What Is Galvanic Corrosion?
Galvanic corrosion (also called ‘ dissimilar metal corrosion’ or wrongly ‘electrolysis’) refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water.
When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.
Either (or both) metal in the couple may or may not corrode by itself (themselves). When contact with a dissimilar metal is made, however, the self-corrosion rates will change:
Corrosion of the anode will accelerate Corrosion of the cathode will decelerate or even stop. Galvanic coupling is the foundation of many corrosions monitoring techniques
The driving force for corrosion is a potential difference between the different materials. The bimetallic driving force was discovered in the late part of the eighteenth century by Luigi Galvani in a series of experiments with the exposed muscles and nerves of a frog that contracted when connected to a bimetallic conductor.
The principle was later put into a practical application by Alessandro Volta who built, in 1800, the first electrical cell, or battery: a series of metal disks of two kinds, separated by cardboard disks soaked with acid or salt solutions.
This is the basis of all modern wet-cell batteries, and it was a tremendously important scientific discovery, because it was the first method found for the generation of a sustained electrical current.
The principle was also engineered into the useful protection of metallic structures by Sir Humphry Davy and Michael Faraday in the early part of the nineteenth century.
The sacrificial corrosion of one metal such as zinc, magnesium or aluminum is a widespread method of cathodically protecting metallic structures.
In a bimetallic couple, the less noble material will become the anode of this corrosion cell and tend to corrode at an accelerated rate, compared with the uncoupled condition.
The more noble material will act as the cathode in the corrosion cell. Galvanic corrosion can be one of the most common forms of corrosion as well as one of the most destructive.
What are the Types of Galvanic Corrosion?
There are two primary types of galvanic cells that cause corrosion: the bi-metallic couple and the concentration cell.
A bi-metallic couple is like a battery, consisting of two dissimilar metals immersed in an electrolyte solution. An electric current (flow of electrons) is generated when the two electrodes are connected by an external, conductive path.
A concentration cell consists of an anode and cathode of the same metal or alloy and a return current path. The electromotive force is provided by a difference in concentration of the surfaces through the external path.
What happens During Galvanic Corrosion?
Galvanic Corrosion occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.
There are four elements necessary for corrosion to occur in a galvanic cell:
- Anode: The electrode where galvanic reaction(s) generate electrons – negative ions are discharged and positive ions are formed. Corrosion occurs at the anode.
- Cathode: The electrode that receives electrons – positive ions are discharged, negative ions are formed. The cathode is protected from corrosion.
- Electrolyte: The conductor through which current is carried.. Electrolytes include aqueous solutions or other liquids.
- Return Current Path: The metallic pathway connecting the anode to the cathode. It is often the underlying metal substrate.
All four elements (anode, cathode, electryolyte, and return current path) are necessary for corrosion to occur. Removing any one of these elements will stop the current flow and galvanic corrosion will not occur.
Substituting a different metal for the anode or cathode may cause the direction of the current to reverse, resulting in a switch to the electrode experiencing corrosion.
The Galvanic Series of Metals (right) lists metals and alloys in decreasing order of electrical activity. Metals nearer the top of the table are fewer noble metals and have a greater tendency to lose electrons than the more noble metals found lower on the list.
How To prevent Galvanic corrosion?
Unfortunately, galvanic corrosion is an expected challenge associated with nearly any seafaring vessel.
Because of the moisture and electrolytes resulting from the constant contact between a vessel’s metallic elements and seawater, along with warmer climates vessels may find themselves in, the process of galvanic corrosion can often occur at an increased rate.
However, there are a few simple guidelines that can help reduce your vessel’s risk of galvanic corrosion, potentially preventing it altogether.
Galvanic corrosion can be prevented by:
- Selecting materials with similar corrosion potentials.
- Breaking the electrical connection by insulating the two metals from each other.
- Applying coatings to both materials. The coating on the cathode is the most important and must be in good condition, otherwise the galvanic corrosion could be worsened.
- Separating the two materials by inserting a suitably sized spacer.
- Installing a sacrificial anode that is anodic to both metals.
- Adding corrosion inhibitor to the environment.
Choose Materials That Have Similar Corrosion Potentials
When selecting materials, such as bolts and screws, you can minimize corrosion potential by matching elements according to metal type.
For example, match the surface metal of the screw or bolt with the metal surface on which it will be fastened. Ideally, fasteners should be comprised of metal less likely to fall victim to corrosion.
