What Galvanized Welded Pipe Is
Galvanized welded pipe is a welded steel pipe carrying a zinc coating on its internal and external surfaces. The tube itself is made from steel strip by forming and high-frequency welding, and the zinc is then applied by hot-dip galvanizing or electro-galvanizing. The coating protects the steel in two distinct ways: it forms a continuous barrier against moisture and oxygen, and it protects sacrificially, so that where the layer is cut or scratched the zinc corrodes in preference to the exposed steel. That combination is why galvanized pipe is specified for outdoor, humid and intermittently wet service where bare carbon steel would corrode quickly.
Production Process
Steel strip or coil is slit to width, formed into a round section and welded longitudinally by high-frequency welding to produce the base tube. The weld bead is dressed or levelled, the tube is straightened, cut to length and then prepared for coating: degreasing, pickling and fluxing so that molten zinc can wet the steel surface.
In hot-dip galvanizing the pipe is immersed in molten zinc at roughly 445-465 degrees C. Zinc-iron alloy layers form at the interface and the outer layer remains essentially pure zinc, giving a coating that is metallurgically bonded to the substrate and therefore highly resistant to mechanical damage. In electro-galvanizing the zinc is deposited electrolytically from a zinc salt solution. The resulting layer is thinner, smoother and more uniform, which suits applications where appearance, tight dimensional control or paint adhesion matter more than maximum coating mass.
Coating Requirements and Standards
| Standard | Coating requirement | Typical use |
|---|---|---|
| ASTM A53 / A53M | Minimum zinc coating of about 0.9 oz/ft2, approximately 275 g/m2, on standard hot-dip galvanized pipe | Water, steam and general piping |
| ASTM A123 / A123M | Hot-dip coating thickness graded by material thickness, from about 45 micrometres on thin sections to about 86 micrometres on heavy sections | Structural and fabricated steelwork |
| ISO 1461 | Minimum average coating thickness of 70 micrometres for steel 6 mm and above, 55 for 3-6 mm, 45 for 1.5-3 mm and 35 below 1.5 mm | Fabricated articles and pipe supports |
| EN 10240 | Internal and external coating classes, with minimum zinc mass defined by wall thickness and service class | European pipework and water systems |
| GB/T 3091 | Hot-dip galvanized welded steel pipe for low-pressure fluid service | Water, gas and heating lines |
Coating mass and coating thickness are directly related, because zinc has a density of 7.14 g/cm3. Dividing the coating mass in g/m2 by 7.14 gives the thickness in micrometres, so a 275 g/m2 coating is approximately 38 micrometres thick. It is that figure, not the generic description galvanized, which governs corrosion life and should be quoted on the purchase order.
Applications
Water supply, fire fighting and irrigation lines where internal and external corrosion must both be controlled.
Greenhouse frames, fencing, gates, handrails, scaffolding and guardrails exposed to weather.
Heating, ventilation and air conditioning ducting and low-pressure gas lines.
Solar mounting structures, transmission poles and outdoor equipment frames.
General engineering where a painted finish will be applied later, in which case electro-galvanized or a suitable surface treatment is often selected.
Selection and Inspection Points
Start from the environment. A coating adequate for a dry inland site is not adequate for a coastal or industrial atmosphere, so agree the coating mass from the applicable standard rather than from a generic product name. At goods receipt, measure coating mass or thickness on a defined number of lengths, examine the surface for bare spots, black spots, flux residues and uncoated areas, and assess adhesion by a bend or impact test agreed between buyer and supplier. Cut ends, threads and weld repairs expose bare steel and should be treated with a zinc-rich repair coating in accordance with the applicable standard.
Two design points are frequently overlooked. First, galvanized steel in contact with copper or copper alloys forms a galvanic couple that accelerates coating loss, so dielectric separation is required at such connections. Second, hot-dip galvanizing demands that fabricated assemblies be properly vented and drained, because trapped air or moisture inside a closed section prevents the zinc from reaching internal surfaces.
FAQ
Q: What is the difference between hot-dip and electro-galvanized welded pipe?
Hot-dip galvanizing produces a thicker, alloy-bonded coating that gives longer corrosion protection and better resistance to mechanical damage. Electro-galvanizing produces a thinner but smoother and more uniform coating, usually chosen where appearance or paint adhesion is the priority.
Q: How is the zinc coating specified?
By coating mass in g/m2 or by coating thickness in micrometres, with the minimum defined in the applicable standard. Dividing coating mass by 7.14 converts g/m2 into micrometres of zinc.
Q: Does galvanizing affect the mechanical properties of the pipe?
Hot-dip galvanizing involves a short immersion at moderate temperature and does not change the specified tensile or yield properties of the base steel. Very high-strength cold-formed sections should nevertheless be reviewed for possible loss of ductility at the coating temperature.
Q: Can galvanized welded pipe be welded on site?
It can, but the zinc coating must be removed from the joint area before welding and the fumes require extraction. After welding, the bare zone should be repaired with a zinc-rich coating to restore corrosion protection.
Q: How long does a galvanized pipe coating last?
Service life depends primarily on coating mass and on the corrosivity of the environment. In dry indoor conditions a hot-dip coating can last several decades, while in coastal or heavily polluted atmospheres the same coating is consumed far more rapidly, so both factors must be considered together.
Q: Are internal and external coatings always the same?
Not necessarily. Some standards define separate internal and external coating requirements, and it is common for water pipe to specify a defined internal coating mass with a heavier external coating for soil or atmospheric exposure.
