A galvanic or sacrificial anode is the key component in a cathodic protection system used to inhibit the oxidation of metal structures exposed to water. Water pipelines, tanks, and marine vessels are fitted with forms of cathodic protection to prevent their metal surfaces from corroding. In order to understand the function of a galvanic anode it is helpful to review the corrosion process.
When water comes into contact with an unprotected metal surface, it acts as an electrolyte forming a voltaic cell across areas of the submerged metal. Then chain, through a of electrochemical reactions that vary depending on the alloy involved, those metallic areas with relatively negative voltage potential (the cell anode) accumulate electrons and combine chemically with oxygen to form rust, ultimately resulting in the decay of the metal structure. This process is accelerated in the presence of salt water.
Cathodic protection systems work by adding an alternative anode material to the water-metal voltaic cell to attract the electrons involved in the corrosion process. Commonly made of zinc or aluminum, this alternative galvanic anode draws electrons away from the protected structure and undergoes the oxidizing corrosion in its stead. Due to this sacrificial deterioration, the galvanic anode must be replaced periodically. Hence it is called “sacrificial.”
Sacrificial anodes on the hull of a tug
There are two main types of cathodic protection systems: passive and impressed current cathodic protection (ICCP) systems. A passive system relies purely on the difference of electrochemical potential between the sacrificial anode and the cathodic structure. For some larger structures this inherent voltage potential is insufficient to ensure electron diversion for the entire protected structure. For these cases, an ICCP system is more suitable. ICCP applies an external DC power supply to the two metals in order to enforce the sacrificial anode’s polarity.
Marine, naval and offshore industries employ the engineering services of companies like Cathodic Marine Engineering (www.cathodicme.com) to design, build, install, and maintain cathodic protection systems for their ships, rigs, and other marine applications.
Expert Marine consultant Jamie D shows the 2 important functions of the often misunderstood sacrificial anode on the outboard engine, as well as what you need to do to keep this important part in perfect operational shape to save your outboard from corrosion.