Hard (engineering) chromium layersa re also deposited directly on a
variety of metals. The purpose in applying these layersi st oo btain wear
resistant surfaces with ahighhardness or to restoreoriginal dimensionstoa
workpiece. In addition, the excellent corrosion resistance resulting from
these layersm ake them suitablef or outdoor applications.
Thick chromium deposits have high residual internal stress and may be
brittle because of the electrodepositionp rocess that hydrogen can be
incorporated in the deposited layer.C racks result during plating when the
stress exceeds the tensile stress of the chromium. As plating continues, some
cracksare filled. This led to the development of controlled cracking patterns
that produce wet-table porous surfaces that can spread oil that is important
for engine cylinders, liners, etc.
Some of the properties of engineering chromium layersa re:
1. Excellent corrosion resistance
2. Wear resistance
3. Hardness up to 950 HV
4. Controlled porosity is possible
7.3.5 TinCoatings (Tinplate)
Tinplateismainlyproduced by the electroplating process. Alkaline and acid
baths are used in the production line. The acid baths are classified as either
ferrostan or halogen baths.
Athermal treatment above the melting point of tin followsthe electrolytic
deposition.T he intermetallic compound FeSn 2 formsa tt he interface
betweent he irona nd tin during this thermal processing. The corrosion
behavior of the tinplate is determined by the quality of the FeSn 2 formed,
particularly when the amount of the free tin is small. The best performing
tinplate is onewhere the FeSn 2 uniformly covers the steel so that the area of
iron exposed is very smalli nc ase the tin should dissolve. Good coverage
requires good and uniform nucleation of FeSn 2 .M any nuclei form when
electrodeposition of tin is carried out from the alkaline stannate bath.
Compared to either iron or tin, FeSn 2 is chemically inert in all but the
strongest oxidizing environments.
Most of the tinplate (tin coating on steel) produced is used for the
manufacture of foodc ontainers (tin cans).T he nontoxic nature of tin salts
makes tin an ideal material for the handling of foods and beverages.
An inspection of the galvanic series will indicate that tin is more noble
than steel and, consequently,the steel would corrode at the base of the pores.
On the outside of atinned container this is what happens—the tin is cathodic
to the steel. However,o nt he inside of the container there is ar eversal of
polarity because of the complexing of the stannous ions by many food
products. This greatly reduces the activity of the stannous ions, resulting in a
change in the potential of tin in the active direction.
Coatings
429
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
variety of metals. The purpose in applying these layersi st oo btain wear
resistant surfaces with ahighhardness or to restoreoriginal dimensionstoa
workpiece. In addition, the excellent corrosion resistance resulting from
these layersm ake them suitablef or outdoor applications.
Thick chromium deposits have high residual internal stress and may be
brittle because of the electrodepositionp rocess that hydrogen can be
incorporated in the deposited layer.C racks result during plating when the
stress exceeds the tensile stress of the chromium. As plating continues, some
cracksare filled. This led to the development of controlled cracking patterns
that produce wet-table porous surfaces that can spread oil that is important
for engine cylinders, liners, etc.
Some of the properties of engineering chromium layersa re:
1. Excellent corrosion resistance
2. Wear resistance
3. Hardness up to 950 HV
4. Controlled porosity is possible
7.3.5 TinCoatings (Tinplate)
Tinplateismainlyproduced by the electroplating process. Alkaline and acid
baths are used in the production line. The acid baths are classified as either
ferrostan or halogen baths.
Athermal treatment above the melting point of tin followsthe electrolytic
deposition.T he intermetallic compound FeSn 2 formsa tt he interface
betweent he irona nd tin during this thermal processing. The corrosion
behavior of the tinplate is determined by the quality of the FeSn 2 formed,
particularly when the amount of the free tin is small. The best performing
tinplate is onewhere the FeSn 2 uniformly covers the steel so that the area of
iron exposed is very smalli nc ase the tin should dissolve. Good coverage
requires good and uniform nucleation of FeSn 2 .M any nuclei form when
electrodeposition of tin is carried out from the alkaline stannate bath.
Compared to either iron or tin, FeSn 2 is chemically inert in all but the
strongest oxidizing environments.
Most of the tinplate (tin coating on steel) produced is used for the
manufacture of foodc ontainers (tin cans).T he nontoxic nature of tin salts
makes tin an ideal material for the handling of foods and beverages.
An inspection of the galvanic series will indicate that tin is more noble
than steel and, consequently,the steel would corrode at the base of the pores.
On the outside of atinned container this is what happens—the tin is cathodic
to the steel. However,o nt he inside of the container there is ar eversal of
polarity because of the complexing of the stannous ions by many food
products. This greatly reduces the activity of the stannous ions, resulting in a
change in the potential of tin in the active direction.
Coatings
429
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485