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.

Potential Adjustment Protection

Normally passive metals when exposed to oxidizing halide environments can suffer from pitting and crevice corrosion. By the application of a direct current the structure potential can be shifted back to the passive region from the transpassive region.

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.

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 of crude oil tanker's cargo tank bottom plate

Source: http://www.jfe-steel.co.jp/en/release/2008/080129.html

Corrosion environment with crude oil tanker's cargo tank

Source: http://www.jfe-steel.co.jp/en/release/2008/080129.html

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.

Oxygen concentration, temperature and corrosion of steel pipes

Effects of oxygen concentration and temperature on the corrosion of low-carbon steel pipes

Source: http://www.engineeringtoolbox.com/oxygen-steel-pipe-corrosion-d_1170.html

Pitting Corrosion

Pitting corrosion on outside surface of cast iron bathtub

Source: http://metallurgist.com/html/corrosion.htm

Undewater Corrosion

severe corrosion damage noted on marine structural pipe pile after 15 years of service

Source: http://www.westcoastcorrosion.com/services.htm

Erosion Corrosion

Rapidly flowing solutions can often disrupt adherent surface films and deposits that would otherwise offer protection against corrosion. Thinning or removal of surface films by erosion from the flowing stream results in accelerated corrosion, called erosion-corrosion. The attack is accelerated at elbows, tube constrictions, burrs, and other structural features that alter flow direction or velocity, and increase turbulence.

Anodic Protection

In circumstances where cathodic protection is not practical, such as in strongly alkaline or acidic environments, anodic protection is a useful corrosion control technique. Specifically, in metal-environment conditions where active-passive behaviour is demonstrated, anodic protection is usually effective.

Sacrificial Anode on Water Pipe

Sacrificial Anode on Water Main Pipe, Peterborough, Canada

Source: http://www.peterboroughutilities.ca/Water/Construction_Projects/Cathodic_Protection.htm

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 (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.

Corrosion Caused by Differences in the Electrolyte

Corrosion can occur due to differences in the electrolyte. These differences may be in the soil resistivity, oxygen concentrations, moisture content and various ion concentrations. The variations produce current flow from one location, through the electrolyte, to another portion of the same metallic structure.

GALVANIC CORROSION OF A SINGE METAL

The same metallic structure, when placed in an electrolyte (e.g. soil) can develop dif-ferences in potential as a result of metal grain composition, milling imperfections, scratches, threads, etc., being exposed.

BI-METALLIC CORROSION

Current will be generated when two dissimilar metals are electrically connected and immersed in an electrolyte. One of the metals will corrode.