Feasibility of Progressive Dipping
How can I determine if it is feasible to progressively dip a customers fabrication at my plant?
It is often considered that progressive dipping can be performed successfully if at least half of the customers article can be placed into the kettle. However, this is sometimes an oversimplified approach to the problem. Instead, kettle capacity should be evaluated in combination with lifting operations at the plant and the effect of a large temperature variation on the article. Such an evaluation is useful when the galvanizer is unsure whether progressive dipping a particular item at the plant is feasible.
The first step in determining if an article can be progressively dipped successfully is to evaluate the maximum theoretical article size which can be accommodated by the internal kettle dimensions. After acquiring the internal dimensions of the kettle, determine whether the article requires progressive dipping due to excessive height or excessive length. The maximum theoretical size of an article to be progressive dipped for excessive height is:
2 x (Kettle Height Freeboard Overlap Length)
The maximum theoretical article length which can be progressively dipped for excess length is a function of the kettle length, kettle depth, and the overall diameter width of the item. In essence, this can nearly double the article length that can be coated.
However, to determine the practical maximum article dimensions for progressive dipping, the galvanizers judgment and experience should be exercised to allow room for handling/movement of the article in the bath and to not disturb any bottom dross. Additionally, the value for overlap length can be optimized where possible.
The maximum article size which can be progressively dipped may be further limited by the layout and lifting capabilities of the hot-dip galvanizing facility. Always confirm the article weight is within the safe working load limit of the available lifting equipment. Then, evaluate the required lifting orientation for galvanizing and determine if the available lift points are suitable. It may be possible for the customer to provide lift points optimized for each lifting orientation required to coat the entire article.
When progressive dipping articles of oversized length, avoid a lifting orientation which results in clashing of the article with nearby walls or equipment during transfer and immersion for each bath in the process. This check should be performed for both lifting orientations required to fully coat the article. Lastly, the portion of the article protruding from the bath should not block or interfere with simultaneous operations in the plant. When progressive dipping articles of oversized height, confirm the crane head height is suitable for lifting the article at a sufficient height above each bath with the planned rigging and lifting equipment to be used.
Uneven heating and cooling of the article are inevitable when progressive dipping since one end of the construction will always be in the molten zinc bath, while the other end is exposed to cooler air. Therefore, progressive dipping results in different expansion rates for the upper and lower part of the component, leading to thermally induced tensions in the component which may lead to distortion. The risk of distortion can be reduced by consulting with the customer to take into account length variations of the assembly and planning for thermal expansion conditions. Additionally, ensuring vent and drain holes are adequately sized will enable rapid immersion and withdrawal of the object from the bath. Bracing (permanent or temporary) can also be incorporated to provide stability during the thermal expansion and contraction cycle.
Especially for welded assemblies, confirm with the engineer/owner that the welds and the physically constrained/framed portions of the assembly are capable of handling the increased stresses from thermal expansion at the zinc bath temperature. Analyzing the conditions of the first dip is most critical as the temperature gradient will be greatest. The temperature differential is less severe for the second dip due to heat retained in the steel from the first immersion.
Progressively dipped pieces often have an overlap area that is visible on the piece. After bringing the first end through the galvanizing process, the coating should be cleaned up and ground down near the center. The second end is then chemically cleaned and galvanized, where overlapping ensures full coating coverage of the article. The overlap area will most likely appear darker and develop a thicker coating. The color and thickness of the overlap area are not within the control of the galvanizer. In order to manage customer expectations, communicate to the customer that an overlap line will occur, but the presence of this line does not affect the coating performance and is acceptable. If required, the excess coating thickness can be buffed or ground down even with the surrounding coating for aesthetic purposes or if the area will be an important connection point with other pieces.
© 2020 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.