences in corrosion rates

between a corroding and noncorroding system are over an order

of magnitude, then even the data with an 86% cveres timation of

corrosion may be acceptable.

Summary

The corrosion rate of steel in concrete has been measured using

polarization techniques with IR compensation. The four methods

used were Stern-Geary ' s , Schwerdtf eger ' s , ^ansfeld's, and

Barnartt's techniques.

The data indicate that the corrosion of steel in chloride free

2

concrete is negligible and less than 0.007 mg/dm *d while in

chloride contaminated concrete the average corrosion rate of

2

steel is well over an order of magnitude greater at 0.33 mg/dm *d.

Comparison of the weight loss calculated from the polarization

data to the weight loss gravimet rically determined indicates

that the electrochemical measurements overestimate weight loss

by 17% to B6%. However, even with an overestimation of 86%

the technique will differentiate between a corroding and noncorroding condition of steel in concrete.
REFERENCES

Escalante, E., and Ito, 5.; A Bibliography on the Corrosion

and Protection of Steel in Concrete, NBS Special Publication 550,

August 1979.

Stratfull, R.F., The Corrosion of Steel in a Reinforced

Concrete Bridge, 1_3, 173t, March 1957.

Dawson, J.L., Callow, L.M., Hladley, K., Richardson, J.A.,

Electrochemical Impedance Measurements Applied to the Corrosion

of Reinforcing, Proceedings NACE 1978, Paper No. 125, (1978).

Stern, M., and A.L. Geary, A Theoretical Analysis of the Shape

of Polarization Curves, J. Electrochemical Society, 104 . 56,

January 1957.

Schwerdtf eger , W.J., and M

c 0orman, O.N., Measurement of the

Corrosion Rate of a Metal from its Polarizing Characteristics,

J. Electrochemical Society, _99, 407, October 1952.

Barnartt, S., Two-Point and Three-Point Methods for the

Investigation of Electrode Reaction Mechanisms, Elec t rochimica

Acta, 15, 1313, 1970.

Mansfeld, F., Tafel Slopes and Corrosion Rates from Polarization Resistance Measurements, 23_, 397, October 1973.

Holler, H.D., Studies on Galvanic Couples, J. Electrochemical

Society, 9_7, 271, September 1950.

Uhlig, H.H., Corrosion and Corrosion Control,

Chapter 2 (J. Wiley 1 Sons, Inc., 1971).

2nd Edition,

2cm:

2cm I ’j

10cm

30cm

10cm

1

3cm

•55cmC

A I S I 1017 Steel

1.27cm diam.

c

Concrete

Figure 1. Concrete Slab With Three Steel Specimens

And Resistivity Probe
6v

Electrode Specimen

Figure 2. A Simplified Form Of The Circuit Used For

Polarization Studies In Concrete
Figure 3. Idealized Response Of Polarization

Circuit When Compensating For IR
Corrosion

Current,

yuA

100

10 —

1.0 —

immersed in

sat. sol n . NaCl

r~~

100k

10k

Ik

Figure 4. The Effect Of Immersion On The Corrosion Current

Of Steel And The Resistivity Of Concrete

Resistivity

Of

Concrete

,

Jicm
Corrosion

Current

wetti ng

stopped

24h immersion

! _in

3.5% NaCl soln.

Slab 1 Specimen B

0 50 100 150 200

Days Since Cast

Figure 5. The Effect Of Immersion And Daily Wetting

On The Corrosion Current Of Steel In Concrete

250

NBS-1 1 4A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-20122. Gov't. Accession No. 3. Recipient” s Accession No.

4. TITLE AND SUBTITLE

Measuring the Rate of Corrosion of Reinforcing Steel

in Concrete

5. Publication Date March 1980

6. Performing Organization Code

561

7. AUTHOR(S)

E. Escalante, S. Ito, and M. Cohen

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

18. Project/Task/Work Unit No. 5610423

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP)

Federal Highway Administration

Paving and Structural Materials Group, HRS-22

Washington, DC 20590

13. Type of Report & Period Covered

Annual

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

|

I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200*word or leas factual summary of moat significant information . If document includes a significant bibliography or

literature survey, mention it here.) The progress on a research program directed at developing a nondestructive

method for measuring the corrosion of steel in concrete as related to bridge deck

deterioration is reported. Several polarization techniques for measuring the

corrosion rate of steel in concrete are correlated to actual weight loss measured

gravimetrically. The design of a prototype automated minicomputer system for

measuring the corrosion of steel in concrete is also described. Included are the

results of a laboratory basic study on the effect of pH, Cl ,02, and moisture on initiating and maintaining corrosion in mortar.

17. KEY WORDS (six to twelve entries ; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Bridge deck corrosion; corrosion in concrete; corrosion of steel; polarization

technique; rebar corrosion

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