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TRANSCRIPT
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Inspection of Hot-Dip Galvanized Steel
This course is intended to train individuals on the proper inspection
techniques and requirements for hot-dip galvanized steel products. There are
four sections in this course:
Hot-Dip Galvanizing Process
Galvanizing Standards
Types of nspection
!epair
"pon completion of this course# you should $e a$le to recognize specification
requirements and perform all inspection steps to ensure conformance %ith the
requirements. &dditionally# any inspector %ho completes the course# and
passes the test '()* or $etter+ %ill receive a printa$le ,ertificate of,ompletion and %ill $e listed on the &G& %e$site as an inspector.
Disclaimer
The information contained in this course has $een compiled $y the &merican
Galvanizers &ssociation '&G&+# a not-for-profit trade association %hose mem$ers
represent the after-fa$rication hot-dip galvanizing industry throughout the "nited
States# ,anada# and eico. The &G& ma/es no endorsement and offers no
evaluation of any vendor0s products# %hether listed here or not.
Galvanizing Process
The term hot-dip galvanizing is defined as the process of immersing iron or steel in a
$ath of liquid zinc to produce a corrosion resistant# multi-layered coating of zinc-iron
alloy and zinc metal. The coating is produced as the result of a metallurgical reaction
$et%een the liquid zinc and the iron in the steel. The coating forms an equal thic/ness
on all surfaces immersed in the galvanizing /ettle. This process# similar to the one
seen inFigure 1# has $een in use since 1234 and has provided long-lasting#
maintenance-free corrosion protection at a reasona$le cost for many years. The three
main steps in the hot-dip galvanizing process are surface preparation# galvanizing# and
post-treatment# each of %hich %ill $e discussed in detail.
Figure 1: Model of the Hot-Dip Galvanizing Process
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Steel structures %ith visi$le evidence of corrosion are pictured in the series of photos
inFigure 2. !ust and corrosion can $e epensive for $usiness o%ners and tapayers
$ecause $uildings# roads# and $ridges# %ithout sufficient corrosion protection# may
need to $e repaired often or even re$uilt.
The process is descri$ed in more detail later in this section. t is inherently simple#and this simplicity is a distinct advantage over other corrosion protection methods.
Figure 2: Corroding teel tructures
Surface Preparation
The first step in the hot-dip galvanizing process is intended to o$tain the cleanest
possi$le steel surface $y removing all of the oides and other contaminating residues.This is achieved $y first hanging the steel using chains# %ires# or specially designed
dipping rac/s# as seen inFigure 3# to move the parts through the process. There are
three cleaning steps to prepare the steel for galvanizing.
Degreasing!Caustic Cleaning
5irst the steel is immersed in an acid degreasing $ath or caustic solution in order to
remove the dirt# oil# and grease from the surface of the steel. &fter degreasing the steel
is rinsed %ith %ater.
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Pic"ling
6et the steel is immersed in an acid tan/ filled
%ith either hydrochloric or sulfuric acid# as seen in
Figure 4# %hich removes oides and mill scale in a
process called 7pic/ling.8 9nce all oidation has$een removed from the steel# it is again rinsed %ith
%ater and sent to the final stage of the surface
preparation.
Figure #: $he Pic"ling $an"
Flu%ing
The purpose of the flu is to clean the steel of all oidation developed since the
pic/ling of the steel and to create a protective coating to prevent the steel from any
oidizing $efore entering the galvanizing /ettle. 9ne type of flu is contained in a
separate tan/# is slightly acidic# and contains a com$ination of zinc chloride and
ammonium chloride. ¬her type of flu# top flu# floats on top of the liquid zinc in
the galvanizing /ettle# $ut serves the same purpose.
&fter $eing immersed in the degreasing# pic/ling# and fluing tan/s# the surface of the
steel is completely free of any oides or any other contaminants that might inhi$it the
reaction of the iron and liquid zinc in the galvanizing /ettle.
Galvanizing
9nce the steel has $een completely cleaned# it is ready for immersion in the liquid
zinc. The galvanizing /ettle contains zinc specified to &ST ;# a document that
specifies any one of three different grades of zinc that are each at least
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Post-$reatment
>hen the steel is removed from the galvanizing /ettle# it may receive
a post-treatment to enhance the galvanized coating. 9ne of the most
commonly used treatments is quenching. The quench tan/ contains
mostly %ater $ut may also have chemicals added to create apassivation layer that protects the galvanized steel during storage and
transportation. 9ther finishing steps include removal of zinc drips# or
icicles# $y grinding them off.
Time to First Maintenance
The estimated time to first maintenance for a hot-dip galvanized coating that
eperiences common atmospheric eposure can $e seen inFigure 7. Time to firstmaintenance is defined as the time to ?* rusting of the su$strate steel. The time to
first maintenance of hot-dip galvanized steel is directly proportional to the zinc
coating thic/ness.
Figure ': $ime to First Maintenance Chart for Hot-Dip Galvanized Coatings
Other orrosion Protection S!stems
There are many other types of corrosion protection# such as coating steel %ith oil#
grease# tar# asphalt# polymer coatings or paints# or corrosion protection materials such
as stainless and %eathering steel# sacrificial anodes# plating systems and impressed
current systems. These are some of the most commonly used corrosion protection
materials and systems and are sometimes used together %ith hot-dip galvanized steel.
ost of these materials rely on $arrier protection# %hile some of them rely on
cathodic properties to prevent corrosion of the steel. The most effective type of
corrosion protection that provides $oth $arrier and cathodic protection is hot-dip
galvanizing.
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There are also a %ide variety of zinc coatings used for corrosion protection. any
people use 7galvanizing8 to descri$e all of these coatings# $ut each has its o%n unique
characteristics and performance. These coatings have several applications $ased on
their properties and respective thic/nesses. The corrosion protection offered $y a zinc
coating is linearly related to its coating thic/ness. The most commonly used coatings
are hot-dip galvanized# metallized# zinc-rich paint# galvannealed or galvanized sheet#and electroplated. The relative thic/ness for each of these zinc coatings can $e seen in
the photomicrograph 'Figure 8+. elo% is a $rief eplanation of each type of zinc
coating.
Figure (: Photomicrogrpah of )inc Coatings* $hic"nesses
Metallizing
etallizing is the general name for the technique of spraying a metal coating on the
surface of non-metallic or metallic o$@ects. This process is accomplished $y feedingzinc in either %ire or po%der form into a heated gun# %here it is melted and sprayed
onto the surface to $e coated using com$ustion gases andAor auiliary compressed air
to provide the necessary velocity. The limitations of this process include a difficulty in
reaching recesses# cavities# and hollo% spaces# even coating thic/ness and cost.
)inc-+ich Paint
Binc-rich paint is applied to a clean# dry steel surface $y either a $rush or spray and
usually contains an organic $inder pre-mi. Paints containing zinc dust are classified
as organic or inorganic# depending on the $inder that they contain# and are discussed
in more detail later in this course.
Continuous Galvanizing
The continuous galvanizing process is a hot-dip
process %here a steel sheet# strip# or %ire is
cleaned# pic/led# and flued on a processing line
approimately ?)) feet '1?3 m+ in length# and
running at speeds $et%een 1)) to ;)) feet per
minute
Figure ,: Continuous Galvanizing Plant
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'=) to 1(? m per minute+. n the coating of a steel sheet or strip# the galvanizing
/ettle contains a small amount of aluminum# %hich suppresses the formation of the
zinc-iron alloys# resulting in a coating that is mostly pure zinc. & post-galvanizing# in-
line heat treatment process /no%n as galvannealing can also $e used to produce a
fully alloyed coating. Galvannealing is usually ordered $y those %anting to paint over
the zinc surface $ecause the presence of alloy layers on the steel surface promotespaint adhesion. & photo of a continuous galvanizing plant is seen inFigure 9and the
common plant setup is sho%n inFigure 10.
