Ajesh Content

8/3/2019 Ajesh Content

1/6

Seminar Topic

On

GALVANIC CORROSION PARAMETERS

Prepared By

Ajesh Babu

Roll No-901

BE IV (MET.& MTRLS ENGG.)

METALLURGICAL & MATERIALS ENGINEERING

DEPARTMENT

FACULTY OF TECHNOLOGY AND ENGINEERING

M.S UNIVERSITY OF BARODA

VADODARA


2/6

1.GALVANIC CORROSION Galvanic corrosion occurs when two metals oralloys with different electrochemical potentials or with different tendencies to corrode are

in metal-to-metal contact in a corrosive electrolyte.

The four essential elements that must be present for the galvanic corrosion to occur :

Electrolyte

A liquid or a corrosive medium that conducts electricity. Cathode One which is more positive or positive emf value. Depostion takes place at

cathode

AnodeOne which is less positive or negative emf value. It will get corroded. Metallic Pathway Surplus electrons at the anode flow through the metallic pathway to

the cathode.


3/6

2.Factors affecting Galvanic Corrosion :-The following factors significantly affect the magnitude of galvanic corrosion :

A. Position of metals in the galvanic series.

B. The nature of the environment.

C. Area, distance and geometric effects

2.1. Position of Metals in the Galvanic Series (Difference in the electrode potential): -

The further apart the metals are in the galvanic series, the greater is the chance for galvanic

corrosion. The magnitude of galvanic corrosion primarily depends on how much potential

difference exists between two metals. For a particular environment, the metals selected

should be close to each other in the galvanic series to minimize galvanic corrosion. Active

metals should not be joined with passive metals.Thus, aluminum should not be joined to

steel,as aluminum being more active would tend to corrode.

The following is the galvanic series for stagnant (that is, low oxygen content) seawater.

The order may change in different environments.
http://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Seawaterhttp://en.wikipedia.org/wiki/Seawaterhttp://en.wikipedia.org/wiki/Oxygen


4/6

2.2. The Nature of Environment : - Due consideration must be given to the environment

that surrounds the metal. For instance, water containing copper ions, like seawater, is likely

to form galvanic cells on a steel surface of the tank. If the water in contact with steel is eitheracidic or contains salt, the galvanic reaction is accelerated because of the increased ionization

of the electrolyte. In marine environments, galvanic corrosion may be accelerated due to

increased conductivity of the electrolyte.


5/6

In cold climates, galvanic corrosion of buried material is reduced because of the increased

resistivity of soil. In warm climates, on the other hand, it is the reverse because of the

decreased resistivity of the soil.

Temperature :- The galvanic corrosion increases with the increase in temperature of the

electrolyte. In case of single metal, due to the difference in the temperature at seperated sites

leads to thermogalvanic corrosion. E.g heat exchanger system.

Concentration :- The galvanic corrosion increases with the increase in concentration of

electrolyte.

Oxygen availability :-In case of Single electrode the part at which there is scarcity of oxygen

will act as anode & the part which has sufficient oxygen will act as cathode. For e.g when a

metal pipe is submerged in water, the part above the level of water gets sufficient oxygen and

act as cathode & the submerged part will get less oxygen and act as anode & get corroded.

2.3Area, Distance and Geometric Effects : -2.3.1 Effect of Area

The anode to cathode area ratio is extremely important as the magnitude of galvanic

corrosion is seriously affected by it. The area ratio can be unfavorable as well as favorable.The area ratio of the anode to cathode plays a dominant role in galvanic corrosion. As a given

amount of current flows in a galvanic couple, the current density at the anode or cathode

controls the rate of corrosion. For a given amount of current, the metal with the smallest area

has the largest current density and, hence, is more damaged if corrosion occurs at it. For

similar reasons, the current density at a large metal is very small. The rate of corrosion

increases with the ratio of cathodic to anodic areas.

2.3.2 Effect of Distance

It is a known principle that the solution conductivity varies inversely with the length of

the conduction path. Most corrosion damage is caused by current which cover short paths.Hence, the greatest galvanic damage is likely to be encountered near the junction of the two

metals and the severity would be decreased with increased length. If two different metals are

far away from each other, there would be no risk of galvanic corrosion, because of very little

current flow. This is possible in designs, like oil rigs and other complex structures requiring a

very large variety of material.


6/6

The effect of area and distance may be best understood by the examples of utility lines in

a large building. Consider, for instance, the copper tubes transporting water and natural gas.

Coated carbon steel pipelines are laid in the same trench in a soil of low resistivity. In the

trench, a corrosion cell would be formed if the pipes touch (metal-to-metal contact), or if

they are bonded together somewhere (for electrical earthing requirements). Copper acts as

cathode and steel acts as anode in an electrolyte of soil and galvanic corrosion would initiate.

Now consider the area effect. Being coated, steel pipe would have a small anodic area at the

sites of coating defects, whereas copper pipes would have a large cathodic area. Due to the

small anode area and large cathode areas, galvanic corrosion would initiate and leaks in the

carbon pipe would soon start. The problem also illustrates the effect of distance. If the above

utility pipe were in two different trenches, a sufficient distance between them would not have

allowed the galvanic current to flow.

2.3.3 Effect of Geometry

Geometry of components and their design also influence galvanic corrosion. As current does

not flow around the corners, the geometry of the circuit affects the degree of galvanic

corrosion. Polarization may be affected by a break in the continuity of the current.

Component design is also a factor in galvanic corrosion as the current circuit geometry affect

the magnitude of galvanic corrosion and the polarization process. Any obstacle to

polarization would accelerate galvanic corrosion.

Reference : -

1. Nace Volume 12. Principles of Corrosion Engineering & Corrosion ControlZaki Ahmad3. Google4. Wikipedia5. Electrochemistry & corrosion scienceNestor Perez6. An introduction to ElectrometallurgySharan & Narain

Ajesh Content

Documents

Transcript of Ajesh Content