Manganese is so negative in electrochemical potential, and active, that its
alloy and duplex coatings provide galvanic protection. Mn/Zn alloy coating
exhibits high corrosion resistance, andthe corrosion potential of manganese
is more negative than that of zinc; therefore, this alloy coating provides
cathodic protection for asteel substrate. The structure of Mn/Zn alloy coating
is composed of the single phase of 3 in the manganesecontent range ! 20%,
and 3 and g phases in the range above 20%. As the manganese content in the
depositisincreased, the percentage of g -Mn phaseisincreased.
7.12 Conversion Coatings
The term conversion coating is used to describe coatings where the substrate
metal provides ions that become part of the protective coating. The coating
layersa re composed of inorganic compounds that are chemically inert.
These inert compounds on the surface reduce bothanodic and cathodic areas
and delay the transit of reactive speciest ot he base metal. This results in
increases in the slopes of anodica nd cathodic polarization curves, thereby,
decreasing the rate of corrosion of the substrate.
Conversion layersare used for variousr easons:
1. To improve the adherence of the organic layers
2. To obtain electrically insulatingbarrier layers
3. To provide au niform grease-free surface
4. To provide active corrosion inhibition by reducing the rate of the
oxygen reduction reaction or by passivating the metallic substrate
Conversion coatings belonging in this group are phosphate, chromate,
oxide, and anodized coatings. These coatings are composed of corrosion
products that have been artificially formed by chemical or electrochemical
reactions in selecteds olutions. The corrosionp roducts formed build a
barrier protection for the substrate metal.T his barrier reduces the active
surfacea rea on the base metal, thereby,d elaying the transport of oxidizers
and aggressive species. By doing so the coating inhibits the formation of
corrosion cells. The degree of secondary barrier action depends on the
compactness,continuity,and stability of the corrosionproduct layer.
Each conversion coating protects the base metal against corrosion with
two or three of the following protective abilities:
1. Secondary barrieraction of corrosionproducts
2. Inhibiting action of soluble compounds containedi nt he corrosion products
3. Improvement in paint adhesion by the formation of au niform
corrosion product layer
Coatings
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CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
alloy and duplex coatings provide galvanic protection. Mn/Zn alloy coating
exhibits high corrosion resistance, andthe corrosion potential of manganese
is more negative than that of zinc; therefore, this alloy coating provides
cathodic protection for asteel substrate. The structure of Mn/Zn alloy coating
is composed of the single phase of 3 in the manganesecontent range ! 20%,
and 3 and g phases in the range above 20%. As the manganese content in the
depositisincreased, the percentage of g -Mn phaseisincreased.
7.12 Conversion Coatings
The term conversion coating is used to describe coatings where the substrate
metal provides ions that become part of the protective coating. The coating
layersa re composed of inorganic compounds that are chemically inert.
These inert compounds on the surface reduce bothanodic and cathodic areas
and delay the transit of reactive speciest ot he base metal. This results in
increases in the slopes of anodica nd cathodic polarization curves, thereby,
decreasing the rate of corrosion of the substrate.
Conversion layersare used for variousr easons:
1. To improve the adherence of the organic layers
2. To obtain electrically insulatingbarrier layers
3. To provide au niform grease-free surface
4. To provide active corrosion inhibition by reducing the rate of the
oxygen reduction reaction or by passivating the metallic substrate
Conversion coatings belonging in this group are phosphate, chromate,
oxide, and anodized coatings. These coatings are composed of corrosion
products that have been artificially formed by chemical or electrochemical
reactions in selecteds olutions. The corrosionp roducts formed build a
barrier protection for the substrate metal.T his barrier reduces the active
surfacea rea on the base metal, thereby,d elaying the transport of oxidizers
and aggressive species. By doing so the coating inhibits the formation of
corrosion cells. The degree of secondary barrier action depends on the
compactness,continuity,and stability of the corrosionproduct layer.
Each conversion coating protects the base metal against corrosion with
two or three of the following protective abilities:
1. Secondary barrieraction of corrosionproducts
2. Inhibiting action of soluble compounds containedi nt he corrosion products
3. Improvement in paint adhesion by the formation of au niform
corrosion product layer
Coatings
453
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485