A SEMINAR ON

GALVANIC CORROSION

BY

SAMEER ALI KHAN M Tech M/c Design

092090001

What is Corrosion?

Corrosion is the deterioration of materials by chemical interaction with their environment. 

The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.

Effects of corrosion:

Losses are economic and safety:

•Reduced Strength•Downtime of equipment•Escape of fluids•Lost surface properties•Reduced value of goods

The consequences of corrosion are many and

varied and the effects of these on the safe, reliable and

efficient operation of equipment or structures are often

more serious than the simple loss of a mass of

metal. Failures of various kinds and the need for

expensive replacements may occur even though the

amount of metal destroyed is quite small. 

The Eight Forms of CorrosionThe Eight Forms of Corrosion

1. Uniform attack (general corrosion);

2. Galvanic corrosion;

3. Crevice corrosion;

4. Pitting;

5. Intergraular attack (“IGA”);

6. Selective leaching;

7. Flow-Accelerated Corrosion;

8. Stress corrosion cracking (“SCC”)

Introduction

Galvanic corrosion (also called “dissimilar metal corrosion” and “Electrolytic corrosion”) occurs when a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte.

When dissimilar metals are electrically coupled (i.e., galvanic coupling), corrosion of the less corrosion resistant metal increases, and the surface becomes anodic, while corrosion of the more corrosion resistant metal decreases, and the surface becomes cathodic. The driving force for corrosion or galvanic current flow is the potential developed between the dissimilar metals.

Galvanic Corrosion Factors (Variables)

A collection of factors affecting galvanic corrosion of metals presented by Oldfield includes the following:

Type of joint:welded, fasteners, separated but with external connection

Total geometry:area ratio, distances involved, surface shape, surface condition, number of galvanic cells

Bulk solution properties:oxygen content, pH, conductivity, corrosivity, pollutant level

Bulk solution environment:temperature, flow rate, volume, height above surface

Mass Transport:migration, diffusion, convection Reaction kinetics:metal dissolution, oxygen reduction overvoltage, hydrogen evolution overvoltage

Electrode potentials:galvanic potential between metals, standard electrode potentials (the latter as a "rough" guide only)

Surface Area Effects in a Galvanic Corrosion

Another important factor in galvanic corrosion is the area effect or the ratio of cathodic to anodic area. The larger the cathode compared with the anode, the more oxygen reduction, or other cathodic reaction, can occur and, hence, the greater the galvanic current. From the standpoint of practical corrosion resistance, the least favorable ratio is a very large cathode connected to a very small anode. This effect is illustrated in the following series of pictures.

Steel rivets on a copper bar submerged in 3% sodium chloride solution at the start of the experiment

Steel rivets on a copper bar submerged in 3% sodium chloride solution after six months

Steel rivets on a copper bar submerged in 3% sodium chloride solution after ten months

The GALVANIC TABLE indicates that iron is anodic with respect to copper and therefore is more rapidly corroded when placed in contact with it. This effect is greatly accelerated if the area of the iron is small in comparison to the area of the copper, as shown below.

However, under the reverse conditions when the area of the iron is very large compared to the copper, the corrosion of the iron is only slightly accelerated.

Copper rivets on a steel bar submerged in 3% sodium chloride solution at the start of the experiment

Copper rivets on a steel bar submerged in 3% sodium chloride solution after six months

Copper rivets on a steel bar submerged in 3% sodium chloride solution after ten months

Galvanic corrosion Examples:Rainwater Guttering:

This rainwater guttering is made of aluminium and would normally resist corrosion well.  Someone tied a copper aerial wire around it, and the localised bimetallic cell led to a “knife-cut” effect.

Tyres in Bicycles:

Galvanic corrosion can be even worse underneath the tyre in bicycles used all winter. Here the corrosion is so advanced it has penetrated the rim thickness.

Tubing of Aircraft Hydraulic System:

The tubing, shown here was part of an aircraft’s hydraulic system.  The material is an aluminium alloy and to prevent bimetallic galvanic corrosion due to contact with the copper alloy retaining nut this was cadmium plated.  The plating was not applied to an adequate thickness and pitting corrosion resulted in addition with the galvanic corrosion.

Aluminium Rim of bicycle:

This polished Aluminium rim was left over Christmas with road salt and mud on the rim. Galvanic corrosion has started between the chromium plated brass spoke nipple and the aluminium rim. 

Auto Body Panel:

Galvanic corrosion of painted steel auto body panel in contact with stainless steel wheel opening molding.

CS Tank wall

Area of Galvanic Corrosion (close to Joint)

Weld

SS Tank bottom

Control of Galvanic Corrosion:

Two different metals of approximately the same area are joined to form a galvanic couple in a corrosive solution; we are to reduce the corrosion by coating (e.g., painting) one component of the couple. Do we coat the anode or the cathode?

So we can conclude that, Galvanic corrosion is under cathodic control … if we reduce the area of the Cathode (by coating, etc.) we reduce the corrosion; if we reduce the area of the Anode, corrosion will continue at the same rate but over a smaller area, so perforation etc. will occur sooner.

TO REDUCE GALVANIC CORROSION BY COATINGS, THE MORE CORROSION-RESISTANT (i.e. THE MORE NOBLE OR CATHODIC) COMPONENT OF THE COUPLE IS COATED.

more

anodic

(a

ctiv

e)

more

cath

odic

(i

nert

)

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium

GALVANIC SERIES

ZINC PLATING (“ GALVANIZING”)

•Steel sheeting is coated with zinc by hot-dipping in the molten metal, by heating with zinc dust (“Sherardizing”), etc.

•The Zn coating acts as a sacrificial anode... at the inevitable imperfections, holes, etc., zinc dissolves preferentially, deposits loose, flocculant Zn(OH)2 from aqueous solution.

Protection continues as long as enough Zn is left ... if large enough areas of steel are exposed steel corrosion will occur usually at the middle of the exposed area.

MINIMIZE GALVANIC CORROSION:

•Select metals as close together as possible in galvanic series;

•Avoid small-anode/large-cathode combinations . . . choose fasteners of more noble materials;

•Insulate dissimilar metals (e.g., sleeve bolts in flange joints, as well as use insulating washers);

•Add inhibitors, if possible, to environment;

•Avoid threaded joints where possible;

•Design for anodic member (make thicker, easily replaceable, etc.);

•Install a third metal that is anodic to BOTH in the couple.