are formulated as zinc-rich coatings. In doings o, the highp igmentc ontent
changes the properties of the formulated coating.
Zinc-rich coatings can be classified as organic or inorganic. The organic
zinc-riches have organic binders with polyamidee poxiesa nd chlorinated
rubber binders beingt he mostc ommon.O thert ypes such as urethane
zinc-rich are also available. These latter coatings arem oree asily applied
than the other zinc-rich coatings.
Inorganic zinc-rich binders are based on silicate solutions that after drying
or curing, crystallize and form an inorganic matrix, holding zinc dust
particles together and to the steel substrate.
For ac oating to be considered zinc-rich, it must contain at least 75% by
weight of zinc dust in the dry film. This may change because conductive
extenders (notably di-ironp hosphite) have been added to improve weldability and burn-through with supposedly equivalent protection at lower
zinc loadings.
The primary advantage of zinc-rich coatings is their ability to galvanically
protect. The zincp igmenti nt he coating preferentially sacrifices itself in
the electrochemical corrosion reaction to protect the underlying steel. This
galvanic reaction, together with the filling and sealing effect of zinc reaction
products (primarily zinc carbonate, zinc hydroxide, and complex zincsalts),
provides more effective corrosionp rotection to steel substratest han does
any other typeofcoating. Zinc-rich coatings cannot be used outside the pH
range of 6–10.5.
Table 8.3p resents as ummary of the properties of various paint
formulations.
8.6S electing aP aint System
The first step in selectingap aint system for corrosion protection is to
determine the environment around the structure or item to be painted. Is
the environment predominantly aw eathering environment subject to heat,
cold, daily or seasonal temperature changes, precipitation, wind (flexing),
exposure to sunlight, or detrimental solar rays? If the structure or item is
located outdoors, aret here chemical plants located nearby,o rp ulp and
paper mills, or other industrial facilities that are apt to discharge airborne
pollutants? Is color,g loss, and overallp leasing effect more important than
corrosion protection, or arethe normal grays,whites, and pastels of the more
corrosion resistant paints satisfactory? If located in achemical facility,what
specificchemicals are used nearby; is there any chance of achemical spill on
the paintedsurface?
Since surface preparation is an important factor in the selectionofapaint
system, the suitability or availability of the surfacef or specificp reparation
techniques must be known. In some instances, certain types of surface
preparation may not be permitted or practical. For example, many companies
Corrosion of Linings and Coatings
496
CAT8247—CHAPTER 8—17/10/2006—14:20—SRIDHAR—XML MODEL B–pp. 487–507
changes the properties of the formulated coating.
Zinc-rich coatings can be classified as organic or inorganic. The organic
zinc-riches have organic binders with polyamidee poxiesa nd chlorinated
rubber binders beingt he mostc ommon.O thert ypes such as urethane
zinc-rich are also available. These latter coatings arem oree asily applied
than the other zinc-rich coatings.
Inorganic zinc-rich binders are based on silicate solutions that after drying
or curing, crystallize and form an inorganic matrix, holding zinc dust
particles together and to the steel substrate.
For ac oating to be considered zinc-rich, it must contain at least 75% by
weight of zinc dust in the dry film. This may change because conductive
extenders (notably di-ironp hosphite) have been added to improve weldability and burn-through with supposedly equivalent protection at lower
zinc loadings.
The primary advantage of zinc-rich coatings is their ability to galvanically
protect. The zincp igmenti nt he coating preferentially sacrifices itself in
the electrochemical corrosion reaction to protect the underlying steel. This
galvanic reaction, together with the filling and sealing effect of zinc reaction
products (primarily zinc carbonate, zinc hydroxide, and complex zincsalts),
provides more effective corrosionp rotection to steel substratest han does
any other typeofcoating. Zinc-rich coatings cannot be used outside the pH
range of 6–10.5.
Table 8.3p resents as ummary of the properties of various paint
formulations.
8.6S electing aP aint System
The first step in selectingap aint system for corrosion protection is to
determine the environment around the structure or item to be painted. Is
the environment predominantly aw eathering environment subject to heat,
cold, daily or seasonal temperature changes, precipitation, wind (flexing),
exposure to sunlight, or detrimental solar rays? If the structure or item is
located outdoors, aret here chemical plants located nearby,o rp ulp and
paper mills, or other industrial facilities that are apt to discharge airborne
pollutants? Is color,g loss, and overallp leasing effect more important than
corrosion protection, or arethe normal grays,whites, and pastels of the more
corrosion resistant paints satisfactory? If located in achemical facility,what
specificchemicals are used nearby; is there any chance of achemical spill on
the paintedsurface?
Since surface preparation is an important factor in the selectionofapaint
system, the suitability or availability of the surfacef or specificp reparation
techniques must be known. In some instances, certain types of surface
preparation may not be permitted or practical. For example, many companies
Corrosion of Linings and Coatings
496
CAT8247—CHAPTER 8—17/10/2006—14:20—SRIDHAR—XML MODEL B–pp. 487–507