The reinforced concrete structures are designed and built for a service life, which is defined by the designer and is controlled by the manufacturer. For this period of time, the structure should not be so deteriorated that it does not fulfill any more its functions. Reinforcements corrode when they are in contact with a high amount of aggressive agents. This is the reason why, the prevention of reinforcement corrosion, in structures to be built, is obtained mainly by controlling the thickness and the quality of the concrete cover.

The presence of microbial cells on a metal surface, as well as their metabolic activities, can cause Microbiologically Influenced Corrosion (MIC). The forms of corrosion caused by bacteria are not unique. Biocorrosion results in pitting, crevice corrosion, selective dealloying, stress corrosion cracking, and under-deposit corrosion. There are some mechanisms of the causes of biocorrosion as follows.

MIC

There are biological organisms, called microorganisms or microbes, which influence corrosion. This type of corrosion is known as microbial influenced corrosion (MIC). The primary, and to many, the only concern is that this influence often results in an extremely accelerated rate of corrosion. It affects most alloys, such as ductile iron, steel (including stainless and galvanized), and copper; but it doesn't seem to affect titanium. The affect does vary between the different alloys with ductile iron corroding slower than steel. There was also a case where MIC caused the water in copper pipe to turn blue.

The tiny hole in the rolled groove in this picture is a sprinkler fitter’s nightmare: a small leak that potentially causes water damage to the building or contents where the sprinkler system is installed. This leak was caused by MIC (also known as bio-corrosion), which is the result of the metabolic action of microbial cells in contact with a metal surface, and can cause pitting, crevice corrosion, selective de-alloying, stress corrosion cracking, and deposit corrosion. Iron-oxidizing bacteria, such as Gallionella, Sphaerotilus, Leptothrix, and Crenothrix, are the leading sources of MIC.

Deposits can not only reduce heat transfer and cause restricting flow problems, but corrosion, specifically under-deposit corrosion can lead to some significant damage. This corrosion can occur both directly and indirectly.

One of the most prevalent materials in the reinforcement of concrete is steel; the main reason is because of its high tensile. A second almost as valid reason is that steel and concrete cement have almost the same thermal expansion coefficient. This means that when concrete/steel composite expands upon heating all the components of the composite. To insure and strengthen the bonding in a concrete steel reinforced composite the surface of the steel members are processed with the incorporation of contours (or ridged) that is called rebar.
As a result of the hydration reactions of cement, the pore solution of concrete tends to be alkaline, with pH values typically in the range 12.5-13.6. Under such alkaline conditions, reinforcing steel tends to passivate and display negligible corrosion rates.

Galvanized coatings are traditionally used for protecting steel rebars in concrete. In some cases, the galvanized steel rebars are also protected using calcium nitrite in the concrete mix. Data from galvanized iron immersed in model solutions of Ca(OH)2, reveals that calcium nitrite does not prevent zinc corrosion in the presence of chloride ions.

The most common and cost effective advanced corrosion protection method:

• reinforcement coatings,
• migrating corrosion inhibitors (MCI), and
• cathodic protection.

The pH condition of the environment is not sufficient for predicting the form in which an element will exist in natural waters. Consider whether the aqueous environment is well aerated (oxidizing) or polluted with organic wastes (reducing). In order to add this variable, we must expand the predominance diagram to include the reduction potential of the environment as well as the pH. This type of predominance diagram is known as a Pourbaix diagram.Eo-pH diagram, or pE-pH diagram.. Low E or pE values represent a reducing environment. High E values represent an oxidizing environment. The pE scale is intended to represent the concentration of the standard reducing agent (the e-) analogously to the pH scale representing the concentration of standard acid (H⁺). PE values are obtained from reduction potentials by dividing E^oby 0.059.

Knowledge of the pH condition of the environment is not sufficient for predicting the form in which an element will exist in natural waters. You have to take consideration whether the aqueous environment is well aerated (oxidizing) or polluted with organic wastes (reducing). In order to add this variable, we must expand the predominance diagram to include the reduction potential of the environment as well as the pH. This type of predominance diagram is known as a Pourbaix diagram.Eo-pH diagram, or pE-pH diagram.

Corrosion is the enemy of most civil engineering structures, particularly maritime structures. Combating corrosion is fundamental in providing durable structures which will not become a liability in the future. This may sound like a statement of the obvious but corrosion is pervasive and has some surprising forms.

Microbe Information
Microbes fall into two basic groups, aerobic and anaerobic. These two groups are based on the kind of environment they prefer, either with or without oxygen. Slime formers form a diverse group of aerobic bacteria. Common anaerobic bacteria include Sulfur/sulfate reducing bacteria (SRB's) and organic acid formers.

Cooling towers can be a critical process in many industrial production facilities. For reducing cost and getting more value out of cooling water system, engineers need multi-disciplinary knowledge of cooling water microbiology, chemistry and mechanics. In today's world of expensive energy, it is more vital than ever for heat exchange equipment to be kept free of insulating deposits that promote high energy consumption.

Mr. J. P. Snow, Chief Engineer of the Boston and Maine Railroad, has called attention to a very significant case of corrosion in connection with the destruction of some railroad signal bridges erected in 1894, and removed and scrapped in 1902. These structures were built at the time that steel was fast displacing puddled iron as bridge material.The result was that the bridges were built from stock material which was partly steel and partly wrought iron. The particular point of interest in this case lies in the fact that while some of the members of the bridge structures rusted to the point of destruction in eight years, others were in practically as good condition as on the day they were erected.