What Is Galvanized Steel Pipe?
Galvanized steel pipe is a steel tube, normally welded and occasionally seamless, whose surface carries a zinc coating. The base tube is usually a low-carbon welded pipe produced to GB/T 3091, and the coating is applied either by dipping the finished pipe into molten zinc or by depositing zinc electrolytically from a zinc salt solution. The value of the product lies in two mechanisms acting at the same time: the coating seals the steel from air and water, and the zinc, being electrochemically less noble than iron, corrodes preferentially at scratches and cut ends and thereby protects the steel beneath. Galvanized pipe is used for water, gas, air and other low-pressure fluid lines, and for structural, fencing and greenhouse frames that are exposed to weather.
The terminology matters when an order is written. Hot-dip galvanizing produces a metallurgically bonded coating in which iron-zinc alloy phases grow from the steel surface. Electrogalvanizing, often called cold-dip galvanizing in trade usage, produces a purely metallic deposit that sits on the surface without alloying, and its performance is correspondingly different.
Hot-Dip Galvanizing: Principle and Process Flow
The principle is simple: a clean steel surface is brought into contact with molten zinc, and the iron at that surface reacts with the zinc to form a series of iron-zinc alloy phases. In practice the bath is held at about 450 degrees Celsius, with 445 to 465 degrees Celsius the normal working window, and the zinc charge is of at least 99.5 percent purity. Dipping time and withdrawal speed are controlled so that the coating builds to the specified mass without excessive drainage marks.
The coating that forms is layered. Next to the steel there is an iron-zinc alloy zone containing phases such as the delta and zeta phases, and above it an outer eta layer of essentially pure zinc. As a working approximation, the alloy zone accounts for roughly one third of the coating and the pure zinc layer for about two thirds. The alloy zone is what gives the coating its adhesion to the steel; the outer zinc layer is what provides the long sacrificial protection.
Incoming inspection and pretreatment of the base pipe, including removal of oil and loose scale.
Pickling in hydrochloric or sulphuric acid to dissolve mill scale and rust.
Rinsing in clean water to remove residual acid before it can attack the steel.
Fluxing in a zinc chloride and ammonium chloride solution, which forms a protective film, prevents re-oxidation before dipping and improves zinc wetting.
Immersion in the molten zinc bath at about 450 degrees Celsius, followed by controlled withdrawal.
Cooling in air or water, then passivation if the pipe is to be stored for a long period or shipped by sea.
Inspection of coating mass, coating continuity, appearance and adhesion, using weighing, magnetic thickness measurement and a bend or impact test on a sample.
Electrogalvanizing and How It Differs
In electrogalvanizing the pipe is the cathode and zinc plates are the anode, both immersed in an electrolyte containing zinc ions such as zinc chloride. A direct current deposits metallic zinc on the pipe surface. No iron-zinc alloy forms, so the coating is thinner, softer and less firmly bonded than a hot-dip coating, and it is more easily damaged in handling. Because the coating is light, passivation becomes an essential part of the process rather than an optional one, and the finished pipe is best reserved for indoor or temporary service. For continuous fluid contact or for outdoor exposure, the hot-dip product is the appropriate choice.
Coating Characteristics, Protection Mechanisms and Service Life
Coating mass is the parameter that should be specified, because it determines how long the pipe will last. The table below gives the values normally seen on mill certificates and the service conditions they suit.
| Coating type | Typical total coating mass | Approximate thickness per surface | Suited to |
|---|---|---|---|
| Hot-dip, standard | About 500 grams per square metre | About 35 micrometres | Normal outdoor and water service |
| Hot-dip, heavy | 600 grams per square metre and above | About 42 micrometres and above | Coastal, buried and high-humidity service |
| Electrogalvanized | About 36 to 143 grams per square metre | About 5 to 20 micrometres | Dry indoor and temporary lines |
The conversion follows directly from the density of zinc: dividing the coating mass per surface by 7.14 grams per cubic centimetre gives the thickness of that surface. A total coating mass of 500 grams per square metre therefore corresponds to roughly 250 grams per square metre on each face, or about 35 micrometres.
Sacrificial anode protection: the standard potential of zinc is about minus 0.76 volt and that of iron about minus 0.44 volt, so at a scratch or a cut end the zinc becomes the anode and dissolves while the steel remains cathodic and protected.
Barrier protection: a continuous, pore-free coating keeps oxygen and water away from the steel, which is why coating continuity is inspected as strictly as coating mass.
Service life: in moderate rural and urban atmospheres, a standard hot-dip coating on pipe is commonly quoted at 15 to 30 years before maintenance becomes necessary; a thin electrogalvanized coating can show rust within one to two years in a humid environment.
Passivation: a chromate-free or chromate passivation film suppresses white rust during transport and storage and is not a substitute for coating mass.
Advantages, Limitations and Typical Applications
Advantages
Long maintenance-free life at a moderate cost compared with painting an uncoated pipe.
Protection at cut ends and threads, where a paint film cannot help but the zinc still can.
Ready availability in standard sizes with threaded and grooved ends, so installation follows familiar trade practice.
Limitations
Corrosion resistance has an upper limit. Strong acids, strong alkalis and media with a high chloride content consume the zinc quickly, so seawater and chemical process lines require a different material.
High-temperature service is unsuitable. Zinc melts at 419 degrees Celsius and its protective film degrades well before that, so steam lines and hot oil circuits are outside the range of the product.
The coating is a consumable. Once the zinc has been consumed at a location, the steel there begins to rust, and repair relies on a zinc-rich coating applied to the affected area.
Threading and welding break the coating locally, so those areas need protection after fabrication.
Typical applications
Water supply, fire fighting and irrigation pipework at low pressure.
Compressed air, nitrogen and inert gas distribution lines.
Structural tube for railings, fencing, greenhouse frames, walkway handrails and scaffolding.
Electrical conduit and cable protection in industrial plants.
General low-pressure fluid transfer in utilities, agriculture and light industry.
Frequently Asked Questions
Q: What is the difference between hot-dip and cold-dip galvanized pipe?
Hot-dip pipe is dipped in molten zinc and carries an alloyed coating of roughly 35 micrometres per surface or more, while cold-dip, correctly called electrogalvanized, pipe carries a thinner electrolytic deposit of about 5 to 20 micrometres and is intended for indoor or temporary use.
Q: How long will galvanized pipe last outdoors?
A standard hot-dip coating in a moderate atmosphere is commonly quoted at 15 to 30 years, but life shortens sharply in coastal air, in industrial pollution or where condensate is trapped against the pipe.
Q: Can galvanized pipe be used for steam or hot water above 100 degrees Celsius?
It should not be. The protective behaviour of zinc depends on a stable surface film, and at sustained temperatures above about 200 degrees Celsius the coating degrades; steam and heat-transfer oil lines need a different material.
Q: Can galvanized pipe be welded?
It can be welded, but the zinc is burned away at the weld and the fumes must be controlled, so the joint has to be cleaned and protected afterwards with a zinc-rich repair coating.
Q: Why does galvanized pipe develop white rust in storage?
White rust is zinc oxide and zinc hydroxide formed when moisture sits on the coating without air circulation, so pipe should be stored under cover, on dunnage, with ventilation between layers.
Q: Which standards apply to galvanized steel pipe?
In China the base product is normally ordered to GB/T 3091 for welded steel pipe, the coating to GB/T 13912 for hot-dip galvanized fabricated articles, and the base steel to GB/T 700 for Q235; equivalent international references include ASTM A53/A53M and EN 10255.