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./ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> 

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.

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.

When connecting pipes of various common piping materials their difference in electrode potential may cause galvanic corrosion and serious damage of pipes, valves and other equipment in the system.

Histories

From- corrosion doctors
by: John Morgan, 1988

Discussion on the AC effects, Causes of AC corrosion, Prediction of AC corrosion , Mitigation of AC corrosion

Discussion on the AC effects.
The intelligent pig survey in 1999, confirmed that the corrosion was ongoing in Longton. pH measurements were carried out on the exposed pipe at the sites of the corrosion pits in Autumn of 1999. Of the ten tests carried out, three were neutral and seven were very alkaline at 11-12. This indicates the cathodic reaction 2H2O+O2+e->40H- was occurring and that the cathodic protection system was working. The high pH also indicates that SRB activity was not the cause of the corrosion as SRB's require a near neutral environment to proliferate.

This article describes the history of the investigations and how the phenomena was ultimately attributed to the effects of induced AC. Discussion and background research findings is given on probable causes of AC induced corrosion, how it can be predicted and how the effects can be mitigated against.

Alternating current induced corrosion is a significant threat to integrity of buried pipelines. The pick up of AC by buried pipelines is a well known phenomenon, in particular in areas of dense infrastructure. Inductive, capacitive, or ohmic coupling to electrical systems like power transmission lines, railways, or grounding systems of power stations may introduce high AC voltages of technical frequencies (50 or 162/3 Hz in Europe). Voltages of more than 100 VAC versus remote earth are reported, which do not only pose a hazardous situation to humans getting in direct contact, but which may also cause high AC currents across the interface metal/soil at coating defects. Such currents were found to be potentially corrosive, also for cathodically protected (CP) pipelines.