In the marine environment, many types of corrosion can affect structures and vessels.
We have worked all over the world on corrosion problems, and we can help you identify the source and propose some solutions.
Below are some examples of corrosion types and their causes.
- Galvanic corrosion, a 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 (such as saltwater). This corrosion process is akin to a simple direct-current cell, in which the more-active metal becomes the anode and corrodes, while the less-active metal becomes the cathode and is protected. (The increased rate of steel corrosion in seawater when in contact with copper alloys is a common example of galvanic corrosion.) 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 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 – due primarily to moisture and oxygen – is amplified by contaminants, such as sulfur compounds and sodium chloride, as well as by salt spray. Research has shown that corrosion of steel on a seacoast can be 400 to 500 times greater than in a desert; also, in one study steel samples located 80 feet from a coastline corroded 12 times faster than those 800 feet from shore. Atmospheric corrosion – an electrochemical process – requires 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 percent in unpolluted areas, but the critical humidity level is variable, depending 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 refers to corrosion damage induced under load and in the presence of repeated relative surface motion, such as vibration. Pits, grooves, and/or oxide debris characterize this type of corrosion, which typically is found in machinery, bearing assemblies, and bolted components. The damage occurs where two highly loaded surfaces – which are not designed to move against each other – come in contact, removing any protective film on the metal surfaces and exposing fresh, active metal to the corrosive action of the atmosphere. Surfaces in contact that are exposed to vibration during transportation are often at risk of fretting corrosion.
Many other specific types of corrosion exist, with numerous causes and manifestations. Some experts divide corrosion into categories based upon causation (e.g., electrochemical corrosion), while others use categories based on how corrosion manifests itself. (The leading proponent of the latter approach, former Ohio State University Professor Mars Fontana, divided corrosion into eight separate forms: uniform, galvanic, crevice corrosion, pitting, inter-granular 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 recommend appropriate remedial and/or preventative measures.
If you need help identifying the source of your corrosion problems, please call us!