FACTORS INFLUENCING THE CORROSION RATEMETAL OBJECTS

Jan Gullman

OF

Introduction

It is a basic principle that all meials (excluding the

noble ones) are chemically unstable in ordinary air,but this can hardly be said to be part of commonknowledge. In spite of this fact many metals appearquite permanent in many everyday surroundings.The reason for this and the reasons why metal cor-rosion occurs under other circumstances are funda-mental questions of corrosion science.

When metal comes in contact with oxygen a chemi-cally unstable state is created. The correspondingstable state is metal oxide. A metal can, for obviousgeometrical reasons, only react with oxygenwhere itis in contact with it (ie on the metal's surface). Thatis why all non-noble metals are covered with a layerof metal oxide (although this layer often is so thinthat it is invisible).

Metals which are covered with a protective oxidehave such properties that they can form a barrieragainst surrounding oxygenand are potentially usefulfor making metal objects. Other properties such as

hardness, ductility etc are of course also determi-nants of their usefulness.

The nobility of the metal (ie its tendency to chemi-cally convert into an oxidized state) is of importancefor the development of corrosion reactions. Forexample, zinc is less noble than iron. That means

that it has a greater electrochemical tendency toconvsrt into chemical compounds than iron has. Ifan object made partly of zinc and partly of iron isplaced in an aerated water solution, the zinc willcorrode first and leave the iron practically uncor-roded as long as the zinc remains in contact with the

iron.

In another situation the relatively less protectiveproperties of corrosion products on surfaces deter-mine the corrosion rate. A practical application ofthis is the common practice of covering iron with

zinc ("galvanizing").This treatment is useful be-

cause zinc corrodes slower in outdoor environmentsthan iron, since the zinc corrosion products in theatmosphere are more protective for the metal com-pared to porous iron corrosion products.

Wet and dry corrosion

The occurrences of corrosion are usually divided intotwo main categories: dry and wet corrosion. Limi-ting the discussion to archaeological metal objects,one could say that dry corrosion is typically found intwo situations.

Objects from, for example, cremation graves, havebeen oxidized in fire before they were deposited inthe soil. As a result of these heat treatments, theobjects were covered with oxide layers. Such oxidelayers often withstand millennia of exposure in soil.This is the case for magnetite layers on iron objects.The other important type of dry oxidation for arch-aeological metal objects is often found in museuru.Polished copper and silver surfaces are stained inatmospheres containing sulphur compounds withsulphur in the oxidation state -II. The sulfide con-taining corrosion products of copper and silver are

less protective since matter can be transportedthrough therir at marked rates by diffusion. Usuallythe diffusion rates in oxides are very slow at ordinarytemperatures and therefore oxides normally consti-tute very effective barriers against the surroundingatmosphere.

The majority of important corrosion phenomena iswet corrosion. The presence of liquid water on metalsurfaces (even with very thin layers) allows the possi-

bility for electrochemical reactions. These reactionsare of two kinds, cathodical and anodical. They can

occur spatially separated from each other but mustthen be connected via an electrical conductor inwhich electrons can flow. The reason for this is thatthe anodic reactions produce electrons which are

used for the cathodic reactions. The two kinds of

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reactions balance each other so that all producedelectrons must be consumed and vice versa. Elec-trochemical corrosion can therefore be limited orcontrolled byintroducing restrictions in one ofthreephenomena:

- the anodic reaction- the cathodic reaction or- the electrical current which unite the reactions

The anodic reactions convert the metals from ele-

ments to metal ions and electrons. The most im-portant cathodic reaction, for corrosion processes, is

the reduction of oxygen dissolved in water to formhydroxide ions by taking up the electrons generated

by the anodic reactions.

The soil environments from which archaeologicalobjects are excavated are more or less wet. In thetemperate climatic zone the prerequisites for wetcorrosion are almost always fulfilled in unfrozensoils. In spite of this, corrosion rates vary a great

deal between different soil environments. Corrosionproblems with technical equipment such as steel

pipes and poles have necessitated the developmentof a field of soil corrosion technology.

Soil corrosion technology

Corrosion research can be divided into the scientificand technological areas. There isno clear distinctionbetween these two fields.

Corrosion technology usually avoids complicatedtheoretical models and is mainly based on experi-ence and evaluations of relatively simple experiments

and standard measurements.

In corrosion science theories and hypotheses are

formulated and tested. For these purposes corrosionconditions in liquids or atmospheres are easier towork with than soil conditions (which are difficult todefine and reproduce). This is probably the reasonfor the relatively little work done on soil corrosionscience compared to soil corrosion technology. An-other reason for this situation is that the need forknowledge concerning soil corrosion emanates fromtechnology. Corrosion rates in soils of the magnitude0. l millimeter per decade or more are considered tobe of technical importance and require countermea-

sures.