Initial physical contact isn’t necessarily the cause of galvanic corrosion, especially when working with water-soluble corrosive elements. In all stages of installation, maintenance, and repair, the utmost attention should be paid to the selection and placement of materials.
Confirm that your marine solutions provider has a detailed understanding of galvanic corrosion and the potential ways certain materials may interact with each other, allowing you to avoid unexpected corrosion resulting from poorly considered material placement.
Ensure Proper Insulation Between Metals
Certain metals, such as copper, are primary conductors of electricity, making them particularly susceptible to electrolysis and galvanic corrosion. Leaving these metals unprotected is a near-guarantee for extensive corrosion damage.
To protect conductive metals such as these, use insulation whenever possible. Options for insulation include teflon or plastic tubing, as well as specific coatings, oils, greases, and paints.
Adequate Maintenance: Washing And Painting
Key to inhibiting the rate of corrosion in general on areas such as lifeboat decks and balconies are their adequate washing, chipping and painting.
Sea water that reaches these “spray” areas is massively harmful, passive metals such as aluminum are subject to stress corrosion and pitting, with cracks and erosion corrosion being visible.
Areas exposed to chlorine such as pool areas and spray areas, due to the high chlorine concentration in both pool and seawater, are particularly vulnerable to this. Removing and protecting from spray is key to maintaining your vessels condition.
What are the Examples of Galvanic Corrosion?
When two metals are in contact, the more active metal may undergo galvanic corrosion. For example, if zinc is in contact with mild steel, the zinc may undergo galvanic corrosion because it is the more active metal of the two.
Statue of Liberty
A spectacular example of galvanic corrosion occurred in the Statue of Liberty when regular maintenance checks in the 1980s revealed that corrosion had taken place between the outer copper skin and the wrought iron support structure.
Although the problem had been anticipated when the structure was built by Gustave Eiffel to Frédéric Bartholdi’s design in the 1880s, the insulation layer of shellac between the two metals had failed over time and resulted in rusting of the iron supports.
An extensive renovation was carried out with replacement of the original insulation with PTFE. The structure was far from unsafe owing to the large number of unaffected connections, but it was regarded as a precautionary measure to preserve a national symbol of the United States.
Royal Navy and HMS Alarm
In 1681, Samuel Pepys agreed to the removal of lead sheathing from English Royal Navy vessels to prevent the mysterious disintegration of their rudder-irons and bolt-heads, though he confessed himself baffled as to the reason the lead caused the corrosion.
The problem recurred when vessels were sheathed in copper to reduce marine weed accumulation and protect against shipworm. In an experiment, the Royal Navy in 1761 had tried fitting the hull of the frigate HMS Alarm with 12-ounce copper plating.
Upon her return from a voyage to the West Indies, it was found that although the copper remained in fine condition and had indeed deterred shipworm, it had also become detached from the wooden hull in many places because the iron nails used during its installation “were found dissolved into a kind of rusty Paste”.
To the surprise of the inspection teams, however, some of the iron nails were virtually undamaged. Closer inspection revealed that water-resistant brown paper trapped under the nail head had inadvertently protected some of the nails: “Where this covering was perfect, the Iron was preserved from Injury”.
The copper sheathing had been delivered to the dockyard wrapped in the paper which was not always removed before the sheets were nailed to the hull. The conclusion therefore reported to the Admiralty in 1763 was that iron should not be allowed direct contact with copper in sea water.
US Navy littoral combat ship Independence
Serious galvanic corrosion has been reported on the latest US Navy attack littoral combat vessel the USS Independence caused by steel water jet propulsion systems attached to an aluminium hull.
Without electrical isolation between the steel and aluminium, the aluminium hull acts as an anode to the stainless steel, resulting in aggressive galvanic corrosion
Corroding lighting fixtures
The unexpected fall in 2011 of a heavy light fixture from the ceiling of the Big Dig vehicular tunnel in Boston revealed that corrosion had weakened its support. Improper use of aluminium in contact with stainless steel had caused rapid corrosion in the presence of salt water.
The electrochemical potential difference between stainless steel and aluminium is in the range of 0.5 to 1.0 V, depending on the exact alloys involved, and can cause considerable corrosion within months under unfavorable conditions. Thousands of failing lights would have to be replaced, at an estimated cost of $54 million.
Lasagna cell
A “lasagna cell” is accidentally produced when salty moist food such as lasagna is stored in a steel baking pan and is covered with aluminium foil. After a few hours the foil develops small holes where it touches the lasagna, and the food surface becomes covered with small spots composed of corroded aluminium.
