HUNTING FREE AND BOUND CHLORIDE IN THE WROUGHT IRON RIVETS FROM THE AMERICAN CIVIL WAR SUBMARINE H. L. HUNLEY
NÉSTOR G. GONZÁLEZ, PHILIPPE DE VIVIÉS, MICHAEL J. DREWS, & PAUL MARDIKIAN
The Confederate submarine H. L. Hunley was built in 1863. It sank in February 1864 off the coast of Charleston, South Carolina, shortly after becoming the first submarine in history to sink an enemy ship in combat. The Hunley was successfully recovered essentially intact in August 2000 after nearly 140 years of immersion in the Atlantic Ocean.
One of the most significant problems facing the conservator with regard to the conservation of iron artifacts recovered from marine archaeological sites is the removal of the chloride ions present in the corrosion layer and the prevention of subsequent active corrosion. This problem has been the topic of many investigations and remains one of the most active areas in conservation research.
While at least 18 different iron corrosion products have been identified on artifacts recovered from burial and marine sites, most of the focus in the literature has been on the chloride (Cl-1) containing corrosion products (Selwyn 1999). The Cl-1 containing compounds most often associated with the corrosion of recovered artifacts include akaganéite (β-FeOOH), hydrated ferrous chlorides (FeCl2•2H2O and FeCl2•4H2O), and green rust (a mixed-valence ferrous (Fe+2) and ferric (Fe+3) hydroxide and oxyhydroxide of variable Cl-1 content (Gilberg and Seeley 1981; Al-Zahrani 1999; Selwyn 1999). Ferric chloride (FeCl3) has only occasionally been mentioned in the literature and would probably be present only under relatively unusual circumstances (North and Pearson 1978b). Of the above-mentioned compounds, most of the attention has been focused on the formation and presence of akaganéite and its role in the postexcavation corrosion processes that have been observed (North and Pearson 1978a, b; Yabuki and Shima 1979; Gilberg and Seeley 1981; Rinuy and Schweizer 1981; Knight 1982; North 1982; Turgoose 1982b; Stalh et al. 1998; Al-Zahrani 1999; Selwyn 1999).
Clearly, any successful conservation of an iron artifact must involve the removal of as much Cl-1 as possible. It has been postulated (Turgoose 1982b):
- Cl-1 may be contained within solid corrosion products such as by being trapped within the β-FeOOH crystalline matrix, as green rust, or as one of the hydrated ferrous chlorides.
- Cl-1 may be adsorbed on the surface of a solid corrosion product, as has been observed on goethite (α-FeOOH) or β-FeOOH (Kaneko 1989; Stalh et al. 1998; Al-Zahrani 1999).
- Cl-1 may be present in the active corrosion solution within the active pores of the corrosion layer and at the anodic sites as a counterion to the Fe+2 as it is being formed electrochemically.
In addition, there has been considerable discussion as to the distribution of the Cl-1 within the corrosion layer. This discussion has recently been reviewed within the context of developing a more sophisticated diffusion model for the removal of Cl-1 during the conservation process (Selwyn et al. 2001). Based on this discussion, it can be argued that the Cl-1 distribution within any artifact can be represented by two boundary cases, one in which it is uniformly distributed throughout the object (North and Pearson 1978b) or one in which a distinct concentration gradient, presumably at the active corrosion layer site, exists (Selwyn et al. 2001). If active corrosion is occurring, it is more reasonable to assume the latter case would represent the Cl-1 within the sample. If the active corrosion has been stopped by treatment, then the former case might better represent the Cl-1 within the artifact.
Finally, the effect of treatment conditions on the removal of the Cl-1 from the artifact must be taken into consideration. For this discussion it will be assumed that three general classes of Cl-1 ions may exist within the artifact:
- Free Cl-1 ions that may be readily displaced in a treatment solution. These would include any precipitated salts on an artifact that had been allowed to dry or mobile counterions that are associated with soluble corrosion products or those that may be displaced by another counterion such as hydroxide (OH-1)
- Adsorbed Cl-1 ions that are chemically or physically adsorbed on the surface of a corrosion product such as α-FeOOH or β-FeOOH
- Bound Cl-1 ions that are chemically or physically trapped within a matrix, such as β-FeOOH, that must be disrupted before they can be allowed to diffuse out of the artifact
With respect to Cl-1 ions present in the structure of β-FeOOH, in studies employing synthetic akaganéite it has been postulated that the β-FeOOH is readily transformed under alkaline conditions to α-FeOOH with the release of the trapped Cl-1 ions (North and Pearson 1978b; Al-Zahrani 1999). Other authors have referred to the difficulty of releasing the Cl-1 from the akaganéite (Turgoose 1982b; Stalh et al. 1998; Selwyn 1999). Perhaps some of this apparent contradiction can be related to the synthetic akaganéite itself. At least 25 different syntheses producing akaganéite with chloride contents ranging from 0.87 to 17 wt% have been reported in the literature (Al-Zahrani 1999). In addition, in at least one study on iron Roman nails from a marine site, the ratio of soluble (1) and (2) to insoluble (3) chloride was studied and reported to be approximately 1:4 (Rinuy and Schweizer 1981).
Given the critical role that the various Cl-1 containing species are suspected to play in the subsequent corrosion of an artifact, before an appropriate conservation plan can be devised it is important to determine as much as possible about the state of the existing corrosion products. In particular, if possible, the lability of the Cl-1 species present should be characterized. The objective of this investigation was to determine the relative ratios of the class 1 and class 2 to the class 3 Cl-1 species present in the rivets of the H. L. Hunley. For the purposes of this study, any Cl-1 that could readily be extracted from the rivet shavings produced by drilling the rivets during the excavation of the submarine's interior was considered as class 1 and 2. The remaining Cl-1 determined by digestion was considered as class 3.