zinc akzo nobel
TRANSCRIPT
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For internal use only – not to be circulated outside AkzoNobel
How much Zinc is needed for galvanic protection?
Mike Winter
Technical Manager – International Paint
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Traditional Wisdom
• Zinc silicates are all the same – they only differ in zinc
content
• The only important consideration is how much is in the
coating
• The more Zinc the better
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Zinc Levels – how much is needed?
• Most standards use % Zn dust by weight in dry film:
• BS5493 >90%
• SSPC Paint 20 Level 1 >85%
• ISO12944 >80%
• SSPC Paint 20 Level 2 77% - 85%
• SSPC Paint 20 Level 3 65% - 77%
• SSPC Paint 29 >65%
• Some use % Zinc metal:
• BS5462 > 85%
• UNE48293 (Spain) > 80%
• Caltrans > 78%
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Laboratory Test Protocols Commonly Used
• ASTM B117 – continuous hot salt spray
– e.g SSPC Paint 20/29 3,000 hr test requirement
• ISO 20340 - 4,200 hrs cyclic salt spray/QUV (3 days/3 days)
with 1 day freeze cycle @ -20ºC
– e.g. Norsok M501
• ASTM D5894 – cyclic prohesion/QUV (7 days/7 days)
– e.g. AASHTO R31-04 5,000 hrs test requirement
• Outdoor exposure
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ASTM B 117 Salt Spray – 3,000 hrs
85% zinc in dry film
80% zinc in dry film
77% zinc in dry film
65% zinc in dry film
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ISO 20340 4,200 hours
85% zinc 80% zinc 77% zinc
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ISO 20340 creep data – 4,200 hours
Single coat IOZ system
% Zn vs creep 1 coat
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10
ISO20340 creep (mm)
% Z
n i
n d
ry f
ilm
% Zn
Zinc epoxy85%
77%
65%
50%
Similar formula types
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ISO 20340 Creep data – 4,200 hours
Zinc silicate/epoxy/polyurethane systems
% Zn vs creep 3 coat system
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12
ISO20340 creep (mm)
% z
n i
n d
ry f
ilm
% Zn
85%
77%
65%
50%
Zinc epoxy
Similar formula types
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Mechanism of Zinc Silicate Protection
• Untopcoated Zinc silicates in corrosive environments protect
galvanically for approx 3 months
• Over time, the open porous film of a zinc silicate coating
becomes “plugged” with reaction products of zinc metal and
oxygen, carbon dioxide, sulfur dioxide, sodium chloride etc
• The porous zinc silicate film becomes a tight, dense, passive
barrier coating
• This mechanism does not occur with topcoated zinc silicates
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Single Coat Zinc Rich Epoxy Systems
3,000 hrs ASTM B117
3,024 hrs ASTM D5894
Paint “A”Paint “B”
Note: both paints have 80% Zn/dry film
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Three Coat Zinc Epoxy/Epoxy/PU Systems
3,000 hrs ASTM B117
Scribe cleaned using ISO
20340 methodScribe cleaned using
ASTM D1654 method
3,024 hrs ASTM D5894
“A” “B” “A” “B”
“A” and “B” both
have 80% Zn in dry
film
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Outdoor Exposure (ISO12944 C3 environment)
85% Zn
85% Zn
77% Zn
65% Zn
50% Zn16 Mth exposure
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Outdoor exposure (ISO12944 C3 environment)
85% Zn
85% Zn
77% Zn
65% Zn
50% Zn
3 coat system:
Zinc sil/epoxy/pu
16 mths exposure
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Zinc Rich Performance Trends
• Single coat Zinc Silicate systems always give less scribe
creep than 3 coat systems
• Single coat zinc epoxy systems usually give less scribe
creep than 3 coat systems
• Different formulations at same zinc level perform differently
• Within similar formula types, higher zinc loads usually give
less scribe creep
• Testing with an acidic spray (e.g. ASTM D5894 type tests),
always gives higher creep values than testing with neutral
(e.g. ASTM B117) spray
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Other Factors Affecting Performance
of Zinc Silicate Systems
• Cure conditions
– Zinc silicates typically require >50% RH, and some require
>65% RH
– Application at low %RH gives poor curing and poor performance
• Cure time prior to topcoating
– most zinc silicates require 24 hrs, but fast recoating is a benefit
for the applicator
• Quality of application – dry spray, film roughness, film
porosity, bubbling of topcoats
• Type of zinc dust – high lead contents may improve
anticorrosive performance, but…..it’s lead. Particle size
distribution may also affect performance.
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Overcoating of Zinc Rich Systems
• Zinc silicates must be well cured prior to overcoating
– Typically 16-24 hrs at 77ºF/60% RH
– Require a “sealer” or “mist” coat to prevent pinholing of topcoats
• Zinc epoxies may be overcoated in as little as 3 hours under
similar conditions
• Time to complete a 3 coat zinc/epoxy/polyurethane system
(actual data from a job*)
– Zinc silicate movable in 100 hrs
– Zinc epoxy movable in 36 hrs
• Faster curing zinc silicates can reduce the time gap between
zinc epoxy and zinc silicate system application
* See “Inorganic Zinc Primer vs Organic Zinc primer, M.Cornago, ENI Exploration & Production,
presented at Corrrosion 2007
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Flaking due to overcoating before full cure
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Pinholing of topcoats over IOZ
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Conclusions
• % Zinc is not sole determining factor in IOZ or OZ performance
• Generally, in equivalent type formulations, % Zinc will affect performance, depending on the system/exposure environment
• Selection of % Zn required (or even if zinc rich is appropriate) should be based on application and exposure environment
• Topcoated IOZ systems perform poorer in lab testing than single coat IOZ systems
• “Performance” of a Zinc rich coating should be considered as a blend of anticorrosive and application related properties (cure speed, application tolerance, ease of application etc)
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Traditional Wisdom?
• Zinc silicates are all the same – they only differ in zinc
content (wrong)
• The only important consideration is how much is in the
coating (no, there’s more to it than that)
• The more Zinc the better (maybe, but depends on
what you’re doing with it)