Galvanic Corrosion

An electrical potential, or voltage, difference usually exists between two different metals that are in electrical contact and immersed in a corrosive solution. This potential difference causes current to flow and the less noble, or more anodic, metal suffers increased corrosion rate. The severity of attack depends upon the relative voltage difference between the metals, the relative exposed areas of each, and the particular corrosive environment.

The most common example is the old-fashioned flashlight battery, or dry cell. It has a shell of zinc metal (less noble, or anodic) filled with a moist corrosive paste that conducts electricity. The center post is made of graphite, which is quite noble (cathodic, does not tend to corrode). The potential (voltage) difference between zinc and graphite is about 1.5 V. 

When an electric connection is made in a flashlight, the zinc corrodes, giving up electrons, which flow through the lightbulb toward the graphite cathode, the positive pole. In this case, because generating electricity is the point, no one minds that the zinc corrodes and gets used up.

The ratio of cathodic (noble) to anodic areas is an important factor in galvanic corrosion. An undesirable situation is a large cathode connected to a small anode, or less noble metal. This can develop high current density, hence severe corrosion, at the anode. In that common zinc dry cell the zinc anode has a much larger area than the graphite cathode, so it has some useful working life before corroding through the zinc case.

For example, a large area of stainless steel in contact with a small surface area of carbon steel is undesirable. The potential difference will tend to corrode the carbon steel, and the very large area of stainless steel will make that corrosion occur quickly. The reverse condition is preferred. That is, a small area of stainless (or more noble) metal may be coupled with a much larger area of carbon steel (anodic) without significant problems. ‘‘Significant’’ depends upon the application. In the past, when ferritic stainless trim was used on carbon steel automobile bodies, the steel would tend to corrode most severely underneath the trim. In

part that was because the crevice trapped salt, but it was accentuated by the galvanic difference

between ferritic stainless and carbon steel. 

There is some potential difference among the various stainless and nickel alloys. In practice, galvanic corrosion is rarely a problem among these various alloys. There is, however, a significant potential difference between copper and stainless alloys. So long as the stainless steel is passive (not actively corroding), it is more noble enough than copper to corrode the copper alloy. An example is when a heat exchanger with a Muntz metal (60% Cu–40% Zn) tubesheet is retubed with AL-6XN alloy instead of the original copper–nickel
 tubes. The potential difference is enough to corrode the copper alloy tubesheet. One ought to either replace the tubesheet with stainless steel, or retube using a copper-base alloy. 

Graphite is at the noble end of the galvanic series. If graphite is in contact with stainless or nickel alloys in a corrosive environment, those alloys may corrode preferentially.

 Galvanic effects have a positive side and may be used to protect equipment from corrosion, a common example being a zinc coating on steel. The zinc corrodes preferentially, and in doing so protects the steel from corrosion (rusting). Zinc or magnesium anodes are often connected to equipment ranging from chemical process to steel ship hulls to protect them from corrosion.

READ MORE.......

STAINLESS STEELS
James Kelly
Rochester, Michigan

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.



for STEP BY STEP GUIDE unigraphics simple tutorial please visit.........
www.unigraphicsimpletutorial.blogspot.com

---or---

www.unigraphic-simple-tutorial.com

Berbagi :