that are the reactants in the cathodic reaction, are normal oxidizers found in
natural environments. In the natural atmosphere, the single-potential of
dissolved oxygenisnearlyconstant. Because of this, metalswith more noble
electrode potentials are used as coating materials. These include copper,
silver, platinum, gold, and theira lloys.Acopper coating system provides
excellent corrosionr esistance under the conditiont hat the defect-free
continuous layer covers the surfaceo ft he iron substrate.I nd oings o, the
EMF of the iron surface is decreased by the copper coating. The corrosion
potential is changedfrom E corr of uncoated ironto E corr of copper by coating
with copper.U nder this condition, the ironc orrodes at the low rate of i corr
of copper. However,i ft he irons ubstrate is exposed to the environment,
as the result of mechanical damage,the substrate is corroded predominantly
at the rate of i corr of exposed copper by coupling ironand copper (galvanic
corrosion).
Organic coatings and paints are also able to provide EMF control protection.Surface conditions are converted to more stablestates by coating with
organic compounds.T hesec oatings delay the generation of electromotive
force, causing the corrosion of the substrate.
How long an organic coating willb es erviceable is dependent upon the
durability of the coating and its adhesive ability on the base metal. The
former is the stability of the coating layer as exposed to various environmentalfactors, and the latter is determinedbythe conditionofthe interface
between the organic film and the substrate.
EMF can also be decreased by the use of glass lining, porcelain enameling,
and temporary coating with greases and oils.
7.1.4C athodic Control Protection
Cathodic control protection protects the substrate by coating with al ess
noble metal, for which the slopes of the cathodic polarization curves are
steep. The cathodic overpotential of the surfacei si ncreased by the coating;
therefore, the corrosion potential becomesm ore negative than that of the
substrate. Coating materials used for this purpose arez inc, aluminum,
manganese,c admium, and their alloys. The electrode potential of these
metalsare more negative than thoseofiron andsteel. When exposed to the
environment, thesec oatings act as sacrificial anodesf or the iron and steel
substrates.
The protective abilitieso fc oatings are as follows:
1. Originalb arrier actionofc oating layer
2. Secondarybarrier actionofc orrosion product layer
3. Galvanic action of coating layer as sacrificial anode
Barrier coatings 1and 2predominate as the protective ability even though
as acrificial metal coating is characterized by galvanic action.
Corrosion of Linings and Coatings
412
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
natural environments. In the natural atmosphere, the single-potential of
dissolved oxygenisnearlyconstant. Because of this, metalswith more noble
electrode potentials are used as coating materials. These include copper,
silver, platinum, gold, and theira lloys.Acopper coating system provides
excellent corrosionr esistance under the conditiont hat the defect-free
continuous layer covers the surfaceo ft he iron substrate.I nd oings o, the
EMF of the iron surface is decreased by the copper coating. The corrosion
potential is changedfrom E corr of uncoated ironto E corr of copper by coating
with copper.U nder this condition, the ironc orrodes at the low rate of i corr
of copper. However,i ft he irons ubstrate is exposed to the environment,
as the result of mechanical damage,the substrate is corroded predominantly
at the rate of i corr of exposed copper by coupling ironand copper (galvanic
corrosion).
Organic coatings and paints are also able to provide EMF control protection.Surface conditions are converted to more stablestates by coating with
organic compounds.T hesec oatings delay the generation of electromotive
force, causing the corrosion of the substrate.
How long an organic coating willb es erviceable is dependent upon the
durability of the coating and its adhesive ability on the base metal. The
former is the stability of the coating layer as exposed to various environmentalfactors, and the latter is determinedbythe conditionofthe interface
between the organic film and the substrate.
EMF can also be decreased by the use of glass lining, porcelain enameling,
and temporary coating with greases and oils.
7.1.4C athodic Control Protection
Cathodic control protection protects the substrate by coating with al ess
noble metal, for which the slopes of the cathodic polarization curves are
steep. The cathodic overpotential of the surfacei si ncreased by the coating;
therefore, the corrosion potential becomesm ore negative than that of the
substrate. Coating materials used for this purpose arez inc, aluminum,
manganese,c admium, and their alloys. The electrode potential of these
metalsare more negative than thoseofiron andsteel. When exposed to the
environment, thesec oatings act as sacrificial anodesf or the iron and steel
substrates.
The protective abilitieso fc oatings are as follows:
1. Originalb arrier actionofc oating layer
2. Secondarybarrier actionofc orrosion product layer
3. Galvanic action of coating layer as sacrificial anode
Barrier coatings 1and 2predominate as the protective ability even though
as acrificial metal coating is characterized by galvanic action.
Corrosion of Linings and Coatings
412
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485