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% |
|
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 |
