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Hellen Christodoulou Ph.D., Eng., B.C.L., LL.B., M.B.A.
Executive Vice President
Engineering, Sustainability and Business Development
Corbec Inc.
Designers opt for galvanization as the corrosion protection system for structural steel fabrication, since it ultimately provides the most sustainable solution with the lowest carbon footprint. Hot-dip galvanizing provides excellent corrosion resistance.
The welding of steel prior to or after galvanization is common, both are compatible with the objective of providing excellent corrosion protection. The processes, however, involve specific considerations, recommended as best practices, to ensure the viability of both the structural integrity and the intended corrosion protection.
It is key to reiterate some of the key considerations for the proper preparation and the importance of adherence to guidelines, which are essential for the successful welding of pre or post galvanized steel. These include the proper welding practices for before or after galvanizing, and how they play a significant role in eliminating distortion and ways welded surfaces can be re-galvanized.
Specific codes and standards for galvanized steel play a crucial role in ensuring safety, quality, and reliability. Specifications for zinc (Hot-Dip Galvanized) coatings are essential for ensuring the quality, durability, and performance of galvanized steel. Here’s why these specifications matter:
• They define coating thickness, surface finish, and adherence requirements to ensure long-lasting protection against rust and environmental exposure.
• They outline the coating application process, including pre-cleaning, fluxing, and dipping. Proper execution ensures uniform coverage, preventing weak spots or bare areas.
• They set standards for bond strength between the zinc layer and the base metal. Strong adhesion prevents peeling or flaking. Adhesion strength is the coating adherence, and it’s critical.
• They address surface finish, including smoothness, spangle size, and visual defects. A consistent appearance enhances aesthetics and quality.
• They guide galvanizers in temperature control, immersion time, and cooling rates. Proper process control ensures optimal coating properties.
• They define inspection methods (visual, thickness measurement, etc.) and acceptance criteria. Inspections verify compliance and conformity with standards.
• They consider galvanized steel’s interaction with other materials, since the proper compatibility prevents issues during fabrication and use.
In summary, adhering to zinc coating specifications ensures reliable, cost-effective, and durable galvanized products for various applications. A partial list is shown below:
AWS D1.1 (Structural Welding Code - Steel)
A123/A123M Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
A153/A153M Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
ASTM A384/A384M Standard Practice for Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies
ASTM A385/A385M Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)
A780 Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings D6386 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
D7803 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating
E376 Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Testing Methods
SSPC-SP 1 Solvent Cleaning
Effective communication between all stakeholders of the project team is critical. An integrated approach to discuss, plan, and anticipate some of the critical matters at the forefront is key to ensure flawless execution of a project. Some key elements needing discussion, education and collaboration are on the process, the steel chemistry, surface requirements, shapes in consideration, the required process temperature/heat, the consideration of venting & drainage, the proper and recommended welding processes and the relevance on the type of connections. Furthermore, it is important to have a comprehensive appreciation of both pre and post galvanizing design requirements and material intended use.
The galvanizing process in principle is simple and it consists of three key basic elements: surface preparation, galvanizing, and inspection. The proper handling of projects requires expertise and quality assurance, and this largely depends on the quality of the facilities, the crew and the knowledge.
Surface Preparation: The surface preparation step in the galvanizing process has its own built-in means of quality control because zinc simply will not react with a steel surface that is not perfectly clean. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the liquid zinc and can therefore be corrected.
Degreasing/Caustic Cleaning: The steel or iron articles are first cleaned to remove organic contaminants like dirt, paint, grease, and oil, using a hot alkali solution. This step ensures proper adhesion of the zinc coating.
Pickling: Next, the articles undergo pickling, where they are immersed in heated sulfuric acid or hydrochloric acid. This removes mill scale and rust from the surface.
Fluxing: After pickling, the articles are fluxed by applying a zinc ammonium chloride solution. Fluxing prevents further oxide formation and prepares the surface for galvanizing.
Drying: The material is then thoroughly dried prior to immersion in liquid zinc.
Galvanizing: The steel is ready to be immersed in a bath of molten zinc (typically maintained at around 450-460°C or 850-860°F). During immersion, zinc reacts with iron in the steel, forming metallurgically bonded zinc-iron alloy layers, topped by a layer of pure zinc. Excess zinc is removed, and the material cools either in a passivation solution or open air. It’s important to have awareness of some of the important parameters that affect the dipping process. Considering size and shape, as well as weight, are important to material handling capacity used in galvanizing plants.
The steel is moved through the process using hoists and overhead cranes.
• Special jigs and racks are also commonly used to simultaneously galvanize large numbers of similar items. Providing lifting points where possible will reduce or eliminate chain or wire marks that can be left on an item when no lifting points are present.
