Monitoring the effect of sacrificial anodes on the large iron artefacts on the Duart Point wreck, 1997

The International Journal of Nautical Archaeology (1999) 28.2: 164-1 13 Article No. ijna.1999.0191

(29 Technical Communication

Monitoring the effect of sacrificial anodes on the large iron artefacts on the Duart Point wreck, 1997

David Gregory National Museum of Denmark, Centre for Maritime Archaeology, Havnevej 7, Postbox 31 7, DK-4000 Roskilde, Denmark

The in situ measurement of electrochemical parameters such as surface pH and corrosion potential, E,,,,, of corroding metal objects on submerged wreck sites has a relatively recent history. Undoubted pioneers in this research are Dr Ian MacLeod and his colleagues of the West- ern Australian Museum. Their work spans over two decades and during this time they have shown that the routine recording of on-site measurements has been an invalu- able tool in understanding the corrosion mechanisms and modes of deterioration of metals on marine archaeological sites. Fur- ther, they have shown that with the aid of such information it is possible to stabilize and start the conservation process of iron artefacts on the seabed with sacrificial anodes, thereby significantly reducing the time and cost of subsequent conservation in the laboratory (North, 1976, 1982; MacLeod, 1987, 1989, 1995, 1996).

The benefits of attaching sacrificial anodes to artefacts to start the conservation process in situ have been discussed by MacLeod (1987: 51). A sacrificial anode normally comprises a metal such as a zinc or aluminium alloy, electrically connected, for example by copper cable, to the artefact which is a less reactive metal, such as iron. The iron artefact gains protection as the electrons released from the corroding

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anode flow through the copper wire into the artefact; that is to say, the artefact is the cathode of the corrosion cell while the seawater completes the circuit. This effec- tively lowers the corrosion potential and hence the corrosion rate. Furthermore, as electrons flow into the artefact they will tend to cause a reduction in the acidity levels of the solution trapped between the concretion/metal artefact interface as hydrogen is evolved. The effective change in polarity of the artefact will assist the removal of chloride ions. The fact that removal of chlorides from artefacts is effected while still in situ will mean that less time is required for conservation in the laboratory.

In 1994 Dr MacLeod visited the Duart Point site and took measurements on five of the cannon and the anchor, leading to his recommendations for future in situ conservation (1995: 57). His results are tabulated in Table 1.

In 1996, based on his findings, a series of aluminium anodes were attached to the cannon and anchor. His recommendations were that cannon 4 and 5 were not in imminent danger of collapse. However, cannon 1 and 3 and the anchor needed urgent attention. The anodes consisted of scrap aluminium smelted into ingots which were attached to clamps by means of a

0 1999 The Nautical Archaeology Society

D. GREGORY: ARTEFACTS ON THE DUART POINT WRECK

Table 1. Duart Point corrosion data from 1994 after MacLeod (1995: 54)

Measured depth of Average corrosion E,,,, volts versus Artefact corrosion (mm) rate (mm/yr) AglAgCl PH

Cannon 1 61.5 0.180* - 0.452 5.42 Cannon 2 62.5 0.183 - 0.505 5.90 Cannon 3 64.5 0.189 - 0.503 5.63 Cannon 4 22.5 0.066 - 0.532 6.65 Cannon 5 6.5 0.019 - 0.512 6-85 Anchor 78.5 0.230 - 0.479 4.61

*Erratum: MacLeod states 0.164 but dividing 61.5 by 341 gives 0.180. However, further calculations were based on the correct value. Voltage of AglAgC1 reference was 0.243 volts versus S.H.E.

length of six-core copper cable (Fig. 1). The anodes were attached to the artefacts by drilling two locating holes through the concretion layer to the metal surface and positioning the clamp over the holes. Two bolts, which were part of the clamp, were then tightened until they made contact with the metal (Fig. 2). Unfortunately at the time of attachment no equipment was available to monitor either the change in the corrosion potential of the artefact, or the current flow from the anode to the artefact in order to check that the anodes were working. Admittedly the anodes started ‘fizzing’ almost as soon as they were attached. However, it was not certain that the artefact was being protected as this phenomenon may have been due to the steel clamps being protected by the anodes. In 1997 their effectiveness was checked by taking further pH and potential measurements on the artefacts.