Figure 1: .%ample of a Continuous Process
.lectroplating
The electroplating process# or zinc-plated coating# has a dull gray color# a matte finish#
and a thin coating that ranges up to one mil '4? Cm+ thic/. This very thin coating
restricts the use of zinc-plated products to indoor eposures. The specification &ST
;== lists the classes of zinc-plated steel coatings as 5eABn ?# 5eABn (# 5eABn 14# and
5eABn 4?# %here 5e represents iron and Bn represents zinc# %hile the num$er indicates
the coating thic/ness in microns. The main uses for this type of coating include
scre%s# light s%itch plates# and other small products or fasteners.
"STM Specifications
There are certain specifications that have $een developed for hot-dip galvanizing in
order to produce a high-quality coating. The most commonly used specifications
design engineers and fa$ricators should $ecome familiar %ith in order to promote a
high-quality coating and ensure their steel design is suita$le for hot-dip galvanizing
are:
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/$M / 120!/ 120M:Standard Specification for Zinc (Hot-Dip a!"ani#ed$
%oating& on 'ron and Stee! roduct&
Single pieces of steel or fa$rications %ith different types of steel products
/$M / 10!/ 10M:Standard Specification for Zinc %oating (Hot-Dip$ on
'ron and Hard)are
5asteners and small products that are centrifuged after galvanizing to removeecess zinc
/$M / '&'!/ '&'M:Standard Specification for Zinc-%oated (a!"ani#ed$
Stee! *ar& for %oncrete +einforce,ent
!einforcing steel or re$ar
/$M / '(:Standard ractice for +epair of Da,aged and ncoated .rea&
of Hot-Dip a!"ani#ed %oating&
Touch-up procedures for coating $are spots on an eisting hot-dip galvanized
product
9ther commonly used specifications in the hot-dip galvanizing industry include:
/$M / 1#0!/ 1#0M:Standard ractice for Safeguarding .gain&t
/,ritt!e,ent of Hot-Dip a!"ani#ed Structura! Stee! roduct& and
rocedure for Detecting /,ritt!e,ent
/$M / 0(#!/ 0(#M:Standard ractice for Safeguarding .gain&t arpage
and Di&tortion During Hot-Dip a!"ani#ing of Stee! .&&e,!ie&
/$M / 0(!/ 0(M:Standard ractice for ro"iding Hig-ua!it Zinc
%oating& (Hot-Dip$
/$M &:Standard Specification for Zinc
/$M D &0(&:Standard ractice for reparation of Zinc (Hot-Dip
a!"ani#ed$ %oated 'ron and Stee! roduct and Hard)are Surface& for aint /$M . 0'&:Standard ractice for ea&uring %oating icne&&
agnetic-Fie!d or /dd-%urrent (/!ectro,agnetic$ /a,ination etod&
C/3!C/ G 1:Hot-Dip a!"ani#ing of 'rregu!ar! Saped .rtic!e&
45 1#&1Hot-Dip a!"ani#ed %oating& on Faricated 'ron and Stee!
.&&e,!ie&
Specification& and e&t etod&
et0s eamine a fe% of these specifications in more detail.
"STM " #$% for Structural Steel Products
The &ST & 14=A& 14= specification covers individual steel pieces as %ell as
assem$lies of various classes of material. The four material categories covered in
&ST & 14=A& 14= include structural steel and plates# strips and $ars# pipes and
tu$ing# and %ires. & fa$rication can have more than one material category such as a
frame assem$ly. &ny com$ination of these products can $e assem$led into a singlefa$rication and then can $e hot-dip galvanized# as seen inFigure 11.
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t is the responsi$ility of the designer and fa$ricator to ensure the product has $een
properly designed and $uilt $efore the hot-dip galvanizing process. The galvanizer
should $e made a%are of any necessary special instructions or requests in advance of
shipping the materials to the galvanizing plant. These requests should $e stated on the
purchase order for the hot-dip galvanizing.
t is the responsi$ility of the galvanizer to ensure compliance %ith the specifications
as long as the product has $een designed and fa$ricated in accordance %ith the
referenced specifications. Ho%ever# if the galvanizer has to perform additional %or/
in order to prepare the product for hot-dip galvanizing# such as drilling holes to
facilitate drainage or venting# it must $e approved $y the customer. 9nce the material
has $een hot-dip galvanized# it can $e fully inspected at the galvanizing plant prior to
shipment.
&ny materials re@ected $y the inspectors for reasons other than em$rittlement may $e
stripped# regalvanized# and resu$mitted for inspection. The &ST specifications &
13=A& 13=# &ST & =(3A& =(3# and &ST & =(? provide guidelines forpreparing products for hot-dip galvanizing. The requirements listed in &ST & 14=A&
14= include coating thic/ness# finish# appearance# and adherence. These are each
defined $elo% and discussed in more detail later in this course.
/$M / 120!/ 120M +e6uirements
Coating $hic"ness ! 7eightE dependent upon material category and steel
thic/ness
FinishE continuous# smooth# uniform
/ppearanceE free from uncoated areas# $listers# flu deposits and gross drossinclusions as %ell as having no heavy zinc deposits that interfere %ith intended
use
/dherenceE the entire coating should have a strong adherence throughout the
service life of galvanized steel
The hot-dip galvanized coating is intended for products fa$ricated into their final
shape that %ill $e eposed to corrosive environmental conditions. 9nce a product has
$een hot-dip galvanized# any further fa$rication# %hich very rarely occurs# may have
negative effects on the corrosion protection of the coating. The coating grade is
defined as the required thic/ness of the coating and is given in microns. &ll coating
thic/ness requirements in specification &ST & 14=A& 14=# as seen in a!e& 1 2# are minimumsF there are no maimum coating thic/ness requirements in either
specification.
$a8le 1: Minimum /verage Coating $hic"ness Grade 89 Material Categor9
From /$M /120;
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$a8le 2: Coating $hic"ness Grade From /$M / 120;
The time to first maintenance of hot-dip galvanized steel is directly proportional to the
thic/ness of the hot-dip galvanized coating. >ith all other varia$les held constant# the
thic/er the zinc coating# the longer the life of the steel. The aim of the finish and
appearance requirements is to ensure no coatings have pro$lem areas that are deficient
of zinc or have surface defects that %ould interfere %ith the intended use of the
product. n addition# the coating should have a strong adherence throughout the
service of the hot-dip galvanized steel.
"STM " #&% for Hard'are
The specification &ST & 1?=A& 1?= applies to hard%are products such as
castings# fasteners# rolled# pressed and forged products# and miscellaneous threaded
o$@ects that %ill $e centrifuged# spun# or other%ise handled to remove the zinc# as
seen inFigure 12.
Figure 12: Galvanized Fasteners
The requirements for &ST & 1?=A& 1?= are very similar to those reported earlier
for &ST & 14=A& 14=# ecept for the addition of threaded products and
em$rittlement requirements.
