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The Cleaning and Protective Coating of Ferrous MetalsAuthor(s): Geoffrey Michael LemmerSource: Bulletin of the American Group. International Institute for Conservation of Historic

and Artistic Works, Vol. 12, No. 2 (Apr., 1972), pp. 97-108Published by: Maney Publishing on behalf of The American Institute for Conservation of Historic &Artistic WorksStable URL: http://www.jstor.org/stable/3179132 .

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Bulletin of the American Group-IIC 12, No. 2, 1972

THE CLEANING AND PROTECTIVE COATING OF FERROUS METALS

Geoffrey Michael Lemmer

Objects of iron and steel provide some of the mtost intract-able problems for the conservator because of the variety and complexityof their corrosion products. The metals corrode easily, being attackedby oxygen in the presence of moisture. The corrosion products are uglyand deformation of an attacked object may be severe. The action of oxygenand moisture can be further compounded by the presence of salts whichcan cause local electrochemical activity, accelerating pitting, and mineral-

ization. The problem for the conservator is to remove and/or stabilizethe layer of corrosion products on the object, and to provide some methodto prevent further corrosion. It is hoped that the following brief surveywill assist to this end.

I. Scope

Cleaning methods and protective coatings that have beendescribed in the literature will be reviewed and subjected to comparativetests. An attempt will not be made to review or test all known processesor coatings, but only the major ones. The chemistry and causes of corrosionwill not be dealt with; this subject has been explored in great depth by others

and their findings are readily available.

The results of the tests carried out by the author will bepresented together with comments based on the author' s own observationsof specific examples. The process adapted for a specific object must bemade by the person doing the work and should be based on a precise knowl-edge of all the factors involved. The author has borrowed freely from thepublications and work of others, and is much indebted to them.

II. Cleaning

Beforecleaning

can besettled upon, a thorough examinationof the object must be made. This examination will be instrumental in

selecting a cleaning method and a coating material. Some objects that maybe completely mineralized, having no metallic core remaining, may havereached a stage of stability and only consolidation would be required.

The surface is the first to be examined. Even though corrodediron and steel do not have the color range of corroded bronze objects,slight differences in surface texture and coloration can be noticed and may

Conservator of Glass, Ceramics, and Metals, The Francis du PontWinterthur Museum, Winterthur, Delaware 19735.

97.

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98.

indicate areas of active corrosion. Dampness on the surface either localor overall, is a sure sign of active corrosion as chlorides in contact withiron give rise to hygroscopic corrosion products. In the case of heavilycorroded pieces, detailed examination with a magnet is recommended to

ascertain the extent of the metallic core remaining. Occasionally X-

radiography is a useful tool for assessing the internal condition of the

object. A needle or metal probe used with magnification can help revealhidden decoration or marks of identification which may be concealed bythe corrosion.

After the examination is completed a course of action mustbe decided on. There are several basic methods of cleaning, with manyvariations of each. For the sake of simplicity, cleaning will be broken down

into the five following classes with a brief description of each.

1. Mechanical: The removal of the corrosion product by use of abrading,

scraping, grinding, chipping, picking,shot

blasting,wire

brushing,or any combination of these.

In general, these methods are to be condemned for use on museum

objects. Usually, they do not differentiate between corrosion or

metal. The high areas of a piece can be damaged, while corrosion

still remains in the crevices and pores. However, there are some

mechanical processes that are acceptable if used in moderation;

gentle brushing during other classes of treatment, picking away of

deep pits with a sharp needle to speed up a process, and the use of

an Airbrasive system. This last mentioned is amplified in the

specific test section.

2. Chemical: This class of cleaning can cover a wide range of differentsolutions. There are the commercial paste and liquid rust solvents

based on phosphoric acid as well as other acids and complexing

agents; proprietary solvents, that are neutral in pH, probably are

sequestering or chelating agents. Several chemical agents are treated

under the specific tests.

