What are some examples of types of corrosion that can affect a marine structure or vessel?
A combination of galvanic and atmospheric corrosion can occur where pilings connect to the structural framework of a pier.
In the marine environment, there are many types of corrosion that can affect structures and vessels.
Galvanic Corrosion
One type of electrochemical corrosion is induced when a difference exists in the electrical potential of dissimilar metals that are coupled together and immersed in an electrolyte. The process, called galvanic corrosion, is akin to a simple DC cell in which the more active metal becomes the anode and corrodes, where as the less active metal becomes the cathode and is protected.
A common example of this corrosion phenomenon is the increased rate of corrosion of steel in seawater when in contact with copper alloys. Galvanic attack can be uniform in nature or localized at the junction between the alloys depending on service conditions.
Galvanic corrosion can be particularly severe under conditions where protective corrosion films do not form or where they are removed by conditions of erosion corrosion. This process can also develop on a single metal surface when there is a difference in electrical potential between different locations on the surface of the metal.
Atmospheric Corrosion
So-called atmospheric corrosion is due primarily to moisture and oxygen but is amplified by contaminants such as sulphur compounds and sodium chloride, or salt spray.
Corrosion of steel on a seacoast is 400 to 500 times greater than in a desert. One researcher has shown that steel samples located eighty feet from a coastline corroded 12 times faster than those eight hundred feet from the coastline.
Atmospheric corrosion is also an electrochemical process, requiring the presence of an electrolyte. “Invisible” electrolytes in the form of a thin film tend to form on metallic surfaces when a certain critical humidity level is reached. For iron, this humidity level is approximately 60% in unpolluted areas.
The critical humidity level is variable - it depends on factors such as the corroding material, the nature of the products of corrosion, the presence of atmospheric pollutants, and the presence of surface deposits.
Fretting
Fretting refers to corrosion damage induced under load and in the presence of repeated relative surface motion (one common example of such motion is vibration). Pits, grooves, and/or oxide debris characterize this type of corrosion typically found in machinery, bearing assemblies, and bolted components.
Surfaces in contact that are exposed to vibration during transportation are often at risk of fretting corrosion. The damage occurs where two highly loaded surfaces which are not designed to move against each other come in contact. This removes any protective film on the metal surfaces and exposes fresh, active metal to the corrosive action of the atmosphere.
Other Types of Corrosion
Many other specific types of corrosion exist. Each has its own cause and each manifests itself in a different way. It should be noted that some authorities divide corrosion into categories based upon causation - an example would be electrochemical corrosion - while others divide corrosion based upon how corrosion manifests itself.
The leading proponent of this latter approach was Professor Mars Fontana. Professor Fontana divided corrosion into eight separate forms: uniform, galvanic, crevice corrosion, pitting, intergranular corrosion, selective leaching, erosion corrosion, and stress corrosion.
Regardless of approach, a licensed and knowledgeable corrosion engineer is able to both assess a particular corrosion problem and to recommend appropriate remedial and/or preventative measures.