Conventional coatings that area pplied to finisheda rticlesa re not
formable. The alloy layer is abrasionresistant but brittle on bending. Special
coatings with little or no alloy layer are readilyf ormed (e.g., on sheet) and
resistance welded.
Achromate conversion coating over the zinc coating prevents wet storage
stain, whereas, phosphate conversion coatings provide ag ood base( on a
new sheet) for paints. Weathered coatings are often painted after 10–30 years
for longer service.
Hot-dip galvanizing is the most important zinc coating process. All mild
steels and cast ironcan be coated by this process. The thickness and structure
of the coating will depend on the alloying elements. Approximately half
of the steel that is coatedisinthe form of sheet, approximately one quarter
is fabricated work, and the remainder is tubeo rw ire. Metallurgically,
the processes used for tubes and fabricatedw ork is similar,w hereas,
the process used for sheeth as small additions to the zinc that reduces the
quantity of iron–zinc alloy in the coating which provides flexibility.
7.4.1.2 Zinc Electroplating
This process is sometimes mistakenly referred to as electrogalvanizing.
In this process, zinc salt solutions areu sed in the electrolytic deposition of
al ayer of zinc on ac leaned steel surface.
This process provides good adhesion, comparable with othere lectroplated coatings. The coating is uniform within the limitations of “throwing
power” of the bath. Pores are not aproblem as exposed steel is protected by
the adjacent zinc.
Coating thickness can be varied at will but is usually 2.5–15 m m. Thicker
layersa re possible but are not generally economical.
Electroplated steel has excellent formability and can be spot welded. Small
components are usually finished before beingp lated.
Chromate conversionc oatings are used to prevent wet storage stain,
whereas, phosphate conversioncoatings are used as ab ase for paint.
The process is normally used for simple,f airly small components. It is
suitable for barrel plating or for continuous sheeta nd wire. No heating is
used in this process exceptf or hydrogen embrittlement relief on high
strength steels.
Electroplated zinc is very ductile, and consequently,this process is widely
used for the continuousplating of strip and wire where severe deformation
may be required.
The coating on steel from this process gives abright and smooth finish. It
is used for decorative effect to protect delicate objects where rough or uneven
finishes cannot be tolerated (e.g., instrument parts).Itisalso used for articles
that cannot withstand the pretreatment or temperatures required in other
coating processes.
It was previously mentioned that the term electrogalvanizing is sometimes
used to describethis process. This is misleadingsince the chiefcharacteristic
Coatings
441
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
formable. The alloy layer is abrasionresistant but brittle on bending. Special
coatings with little or no alloy layer are readilyf ormed (e.g., on sheet) and
resistance welded.
Achromate conversion coating over the zinc coating prevents wet storage
stain, whereas, phosphate conversion coatings provide ag ood base( on a
new sheet) for paints. Weathered coatings are often painted after 10–30 years
for longer service.
Hot-dip galvanizing is the most important zinc coating process. All mild
steels and cast ironcan be coated by this process. The thickness and structure
of the coating will depend on the alloying elements. Approximately half
of the steel that is coatedisinthe form of sheet, approximately one quarter
is fabricated work, and the remainder is tubeo rw ire. Metallurgically,
the processes used for tubes and fabricatedw ork is similar,w hereas,
the process used for sheeth as small additions to the zinc that reduces the
quantity of iron–zinc alloy in the coating which provides flexibility.
7.4.1.2 Zinc Electroplating
This process is sometimes mistakenly referred to as electrogalvanizing.
In this process, zinc salt solutions areu sed in the electrolytic deposition of
al ayer of zinc on ac leaned steel surface.
This process provides good adhesion, comparable with othere lectroplated coatings. The coating is uniform within the limitations of “throwing
power” of the bath. Pores are not aproblem as exposed steel is protected by
the adjacent zinc.
Coating thickness can be varied at will but is usually 2.5–15 m m. Thicker
layersa re possible but are not generally economical.
Electroplated steel has excellent formability and can be spot welded. Small
components are usually finished before beingp lated.
Chromate conversionc oatings are used to prevent wet storage stain,
whereas, phosphate conversioncoatings are used as ab ase for paint.
The process is normally used for simple,f airly small components. It is
suitable for barrel plating or for continuous sheeta nd wire. No heating is
used in this process exceptf or hydrogen embrittlement relief on high
strength steels.
Electroplated zinc is very ductile, and consequently,this process is widely
used for the continuousplating of strip and wire where severe deformation
may be required.
The coating on steel from this process gives abright and smooth finish. It
is used for decorative effect to protect delicate objects where rough or uneven
finishes cannot be tolerated (e.g., instrument parts).Itisalso used for articles
that cannot withstand the pretreatment or temperatures required in other
coating processes.
It was previously mentioned that the term electrogalvanizing is sometimes
used to describethis process. This is misleadingsince the chiefcharacteristic
Coatings
441
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485