wherez inc–5% aluminum coatings provideg ood generalc orrosion
resistance, and when sheared edges are exposed or localizedd amage to
the coating occurs during fabrication or service,t he galvanic protection is
retained for al onger period.
7.7Z inc–15%AluminumT hermalS pray
Zinc–15% aluminum coatings are available as thermally sprayed coatings.
These coatings have at wo-phases tructure consisting of az inc-rich and an
aluminum-rich phase. The oxidation products formed are encapsulated in
the porous layer formed by the latter and do not build up ac ontinuous
surfacelayer as with pure zinc coatings. As aresult, no thickness or weight
loss is observed even after several years of exposure in atmospheric field
testing.
It is normally recommended that thermally sprayed coatings be sealed
to avoid initial rust stains, to improve appearance, and to facilitate maintenance painting. Sealing is designed to fill pores and give onlyathin overall
coating, toot hin to be directly measurable. Epoxy or acrylic system resin,
havingalow viscosity,isu sed as asealer.
7.8Z inc–Iron AlloyC oatings
As compared with pure zinc, the zinc–iron alloy coatings provide increased
corrosion resistance in acid atmospheres, but slightly reduced corrosion
resistance in alkaline atmospheres.
Electroplated zinc–iron alloyl ayersc ontaining more than 20% iron
provide ac orrosion resistance 30%h igher than zinci ni ndustrial atmospheres. In other atmospheres, the zinc–irongalvanized coatings provide as
good acoating as coatings with an outer zinc layer.Sherardized coatings are
superiort oe lectroplated coatings and equal to galvanized coatings of the
same thickness. However,t he structure of the alloy layer,a nd its composition, affects the corrosion resistance.
If the zinc layer of ag alvanized coating has weathered, or the zinc–iron
alloy layer formst he top layer after galvanizing, brown areas may form.
Brown staining can occur on sherardized or hot dip galvanizedc oatings in
atmospheric corrosion through the oxidation of ironf rom the zinc–iron
alloy layers or from the substrate. Such staining is usually ad ull brown
rathert han the bright red–browno fu ncontrolled rust. Usually,t herei sa
substantial intact galvanized layer underneath, leavingt he life of the
coating unchanged. Unless the aesthetic appearance is undesirable, no
action need be taken.
Coatings
451
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
resistance, and when sheared edges are exposed or localizedd amage to
the coating occurs during fabrication or service,t he galvanic protection is
retained for al onger period.
7.7Z inc–15%AluminumT hermalS pray
Zinc–15% aluminum coatings are available as thermally sprayed coatings.
These coatings have at wo-phases tructure consisting of az inc-rich and an
aluminum-rich phase. The oxidation products formed are encapsulated in
the porous layer formed by the latter and do not build up ac ontinuous
surfacelayer as with pure zinc coatings. As aresult, no thickness or weight
loss is observed even after several years of exposure in atmospheric field
testing.
It is normally recommended that thermally sprayed coatings be sealed
to avoid initial rust stains, to improve appearance, and to facilitate maintenance painting. Sealing is designed to fill pores and give onlyathin overall
coating, toot hin to be directly measurable. Epoxy or acrylic system resin,
havingalow viscosity,isu sed as asealer.
7.8Z inc–Iron AlloyC oatings
As compared with pure zinc, the zinc–iron alloy coatings provide increased
corrosion resistance in acid atmospheres, but slightly reduced corrosion
resistance in alkaline atmospheres.
Electroplated zinc–iron alloyl ayersc ontaining more than 20% iron
provide ac orrosion resistance 30%h igher than zinci ni ndustrial atmospheres. In other atmospheres, the zinc–irongalvanized coatings provide as
good acoating as coatings with an outer zinc layer.Sherardized coatings are
superiort oe lectroplated coatings and equal to galvanized coatings of the
same thickness. However,t he structure of the alloy layer,a nd its composition, affects the corrosion resistance.
If the zinc layer of ag alvanized coating has weathered, or the zinc–iron
alloy layer formst he top layer after galvanizing, brown areas may form.
Brown staining can occur on sherardized or hot dip galvanizedc oatings in
atmospheric corrosion through the oxidation of ironf rom the zinc–iron
alloy layers or from the substrate. Such staining is usually ad ull brown
rathert han the bright red–browno fu ncontrolled rust. Usually,t herei sa
substantial intact galvanized layer underneath, leavingt he life of the
coating unchanged. Unless the aesthetic appearance is undesirable, no
action need be taken.
Coatings
451
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485