3. Electrochemical Reduction: The procedure is to bury the object in

a sacrificial metal baser than the metal the object is made of; zinc

is usually chosen. This is done in an inert container such as enamel,

ceramic, glass and sometimes iron. Everything is then covered

with an electrolyte, usually ten to twenty percent caustic soda solu-

tion, and heated to promote the chemical activity. This procedure

will soften the corrosion layer, which can then be brushed away.The solution is spent rather quickly, usually one-half to one hour,

and must be replaced with fresh. The process is continued until all

of the corrosion is removed. The amount of heat applied is dependenton the strength of the object. A heavy adz head with a continuous

metallic core remaining could be brought to a boil without fear of

damage, whilst a flimsy or discontinuous belt buckle may requirethe use of a steam oven. Needless to say, caution is needed with

this method as the caustic electrolyte is hazardous, as are the fumes.





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99.

4. Electrolytic Reduction: This class of cleaning is similar in opera-tion to electrochemical reduction except that the EMF (Electromotive

Force) is supplied externally rather than being generated duringthe

process. Electrolyticreduction is

probablythe most

widelyused, but it has some drawbacks, one of which is the probabilityof an object with a discontinuous core being mechanically weakened.

Many variations of this procedure have been developed: differentmaterials for the anode plates, and different electrolytes bothcaustic and acid.

5. Other: This class involves any other method not already outlined.Some of the procedures, of course, overlap.

a. Heating and Cooling: Here expansion and contraction will causesome of the corrosion to flake off. Not to be recommended, butis sometimes useful in separating large masses of objects corroded

together.b. Ultrasonic Baths and Probes: The ultrasonic probes, such as

the Cavitron unit, are classified with the mechanical processes.The unit has been found to be of very limited use in our work.

Only limited tests have been made with ultrasonic tank cleaning,and more work needs to be done.

Sample Preparation

Each sample used for the specific cleaning tests was one-halfof a lock plate with an even layer of corrosion on the surface. Each samplewas prepared for testing by grinding away the corrosion

layerfrom one

corner so as to have a bare metal surface. An identification letter was

stamped into this area. By observing the appearance of this area at the con-clusion of the tests any damage to the metal could be observed.

Specific Cleaning Tests

SAMPLE A: Class of Cleaning: None

Type: NoneProcedure: NoneResults: Control piece

SAMPLE B: Class of Cleaning: ChemicalType: Chelating Agent, sodium gluconateProcedure: Soaked in a solution of 700 ml. water,

70 gm. sodium gluconate, 70 gm.sodium hydroxide. Alternately heatedto 190?F and cooled for a period of 24

days, scrubbed with a soft brushseveral times during treatment.

Results: The bulk of the corrosion was removedwith no indication of attack to the metal.





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SAMPLE C:?

Class of Cleaning:Type:

Procedure:Results:

ChemicalBrookstone Rust Remover No. 1595

(Brookstone Co., Dept. C, 9Brookstone Bldg., Peterborough,

N.H. 03458), pH 6.75, probablya complexing agent.

Followed instructions on the container.Most of the corrosion had been removed,

except some thick deposits. However,the metal has also been attacked tothe extent that the identification numberwas almost obliterated.

SAMPLE D:

SAMPLE E:


Procedure:

Results:


Procedure:Results:

ChemicalRust-Removo (Ace Chemical Co., 4401

North Ravenswood, Chicago, Ill. 60640)acid type.

Followed directions on container for a

period of twenty days.All of the corrosion on the area treated

was removed, but there is evidence

of attack to the metal.

ElectrochemicalCaustic soda-zinc process at elevated

temperature.StandardAll of the corrosion had been removed

with no indication of attack to the metal.

SAMPLE F: Class of Cleaning:Type:Procedure:Results:

ElectrolyticStandardStandardAll of the corrosion

with no indicationmetal.

had been removed

of attack to the


Procedure:

Results:

Class of Cleaning:Type:Procedure:

ChemicalNaval Jelly (Woodhill Chemical Sales

Corp., Cleveland, Ohio 44128, Stock

No. NJ-l), acid type.Followed directions on container for a

period of twelve days.All of the corrosion was removed from

the area treated, but there was evi-

dence of attack to the metal.

Chemical

Thioglycolic acid

As outlined in the article by T. Stambolov

and B. Van Rheeden in Studies in Con-

servation, Vol. 13, No. 3.

100.

SAMPLE G:

SAMPLE J:





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101.

Results:

SAMPLE K:

SAMPLE L:

SAMPLE M:

SAMPLE N:

All of the corrosion was removed,but some fine pitting was producedin the metal.

