Environment

Example of an environment where bacterial corrosion can be found : seawater

Seawater is a complex, agressive environment towards the material that are immersed in it. The agressivity of a seawater environment can be defined as a whole of parameters which conditionate the act of mechanical, physical, chemical and electrochemical degradation.

Salinity

Seawater is characterized by its salinity, which is defined as the weight in electrolyte grams contained in one kilogram of water. The chloride element represents 55% of the total amount of the dissolved salts. This salinity varies in relatively low proportions for oceans (32%° to 37%°) but in much higher ones for border seas (below 4%° in the Baltic sea areas) and over 41%° in the north of the Red Sea. In general, the global salinity is around 35%.

Organic materials

The low concentrations of organic materials in seawater (0,5mg.l-1) make their chemical analyses very difficult. Amongst these organic materials, we find chlorophylles, carotenoides, monosaccharines, polysaccharines, aminoacids, organic acids and fatty acids. Particular organic materials are mainly constituted of animal or vegetal wastes.

pH variations

Seawater is slightly alkaline and its average pH is around 8.2, varying between 7 and 8.4. pH variations can be induced by those of the dissolved oxygen. It can be explained by the photosynthetic activity of certain organisms : a heavy production of oxygen implies a decrease of carbon dioxide and leading to an increase of pH ; the oxidation of the organic materials reduces the amount of oxygen in the water, while producing carbon dioxide and thus decreasing the pH. The pH of the water decreases with the temperature increase.

 Dissolved gas

Some gas, originating from the atmosphere, are dissolved in seawater. These gas are dissolved in seawater according to the Henry’s Law. The dissolution of the carbonic gas is more complicated because it interacts with water to form bicarbonate  and carbonate ions and it participates to the pH regulation. As it is the case for all atmospheric gas, oxygen solubility varies with  water temperature and salinity. For a water with a salinity of 35%°, oxygen solubility is 7 mg.l-1 of O² per liter of water at 25°C and of 11 mg.l-1 at 0°C. Under natural conditions of oxygenation, the potential of oxidation reduction of seawater is determined by the concentration of dissolved oxygen (around 8 mg.l-1 at 20°C for a salinity of 35%°). Certain researchers mention the intervention of the redox couple  O2/H2O2 (standard potential 0.69 5V) with a stationary concentration in hydrogen peroxide. The concentration of hydrogen peroxide is around one nanomole per liter.