Uncoated vs Galvanized Rebar

Zinc coatings have a higher chloride (Cl -) corrosion threshold (2-4 times) than that of uncoated steel, significantly extending the time until corrosion initiation.

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.

Coating Defect Survey - DC Voltage Gradient

Today, DC voltage gradient surveys have evolved as the most accurate and economic means of locating coating defects.

When a DC current is applied to a pipeline in a similar manner to cathodic protection, ground voltage gradients are created due to passage of current through resistive soil. Well coated pipelines have a high resistance to earth. However, at locations where there are coating defects the resistance to earth is of such way that current can flow through the soil to be picked-up by the pipe. In the vicinity of these defects measurable voltage gradients can be detected at ground level. The larger the defect, the greater the current flow. Increasing the current flow also results in an increased voltage gradient for a given soil resistivity.

Atmospheric test-sites

Atmospheric corrosion tests consist in the exposure of samples to atmospheric conditions at test-sites and in their periodic evaluation. To expose materials and their surface coatings to all factors of atmospheric environment is only possible by their exposure to a realistic environment whilst studying all parameters of that environment. The performance of corrosion testing is experimentally and time demanding.

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.

Hydrogen Embrittlement (HE)

Hydrogen embrittlement (HE) is a process resulting in a decrease of the toughness or ductility of a metal due to the presence of atomic hydrogen. Hydrogen embrittlement has been recognized classically as being of two types. The first, known as internal hydrogen embrittlement, occurs when the hydrogen enters molten metal which becomes supersaturated with hydrogen immediately after solidification. The second type, environmental hydrogen embrittlement, results from hydrogen being absorbed by solid metals. This can occur during elevated-temperature thermal treatments and in service during electroplating, contact with maintenance chemicals, corrosion reactions, cathodic protection, and operating in high-pressure hydrogen.

Stainless Steel - Corrosion Resistance: Stress Corrosion Cracking (SCC)

Under the combined effects of stress and certain corrosive environments stainless steels can be subject to this very rapid and severe form of corrosion. The stresses must be tensile and can result from loads applied in service, or stresses set up by the type of assembly e.g. interference fits of pins in holes, or from residual stresses resulting from the method of fabrication such as cold working. The most damaging environment is a solution of chlorides in water such as sea water, particularly at elevated temperatures.

Stainless Steel - Corrosion Resistance: Crevice Corrosion

The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer on its surface, but it is possible under certain conditions for this oxide layer to break down, for example in reducing acids, or in some types of combustion where the atmosphere is reducing. Areas where the oxide layer can break down can also sometimes be the result of the way components are designed, for example under gaskets, in sharp re-entrant corners or associated with incomplete weld penetration or overlapping surfaces.

Stainless Steel - Corrosion Resistance: Pitting Corrosion

Pitting Corrosion

Under certain conditions, particularly involving high concentrations of chlorides (such as sodium chloride in sea water), moderately high temperatures and exacerbated by low pH (ie acidic conditions), very localised corrosion can occur leading to perforation of pipes and fittings etc. This is not related to published corrosion data as it is an extremely localised and severe corrosion which can penetrate right through the cross section of the component. Grades high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.

The Galvanic Corrosion Trap

Abstract:

The Galvanic Corrosion Trap. The wrong combination of metals can produce a corrosion cell of unequal voltage where one metal gives up its electrons and corrodes away. If the corrosion is not detected it will result in eventual failure of the equipment. Galvanic corrosion prevention is a design selection issue that one must always be aware of, as it can arise with the simplest of decisions. This article highlights a galvanic corrosion problem often seen in industry and sometimes missed by the person selecting equipment.

Keywords: rusting, corrosion scale, galvanic corrosion cell

source: http://lifetime-reliability.com/tip_016_pipe_galvanic_corrosion.html

Stainless Steel - Corrosion Resistance: Background

Although one of the main reasons why stainless steels are used is corrosion resistance, they do in fact suffer from certain types of corrosion in some environments and care must be taken to select a grade which will be suitable for the application. Corrosion can cause a variety of problems, depending on the applications:

Galvanic Corrosion

Metals themselves can also set up corrosion cells. When a pipe consists of only one type of metal, impurities in the pipe wall can develop into anodes and cathodes. Alternatively, when two dissimilar metals come into contact, galvanic corrosion will occur. Galvanic corrosion is often set up in the distribution system in meter installations and at service connections and couplings.

Pitting Corrosion: Mechanism

For a defect-free "perfect" material, pitting corrosion is caused by the ENVIRONMENT (chemistry) that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks.

The environment may also set up a differential aeration cell (a water droplet on the surface of a steel, for example) and pitting can initiate at the anodic site (centre of the water droplet).

Pitting Corrosion (2)

ASTM-G46 has a standard visual chart for rating of pitting corrosion

The shape of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the shape the size and the depth of penetration can be determined.

Source: http://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm

Pitting Corrosion (1)

Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting is one of the most damaging forms of corrosion. Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. This following photo show pitting corrosion of SAF2304 duplex stainless steel exposed to 3.5% NaCl solution.

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.