In our last article, we reviewed electrochemical corrosion and introduced – differences and similarities
the major areas where cathodic protection is used to protect against corrosion in aggressive environments such as chloride-contaminated soil, water, and concrete.
In this article, we will examine how cathodic protection works.
Two types of cathodic protection
There are two types of cathodic protection: galvanic anode and impressed current cathodic protection.
Both provide cathodic protection currents from cathodic protection anodes that are in the same metal electrolyte as the one to be protected. Current flows from the anode into the electrolyte. It is discharged on the metal and controls corrosion. It must flow through the metal circuit (metal plus cables) and back to the anode to complete the circuit.
Galvanic anode cathodic protection (GACP)
Galvanic anode cathodic protection works as summarized above.
Anode materials are alloys of zinc, aluminum, or magnesium, all of which are more active metals than carbon steel. These more active metals preferentially corrode the steel when metallically bound to the steel in an electrolyte.
The corrosion current of the anode material is the protective current of the steel cathodic. The current passes through the electrolyte to the steel and controls its corrosion. The current in the metallic circuit returns to the anode.
You may have heard the term “sacrificial anodes”. However, although this term describes the anode materials and how they work (galvanic anodes are preferably corroded to steel), it was changed to “galvanic anodes” in Europe in the 1980s.
You may see galvanic cathodic protection denoted by the abbreviation SACP or GACP.
· How is galvanic anode cathodic protection used?
Offshore, anodes are typically cast on structural tubular cores that are welded to the offshore structure during onshore construction.
Offshore oil and gas pipelines are protected with aluminum alloy or zinc bracelet anodes, which are sealed over the protective cover and connected to the pipeline by short cables or welded joints. Such protection must last 30 years or more.
Short pipelines on land are often protected using magnesium anodes. These are cast on steel cores and connected to the pipeline by cables. In soils with low electrical resistance, extruded zinc ribbon or continuous casting and hot rolling are used. Zinc tape is widely used as a grounding electrode to reduce induced alternating current (AC) in buried pipelines.
