Inspection of
Hot Dip Galvanizing
1
Reasons for Inspecting
• Compliance
– Standard
– Specifications
• Quality of work
• Assured corrosion protection
2
Inspection Before Galvanizing
• Suitable size for bath
• Temporary identification markings
• Appropriate vent, fill & drainage holes
• Venting of overlapping surfaces
• No weld slag/splatter
3
Inspection Before Galvanizing
• Unsuitable joining materials
• Clearance of moving parts
• Check for distortion caused during rolling or
fabrication
• Potential for distortion
4
Inspection After Galvanizing
• Visual
– Appearance
– Surface conditions
• Coating Thickness Measurement
5
Appearances – Reasons for Differences
• Steel Chemistry
• Cooling Rate
• Steel Processing
– Smooth
– Rough
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Steel Chemistry - Silicon
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Steel Chemistry - Silicon
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Steel Chemistry - Silicon
Resistance to
Category Si and P relationship Appearance mechanical Mass of coating Typical use
damage
Minimum. Can
For aesthetic and
X Si ≤ 0.010% Excellent, typically shiny Excellent sometimes be
corrosion protection
under Standard
Si ≤ 0.040%
Hot rolled Standard.
Si+2.5P ≤ 0.090% For compliance with
Generally superior
A Excellent, typically shiny Excellent Standard and excellent
to the normal
corrosion protection
requirement
Cold rolled Si + 2.5P ≤ 0.04%.
Always heavier
Good, can tend to mottled than normal. Best
Optimum long-term
B 0.14% < Si ≤ 0.25% or dull with increasing Good specification for
corrosion protection
steel thickness corrosive
environments
C 0.04% < Si ≤ 0.14% In non-abrasive
environments can
Can be dark and coarse Reduced Extra Heavy
provide extreme
D Si > 0.25% corrosion protection
[i]
Category X has been derived from “Galvanizing reactive steels – a guide for galvanizers and specifiers”, ILZRO 1996 and “Galvanizing Difficult Steels”,
John Robinson, Director – Mount Townsend Solutions Pty Ltd, 2007.
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Steel Chemistry - Silicon
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Steel Chemistry - Silicon
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Steel Chemistry - Silicon
12
Steel Chemistry - Silicon
13
Steel Chemistry - Silicon
14
Steel Chemistry - Silicon
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Steel Chemistry - Silicon
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Steel Chemistry
17
Steel Chemistry
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Steel Chemistry – Cooling Rate
19
Visual Inspection
SURFACE CONDITIONS
20
Ash deposits or entrapments
• Zinc oxide attached
going into or out of
the zinc bath.
• The coating is
normally intact.
Remove ash and
coating thickness
verified
• Galvanizers
responsibility to
remove
21
Bare spots
22
Bare spots
• Number of causes
including:
– Contaminants on steel surface
not removed by pre-treatment
(e.g. oil paints & lacquers)
– Blowouts, flux deposits, touch
marks, scale or sand
• Acceptable, depending
on size, amount and
cause
• Bare areas should be
repaired according to
AS/NZS 4680
23
Blasting damage
• Incorrect nozzle
pressure, nozzle
angle, sweeping
distance, size of
abrasive grit
• Excessive blasting
can destroy the
coating
• This is out of the
control of the
galvanizer
24
Chain & wire marks
• Lifting device can leave
superficial marks on
finished product
• Repair to AS/NZS 4680
unless bare steel is
exposed
25
Clogged holes/threads
• Molten zinc has high
surface tension
• Holes must be more than
8mm in diameter
• Utilization of vibrators /
centrifuge will reduce
clogging
• Holes should be cleaned
by the galvanizers where
possible
• Clogged threads are
rejectable as they should
be clean of excess zinc
26
Delamination or Peeling
• Top, pure layer of zinc
peeling from surface
• Possible causes:
– result of slow cooling
(Kirkendall Effect)
– Sharp, thermal cut edges
• Acceptable but may
need to repaired
depending on
remaining coating
thickness
27
Distortion
• Any inherent rolling or
welding stresses in
fabrication can cause
dimensional change
• Use symmetrical design and
sections of similar thickness
• Acceptable unless it prevents
part from fulfilling intended
use
• May be bent after galvanizing
to achieve acceptance final
condition
28
Drainage spikes
29
Drainage spikes
• Edge last to leave the
bath is likely to have
spikes, particularly
complex fabrication
• Should be removed by
galvanizer
30
Dross Inclusions
• Dross are zinc-iron
intermetallic alloy
• Become entrapped or
entrained in zinc coating
• May be avoided by
changing lifting
orientation or design
• Doesn’t affect corrosion
protection
• Excessive dross
inclusions can be a cause
for rejection
31
Excessive grinding/cleaning
• Cleaning by
mechanical methods
can result in uncoated
areas
• Check coating
thickness and repair
according to AS/NZS
4680
32
Flaking
33
Flaking
• Thick-walled steel components
with >0.25% Si are at risk
• The risk is enhanced by rapid
cooling of articles in quenching
• Remaining coating thickness
generally doesn’t meet the
Standard
• If the area of flaked coating is
small, it may be repaired
• If the area is large, this is
rejectable and should be re-
galvanized
• The steel grade/source of the
item may need to be changed
34
Marks and Mechanical damage
• Generally happens during transportation.
