i corr of exposed iron .Z inci ons predominately dissolved from the zinc coating
form the surrounding barrier of corrosion products at the defect and,
thereby,p rotect the exposed iron.
Sacrificial metal coatings protect ironand steel by two or three protective
abilities such as:
1. Originalb arrier actionofc oating layer
2. Secondarybarrier actionofc orrosion product layer
3. Galvanic action of coating layer
The surface oxide film and the electrochemical properties based on the
metallography of the coating metal provide the original barrier action.
Electrode potential
E corr
E corr
of iron
of zinc
i oc
i oc
i corr
i corr
i corr
of
exposed
iron
of
zinc
coating
of
uncoated
iron
on zinc on iron
Log current density
O
2 + 2 H
2 O + 4 e
4 O H −
4
O
H
−
F e
F e
2 +
+
2 e
Z n
Z n
2 + +
2 e
O 2
+
2 H 2
O
+
4 e
FIGURE 7.14
Cathodic control protection.
Corrosion of Linings and Coatings
436
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485