Properly Specifying Powder Coatings Over Galvanized Steel
Powder Coating Over Galvanized Steel
A duplex system is a protective system consisting of hot-dip galvanizing combined with an additional organic coating such as powder coating or paint. This combination provides longer service life than either system used independently because the coatings work synergistically to resist corrosion.
Powder coating over hot-dip galvanized (HDG) steel can provide excellent aesthetics and enhanced corrosion protection when properly specified and applied. However, failures in powder-coated galvanized systems are often traced to inadequate surface preparation, failure to perform outgassing, or insufficient consideration of the unique characteristics of galvanized coatings.
Unlike bare steel, galvanized steel has a zinc surface that changes with age and environmental exposure. The galvanized steel coating is also known to retain air or moisture which can form a small crater or blister in the powder coat during the curing stage of the coating process if outgassing is not performed. Specifications developed for conventional powder coating applications may not adequately address the requirements necessary to achieve reliable adhesion and appearance when coating galvanized steel.
Successful duplex systems rely on clearly defined requirements for surface preparation, pretreatment, outgassing control, coating application, and verification testing. Incorporating these requirements into project specifications helps establish clear responsibilities among galvanizers, coaters, inspectors, and owners while reducing the potential for coating failures and project disputes.
Can Galvanized Steel Be Powder Coated Successfully?
Yes. Hot-dip galvanized steel can be successfully powder coated when the galvanized surface is properly prepared and the coating process is designed to accommodate the characteristics of zinc coatings.
Most performance issues associated with powder coating galvanized steel can be traced to one or more of the following:
- Outgassing during the curing process
- Inadequate surface profiling or pretreatment
- Contamination of the galvanized surface
- Insufficient coating coverage in complex geometries
- Improper cure schedules or process control
For this reason, specifications should address not only the powder coating itself but also the condition of the galvanized surface and the procedures used before and during application.
AGA provides numerous resources to help you prepare for powder coating over hot-dip galvanizing:
What Surface Preparation Should Be Specified When Applying Powder Coats to Galvanized Steel?
Surface preparation is one of the most critical variables affecting the performance of powder coatings applied over galvanized steel.
ASTM D7803 details recommended practices for preparing hot-dip galvanized surfaces for powder coating, including available practices for surface smoothing, cleaning, and profiling based on the identified initial HDG surface condition. However, D7803 also provides guidance on a thermal pretreatment to be performed after surface preparation which is necessary to prevent outgassing of the HDG coating during the baking step in the powder coating curing process.
While ASTM D7803 describes the established methods for cleaning, roughening, and outgassing the HDG surface, they do not address project-specific requirements. Due to variations in powder coating products and preferences among structure owners and powder coaters, it is necessary to clearly communicate all obligations to galvanizer and powder coater in the project documents.
HDG Cooling Methods & Galvanizer Post Treatments
Galvanizing facilities may choose to immerse steel immediately after galvanizing in a water bath (“water quench”) or passivation bath (“passivation quench”) to cool the parts rapidly in preparation for handling and inspection. Alternatively, galvanizers may choose natural air cooling or forced air cooling using fans. If cooling requirements are not addressed by the project specifications nor communicated to the galvanizer, galvanizers will perform any cooling method of their choice as ASTM A123 does not prohibit the use of passivation coatings. Although there are exceptions, traditional industry best practice is to request the galvanizer avoid quench cooling (water quench and passivation quench). The most common passivation coatings used in hot-dip galvanizing are known to compromise the adhesion of coatings applied over HDG. If a passivation quench has been performed (or if use of a passivation quench is unknown), it can be detected and removed. Procedures for removing a passivation coating by sweep blasting and an inspection to confirm removal of the passivation coating are provided in ASTM D7803. Many powder coatings require the galvanized surface to be roughened by sweep blasting anyway, meaning a separate step for passivation removal is not necessary but removal should be confirmed.
Selection of HDG Repair Materials
When specifying zinc-rich paint for HDG repairs at the galvanizing facility or by the painter, evaluate the material compatibility of the zinc-rich paint and the selected coating system. Additionally, consult the manufacturer’s product data sheet regarding re-coating intervals. It may be practically necessary to agree on whether or not the galvanizer should apply zinc-rich paint repair material if the galvanizer and painter are two different parties.
Surface Smoothing
Some surface conditions and prominences present on hot-dip galvanized coatings do not affect the corrosion protection and are acceptable under ASTM A123 (general roughness, small dross inclusions, skimmings, zinc runs, etc.). However, these same surface conditions are understood to affect adhesion of the powder coating and must be smoothed or removed from the galvanized surface prior to cleaning and surface roughening. A description of common surface conditions is provided in AGA Publication, Inspection of Hot-Dip Galvanized Steel Products.
ASTM A123 does not define acceptable or rejectable surface conditions for duplex systems, so the required level of smoothing before powder coating must be agreed upon by the galvanizer and purchaser.
