Tubular Fabrications & Hollow Structurals Cleaning

Tubular Fabrications & Hollow Structurals Cleaning

Tubular assemblies (handrails, pipe columns, pipe girders, street light poles, transmission poles, pipe trusses, sign bridges) are commonly galvanized because corrosion protection is afforded to the interior and exterior of the product. To provide an optimal galvanized coating, hollow products require proper cleaning, venting, and draining.

Cleaning

As with all steel, pipe and other hollow materials must be thoroughly cleaned before the molten zinc will metallurgically bond with the steel. Pipe can present two special cleaning challenges. First, the mill coating (varnish, lacquer, and similar materials) applied by pipe manufacturers requires extra time and effort to remove at the galvanizing plant. Some galvanizers do not have the capability to remove this coating. Some organic mill coating formulations, both foreign and domestic, are

extremely difficult to remove with common cleaning solutions, so blasting may be required. Ordering uncoated pipe avoids costly attempts to remove these mill coatings. In some cases, it may be more cost effective to substitute tube for pipe.

Second, welding around mill coatings burns and carbonizes the varnish in the surrounding areas and cannot be removed

by the normal cleaning process at a galvanizer. This soot must be removed by blasting or other mechanical cleaning methods prior to delivering steel to the galvanizing facility. 

Allowing for Proper Drainage

For effective galvanizing, cleaning solutions and molten zinc must flow without undue resistance into, over, through, and out of the fabricated article. Failure to provide for this free, unimpeded flow can result in complications for the galvanizer and the customer. Improper drainage design results in poor appearance, bare spots, and excessive build-up of zinc. All of these are unnecessary and costly, and another example of why communication throughout the project is key.
A few common fabrications where drainage is important are gusset plates, stiffeners, end-plates, and bracing. Following these best design practices will help ensure the highest quality coatings:
• Where gusset plates are used, generously cropped corners provide for free drainage. When cropping gusset plates is not possible, holes at least 112-inch (13 mm) in diameter must be placed in the plates as close to the corners as possible (Figure 1).

Figure 7: Cropped bracing

• To ensure unimpeded flow of solutions, all stiffeners, gussets, and bracing should be cropped a minimum of 3/4-inch (19 mm) (Figure 8). Provide holes at least 112-inch (13 mm) in diameter in end-plates on rolled steel shapes to allow molten zinc access during immersion in the galvanizing bath and drainage during withdrawal.
corners as possible (Figure 1).

Cropped Corners (preferred) ​​Holes close to corners (alternatively)

Figure 8: Cropped gusset plate corners

Alternatively, holes at least 112-inch (13 mm) in diameter can be placed in the web within 1I4-inch (6 mm) of the
end-plate. To facilitate drainage, end-plates should have holes placed as close to interior corners as
possible (Figure 9).

Reactive Steels

Atypical coatings produced from reactive steels exhibit different coating characteristics than a typical galvanized coating such as:

Appearance: The atypical galvanized coating may have a matte gray appearance and/or rougher surface due to the absence of the free zinc layer. The free zinc layer present on typical coatings imparts a shinier finish to a galvanized coating.

Adherence: The zinc-iron alloy coating tends to be thicker than a typical galvanized coating. In the rare situation where the coating is excessively thick, there is the possibility of diminished adhesion under external stress (thermal gradients, sharp impact).

Reactive steels are still galvanized on a regular basis, and it is important to note differences in appearance have no effect on the corrosion protection afforded by the galvanized coating. The performance of the coating is based on the thickness of the zinc; therefore, often the duller (and thicker) coatings produced by reactive steels last longer. Furthermore, all galvanized coatings as they weather over time will develop a uniform matte gray appearance.

It is difficult to provide precise guidance in the area of steel selection without qualifying all steel grades commercially available. However these guidelines discussed will assist you in selecting steels that provide good galvanized coatings.

• Levels of carbon less than 0.25%, phosphorus less than 0.04%, or manganese less than 1.35% are beneficial

• Silicon levels less than 0.04% or between 0.15% - 0.22% are desirable

Silicon may be present in many steels commonly galvanized even though it is not a part of the steel's controlled composition, because silicon is used in the steel deoxidation process and is found in continuously cast steel. Both silicon and phosphorous act as catalysts during the galvanizing process, resulting in rapid growth of zinc-iron alloy layers.And even when both elements are individually held to desirable limits, the combined effect between them can still produce an atypical coating of all or mostly zinc-iron alloy layers. When possible, your galvanizer should be advised of the grade of steel selected in order to determine whether specialized galvanizing techniques are suggested.

