Anode material steadily oxidizes to metal ions that move through the electrolyte. (USPS Education Department)
Marine corrosion is a challenge for all boat owners. It has implications in both boat maintenance and boat safety. You can only cope with corrosion by diligence and proper maintenance. Certain kinds of corrosion can devalue your investment in a matter of months or even days.
To protect your boat it is critical to understand the types of corrosion. These are:
- Simple Corrosion
- Galvanic Corrosion
- Electrolytic or Stray Current Corrosion
- Crevice Corrosion
In this first article in a series, we will look at simple corrosion and galvanic corrosion.
Simple Corrosion
Simple corrosion is the process of oxidation of marine metals. Oxidation occurs when molecules on a metal surface react with oxygen. For example, iron/steel alloys develop a layer of iron oxide or rust. Aluminum metal becomes aluminum oxide, a white or gray powder.
Painting, galvanizing, plating, chemical passivation and use of alloys slow the progress of simple corrosion.
Galvanic Corrosion
The more any two metals are separated in this chart, the worse the corrosion of the less noble metal when immersed in electrolyte. (USPS Education Department)
Galvanic corrosion is an electrochemical reaction between two of more different metals. Galvanic corrosion is differentiated from stray current corrosion in that there is no applied source of electrical current.
Galvanic corrosion involves the same DC current generation process that is found in a storage battery. It is an electrical exchange between connected metals, whether touching or in a conductive solution. The conductive solution may be salt water, “hard” fresh water with a high concentration of dissolved mineral ions, or polluted fresh water.
Dissimilar metals used in marine components in a conductive solution form a galvanic corrosion cell, like a battery. Electrons flow from the more chemically active anode metal (e.g., outboard or stern drive leg) to the less chemically active cathode (e.g., stainless steel propeller, etc.). The anode metal that loses electrons becomes a positively charged metal ion and reacts with oxygen, becoming a metal oxide.
The oxide may become a stable coating or may dissolve away as a powder or rust flakes. The base metal then becomes pitted or crevised. The stray ions attracted toward the cathode metal become alkaline hydroxides. The rate of corrosion of the less noble, higher potential metal depends on its potential difference with other metals in contact with the solution.
For example, if zinc is connected to aluminum, the zinc corrodes. This is what makes zinc a good sacrificial anode, protecting submerged aluminum components such as lower units. If aluminum is connected to copper or bronze, the aluminum corrodes rather rapidly. This is why pressure-treated wood (containing copper) should never be used in aluminum boats as decking or in transoms.
Rate factors
The rate of galvanic corrosion is temperature dependent. The total charge in the electrolytic reaction between the two metals will always balance. A very large anode connected to a small cathode will corrode very slowly. However, if a very large cathode is connected to a small anode, the small anode will corrode rapidly.
Practical considerations
Nearby boats with steel alloys overwhelmed the anode on this kicker and destroyed the lower unit while docked. (courtesy Boat-US)
You may do everything right to limit galvanic corrosion, but that doesn’t mean that boats and metal structures near you are protected. You may have no steel on your boat, but if you dock near stainless steel or mild steel-containing structures or boats and connect to shore power, you must take special precautions.
The green shore power ground wire connects your aluminum components with the grounded, submerged steel and submerged metals on nearby boats. This galvanic cell can overwhelm you anodic protection system. Your submerged aluminum components will sustain serious damage unless you install a galvanic isolator. Any submerged stainless steel components will accelerate galvanic corrosion of submerged aluminum components, especially when connected to the engine ground. Most submerged metals form small anodes and cathodes on their surfaces because of alloy composition, impurities or surface disruption.
Next time, we will look at measuring your vessels susceptibility to galvanic corrosion, as well as preventing or slowing down the process
Sources:
https://www.boatus.com/seaworthy/magazine/2015/july/marine-corrosion-101.asp
http://www.boatus.com/boattech/articles/marine-corrosion.asp
https://www.goldeagle.com/tips-tools/understanding-corrosion-around-boat/
https://www.jetdock.com/knowledge-center/prevent-aluminium-boat-corrosion.asp
Learning More
A nine-session course is well worth the time and effort expended to manage your electrical system for safety and longevity. (USPS Education Department)
Does marine corrosion seem complicated to you? Are you not sure where to start in applying best practices to your boat? United States Power Squadrons has a solution for you. Marine Electrical Systems is an elective course dedicated to onboard electrical power–-both AC and DC. You will learn about electrical circuits, wiring, terminating, and routing wiring for safety and trouble-free operation. You will learn how to use multimeters and tools to check, diagnose and repair electrical problems. You also will learn about galvanic corrosion, stray currents, isolation, grounding, and safety of electrical systems.
The course material is organized in seven chapters and is intended to be presented in the classroom over nine two-hour sessions:
Day 1 – Introduction, Chapter 1: Properties of Electricity
Day 2 – Chapter 2: Boat Electrical Wiring Practices
Day 3 – Chapter 3: Direct Current Power
Day 4 – Chapter 4: Alternating Current Power
Day 5 – Chapter 5: Galvanic and Stray Current Corrosion
Day 6 – Chapter 6: Lightning Protection
Day 7 – Chapter 7: Troubleshooting Part 1; on-the-boat optional
Day 8 – Chapter 7: Troubleshooting Part 2
Day 9 – Review and Examination
This course is the best way to learn about electrical factors that can save you from sustaining considerable damage, especially if you berth your boat at a marina, or if you make any repairs or modifications to your OEM electrical system.
If you are interested in taking this class, indicate your interest using the contact page.
That’s good to know that there are several different methods by which your boat could be corroded. I am considering picking up a boat, but I wouldn’t want to have it corrode if I am unable to use it for a while. I’ll have to make sure that I inspect it for signs of corrosion damage so it can know when I need to have it repaired if I end up getting one.
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