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ANODIC PROTECTION. Feasibility of anodic protection is firstly demonstrated and tested by Edeleanu in 1954. Corrosion control of metal structure by impressed anodic current. Interface potential of the structure is increased into passive corrosion domain.

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anodic protection

ANODIC PROTECTION

Feasibility of anodic protection is firstly demonstrated and tested by Edeleanu in 1954

slide2
Corrosion control of metal structure by impressed anodic current.

Interface potential of the structure is increased into passive corrosion domain.

Protective film is formed on the surface of metal structure which decrease the corrosion rate down to its passive current.

Can be applied for active-passive metals/alloys only.

anodic protection can decrease corrosion rate substantially
Anodic protection can decrease corrosion rate substantially.

Anodic protection of 304SS exposed to an aerated H2SO4 at 300C at 0.500 vs. SCE

metals which can be passivated and de activated
Metals which can be passivated and de-activated
  • The metals which can be passivated by oxidation and activated by reduction are those which have a higher oxide less soluble than a lower oxide and will thus each corrosion domain forms an angle.
  • The lower the apex of this angle in the diagram (such as titanium, chromium and tin etc.), the easier it will be to passivate the metal by oxidation and it will be difficult to reactivate the passivated metals by reduction.
slide5

Titanium and chromium can be passivated very easily and their passivation process will occur more often than not, spontaneously, even in the absence of oxidizing agent.

anodic protection parameters
Anodic protection parameters :

(can be obtained from anodic polarization measurement)

  • Range of potential in which metal is in passivation state (protection range)
  • Critical current density
  • Flade potential

Optimum potential for anodic protection is midway in the passive region

flade potential e f
Flade potential (EF)

In which EFO : Flade potential at pH = 0

n : a constant (between 1 and 2) depends of metal composition and environment conditions

  • Metals having EF < equilibrium potential of hydrogen evolution reaction (HER) can be passivated by non oxidizing acid (i.e. titanium)
  • Increasing temperature will reduce the protection potential range and increase the critical current density and therefore anodic protection will be more difficult to be applied.
slide10

Parameters that should be considered for anodic protection design (Flade potential is not included in the figure)

slide11

Influences of temperature and chloride concentration on anodic polarization curve of stainless steels

(schematic figure)

slide13
For metals exposed in aggressive ions containing - environment
  • Interface potential of metal should be :

Eprot>Elogam>Eflade

  • Basically : Eflade is equal or slightly lower than Epp.
slide14

Schematic figure of potential range for anodic protection of a stainless steel which is susceptible to pitting corrosion in an environment containing aggressive ions

slide15
Increasing of chloride ions concentration results in a significant decrease of protection potential range.
  • Consequently, in aggressive ions containing-environment anodic protection is applied only for metals which have relatively high protection potential and high pitting potential.
  • Increasing temperature leading to a decrease of Eprot
cathodes for anodic protection
CATHODES FOR ANODIC PROTECTION
  • Should be permanent and can be used as current collector without any significant degradation.
  • Having large surface area in order to suppress cathodic overpotential.
  • Low cost.

Platinum clad brass can be used for anodic protection cathodes because this cathode has low overpotential and its degradation rate is very low, however it is very expensive.

slide22
Anodic protection has been applied to protect storage tanks, reactors, heat exchangers and transportation vessels for corrosive solutions.
  • Heat exchangers (tubes, spirals and plates types) including their anodic protection systems can be easily to purchase in the market.
  • i.e. AISI 316 SS HE is used to handle 96-98% sulfuric acid solution at 1100C. Anodic protection decreases corrosion rate of the stainless steel, initially from 5mm/year down to 0.025mm/year and therefore less contaminated sulfuric acid can be obtained.
slide23

DATA

Effect of chromium content on critical current density and Flade potential of iron exposed in 10% sulfuric acid.

slide24

Effects of nickel and chromium contents on critical current density passivation potential in 1N and 10 N H2SO4 containing 0.5 N K2SO4

slide25

Requirement of critical protection current densities for several austenitic stainless steels (18-20 Cr , 8-12 Ni) exposed in different electrolytes

Protection current density :current density required to maintain passivity

slide26

Effect of sulfuric acid concentration at 240C on the corrosion rate and critical current density of stainless steel

slide27

Effect of stirring of electrolyte on the corrosion rate and requirement of current density to maintain passivity on a stainless steel at 270C

slide29
Anodic Protection Using a Galvanic Cathode

A cylindrical tank of 304 stainless steel for storing deaerated sulfuric acid (pH=0) is found to corrode rapidly. To provide anodic protection, a galvanic cathode of platinum will be installed. The tank has a diameter of 5 m and the depth of acid is 5 m.

  • Draw a labeled sketch of the polarization diagram for the tank and calculate the passivation potential versus SHE.
  • What is the area of platinum required to ensure stable passivity?
  • What will the corrosion potential be when the tank achieves passivity?
slide30
Data:
  • 304 stainless steel:
  • Ecor = -0.44 V vs SCE
  • icor = 10-3 A/cm2
  • Tafel slope anodic = 0.07 V/decade
  • icrit = 1.4 x 10-2 A/cm2
  • ipas = 4 x 10-7 A/cm2
  • H+ reduction on platinum
  • i0 = 10-3 A/cm2
  • Tafel slope cathodic = 0.03 V/decade
  • SCE = +0.2416 V vs.SHE