/$M / 10!/ 10M +e6uirements
Coating $hic"ness!7eightE depends on the material category and steel
thic/ness# values are listed in a!e 3
$hreaded ProductsE areas %ith threads are not su$@ect to the coating
thic/ness requirement
FinishE continuous# smooth# uniform
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.m8rittlementE high tensile strength fasteners '1?)/si+ and castings can $e
su$@ect to em$rittlement
/ppearanceE free from uncoated areas# $listers# flu deposits and gross dross
inclusions as %ell as having no heavy zinc deposits that interfere %ith intended
use
/dherenceE the entire coating should have a strong adherence throughout theservice life of hot-dip galvanized steel
There are fa$rication steps that may impair the corrosion protection of the hot-dip
galvanized coating# ho%ever# fla/ing or damage to the coating $ecause of this is not
case for re@ection. n all cases# good steel selection results in the formation of a higher
quality coating and finish on the product. The corrosion protection coating for
threaded products is applied after the product has $een fa$ricated and further
fa$rication may compromise the corrosion protection system. The one eception to
this rule is the internal threads of a nut that should $e over-tapped after the coating is
applied in order to accommodate the coating thic/ness change on the thread of the
$olts. n this case# the zinc on the $olt threads provides the corrosion protection to theuncoated threads in the nut.
There are certain fa$rication techniques that can induce stresses into the steel and lead
to $rittle failure. There are precautions given in &ST & 13=A& 13= that should $e
ta/en in order to prevent em$rittlement. n addition# selecting steels %ith appropriate
chemistries can help prevent em$rittlement of mallea$le castings. & reproduction and
summary of the ta$le given in &ST & 1?=A& 1?=# %hich is seen in a!e 3# gives
the different classes of products and the minimum coating thic/ness required $y the
specification.
$a8le 0: Minimum /verage Coating $hic"ness 89 Material Class From /$M /
10;
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"STM " ()( for *einforcing Steel
The specification &ST & 2;2A& 2;2 is applica$le eclusively to the hot-dip
galvanizing of reinforcing steel# other%ise /no%n as re$ar# as seen inFigure 13# and
is applica$le to all types of re$ar# $oth smooth and deformed. Ho%ever# %ire is not
included.
Figure 10: Hot-Dip Galvanized +e8ar
The requirements in &ST & 2;2A& 2;2 are also intended to produce a high qualityzinc coating for corrosion protection.
/$M / '&'!/ '&'M +e6uirements
4dentit9E the galvanizer is responsi$le for consistent material trac/ing if
necessary
Coating $hic"ness!7eightE material category and steel thic/ness
ChromatingE to prevent reaction $et%een cement and recently galvanized
material
FinishE continuous# smooth# and uniform
/ppearanceE free from uncoated areas# $listers# flu deposits and gross drossinclusions as %ell as having no heavy zinc deposits that interfere %ith intended
use
/dherenceE should $e tightly adherent throughout intended use of the
product
end DiametersE fla/ing and crac/ing due to fa$rication after the hot-dip
galvanizing process are not re@ecta$le
9nce re$ar is delivered to $e hot-dip galvanized# it is the galvanizer0s responsi$ility to
trac/ and maintain the identity of the product throughout the hot-dip galvanizing
process until shipment of the finished product. &gain# the analogous coatingrequirements in the areas of coating thic/ness# finish# and adherence are present in
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&ST & 2;2A& 2;2. Ho%ever# this single product specification introduces a fe%
ne% requirements that apply solely to hot-dip galvanized re$ar. n &ST & 2;2A&
2;2# the coating requirement is given in 7mass of the zinc coating per surface area8.
& summary of the ta$le given in &ST & 2;2A& 2;2 and the minimum required
coating thic/ness A %eight of the classes is seen in a!e 4.
$a8le #: Mass of )inc Coating From /$M / '&';
This specification also introduces a ne% requirement to the galvanized coating /no%n
as chromating. 6e%ly galvanized steel can react %ith %et cement and potentially form
hydrogen gas as a product. &s this evolved hydrogen gas travels through the concrete
matri to%ard the surface# voids can $e created %hich %ea/en the $onding %ith the
concrete or distur$ the smoothness of the concrete surface. n order to help prevent
and suppress this reaction# hot-dip galvanized re$ar is dipped into a %ea/ chromate
quench solution after $eing removed from the galvanizing /ettle.
The finish requirement for re$ar is along the same lines as the finish requirements
given in specifications &ST & 14=A& 14= and & 1?=A& 1?=. The coating is
intended for corrosion protection# so deficiencies that affect the coating0s corrosion
performance are grounds for re@ection. n addition# since re$ar is handled frequently
during its installation# any tears or sharp spi/es that ma/e the material dangerous to
handle are grounds for re@ection.
!e$ar is commonly $ent prior to the hot-dip galvanizing process. The ta$le $elo%
gives recommendations for $end diameters $ased upon the $are steel $ar diameter
$efore coating. Steel reinforcing $ars that are $ent cold prior to hot-dip galvanizing
should $e fa$ricated to a $end diameter equal to or greater than the specified values.Ho%ever# steel reinforcing $ars can $e $ent to diameters tighter than specified in
a!e 5providing they are stress relieved at a temperature of
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$a8le : Minimum Finished end Diameters From /$M / '&';
Other Galvanizing Standards
There are ,anadian and international specifications that could $e used to specify hot-
dip galvanizing on a pro@ect. The differences in these specifications and the &ST
specifications are minimal# and for the most part# only differ slightly in the minimum
coating thic/nessA%eight required for each type and thic/ness of product $eing hot-
dip galvanized.
Other Specifications for Hot-Dip Galvanizing (Taken from
CAN/CSA an !SO Stanars"
C/3!C/-G1 Hot Dip Galvanizing of 4rregularl9 haped /rticles
Scope
1. This standard specifies the requirements for zinc coating 'galvanizing+ $y the
hot-dipping process on iron and steel products made from rolled# pressed# or
forged shapes such as structural sections# plates# $ars# pipes# or sheets 1 mm
thic/ or thic/er.
4. &pplies to $oth unfa$ricated and fa$ricated products such as assem$led steelproducts# structural steel fa$rications# large hollo% sections $ent or %elded
$efore galvanizing# and %ire %or/ fa$ricated from uncoated steel %ire.
=. &pplies to steel forgings and iron castings that are to $e galvanized separately
or in $atches.
3. Does not apply to continuous galvanizing of chain lin/ fence fa$ric# %ire#
sheet# and strip.
?. Does not apply to pipe and conduit that are normally hot dip galvanized $y a
continuous or semicontinuous automatic process.
;. The values stated in S units are to $e regarded as the standard. The values in
parentheses are imperial units and are included for information only.
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45 1#&1 Hot Dip Galvanized Coatings on Fa8ricated 4ron and teel
/rticles
Scope: This Standard specifies the general properties of and methods of test for
coatings applied $y hot dipping in zinc 'containing not more than 4* of other metals+
on fa$ricated iron and steel articles.
t does not apply to:
1. Sheet and %ire continuously hot dip galvanizedF
4. Tu$e and pipe hot dip galvanized in automatic processF
=. Hot dip galvanizing products for %hich specific standards eist and %hich
may include additional requirements or requirements different from those of
this uropean Standard.
3. &fter-treatmentAovercoating of hot dip galvanized articles is not covered $y
this standard.
35$.ndividual product standards can incorporate this standard for the coating $y
quoting its num$er# or may incorporate it %ith modifications specific to the product.
n this section# the type of inspections performed on hot-dip galvanized steel %ill $e
discussed. The various inspections are used to verify the necessary specifications for
the galvanized product are met. These techniques for each test method are specified in
&ST & 14=A& 14=# & 1?=A& 1?=# or & 2;2A& 2;2# depending upon the type of
product $eing inspected. The most common inspections# listed $elo%# range from a
simple visual inspection to more sophisticated tests to determine em$rittlement or
adhesion.
