A number of metals, such as structural steels, copper alloys etc., tend to corrode generally over the entire surface in the absence of crevices or galvanic effects. In such cases, corrosion is determinated by the rate at which dissolved oxygen can be delivered to the metal surface. Biological organisms present in the aqueous medium often have the potential to increase or decrease oxygen transport to the surface; consequently, these organisms have a role in increasing or decreasing general corrosion. Most MIC, however, manifests as localized corrosion because most organisms do not form in a continuous film on the metal surface. Microscopic organisms also tend to settle on metal surfaces in the form of discrete colonies or at least spotty, rather than continuous films. Biological organisms fall under two groups based on the type of corrosion they engender: (a) anaerobic corrosion (b) aerobic corrosion. Sulfate reducing bacteria (SRB) from the genera desulfovibrio are a typical example of anaerobic MIC. Aerobic sulfur oxidizing bacteria of the type thiobacillus can create an environment of up to 10 percent sulfuric acid, thereby encouraging rapid corrosion.
(From www.clihouston.com).
5. Read the text again. Decide whether the following statements are true or false.
1. Microbiological corrosion (MIC) refers to corrosion and ensuing alteration of metal surface caused by biological organisms.
2. Most MIC is defined as limited corrosion because most organisms do not form in a continuous film on the metal surface.
3. The type thiobacillus can form an environment of up to 70 percent ferrum acid.
6. Work in pairs. Translate the following text from English into Russian.
MIC is not caused by a single microbe, but is attributed to many different microbes. These are often categorized by common characteristics such as by-products (i.e., sludge producing) or compounds they effect (i.e. sulfur oxidizing). In a general sense, they all fall into one of two groups based upon their oxygen requirements; one being aerobic (requires oxygen) such as sulfur oxidizing bacteria, and the other being anaerobic, (requires little or no oxygen), such as sulfate reducing bacteria.
General corrosion affects the entire surface or at least the wetted surface. MIC, on the other hand, is very localized. It creates a nodule and a pit beneath the nodule. There can be only a few nodules or there can be many. Within these nodules microbes rarely work alone but operate as a mixed community of differing types and groups. The different microbes perform different functions within the community. This interaction allows a community to thrive in environments that are actually hostile to some of its members. For example, in an aerobic environment, anaerobic bacteria are generally inhibited or killed. But within a community the aerobic bacteria reside in the outer layer of the nodule consuming the oxygen in the water as it penetrates the nodule. Thus, the inner portion of the nodule experiences a reduced oxygen level allowing anaerobic bacteria to thrive.