Anodic Protection 3h4m1m

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 ive corrosion domain.

Protective film is formed on the surface of metal structure which decrease the corrosion rate down to its ive current. Can be applied for active-ive metals/alloys only.

Anodic protection can decrease corrosion rate substantially. Anodic protection of 304SS exposed to an aerated H2SO4 at 300C at 0.500 vs. SCE Acid concentration, M

NaCl, M

Cor. Rate μm/y (Unprotected)

Cor. Rate μm/y (Protected)

0.5

10-5

360

0.64

0.5

10-3

74

1.1

0.5

10-1

81

5.1

5

10-5

49000

0.41

5 5

10-3 10-1

29000 2000

1.0 5.3

Metals which can be ivated and deactivated 

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The metals which can be ivated 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 ivate the metal by oxidation and it will be difficult to reactivate the ivated metals by reduction.

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

Experimental potential - pH diagram for chromium

Anodic polarization curve of AISI 304 SS in 0.5 M H2SO4

Anodic protection parameters : (can be obtained from anodic polarization measurement)   

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

Optimum potential for anodic protection is midway in the ive region

Flade potential (EF) E F  E  n 0,059 pH O F

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 ivated 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.

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

10

Influences of temperature and chloride concentration on anodic polarization curve of stainless steels (schematic figure)

Anodic polarization curves of a mild steel in 10% sulfuric acid at 22 and 600C

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For metals exposed in aggressive ions containing - environment

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Interface potential of metal should be : Eprot>Elogam>Eflade

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Basically : Eflade is equal or slightly lower than Epp.

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

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Increasing of chloride ions concentration results in a significant decrease of protection potential range. Consequently, in aggressive ions containingenvironment 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

Schematic figure of anodic protection system for protecting inner surface of storage tank

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. 

Cathodes used in recent anodic protection systems

Comparison of anodic and cathodic protection :

Applicability

Corrosives

Anodic protection Active-ive metals only Weak to aggressive High

Relative investment cost Relative Very low operation cost Equipment Potentiostat + cathode/s

Cathodic protection All metals

Weak to moderate Low Mediums to high Sacrificial anodes or DC power supply + IC anode/s

Throwing power

Very high

Low to high

Significant of applied current

Often a direct measure of protected corrosion rate

Operating conditions

Can be accurately and rapidly determined by electrochemical measurement

Complex Does not indicate corrosion rate Must usually be determined by empirical testing

Typical applications of anodic protection

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Anodic protection has been applied to protect storage tanks, reactors, heat exchangers and transportation vessels for corrosive solutions.

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Heat exchangers (tubes, spirals and plates types) including their anodic protection systems can be easily to purchase in the market.

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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.

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

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

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 ivity

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

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

Current density requirements for anodic protection

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. a. Draw a labeled sketch of the polarization diagram for the tank and calculate the ivation potential versus SHE. b. What is the area of platinum required to ensure stable ivity? c. What will the corrosion potential be when the tank achieves ivity?

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