NBSIR 73-405

Use of Organic Coatings on the

interior Surfaces of Equestrian

Statues at Memorial Bridge Plaza

Fielding Ogburn

National Bureau of Standards

Corrosion and Electrodeposition Section

Washington. D. C. 20234

November 1973

Final

Prepared for IMational Capital Parks

U. S. Department of Interior Washington, D. C. 20242

NBSIR 73-405

USE OF ORGANIC COATINGS ON THE

INTERIOR SURFACES OF EQUESTRIAN

STATUES AT MEMORIAL BRIDGE PLAZA

Fielding Ogburn

National Bureau of Standards

Corrosion and Electrodeposition Section

Washington, D. C. 20234

November 1973

Final

Prepared for National Capital Parks

U. S. Department of Interior Washington, D. C. 20242

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

NATIONAL BUREAU OF STANDARDS

Washington, D.C. 20234

USE OF ORGANIC COATINGS ON THE INTERIOR SURFACES OF

EQUESTRIAN STATUES AT MEMORIAL BRIDGE PLAZA

INTRODUCTION

In March of 1971, the National Bureau of Standards submitted to

National Capital Parks a report outlining a number of methods for the

restoration of the equestrian statues at Memorial Bridge Plaza. At

that time and ever since there have been intermittent discussions of

the advisability of applying an organic coating to the interior

surfaces in order to retard or prevent deterioration of the statues

with time. These discussions have not lead to conclusions clearly

acceptable to everybody taking an active interest in the matter. There

is an apparent need to make some evaluation of the matter in a systematic

way.

This report was prepared by Fielding Ogburn of the Corrosion Section

and reviewed by: Elio Passaglia, Chief, Metallurgy Division; Jerome

Kruger, Chief, Corrosion Section; Marion L. Picklesimer, Chief, Mechanical Properties Section; and Paul T. Howard, Structures, Materials,

and Life Safety Division.

Drafts of this report were sent to: Mildred Patterson, Materials

Engineer, Dept. of the Navy; Louis D. Fitzgerald, Internation Copper

Research Assoc.; Robert Brookshire, Brooktronics Engineering Corp.;

and Wilburn A. Boggs, Corrosion Engineer, Lockheed-Georgia Co. Their

comments were solicited and given careful consideration. It is our

understanding that the opinions of Mrs. Patterson, Mr. Brookshire, and
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Mr. Boggs have been communicated to personnel of National Capital Parks.

The Problem

The interiors of the statues have a rather hostile environment.

The temperature is continually cycling between night and daytime

conditions and between summer and winter conditions. The temperature

range extends from below freezing to that produced by hot summer sun

on the bronze castings. The humidity is very high at all times and

water is always present in standing pools and as condensate on the

cooler surfaces

.

Exposed to this hostile environment is a combination of metals

consisting of large areas of bronze and brass of various compositions

and very small areas of a tin-silver alloy, copper, nickel, and gold.

Some steel may also be present. Also present is residual material

from the casting molds, mostly gypsum and sand.

The concern is that this combination of materials and environment

will result in detrimental corrosion, especially in the pores extend-

| ing from the interior to the exterior lacquer. There is also the concern that alternate freezing and thawing of water in these pores will

disrupt the lacquer seal and, hence, contribute to deterioration of

the exterior appearance.

An Evaluation of the Problem

Brass and bronze are known to withstand weathering and many wet

environments very well. Those knowledgeable about corrosion of copper

alloys are agreed that the general interior surfaces of the bronze j
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castings can be expected to satisfactorily withstand the action of the

interior environment. The same can be said of the brass members that

have been installed for structural purposes.

The corrosion engineer always concerns himself with the contact

of dissimilar metals. Again there is agreement that the combinations

of brass and bronze of various compositions are not expected to have

adverse effects. Nor is the presence of small areas of tin-silver alloy

considered detrimental, though not with complete assurance.

The specific concerns are the pores and cracks extending through

the bronze castings, especially at the junctions of flanges of neighboring sections. Those pores and cracks which are open to the exterior

are passages for water carrying gypsum and corrosion products from the

interior to the exterior. Such passages have already been encountered

as evidenced by white deposits on the exterior surfaces. If these

pores are cleaned out and plugged with Incralac, they can be satisfactorily controlled. If such a maintenance program is not followed, then

there will be a problem.

At the exterior end of a pore is a potential source of trouble.

Here is to be found a combination of gold, nickel, and bronze and if

this combination of metals is exposed to water, very serious corrosion

will occur. The purpose of the exterior lacquer coating is to cover

the gold and nickel, especially near exposed bronze so that water would

be excluded from the areas where the 3 metals might contact each other.

The problem is that all such areas may not have received an adherent

coat of lacquer or some may become exposed by the action of the elements

such as the freezing and thawing of water in pores.
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The Need for an Interior Organic Coating

The maintenance program which we have recommended anticipates

the open pores and the gradual removal of Incralac by external

weathering. It also provides for those defects arising from other

causes, but only as infrequent occurrences. If the disruption of

the lacquer from interior conditions should prove to be very frequent,

then the maintenance program could be ineffective.

Since the completion in January 1972 of the refinishing of Music

and Harvest and of Aspiration and Literature, a number of white gypsum

and green-blue corrosion deposits have formed as was expected. The

white spots might have been prevented by an interior coating. The greenblue formations may have been associated with pores extending through

the castings, but an interior coating would probably not have prevented

this corrosion since it is most likely that water got to the gold-nickelbronze junctions from the exterior.