The technological aims of corrosion research in soilcan be taken up in one of two fields. The first oneconcerns the construction of a set of reliable criteria

for determination of whether there is high potentialcorrosivity in a given soil or not. The second one

concerns development of cheap and reliable corro-sion countermeasures, such as coatings or cathodicprotection. Therefore soil corrosion technology is

mainly concerned with problems in environmentswhere only relatively young iron artifacts are notcompletely corroded.

The technological criteria used in estimating soilcorrosivity are based on measurements of electricalresistivity, pH and concentration of some ions suchas hydrogen carbonate and chloride (German Stand-ard DIN 50 929) in the soil. Sulphur content of thesoil is also considered to have a gre,at influence oncorrosion rates in soil. The most important factor forthe corrosion rate of iron in soil is, however, whe-ther the corroding object in question is above orbelow the ground water table. Below the groundwater table the transport rate of oxygen is the step

that determines the rate for the corrosion of iron(Iversen 1988, Camitz and Vinka 1989).

Archaeological metals in soil

The conditions which are prevalent at the time ofthe excavation may often be very different from theones that prevailed some time after the deposition ofthe material. The objects may have been placed incontainers made of organic materials which created

a microclimate. This should have resulted in cor-rosivity similar to a den in the earth rather than soilconditions. In other cases, the environment mighthave been dominated by organic material from, forexample, a decaying body in a grave or waste mate-rials in a compost heap. In order to gain knowledgeabout the possible changes in the environment overtime and their nature, archaeological evidence fromexcavations should provide valuable information.Physical conditions, such as particle size of the soiland factors influencing the water flow around theobject can be expected to influence the corrosionrate. As well, as various chemical factors such as thechemical composition of the soil constituents, thepore water composition and the pH levels can alsoaffect corrosion rates.

It might be assumed that biological processes can

also have an important influence on corrosion pro-cesses in soil. There are at least two kinds of pro-cesses which seem to have great potential influence.The first is oxygen-consuming and carbon dioxide-producing fermentation of organic materials. Thiscan on one hand lower the availability of oxygen so

that the corrosion processes are slowed. On the

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other hand carbon dioxide dissolved in water in-creases the solubility of certain corrosion products.The second kind ofprocesses are caused by sulphurbacteria of different kinds which promotes transfor-mation of sulphur between the oxidation states -IIand +VI which may have influence on corrosionmechanisms.

Bronze corrosion

Archaeological bronze objects are attacked to a

lesser degree by corrosion than iron objects. Thecorrosion products usually consist almost entirely ofcuprite, malachite and stannic oxide (Nöggerath1825, Becquerel 1832, Fink and Polushkin 1936,

Gettens 1951, Geilmann 1956, Otto 1961, Walker1980, Ullrich 1985, Tylecote 1979, Grauer 1980).

Malachite is typically expected to form a protectivelayer under appropriate conditions. This can also be

true of cuprite. The stannic oxide is amorphous and

can probably be described as a hydrogel. It shouldtherefore not be expected to act as a protectivebarrier.

The stannic oxide is precipitated within the bound-aries of the original shape of the object. This oftenallows even completely corroded bronze objects tokeep their original shape.

According to the classical article by Geilmann(Geilmann 1956) bronze artifacts can be completelycorroded in sand with high organic content. Theycan even be transformed so extensively that theybecome white objects of tin oxide. This shows thatleaching - or using the pedological term "eluviation"- ofsoluble copper compounds from bronze objects,

is an important factor for the deterioration mecha-

nisms of bronze in soil. Low pH from carbonic acidand substantial water transport are bound to pro-mote this process. These circumsiances are also

known from the field of corrosion technology. Con-ditions which lead to dense coatings of corrosionproducts on copper alloyslead to lowcorrosion rates(Linder 1984). Unsuitable pH values ie below pH 7lead to relatively rapid corrosion attacks.

It can therefore be concluded that the most detri-mental factors for copper alloys in soil seem to be

those which promote eluviation of inter alia mala-chite and cuprite. These processes include loweringthe carbonate activity (eg by lowering the pH) and

lowering the copper(Il) ion activity (by reactionssuch as complexation). Ion exchange with clayminerals can also play a role.

The literature describes several cases of bronze

artifacts with banded structures of malachite and

cuprite in the stannic oxide layer. These have been

found covering the remaining metal core. The me-

chanisms of formation of these banded structures

does not seem to be completely understood, but the

interpretation of the so called Liesegang type pat-terns indicate influence of diffusion in the tin hy-droxide gel (Scott 1985,Robbiola et al 1988).

