Corrosion Control

Corrosion Control

5.1 Corrosion protection in aqueous solutions5.2 Material selection and design, phosphating, anodizing

and galvanizing5.3 Organic coatings and lining5.4 Cathodic/anodic protection

Cathodic Protection

• The corrosion rate of a metal surface in contact with an electrolyte solution is strongly dependent on the electrode potential

• In most cases the corrosion rate can be reduced considerably by shifting the electrode potential to a lower value

• This can usually be brought by loading the surface of the object to be protected with a cathodic current, so that a negative polarization is produced

• Ex: consider iron corroding in a dilute aerated neutral electrolyte solution

• The anode and cathode reaction are:

(2) 4OH4eO2HO

(1) 2eFeFe

22

2

• Cathodic polarization reduces the rate of the half reaction 1 with an excess of electrons

• Which also increases the rate of oxygen reduction and OH- production by 2

• There are two ways to cathodically protect a structure:

1) By an external power supply2) By appropriate galvanic coupling

Impressed current cathodic protection (ICCP)

• Figure 6.1 shows cathodic protection by impressed current

Impressed current cathodic protection (ICCP)

• The negative terminal of the power supply is connected to an underground tank.

• The positive terminal is connected to an inert anode such as graphite.

Impressed current cathodic protection (ICCP)

• The electric leads to the tank and the inert electrode are carefully insulated to prevent current leakage

• As shown in Fig 6.1, current passes to the metallic structure and corrosion is suppressed

Example

• Underground piping for water, oil and natural gas

• External protection of underground oil or gasoline tanks

• Underground telecommunication cables with lead sheathing

• Steel piling in the ground

The polarization diagram

• Ex: Cathodic protection of steel in seawater.

The polarization diagram

• Diffusion of dissolved oxygen to the corroding surface controls corrosion at about 100µA/cm2

• Corrosion rates in the nearly neutral pH range may be reduced to about 20µA/cm2

• Any degree of stirring or agitation restores the corrosion rate to the higher level

• Assuming βa=0.040V, a cathodic polarization of 120mV reduces the corrosion rate now to 0.1µA/cm2

• Because the applied current and corrosion rate is limited by iL, the iapp is max at 100 µA/cm2

Cathodic protection

• There are two ways to cathodically protect a structure:

1) By an external power supply2) By appropriate galvanic coupling

Galvanic coupling (sacrificial anode)

• A metal structure can be cathodically protected by connection to a second metal, called a sacrificial anode, which has a more active corrosion potential

• The more noble (positive) structure in this galvanic couple is cathodically polarized, while the active metal is anodically dissolved.

Mechanism

• Electrons flow from active sacrificial anode to the noble cathode structure.

• The anodic reaction at the cathode structure is reduced by the surplus of electrons provided by the sacrificial anode

• At the same time, the reduction of dissolved oxygen by reaction 2 is accelerated

• Ex: cathodic protection by galvanic coupling to magnesium.

• Magnesium is anodic with respect to steel and corrodes preferentially when galvanically coupled.

• Other materials used in sacrificial anodes are zinc or aluminium alloys

• Iron is also used as a sacrificial anode for copper alloy

• The sacrificial anodes are consumed or ‘sacrificed’ as a result of their protective action.

• Cathodic protection using sacrificial anodes can also be used to protect buried pipelines

• Ex: figure 6.3.

• Protection of an underground pipeline with a magnesium anode.

Principles

• When any two metals or alloys are galvanically coupled, the more active of the two in the galvanic series becomes the sacrificial anode and cathodically protects the other

• Thus, fasteners are cathodically protected when attached to alloys.

Other examples

• The underwater parts of ships, especially near the propeller

• Internal protection of the tanks in oil tankers when the tanks are filled with water as ballast; used zinc or aluminium anodes

• The underwater parts of offshore platforms and pipelines on the bottom of the sea

Other examples

• Internal protection of hot-water tanks made of steel; in this case a centrally-placed magnesium anode can be suitable

Galvanic series

Stray current effect

• Often encountered in cathodic-protection systems

• Refers to extraneous direct currents in the earth

• If a metallic object is placed in a strong field, a potential difference is develops across it

• Corrosion is accelerated at points where current leaves the object and enters the soil

• Common source of stray currents is from cathodic protection systems especially in – densely populated oil production fields and – within industrial complexes containing numerous

buried pipelines

• Figure 6.6 illustrates stray currents resulting from a cathodic protection

• The owner of the buried tank installed cathodic protection

• He did not know of the nearby pipeline that failed rapidly due to the stray current field

Solution

• The solution to this problem is cooperation between operators

• 1)the stray current problem could be prevented by electrically connecting the tank and pipe by a bus connector

• 2)rearranging anodes

• Here, both pipe and tank are protected without stray-current effects, with the owners sharing the cost of installation and operation.

Anodic Protection

• Anodic protection is relatively new• An increase in electrode potential to more

nobles values converts certain metals from the active to the passive state

• Ex: stainless steel in sulphate solution• When this takes place, the corrosion current is

reduced by several orders of magnitude

• Anodic protection is based on the formation of a protective film on metals by externally applied anodic current

• The application of anodic current to a structure tend to – increase the dissolution rate of a metal– Decrease the rate of hydrogen evolution

• This usually occur except for metals with active-passive transitions such as nickel, iron, chromium, titanium and their alloys.

• If carefully controlled anodic current are applied to these materials, they are passivated and the rate of metal dissolution is decreased.

• To anodically protect a structure, a device called a potentiostat is required

• A potentiostat: an electronic device that maintains a metal at a constant potential with respect to a reference electrode

• Ex: the anodic protection of a steel storage tank containing sulfuric acid

• The potentiostat has 3 terminals:– One connected to the tank– Another to an auxiliary cathode (a platinum or

platinum-clad electrode)– To a reference electrode (e.g. calomel cell)

• The potentiostat maintains a constant potential between the tank and the reference electrode

• The optimum potential for protection is determined by electrochemical measurement

• Anodic protection can decrease corrosion rate substantially

• Table 6.4: lists the corrosion rates of austenitic stainless steel in sulfuric acid solutions containing chloride ions with and without anodic protection

Alloy 304 (19Cr-9Ni)

Comparison of anodic and cathodic protection

• Both anodic and cathodic protection utilize electrochemical polarization to reduce corrosion rates.

• Table below shows the difference between anodic and cathodic protection

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• The incorporation of anodic protection has occurred very slowly since its introduction

• Utilizing this technique, it is possible to reduce the alloy requirements for a particular corrosion service

• Anodic protection can be classed as one of the most significance advances in the entire history of corrosion science