Cathodic protection of steel by Zinc Coating: What is the best anti-corrosive primer for steel? How does a Zinc coating works as anti-corrosive?

 

Cathodic protection of steel by Zinc Coating

SCOPE:   

Material: Stainless steel up to 120°C

Environment: Atmospheric

Exposure category C5 (high)

Standard ISO 12944 - 5

 

Abstract

According to AGA (American Galvanizers Association) explanation, Zinc is a natural, healthy and abundant element. Zinc metal has a number of characteristics that makes it a well suited corrosion protective coating for iron and steel products.

 

Zinc coating protect steel from corrosion due to it’s anodic properties where it will sacrificially corroded before the underlying steel.

 

Zinc Coating

The Film Galvanizing System ZINC coating contains 96% zinc in the dry film and provides cathodic protection of ferrous metals. Paint can be used as a unique system as an alternative to hot-dip galvanization or metallization, as primer in a duplex system (active + passive) or as a recharging system for hot-dip galvanization, metallization or zinced surfaces. Paint can be applied by brushing, rolling or spraying on a clean and rough substrate in a wide range of atmospheric circumstances.

 

Product physical & technical parameters

Components: - Zinc powder, Aromatic hydrocarbons, Binder

Density: 2.7kg/dm³

Volume Solids: 60 (±2%)

Solid content: 80% by weight (±2%) - 58% by volume (±2%) according to ASTM D2697

Color: Grey (color darkens after contact with humidity)

Zinc content: 96% (±1%) by weight, with a purity of 99,995%.

Special characteristics: Atmospheric temperature resistance of dry film

» Minimum: -40°C

» Maximum: 120°C with peaks up to 150°C

- pH resistance in immersion: 5,5 pH to 9,5 pH.

- pH resistance in atmospheric conditions: 3,5 pH to 12,5 pH.

- Excellent UV resistance

Non-toxicity:  A dry layer is not toxic. Have to test according to the standard AS/NSZ 4020.

Bellow table shows an overview of the steel preparation system

Step no.	Action	Method	Purpose
1	Pre-preparation	Visual inspection + wash down	Remove contamination
2	Salt Check	Conductivity Test	Check contamination
3	Preparation	Blast-clean all surfaces of the steelwork	Cleanliness / profile
4	De-dusting	By appropriate means	Cleanliness

 

Surface preparation

1.       Surface salt test: according with ISO 8502-6 and testing according to ISO 8502-9 (conductivity test).

2.       Blast cleaning:

Abrasive: all abrasives must comply with SSPC-AB1 grade 3

Abrasive salt contamination: test according with ISO 11127 or ASTM D 4940.

3.       Blast-profiles and cleanliness: All blast-profiles shall fall inside the range of 50 – 70 μm @110-130 PPI (peaks per inch) according to ISO 8503. Measurement according to ASTM 4417

The cleanliness of the overall surface must be in accordance with SA 2.5 standards - ISO 8501-1.

4.       De-dusting: The cleanliness should be class 1 (according to ISO 8502-3)

Surface Pre-preparation	Steam Clean @ 150 bar (min) @ 80°C or High Pressure Fresh Water Wash @ 150 bar (min)
Surface Preparation	Sa 2.5 (ISO 8501-1) SSPC-SP10 (Grit-blast or slurry blast. Also UHPAB to SSPC-SP12
Surface Profile	Rz 50 – 70µm (ISO 8503-2)
Surface Roughness	Ra 12.5 – 15µm (ISO 8503-1)

 

Application:

Brush / Roller	Dilution is not required. Only when the product thickens, use thinner within 5% of paint weight.
Conventional spray	Use Gravity feed spray gun. Nozzle size: 1.5 ~ 2.0 mm, Pressure: 0.3 MPa Dilution: 0 ~ 5%, Paint strainer: #100
Air-less spray	Tip size: above 0.017 inches (e.g. 517) Pressure: above 20 MPa, Gun filter: #50 ~ 60 Dilution: 0 ~ 5%

 Application system:

Coats	Product	Application Type	Application Method	Required DRY Film Thickness (µm)
Coat-1	Zinc Coating	Full coat	Airless Spray, Conventional Spray & Brush	(60-120) µm
Coat-2	Zinc Coating	Full coat	Airless Spray, Conventional Spray & Brush	(60-120) µm

 

Properties

Test item	Methods	Result should be
Hardness	Pencil scratching (ISO/DIS 15184:1996)	Pencil B (minimum)
Adhesion	Peel off a sticky tape on 25 cells check pattern (square: 2mm x 25)- (ISO 2409:1992)	No abnormality
Heat resistance	Electrical constant temp. drier 170 °C, 24 hours	No abnormality
Cold resistance	Low temp. cycle test for 72 cycles <1 cycle>Left at -30 °C for 5 hours and Left at +10°C for 1 hour	No abnormality

 

 

Anti-corrosion performance

 

Test item	Testing method	Duration	Result Should be
Water immersion	Immersed in ion exchanged water	3 months	No abnormality other than white rust
Salt water immersion	Immersed in 3% salt water	3 months	No abnormality other than white rust
Salt spray	5% NaCl (pH6.5~7.2) Temp of spray chamber: 35±1°C (ISO 7253:1984)	1440 hours	No abnormality other than white rust

 

Image: Anti-corrosive Zinc Primer

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Sorwar Jahan

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