Corrosion of an iron nail wrapped in bright copperwire, showing cathodic protection of copper; aferroxyl indicator solution shows colored chemicalindications of two types of ions diffusing through amoist agar medium

Galvanic corrosionFrom Wikipedia, the free encyclopedia

Galvanic corrosion is an electrochemical process inwhich one metal corrodes preferentially to anotherwhen both metals are in electrical contact, in thepresence of an electrolyte. This same galvanic reactionis exploited in primary batteries to generate anelectrical voltage.

Contents

1 Overview2 Examples of corrosion

2.1 Statue of Liberty2.2 Royal Navy and HMS Alarm2.3 US Navy Littoral Combat ShipIndependence2.4 Lasagna cell2.5 Electrolytic cleaning

3 Preventing galvanic corrosion4 Galvanic series5 Anodic index6 See also7 References8 External links

Overview

Dissimilar metals and alloys have different electrode potentials, and when two or more come into contactin an electrolyte, one metal acts as anode and the other as cathode. The electropotential differencebetween the dissimilar metals is the driving force for an accelerated attack on the anode member of thegalvanic couple. The anode metal dissolves into the electrolyte, and deposit collects on the cathodicmetal.

The electrolyte provides a means for ion migration whereby metallic ions move from the anode to thecathode within the metal. This leads to the metal at the anode corroding more quickly than it otherwisewould and corrosion at the cathode being inhibited. The presence of an electrolyte and an electricalconducting path between the metals is essential for galvanic corrosion to occur.

Galvanic corrosion in the Statue ofLiberty

In some cases, this type of reaction is intentionally encouraged. For example, low-cost household batteriestypically contain carbon-zinc cells. As part of a closed circuit (the electron pathway), the zinc within thecell 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. In this case, sacrificialanodes work as part of a galvanic couple, promoting corrosion of the anode, while protecting the cathodemetal.

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 inhibitorssuch as sodium nitrite or sodium molybdate can be injected into these systems to reduce the galvanicpotential. However, the application of these corrosion inhibitors must be monitored closely. If theapplication of corrosion inhibitors increases the conductivity of the water within the system, the galvaniccorrosion potential can be greatly increased.

Acidity or alkalinity (pH) is also a major consideration with regard to closed loop bimetallic circulatingsystems. 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 tobe applied within the plumbing of the system and, over time, would corrode and release particles thatcould cause potential mechanical damage to circulating pumps, heat exchangers, etc.[1]

Examples of corrosion

A common example of galvanic corrosion is the rusting of corrugated iron sheet, which becomeswidespread when the protective zinc coating is broken and the underlying steel is attacked. The zinc isattacked preferentially because it is less noble, but once it has been consumed, rusting of the base metalcan occur in earnest. By contrast, with a traditional 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 beneathis immediately attacked preferentially.

Statue of Liberty

A spectacular example of galvanic corrosion occurred in the Statueof Liberty when regular maintenance checks in the 1980s revealedthat corrosion had taken place between the outer copper skin andthe wrought iron support structure. Although the problem hadbeen anticipated when the structure was built by Gustave Eiffel toFrdric Bartholdi's design in the 1880s, the insulation layer ofshellac between the two metals had failed over time and resultedin rusting of the iron supports. An extensive renovation requiringcomplete disassembly of the statue replaced the original insulationwith PTFE. The structure was far from unsafe owing to the largenumber of unaffected connections, but it was regarded as aprecautionary measure for what is considered a national symbol ofthe United States.[2][3]

Royal Navy and HMS Alarm

Regular maintenancechecks discovered thatthe Statue of Libertysuffered from galvaniccorrosion

In 17th-century England, Samuel Pepys (then serving as Admiralty Secretary),agreed to the removal of lead sheathing from British Royal Navy vessels toprevent the mysterious disintegration of their rudder-irons and bolt-heads,though he confessed himself baffled as to the reason the lead caused thecorrosion.[4]

The problem recurred when vessels were sheathed in copper to reducemarine weed accumulation and protect against shipworm. In an experiment,the Royal Navy in 1761 had tried fitting the hull of the frigate HMS Alarm with12-ounce copper plating. Upon her return from a voyage to the West Indies, itwas found that although the copper remained in fine condition and hadindeed deterred shipworm, it had also become detached from the woodenhull in many places because the iron nails used during its installation "...werefound dissolved into a kind of rusty Paste".[5] To the surprise of the inspectionteams, however, some of iron nails were virtually undamaged. Closerinspection revealed that water-resistant brown paper trapped under the nailhead had inadvertently protected some of the nails: "Where this covering wasperfect, the Iron was preserved from Injury". The copper sheathing had beendelivered to the dockyard wrapped in the paper which was not always removed before the sheets werenailed to the hull. The conclusion therefore reported to the Admiralty in 1763 was that iron should not beallowed direct contact with copper in sea water.[6][7]

