of galvanizing is the formation of am etallurgical bond at the zinc–iron
interface. This does not occur in electroplating.
7.4.1.3 Mechanical Coating
This process involves the agitating of suitablyp repared parts to be coated
with am ixture of nonmetallic impactors (e.g., glass beads),z inc powder,a
chemical promoter,and water.All types of steel can be coated. However, this
process is less suitablef or parts heavier than 1/2 lb (250 g) because the
tumbling process reduces coating thickness at the edges.
The adhesion is good compared with electroplated coatings. Thickness
can be varieda sd esired from 5 m mt om ore than 70 m m. However,t he
coating is not alloyed with the steel nor does it have the hard, abrasion
resistance iron–zinc alloy layerso fg alvanized or sherardized coatings.
Conversion coatings can be applied.
7.4.1.4 Sherardizing
The articlest ob ec oateda re tumbledi nabarrel containing zinc dust at a
temperature just below the melting point of zinc, usually around 7168 F/
3808 C. In the case of spring steels, the temperature used is somewhat lower.
By means of adiffusion process, the zinc bonds to the steel forming ahard,
even coating of zinc–iron compounds. The coating is dull gray in color and
can readilyb ep aintedi fnecessary.
The finish is continuous and very uniform, even on threaded and irregular
parts. This is av ery useful finishf or nuts and bolts that,w ith proper
allowance for thickness of coats,c an be sherardized after manufacture and
used without retapping the threads.
The thickness of coating can be controlled. Usually,athickness of 30 m mis
used for outdoor applications while 15 m misu sed indoors.
7.4.1.5 Thermally SprayedC oatings
In the process, droplets of semimolten zinc are sprayed from as pecial gun
that is fed with either wire or powder onto ag rit-blasted surface. The
seimmolten droplets coalescewith somezinc oxide present at each interface
between droplets. Electrical continuity is maintained both throughout the
coating and with the iron substrate so that full cathodic protection can be
obtained since the zinc oxide formso nly as mall percentage of the coating.
The sprayed coating contains voids (typically 10–20% by volume) between
coalesced particles. These voids have little effect on the corrosion protection
because they soon fill up with zinc corrosionp roducts and aret hereafter
impermeable. However, the use of as ealer to fill the voids improves
appearance in service and adds to life expectancy,b ut more importantly,i t
provides abetter surface forsubsequent application of paints.
There are no size or shapel imitations regarding the use of this process.
Corrosion of Linings and Coatings
442
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
interface. This does not occur in electroplating.
7.4.1.3 Mechanical Coating
This process involves the agitating of suitablyp repared parts to be coated
with am ixture of nonmetallic impactors (e.g., glass beads),z inc powder,a
chemical promoter,and water.All types of steel can be coated. However, this
process is less suitablef or parts heavier than 1/2 lb (250 g) because the
tumbling process reduces coating thickness at the edges.
The adhesion is good compared with electroplated coatings. Thickness
can be varieda sd esired from 5 m mt om ore than 70 m m. However,t he
coating is not alloyed with the steel nor does it have the hard, abrasion
resistance iron–zinc alloy layerso fg alvanized or sherardized coatings.
Conversion coatings can be applied.
7.4.1.4 Sherardizing
The articlest ob ec oateda re tumbledi nabarrel containing zinc dust at a
temperature just below the melting point of zinc, usually around 7168 F/
3808 C. In the case of spring steels, the temperature used is somewhat lower.
By means of adiffusion process, the zinc bonds to the steel forming ahard,
even coating of zinc–iron compounds. The coating is dull gray in color and
can readilyb ep aintedi fnecessary.
The finish is continuous and very uniform, even on threaded and irregular
parts. This is av ery useful finishf or nuts and bolts that,w ith proper
allowance for thickness of coats,c an be sherardized after manufacture and
used without retapping the threads.
The thickness of coating can be controlled. Usually,athickness of 30 m mis
used for outdoor applications while 15 m misu sed indoors.
7.4.1.5 Thermally SprayedC oatings
In the process, droplets of semimolten zinc are sprayed from as pecial gun
that is fed with either wire or powder onto ag rit-blasted surface. The
seimmolten droplets coalescewith somezinc oxide present at each interface
between droplets. Electrical continuity is maintained both throughout the
coating and with the iron substrate so that full cathodic protection can be
obtained since the zinc oxide formso nly as mall percentage of the coating.
The sprayed coating contains voids (typically 10–20% by volume) between
coalesced particles. These voids have little effect on the corrosion protection
because they soon fill up with zinc corrosionp roducts and aret hereafter
impermeable. However, the use of as ealer to fill the voids improves
appearance in service and adds to life expectancy,b ut more importantly,i t
provides abetter surface forsubsequent application of paints.
There are no size or shapel imitations regarding the use of this process.
Corrosion of Linings and Coatings
442
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485