In this example, the salty food (lasagna) is the electrolyte, the aluminium foil is the anode, and the steel pan is the cathode. If the aluminium foil touches the electrolyte only in small areas, the galvanic corrosion is concentrated, and corrosion can occur fairly rapidly.
If the aluminium foil was not used with a dissimilar metal container, the reaction was probably a chemical one.
It is possible for heavy concentrations of salt, vinegar or some other acidic compounds to cause the foil to disintegrate. The product of either of these reactions is an aluminum salt. It does not harm the food, but any deposit may impart an undesired flavor and color.
Electrolytic cleaning
The common technique of cleaning silverware by immersion of the silver or sterling silver and a piece of aluminum in a hot electrolytic bath (usually composed of water and sodium bicarbonate, i.e., household baking soda) is an example of galvanic corrosion.
Silver darkens and corrodes in the presence of airborne sulfur molecules, and the copper in sterling silver corrodes under a variety of conditions.
These layers of corrosion can be largely removed through the electrochemical reduction of silver sulfide molecules:
the presence of aluminum in the bath of sodium bicarbonate strips the sulfur atoms off the silver sulfide and transfers them onto and thereby corrodes the piece of aluminum foil, leaving elemental silver behind. No silver is lost in the process.
What metals should not be used together?
To mitigate the risk of galvanic corrosion, there are certain metals that should not be used in conjunction. Below, we have provided a galvanic series corrosion chart showcasing the metal combinations that carry the highest risk.
Stainless Steel (Active) + Aluminum
Stainless steel acts as the cathode, and aluminum acts as an anode. Aluminum negatively reacts to stainless steel but with a conducive environment (marine environment); if the surface area of aluminum is larger than stainless steel, corrosion of the aluminum surface is likely to occur.
In this regard, there is a risk with hanging heavy aluminum equipment with stainless steel fasteners in any sea vessel. Gradually, the aluminum will start to corrode at the point of coupling and may fall if not quickly amended.
Copper + Steel
Let’s consider a case where a copper water tube is connected to a steel pipe through an adapter, and the water inside is an electrolyte. As steel is electro-negative to copper, electrons will flow from steel to copper, and the steel pipe will corrode.
For economic reasons, making an all-copper system can be difficult in most cases. Breaking the electrical contact through a thick six-inch coupling made of insulating material is an easier way to avoid galvanic corrosion.
Copper + Aluminum
As aluminum is lightweight, cheap, and has heat-transfer properties similar to copper, aluminum-copper joints are commonly used in most HVAC applications. Under a non-protective, damp environment, aluminum will act as a sacrificial anode, and copper as a cathode.
The aluminum will corrode, leading to joint failures. However, applying zinc coating to the joint saves the aluminum, as the zinc acts as a sacrificial anode to both copper and aluminum.
Tips to Avoid Galvanic Corrosion
While referring to our galvanic corrosion chart is one of the easiest and fastest methods of avoiding dangerous metal combinations, there are other ways of avoiding galvanic corrosion, including the following.
Choose the Right Metals
It’s essential to choose metals with small nobility differences (not more than 0.2 volts). For instance, there is a 0.15-volt difference between nickel and silver, which is fairly acceptable.
Minimize the Cathode Surface Area
The rate of galvanic reaction directly depends on the area of the cathode. Minimizing the large cathode surface area in comparison to the anode will lower the corrosion speed of the cathode.
Coat Cathodes and Anodes with Less Noble Metals
Coating the cathodes and anodes with a less noble metal will protect both from galvanic corrosion. The application of zinc on stainless steels is one of the common examples of galvanic coating.
Use Inhibitors
Nowadays, galvanic corrosion inhibitors are also popular in all industries. It’s a zinc-rich coating that can be applied between the joints of any dissimilar metals and electrical connections. The powerful formula easily absorbs moisture and any energy created by dissimilar metal reactions.
Use Sacrificial Anodes
Using a sacrificial anode that is more electrochemically reactive than the base material helps save the protected material. For instance, instead of zinc coating, a block of zinc positioned close to the galvanized steel plate can save it from galvanic corrosion.
Insulate Dissimilar Metals
Another method of avoiding galvanic corrosion is adding dielectric insulations between the coupling of two dissimilar metals to break the electrical contact.
For example, neoprene or nylon washers and bolt sleeves offer full isolation to the stainless-steel bolts from aluminum or galvanized steel members. However, when dealing with high-strength connections, you must check the dielectric material against the loading condition.