Aluminumand 55% Al–Zn alloy provide galvanic protection forthe steel
substrate. In rural and industrial atmospheres, an aluminumc oating does
not act as as acrificial anode.H owever,i nachloride atmosphere sucha sa
marinea rea, it does act as as acrificial anode.
The choice of what sacrificial metal coating to use willb eb ased on the
environment that it willb ee xposed to and the service life required. The
service life required will also determine the coating thickness to be applied
that,i nt urn,w illi nfluence the coating process to be used. Sacrificial metal
coatings have been used successfully forroofs, walls, ducts, shutters, doors,
and window frames in the housing industry; and on structural materials
sucha st ransmission towers, structural members of ab ridge, antennae,
chimney structures, grand-stands, steel frames, high-strength steel bolts,
guard rails, corrugated steel pipe, stadium seats, bridge I-beams,f ootway
bridges, road bridges, and fencing.
7.4.1Z inc Coatings
Approximately half of the world’sp roduction of zinc is used to protect
steel from rust. Zinc coatings are probably the mosti mportant typeo f
metallic coating for corrosion protection of steel. The reasons for the wide
application are:
1. Prices are relatively low.
2. Because of large reserves, an ample supplyo fzincisa vailable.
3. There is great flexibility in application procedures resulting in
many different qualitiesw ith well-controlled layer thicknesses.
4. Steel provides good cathodic protection.
5. Many special alloy systems have been developed with improved
corrosionprotection properties.
The ability to select aparticular alloy or to specify aparticular thickness of
coating depends on the type of coating process used.Z incc oatings can be
applied in many ways.The six mostcommonly used procedures follow.
7.4.1.1 Hot Dipping
This is aprocess where cleanedsteel is immersed in moltenzinc or zinc alloy,
and ar eaction takes place to form am etallurgically bonded coating.
The coating is integral with the steel because the formation process
produces zinc–iron alloyl ayerso vercoated with zinc. Continuitya nd
uniformityi sg ood since any discontinuities are readily visible as “black
spots.”
Coating thicknesses can be varied from approximately 50–125 m montube
and products. Thicker coatings up to 250 m mcan be obtained by grit-blasting
before galvanizing. Sheet and wire normally receive thicknesses of 10–30 m m.
Corrosion of Linings and Coatings
440
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
substrate. In rural and industrial atmospheres, an aluminumc oating does
not act as as acrificial anode.H owever,i nachloride atmosphere sucha sa
marinea rea, it does act as as acrificial anode.
The choice of what sacrificial metal coating to use willb eb ased on the
environment that it willb ee xposed to and the service life required. The
service life required will also determine the coating thickness to be applied
that,i nt urn,w illi nfluence the coating process to be used. Sacrificial metal
coatings have been used successfully forroofs, walls, ducts, shutters, doors,
and window frames in the housing industry; and on structural materials
sucha st ransmission towers, structural members of ab ridge, antennae,
chimney structures, grand-stands, steel frames, high-strength steel bolts,
guard rails, corrugated steel pipe, stadium seats, bridge I-beams,f ootway
bridges, road bridges, and fencing.
7.4.1Z inc Coatings
Approximately half of the world’sp roduction of zinc is used to protect
steel from rust. Zinc coatings are probably the mosti mportant typeo f
metallic coating for corrosion protection of steel. The reasons for the wide
application are:
1. Prices are relatively low.
2. Because of large reserves, an ample supplyo fzincisa vailable.
3. There is great flexibility in application procedures resulting in
many different qualitiesw ith well-controlled layer thicknesses.
4. Steel provides good cathodic protection.
5. Many special alloy systems have been developed with improved
corrosionprotection properties.
The ability to select aparticular alloy or to specify aparticular thickness of
coating depends on the type of coating process used.Z incc oatings can be
applied in many ways.The six mostcommonly used procedures follow.
7.4.1.1 Hot Dipping
This is aprocess where cleanedsteel is immersed in moltenzinc or zinc alloy,
and ar eaction takes place to form am etallurgically bonded coating.
The coating is integral with the steel because the formation process
produces zinc–iron alloyl ayerso vercoated with zinc. Continuitya nd
uniformityi sg ood since any discontinuities are readily visible as “black
spots.”
Coating thicknesses can be varied from approximately 50–125 m montube
and products. Thicker coatings up to 250 m mcan be obtained by grit-blasting
before galvanizing. Sheet and wire normally receive thicknesses of 10–30 m m.
Corrosion of Linings and Coatings
440
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485