The service life of asingle layer coating is affected by any discontinuity in
the coating such as that caused by pores and cracks. For metals to form
ap rotective barrier,t he coating thickness must be greater than 30 m mt o
ensure absence of defects. The surfaceofabright nickel coating willremain
bright in aclean atmosphere but willchange to adull color when exposed in
an SO x atmosphere.
Chromium coatings are applied as at hin layer to maintain ab right,
tarnish-free surface. Cracking of chromium coatings begins at athickness of
0.5 m ma fter whichanetworkoffine cracks is formed.
For protection of steel in an SO x atmosphere, lead and its alloys (5–10% tin)
coatings are employed. Pitting will occur in the lead coating at the time
of initiale xposure, but the pits are self-healing, and the lead surfacei s
protected by the formationofi nsoluble lead sulfate.
7.1.5.2 MultilayerCoatings
There aret hree types of nickel coatings: bright nickel, semibright, and
dull. The difference is the quantity of sulfur contained in them as shown
below:
Bright nickeld eposits
O 0.04% Sulfur
Semibright nickel deposits
! 0.005% Sulfur
Dull nickel deposits
! 0.001% Sulfur
The corrosionp otentials of the nickeld eposits are dependent on the
sulfur content. Figure 7.5 shows the effect of sulfur contentonthe corrosion
potential of anickel deposit.
As the sulfur content increases, the corrosionpotential of anickel deposit
becomes more negative. Ab right nickel coating is less protective than
as emibright or dull nickelc oating. The difference in potential of bright
nickel and semibright deposits is more than 50 mV.
Use is made in the differences in the potential in the application of
multilayerc oatings. The more negative bright nickel deposits are used as
sacrificial intermediate layers. When bright nickel is used as an intermediate
TABLE 7.3
Corrosion Potentials of Noble Metals
CorrosionP otentials(V, SCE)
pH
2.9
6.5
Chromium
K 0.119
K 0.186
Nickel
K 0.412
K 0.430
Tin
K 0.486
K 0.554
Lead
K 0.435
K 0.637
Steel
K 0.636
K 0.505
Coatings
415
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
the coating such as that caused by pores and cracks. For metals to form
ap rotective barrier,t he coating thickness must be greater than 30 m mt o
ensure absence of defects. The surfaceofabright nickel coating willremain
bright in aclean atmosphere but willchange to adull color when exposed in
an SO x atmosphere.
Chromium coatings are applied as at hin layer to maintain ab right,
tarnish-free surface. Cracking of chromium coatings begins at athickness of
0.5 m ma fter whichanetworkoffine cracks is formed.
For protection of steel in an SO x atmosphere, lead and its alloys (5–10% tin)
coatings are employed. Pitting will occur in the lead coating at the time
of initiale xposure, but the pits are self-healing, and the lead surfacei s
protected by the formationofi nsoluble lead sulfate.
7.1.5.2 MultilayerCoatings
There aret hree types of nickel coatings: bright nickel, semibright, and
dull. The difference is the quantity of sulfur contained in them as shown
below:
Bright nickeld eposits
O 0.04% Sulfur
Semibright nickel deposits
! 0.005% Sulfur
Dull nickel deposits
! 0.001% Sulfur
The corrosionp otentials of the nickeld eposits are dependent on the
sulfur content. Figure 7.5 shows the effect of sulfur contentonthe corrosion
potential of anickel deposit.
As the sulfur content increases, the corrosionpotential of anickel deposit
becomes more negative. Ab right nickel coating is less protective than
as emibright or dull nickelc oating. The difference in potential of bright
nickel and semibright deposits is more than 50 mV.
Use is made in the differences in the potential in the application of
multilayerc oatings. The more negative bright nickel deposits are used as
sacrificial intermediate layers. When bright nickel is used as an intermediate
TABLE 7.3
Corrosion Potentials of Noble Metals
CorrosionP otentials(V, SCE)
pH
2.9
6.5
Chromium
K 0.119
K 0.186
Nickel
K 0.412
K 0.430
Tin
K 0.486
K 0.554
Lead
K 0.435
K 0.637
Steel
K 0.636
K 0.505
Coatings
415
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485