There has been a suggestion that the darkening observed on two of

the statues in local areas may be due to corrosive action from the

interior. This is very unlikely. There is no evidence of such generalized porosity. Also if corrosion were to occur over such extensive

areas, one would expect the lacquer to blister or lift off. This has

not occurred. '

•

Our opinion is that there is little evidence yet for the need of

an interior coating. It is possible that such a need may become evident,

but we feel that is unlikely.
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The Benefits of an Interior Organic Coating

Though there may be no apparent need for an interior coating, it

may still be desirable. It could prevent some unforeseen corrosion

problems or it might be regarded as insurance or an extra precaution.

It is unlikely that an organic coating would do any harm, at least not

the ones mentioned later in this report, and it might prove to be worth

the extra cost and effort involved.

One objection to an organic coating that has been made is that a

corrosion problem could develop underneath the coating and go undetected.

This seems unlikely for several reasons. The only corrosion problems

we anticipate are those involving the gold and nickel on the exterior.

Such corrosion could never be evident in the interior of the statue.

We should, perhaps, consider the unanticipated corrosion problems in

the interior. A transparent coating like Incralac would not hide a

corrosion problem.

A heavy bituminous coating would probably hide a corrosion problem.

The polysulfide rubber-polyurethane system suggested in the next section

of this report could be expected to hinder observation of a mild corrosion problem, but would probably visibly lift or blister if the corrosion were extensive.

There is one situation for which we do recommend an organic coating. If on future inspections, corrosion deposits are found on the

exterior surface and if there is any indication that it may be associated

with a crack or pore extending to the interior, then the corresponding

interior surface, if accessible, should be given an organic coating to

plug up the pores and cracks from the inside.
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Coating Materials

Many coating systems hdve been suggested and discussed for the

interior of these statues and there are probably many that could be

used. There are two which have come to our attention from more than

one source. One is Incralac and the other is magriesium chroma te

inhibited poly sulfide rubber with a clear polyurethane overcoat. The

latter has been recommended to NCP by Mr. Goldberg of the Naval Air

Systems Cornnand and by Mr. Wilburn A. Boggs, Corrosion Engineer of

Lockheed-Georgia Co.

Since the purpose of painting the interior is to plug the pores,-

rather than to protect the bronze, consideration should be given to

emulsion coatings (coal tar, asphalt, or latex emulsions). These are

water base systems and as such would not involve the hazards of organic

solvents during application. They might be effective at plugging pores,

even though they do not constitute good corrosion preventive sys terns

,

Summary

There is no positive indication for the need of an interior organic

coating and, in our opinion, a catastrophic corrosion failure could not

occur in the near future. Our recommendation, therefore, is that no

organic coating be applied this year, and that we keep the statues under

close observation until the fall of 1974 and see what corrosion problems

develop. If a chronic corrosion becomes evident or if NCP wishes to

take the extra precaution, then interior coating would be in order,

especially on surfaces corresponding to exterior areas showing porosity
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corrosion.

We also feel there is merit to a suggestion that has come from

several sources. That is to paint a portion of the interior of one

statue and observe the consequences during the following years. These

observations might be very informative and give the guidance the NCP

is looking for. We should point out that such a trial with a water

base paint would be relatively inexpensive.

In preparing this report, little consideration was given to the

economics involved or to the hazards of applying organic coatings in

an essentially closed chamber. This latter consideration, of course,

is of overriding importance.

No. NBSIR 73-405

' BIBLIOGRAPHIC DATA

, Til Li-; AM) >l hi ll 1-1.

USE OF ORGANIC COATINGS ON THE INTERIOR SURFACES OF

EQUESTRIAN STATUES AT MEMORIAL BRIDGE PLAZA

5. Publication Dare November 1973

6. Performing Organization Code

Fielding Ogburn

8. Performing Organ. Report No. November 1973

rPERFOKMLNl. OKC, AN I/A 1 1< )\ \AM1-: AND ADDRKSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 2C234

10. Proiect Task /Work Unit No. 3120464 (NBS)

11. Contract/ Grant No.

!. "^pon-ioring iVt.in u-.at i, n Nami- aiui Li)niplctc Address (Street, City, State, ZIP)

National Capital Parks

U. S. Department of Interior

Washington, u. C. 20242

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

S. SL P P L i: M E N TA R V N O 1 h S

S. At'STRAC!!' (A 200-word or /ess (actual summary of most si^idcant information. If document includes a significant

bibliography or literature survey, mention it here.)

The equestrian statues at Memorial Bridge Plaza in the District of Columbia

are bronze castings. The exterior finish is subject to corrosion processes associated with pores and cracks in the castings. The interior surfaces are subjected to high humidity and condensation. Detrimental corrosion is expected only

at discontinuities in the plated coatings on the exterior surfaces. Painting of

interior surfaces has been recommended, but the need for painting is not clear.

The NCP is advised to keep the statues under close observation till the fall of

1974 and then reconsider their course of action.

\ KEY UORHS (six to twelve entries; alphabetical order; capitalize only tfie first letter of the first key word unless a proper

name; separated by semicolons) •

bronze statuary; organic coatings; restoration; statues

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