US Navy Littoral Combat Ship Independence

Serious galvanic corrosion has been reported on the latest US Navy attack littoral combat vessel the USSIndependence caused by steel water jet propulsion systems attached to an aluminium hull. Withoutelectrical isolation between the steel and aluminium, the aluminium hull acts as an anode to the stainlesssteel, resulting in aggressive galvanic corrosion.[8]

Lasagna cell

A "lasagna cell" is accidentally produced when salty moist food such as lasagna is stored in a steel bakingpan and is covered with aluminum foil. After a few hours the foil develops small holes where it touchesthe lasagna, and the food surface becomes covered with small spots composed of corroded aluminum.[9]

In this example, the salty food (lasagna) is the electrolyte, the aluminum foil is the anode, and the steelpan is the cathode. If the aluminum foil only touches the electrolyte in small areas, the galvanic corrosionis concentrated, and corrosion can occur fairly rapidly.

Electrolytic cleaning

The common technique of cleaning silverware by immersion of the silver and a piece of aluminum in anelectrolytic bath (usually sodium bicarbonate) is an example of galvanic corrosion. (Care should beexercised because this will also strip silver oxide from the silverware, which may be there for decoration.Use on silverplate is inadvisable, as this may cause unwanted galvanic corrosion of the base metal.)

Aluminum anodes mounted on asteel-jacketed structure

Electrical panel for a cathodicprotection system

Preventing galvanic corrosion

There are several ways of reducing and preventing this form ofcorrosion.

Electrically insulate the two metals from each other. If theyare not in electrical contact, no galvanic couple will occur.This can be achieved by using non-conductive materialsbetween metals of different electropotential. Piping can beisolated with a spool of pipe made of plastic materials, ormade of metal material internally coated or lined. It isimportant that the spool be a sufficient length to beeffective.

Metal boats connected to a shore line electrical power feedwill normally have to have the hull connected to earth forsafety reasons. However the end of that earth connection islikely to be a copper rod buried within the marina, resultingin a steel-copper "battery" of about 0.5V. For such cases, theuse of a galvanic isolator is essential, typically twosemiconductor diodes in series. This prevents any currentflow while the applied voltage is less than 1.4V (i.e. 0.7V perdiode), but allows a full flow of current in case of an electrical fault. There will still be a very minorleakage 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-repellentcompounds such as greases, or by coating the metals with an impermeable protective layer, such asa suitable paint, varnish, or plastic. If it is not possible to coat both, the coating should be applied tothe more noble, the material with higher potential. This is advisable because if the coating is appliedonly on the more active material, in case of damage to the coating there will be a large cathode areaand a very small anode area, and for the exposed anodic area the corrosion rate will becorrespondingly high.

Using antioxidant paste is beneficial for preventing corrosion between copper and aluminumelectrical connections. The paste consists of a lower nobility metal than aluminum or copper.

Choose metals that have similar electropotentials. The more closely matched the individualpotentials, the lesser the potential difference and hence the lesser the galvanic current. Using thesame metal for all construction is the easiest way of matching potentials.

Stainless steel cable ladder with mildsteel bolts

Electroplating or other plating can also help. This tends to use more noble metals that resistcorrosion better. Chrome, nickel, silver and gold can all be used. Galvanizing with zinc protects thesteel base metal by sacrificial anodic action.

Cathodic protection uses one or more sacrificial anodes made of a metal which is more active thanthe protected metal. Alloys of metals commonly used for sacrificial anodes include zinc, magnesium,and aluminium. This is commonplace on many buried or immersed metallic structures.

Cathodic Protection can also be applied by connecting a direct current (DC) electrical power supplyto oppose the corrosive galvanic current. (See impressed current cathodic protection)

Galvanic series

All metals can be classified into a galvanic series representing theelectrical potential they develop in a given electrolyte against astandard reference electrode. The relative position of two metalson such a series gives a good indication of which metal is morelikely to corrode more quickly. However, other factors such aswater aeration and flow rate can influence the rate of the processmarkedly.

Anodic index

The compatibility of two different metals may be predicted by consideration of their anodic index. Thisparameter is a measure of the electrochemical voltage that will be developed between the metal andgold. To find the relative voltage of a pair of metals it is only required to subtract their anodic indices.[10]

For normal environments, such as storage in warehouses or non-temperature and humidity controlledenvironments, there should not be more than 0.25 V difference in the anodic index. For controlledenvironments, in which temperature and humidity are controlled, 0.50 V can be tolerated. For harshenvironments, such as outdoors, high humidity, and salt environments, there should be not more than0.15 V difference in the anodic index. For example; gold/silver would have a difference of 0.15V beingacceptable [11]

Often when design requires that dissimilar metals come in contact, the galvanic compatibility is managedby finishes and plating. The finishing and plating selected allows the dissimilar materials to be in contact,while protecting the base materials from corrosion.[11] Note that it will always be the metal with the mostnegative anodic index which will ultimately suffer from corrosion when galvanic incompatibility is in play.This is why you should never place sterling silver and stainless steel tableware in a dishwasher at the sametime, as the steel items will likely experience corrosion by the end of the cycle (soap and water havingserved as the chemical electrolyte, and heat having amplified the process).

