Most engineers know about the danger of galvanic corrosion caused by physical contact between dissimilar metals exposed to a corrosive liquid. However, this possibility is often overlooked and other features of this type of corrosion may not be widely known.
In galvanic attack the more active or electrochemically susceptible metal in the electrolyte is rapidly attacked while the more noble (less susceptible) metal suffers little or no corrosion. Often this relationship is harmful and in others it is intended and useful. The accelerated corrosion of steel or cast iron when electrically coupled to copper or brass in potable water or in an acid are examples of detrimental galvanic couples. The use of magnesium anodes electrically connected to an underground, steel pipeline is an example of a beneficial galvanic couple application, i.e., sacrificial anode cathodic protection of the steel as the magnesium is consumed.
One important feature of undesirable galvanic attack is the effect of the wetted area of the more susceptible metal (the anode in the couple) to the area of the more noble metal (the cathode in the couple). A small exposed anodic area in comparison to a large exposed cathodic area results in a much higher rate of corrosion of the anodic metal than would be the case if the area relationship was reversed. Thus it is always desirable to have a small cathodic metal area and a large anodic metal area to minimize the corrosion rate of the latter.
The most common method to quickly judge the severity of galvanic corrosion, select materials and thus avoid higher rates of attack is to use a standard galvanic series that lists several metals and alloys in seawater at room temperature. Generally using metals that are far apart in the series will produce much higher corrosion rates of the more susceptible metal in the couple than using two different metals that are close together in the listing. Such a series is formed by measuring the free corrosion electrochemical potential of each metal and then rank ordering the results. Very noble metals then fall at one end of the series while very susceptible metals are placed at the other end.
Using a standard galvanic series can give general guidance but it has shortcomings. Most importantly free corrosion potentials are thermodynamic properties measured for uncoupled, individual metals and alloys. When two metals are electrically coupled in a given electrolyte their individual free corrosion potentials affect the reactions that occur but so too does their kinetic interactions, i.e., the polarization effects that occur. The standard galvanic series only accounts for thermodynamic effects. For certain alloys and electrolytes, e.g., nickel alloys in acids, coupled polarization properties can be more important than thermodynamic values of the given metals. Other issues with using a standard galvanic series are that not all applications involve room temperature seawater. This does not mean that predicting rates of attack by the traditional galvanic series method is worthless. It just indicates that the user should be aware of the possible problems in assuming it is valid in all situations.
The best "fix" to avoid detrimental galvanic attack is to completely avoid the use of contacting dissimilar metals when both metals are wetted by the electrolyte. Where that is impossible the next best option is to electrically separate the two metals by an insulator, e.g., by use of an insulated union in a piping system. When that option is not viable make sure the wetted area of the anodic metal is large relative to the area of the cathodic metal. Traditional use of a standard galvanic series will provide general guidance when alternative materials selections are possible but be aware of the issues raised here. When dissimilar, contacting materials must attain a low corrosion rate in a critical application (and no other options exist) consider completing polarization tests by a competent laboratory using alternative alloy selections coupled in the actual electrolyte and service temperature to be used.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
©Copyright - All Rights Reserved
DO NOT REPRODUCE WITHOUT WRITTEN PERMISSION BY AUTHOR.