To solve this problem, ad uplexn ickel coating was developed as shown
in Figure 7.10.A ni nitial layer of sulfur-free nickeli sa pplied to the steel
substrate, followed by an inner layer of ab right nickel containing sulfur,
with an outer layer of microcracked chromium.
Any corrosion that takesp lace is limited to the bright nickel layer
containing sulfur.T he corrosion laterally spreads between the chromium
and sulfur free nickel deposits because the outer members of this sandwich
(chromium and sulfur free nickel)are cathodic to the sulfur containing nickel.
Apotential problem that could result from this system of corrosion control
would be the undermining of the chromium and the possibility that the
brittlechromium deposits could flake offthe surface. This potential problem
was prevented by the development of am icrocracked or microporous
chromium coating. These coatings contain microcracks or microporest hat
do not detract from the bright appearance of the chromium. Theya re
uniformly formed over the exterioro ft he plated material and serve to
distribute the corrosionp rocess over the entire surface. The result has
been to extend the life of chromium-plated steel exposed to outdoor
atmospheric conditions.
Microcracked chromium coatings are produced by first depositing ahigh
stress nickel strikeo nasulfur free nickell ayer and then ad ecorative
chromium deposit. The uniform crack network results from the interaction
of the thin chromium layer and the high stress nickel deposit. The result is a
mirror surfacea sw ell as ad ecorative chromium coating.
Microporous chromium coatings are produced by first electroplating
ab right nickel layer containing suspended nonconductive fine particles.
Over this, ac hromium layer is depositedw hich results in am irror finish.
As the chromium thickness increases the number of pores decrease. For
ac hromiumd eposit of 0.25 m mt hickness,ap orosityo fm oret han
10,000 pores/cm
2
are required. Ap orosity of 40,000 pores/cm
2
provides
the best corrosion resistance.
Sulfur-free nickel
Bright nickel
containing sulfur
Corrosion contained to
nickellayer containing sulfur
Microcrackedchromium
Steel substrate
FIGURE 7.10
Duplex nickel electrode deposit to prevent corrosion of steel substrate.
Corrosion of Linings and Coatings
428
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
in Figure 7.10.A ni nitial layer of sulfur-free nickeli sa pplied to the steel
substrate, followed by an inner layer of ab right nickel containing sulfur,
with an outer layer of microcracked chromium.
Any corrosion that takesp lace is limited to the bright nickel layer
containing sulfur.T he corrosion laterally spreads between the chromium
and sulfur free nickel deposits because the outer members of this sandwich
(chromium and sulfur free nickel)are cathodic to the sulfur containing nickel.
Apotential problem that could result from this system of corrosion control
would be the undermining of the chromium and the possibility that the
brittlechromium deposits could flake offthe surface. This potential problem
was prevented by the development of am icrocracked or microporous
chromium coating. These coatings contain microcracks or microporest hat
do not detract from the bright appearance of the chromium. Theya re
uniformly formed over the exterioro ft he plated material and serve to
distribute the corrosionp rocess over the entire surface. The result has
been to extend the life of chromium-plated steel exposed to outdoor
atmospheric conditions.
Microcracked chromium coatings are produced by first depositing ahigh
stress nickel strikeo nasulfur free nickell ayer and then ad ecorative
chromium deposit. The uniform crack network results from the interaction
of the thin chromium layer and the high stress nickel deposit. The result is a
mirror surfacea sw ell as ad ecorative chromium coating.
Microporous chromium coatings are produced by first electroplating
ab right nickel layer containing suspended nonconductive fine particles.
Over this, ac hromium layer is depositedw hich results in am irror finish.
As the chromium thickness increases the number of pores decrease. For
ac hromiumd eposit of 0.25 m mt hickness,ap orosityo fm oret han
10,000 pores/cm
2
are required. Ap orosity of 40,000 pores/cm
2
provides
the best corrosion resistance.
Sulfur-free nickel
Bright nickel
containing sulfur
Corrosion contained to
nickellayer containing sulfur
Microcrackedchromium
Steel substrate
FIGURE 7.10
Duplex nickel electrode deposit to prevent corrosion of steel substrate.
Corrosion of Linings and Coatings
428
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485