Hot dipped galvanized pipe
What is Hot Dipped Galvanized Pipe?
Hot-dip galvanizing is the process of coating steel pipe with a layer of zinc. The pipe is first cleaned and then heated to a temperature that is above the melting point of zinc. The steel is then dipped into a vat of molten zinc, where it is held for a period of time. During this process, the zinc reacts with the steel to form a thin layer of silver-colored zinc alloy. This layer helps to protect the steel from corrosion and rust. HDG pipe is often used in construction and other applications where steel is exposed to the elements. Pre-galvanized steel pipe is also available, which has been coated with a layer of zinc before it is formed into a pipe.
Table of Contents
Hot Dipped Galvanized Pipe Application
Hot-dip galvanized steel pipe is mainly used to transport water, gas and steam. Hot-dip galvanized pipe is increasingly being used in construction projects due to its corrosion resistance and lower cost than other coatings.
Hot Dipped Galvanized Pipe Dimensions and Sizes
| DN | O.D. | W.T. | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Inch | mm | SCH5S | SCH10S | SCH10 | SCH20 | SCH30 | SCH40 | SCH60 | SCH80 | SCH100 | SCH120 | SCH140 | SCH160 | Sth | XS | XXS | |
| 50 | 2″ | 60.3 | 1.65 | 2.77 | – | – | – | 3.91 | – | 5.54 | – | – | – | 8.74 | 3.91 | 5.54 | 11.07 |
| 65 | 2 1/2″ | 73 | 2.11 | 3.05 | – | – | – | 5.16 | – | 7.01 | – | – | – | 9.53 | 5.16 | 7.01 | 14.02 |
| 80 | 3″ | 88.9 | 2.11 | 3.05 | – | – | – | 5.49 | – | 7.62 | – | – | – | 11.13 | 5.49 | 7.52 | 15.24 |
| 90 | 3 1/2″ | 101.6 | 2.11 | 3.05 | – | – | – | 5.74 | – | 8.08 | – | – | – | – | 5.74 | 8.08 | – |
| 100 | 4″ | 114.3 | 2.11 | 3.05 | – | – | – | 6.02 | – | 8.58 | – | 11.13 | – | 13.49 | 6.02 | 8.56 | 17.12 |
| 125 | 5″ | 141.3 | 2.77 | 3.4 | – | – | – | 6.55 | – | 9.53 | – | 12.7 | – | 15.88 | 6.55 | 9.53 | 18.05 |
| 150 | 6″ | 168.3 | 2.77 | 3.4 | – | – | – | 7.11 | – | 10.97 | – | 14.27 | – | 18.26 | 7.11 | 10.97 | 21.95 |
| 200 | 8″ | 219.1 | 2.77 | 3.76 | – | 6.35 | 7.04 | 8.18 | 10.31 | 12.7 | 15.09 | 18.26 | 20.62 | 23.01 | 8.18 | 12.7 | 22.23 |
| 250 | 10″ | 273.1 | 3.4 | 4.19 | – | 6.35 | 7.8 | 9.27 | 12.7 | 15.09 | 18.26 | 21.44 | 25.4 | 28.58 | 9.27 | 12.7 | 25.4 |
| 300 | 12″ | 323.9 | 3.96 | 4.57 | – | 6.35 | 8.38 | 10.31 | 14.27 | 17.48 | 21.44 | 25.4 | 28.58 | 33.32 | 9.53 | 12.7 | 25.4 |
| 350 | 14″ | 355.5 | 3.96 | 4.78 | 6.35 | 7.92 | 9.53 | 11.13 | 15.09 | 19.05 | 23.83 | 27.79 | 31.75 | 35.71 | 9.53 | 12.7 | – |
| 400 | 16″ | 406.4 | 4.19 | 4.78 | 6.35 | 7.92 | 9.53 | 12.7 | 16.66 | 21.44 | 26.19 | 30.96 | 36.53 | 40.49 | 9.53 | 12.7 | – |
| 450 | 18″ | 457.2 | 4.19 | 4.78 | 6.35 | 7.92 | 11.13 | 14.27 | 19.05 | 23.83 | 39.36 | 34.93 | 39.67 | 45.24 | – | – | – |
| 500 | 20″ | 508 | 4.78 | 5.54 | 6.35 | 9.53 | 12.7 | 15.09 | 20.62 | 26.19 | 32.54 | 38.1 | 44.45 | 50.01 | – | – | – |
| 550 | 22″ | 558.8 | 4.78 | 5.54 | 6.35 | 9.53 | 12.7 | – | 22.23 | 28.58 | 34.93 | 41.28 | 47.63 | 53.98 | – | – | – |
| 600 | 24″ | 609.6 | 5.54 | 6.35 | 6.35 | 9.53 | 14.27 | 17.48 | 24.61 | 30.96 | 38.89 | 46.02 | 52.37 | 59.54 | – | – | – |
Hot Dipped Galvanized Pipe Manufacturing Process
The manufacture of hot dipped galvanized pipe involves three basic steps: surface preparation, galvanizing, and inspection.