Coating $hic"nessE magnetic gauges# optical microscopy
Coating 7eightE %eigh-galvanize-%eigh# and %eigh-strip-%eigh
Finish and /ppearanceE visual inspection
/dditional $ests
o /dherenceE stout /nife
o .m8rittlementE similar $end radius# sharp $lo%# and steel angle
o ChromatingE spot test
o endingE minimum finished $end diameter ta$le
ampling
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oating Thic+ness
The term coating thic/ness refers to the thic/ness of zinc applied to steel# %hile
coating %eight refers to the amount of zinc applied to steel for a given surface area.
T%o different methods are used in order to measure the coating thic/ness of hot-dip
galvanized steel.
Figure 1#: Pencil-t9le Gauge
The first method to measure coating thic/ness involves using magnetic thic/ness
gauges. There are three different types of magnetic thic/ness gauges and all can $e
used quite easily in the galvanizing plant or in the field.
The first type of magnetic thic/ness gauge is very small and utilizes a spring-loaded
magnet encased in a pencil-li/e container# as seen inFigure 14. The tip of the gauge is
placed on the surface of the steel and is slo%ly pulled off in a continuous motion.
>hen the tip of the gauge is pulled a%ay from the surface of the steel# the magnet#
near the tip# is attracted to the steel. & graduated scale indicates the coating thic/ness
at the instant immediately prior to pulling the magnet off the surface of the steel. The
accuracy of this gauge requires it to $e used in the true vertical plane $ecause# due to
gravity# there is more error associated %ith measurements ta/en in the horizontal
plane or overhead positions. The measurement should $e made multiple times $ecause
the a$solute accuracy of this type of gauge is $elo% average and it is difficult to
determine the true coating thic/ness %hen only one reading is ta/en.
Figure 1: anana Gauge
& $anana gauge# as seen inFigure 15is the second type of thic/ness gauge. >ith thisgauge# coating thic/ness measurements are ta/en $y placing the ru$$er magnet
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housing on the surface of the product %ith the gauge held parallel to the surface. &
scale ring is rotated cloc/%ise to $ring the tip of the instrument in contact %ith the
coated surface and rotated counter-cloc/%ise until a $rea/ in contact can $e heard and
felt. The position of the scale ring %hen the magnetic tip $rea/s from the coated
surface displays the coating thic/ness. This type of gauge has the advantage of $eing
a$le to measure coating thic/ness in any position# %ithout recali$ration or interferencefrom gravity.
Figure 1&: .lectronic!Digital $hic"ness Gauge
The electronic or digital thic/ness gauge# as seen inFigure 16is the most accurate
and argua$ly# the easiest thic/ness gauge to operate. The electronic thic/ness gauge is
operated $y simply placing the magnetic pro$e onto the coated surface and then a
digital readout displays the coating thic/ness. lectronic gauges have the advantage of
not requiring recali$ration %ith pro$e orientation# $ut do require cali$ration %ith
shims of different thic/nesses in order to verify the accuracy of the gauge at the time
it is $eing used. These shims are measured and the gauge is cali$rated according to
the thic/ness of the shim# and then this process is repeated for shims of different
thic/nesses until the gauge is producing an accurate reading in all ranges of thic/ness.
/$M . 0'&
The specification &ST =2; contains information for measuring coating thic/ness
using magnet or electromagnetic current. t also provides some tips for o$taining
measurements %ith the greatest accuracy# as %ell as descri$ing ho% the physical
properties# the structure# and the coating can interfere %ith the measurement methods.
The requirements for &ST =2;# as seen $elo%# are intended to ma/e the coating
thic/ness measurements using magnet or electromagnetic current as accurate as
possi$le.
/$M . 0'& +e6uirements
easurements on large products should $e made at least four inches from the
edge to avoid edge effects
easurement readings should $e as %idely dispersed as possi$le
There are some general guidelines# as seen $elo%# for reducing error and ensuring the
most accurate readings are $eing collected %hen using magnetic thic/ness gauge
instruments.
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Guidelines for +educing .rror
!ecali$rate frequently# using non-magnetic film standards or shims a$ove and
$elo% the epected thic/ness value
!eadings should not $e ta/en near an edge# a hole# or inside corner
!eadings ta/en on curved surfaces should $e avoided if possi$le Test points should $e on 7regular areas8 of the coating
Ta/e at least five readings to o$tain a good# 7true8 value %hich is
representative of the %hole sample
Figure 1': 5ptical Microscop9
The second method used to measure the coating thic/ness involves optical
microscopy# as seen inFigure 17. This is a destructive technique and is typically only
used for inspection of the coating of single specimen samples that have failed
magnetic thic/ness readings or for research studies. Since it is not a common method#
the accuracy is highly dependent on the epertise of the operator.
oating ,eight
The term coating %eight refers to the amount of zinc applied to a product for a given
surface area. T%o different methods can $e used to measure the coating %eight of hot-
dip galvanized steel.
The first method to measure the coating %eight involves using a process called %eigh-
galvanize-%eigh# and is only appropriate for single specimen samples. The zinc
coating %eight from this technique is underestimated $ecause the actual coating is
made up of $oth iron and zinc and this method %ill only measure the added zinc
%eight in the coating. n addition# it can $e very difficult to measure and calculate the
surface area of a comple steel fa$rication# and this ma/es coating %eight values even
less accurate.
>eigh-strip-%eigh is the second method used to measure coating %eight# and again isonly appropriate for single specimen samples. This method is destructive since it
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removes the hot-dip galvanized coating during the measurement. This process
involves first %eighing the specimen# stripping it of all zinc coating that %as added#
and then %eighing it again. The difference in the %eights is then equal to the amount
of coating added during the galvanizing process. Ho%ever# this method is usually only
used on very small products li/e nails# and can $e inaccurate $ecause %hen the
coating is stripped there may $e some $ase metal stripped along %ith the coating. Thismeans that there may $e etra iron included in the %eight measurement# ma/ing for a
higher than actual zinc coating %eight.
Finish "ppearance
The inspection of finish and appearance is done %ith an unmagnified visual
inspection. This inspection is performed $y fully o$serving all parts and pieces of a
hot-dip galvanized product to ensure all necessary components and specifications
have $een met. t is done in order to o$serve surface conditions# $oth inside and out#
and chec/ all contact points# as %ell as %elds# @unctions# and $end areas.
/ppearance
The appearance of the hot-dip galvanized coating can vary from piece to piece# and
even section to section of the same piece. There are a num$er of reasons for the non-uniform appearance# $ut it is important to note appearance has no $earing on the
corrosion protection of the galvanized piece. This section %ill overvie% the resons for
differences in appearance.
Finish
This section %ill revie% a num$er of possi$le surface defects visi$le on the
galvanized coating. Some of these surface defects are re@ecta$le# as they %ill seriously
lo%er the corrosion protection# %hile others have little or no effect on the corrosion
performance and are accepta$le.
Different "ppearances
The appearance of hot-dip galvanized steel immediately after galvanizing can $e
$right and shiny# spangled# matte gray# or a com$ination of these. There are a num$er
of reasons for the difference in appearance# as eplored here# $ut regardless if the
piece is shiny or dull# the appearance has no effect on the corrosion performance. &nd
in time after eposure to the environment# all galvanized coatings %ill ta/e on a
uniform matte gray appearance.
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#easons for Different Appearances
teel Chemistr9
The most common reason for galvanized steel to have different appearances is the
chemistry of the steel pieces. There are t%o elements of steel chemistry %hich moststrongly influence the final appearanceF silicon and phosphorous. oth silicon and
phosphorous promote coating gro%th# and this thic/er coating is responsi$le for the
differing appearance.