Without clear specifications assigning responsibilities, required surface preparation may be missed, especially when the galvanizer and powder coater are separate parties. Specifications should define each smoothing requirement and the responsible party. Even when the galvanizer performs primary smoothing, powder coaters may still need to fill indentations or smooth prominences caused by transport or handling. Hot-dip galvanizing does not hide fabrication irregularities such as burrs, raised areas, weld porosity, crevices, or thermally cut edges. If these project above the surrounding coating, smoothing after HDG for powder coating can expose bare steel, so repair methods should be agreed upon in advance. Smoothing these areas before galvanizing can avoid later smoothing and coating repairs.
Surface Cleaning
Project specifications should require the powder coater to remove organic contaminants during surface preparation in accordance with ASTM D7803. Solvent cleaning is often overlooked, but it is necessary to remove surface contaminants before roughening the galvanized surface.
Surface Roughening
Specifying ASTM D7803 alone does not fully define the surface roughening method. Confusion can arise on projects when the roughening options are not clearly selected from or agreed upon. To ensure best results, the powder coating manufacturer should be consulted when the methods from ASTM D7803 are selected.
In many applications, SSPC-SP 16, Brush-Off Blast Cleaning of Non-Ferrous Metals, describes the most common method for roughening galvanized surfaces for powder coating. Sweep blasting removes surface contaminants and zinc corrosion products while creating a surface profile that promotes adhesion without significantly reducing coating thickness. Sweep blasting is a light abrasive blasting process intended to roughen the galvanized surface and remove zinc oxides while maintaining the protective galvanized coating. The surface profile requirement in SSPC-SP 16 is minimum 0.75 mil unless otherwise specified in the project specification, procurement documents, or the product data sheet for the powder coating to be applied. There exists significant overlap between ASTM D7803 and SSPC-SP 16 regarding both mandatory and non-mandatory information.
An additional consideration to maintain the integrity of the galvanized coating is the use of appropriate blasting abrasives and techniques. Abrasives are known to achieve different peak heights and peak densities when used on HDG surfaces compared to bare steel surfaces. AGA publishes additional research and guidance: Abrasive Blast Media for Preparing HDG for Painting and Powder Coating. Additional mitigations by the powder coater will significantly reduce the risk of HDG coating damage when sweep blasting, but come with a known tradeoff in productivity. The most effective techniques include increased distance between blast nozzle and galvanized surface, reduced nozzle pressure, faster nozzle movement, and use of a finer abrasive size.
Table 1. Established Roughening/Preparation Methods for Powder Coating over HDG
Pretreatment Method | Best for Surface Condition | Potential Concerns | Specification Considerations |
SSPC-SP 16 Sweep Blasting | All | Excessive blasting may reduce coating thickness or cause damage (flaking). Avoid wet abrasive blasting. | Define acceptable abrasive type and blasting techniques. Define any surface profile requirements. |
SSPC-SP 11 Grinding | All | Excessive grinding may reduce coating thickness or cause bare spots. Start with lower pressure or RPM and light pressure and adjust as needed. | Define any surface profile requirements. |
Chemical Surface Treatment | Controlled shop environments | Residual contamination from improper rinsing. Conflicting specification requirements related to the inspection of a surface profile prior to commencing powder coating. | Confirm compatibility with powder coating manufacturer. Apply according to treatment manufacturer instructions. |
Regardless of the preparation method selected, responsibilities should be clearly assigned within the specification to avoid ambiguity between the galvanizer and the powder coater.
How Can Specifications Address Outgassing?
Outgassing occurs when entrapped air, moisture, or other volatiles within the galvanized coating expand during the curing process and pass through the powder film during curing.
The resulting defects may include:
- Pinholes
- Blisters
- Craters
- Surface voids
- Localized texture variations
Because powder coatings are cured at elevated temperatures, outgassing control should be addressed separately from the manufacturer's curing requirements.
ASTM D7803 describes the outgassing requirement and procedure. Pre-baking in a drying/curing oven helps reduce outgassing by releasing trapped air or moisture and drying the steel surface before powder coating. Industry best practice is to pre-bake at 65°F above the curing temperature. If chemical surface treatment is used before coating, consult the treatment manufacturer for maximum temperature limits to avoid damaging the applied film. The part should be pre-baked until the galvanized surface reaches the oven temperature (typically about 1 hour) to assure all moisture and entrapped gasses are expelled. The part should then be cooled to a temperature that allows application of the powder coating in accordance with the manufacturer’s instructions. Powder coating should follow pre-baking as soon as possible so there is little or no time for zinc oxidation to begin.
Summary
Powder coating over galvanized steel is a proven duplex coating system, but its success depends on more than simply applying powder over a galvanized surface. Surface preparation, pretreatment, and outgassing control all play critical roles in overall performance.
Specifications that clearly define these requirements help reduce ambiguity, improve coating quality, and increase the likelihood of long-term success in service.