- Surface Pretreatment
The surface pretreatment of hot-dip galvanized steel pipe is a very critical core link, do a good job of surface cleaning in order to get a high quality galvanized layer on the surface of the steel pipe.
- Three Steps during Surface Pretreatment
Degreasing – first of all, the surface of the steel pipe should be cleaned first, generally can use acid washing or mechanical cleaning. For example, 35MPa high pressure water jetting system and alkaline degreasing tank. After the degreasing step, 99.8% of the surface pollutants are basically removed.
Pickling – activates metal surface activity to remove impurities and oxides from the surface of the pipe. Pickling immerses the pipe in an acidic solution. The chemical solution breaks down the iron oxide on the surface of the pipe, leaving only a clean surface for subsequent galvanizing. Pickling is usually followed by sandblasting or chemical cleaning to further prepare the surface for hot-dip galvanizing.
Flux – A zinc ammonium chloride solution formulation is typically used. ZnCl₂ is mixed with NH₄Cl at a ratio of 3:1, the concentration is maintained at 400-600 g/L, and the pH is stabilized at 3.5-4.5 by an automatic ammonia dosing system.
- Hot Dipped Galvanizing
The heated steel pipe is put into the hot dip galvanizing tank for hot dip galvanizing treatment star.
The molten zinc in the hot-dip galvanizing tank is piped into the tank, into which the steel pipe is immersed so that its surface is in contact with the molten zinc. After a certain period of immersion, the steel pipe is removed from the hot-dip galvanizing tank to form a uniform zinc layer on its surface.
- Post-treatment
After hot-dip galvanized steel pipe is removed, it requires post-treatment to improve its corrosion resistance. This is usually done by ammonium chloride dipping or washing to form a strong epoxy film on the surface of the zinc layer.
-Inspection and Packaging
The hot-dip galvanized steel pipe is subjected to appearance quality inspection and dimensional specifications to ensure that it meets the relevant standards and specifications. Then the qualified hot-dip galvanized steel pipes are packed at to prevent damage during transportation and use.
Hot Dipped Galvanized Pipe with Threaded End
Threaded connections, which are commonly utilized on 3-inch hot-dip galvanized pipe or lesser sizes, are referred to as screwed pipe. Screwed pipe and screwed fittings can be quickly built without welding or other permanent means of attachment by using tapered grooves carved into the ends of a run of pipe. National Pipe Thread is the most used pipe thread (NPT).
Difference between Hot Dip Galvanizing and Pre-Galvanizing
An automated approach may also be used to galvanize pre-galvanized pipe to EN 10240. Immersion durations are short, and steam may be forced into the bore of the tube after extraction to create a smooth surface finish. Although coating thicknesses of 45-55 m are possible, the bulk of products manufactured have a significantly thinner covering of 20-30 m.
Because pre-galvanized pipe components are often only submerged in the galvanizing solution for a brief amount of time, the coating is rather thin. Hot-dip galvanization results in a thicker zinc coating than pre-galvanization, which offers superior rust and corrosion protection, particularly for pipes that are regularly used outdoors and exposed to snow, ice, rain, and salt.
The zinc coating on the steel sheet must meet the requirements of ASTM A525 Grade G90 and G120. The total zinc weight of the exterior and interior surfaces shall be between 0.90 oz/sq ft (or 0.45 oz/sq ft each side) and 1.20 oz/sq ft (or 0.60 oz/sq ft each side). These coating weights do not apply to the inner welded region.
| Feature | Hot Dip Galvanization | Pre-Galvanizing |
|---|---|---|
| Coating Thickness | Thick coating with minimum average requirements of 45-85μm within BS EN ISO 1461 | The coating thickness may vary, but it is typically circa 20μm for sheets and 20-30μm for tubes |
| Coating Continually | Continuous coating over the whole object | The uncoated area at cut edges |
| Coating Bond | Strong metallurgical bond with the base steel | Strong metallurgical bond with the base steel |
| Coating Formability | Forming after hot dipped galvanizing is not advised as it may damage the coating | A thin coating may be normally formed without any damage |
| Coating Appearance | Normally bright, but can be variable | Normally bright |
| Abrasion Resistance | A thicker layer of hard zinc-iron alloy gives a high resistance to abrasion | Thin alloy layer with reduced resistance to abrasion |
| Sacrificial Protection | Offer the highest level of sacrificial protection | Reduced sacrificial protection due to thin layer coating and at some uncoated areas (cut edges) |
Hot Dip Galvanizing History
Since 1742, the hot-dip galvanizing method has been utilized to provide long-lasting, maintenance-free corrosion protection at a reasonable cost. Although hot-dip galvanizing has been used to protect steel for decades, the galvanizing process is still evolving with new technology and innovative chemistries. Surface preparation, galvanizing, and post-treatment are the three primary processes in the hot-dip galvanizing process, and each will be addressed in detail. The technique is essentially simple, which gives it a significant advantage over other corrosion prevention approaches.
