Metals uncorroded

Figure 17.3 Graphite flakes surrounded by graphitically corroded iron. Bright background is uncorroded metal. (Magnification lOOOx.)...

Critical relative humidity The primary value of the critical relative humidity denotes that humidity below which no corrosion of the metal in question takes place. However, it is important to know whether this refers to a clean metal surface or one covered with corrosion products. In the latter case a secondary critical humidity is usually found at which the rate of corrosion increases markedly. This is attributed to the hygroscopic nature of the corrosion product (see later). In the case of iron and steel it appears that there may even be a tertiary critical humidity . Thus at about 60% r.h. rusting commences at a very slow rate (primary value) at 75-80% r.h. there is a sharp increase in corrosion rate probably attributable to capillary condensation of moisture within the rust . At 90% r.h. there is a further increase in rusting rate corresponding to the vapour pressure of saturated ferrous sulphate solution , ferrous sulphate being identifiable in rust as crystalline agglomerates. The primary critical r.h. for uncorroded metal surfaces seems to be virtually the same for all metals, but the secondary values vary quite widely. 

Halogens Although tantalum is severely attacked by flourine at room temperature it does not react with liquid chlorine, bromine and iodine up to 150°C and the metal suffers no appreciable attack in wet or dry bromine, chlorine and iodine below 250°C. It is virtually uncorroded by hydrogen bromide and hydrogen chloride below 370°C, attack starting at about 375 and 410°C respectively. 

In calculating the strength properties of the corroded specimens and comparing them with those of the uncorroded control specimens after appropriate mechanical tests, it will be necessary to take into account the actual area of the cross-section of the corroded metal and report results on this basis instead of, or as well as, on the basis of the original cross-section prior to exposure such as would be represented by the uncorroded control specimens. 

The stability of buried metals largely depends on a combination of pH and redox (Edwards 1996). Under high redox values (oxidizing conditions) most metals will easily corrode, whereas under low redox values (reducing conditions) they will tend to remain as uncorroded metal. In addition, acidic conditions (low pH) will assist corrosion, whereas alkaline conditions will tend result in the formation of a stable corrosion matrix in most metals. Thus, in a well-drained, acidic sand or gravel site, all metals except the most inert (e.g., gold) will corrode rapidly and extensively. However, under most other burial conditions, most metal will be capable of recovery, albeit in a corroded state even after many centuries. 

The SEM photomicrographs in Figure 11 show that copper corrodes locally and nonuniformly. Figure llB shows a sample, exposed for 1 year at the North Carolina site, which has had its corrosion film stripped. Flat uncorroded areas, identified by the rolling marks from the original copper surface, are present. Figure llA shows a similar sample with the corrosion film intact. Thick mounds formed in the corrosion film appear to be associated with the areas of localized metal attack seen in Figure IIB. 

If the extent of corrosion of the artefact while buried is uncertain, a radiograph may be taken to ascertain the amount of metal left uncorroded. This was often helpful in deciding the most suitable method for subsequent conservation. 

By altering the pH of the solution, it may be possible to dissolve out the chlorides without corroding the metal. This is achieved by the formation of a thin, passive film, approximately 10 nm thick on the exposed uncorroded metal. The pH required to passivate any given metal or alloy can be determined by inspection of the relevant E-pH diagram. For wrought iron, a passive film will form above a pH value of 9.5. This would be a disaster for aluminium artefacts... 

Exfoliation is a very deleterious form of corrosion because the splitting off of uncorroded metal rapidly reduces load-carrying ability. The splitting action continually exposes film-free metal, so the rate of corrosion is not self-limiting. Exfoliation generally proceeds at a nearly linear rate. 

These results confirmed suspicions that failure was due to excessive amounts of nitrogen, carbon, and oxygen. To characterize the condition of the vessel further, Charpy V-notch impact tests were run on the unaffected base metal, the HAZ, and the uncorroded (sound) weld metal. These tests gave the data in Table 12.7 for ductile-to-brittle transition temperatures. 

Characteristics Exfoliation is characterized by leafing or splitting off of alternate layers of thin, relatively uncorroded metal and thicker layers of corrosion product that are more bulky than the metal from which they came (Fig. 7). The layers of corrosion products cause the metal to swell. In an extreme case, a 1.3 mm (0.050 in.) thick sheet was observed to swell to a 25 mm (1 in.) thickness. 

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