Lead coatings are usually applied by either hot dipping or by electrodeposition. When the coating is to be applied by hot dipping, as mall
percentage of tin is added to improve the adhesion to the steel plate. If 25%
or more of tin is added, the resulting coating is termed terneplate.
Caution:D on ot use lead coatings where they will come into contact
with drinking water or food products. Lead salts can be formed that are
poisonous.
7.3.7T erneplate
Terneplate is at in-lead alloy coated sheet steel and is produced eitherb y
hot dipping or electrodeposition. The hotd ipping process with ac hloride
flux is used to produce most terneplates. The coating layer,whose electrode
potential is more noble than that of the steel substrate, contains8 –16% Sn.
Since the electrode potential of the coating layer is more noble than the steel
substrate, it is necessary to build auniform and dense alloy layer (FeSn 2 )in
order to form apinhole free deposit.
Terneplate exhibits excellent corrosionr esistance, especially underw et
conditions, excellentw eldability and formability,w ith only small amounts
of corrosionp roducts forming on the surface. At hin nickeld eposit can be
applied as an undercoat for the terne layer.Nickelreacts rapidly with the tinlead alloy to form an ickel–tin alloy layer.T hisa lloy layer provides good
corrosion resistance and inhibits localized corrosion.
The maina pplication for terneplate is in the production of fuel tanks
for automobiles.
7.3.8G oldCoatings
Gold electrodepositsa re primarilyu sedt oc oatc opperi ne lectronic
applications to protect the copper connectors and other copper components
from corrosion. It is desirable to obtain the corrosion protection with the
minimum thickness of gold because of the costofthe gold. As the thickness
of the electrodeposit is decreased, there is at endencyf or the depositt o
provide inadequate coverage of the copper.F or this reason, it is necessary
that therebeameans whereby the coverage of the coppercan be determined.
Such atest,usingc orrosion principles as ag uide,has been developed. In a
0.1 MN H 4 Cl solution, gold serves as the cathode and copper servesa st he
anode. At ah igh cathode/anode surfaceareaf raction, the corrosionpotential is linearly related to the area fraction of copper exposed, as shown in
Figure 7.13.Bymeasuring the corrosion potential of the gold plated copper
in a0.1 MNH 4 Cl solution, the area fraction of copper exposed is determined.
Gold coatings can also be deposited by means of electroless plating.
Borohydride or DMABa re used as reducers with as table gold cyanide
complex. Thing old coatings may be deposited on plastics by an aerosol
spraym ethod using gold complexesw ith amines and hydrazine as a
reducer.Arelatively thick coat may be obtained.
Corrosion of Linings and Coatings
432
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
percentage of tin is added to improve the adhesion to the steel plate. If 25%
or more of tin is added, the resulting coating is termed terneplate.
Caution:D on ot use lead coatings where they will come into contact
with drinking water or food products. Lead salts can be formed that are
poisonous.
7.3.7T erneplate
Terneplate is at in-lead alloy coated sheet steel and is produced eitherb y
hot dipping or electrodeposition. The hotd ipping process with ac hloride
flux is used to produce most terneplates. The coating layer,whose electrode
potential is more noble than that of the steel substrate, contains8 –16% Sn.
Since the electrode potential of the coating layer is more noble than the steel
substrate, it is necessary to build auniform and dense alloy layer (FeSn 2 )in
order to form apinhole free deposit.
Terneplate exhibits excellent corrosionr esistance, especially underw et
conditions, excellentw eldability and formability,w ith only small amounts
of corrosionp roducts forming on the surface. At hin nickeld eposit can be
applied as an undercoat for the terne layer.Nickelreacts rapidly with the tinlead alloy to form an ickel–tin alloy layer.T hisa lloy layer provides good
corrosion resistance and inhibits localized corrosion.
The maina pplication for terneplate is in the production of fuel tanks
for automobiles.
7.3.8G oldCoatings
Gold electrodepositsa re primarilyu sedt oc oatc opperi ne lectronic
applications to protect the copper connectors and other copper components
from corrosion. It is desirable to obtain the corrosion protection with the
minimum thickness of gold because of the costofthe gold. As the thickness
of the electrodeposit is decreased, there is at endencyf or the depositt o
provide inadequate coverage of the copper.F or this reason, it is necessary
that therebeameans whereby the coverage of the coppercan be determined.
Such atest,usingc orrosion principles as ag uide,has been developed. In a
0.1 MN H 4 Cl solution, gold serves as the cathode and copper servesa st he
anode. At ah igh cathode/anode surfaceareaf raction, the corrosionpotential is linearly related to the area fraction of copper exposed, as shown in
Figure 7.13.Bymeasuring the corrosion potential of the gold plated copper
in a0.1 MNH 4 Cl solution, the area fraction of copper exposed is determined.
Gold coatings can also be deposited by means of electroless plating.
Borohydride or DMABa re used as reducers with as table gold cyanide
complex. Thing old coatings may be deposited on plastics by an aerosol
spraym ethod using gold complexesw ith amines and hydrazine as a
reducer.Arelatively thick coat may be obtained.
Corrosion of Linings and Coatings
432
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485