THERMAL SPRAY METALLIZING
Thermal Spraying or metalizing is the most effective and durable method available today for applying a protective coating to standing steel structures such as bridges. LTL Galvanizers (Pvt) Ltd has the technology to apply Zn, Al, and Zn/Al alloy metallic sprays using thermal spray technology.
What is Thermal Spraying?
- A set of coating processes using finely divided metallic or non-metallic materials are deposited in a molten or semi-molten state to form a coating on a given surface
- The coating material may be in the form of powder, ceramic rod, wire or molten materials
METALLIZING PROCESS
1. Abrasive Blasting
2. Zinc/Al/Zinc-Aluminium Spraying
3. Application of Sealer coat
4. Application of top Paint Coats
Electric Arc Wire Spraying – Metallizing
This is a thermal spray process where an electric arc is created between two electrodes made of consumable coating material. The material is heated by the arc atomized by highly compressed gas and propelled onto the surface being treated.
ADVANTAGES OF METALLIZING
- Metalizing is the most versatile and effective option for applying a protective coating to already fixed structures such as bridges.
- The life cycle cost is lower than that of painting.
- The shape and size of the structure is no barrier to the delivery of an effective coating
- Metalizing provides a higher bond strength when compared with painting.
- Higher service life compared with painting.
- There is no limit to coating thickness as in Hot Dip Galvanizing.
REAL APPLICATIONS OF METALLIZING
C5 Environment – Coastal Areas and Industrial Areas
- Zn/Al metalizing is the most suitable coating method for the C5 Environment. A top primer and one or two paint coats are typically applied as sealers during metalizing.
- Metalizing is the ideal process for corrosion protection when restoring already-built bridges and other large structures that cannot be submerged in a galvanizing bath.
Corrosion Performance Study
U.S. Federal Highway Administration (FHWA) Research, Development, and Technology
- The Ohio Department of Transportation has applied 85% zinc/15% aluminum alloy to approximately 10 bridges over the past 8 years with no reported failures.
- These bridges have 200 µm of metal by specification and are sealed with a clear phenolic topcoat.
- Ohio has developed a detailed specification for the application of metallized coatings and has recently mandated metallizing in all expansion joint areas for new construction.
- The American Welding Society and the U.S. Navy have performed extensive testing of metallized coatings applied to steel.
- These studies show that properly applied metallized coatings (zinc, 85% Zinc/ 15% Aluminum, and Aluminum) of at least 150 µm thickness provide at least 20 years of maintenance-free corrosion protection in wet, salt-rich environments and are expected to provide 30 years of protection in most bridge exposure environments.
- A study performed on the Thomas Mathis Bridge by New Jersey DOT shows perfect performance of two metallized bridge spans after 8 years of marine exposure.
- One each of these spans received metallized zinc and 85% zinc/15% aluminum with epoxy and polyurethane topcoats.
- Other spans painted with different conventional coatings showed varying performance.
- The two metallized spans were the only ones showing absolutely no deterioration over the test period.
QUALITY ASSURANCE
- Thermal Spraying of Zinc and Zinc-Aluminium alloy is done in accordance with the ISO 2063:2019 standard.
- Metallized coating thickness is greater than the minimum coating thickness as mentioned in TableB.1 of ISO 2063:2019.
- Metallized coating thickness and thickness of sealer coats can be customized according to the requirement of the customer and according to the Environmental conditions where the item is installed.
- Adhesion tests of coating can be done if requested by the customer.
- Painting over metallized surface is done under the guidance of a certified paint specialist and in accordance with table A.8 of ISO 12944-5:2007.
Inspection
Environmental condition
- Humidity test
- Surface temperature
Surface Condition
- Abrasive test
- Salt contamination test
- Surface Profile test
- Dust test
Coating thickness
- DFT Testing
- WFT Testing
- Cross Hatch testing
- Pinhole detection testing