Galvanic Corrosion and The Little Charges that Save Our Ships

Corrosion has been a significant problem since the use of metals in pipes and marine structures began. In fact, chemists and engineers have studied the problem since Sir Humphry Davy presented the first research on the subject to the Royal Society in 1824.

Today, metals are better protected than ever. A chemical process, called galvanic corrosion, bonds a sacrificial metal onto the surface of the core structural metal. Corrosion then attacks the sacrificial metal first, sparing the core structure.

How Cathodic Protection Systems Work

Galvanic corrosion is a process that submerses two metals in an electrolyte substance. The sacrificial, more easily corroded, metal is bonded to the core structure metal using electric current.

This simple but effective technique enables a sacrificial metal to become an anode, a positively charged electrode. A protected metal then becomes a cathode, a negatively charged electrode.

The Cost of Corrosion

The economic and personal costs of corrosion are high and can be devastating both for public health and the environment. The following applications are just a few of the more important uses for galvanic corrosion.
• Pipelines. At the top of the list of structures requiring significant protection from corrosion are pipelines carrying potentially valuable or harmful substances. Whether natural gas, oil, water or chemicals, structural integrity is of the highest importance.
• Marine Structures. The lifespans of structures surrounded by salt water, such as jetties and other harbour structures, are greatly improved by galvanic corrosion techniques.
• Boats and Ships. The first work on cathodic protection systems addressed the integrity of the earliest iron vessels. Now, to protect the vessel’s metal, galvanic anodes are attached to the hulls and replaced regularly.

Problems Associated with Galvanic Corrosion

The galvanic corrosion process is far from perfect. In addition to regular maintenance, the following are some of the more immediate challenges.

•Escape of Hydrogen Ions. If the process is improperly applied, it will produce atomic hydrogen. Hydrogen gas cannot penetrate the metals, but the smaller hydrogen atoms can do so. This causes an effect known as “hydrogen embrittlement” which causes early metal failure.
•Disbonding. Excessive polarization of the anode and cathode systems occurs in metals that have imbalanced charges. The bonding between the metals is thereby compromised.
•Shielding Problems. Cathodic systems are compromised if substances such as tapes, sleeves or solid film coatings are used on pipes. Those substances are highly resistive to the electrical charges that keep the cathodic system intact.

Cathode protection systems may seem complicated and esoteric. When faced with the environmental disaster of a chemical pipeline break or the sinking of a ship, however, it is good for everyone to have a better idea of what happened, who is at fault and how to fix it.