Class of Cleaning:Type:Procedure:Results:


Procedure:

Results:

Class of Cleaning:Type:Procedure:

Results:


Chemical

Phosphoric acid

10% immersion for 16 hoursAll of the corrosion was removed,

some fine pitting to the metal.

Chemical

Jelly de-Rust, Rust Remover (FlexabarProducts, Inc., Northvale, New

Jersey; available from Edmund

Scientific). The pH is less than one.The label states Contains Acid .

The sample was immersed for 24 hoursrather than coated.

All of the corrosion was removed withthe exception of some deep pockets.Very fine pitting to the metal.

Other

Ultrasonic tankImmersed in a solution of Edmund

Scientific Phosphoric Cleaner, S. N.71200, P-401, approx. 5%, 5 minutesat 120?F. (20 kilohertz). This cyclewas

repeatedsix times over a

periodof several days, the object remainingin the solution between cycles.

Most of the corrosion has been removed,but the metal itself is heavily pitted.

NOTE: The pitting of the metal, Ibelieve, was caused by the action ofthe acid cleaner and not the effect ofultrasonic radiation. I feel that a greatdeal more work is required in the areaof ultrasonic cleaning and that the abovetest is not indicative of the potentialof ultrasonic

cleaning.Abrasive

Airbrasive, Industrial Unit, Model K(Penwalt--S. S. White IndustrialProducts; available from the PrecisionSales Company, 104 West Joppa Road,Baltimore, Maryland 21204).





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102.

Procedure: As per instructions using 27 micronaluminum oxide.

Results: All of the corrosion was removed in

about 20 minutes. The metal was

left with a micro-pitted surface.

NOTE: The micro-pitting of the sur-

face could be reduced even further by

using a finer abrasive, such as ten

micron aluminum oxide.

Conclusions of Cleaning Tests

1. If the loss of any metal from an object cannot be tolerated; the use

of electrochemical and electrolytic reduction would be applicable.

2. If very small and delicate objects made up of several differentmaterials besides metal are involved, the Airbrasive process is

applicable, although the expense involved for the equipment is con-

siderable.

3. If large and bulky objects are to be treated with cost as a factorand if some loss of metal is acceptable, some of the acid treatments

would be preferred, particularly the phosphoric-acid-based solu-

tions.

Many other combinations of cleaning methods can be used to

solve a variety of corrosion problems on ferrous metal objects. It is hoped

that thereader will use the information

presentedas a

guideto formulate

methods for treating specific objects based on additional testing.

III. Protective Coatings

Upon completion of cleaning and proper washing of an iron

or steel object, a protective coating needs to be applied in order to halt

the outbreak of further cor.rosion. A number of different coating materials

have been tested at Winterthur and the results are included in this report.

No special criteria were used in the selection of coatings to test, but several

points were kept in mind when the selections were made. They are:

1. The coating must be transparent, or nearly so. This rules out thelarge selection of pigmented coatings that are so widely used in

industry.

2. From a museum point of view reversibility of the coating for future

removal is of prime importance, although, one crosslinking silicone

epoxy formulation was tested.

3. Ease of availability and application are also to be considered.

Most private collectors and small museums do not have the resources

of a chemical laboratory available.





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103.

The tests described in this survey were performed on

polished ferrous metal samples of two different types and shapes, andin two different types of atmospheres under accelerated aging conditions.However, accelerated

agingtests

maynot be indicative of how a

coatingis going to perform under normal conditions. The results must be con-sidered with this in mind.

Procedure for Testing Coatings

Test chamber specifications:

a. An aquarium with a fitted aluminum hood, loose enough to allowair circulation in and out of the chamber

b. A 250-watt light source mounted inside the top of the hood. This

provides lightfor

photographyand the heat source for

cyclingthe

environmental conditions

c. A suitable liquid in the bottom of the chamber to provide the de-sired atmosphere

d. Controls on the heat source to enable the atmosphere conditionto be cycled from ambient of 73?F and 88% R. H. to extremes of106?F and 35% R. H. (These figures were obtained with a recordinghydro-thermograph placed in the chamber, and operated for twoweeks prior to the start of actual testing. )

Test A:

1. Sample material: soft iron cylindrical sections cut from the barrel ofa model 1816 common musket, filed down to bare metal, and polished.

2. Prior to coating the samples were cleaned with petroleum benzine.

3. Each of the samples were coated with the material to be tested, hungfrom a glass rod, and placed into the test chamber.

