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For internal use only not to be circulated outside AkzoNobel
How much Zinc is needed for galvanic protection?
Mike Winter
Technic al Manager International Paint
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Traditional Wisdom
Zinc silicates are all the same they only differ in zinccontent
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 @ -20C
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
nindryfilm
% 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
nindryfilm
% 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 APaint 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 benefitfor 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..its 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 77F/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 affectperformance, depending on the system/exposureenvironment
Selection of % Zn required (or even if zinc rich isappropriate) should be based on application and exposureenvironment
Topcoated IOZ systems perform poorer in lab testing than
single coat IOZ systems
Performance of a Zinc rich coating should be considered asa 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, theres more to it than that)
The more Zinc the better(maybe, but depends on
what youre doing with it)