Equipment and methodology for determining pH and corrosion potential The pH was determined using a BDH GelPlas flat surface pH electrode (Model 309/1070/09), with the electrodekable junc- tion sealed in flexible heat shrink plastic to make it watertight. This was attached to a Jenway pH meter (Model 3051). The elec-

trode and meter were calibrated according to the manufacturer’s instructions using p H 4 and seven aqueous buffers. As the accuracy was not required to be better than f 0-05 pH units, additional calibration allowing for salinity was not carried out.

E,,,, was determined using a commercially available working and reference electrode system for determining potentials in seawater (Corrintec). This consisted of a two-component probe assembly compris- ing a silvedsilver chloride reference electrode and a contact probe (‘stabber’) made of AISI 316 Stainless Steel fastened together. These were attached to a high impedance digital multimeter, the reference electrode being connected to the ‘comm’ terminal and the working electrode to the ‘positive’ terminal. The multimeter was set to read at 2 volts direct current. The reference electrode was immersed in a bucket of seawater taken from the site. It was left to equilibrate overnight and then calibrated against a saturated calomel electrode according to the manufacturer’s instructions, the calomel electrode having been previously calibrated against a plati- num electrode in aqueous pH 7 buffer saturated with quinhydrone. The multimeter and pH meter were then sealed into a custom-built polycarbonate waterproof housing (Fig. 3).

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NAUTICAL ARCHAEOLOGY, 28.2

Figure 1. Aluminium anodes and clamps used for in situ conservation. (Photo: C . Martin).

In order to drill through the concretion on the artefacts an automotive air drill running off a SCUBA tank was used with a 15 mm diameter drill bit designed for drilling metal.

The waterproof housing and electrodes were taped to the side of the SCUBA tank (Fig. 4) so that the equipment could be taken underwater as one complete unit.

Once on site the procedure for taking measurements was to drill through the marine growth and concretion layer on the artefact until the metal surface was reached, measurements being taken approximately 10 cm from where the 1994 data were collected. The pH electrode was

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then inserted into the hole and held against the metal surface until a stable reading was obtained. In practice, this often took several minutes due to the slow outward diffusion of the acidic and chloride-rich water which had built up under the concretion layer of the encapsulated artefact. The acidity arises from the hydrolysis of metal ions while the increased concentration of chloride ions is due to inward diffusion from the surrounding seawater to achieve electrical neutrality of the corrosion products (North, 1976). The E,,,, was then measured by placing the stabber into the hole and establishing electrical contact


Figure 2. Placing of anode on cannon. (Photo: C . Martin).

with the metal surface. Good contact was indicated by an immediate stable reading which fluctuated only 1 or 2 millivolts over several minutes. Finally the corrosion depth was measured by re-inserting the drill bit into the hole and measuring the depth of penetration against a ruler.

The water temperature and depth were recorded on site using a digital dive com- puter and a water sample was taken from the site to determine dissolved oxygen using a dissolved oxygen meter (Jenway Model 9071). From this information the salinity could be determined by reference to tables of seawater saturated with dissolved oxygen content (Riley & Skarrow, 1975: 561-2).

Results of the 1997 monitoring The results of the 1997 monitoring are tabulated in Table 2. The voltage of the reference cell at 13°C was +0.250V [versus the standard hydrogen electrode (S.H.E)]. The average water depth over the time the measurements were taken was 11.3 m. The average temperature was 13°C and dissolved oxygen was 6.0cm3 dm3 which equates to a salinity of between 32 and 34 parts per thousand.

If the corrosion potentials and pH data from Table 2 are plotted onto a Pourbaix diagram similar to that used by MacLeod (1995: 56), the artefacts are seen to be still located in the area where ferrous ions, Fe2+, are the thermodynamically favoured

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Figure 3. Equipment for determining surface pH and E,,,,, showing from left to right: polycarbonate waterproof housing; pH meter, high impedance multimeter, flat surface pH electrode and ‘stabber’ with reference cell. (Photo: W. Karrasch).

state of iron (Fig. 5). That is to say, the metal is still corroding freely in the absence of any passivating film. However, the corrosion potentials of all the artefacts indicate that there has been a shift towards the region of immunity. Established criteria for cathodic protection (British Standards Institute, 1991: 20) state that the minimum potential for protection of iron in aerobic seawater should be - 0.6 volts (versus S.H.E.). However, the potentials are not as low as this, but the results are most encour- aging, particularly in the cases of cannon 1, 2, 3 and 5 . There has also been an increase in the surface pH of all the artefacts indicating that the acid resulting from the hydrolysis of the corrosion products is being neutralized. These results show that the anodes have been effective in slowing the corrosion rate of the artefacts and decreasing the acidity at the metal surface. In addition the anodes will have initiated the removal of chlorides from the artefacts.