• It is also good practice to discuss the weight-handling capacity with the galvanizer to ensure capability and/or the best places to put lifting points.
Inspection: Coating thickness and appearance are closely scrutinized to ensure quality.
Most ferrous materials are suitable for hot-dip galvanizing. Plain carbon steel (on an average up to 150 ksi / 1100 MPa) and low alloy materials. Hot-rolled steel, cold-rolled steel, cast steel, ductile iron, cast iron, castings, stainless steel and even weathering steel can be galvanized for enhanced protection. Since the chemical composition influences the characteristics of the galvanized coating, this is when information and info exchange help all stakeholders.
General guidance for steel selection:
Levels of carbon less than 0.25%
Phosphorus less than 0.04%
Manganese less than 1.3%
Silicon levels less than 0.04% or between 0.15% - 0.22%
The maximum thickness of steel that can be galvanized without challenges is around 3 inches and as thin as 1/8 inch. We often talk about thin steel surfaces or sheeting; contrary to misinformation they can also be properly galvanized with the right expertise. The proper dipping procedures, drip hole inclusion and time of immersion require expertise, to avoid any possibility of embrittlement.
Various surface conditions, different fabrication methods, or ferrous metals with special chemistries, when combined, make it difficult to produce coatings with uniform appearance. This is because different parameters for pickling (immersion time, solution concentrations, temperatures) and galvanizing (bath temperatures, immersion time) are required.
It is important to coordinate with the galvanizer, to get the proper answers and understand the requirements for these different parameters.
The welding of steel prior to or after galvanization is common, both are compatible with the objective of providing excellent corrosion protection - to ensure the viability of both the structural integrity and the intended corrosion protection. Welding steel before or after galvanizing requires careful consideration to ensure the quality of the weld and the integrity of the galvanized coating. The American Welding Society has a book detailing all aspects of welding galvanized items.
Welding steel prior to galvanizing ensures that the entire structure is coated with corrosion-inhibiting zinc. It is quite simple to achieve this, by simply following certain guidelines:
Cleanliness: Steel surfaces are clean and free from any contaminants: oil, grease, paint, or rust, by using methods as solvent cleaning, wire brushing, or abrasive blasting.
Joint Preparation: Joint preparation is crucial for strong welds. Appropriate welding techniques, such as beveling or chamfering to create clean, well-fitted joints, should be used.
Welding Technique: Based on the steel thickness, the right process should be selected, the proper joint configuration, and application requirements. Common: MIG (Metal Inert Gas), TIG (Tungsten Inert Gas), and stick welding.
Preheating: Depending on the steel grade and thickness, preheating may be necessary to reduce the risk of hydrogen cracking. The welding procedure specification (WPS) should be verified.
Parameters: The voltage, amperage, and travel speed should be set according to the WPS and welding procedure qualification record (PQR) to achieve proper weld penetration and fusion.
Post-Weld Treatment: The welds should be inspected for defects such as cracks, porosity, or incomplete fusion. Necessary repairs should be performed before proceeding to galvanizing.
To better achieve an even zinc coating in the weld area, as well as the rest of the steel section, it is recommended that weld filler metal is chemically compatible to base material steel. Dissimilar composition of steels joined may react with the zinc at a different rate and give a thicker, or thinner coating over the weld. Also, to avoid an increased reaction of the welded material with the molten zinc, the filler material to be used should have less than 0.3 percent silicon content.
When welding prior to galvanizing an uncoated electrode should be used to prevent flux deposits. If using a flux-coated electrode is unavoidable, mechanical cleaning methods will be needed to prepare the steel for galvanizing. Also, anti-spatter sprays should be avoided when welding prior to galvanizing, as they often are not removed by chemical cleaning and must be mechanically removed.
Weld slag are flux melts from the flux-coated welding wire/stick, which then deposit onto the weld rea. Welds performed with flux-coated wire, or a flux blanket - not bare wire, are susceptible to form weld slag. As a result, weld flux and slag must be removed by grinding, abrasive blast cleaning, wire brush, flame-cleaning, or chipping. Different welding processes vary in potential for slag formation.
Seal Welds: A seal weld is used to prevent the inflow of fluid causing corrosion and prevent the infiltration of oxygen-laden air and moisture. Galvanized parts specify seal welds to prevent liquid zinc and pickling acids from entering spaces. Seal welds are applied on exposed steel columns that are to be painted to stop rust bleeding; it is easier to clean a sealed joint rather than an exposed joint. It should be noted that seal welds are not used for strength purposes.