The amount of silicon added during the steel ma/ing process to deoidize the steel
can create differences in appearance of galvanized products. The recommended
silicon composition is either less than ).)3* or $et%een ).1?* and ).4?*. &ny steels
not %ithin these ranges are considered reactive steels and are epected to form zinc
coatings that tend to $e thic/er.
n addition to producing thic/er coatings# highly reactive steels tend to have a matte
gray or mottled appearance instead of the typical $right coating. This difference in
appearance is a result of the rapid zinc-iron intermetallic gro%th that consumes all of
the $right# pure zinc. This gro%th of the intermetallic layer is generally out of the
galvanizer0s control# $ecause they usually do not have prior /no%ledge of the steel0s
composition. Ho%ever# this increased coating thic/ness can $e $eneficial in some
respects $ecause time to firrst maintenance is directly proportional to coating
thic/ness.
nFigure 18# the micrograph on the left sho%s a regular zinc-iron alloy# %hile the
micrograph on the right sho%s an irregular zinc-iron alloy. These clearly sho% themicroscopic level differences that can occur due to the amount of silicon in the steel
$eing hot-dip galvanized.
Figure 1(: +egular vs C
are Spots
lasting Damage
,hain and >ire ar/s
,logged Holes
,logged Threads
D > .
Delamination
Distortion
Drainage Spi/es Dross nclusions
cess &luminum in Galvanizing ath
F > 5
5ish oning
5la/ing
5lu nclusions
9ide ines
P > +
Products in ,ontact
!ough Surface ,ondition
!uns
!ust leeding
> $
Sand m$edded in ,asting
Striations Steel Surface ,ondition
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Surface ,ontaminant
Touch ar/s
? > )
>eeping >eld >elding lo%outs
>elding Spatter
>et Storage Stain
Binc S/immings
Binc Splatter
/isual Defects. "-0are Spots
are spots# defined as uncoated areas on the steel surface# are the most common
surface defect and occur $ecause of inadequate surface preparation# %elding slag#
sand em$edded in castings# ecess aluminum in the galvanizing /ettle# or lifting aids
that prevent the coating from forming in a small area. 9nly very small areas# less than
1 inch in the narro%est dimension %ith a total of no more than ).?*of the accessi$le
surface area# may $e renovated using &ST & 2(). This means narro%# $are areas
may $e repairedF ho%ever# if they are greater than one inch-square areas# the product
must $e regalvanized. n order to avoid $are spots# li/e those seen in Figure 24# thegalvanizer must ensure the surfaces are clean and no contaminants are present after
pretreatment. f the size of the $are spot or total surface area causes re@ection# the
parts may $e stripped# regalvanized# and then re-inspected for compliance to the
standards and specifications.
Figure 2#: are pots
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Figure 2: lasting Damage
lasting Damage
lasting damage creates $listered or fla/ing areas on the surface of the galvanized
product.lasting damage follo%s a$rasive $lasting prior to painting of the galvanized
steel. t is caused $y incorrect $lasting procedures creating shattering and
delamination of the alloy layers in the zinc coating. lasting damage# as seen in
Figure 25# can $e avoided %hen careful attention is paid to preparation of the product
for painting. n addition# $last pressure should $e greatly reduced according to &ST
D ;=(;. Since $lasting damage is induced $y a post-galvanizing process# the
galvanizer is not responsi$le for the damage.
Figure 2&: Chain and 7ire Mar"s
Chain and 7ire Mar"s
¬her type of surface defect occurs %hen steel is lifted and transported around the
galvanizing plant using a chain or %ire. These lifting aids can leave uncoated areas on
the finished product that %ill need to $e repaired. The superficial mar/s# li/e those
seen inFigure 26# left on the galvanized coating from the lifting attachments are not
grounds for re@ection as long as mar/s can $e repaired. &ST specifications do not
allo% any $are spots on the finished galvanized part.
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Figure 2': Clogged Holes
Clogged Holes
,logged holes are holes partially or completely clogged %ith zinc metal. & goodeample is the screen sho%n inFigure 27. The zinc %as trapped $ecause liquid zinc
%ill not drain easily from holes less than =A1)8 '(mm+ in diameter due to its high
surface tension. ,logged holes can $e minimized $y ma/ing all holes as large as
possi$le. The trapped zinc can $e removed $y using active fettling %hen the part is in
the galvanizing /ettle# vi$rating the cranes to @ostle the parts# or $lo%ing compressed
air onto the galvanized products. This condition is not a cause for re@ection# unless it
prevents the part from $eing used for its intended purpose.
Figure 2(: Clogged $hreads
Clogged $hreads
,logged threads are caused $y poor drainage of a threaded section after the product is
%ithdra%n from the galvanizing /ettle. These clogged threads# as seen inFigure 28#
can $e cleaned $y using post-galvanizing cleaning operations such as a centrifuge or
$y heating them %ith a torch to a$out ?)) 5 '4;) ,+ and then $rushing them off %ith
a %ire $rush to remove the ecess zinc. ,logged threads must $e cleaned $efore the
part can $e accepted.
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/isual Defects. D-1
Figure 2,: Delamination
Delamination
Delamination or peeling creates a rough coating on the steel %here the zinc has peeled
off.There are a num$er of causes for zinc peeling. any large galvanized parts ta/e a
long time to cool in the air and form zinc-iron layers after they have $een removed
from the galvanizing /ettle. This continued coating formation leaves $ehind a void
$et%een the top t%o layers of the galvanized coating. f there are many voids formed#
the top layer of zinc can separate from the rest of the coating and peel off the part. f
the remaining coating still meets the minimum specification requirements# then the
part is still accepta$le. f the coating does not meet the minimum specification
requirements then the part must $e re@ected and regalvanized. f delamination# as seen
inFigure 29# occurs as a result of fa$rication after galvanizing# such as $lasting
$efore painting# then the galvanizer is not responsi$le for the defect.
Figure 0: Distortion
Distortion
Distortion# as seen inFigure 30# is defined as the $uc/ling of a thin# flat steel plate or
other flat material such as %ire mesh.The cause of this is differential thermal
epansion and contraction rates for the thin# flat plate and mesh than the thic/er steel
of the surrounding frame. n order to avoid distortion# use a thic/er plate# ri$s# or
corrugations to stiffen flat sections or ma/e the entire assem$ly out of the same
thic/ness steel. Distortion is accepta$le# unless distortion changes the part so that it is
no longer suita$le for its intended use.
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Figure 01: Drainage pi"es
Drainage pi"es
Drainage spi/es or drips are spi/es or tear drops of zinc along the $ottom edges of the
product.These result %hen the surfaces of the product are processed horizontal to the
galvanizing /ettle# preventing proper drainage of the zinc from the surface as the
product is %ithdra%n from the /ettle. Drainage spi/es# as seen inFigure 31# are
typically removed during the inspection stage $y a $uffing or grinding process.
Drainage spi/es or drips are ecess zinc and %ill not affect corrosion protection# $ut
are potentially dangerous for anyone %ho handles the parts. These defects must $e
removed $efore the part can $e accepted.
Dross 4nclusions
Dross inclusions are a distinct zinc-iron intermetallic alloy that $ecomes entrapped or
entrained in the zinc coating. This is caused $y pic/ing up zinc-iron particles from the
$ottom of the /ettle. Dross# as seen inFigure 32# may $e avoided $y changing the
lifting orientation or redesigning the product to allo% for proper drainage. f the dross
particles are small and completely covered $y zinc metal# they %ill not affect the
corrosion protection and are accepta$le. f the dross particles are large# then the drossmust $e removed and the area repaired.