4. Ordinary tap water was placed in the bottom of the chamber and thehood put on.

5. The chamber was cycled twice per week day for a period of twenty-ninedays. At the end of five days acetic acid was added to the water tomake a 1% solution.

Test B:

1. Sample material: mild steel strips, wire brushed, and polished.

2. The samples were degreased with 1, 1, 1-trichloroethane and coatedon the identification end with epoxy (Epo-Tek 301).





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104.

3. A solution of 2000 ml. water and 10 ml. acetic acid was placed in

the bottom of the chamber and the chamber cycled once per week-

day for thirty-three days.

Test C:

Same material and procedures as Test B, except that plain water was used

instead of the acetic acid solution. The chamber was cycled for twenty-three days.





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TABLE I

List and Sources of Materials Used in Coating Tests

MANUFACTURER & SOURCE

1 Sperm Whale Oil

2 3 in 1 Oil

3 Vaseline Petroleum Jelly

4 3 in 1 Oil - 50% andVaseline Petroleum Jelly 50%

5 Micro-crystalline Wax

(Renaissance)

6 Micro-crystalline Wax plus5% Sodium Benzoate

7 Fanox8 Butcher' s Hard Paste Wax

9 Krylon Acrylic Spray

11

12Acryloid

B-72

PVA-AYAC

13 Hoppes Gun Grease

14 Frigilene (Cellulose Acetate)

15 Du Pont Rally Car Wax

16 Amway Shoe Spray

17 Amway Silicone Polish

18 Crosslinking Silicone

(Dow XR-6-2121)19 CRC 3-36

20 Magnus FF-111

Dan & Wes Kindig, Lodi, Ohio orDixie Gun Works, Union City, Tenn.

Boyle-Midway, Inc., New York City,Available at most Hardware Stores

J. Strikland & Co., Memphis, Tenn.,Available at most Drug Stores

(See 2 and 3)

Picreator Renaissance Products,Picreator Enterprises, Ltd.44 Park View Gardens, London N. W. 4

Sodium Benzoate, available at mostChemical Supply Houses

Standard Oil of New JerseyThe Butcher Polish Company, Boston,

Mass.; Available at most HardwareStores

Borden, Inc., Dept. CP, New York City;Available at most Hardware Stores

Rohm-Haas,Independence Square,

Phila.,Pa.

Union Carbide (any local distributor)

Frank A. Hoppe Division, PenquinIndustries, Inc., Parkesburg, Pa.; alsomost Gun Shops

W. Canning & Company, Ltd., Birmingham18, England

E. I. du Pont de Nemours & Co., Inc.,Wilm., Del.; also most Auto SupplyStores

Amway Corp., Ada, Mich.; Local Dis-tributor

(See 16)

Dexter Corp., Midland Div., 9001 Kinsman

Road, Cleveland, Ohio (Code-292-7374)CRC Chemicals, Div. of C. J. Webb, Inc.,

Dresher, Pa.; Also many Auto SupplyStores

Magnus Chemical, Div. of Economics

Laboratory, Inc., Garwood New Jersey

COATINGNO. NAME





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106.

TABLE II

Results of Tests on Coatings

Coating Method of Test ANo. Application* Coupon No. Points

12345678

911

12131415161718

1920

a

a

a

a

c

c

a

c

c

b

bc

bc

c

c

bc

b

23456789

1013

1415161718

19202122

6666666624

666666655

Test B Test CCouponNo. Points Coupon No. Points

32456789

10

12

131415161718

192021

553366653

5

65656

56

56

2345678

91012

131415161718

192021

1433553334

6151

32514

TotalPoints

12 :

151212171715148

13

1812171215131711

15

Application methods used:

a. Protective coating wiped on with a soft lint-free cloth and the excess wiped off.

b. Protectivecoating

diluted to theproper consistency

andapplied

with a brush.

c. Protective coating applied according to the directions printed on the container.d. The sample heated up and dipped into the protective coating, which is heated

to a liquid state, and the excess material wiped off.

,The coating has run due to the elevated temperature, leaving the coupon less pro-tected in some areas.

' Material formulated as a temporary protective coating.