Materials Suitable for Galvanizing -

Most iron-containing (ferrous) materials are suitable for hot­dip galvanizing. Plain carbon steel (under 150 ksi/llOO 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 and are galvanized for enhanced corrosion protection. However, the material's chemical composition influences the characteristics of the galvanized coating. During galvanizing, the iron in the material reacts with the molten zinc to form a series of zinc-iron alloy layers, which are covered by a layer of iron-free zinc. For most hot-r

olled steels, the zinc-iron alloy portion of the coating will represent 50-70% of the total coating thickness, with the free zinc outer layer accounting for the balance (Figure 1).

Steel compositions vary depending on strength and service requirements. Trace elements in the steel (silicon, phosphorus) affect the galvanizing process as well as the structure and appearance of the galvanized coating. Steels with these elements outside of the recommended ranges are known in the galvanizing industry as highly reactive steel, and may produce a coating composed entirely, or almost entirely, of zinc-iron alloy layers (Figure 2).

Communication among Design Engineer, Architect, Fabricator, & Galvanizer

Corrosion protection begins at the drawing board, and regardless of what protection system is specified, it must be factored into the product's design. Similarly, all corrosion protection systems require certain design details and proper planning to ensure the highest quality coating. For hot-dip galvanizing, a total immersion process in molten zinc, the design engineer will want to ensure all pieces are fabricated suitably for the process. Most design principles necessary for success throughout the galvanizing process are easily and readily followed, and in most cases, ensure maximum corrosion protection. Incorporating these design practices along with those listed in ASTM A 385 Practice for Providing High Quality Zinc Coatings (Hot-Dip), will not only produce optimum quality galvanized coatings, but also help reduce costs and improve turnaround times.

One key to providing the best design for the hot-dip galvanizing process is communication between the architect, engineer, fabricator and galvanizer. Opening the lines of communication early in the design process can eliminate potential costly pitfalls later in the process. A few discussion topics good to cover while the project is being designed include:

• Steel Chemistry and Surface Condition

• Size & Shape

• Process Temperature/Heat

• Venting & Drainage

• Welding

• Threaded Parts/Connections

• Post Galvanizing Design/Use

Understanding these aspects of the galvanizing process and how they can affect the coating and finished product's outcome will help ensure everyone's expectations are met.

Introduction - To Hot Dip Galvanizing

Hot Dip Galvanizing

The galvanizing process has existed for more than 250 years and has been a mainstay of North American industry since the 1890s. Galvanizing is used throughout various markets to provide steel with unmatched protection from the ravages of corrosion. A wide range of steel products ­from nails to highway guardrail to the Brooklyn Bridge's suspension wires to NASA's launch pad sound-suppression system - benefit from galvanizing's superior corrosion protection properties.

The uses of hot-dip galvanized steel continue to evolve, and new markets are emerging all the time. As with all materials and coatings, there are certain practices which yield better quality finished products. In order to meet the expectations and demands of many different markets, it is important to be cognizant of these best design practices for steel to be galvanized. Often no or only minor adjustments to the design are necessary, and worth the extra time and/or effort up front to alleviate certain future headaches related to the utilization of other coating systems.

Hubbell wins award for environmental energy project

Hubbell Galvanizing has won the top award for Electrical, Utility and Communications projects from the American Galvanizers Association for its work with Optiwind Inc. on a wind turbine project in Torrington, Conn.

The Klug Hill Farm windmill was designed to support the green movement, reduce cost and increase efficiency for Connecticut Light and Power. Optiwind designers wanted to create a sustainable, affordable, environmentally conscious way to generate electricity from wind for smaller markets.

The hot-dip galvanized steel that Hubbell provided throughout the structure helps the project achieve all of these goals. The structure is made entirely of galvanized steel, using infinitely recyclable zinc to protect it. The unbeatable corrosion protection means less energy or materials will be spent on routine maintenance.

Optiwind engineers briefly considered painting the structure, but realized the maintenance cost was too high. Hot-dip galvanized steel's cathodic protection is the perfect marriage of economic and environmental goals. We needed good communications to make all 100 tons of steel work. Many connection points required very tight tolerances, and we had to mask many of the movable parts. We made sure Optiwind's engineers knew what to expect from the hot-dip galvanizing process, and made modifications to ensure success.

This project breaks new ground, allowing small institutions to use wind generators for electricity. And hot-dip galvanized steel is an essential part of its success. The long-lasting, no-maintenance, and sustainable nature of galvanized steel is a crucial element of any affordable, environmentally friendly project.

We needed good communications to make all 100 tons of steel work. Many connection points required very tight tolerances, and we had to mask many of the movable parts. We made sure Optiwind's engineers knew what to expect from the hot-dip galvanizing process, and made modifications to ensure success.

This project breaks new ground, allowing small institutions to use wind generators for electricity. And hot-dip galvanized steel is an essential part of its success. The long-lasting, no-maintenance, and sustainable nature of galvanized steel is a crucial element of any affordable, environmentally friendly project