0.0476–0.515 mil/year in industrial atmospheres
0.0198–0.0562 mil/year in marinea tmospheres
Until the base metal is exposed, the corrosionprocess of ac opper coated
layer is similar to that of copper plate. Galvanic corrosion of copper coated
steel is induced when the steel substrate is exposed. However,inthe case of
copper coated stainless steel, the occurrence of galvanicactionisdependent
on the composition of the stainless steel.
In chloride atmospheres galvanic pitting takesplace at the pores in copper
layersa nd galvanic tunneling at cut edges on types 409 and 430 stainless
steels, whereas, in SO x atmospheres uniform corrosiont akes place on the
copper coating.
Copper coatings are used both for decorativeand for corrosion protection
from the atmosphere. Copper coateds teels are used as roofs, flashings,
leaders, gutters, andarchitectural trim. Copper undercoats also improve the
corrosion resistance of multilayeredc oatings, specifically in the platingo f
nickel and chromium.
7.4N onnobleCoatings
Nonnoble metals protect the substrateb ym eans of cathodicc ontrol.
Cathodic overpotential of the surface is increased by coating which makes
the corrosion potential more negative than that of the substrate.The coating
metals used for cathodic control protection arezinc, aluminum, manganese,
and cadmium-and theira lloys, of which the electrode potentials arem ore
negativet han those of iron and steel. Consequently,t he coating layers
of these metals act as sacrificial anodesf or iron and steel substrates, when
the substrates aree xposed to the atmospheres. The coating layer provides
cathodic protection for the substrate by galvanic action. These metals are
called sacrificial metals.
The electrical conductivity of the electrolyte, the temperature, and
the surfacec ondition determines the galvanic action of the sacrificial
metal coating. An increase in the cathodic overpotential is responsible
for the corrosion resistance of the coating layer.F igure 7.14 shows the
principle of cathodic control protection by as acrificial metal coating.
The corrosion rate of zinc coated iron i corr of zinc coating becomes lower than
that of uncoated iron i corr of zinc uncoated iron because the cathodic overpotential of the surfacei si ncreased by zinc coating and the exchange
current densityo fd issolvedo xygen on zinc i oc on zinc is lower than that on
iron i oc on iron .I fasmallp art of ironi se xposed to the atmosphere, the
electrode potential of the exposed ironisequal to the corrosion potential of
zinc E corr of zinc coating because the exposed ironi sp olarized cathodically by
the surrounding zincs ot hat little corrosiono ccurs on the exposed iron
Coatings
435
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
0.0198–0.0562 mil/year in marinea tmospheres
Until the base metal is exposed, the corrosionprocess of ac opper coated
layer is similar to that of copper plate. Galvanic corrosion of copper coated
steel is induced when the steel substrate is exposed. However,inthe case of
copper coated stainless steel, the occurrence of galvanicactionisdependent
on the composition of the stainless steel.
In chloride atmospheres galvanic pitting takesplace at the pores in copper
layersa nd galvanic tunneling at cut edges on types 409 and 430 stainless
steels, whereas, in SO x atmospheres uniform corrosiont akes place on the
copper coating.
Copper coatings are used both for decorativeand for corrosion protection
from the atmosphere. Copper coateds teels are used as roofs, flashings,
leaders, gutters, andarchitectural trim. Copper undercoats also improve the
corrosion resistance of multilayeredc oatings, specifically in the platingo f
nickel and chromium.
7.4N onnobleCoatings
Nonnoble metals protect the substrateb ym eans of cathodicc ontrol.
Cathodic overpotential of the surface is increased by coating which makes
the corrosion potential more negative than that of the substrate.The coating
metals used for cathodic control protection arezinc, aluminum, manganese,
and cadmium-and theira lloys, of which the electrode potentials arem ore
negativet han those of iron and steel. Consequently,t he coating layers
of these metals act as sacrificial anodesf or iron and steel substrates, when
the substrates aree xposed to the atmospheres. The coating layer provides
cathodic protection for the substrate by galvanic action. These metals are
called sacrificial metals.
The electrical conductivity of the electrolyte, the temperature, and
the surfacec ondition determines the galvanic action of the sacrificial
metal coating. An increase in the cathodic overpotential is responsible
for the corrosion resistance of the coating layer.F igure 7.14 shows the
principle of cathodic control protection by as acrificial metal coating.
The corrosion rate of zinc coated iron i corr of zinc coating becomes lower than
that of uncoated iron i corr of zinc uncoated iron because the cathodic overpotential of the surfacei si ncreased by zinc coating and the exchange
current densityo fd issolvedo xygen on zinc i oc on zinc is lower than that on
iron i oc on iron .I fasmallp art of ironi se xposed to the atmosphere, the
electrode potential of the exposed ironisequal to the corrosion potential of
zinc E corr of zinc coating because the exposed ironi sp olarized cathodically by
the surrounding zincs ot hat little corrosiono ccurs on the exposed iron
Coatings
435
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485