Test Rating Points: The amount of corrosion on a coated coupon after the

completion of the tests is indicated by the number of points assigned:

6 points

5 points4 points3 points2 points1 point

total or near total corrosion

75% - 100% corrosion50% - 75% corrosion

25% - 50% corrosion

less than 25% corrosionno corrosion or very little

The total number of points for each coating were added together. Presumablythe coating with the lowest total should be the best, but this is not alwaysthe case because some coatings performed erratically.

-





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107.

Test Observations of Protective Coatings

1. If the object to be coated is going to be handled a great deal, the

greaseand oil

typesof

coatingswould not be desirable, even though

in general they tested quite well.

2. The coatings seem to perform much better in a non-polluted

atmosphere; therefore, proper climatic control can improve the

corrosion resistance of almost any coating.

3. The ingredients in proprietary coatings are not always listed on the

container, making the permanence of the coating an unknown factor.

Moreover, the ingredients are subject to change without notice.This further strengthens the argument for periodic testing of pro-tective coatings.

Bibliography

Biek, L., Cripps, E.S., and Thacker, D. M. D., Some Methods for Pro-

tecting Cleaned Iron Objects , Museums Journal, Vol. 54, No. 2

(May, 1954) pp. 32-36.

Bird, Vera and Hodges, Henry, A Metallurgical Examination of Two IronSwords From Luristan , Studies in Conservation, Vol. 13, No. 4

(November, 1968) pp. 215-223.

Burns, R. M. and Bradley, W. W., Protective Coatings for Metals, NewYork: Reinhold, 1962.

Cushing, Daniel, Principals of Corrosion Applicable to Ancient Metalsand Methods of Identifying Corrosion Products , Application of Sciencein Examination of Works of Art, Boston: Museum of Fine Arts, 1967,

pp. 53-65.

Farrier, T. W., Biek, L., and Wormwell, I. F., The Role of Tannatesand Phosphates in the Preservation of Ancient Buried Iron Objects ,Journal of Applied Chemistry, Vol. 53 (February, 1953) pp. 80-84.

Gettens, Rutherford J., Mineral Alteration Products on Ancient Metal Ob-

jects , Recent Advances in Conservation, edited by G. Thomson, London:

Butterworths, 1963, pp. 89-92.

Gibson, Bethune M., The Use of the Airbrasive Process for Cleaning Ethno-logical Materials , Studies in Conservation, Vol. 14, No. 4 (November,1969) pp. 155-164.

Henson, William, Restoration of Modern Machinery , Museum News, Vol.49, No. 10 (June, 1971) pp. 13-17.





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108.

Knowles, E. and White, T., The Protection of Metals with Tannins ,Journal of the Oil & Colour Chemists Association, Vol. 41 (January,1958) pp. 10-23.

Norton, John T., Metallography and the Study of Art Objects , Applicationof Science in Examination of Works of Art, Boston: Museum of Fine

Arts, 1967, pp. 13-19.

Organ, R. M., Design for Scientific Conservation of Antiquities, Washington,D. C.: Smithsonian Institution, 1968.

Organ, R. M., The Washing of Treated Bronzes , Museums Journal, Vol.

55, No. 5 (August, 1955) pp. 112-119. (This entry is included becausethe washing and testing procedures outlined in it are in general applicableto all metals. )

Pelikan, J. B.,The

Useof

Polyphosphate Complexesin the Conservation

of Iron and Steel Objects , Studies in Conservation, Vol. 9, No. 2

(May, 1964) pp. 59-66.

Pekikan, J. B., Conservation of Iron with Tannin , Studies in Conservation,Vol. 11, No. 3 (August, 1966) pp. 109-116.

Peterson, Mendel, History Under the Sea, Washington, D.C.: Smithsonian

Institution, 1965.

Plenderleith, H. J., The Conservation of Antiquities and Works of Art,London: Oxford University Press, 1956.

Plenderleith, H. J. and Toracca, G., The Conservation of Metals in theTropics , The Conservation of Cultural Property, Paris: UNESCO,

1968, pp. 237-249.

Stambolov, T., The Corrosion and Conservation of Metallic Antiquities and

Works of Art - A Preliminary Survey, Amsterdam: Central Research

Laboratory for Objects of Art and Science, N. D.

Stambolov, T. and van Rheeden, B., Notes on the Removal of Rust from Old

Iron with Thioglycolic Acid , Studies in Conservation, Vol. 13, No. 3

(August, 1968) pp. 142-144.

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