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Hot Dip Galvanized Pipe

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What is Hot Dip 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

The Application of Hot Dip Galvanized Pipe

Hot-dip galvanized steel pipe is mostly utilized in the transmission of coal, gas, and steam. It was utilized as a water pipe, but after a few years, there were many rusts in the pipe, and the water turned yellow from carrying iron oxide. The water not only contaminated dishes or other sanitary equipment but also carried bacteria generated on the interior surface of the non-smooth internal surface. As a result of corrosion, the water includes an excessive amount of heavy metal elements, which is damaging to people’s health. As a result, since the 1960s, developed countries have steadily prohibited its use as a water pipe. While hot dip galvanized pipes are increasingly being used in building projects due to their excellent corrosion resistance and lower cost than other coatings.

Dimensions and Sizes of Hot Dip Galvanized Pipe

DNO. D.W. T.
InchmmSCH5SSCH10SSCH10SCH20SCH30SCH40SCH60SCH80SCH100SCH120SCH140SCH160STDXSXXS
502″60.31.652.773.915.548.743.915.5411.07
652 1/2″732.113.055.167.019.535.167.0114.02
803″88.92.113.055.497.6211.135.497.5215.24
903 1/2″101.62.113.055.748.085.748.08
1004″114.32.113.056.028.5811.1313.496.028.5617.12
1255″141.32.773.46.559.5312.715.886.559.5318.05
1506″168.32.773.47.1110.9714.2718.267.1110.9721.95
2008″219.12.773.766.357.048.1810.3112.715.0918.2620.6223.018.1812.722.23
25010″273.13.44.196.357.89.2712.715.0918.2621.4425.428.589.2712.725.4
30012″323.93.964.576.358.3810.3114.2717.4821.4425.428.5833.329.5312.725.4
35014″355.53.964.786.357.929.5311.1315.0919.0523.8327.7931.7535.719.5312.7
40016″406.44.194.786.357.929.5312.716.6621.4426.1930.9636.5340.499.5312.7
45018″457.24.194.786.357.9211.1314.2719.0523.8339.3634.9339.6745.24
50020″5084.785.546.359.5312.715.0920.6226.1932.5438.144.4550.01
55022″558.84.785.546.359.5312.722.2328.5834.9341.2847.6353.98
60024″609.65.546.356.359.5314.2717.4824.6130.9638.8946.0252.3759.54

The Process of Producing Hot Dip Galvanized Pipes

The manufacture of hot dip galvanized pipe involves three basic steps: surface preparation, galvanizing, and inspection. 

shows the processes of hot dip galvanizing

- Surface Preparation

Surface preparation is perhaps the most important step in the process because it determines the quality of galvanizing and the service life of the pipe. We have to pay attention to surface preparation, otherwise, the unclean part of the pipe will remain uncoated.

- Three Steps during Surface Preparation

  1. Degreasing – The degreasing stage is an important part of the surface preparation process for hot-dip galvanized (HDG) pipe. This stage removes contaminants such as oils, greases, and other organic matter from the surface of the pipe. The most common method for degreasing HDG pipe is to immersion in a solution bath. The solution bath may be either alkaline or solvent-based solution. HDG pipe can also be decreased by blasting with dry ice, sand, or other abrasive media. Once the degreasing stage is complete, it is important to thoroughly clean the surface of the pipe to remove any solution residues. This will ensure that the HDG coating can properly adhere to the surface of the pipe.
  2. Pickling – When preparing the surface of a pipe for hot-dip galvanizing, it is important to remove any impurities that could potentially affect the quality of the coating. One way to do this is through pickling, which involves immersing the pipe in an acidic solution. This solution breaks down the iron oxide that is present on the surface of the pipe, leaving a clean surface that is ready to be coated with zinc. Pickling is typically followed by sandblasting or chemical cleaning, which further prepares the surface for hot-dip galvanizing.
  3. Fluxing – Fluxing is an important part of the surface preparation process for hot-dip galvanized pipes. By removing oxides and other impurities from the surface of the pipe, fluxing helps to ensure a stronger bond between the zinc coating and the steel. Many different fluxes can be used for this purpose, but zinc ammonium chloride is a common choice. This solution helps to prevent zinc from being oxidized during the galvanizing process, which can result in a weaker coating. To flux the surface of the pipe, it is first cleaned with a solvent or abrasive material. The zinc ammonium chloride solution is then applied to the surface, and the pipe is heated to remove any remaining impurities. This process ensures that the surface of the pipe is clean and ready for hot-dip galvanizing.

- Galvanizing

Galvanizing is a process in which a metal is coated with zinc to protect it from corrosion. The bath can be either cold or hot, but most galvanizing is done at temperatures between 840 and 850 degrees Fahrenheit. The zinc becomes molten at these temperatures, and it bonds to the surface of the metal. Once the piece is removed from the bath, it is allowed to cool in the air. This forms a thin coat of zinc on the surface of the metal that protects it from corrosion. Galvanizing is often used on steel to protect it from rust, but it can also be used on other metals such as iron and copper.

- Inspection

Inspecting a hot dip galvanized pipe coat is critical to ensure the quality of the zinc coating. The coat must be of sufficient thickness to protect the zinc from corrosion. The coat thickness and appearance should be checked regularly with the appropriate quality control equipment. The coat thickness can be measured with a micrometer or a coating thickness gauge. Additionally, the zinc coat should evenly cover all surfaces of the pipe, including the inside diameter and welds. The appearance of the coat should be checked for blisters, pinholes, or bare spots. Any areas that are not properly coated should be repaired immediately. By regularly inspecting hot dip galvanized pipes, coat failures can be prevented and quality controlled.

Hot Dip 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.

FeatureHot Dip GalvanizationPre-Galvanizing
Coating ThicknessThick coating with minimum average requirements of 45-85μm within BS EN ISO 1461The coating thickness may vary, but it is typically circa 20μm for sheets and 20-30μm for tubes
Coating ContinuallyContinuous coating over the whole objectThe uncoated area at cut edges
Coating BondStrong metallurgical bond with the base steelStrong metallurgical bond with the base steel
Coating FormabilityForming after hot dipped galvanizing is not advised as it may damage the coatingA thin coating may be normally formed without any damage
Coating AppearanceNormally bright, but can be variableNormally bright
Abrasion ResistanceA thicker layer of hard zinc-iron alloy gives a high resistance to abrasionThin alloy layer with reduced resistance to abrasion
Sacrificial ProtectionOffer the highest level of sacrificial protectionReduced sacrificial protection due to thin layer coating and at some uncoated areas (cut edges)

The History of Hot Dip Galvanizing

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.

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