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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
Continue…
Continue…
• 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