Overview[ edit ] Dissimilar metals and alloys have different electrode potentials , and when two or more come into contact in an electrolyte, one metal acts as anode and the other as cathode. This leads to the metal at the anode corroding more quickly than it otherwise would and corrosion at the cathode being inhibited. The presence of an electrolyte and an electrical conducting path between the metals is essential for galvanic corrosion to occur. The electrolyte provides a means for ion migration whereby ions move to prevent charge build-up that would otherwise stop the reaction.
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Overview[ edit ] Dissimilar metals and alloys have different electrode potentials , and when two or more come into contact in an electrolyte, one metal acts as anode and the other as cathode. This leads to the metal at the anode corroding more quickly than it otherwise would and corrosion at the cathode being inhibited.
The presence of an electrolyte and an electrical conducting path between the metals is essential for galvanic corrosion to occur.
The electrolyte provides a means for ion migration whereby ions move to prevent charge build-up that would otherwise stop the reaction. In some cases, this type of reaction is intentionally encouraged. For example, low-cost household batteries typically contain carbon-zinc cells. As part of a closed circuit the electron pathway , the zinc within the cell will corrode preferentially the ion pathway as an essential part of the battery producing electricity. Another example is the cathodic protection of buried or submerged structures as well as hot water storage tanks.
In this case, sacrificial anodes work as part of a galvanic couple, promoting corrosion of the anode, while protecting the cathode metal. In other cases, such as mixed metals in piping for example, copper, cast iron and other cast metals , galvanic corrosion will contribute to accelerated corrosion of parts of the system. Corrosion inhibitors such as sodium nitrite or sodium molybdate can be injected into these systems to reduce the galvanic potential.
However, the application of these corrosion inhibitors must be monitored closely. If the application of corrosion inhibitors increases the conductivity of the water within the system, the galvanic corrosion potential can be greatly increased. Acidity or alkalinity pH is also a major consideration with regard to closed loop bimetallic circulating systems.
Should the pH and corrosion inhibition doses be incorrect, galvanic corrosion will be accelerated. In most HVAC systems, the use of sacrificial anodes and cathodes is not an option, as they would need to be applied within the plumbing of the system and, over time, would corrode and release particles that could cause potential mechanical damage to circulating pumps, heat exchangers, etc.
Even when the protective zinc coating is broken, the underlying steel is not attacked. Instead, the zinc is corroded because it is less "noble"; only after it has been consumed can rusting of the base metal occur.
By contrast, with a conventional tin can , the opposite of a protective effect occurs: because the tin is more noble than the underlying steel, when the tin coating is broken, the steel beneath is immediately attacked preferentially. Statue of Liberty[ edit ] Galvanic corrosion in the Statue of Liberty Regular maintenance checks discovered that the Statue of Liberty suffered from galvanic corrosion A spectacular example of galvanic corrosion occurred in the Statue of Liberty when regular maintenance checks in the s revealed that corrosion had taken place between the outer copper skin and the wrought iron support structure.
An extensive renovation was carried out requiring complete disassembly of the statue and 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. In an experiment, the Royal Navy in had tried fitting the hull of the frigate HMS Alarm with 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".
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 was that iron should not be allowed direct contact with copper in sea water. Without electrical isolation between the steel and aluminium, the aluminium hull acts as an anode to the stainless steel, resulting in aggressive galvanic corrosion.
Improper use of aluminum in contact with stainless steel had caused rapid corrosion in the presence of salt water. 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 aluminum. If the aluminum foil touches the electrolyte only in small areas, the galvanic corrosion is concentrated, and corrosion can occur fairly rapidly.
If the aluminum 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. 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 which is less noble than either silver or copper 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 a much more reactive metal , leaving elemental silver behind.
No silver is lost in the process. Electrically insulate the two metals from each other. If they are not in electrical contact, no galvanic coupling will occur. This can be achieved by using non-conductive materials between metals of different electropotential.
Piping can be isolated with a spool of pipe made of plastic materials, or made of metal material internally coated or lined.
It is important that the spool be a sufficient length to be effective. For reasons of safety, this should not be attempted where an electrical earthing system uses the pipework for its ground or has equipotential bonding. Metal boats connected to a shore line electrical power feed will normally have to have the hull connected to earth for safety reasons. However the end of that earth connection is likely to be a copper rod buried within the marina, resulting in a steel-copper "battery" of about 0.
For such cases, the use of a galvanic isolator is essential, typically two semiconductor diodes in series, in parallel with two diodes conducting in the opposite direction antiparallel. This prevents any current while the applied voltage is less than 1. There will still be a very minor leakage of current through the diodes, which may result in slightly faster corrosion than normal.
Ensure there is no contact with an electrolyte. This can be done by using water-repellent compounds such as greases, or by coating the metals with an impermeable protective layer, such as a suitable paint, varnish, or plastic. If it is not possible to coat both, the coating should be applied to the more noble, the material with higher potential. This is advisable because if the coating is applied only on the more active material, in case of damage to the coating there will be a large cathode area and a very small anode area, and for the exposed anodic area the corrosion rate will be correspondingly high.
Using antioxidant paste is beneficial for preventing corrosion between copper and aluminum electrical connections. The paste consists of a lower nobility metal than aluminum or copper. Choose metals that have similar electropotentials. The more closely matched the individual potentials, the smaller the potential difference and hence the smaller the galvanic current. Using the same metal for all construction is the easiest way of matching potentials.
Electroplating or other plating can also help. This tends to use more noble metals that resist corrosion better. Chrome , nickel , silver and gold can all be used. Galvanizing with zinc protects the steel base metal by sacrificial anodic action. Cathodic protection uses one or more sacrificial anodes made of a metal which is more active than the protected metal. Alloys of metals commonly used for sacrificial anodes include zinc, magnesium , and aluminium.
This approach is commonplace in water heaters and many buried or immersed metallic structures. Galvanic series[ edit ] Galvanized mild steel cable ladder with corrosion around stainless steel bolts All metals can be classified into a galvanic series representing the electrical potential they develop in a given electrolyte against a standard reference electrode.
The relative position of two metals on such a series gives a good indication of which metal is more likely to corrode more quickly. However, other factors such as water aeration and flow rate can influence the rate of the process markedly. Anodic index[ edit ] Sacrificial anode to protect a boat The compatibility of two different metals may be predicted by consideration of their anodic index.
This parameter is a measure of the electrochemical voltage that will be developed between the metal and gold. To find the relative voltage of a pair of metals it is only required to subtract their anodic indices.
For controlled environments in which temperature and humidity are controlled, 0. For harsh environments such as outdoors, high humidity, and salty environments, there should be not more than 0. For example: gold and silver have a difference of 0. The finishing and plating selected allow the dissimilar materials to be in contact, while protecting the more base materials from corrosion by the more noble.
This is why sterling silver and stainless steel tableware should never be placed together in a dishwasher at the same time, as the steel items will likely experience corrosion by the end of the cycle soap and water having served as the chemical electrolyte, and heat having accelerated the process.