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With regard to the rate of corrosion, MacLeod (1995: 54, 1996: 360-361) has previously shown the linear relationship, Equation 1, between the logarithm of the corrosion rate and the corrosion potential for iron artefacts in seawater. The data for the equation are obtained by measuring the depth of the concretion layer which is then divided by the number of years of submersion of the artefact in order to give a yearly corrosion rate:

log d, = M E,,,,+ C (Equation 1)

Where: log d, = logarithm of corrosion rate in mm/year

M = A log dJA EL,,, E,,,, = corrosion potential

in volts versus the Standard Hydrogen Electrode (S.H.E.)

C= intercept

At Duart Point the values determined for Equation 1 in 1994 were:


Figure 4. Equipment attached to a SCUBA cylinder with air drill ready for in situ measurements. (Photo: W. Karrasch).

Table 2. 1997 in situ corrosion parameters for the Duart Point wreck

Measured depth of Average corrosion Potential Potential Artefact corrosion (mm) rate (mdyr ) pH (vs Ag/AgCl) (vs S.H.E.)

Cannon 1 35 Cannon 2 24 Cannon 3 27 Cannon 4 42 Cannon 5 14 Anchor 16

0.103 6.9 - 0.578 - 0.328 0.070 7.1 - 0.678 - 0.428 0.079 7.4 - 0.672 - 0.422 0.123 7.4 - 0.574 - 0.324 0.04 1 8.0 - 0.71 1 - 0.461

- 0.280 0.047 6.9 - 0.530

log d,=3-70 E,,,,+0-228 (Equation 2).

The effects of the anodes on the cannon and anchor at Duart Point will be considered using Equation 2 as the original

results serve as the baseline for comparison before and after attachment of the sacrificial anodes.

By substituting the. corrosion potential data values from Table 2 into Equation 2

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-0.6

0.6 I I I

- I I I

Fe I I

I I I

I I FeOH’

-0.8 I I 1 I I I I I I I I 1

PH

Data from 1994 Measurements

Data from 1997 Measurements

Figure 5. Pourbaix diagram for iron at 13°C and a dissolved ion concentration of 1.0 x showing the in situ corrosion parameters for iron objects at Duart Point.

Table 3. 1997 Corrosion rates of iron artefacts on the Duart Point wreck site

Corrosion potentials Calculated corrosion Artefact vs S.H.E. Log d, rate (mmlyr) 1997

Cannon 1 - 0.328 - 0.9856 0.103 Cannon 2 - 0.428 - 1.3556 0.044 Cannon 3 - 0.422 - 1.3334 0.046 Cannon 4 - 0.324 - 0.9708 0.107 Cannon 5 - 0.461 - 1.4777 0.033 Anchor - 0.280 - 0.8080 0.156

the present corrosion rate of the artefacts is found, and these results are tabulated in Table 3. By comparing the calculated corrosion rates of cannon 1, 2, 3, 4 and 5 and the anchor from the 1997 data values with the average corrosion rates measured in 1994, the percentage corrosion rate of the artefacts can be calculated. In this way it is possible to see how effective the anodes

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have been. These results are tabulated in Table 4.

From this the anodes are seen to have been effective in that the percentage rate of corrosion has decreased significantly on all the artefacts when compared with the corrosion rates determined prior to the attachment of the anodes. However, the present appearance of the anodes and clamps


Table 4. Comparison of the 1997 corrosion rate with the 1994 corrosion rate of iron artefacts from Duart Point

Average corrosion rate

Artefact (mdyr) 1994

Cannon 1 0.180 Cannon 2 0.183 Cannon 3 0.189 Cannon 4 0.066 Cannon 5 0.0 19 Anchor 0.230

Calculated Percentage Calculated corrosion rate rate of corrosion corrosion rate (mdyr) 1994 1994 (rndyr) 1997

