Reinforced Concrete: CP

The pH of environment in concrete usually is greater than 12.5, which causes a passive film on the surface of embedded steel reinforcement. However, this protective film breaks down when concrete reacts with atmospheric carbon dioxide or chloride ions (present in aggressive environments such as seawater or de-icing salt) penetrate into the hardened concrete and reach the surface of reinforcing steel.

How Do We Know When We Have Enough Cathodic Protection?

We know whether or not we have enough current by measuring the potential of the steel against a standard reference electrode, usually silver silver/chloride (Ag/AgCl sw.), but sometimes zinc (sw.).

Impressed Current Cathodic Protection Systems

Due to the high currents involved in many seawater systems it is not uncommon to use impressed current systems. Impressed current systems use anodes of a type that are not easily dissolved into metallic ions, but rather sustain an alternative reaction,

Basic Considerations When Designing Sacrificial Anode Systems

The electrical current which an anode discharges is controlled by Ohm's law; that is:


I=E/R

I= Current flow in amps
E= Difference in potential between the anode and cathode in volts
R= Total circuit resistance in ohms

Initially current will be high because the difference in potential between the anode and cathode are high, but as the potential difference decreases due to the effect of the current flow onto the cathode, current gradually decreases due to the polarization of the cathode. The circuit resistance includes both the water path and the metal path, including any cable in the circuit. The dominant value here is the resistance of the anode to the seawater.

How Does Cathodic Protection Stop Corrosion?

Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.

Usually this takes the form of galvanic anodes, which are more active than steel. This practice is also referred to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or pipeline from corrosion.

How Does Steel Corrode in Water?

To understand cathodic protection one must first understand the corrosion mechanism. For corrosion to occur, three conditions must be present.

1. Two dissimilar metals
2. An electrolyte (water with any type of salt or salts dissolved in it)
3. A metal (conducting) path between the dissimilar metals

The two dissimilar metals may be totally different alloys, such as steel and aluminum, but are more usually microscopic or macroscopic metallurgical differences on the surface of a single piece of steel.

Installation of Impressed Current Cathodic Protection System

The Division provides maintenance to 108 numbers of suspended marine structures all over Hong Kong. Suspended deck structures are normally built of reinforced concrete (RC) and are subject to reinforcement corrosion in the aggressive marine environment.

The durability of RC structures is largely depended on the conditions of the embedded reinforcement. In the tidal zone and the splash zone, water and oxygen contents are optimal for reinforcement corrosion to occur.

Zinc Ribbon Anode for Cathodic Protection

Source: http://www.farwestcorrosion.com/fwst/anodgalv/platt01.htm

Cathodic Protection

Cathodic protection is the most widely applied electrochemical corrosion control technique. This is accomplished by applying a direct current to the structure which causes the structure potential to change from the corrosion potential (E_corr) to a protective potential in the immunity region.

Cathodic Protection: Underground Bare Structures

The problems presented in attempting to provide cathodic protection for existing bare structures are much more difficult than those on coated structures. The major difficulty arises because of the much greater magnitude of current required. On a well-coated underground storage tank, it is not unusual to be able to provide protection with one or two galvanic anodes while it is not uncommon to have several rectifier units in a large complex tank farm.

Cathodic Protection: Underground Coated Structures

The economics favoring cathodic protection of cross country pipelines are so overwhelming, particularly on high pressure gas and oil lines, that practically every new line of consequence is provided with cathodic protection almost immediately after completion. The Department of Trans-portation has passed Federal legislation requiring that all oil, gas and gas products pipelines be cathodically protected and that the level of protection meets designated standards and regulations.

Impressed Current Anodes

When a rectifier type system is used, the current is derived from an outside source and is not generated by the corrosion of a particular metal as is the case with galvanic anodes. However, materials used as energized anodes do corrode. Thus, junk pipe and steel rails that were at one time used extensively as anode materials in rectifier type systems, corrode at the rate of 20 lbs. per ampere year.

Sacrificial Anode on the Hull of a Tug / Ship

Steel hulls of ships, which come into contact with seawater (an electrolyte) and consequently corrode (rust) are directly connected with base metals (e.g. zinc) to provide protection against corrosion.

Application Method of Cathodic Protection (3)

Regardless of the type of system used, current flows from the cathodic protection anode through the soil to the structure to be protected. Where this current flows onto a structure from the surrounding electrolyte (soil), the potential of the structure is made more negative. Cathodic protection is achieved when this change in potential is sufficient to arrest corrosion.

Application Method of Cathodic Protection (2)

In most cases, the rectifier type system is designed to deliver relatively large currents from a limited number of anodes, and the galvanic anode type system is designed to deliver relatively small currents from a large number of anodes.

Application Method of Cathodic Protection (1)

There are basically two methods of applying cathodic protection. One of these methods makes use of anodes which are energized by an external DC power source. In this type of cathodic protection system, anodes are installed in the electrolyte and are connected to the positive terminal of a DC power source and the structure which is to be protected is connected to the negative terminal of that source.

Pipe-to-Soil Potential as a Criterion of Cathodic Protection

It is almost universally accepted that a steel structure under cathodic protection is fully protected if the potential is at least 0.85-volt negative, referred to a standard copper-saturated copper sulfate electrode placed in the electrolyte immediately adjacent to the metal surface.

Galvanic and CP

Nature has endowed each metallic substance with a certain natural energy level or potential. When two metals having different energy levels or potentials are coupled together, current will flow. The direction of positive current flow will be from the metal with the more negative potential through the soil to that which is more positive. Corrosion will occur at the point where positive current leaves the metal surface. A dry cell battery is one example of a corrosion cell.

Cathodic Protection: Historical

The first application of cathodic protection (CP) can be traced back to 1824, when Sir Humphrey Davy, in a project financed by the British Navy, succeeded in protecting copper sheathing* against corrosion from seawater by the use of iron anodes. This limited use of CP on copper sheathing perdured and when wooden hulls were replaced by steel the fitting of zinc protector blocks on the sterns of naval vessels became traditional. These zinc slabs, although they offered some protection to steel hulls against local galvanic effects due to the presence of the bronze propellers, were generally not deemed to be effective.