Figure 02: Dross 4nclusions
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Figure 00: .%cess /luminum in Galvanizing ath
.%cess /luminum in Galvanizing ath
¬her type of surface defect# sho%n inFigure 33# is caused $y an ecess amount of
aluminum in the galvanizing $ath. This creates $are spots and $lac/ mar/s on the
surface of the steel.The ecess aluminum can $e avoided $y ensuring proper control
of the aluminum level in the galvanizing $ath $y means of regular sampling and
analysis# and $y ad@usting the levels in a regular and controlled manner. 5or small
areas of $are spots# the part may $e repaired as detailed in the specification. f this
condition occurs over the entire part# then it must $e re@ected and regalvanized.
/isual Defects. F-O
Figure 0#: Fish oning
Fish oning
5ish $oning is an irregular pattern over the entire surface of the steel part.This is
caused $y differences in the surface chemistry of a large diameter steel piece and
variations in the reaction rate $et%een the steel and zinc. These reaction differences
cause the thic/ness of the galvanized coating to vary in sharply defined zones across
the surface. 5ish $oning# as seen inFigure 34# has no effect on the corrosion
protection provided $y the zinc coating and is not cause for re@ection of the hot-dip
galvanized part.
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Figure 0: Micrograph of Fla"ing
Fla"ing
5la/ing results %hen heavy coatings develop in the galvanizing process# usually 14
mils or greater. This generates high stresses at the interface of the steel and the
galvanized coating and causes the zinc to $ecome fla/y and separate from the surface
of the steel. 5la/ing can $e avoided $y minimizing the immersion time in the
galvanizing /ettle and cooling of the galvanized steel parts as quic/ly as possi$le.
Figure 35sho%s a micrograph of fla/ing. n addition# using a different steel grade# if
possi$le# may also help avoid fla/ing. f the area of fla/ing is small# it can $e repaired
and the part can $e acceptedF ho%ever# if the area of fla/ing is larger than allo%ed $y
the specifications# the part must $e re@ected and regalvanized.
Figure 0&: Flu% 4nclusion
Flu% 4nclusions
5lu inclusion can $e created $y the failure of the flu to release during the hot-dip
galvanizing process. f this occurs# the galvanized coating %ill not form under this
flu spot.f the area is small enough# it must $e cleaned and repairedF other%ise# the
part must $e re@ected. 5lu spots can increase if the flu is applied using the %et
galvanizing method# %hich is %hen the flu floats on the zinc $ath surface. 5lu
deposits on the interior of a hollo% part# such as a pipe or tu$e# as seen in Figure 36#
cannot $e repaired# thus the part must $e re@ected. &ny flu spots or deposits#pic/ed
up during %ithdra%al from the galvanizing /ettle do not %arrant re@ection if theunderlying coating is not harmed# and the flu is properly removed.
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Figure 0': 5%ide =ines
5%ide =ines
9ide lines are light colored oide film lines on the galvanized steel surface . 9ide
lines are caused %hen the product is not removed from the galvanizing /ettle at a
constant rate. This may $e due to the shape of the product or the drainage conditions.9ide lines# as seen inFigure 37# %ill fade over time as the entire zinc surface
oidizes. They %ill have no effect on the corrosion performanceF only the initial
appearance %ill $e affected. This condition is not a cause for re@ection of the hot-dip
galvanized parts.
/isual Defects. P-*
Figure 0(: Products in Contact
Products in Contact
¬her type of surface defect is caused $y products that come in contact %ith each
other or are stuc/ together. This usually occurs %hen many small products are hung
on the same fiture# %hich creates the chance products may $ecome connected or
overlapped during the galvanizing process# as seen inFigure 38. The galvanizer is
responsi$le for proper handling of all products in order to avoid this defect. naddition# if the surface of a product has a larger $are area than the specified repair
requirement allo%s# then that product must $e re@ected and regalvanized.
+ough urface Condition
!ough surface condition or appearance is a uniformly rough coating %ith a tetured
appearance over the entire product.The cause for this rough surface condition is hot-
rolled steel %ith a high level of silicon content. This can $e avoided $y purchasing
steel %ith a silicon content less than ).)=* of the steel $y %eight. !ough surface
condition# as seen inFigure 39# can actually have a positive effect on corrosion
performance $ecause of the thic/er zinc coating produced. 9ne of the fe% situations
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%here rough coating is cause for re@ection is if it occurs on handrails. The corrosion
performance of galvanized steel %ith rough coatings is not affected $y the surface
roughness.
Figure 0,: +ough urface Condition
Figure #: +uns
+uns
!uns are localized thic/ areas of zinc on the surface. !uns occur %hen zinc freezes on
the surface of the product during removal from the zinc $ath. This is more li/ely to
occur on thinner sections %ith large surface areas that cool quic/ly. n order to avoid
runs# as seen inFigure 40# ad@ustments of the dipping angles can $e made# if possi$le#
to alter the drainage pattern to a more accepta$le mode. f runs are unavoida$le and
%ill interfere %ith the intended application# they can $e $uffed. !uns are not cause forre@ection.
+ust leeding
!ust $leeding appears as a $ro%n or red stain that lea/s from unsealed @oints after the
product has $een hot-dip galvanized.t is caused $y pre-treatment chemicals that
penetrate an unsealed @oint. During galvanizing of the product# moisture $oils off the
trapped treatment chemicals leaving anhydrous crystal residues in the @oint. 9ver
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time# these crystal residues a$sor$ %ater from the atmosphere and attac/ the steel on
$oth surfaces of the @oint# creating rust that seeps out of the @oint. !ust $leeding# as
seen inFigure 41# can $e avoided $y seal %elding the @oint %here possi$le or $y
leaving a gap greater than =A=48 '4.3mm+ %ide in order to allo% solutions to escape
and zinc to penetrate during hot-dip galvanizing. f $leeding occurs# it can $e cleaned
up $y %ashing the @oint after the crystals are hydrolyzed. leeding from unsealed@oints is not the responsi$ility of the galvanizers and is not cause for re@ection.
Figure #1: +ust leeding
/isual Defects. S-T
Figure #2: and .m8edded in Casting
and .m8edded in Casting
¬her type of surface defect occurs %hen sand $ecomes em$edded in the castings
and creates rough or $are spots on the surface of the galvanized steel. Sand inclusions
are not removed $y conventional acid pic/ling# so a$rasive cleaning should $e done at
the foundry $efore the products are sent to the galvanizer. This type of defect also
leaves $are spots and must $e cleaned and repaired or the part must $e re@ected#stripped# and regalvanized. Sand em$edded in a casting can $e seen in Figure 42.
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Figure #0: triations
triations
Striations are characterized $y raised parallel ridges in the galvanized coating# mostly
in the longitudinal direction. This can $e caused %hen sections of the steel surface are
more highly reactive then the areas around them. These sections are usually associated
%ith segregation of steel impurities# especially phosphorous# created during the rolling
process in steel ma/ing. Striations# as seen inFigure 43# are related to the type of steel
galvanized and %hile the appearance is affected# the performance of the corrosion
protection is not. Striations are accepta$le on most partsF ho%ever# if the striations
happen to occur on handrails# then the parts must $e re@ected and regalvanized.
Sometimes regalvanizing does not improve the striations and the handrail must $e
refa$ricated out of $etter quality steel.
Figure ##: urface Contaminents
urface Contaminant
>hen surface contaminants create an ungalvanized area %here the contaminant %as
originally applied# a surface defect may occur.This is caused $y paint# oil# %a# or
lacquer not removed during the pretreatment cleaning steps. Surface contaminants# as
seen inFigure 44# should $e mechanically removed prior to the galvanizing process.
f they result in $are areas# then the repair requirements apply and small areas may $e
repaired# $ut a large area is grounds for re@ection and the entire part must $e
regalvanized.