• Marks should be cleaned where possible
• If coating chipped or damage, should be repaired
• Appropriate strapping/padding should be used
during transportation, especially for heavier coatings
35
Oxide lines
• Light aluminium oxide film lines on a hot dip galvanized surface
• Occurs when items are not withdrawn from the zinc bath at a
constant rate
• Acceptable. No effect of corrosion resistance, will weather to
uniform appearance
36
Pimples and blisters
• Usually associated with surface imperfections such as
dross inclusions.
• Bare spots should be repaired
• Generally acceptable, but excessive amounts can be
cause for rejection
37
Products in contact or touch marks
• Where a component has been resting on jigging or dipping
equipment, an uncoated area or touch mark could appear
• Can be cause for rejection, depending on size
• Should be repaired by galvanizer where appropriate
38
Rolling defects in steel
• Defect in the steel
• Can cause
flaking/uncoated area
• Acceptable and
should be repaired
where needed
39
Rough coatings & surface condition
• Generally a result of steel composition or processed
steel surface
• Doesn’t effect corrosion resistance
40 • Acceptable
Runs & uneven drainage
• The cause can be withdrawal speed too high or low
galvanizing temperature.
• Can be design issue (poor drainage or hanging points)
• Generally acceptable or cleaned if necessary
41
Rust staining
• Usually due to contact with or drainage from other corroded steel
surfaces.
• Steel filings or saw-chips produced during erection and
fabrication operations should be removed from galvanized
surfaces to prevent possible localised rust staining.
• May also be caused by the weeping of pickling acid from seams
and joints
42 • Generally acceptable, but should be cleaned off
Iron castings
• Sand on cast iron is
generally caused by the
process used to form the
product
• A localised ungalvanized
area in an otherwise
continuous coating can
occur if sand from the
moulding is not removed by
acid pickling or abrasive
blasting
• Can be either cleaned &
repaired or regalvanized
43
Striations/fish-boning
• Generally caused by variations is surface chemistry
which are thought to occur in the manufacturing
process of large diameter tube/pipes
• No affect of corrosion resistance and is acceptable
44
Weeping weld
• Acid or flux liquid penetrate
porous welding or areas
between contact surfaces
during cleaning process can
weep out after galvanizing and
water quenching
• Produce a stained area
• Generally due to poor welding
or design for galvanizing
• Acceptable
• Stained areas should be
cleaned
• Advisable to seal crevice with a
sealant
45
Welding blowouts
• Staining and coating defects (bare spots) around
unsealed weld areas and vent holes
• Cleaning solutions penetrate sealed overlap areas
through required vent holes and escape during
immersion into the molten zinc
46
Welding splatter
• Weld spatter should be removed prior to hot dip galvanizing
• Presence of tightly adherent weld spatter after hot dip galvanizing will
not affect corrosion resistants of the coating
• Generally acceptable
47
Wet storage stain
48
Wet storage stain
• Occurs when new galvanizing
stored close together in
moisture environment
• Zinc corrosion product
• Generally superficial
• Should clean from surface and
repair any areas if case is
severe
• If formed at the galvanizer
plant, should be removed prior
to leaving the plant
• If occurs after leaving
galvanizers site this is not
cause for rejection
49
TOUCH-UP & REPAIR
50
Touch-up & Repair
• Area to be repaired • Methods of repair
– Total area = lesser of – Organic zinc rich
0.5% of article or epoxy
250cm2 – Inorganic zinc silicate
– Each area ≤ 40 cm2 – Thermal zinc metal
spray
– Zinc alloy solder stick
51
Repair – Organic Zinc Rich Epoxy
• Most common repair
method
• Paint must comply
with AS/NZS 3750.9
• 2 x coats of 50µm
• Applied via spray or
brush
52
Repair – Inorganic Zinc Silicate
• Paint to comply with AS/NZS 3750.15
• 1 x coat 100µm
• Applied via spray or brush
53
Repair – Thermal zinc metal spray
• To ISO 2063 or AS/NZS 2312
• Required blast cleaning for surface preparation
• Generally not used
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Repair – Zinc alloy solder stick
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Field Inspection - Maintenance
• Good corrosion
protection strategy
• Periodic inspection
– Potential accelerated
corrosion areas
– Aesthetic surface
defects
• Repaired areas
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