0.317 + 76 0.103 0.044 0.046

0.181 + 174 0.107 0.173 +810 0.033

0.156

Percentage rate of corrosion

1997

- 43 - 76 - 76 + 62 + 74 - 32

suggests that they may now not be as effective as they were. All of the anodes show a large build up of corrosion products (aluminium oxide/hydroxide) which may be decreasing their efficiency. This may be expected considering that the anodes were scrap aluminium rather than commercially available anodes designed for cathodic protection (British Standards Institute, 1991: 20). Aluminium corrodes to form an oxide film which is tightly adherent and causes rapid polarization when the pure metal is used as an anode. In chloride containing electrolytes, such as seawater, this breaks down to give very non-uniform attack by pitting. The addition of an alloying element that leads to the total breakdown of the oxide film (activation) is necessary to make aluminium useful as an anode.

Apart from the clamp on cannon 5, which is in an excellent state of preservation, all clamps show signs of corrosion, in particular the clamp on cannon 1 which has severely corroded suggesting a bad connection with the anode. If the stabiliz- ation of the artefacts is to continue the anodes should be cleaned or, ideally, replaced and a better system of attachment to the artefacts devised.

Discussion of the 1994 and 1997 results Problems were encountered in determining the corrosion rate of the artefacts post

anode attachment and with derivation of Equation 2 based on MacLeod’s initial findings, as will be discussed.

Consideration of the corrosion depths in Table 2 shows that there are significant differences between these and MacLeod’s 1994 results (Table 1). This may be the result of differences in corrosion thickness in the areas where the 1994 and 1997 measurements were taken. However, the measurements were deliberately taken in close proximity (10cm) to each other in order to negate such differences and it seems unreasonable that discrepancies in the magnitudes observed (up to 60mm in the case of the anchor) can be solely attrib- uted to spatial corrosion variations. Operator error may have contributed, yet with the aid of the air drill it was extremely easy to sense when the metal surface had been reached. However, in order to get an accurate measurement of the corrosion depth, the original metal surface within the corrosion products as well as the present metal surface should be found. This proved exceedingly difficult to find; hence measurements were taken at the present metal surface. Nevertheless the problem of precision needs to be addressed-how many measurements need to be taken per artefact in order to get an accurate result?

Furthermore, with reference to the 1994 Duart Point data, MacLeod used three of his six data points, cannon 2 and 3

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and the anchor, to correlate the relationship between corrosion potential and the annual corrosion depth and from this is derived Equation 2. Personal communication with Dr MacLeod established that the selection criteria for the three points had been based on knowledge gained by working on dozens of sites. This enabled him to state that objects which have been stable for centuries and have not recently been disturbed will generally have a strong correlation between the logarithm of the annualised depth of corrosion and the corrosion potentials. Thus his approach was that these three points were the only ones which made sense, in terms of the general corrosion model: the slope of the logarithm of the annual corrosion rate versus corrosion potential plot being the inverse of the Tafel Corrosion slope (MacLeod, pers. comm.). Thus in the case of the Duart Point data it was not possible to correlate the data from cannon I with cannon 2 and 3 because the corrosion potential was too high to sustain the lower corrosion rate that had been observed for cannon 2 and 3 along with the anchor. It was clear to Dr MacLeod that the environment of cannon 1 had undergone a recent change that related to the new environment and not the long-term one; thus number 1 was ex- cluded. Inspection of the other cannon’s concretion layer suggested that number 4 had been alternately buried and exposed. Similarly cannon 5 was just sticking out of the seabed and could not be considered.

However, two questions arise from the use of the anchor and cannon 2 and 3 to generate Equation 1. First, a recent publication by Dr MacLeod (1996: 421) refers to measurements taken on cast and wrought iron artefacts from the wreck of the Sirius. He states ‘direct comparison of corrosion potential values of cast and wrought iron is not possible owing to their different composition’. This poses a problem for the Duart data because the anchor was wrought iron and the cannon cast

I72

iron. Admittedly in his consideration of the Duart Point data MacLeod does discuss the different composition of the artefacts and comments that the higher corrosion rate of the anchor may be due to its lower carbon content. Furthermore since Dr MacLeod’s work at the Duart Point site and the 1996 publication his work has addressed this problem and pre- liminary results (forthcoming) seem to sup- port the validity of using data from wrought and cast iron artefacts (MacLeod, pers. comm.).