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Figure #: $ouch Mar"s
$ouch Mar"s
¬her type of surface defect is /no%n as touch mar/s# %hich are damaged or
uncoated areas on the surface of the product. Touch mar/s are caused $y galvanized
products resting on each other or $y the material handling equipment used during the
galvanizing operation. Touch mar/s# as seen inFigure 45# are not cause for re@ection
if they meet the size criteria for repaira$le areas. They must $e repaired $efore the
part is accepted.
/isual Defects. 2-3
Figure #&: 7eeping 7eld
7eeping 7eld
>eeping %elds stain the zinc surface at the %elded connections on the steel.They arecaused $y entrapped cleaning solutions that penetrate the incomplete %eld. n order to
avoid %eeping %elds for small overlapping surfaces# completely seal %eld the edges
of the overlapping area. 5or larger overlapping areas# the area cannot $e seal %elded
since the volume epansion of air in the trapped area can cause eplosions in the
galvanizing /ettle. To avoid %eeping %elds in large overlapping areas# the $est plan is
to provide a =A=48 '4.3mm+ or larger gap $et%een the t%o pieces %hen %elding them
and let the zinc fill the gap $et%een the pieces. This %ill actually ma/e a stronger
@oint %hen the process is complete. >eeping %elds# as seen inFigure 46# are not the
responsi$ility of the galvanizer and are not cause for re@ection.
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Figure #': 7elding lo@outs
7elding lo@outs
>elding $lo%out is a $are spot around a %eld or overlapping surface hole.These are
caused $y pre-treatment liquids penetrating the sealed and overlapped areas that $oil
out during immersion in the liquid zinc. This causes localized surface contamination
and prevents the galvanized coating from forming. n order to avoid %elding
$lo%outs# as seen inFigure 47# chec/ %eld areas for complete %elds to insure there is
no fluid penetration. n addition# products can $e preheated prior to immersion into
the galvanizing /ettle in order to dry out overlap areas as much as possi$le. >elding
$lo%outs cause $are areas that must $e repaired $efore the part is accepta$le.
Figure #(: 7elding patter
7elding patter
>elding spatter appears as lumps in the galvanized coating ad@acent to %eld areas. t
is created %hen %elding spatter is left on the surface of the part $efore it is hot-dip
galvanized. n order to avoid %elding spatter# %elding residues should $e removed
prior to hot-dip galvanizing. >elding spatter# as seen inFigure 48# appears to $e
covered $y the zinc coating# $ut the coating does not adhere %ell and can $e easily
removed. This type of defect can leave an uncoated area or $are spot if the zinc
coating is damaged and must $e cleaned and properly repaired.
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7et torage tain
>et storage stain is a %hite# po%dery surface deposit on freshly galvanized surfaces.
t is caused $y ne%ly galvanized surfaces $eing eposed to fresh %ater# such as rain#
de%# or condensation that react %ith the zinc metal on the surface to form zinc oide
and zinc hydroide. t is found most often on tightly stac/ed and $undled items# suchas galvanized sheets# plates# angles# $ars# and pipes. >et storage stain can have the
appearance of light# medium# or heavy %hite po%der on the galvanized steel product.
ach of these appearances can $e seen from right to left inFigure 49.
9ne method to avoid %et storage stains is to passivate the product after galvanizing
$y using a chromate quench solution. ¬her precaution is to avoid stac/ing products
in poorly ventilated# damp conditions. ight or medium %et storage stain %ill %eather
over time in service and is accepta$le. n most cases# %et storage stain does not
indicate serious degradation of the zinc coating# nor does it necessarily imply any
li/ely reduction in the epected life of the product. Ho%ever# heavy %et storage stain
should $e removed mechanically or %ith appropriate chemical treatments $efore the
galvanized part is put into service. Heavy storage stain must $e removed or the part
must $e re@ected and regalvanized.
Figure #,: 7et torage tain
Figure : )inc "imming 4nclusions
)inc "immings
S/imming deposits are usually caused %hen there is no access to remove the
s/immings during the %ithdra%al of the steel from the galvanizing /ettle. Thes/immings on the liquid zinc surface are trapped on the zinc coating. n order to
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remove zinc s/immings %ithout harming the soft zinc coating underneath# lightly
$rush them off the surface of the galvanized steel during the in-house inspection stage
%ith a nylon-$ristle $rush. Binc s/immings# as seen inFigure 50# are not grounds for
re@ection. The zinc coating underneath is not harmed during their removal and it meets
the necessary specifications.
)inc platter
Figure 1: )inc platter
Binc splatter is defined as splashes and fla/es of zinc that loosely adhere to the
galvanized coating surface.Binc splatter is created %hen moisture on the surface of
the galvanizing /ettle causes liquid zinc to 7pop8 and splash droplets onto the product.
These splashes create fla/es of zinc loosely adherent to the galvanized surface. Binc
splatter# as seen inFigure 51# %ill not affect the corrosion performance of the zinc
coating and is not cause for re@ection. The splatter does not need to $e cleaned off the
zinc coating surface# $ut can $e if a consistent# smooth coating is required.
"dditional Tests
/dherence $est
Figure 2: tout Anife $est
Testing of the zinc coating adherence to the steel is achieved using a stout /nife. The
steps used in this test are listed $elo% and a photo of the test $eing performed can $e
seen inFigure 52. The coating shall $e deemed 7not adherent8 if it fla/es off and
eposes the $ase metal in advance of the /nifepoint. The test is not an attempt to pare
or %hittle the zinc coating. f the coating is adherent the /nife should put a slight mar/in the zinc metal surface# $ut should not cause any delamination of the coating layers.
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/dhesion $est @ith a tout Anife
Push do%n point of stout /nife
,oating must not fla/e off eposing the $ase metal
Do not perform at edges or corners of the product
6o paring or %hittling %ith /nife is accepta$le
ending $est
The hot-dip galvanized coating on a steel $ar must %ithstand $ending %ithout fla/ing
or peeling %hen the $ending test is preformed in accordance %ith the specifications in
&ST & 13=. There are various tests used to assess the ductility of steel %hen
su$@ected to $ending. 9ne test may include the determination of the minimum radius
or diameter required to ma/e a satisfactory $end. ¬her test may include the
num$er of repeated $ends that the material can %ithstand %ithout failure %hen it is
$ent through a given angle and over a definite radius.
!e$ar is commonly $ent prior to the hot-dip galvanizing process. Steel reinforcing
$ars $ent cold prior to hot-dip galvanizing should $e fa$ricated to a $end diameter
equal to or greater than the specified value in &ST & 2;2A& 2;2. Ho%ever# steel
reinforcing $ars can $e $ent to diameters tighter than the specified values if they are
stress relieved at a temperature of hen there is suspicion of potential em$rittlement of a product# it may $e necessary
to test a small group of the products to measure the ductility. These tests are usually
destructive to the zinc coating and possi$ly to the product as %ell. Products suspected
of em$rittlement shall $e tested according to the specification &ST & 13=.Depending on the service conditions the product %ill $e eposed to# one of three
em$rittlement tests may need to $e performed. These em$rittlement tests include the
similar $end radius test# sharp $lo% test# and steel angle test. The em$rittlement test
uses a /no%n force to provide a stress that should $e lo%er than the yield stress of the
part. f there is a fracture or permanent damage created during the testing process# the
parts must $e re@ected.
