Electrode Potential and kinetics

[MrReds]

Hallo to everybody !
Please, I’d like to know if somebody can help me in this issue:
When I consider the galvanic couple of
1) aluminum and copper, the electrode potential of aluminum is -1.66V, while copper is +0.34V.
As a consequence aluminum is the less noble metal and will be subjected to corrosion.
2) copper and silver. Copper +0.34V Silver 0.80. Therefore copper will be corroded
3) Copper and Tin. Copper +0.34V, Tin +0.1335V. Tin will be corroded
4) Aluminum with silver or tin = > aluminum will be corroded anyhow.

Let’s consider that all of these reactions are in the same environment.

Question 1: Is there a relationship between the potential difference and the kinetics of corrosion ? Ex: higher the potential difference, faster the corrosion
Question 2: If there is, which is this relation ?
Question 3: is the corrosion affected also by the quantity of the two materials ? Ex: in case 1, should the aluminum be present in a small quantity, the quantity will disappear in a shorted time that a bigger one, shouldn’t it ?

Thanks in advance !

 

[mshimko]

The potential difference is a thermodynamics aspecty, it best describes WHAT reaction will occur. Often, attems to translate a smaller potential difference into a kinetics results is invalid and provides false predictions.

Kinetics of galvanic corrosion IS impacted by the relative surface areas of the two materials. You want to avoid having a small surface area of the more active material, which is called an "Adverse Area Ratio"; unless, that is, you WANT the smaller material to provide sacrificial cathodic protection to the other material.

More information on kenetics can be derived from thinking about, or studinging "evans diagrams" (potential versus current density) of the two materials involved.

 

[SJones]

http://resource.npl.co.uk/.../publications/online_guides/pdf/the_electrochemistry_of_corrosion.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[MrReds]

Many thanks mshimko & SJones !

Please mr. Jones, would you be so kind and putting the complete link, since I cannot download the pdf from the link you have put !

Many thanks

 

[SJones]

My apologies: NPL have just altered their web site.

http://www.npl.co.uk/upload/pdf/the_electrochemistry_of_corrosion_with_figures.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[strider6]

To eavluate the corrosion rate of a couple of metal you nedd to consider the potential difference, the exchange current densities and the slope of the evans diagrams of the metals. High potential difference doesn't always means high corrosion rate.

Look also at this presentation:

http://www.unene.ca/un1001/UN1001_Galvanic Corrosion.ppt

hope this help u

S

http://www.corrosionist.com