Cathodic protection fundamentals

Cathodic protection
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Aluminium anodes mounted on a steel jacket structure
Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.
It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings.
A side effect of improperly performed cathodic protection may be production of molecular hydrogen, leading to its absorption in the protected metal and subsequent hydrogen embrittlement.
Cathodic protection is an effective method of preventing stress corrosion cracking.
Contents[hide]
1 Origins
2 Galvanic CP
3 Impressed Current CP
4 Testing
5 Galvanized Steel
6 Standards
7 External links
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Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.
It is recognizable by the presence of a buildup of corrosion at the joint between the dissimilar metals. For example, when aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium. This can be seen on the photo above where the aluminum helicopter blade has corroded near where it was in contact with a steel counterbalance.
Galvanic Series In Sea Water
Noble(least active)
PlatinumGoldGraphiteSilver18-8-3 Stainless steel, type 316 (passive)18-8 Stainless steel, type 304 (passive)Titanium13 percent chromium stainless steel, type 410 (passive)7NI-33Cu alloy75NI-16Cr-7Fe alloy (passive)Nickel (passive)Silver solderM-BronzeG-Bronze70-30 cupro-nickelSilicon bronzeCopperRed brassAluminum bronzeAdmiralty brassYellow brass76NI-16Cr-7Fe alloy (active)Nickel (active)Naval brassManganese bronzeMuntz metalTinLead18-8-3 Stainless steel, type 316 (active)18-8 Stainless steel, type 304 (active)13 percent chromium stainless steel, type 410 (active)Cast ironMild steelAluminum 2024CadmiumAlcladAluminum 6053Galvanized steelZincMagnesium alloysMagnesium
Anodic(most active)
The natural differences in metal potentials produce galvanic differences, such as the galvanic series in sea water. If electrical contact is made between any two of these materials in the presence of an electrolyte, current must flow between them. The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate will be. Metals or alloys at the upper end are noble while those at the lower end are active. The more active metal is the anode or the one that will corrode.
Control of galvanic corrosion is achieved by using metals closer to each other in the galvanic series or by electrically isolating metals from each other. Cathodic protection can also be used to control galvanic corrosion effects.


The scuba tank above suffered galvanic corrosion when the brass valve and the steel tank were wetted by condensation. Electrical isolation flanges like those shown on the right are used to prevent galvanic corrosion. Insulating gaskets, usually polymers, are inserted between the flanges, and insulating sleeves and washers isolate the bolted connections.
KSC conducts research on the effects of galvanic corrosion. The photo below shows the corrosion caused by a stainless steel screw causing galvanic corrosion of aluminum. The picture shows the corrosion resulting from only six months exposure at the Atmospheric Test Site.

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