In soils with high chloride content, copper(I) chlo-ride (nantokite) is often formed in the inner parts ofthe corrosion product layer.

It should be remembered that archaeological brass

objects are often referred to as bronze. The corro-sion of brass objects is similar to that of bronzeobjects with the main difference being that no tinoxide hydrogel layer can then be formed. Thereseems to be very little known regarding details ofinfluence by humic and fulvic acids or other soilcomponents with high ion exchange capacities oncorrosion mechanisms.

Iron corrosion

The details of the corrosion process of iron objectsin soil seem to be more complicated than those ofbronze. According to Stambolov (1985), who quotes

many publications, the most commonly found corro-sion products in soil are magnetite and goethite. Thereason for this is explained by Turgoose (1982), whoalso proposes a model for corrosion of iron in soil.This model includes the anodic reaction of iron oxi-dation to ferrous ions and the cathodic reduction ofoxygen to hydroxide ions. These two reactions are

assumed to occur spatially separated by a layer of"largely magnetite and goethite with perhaps some

other stable ferrous-compounds". A water solutionwhich may contain high concentrations of ferrousions is assumed to be contained in pores of thedeposited corrosion products. The counter ion isusually chloride. The accumulation of chloride ionsis explained by the flow of ionic current betweenanodes on the metal surface and cathodes on the

surface of the deposited corrosion products. These

are assumed to possess electronic conductivity.

The corrosion model described above is in accor-dance with established corrosion theory and seems

to givea reasonable explanation in qualitative terms.However, a semiquantitative treatment of the accu-mulation of ferrous chloride raises some questions.

Corrosion rates on archaeological iron objects withchloride enrichments are usually on the order of

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millimeters per millennium. The corrosion currentdensity for a corrosion rate of 1 mm/millennium can

be calculated by considering I cm' of an iron sur-face.

- 1 mm penetration corresponds to O.786155.847 :0.014moles of iron.

- In the corrosion reaction electrons are delivered togenerate the charge 2x0.014x9650F 2700 As.

- Division by the number of seconds in 1000 years,

31536000000,gives as a result that it is equivalentto a current density of 0. 1 pAlcm'.

This is a very low current density. From the above

we can also calculate the case if all anionic charge

transfer is accomplish"d by chloride ions. Therewould then be a flow of:

2x0.014/1000 : 28x10" mole

chloride ions towards 1 cm' of anodic surface peryear.

In the opposite direction there must be a diffusionof ferrous chloride from a concentration on themetal surface of the magnitude l molar compared tothe magnitude of some millimolars in chloride (and

much lower in iron(Il) ions) in the pore water so-

lution of the surrounding soil. This is assumed tohave a composition in the normal range for groundwaters.

Diffusion coefficients for salts are of the magnitude10-5 cm'/s. Approximating the rust layer to a thick-ness of ca 10 mm, there is a concentration gradientof 10-3 mole/cm2.year. Based on these data Fick'sfirst law gives a diffusion flux equivalent to 0.3mole.cm-'.year-'. This is four orders of magnitudelarger than the assumed flux of chloride ions whichserye as charge carriers. Even with very small poresizes it seems unlikely that electrophoresis alone

could counteract diffusion to such an extent, and

that this could lead to substantial accumulations offerrous chloride between the layer ofsolid corrosion

products and the metal surface. With small pores rnthe layer, ion selective properties should be expectedin ferric hydroxide membranes. This has been in-vestigated by N. Sato and co-workers (eg Sakashitaet al 1984). In these studies it was found that ferrichydroxide membranes are anion selective below a

pH ofabout lO.This property leads to an electro-os-

motic flowof chloride ions. Sato et al concluded thatthese play a significant role in the corrosion ofmetals. Perhaps these matters deserve additionalattention in research regarding corrosion in soils.

There has so far been limited interest in these

questions among those who work with corrosionproblems in soil. This is in contrast to the great

development in similar fields described in modemliterature on surface and soil science (Lindsay 1979,Stumm 1987). Many examples can be found of fruit-ful approaches to phenomena in soil that also shouldbe possible to apply on questions regarding corro-sion in soil.

Conclusions

The above discussion gives a somewhat simplifiedsurvey of the most important factors for metal cor-rosion, with special emphasis on soil corrosion ofarchaeological metal objects. The following conclu-sions can be drawn:

- The principle inorganic chemical equilibria andthermodynamic data for virtually all corrosionproducts formed by corrosion of metal artifacts insoils seem to be known today.

- There are, however, important gaps of knowledgeregarding details in reaction mechanisms.

- For an understanding of these questions morecooperation between corrosion science and mod-ern soil chemistry may contribute to greaterunderstanding of the problems addressed in thispaper. Of course the close cooperation of archae-ologists will be necessary to the success of suchwork.

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