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Corrosion in Boats FAQ

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Corrosion is the process of degrading or weakening of a metal by something that reacts with the metal.
Corrosion can occur from exposure to air in the form of rust. However, when the metal is introduced to water it speeds up this reaction. Also, if minerals in the water such as salt, calcium, etc. are present the speed of the process increases again. See Sacrificial Zinc Anode
Every combination of metals in different locations on our boats corrodes for different reasons. All forms of corrosion are the same electro-chemical process caused by different circumstances
The two basic types are erosion and electro-chemical.
Erosion is a mechanical form of corrosion that is caused by friction.
Sandy water flow acting just like sand paper and the pitting caused by high speed water flow.
Electro-chemical corrosion is the number one type of corrosion in boats. All corrosion except mechanical erosion is electro-chemical in nature.
All particles of basic elements have positive or negative electrical charges. When materials have the same electrical charge, no corrosion will happen and they are said to be compatible. Certain types of stainless steel and bronze are compatible.
If two materials have a sufficient different charge, then a flow of current (electrons) will occur. This is the principle that makes a dry cell battery work. Dry cells use carbon and another metal to generate an electrical current flow between the negatively charged carbon, and a highly charged metal.
Electrolysis is a reaction between metal and electrical energy. Electrolysis occurs when electrical current is "leaking" into the water and can come from a variety of things such as improperly grounded electrical devices and power circuits, old electrical devices in contact with the water, batteries in boats, etc. This process produces a very quick reaction.
Substances that conduct electric current are called electrolytes. They are formed as a result of a separation into positively and negatively charged particles called ions, which migrate toward and ordinarily are discharged at the negative and positive terminals of an electric circuit, respectively. The most familiar electrolytes are acids, bases, and salts, which ionize when dissolved in such solvents as water. Many salts, such as sodium chloride, behave as electrolytes when dissolved in water.
Pure water will not behave as an electrolyte. Seawater is an excellent electrolyte. The presences of a large amount of dissolved salts, sodium chloride (NaCl), that are ionized make it an excellent conductor. The chloride ions is particularly aggressive as it causes a breakdown of passivity causing pitting, crevice corrosion and stress corrosion cracking on most stainless steel grades.
Metals are rated on what is called a Scale of Nobility or Nobel Scale, in the order of the materials ability to resist corrosion. A metal that has a neutral or negative electrical potential will not generate a flow of positive ions, and is called "noble." The reverse of this is the least noble metal, which has a high positive charge, and which will generate an electrical current. These include such metals as zinc, unalloyed aluminum and copper, iron and steel. Graphite and carbon bottom out the list, being the most highly charged metals.
Each metal when placed into an electrolyte, water, seawater, soil or any other good electrolyte, has an electric potential. This potential is different for every metal. The galvanic series is a list of this potential arranged from the most "active", lower potential to the most "noble", higher potential. The terms "noble" and "active" means that when two metal are connected the most "active", with the lower potential will corrode, while the metal with the higher potential, the "noble" material will not corrode. The galvanic series, the potentials, are different in different electrolyte.
Stray current corrosion occurs when metal with an electrical current flowing into it is immersed in water that is grounded (such as in any lake, river, or ocean). The current can leave the metal and flow through the water to ground. This will cause rapid corrosion of the metal at the point where the current leaves. Stray direct current (or battery current) is particularly destructive. Stray current corrosion can cause rapid deterioration of the metal. If the metal in question happens to be an aluminum part like your drive unit, it can be destroyed in a matter of days.
Stray current corrosion is different from galvanic corrosion in that galvanic corrosion is caused by connections between dissimilar metals of your boat’s drive components, and utilizes the electrical potential of those dissimilar metals. Electrons flow from one dissimilar metal (the anode) to another dissimilar metal (the cathode). In stray current corrosion, electricity from an outside source flows into your boat’s metal components and out through the water for a ground.
Your boat may be sitting between a boat leaking DC current and the best ground for that current. Rather than the DC current moving through the water to ground, your boat could provide a path of lower resistance. The DC current could enter a throughhull fitting, travel through the bonding system, and leave via your drive to the ground. Corrosion occurs at the locations where DC current leaves metal and enters water.
Stray current can come from an outside source either internal or external to your boat. Internal sources involve a short in your boat’s wiring system, such as a poorly insulated wire in the bilge, an electrical accessory that may be improperly wired, or a wire with a weak or broken insulation that is intermittently wet. External sources are almost always related to shorepower connections.
A boat with internal stray current problems can cause accelerated corrosion to other boats plugged into the same shorepower line if they provide better ground. The stray current would be transmitted to other boats through the common ground wire, but can and should be blocked by installing a galvanic isolator.
Supposed you cruise back to your marina, and plug into shorepower to recharge batteries using your automatic trickle charger. Later, a large steel hulled boat (with scratched and scraped paint) ties up next to your boat. This boat is also plugged into shorepower. A battery has just been formed—the large steel hull and your small aluminum drive connected by the shorepower and ground wire. Depending on the proximity, relative sizes, and how long your neighbor is ashore, when you go out the next weekend you may find your drive highly deteriorated. This unfortunate scenario can also be prevented by the installation of a galvanic isolator.