Stitch Versus Continuous Weld: It is often beneficial to use stitch-welding techniques. If air is trapped in a seal-welded space it will expand rapidly upon immersion into the zinc bath, which may be the cause of sudden fracture of the steel structure. A stitch weld in conjunction with a gap of 3/32 (2.5mm) or more between pieces is the suggested procedure to avoid this phenomenon. Residues around the base, like welding slag, should also be cleaned prior to galvanizing.
Distortion during welding of steel before galvanizing can be minimized, by simply implementing some known best practices. This will ensure the integrity of the structure is maintained, while preparing it for the galvanizing process. Some of the recommended best practices.
Joint Design: Joint designs that minimize the concentration of heat should be preferred. Consider the use of fillet welds instead of butt welds, when possible, to better distribute heat more evenly, reducing distortion.
Fixture and Clamping: Clamps or fixtures should be used to secure pieces and to prevent movement during welding. Proper fixation helps maintain alignment and reduces the risk of distortion.
Preheating: Preheating the steel before welding can help reduce thermal gradients and minimize distortion, when necessary. This option depends on factors such as material thickness and welding process.
Weld Sequence: The selected welding sequence should evenly distribute heat across the structure. Welding sequentially in one direction should be avoided, for this leads to localized heating and distortion; alternating between welding locations is preferable, to result in uniform heat distribution.
Intermittent Welding: Intermittent welding techniques, like stitch or skip welding allow the welded area to cool between passes. This minimizes heat buildup and reduces distortion.
Tack Welding: Components should be temporarily secured, using tack welds before the final welding; they should be small and strategically placed to minimize their impact on distortion.
Controlled Parameters: Control on voltage, current, and travel speed etc. should be maintained to achieve consistent welds and avoid excessive heat input, which can contribute to distortion.
Backstep Welding: Backstep welding techniques should be used to help distribute heat more evenly and reduce the likelihood of distortion.
Cooling Rate Control: Welded structures must be cooled gradually after welding to minimize residual stresses; quick water quenching can induce distortion.
Post-Weld Heat Treatment: Post-Weld heat treatment processes, such as stress relieving, reduce residual stresses and minimize distortion.
Welding on galvanized steel after the galvanizing process is common and can be necessary for structures that are too large to be dipped in a galvanizing kettle all at once or for structures for which field-welding is required. Here are some key points regarding welding after galvanizing:
Surface Preparation: The galvanized coating should be removed in the area to be welded: via mechanical methods - grinding or sanding, or chemical methods such as pickling.
Ventilation: Proper ventilation should be available to prevent exposure to zinc fumes by using local exhaust ventilation or well-ventilated areas.
Welding Technique: Proper techniques to minimize the formation of zinc fumes and maintain the quality of the welding must be applied: intermittent welding and back-stepping can help reduce heat input and zinc vaporization.
Welding Consumables: Welding consumables selected must be compatible with welding galvanized steel: low-hydrogen electrodes or wire with flux-cored arc welding (FCAW), should be selected, to minimize the risk of hydrogen embrittlement.
Post-Weld Cleaning: The welded area must be cleaned to remove any zinc spatter or residue, using a wire brush or grinder.
Repair of Galvanized Coating: Damage during welding should be repaired using a zinc-rich paint or thermal spray zinc coating to restore corrosion protection.
Inspection: Welded joints should be inspected for quality and integrity, using non-destructive testing- visual inspection, dye penetrant testing, or ultrasonic testing, as necessary.
The maximum allowable area of galvanized steel that can be touched-up after welding must be compliant to the requirements of ASTM A123/A123M. Welding galvanized steel specifications are derived from the American Welding Society, specification D-19.0: “Welding Zinc Coated Steel”. The welding is done on steel free of zinc, to prevent strength reduction through zinc inclusion in the weld itself. The zinc coating should be removed at least one to four inches from either side of the intended weld zone and on both sides of the steel section. Grinding is the effective means of removing the galvanized coating. After the welding is completed, the area of the weld can be repaired using procedures ed in ASTM A780.
Section 6.2.2 of ASTM A123/A123M, requires that the total area subject to modification, repair or change on each member, shall be no more than ½ of 1% of the accessible surface area to be coated on that piece, or 36 square inches per short ton of piece weight, whichever is less.
After welding, any of the touch-up methods described in ASTM A780 can be used to repair the areas where the galvanized coating was removed for welding.
Projects vary in size and complexity and come with different welding requirements, fitting the project scope. The welding of steel prior to or after galvanization is common and both will result in excellent corrosion protection to the steel. The key takeaway is to consistently follow the proper standards and procedures, while adhering to best practices and guidelines. This practice will result in efficiency, viability of structural integrity and better execution of the project and ultimately a way to galvanize better!
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