Second, the argument for not using cannon 4 was because it showed indications of periodic covering and uncovering. However, cannon 4 lies adjacent to and approximately 1.5 m distant from the anchor that was used in his calculation (see Martin, 1995: 17). With knowledge of the topography of the site it seems unlikely that cannon 4 would be subject to covering and uncovering while the anchor remained in a stable environment. While it is true that in the case of a partially covered artefact if a measurement is taken on the exposed part it has a potential which may not reflect the history of the artefact. How- ever, can the burial history of an artefact be completely certain without analysis of its covering concretion? Certainly at Duart Point, fluctuations in the seabed topography have been seen over the 6 years the site has been worked on.

Conclusion Following Dr MacLeod’s initial work on the large iron artefacts at Duart Point in 1994, observations were made that cannon 1 and 3 and the anchor were corroding at a faster rate than the other artefacts on the site. In 1996 sacrificial anodes were attached to these artefacts and the remain- ing cannon. In 1997 monitoring of the effects of the anodes showed that they had been successful in slowing the rate of corrosion. The corrosion potentials indicate a shift towards the region of immunity


of iron in seawater and the pH at the metal surface has increased indicating the removal of chlorides and acid.

Using Ian MacLeod’s rate equation method it was calculated that cannon 1, 2 and 3 were, respectively, corroding at 43%, 76% and 76% less than those indicated by the total depth of corrosion that has occurred over the past 341 years. Cannon 4 and 5 were, respectively, corroding 62Y0 and 74% faster. However, if we consider prior to attachment of the anodes they were, respectively, corroding 124% and 810% faster than that indicated by the total corrosion depth, the anodes have drastically reduced their corrosion rate.

In addition this work has highlighted possible problems with methodology and interpretation of data. First, the corrosion depths recorded in 1994 and 1997 show significant discrepancies which raise questions about the accuracy and precision of taking measurements and requires further research. Second, there is still some con- cern over the validity of comparing E,,,, data from wrought and cast iron artefacts. However, recent research by Dr MacLeod may have overcome this problem. Third, the burial history of an artefact cannot adequately be assessed in situ by observing its covering concretion, although this is where Dr MacLeod’s experience may be able to help in his interpretation of data to generate the corrosion rate equation.

This investigation has highlighted the effectiveness of in situ monitoring and conservation. It is hoped more researchers will use this method of managing sites with iron artefacts in situ. This would add to the corpus of information on corrosion in underwater environments and undoubt- edly preserve more of our submerged heritage, which after all, is one of the main aims of such research.

Acknowledgements I am grateful to the following people and Institutions for funding and helping collect the data for this paper: the Danish National Museum’s Centre for Maritime Archaeology, a Centre sponsored by The Danish National Research Foundation, and its Director Ole Crumlin Pedersen; Dr Colin Martin, of the Scottish Institute of Maritime Studies, and Director of the Duart Point project; Neil Dobson, Kevin Robinson, Ray Sutcliffe, Billy MacGregor and Peter and Edward Martin.

I am grateful to Birgit S~rensen, Barry Kaye and Ian Oxley for comments on draft versions of this paper. Finally, and certainly not least, Dr Ian MacLeod, not only for his comments on this paper but his enthusiasm and patience in answering numerous queries regarding this paper and other questions concerning corrosion.

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MacLeod, I. D., 1987, Conservation of concreted iron artefacts-new methods for on-site preservation

MacLeod, I. D., 1989, The application of corrosion science to the management of maritime

MacLeod, I . D., 1995, In situ corrosion studies on the Duart Point wreck, 1994. IJNA, 2 4 53-59. MacLeod, I. D., 1996, The electrochemistry and conservation of iron in seawater. In G. Kuppuram &

Martin, C. J. M., 1995, The Cromwellian shipwreck off Duart Point, Mull: an interim report. IJNA, 2 4

North, N. A., 1976, Formation of coral concretions on marine iron. IJNA, 5: 253-258. North, N. A., 1982, Corrosion products on marine iron. Studies in Conservation, 27: 75-83. Riley. J. P. & Skarrow, G., 1975, Chemical Oceanography, (2nd edn). New York.

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Monitoring the effect of sacrificial anodes on the large iron artefacts on the Duart Point wreck, 1997