Galvanic isolators are solid-state devices that are part of a series connected in line to the boat's green safety ground lead ahead of all grounding connections on the boat. This device functions as a filter, blocking the flow of destructive low voltage galvanic (DC) currents but still maintaining the integrity of the safety grounding circuit. These currents could cause corrosion to your underwater metals; through hulls, propeller, shaft etc. Boats in a marina plugged into shore power all act as a giant battery. They are all connected together by the green shore power ground wire, which is (or should be) connected to their DC grounds, engine block, and bonded underwater metals. If the boats are in salt water then that forms an electrolyte and the dissimilar metals connected together act as a battery, causing corrosion.
The galvanic isolator has two pairs of diodes set up so that a voltage of about 1.2 volts is required to cause them to conduct. As most DC voltages caused by galvanic action will be less than this, they are blocked. Good quality isolators also contain a capacitor, which only conducts AC current, as a backup. Normally no AC current is carried on the shore power ground wire, but it has to be able to carry the full load of the circuit in the event of a fault. Therefore it is important to have a good quality unit that will not overheat when required to carry the rated load. Some heat will be generated by the voltage drop and the unit must be able to withstand this. As the galvanic isolator fulfills such a key function in the AC circuit it is only prudent to use the best quality unit available.
Safety regulations require a three-wire cable for carrying shorepower aboard any boat, and that one of these leads grounds all electrical and propulsion equipment to shore. This safety procedure reduces the danger of shock, but also connects the underwater metal components on your boat with metal on neighboring boats using shorepower, steel piers, and metal objects on shore that extend into the water. This interconnecting of dissimilar metals allows destructive galvanic currents to flow between them. If these currents are allowed to continue, your drive unit will experience severe corrosion damage in a very short time—as little as a few days.
There is a common misconception that you can overprotect your drive by using too many zinc or sacrificial aluminum anodes. This is not true. The corrosion potential of any metal is a voltage that can be measured by a reference electrode. Such measurements in water commonly are made with a silver/silver chloride reference electrode. The corrosion potential of a sacrificial anode is a characteristic value for that metal, and it does matter if you have one piece of the metal or 100 pieces. The corrosion potential stays the same. Of course, 100 anodes would be expensive, heavy, and a considerable drag under water. Only by increasing the corrosion potential by using a different anode material (such as magnesium in seawater) can you overprotect your drive.
Electrolysis is the result of stray current. Galvanism and electrolysis produce similar results, only they have different causes. We would be better using the term "stray current corrosion" because this would identify the cause.
Galvanic corrosion occurs when two different types of metals are put into physical contact with each other while they are immersed in an electrolyte, such as seawater. The reason corrosion takes place between two different, coupled metals is that a voltage difference (potential) exists between them. For example, many mid-sized sailboat propulsion shafts are made of 300 series stainless steel, which possesses an average voltage of -0.07V. These sailboat shafts are frequently connected to bronze propellers, which possess an average voltage -0.26V. The voltage difference between these two coupled metals is approximately .19V. The most successful method in protecting both metals is to connect both of them to a third metal that is more anodic than the original coupled metals. In this case, the third metal becomes a “sacrificial anode” and the two original metals will remain passive.
The purpose of a electrical bonding system is to ensure that metals on a boat are at the same voltage potential. This provides two major benefits: electrical safety for the above water metals, and corrosion protection for the underwater metals. When bonded, underwater metals cannot be damaged by stray electrical currents originating from within the boat (e.g., a defective bilge pump, float switch or wire splice). When the bonding system is then connected to sacrificial anodes, all bonded underwater metals are protected against galvanic corrosion.
Metals are ranked on a Noble Scale as to how well they resist corrosion. If two types of metals are present (Stainless Steel & Zinc for example), the metal that resists corrosion the least (Zinc) will deteriorate before the metal that resists corrosion the best (Stainless Steel). Therefore, the Sacrificial Zinc Anode is designed to protect a unit from corrosion because the Zinc will deteriorate before the Stainless Steel. It is crucial to inspect the unit and Sacrificial Zinc Anode routinely, and if the Sacrificial Zinc Anode is corroded or deteriorated to half of its original size or less, it should be replaced. The more minerals, such as salt content, the quicker the corrosion will occur, therefore, the Sacrificial Zinc Anode must be checked more often. In areas of high mineral content, it is suggested to check the Anode every couple months.
The Sacrificial Zinc Anode does offer some protection against electrolysis. However, a Sacrificial Zinc Anode can be completely deteriorated and damage to the unit can occur in as little as a couple weeks or less. This is largely dependent on the amount of electrical current in the water and how well the current travels through the water. Electricity uses particles in the water to travel, not the water itself, so the more minerals, such as salt, the further and quicker electricity can travel through the water to attack the unit.
There are basically two options to protect your equipment from electrolysis. One is to keep replacing the Sacrificial Zinc Anode as often as needed. Two is to contact a qualified electrician to find the source of the stray voltage and eliminate it. Option two is a much more effective and reliable option than option one since damage can occur before an Anode can be changed and it becomes costly to continually change the Anode.
Boats and vessels that are in salt water use either zinc alloy or aluminum alloy. If boats are only in fresh water, a magnesium alloy is used. Magnesium has one of the highest galvanic potentials of any metal. If it is used in a salt water application on a steel or aluminum hull boat, hydrogen bubbles will form under the paint, causing blistering and peeling.

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