La tradición oral desde la perspectiva de los pueblos originarios

2.2.2.1 MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)

2.2.2.1.1 It can be said that all bacteria influence corrosion and that they fall into two distinct groups. One type of bacteria is directly involved in the dissolution of the metal and the other is indirectly involved by setting up the conditions for direct action bacteria or by altering the electrochemical cell. Generally anaerobic bacteria are those directly responsible for corrosion. Other bacteria may create an oxygen gradient across a biofilm setting up the conditions in which these bacteria can exist. Wolfaardt and Archibald (1990) summarised the following mechanisms of MIC within water distribution systems:

- Respiring bacteria may deplete oxygen levels causing differential oxygen concentration cells. The oxygen deficient area can become anodic and/or anaerobic.

- Certain bacteria can produce organic or inorganic acids causing localised metal dissolution.

- The use of cathodic hydrogen can depolarise the cathode maintaining the electrochemical cell (Tuovinen et aL, 1980).

- Bacterial metabolic products may create localised concentrations of certain ions giving rise to changes in potential. These ions may also cause the dissolution of the metal. The biofilm may also increase the difiusion resistance of ions in solution.

- Bacteria can degrade organic protective coatings. The exposed area becomes anodic to the rest o f the pipe, hence corrosion can be rapid, resulting in loss of inhibition.

- Chemical inhibitors have been metabolised by bacteria, resulting in a loss of inhibition.

2.2.2.1.2 The prevalent mechanism will depend on the species of bacteria at the pipe surface. Anaerobic sulphate-reducing bacteria have been identified within corrosion tubercles on cast iron pipes (Tuovinen et a l, 1980). These bacteria use hydrogen produced at cathodic sites, under acidic conditions, to reduce sulphate with the production of hydrogen sulphide. Hydrogen sulphide has been detected dissolved in water distribution samples (Lee et a l, 1978) or it may then react with the metal producing ferrous sulphide. Sulphur-oxidising bacteria, such as Thiobacillus thiooxidans, utilise the hydrogen sulphide to generate sulphuric acid, which vdll attack the metal. Iron-fixing bacteria use ferrous iron as an energy source, producing ferric hydroxide. This corrosion product may help create the tubercles providing additional anaerobic sites and restricting flow. Heterotrophic bacteria, such as Pseudomonas, Enterobacter and Mycobacterium, form precipitates with iron. This activity may result in high visible iron concentrations due to precipitates or bioflocculation (Lee et aL,

1978). This action gives rise to ‘red water’ complaints but does not affect the corrosion rate.

2.2.2.1.3 The corrosion products of MIC can be differentiated fi"om those of the chemical process. Wolfaardt and Archibald (1990) consider MIC deposits to be slimy and generally soft and easily deformed.

2.2.2.2 EFFECT OF ORGANIC MOLECULES

2.2.2.2.1 Natural organics can act as corrosion inhibitors. Ryder and Wagner (1985) showed how the corrosion of a zinc layer depends on the humic acid content o f a water. They also stated that the inhibiting effect is greater, the greater the polarity of the molecules. Molecular weight and polarity both affect the adsorption of organics on the metal surface. Rudek (1979) postulated that the adsorption of organics can delay the precipitation of carbonate and retard the oxidation of ferrous to ferric iron resulting in a more dense protective deposit. Ryder and Wagner (1985) further state that organics are of most influence on unlined systems and that red water problems have been seen after reducing the organic content. Singley et al. (1985) suggest that the effect of natural

organics is a function of the type and nature of the organics. The inhibiting effects of organics may be due to the organic molecules adsorbing onto the corrosion sites preventing the transport of corrosion reactants (adsorption inhibitors). It may therefore be the case that organics are of little influence in a system already covered with a porous metal oxide corrosion deposit. Polar organics, such as acetyleric alcohols, amines and thiourea, are used as corrosion inhibitors in non-potable, strongly acidic water systems. Their action is to adsorb to the surface and exclude hydrogen ions from reaching cathodic areas. Kolle and Rosch (1978) show that ozone treatment produces a scale that is very protective and hence reduces corrosion. They postulate that the chemical structure of the humic substances are altered leading to more polar and better adsorbable substances with regard to the calcium carbonate and other scale constituents. This improved adsorption gives a denser deposit. Ryder and Wagner (1985) consider that despite the increased polarity, the lower molecular weight of the organic molecules will give lower adsorption. Again the effect is likely to depend on the ozone dose and the organic profile of the water. The use of granular activated carbon (GAC) post ozonation may result in increase removal of the polar organics via adsorption and biological removal of the low molecular weight organics. The water may therefore be less protective with ozone treatment than without, due to the overall reduction in the humic substances. Tuovinen et al. (1980) confirmed that humic material formed up to 2% of corrosion deposits. They further found that tubercle interiors contain only a fraction of the organic carbon found on the exterior. Organic carbon retained on the metal surface may adsorb iron ions forming complexes (Tuovinen et al., 1980).

1.1.2.22 Research undertaken by Turrell (1991) for Anglian Water Services also looked at the effects of organic molecules on corrosion. The data can be re-presented in a different form to that used by Turrell (1991). Turrell (1991) concluded that “....the influence of organic carbon was not a significant factor with regard to the structure and stability of coupon deposits.”. Organic carbon therefore did not, in his opinion, affect the corrosion rate. Turrell (1991) obtained his data from a pipe rig of a similar design to that of the Grafham pipe rig. Turrell (1991) also investigated the effect different sources of water, hence different levels of TOC, had on the corrosion rate. If one assumes that

the most informative figure is an average corrosion rate across all samples in the study period then the following table shows a recalculated summary o f Turrell’s data.

Table 2.1. Data taken from Turrell (1991) table 3.5.1. recalculated. Exposure time > 84 days, average o f all data.

WATER TOC (mg/1) Ave. TOTAL TOTAL

SOURCE CORROSION ALKALINITY HARDNESS

RATE