This new manifold for water metershas replaced the old one after 4years of use, having been connecteddirectly to a copper pipe in thebuilding (0.85 V of potentialdifference, see the point of corrosionin the bottom left). With a PVCbattery support, lifetime is unlimited.

Zinc sacrificial anode (photo center)to protect a boat

Anodic index[11]

Metal Index(V)

Most Cathodic

Gold, solid and plated, Gold-platinum alloy 0.00

Rhodium plated on silver-plated copper 0.05

Silver, solid or plated; monel metal. High nickel-copper alloys 0.15

Nickel, solid or plated, titanium an s alloys, Monel 0.30

Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickelalloys; nickel-chromium alloys 0.35

Brass and bronzes 0.40

High brasses and bronzes 0.45

18% chromium type corrosion-resistant steels 0.50

Chromium plated; tin plated; 12% chromium type corrosion-resistant steels 0.60

Tin-plate; tin-lead solder 0.65

Lead, solid or plated; high lead alloys 0.70

2000 series wrought aluminum 0.75

Iron, wrought, gray or malleable, plain carbon and low alloy steels 0.85

Aluminum, wrought alloys other than 2000 series aluminum, cast alloys of the silicon type 0.90

Aluminum, cast alloys other than silicon type, cadmium, plated and chromate 0.95

Hot-dip-zinc plate; galvanized steel 1.20

Zinc, wrought; zinc-base die-casting alloys; zinc plated 1.25

Magnesium & magnesium-base alloys, cast or wrought 1.75

Beryllium 1.85

Most Anodic

See also

CorrosionGalvanizationGalvanic anode

References

1. M. Houser, Corrosion Control Services, Inc., introduction handbook2. Corrosion Doctors [1] (http://corrosion-doctors.org/Landmarks/Statue.htm) (Retrieved January 2011)3. Copper.org [2] (http://www.copper.org/education/liberty/liberty_reclothed2.html) (Retrieved January 2011)

Wikimedia Commons hasmedia related to Galvaniccorrosion.

4. Bryant, Arthur (1935). Samuel Pepys: The Years of Peril. Cambridge: Macmillan. p. 370.5. "Galvanic corrosion... What it is and how to fight it" (http://books.google.com/books?

id=RvM1AQAAMAAJ&pg=PA50). Motorboating (Hearst Magazines Inc.) 82 (1): 50. July 1948.6. "CLI Houston" (http://www.clihouston.com/news/galvanic-corrosion-processes-environmental-

deterioration.html). Retrieved January 2011.7. "Historic Corrosion Lessons" (http://corrosion-doctors.org/Corrosion-History/Lessons.htm). Corrosion Doctors.

1988. Retrieved 2014-02-27. |first1= missing |last1= in Authors list (help)8. David Axe. "Builder Blames Navy as Brand-New Warship Disintegrates"

(http://www.wired.com/dangerroom/2011/06/shipbuilder-blames-navy-as-brand-new-warship-disintegrates/).

9. Water (http://books.google.es/books?id=5mP6BSBeqPIC&pg=PA826). Hemat, R.A.S. Editor: Urotext. ISBN 1-903737-12-5. p. 826

10. Wheeler, Gerson J., The design of electronic equipment: a manual for production and manufacturing, Prentice-Hall, 1972

11. Handbook of Corrosion Engineering by Pierre R. Roberge (http://www.corrosion-doctors.org/Books/Handbook.htm)

External links

Galvanic Corrosion and Other Types of Corrosion

(http://www.corrosionclinic.com/different_types_of_corrosion.htm)Corrosion Doctors (http://www.corrosion-doctors.org/Forms-galvanic/galvanic-corrosion.htm)Galvanic Corrosion Theory and documents (http://www.corrosionist.com/Galvanic_Corrosion.htm)Galvanic series (http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm)Electrochemistry of corrosion (http://electrochem.cwru.edu/encycl/art-c02-corrosion.htm) Fromthe Yeager Center at CWRU.Bimetallic corrosion (http://www.roymech.co.uk/Useful_Tables/Corrosion/Cor_bi_met.html)The Straight Dope: why does ketchup dissolve aluminum?(http://www.straightdope.com/classics/a1_121a.html)PIRA physics lecture demonstration 5e40.25(http://physicslearning.colorado.edu/PIRA/PiraSubTOC.asp?STopic=5E40)Cathodic Protection 101: A basic tutorial (http://www.cathodicprotection101.com)

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Categories: Corrosion

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