Common Types of Corrosion
It’s the boat owner’s greatest fear yet also one of the most misunderstood. Corrosion expert Brian Gatt gives us a primer on the 5 most prevalent kinds.
If you know what type of corrosion you’re dealing with, you’ll be in a better position to take the correct steps to stop it from developing further and causing even more damage.
Corrosion falls into five main categories:
1. Galvanic: where two dissimilar metals submerged in the same electrolyte (in this case, seawater) are connected either mechanically or electrically. The more active (softer) metal will corrode to protect the nobler (harder) metal. NB. Refer to a simple galvanic series table, which grades metals by their reactivity. A simple example of this: A bronze propeller will corrode to protect a stainless-steel shaft. This type of corrosion can be prevented by using sacrificial anodes or impressed current systems such as MerCathode or Volvo ACP.
Galvanic corrosion of bronze p-bracket due to lack of cathodic protection.
Galvanic corrosion of bronze through hull fittings due to lack of cathodic protection
2. Electrolytic: corrosion induced by the presence of a stray electrical current. Depending on the type of stray current—alternating current (AC), direct current (DC) or radio frequencies (VHF, HF)—this type of corrosion can:
- Affect coatings, resulting in premature failure and increased marine growth
- Cause the aggressive disintegration of underwater metals
- Affect metallic components above the water line, e.g., “tea staining” of stainless-steel stays
- Increase or stop anode consumption by reversing or manipulating the effects of cathode/anode action
- Cause corrosion and rapid anode consumption within cooling systems
Electrolytic corrosion on a saildrive
Electrolytic corrosion on a propeller shaft
3. Oxidative: Commonly referred to as surface corrosion, this type occurs when a bare metallic surface reacts with oxygen, causing a local reaction and the general degradation of the surface. Galvanic and electrolytic activity can further exacerbate this reaction. Metals vulnerable to oxidation include aluminum, brass, and bronze. The best defence is using the appropriate coatings to isolate the substrate from the environment, thereby removing oxygen.
Oxidation of bare aluminium surface
4. Environmental: Corrosion can be exacerbated and/or caused by a number of environmental factors. These may include water temperature, salinity, water movement, humidity, heat, and exposure to oxygen. Understanding these factors will allow you to take the appropriate measures to protect against corrosion. These may involve:
- Applying protective coatings
- Selecting the right anode type
- Adding/subtracting anode mass and/or adjusting placement
- Electrical protection and design considerations
5. Atrolytic and crevice: a very specific type of corrosion that likes to attack stainless steel in particular. It usually occurs within closed systems and therefore goes unidentified until failure occurs. Micro-galvanic coupling between two metal surfaces with small amounts of water trapped in between causes ion transfer and the subsequent acidification of the water. This highly acidic water will bore holes into metal surfaces and can be particularly aggressive. It is commonly seen between pipe threads, stern tubes, and other closed-water systems where freshwater can’t be flushed constantly.
Atrolytic corrosion on stainless-steel hose fitting
Atrolytic corrosion on stainless-steel shaft within stern tubes
Want to know more about how to identify and rectify corrosion? Feel free to drop us a line at firstname.lastname@example.org
by Brian Gatt