Zn Coating

7/30/2019 Zn Coating

1/20

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


2/20

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





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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)

Zn Coating

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