For internal use only – not to be circulated outside AkzoNobel

How much Zinc is needed for galvanic protection?

Mike Winter

Technical Manager – International Paint

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

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%

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

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

ISO 20340 4,200 hours

85% zinc 80% zinc 77% zinc

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

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

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

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

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

Outdoor Exposure (ISO12944 C3 environment)

85% Zn

85% Zn

77% Zn

65% Zn

50% Zn16 Mth exposure

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

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

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.

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

Flaking due to overcoating before full cure

Pinholing of topcoats over IOZ

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)

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)