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Sampling
& sampling protocol has $een developed $y &ST to ensure high quality products
$ecause the inspection of the coating thic/ness for every piece of material galvanized
in a pro@ect %ould not $e practical. &ST & 14=A& 14= states for a unit of products
%hose surface area is equal to or less than 1;) inI '1)=4 cmI+# the entire surface of
each test product constitutes a specimen. n the case of a product containing more
than one material category or steel thic/ness range# that product %ill contain more
than one specimen. n addition# products %ith surface areas greater than 1;) inI '1)=4
cmI+ are multi-specimen products. There are four important terms used in the &ST
specifications and each is defined $elo%.
ampling $erms
=otE unit of production or shipment from %hich a sample is ta/en for testing
ampleE a collection of individual units of product from a single lot
pecimenE the surface of an individual test product or a portion of a test
product %hich is a mem$er of a lot or a mem$er of a sample representing that
lot
$est ProductE an individual unit of product that is a mem$er of the sample
5or single specimen products# each randomly selected product is a specimen. n
thic/ness measurement tests# five measurements are ta/en %idely dispersed over the
surface area of the specimen in order to represent the total coating thic/ness. The
mean value of the five coating thic/nesses for one specimen must have a minimumaverage coating thic/ness grade of not less than one grade $elo% the minimum
average coating thic/ness for the material category. nFigure 53# the separation of a
lot into a sample and individual specimen is sho%n.
Figure 0: ingle pecimen Product ampling
& multi-specimen product is defined as having a surface area that may $e larger than
1;) inI '1)=4 cmI+# have multiple steel thic/nesses# or contain more than one coating
category. n order to test coating thic/ness of products %hose surface area is greater
than 1;) inI '1)=4 cmI+# they are su$divided into three continuous local sections %ith
equivalent surface areas# each of %hich constitutes a unique specimen. n the case of
any such local section containing more than one material category or steel thic/ness
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range# that section %ill contain more than one specimen. n Figure 54# the separation
of a lot into a sample and individual specimen is sho%n.
Figure #: Mutli-pecimen Product ampling
5or products hot-dip galvanized to either &ST & 14=A& 14= or & 1?=A& 1?=#
a!e 6is used to determine the minimum num$er of specimens for sampling from a
given lot size.
3o< of Pieces in =ot 3o< of pecimens
= or less &ll
3 to ?)) =
?)1 to 14)) ?
14)1 to =4)) (
=4)1 to 1)#))) 1=
1)#))1J 4)
$a8le &: Minimum 3um8er of pecimens for /$M / 120 and / 12
5or re$ar hot-dip galvanized according to &ST & 2;2# the information $elo% is
used to determine the minimum num$er of samples per lot# measurements per sample#
and the total num$er of measurements required for each of the different coating
thic/ness measurement techniques.
Magnetic $hic"ness:
o = samples per lot
o ? or more measurements per sample
o 1? measurements# at the minimum# comprise the average
Microscop9 Method:
o ? samples per lot
o 3 measurements per sample
o 4) measurements# at minimum# comprise the average
tripping and 7eighing:o = samples per lot
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The minimum average coating thic/ness for a lot is the average of the specimen
values and must meet the minimum for the material category. The minimum for an
individual specimen is one grade $elo% the minimum for the material category. &n
individual measurement has no minimum# $ut $are areas are not allo%ed on the part.
The final inspection of a part shall include thic/ness measurements and visual
inspection. &ll parts that do not meet the requirement must $e resorted andreinspected or re@ected and then regalvanized.
*epair
f the galvanized product does not meet all of the requirements of the specification# it
must $e repaired or re@ected along %ith the lot it represents. >hen repair of the
product is allo%ed $y the specification or $are spots are present# the galvanizer is
responsi$le for the repair unless directed other%ise $y the purchaser. The
specifications allo% for some retesting of products that represent lots or retesting after
the lot has $een sorted for non-conformance. The coating thic/ness of the repairedarea must match the coating thic/ness of the surrounding area. Ho%ever# if zinc-rich
paint is used for repair# the coating thic/ness must $e ?)* higher than the
surrounding area# $ut not greater than 3.) mils $ecause mud crac/ing tends to result
%hen the paint coating is too thic/. The maimum sizes for allo%a$le areas that can
$e repaired during in-plant production are defined in the specifications as summarized
$elo%.
Ma%imum ize of +epaira8le /rea
/$M / 120!/ 120M:
o 9ne inch or less in narro%est dimensiono Total area can $e no more than ).?* of the accessi$le surface area to
$e coated or =; square inches per piece# %hichever is less
/$M / 10!/ 10M:
o The $are spots shall have an area totaling no more than 1* of the total
surface area to $e coated# ecluding threaded areas of the piece
/$M / '&'!/ '&'M:
o 6o area given
o f the coating fails to meet the requirement for finish and adherence#
the $ar may $e stripped# regalvanized# and resu$mitted
o Damage done to the coating due to fa$rication or handling shall $e
repaired %ith a zinc-rich formulation
o Sheared ends shall $e coated %ith a zinc-rich formulation
+epair Methods
&ny repairs made to galvanized products must follo% the requirements of &ST &
2()# %hich defines the accepta$le materials and the required procedures. !epairs are
normally completed $y the galvanizer $efore the products are delivered# $ut under
certain circumstances# the purchaser may perform the repairs on their o%n. The touch-
up and repair materials are formulated to deliver an ecellent color that matches either
$rightly coated# ne%ly galvanized products or matte gray# aged galvanized products.
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aterials used to repair hot-dip galvanized products include zinc-$ased solder# zinc-
rich paint# and zinc spray metallizing# and are eplained in the follo%ing sections.
3inc-0ased Solder
Figure : )inc-ased older
Soldering %ith zinc-$ased alloys is achieved $y applying zinc alloy in either a stic/ orpo%der form. The area $eing repaired needs to $e preheated to approimately ;)) 5
'=1? ,+. The most commonly used solders for repair# as seen inFigure 55# include
zinc-tin-lead# zinc-cadmium# and zinc-tin-copper alloys.
urface Preparation
&ccording to &ST & 2()# the surface to $e reconditioned shall $e %ire $rushed#
lightly ground# or mildly $last cleaned. n addition# if %ire $rushing or light $lasting is
inadequate# all %eld flu and spatter must $e removed $y mechanical methods. The
cleaned area also needs $e preheated to ;)) 5 '=1? ,+ and %ire $rushed %hile heated.
Pre-flu may also $e necessary to provide chemical cleaning of the $are spot. 5inally#special care should $e given to insure that the surrounding galvanized coating is not
overheated and $urned $y the preheating.
/pplication
The soldering method is the most difficult of the three repair methods to complete. &
high level of caution must $e ta/en %hile heating the $are spot to prevent oidizing
the eposed steel or damaging the surrounding galvanized coating. Solders are
typically not economically suited for touch-up of large areas $ecause of the time
involved in the process and $ecause heating of a large surface area to the sametemperature is very difficult. >hen the repair has $een completed# the flu residue
needs to $e removed $y rinsing the surface %ith %ater or %iping %ith a damp cloth.
Final +epaired Product
The final coating thic/ness for this repair shall $e agreed upon $et%een the galvanizer
and the purchaser# and is generally in the 1 to 4 mil range. The thic/ness shall $e
measured $y any of the methods in &ST & 14=A& 14= that are non-destructive.
Binc-$ased solder products closely match the surrounding zinc and $lend in %ell %ith
the eisting coating appearance.
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3inc-*ich Paint
Figure &: )inc-+ich Paint
Binc-rich paint is applied to a clean# dry steel surface $y either a $rush or spray as
seen inFigure 56# and usually contains an organic $inder pre-mi. Binc-rich paints
must contain either $et%een ;?* to ;
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3inc Spra! Metallizing
Figure ': )inc pra9 Metallizing
Binc spray# %hich is also referred to as metallizing# is done $y melting zinc po%der or
zinc %ire in a flame or electric arc and pro@ecting the liquid zinc droplets $y air or gas
onto the surface to $e coated# as seen in Figure 57. The zinc used is nominally