Lecture 3

Corrosion rates Faraday law

Michael Faraday (1791-1867)

For example: to calculate how many gram of iron will be oxidized by a direct current charge

from metals surface into the surrounding electrolyte at a current flow of one ampere for one

year.

Corrosion current density

Corrosion current values may be obtained from galvanic cells and polarization

measurements

nF

irnF

M

WLi

)3600.24(

Where:

n is the number of electrons associated with the ionization of each metal atom,

F is 96,500 C/mol, I current density in A/cm2, M molecular weight of metal, WL in

g/cm2.day, (24x3600) days time in second. r is the reaction rate

Corrosion current density can be also determine by expression as below

Equivalent Weight

Equivalent Weight

For alloy: consider a unit mass of alloy oxidized

Therefore, the alloy equivalent weight, EW, is the reciprocal of this quantity:

Calculation of corrosion rate

Penetration rate (CR)

Mass loss rate (MR)

Weight loss (WL)

The corrosion rate or the rate of material removal as a consequence of the chemical

action, is an important corrosion parameter. It may be present by weight loss WL:

Where:

W is the weight loss after exposure corrode time t [days],

A represent the exposed specimen area [cm2].

The WL is conveniently expressed in terms of [g/cm2.day] or [g/cm2.year].

At

WWL

Corrosion penetration rate

This may be expressed as the corrosion penetration rate (CPR), or the thickness loss

of material per unit of time. The formula for this calculation is

where W is the weight loss after exposure time t; = density of metal and A = exposed

specimen area, respectively. K is a constant to change unit.

The CPR is conveniently expressed in terms of either mils per year (mpy) or

millimeters per year (mm/yr).

In the first case, K = 534 to give CPR in mpy (where 1 mil = 0.001 in.),

KWL

At

KWCPR

Classification of

corrosion resistance

after ASTM

Sign CPR, mm/yrCPR.

m/yr

Very good A 0,02 0,1 25 100

Good B > 0,1 0,5 > 100 500

Average C > 0,5 1 > 500 1000

Poor D > 1 5 > 1000 5000

Very poor E > 5 > 5000

Classification of materials base on CPR

Base on corrosion penetration rate (CPR), materials can be divided into 5 groups as

below:

Classification SignCPR

mm/yr

Outstanding < 0.02

Very good A 0.02 0.1

Good B 0.1 0.5

Average C 0.5 1

Poor D 1 5

Very poor E > 5