Masking and Prevention of Hot Dip Galvanized Coating

When galvanizing a steel article, it may be desirable to prevent a coating from forming in certain areas which will have special functions such as:

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• A threaded component on a fabrication
• Drilled holes
• Surfaces which will be subject to welding after galvanizing

There are several different methods that may be used to prevent a coating from forming on the steelwork, the preferred option being dependent upon the nature of the area which requires masking off.

High Temperature Masking Tape

Articles such as threaded bars may be protected by tightly wrapping a high temperature masking tape (typically silicone polyimide based) around relevant areas so as to prevent the ingress of molten zinc (see image below). Such tapes will withstand the galvanizing bath temperature of 450°C although charring or a carbon residue might be produced with some products, which subsequently can be easily removed by, for example wire brushing.

Masking with fabric tape prevents a galvanized coating

Anti-Galvanizing Solutions

On larger flat surfaces, the use of an anti-galvanizing solution may be used to prevent the formation of a coating. Some charring may occur resulting in a residue but this can be easily removed by wire brushing. In order to the anti-galvanizing solution to work effectively it is important that a sufficiently thick coating is achieved and the manufacturer`s or supplier`s instructions should therefore be closely followed. It is also important to ensure the product used in resistant to attack by acid as it will normally be applied prior to chemical cleaning in hydrochloric acid.

Masking Grease

Both flat or threaded areas may be masked off by using a grease which will act as a barrier between the steel surface and molten zinc, so preventing a coating from forming. However where such products are used, there is an increased risk of the grease contaminating the steel surface away from the area that requires masking so resulting in an uncoated area.

After galvanizing, the fabric tape residue must be removed by means of a wire brush

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Other methods

For threaded females holes the processes outlined above will not be efficient and instead a suitably sized bolt may be inserted into the hole prior the galvanizing. This should prevent zinc clogging the thread but the bolt will become `welded` into position by formation of the coating on the surrounding surface. As a result, after galvanizing, the area around the bolt needs to be gently heated with a blow torch so as to soften the coating and allow the bolt to be removed.

Use of an anti-galvanizing paint solution as a masking system

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Best practice also recommends studs and bolted assemblies be sent to the galvanizer disassembled.  Masking to prevent galvanizing threads on pipe or fittings is very difficult. The recommended practice is to clean and tap after galvanizing. Anchoring devices (such as threaded rods and anchor bolts)  sometimes are specified to be galvanized in the threaded areas only or in the areas to be exposed above ground. This can be more expensive than galvanizing the complete unit because of the additional handling required. Complete galvanizing can be specified for items to be anchored in concrete.

Manufacturers of threaded parts recognize special procedures must be followed in their plants when certain items are to be galvanized. Following are some examples:

• Low carbon bars are recommended since high carbon or high silicon cause a heavier, rougher galvanized coating on the threads.
• Hot-formed heading or bending requires cleaning at the manufacturing plant to remove scale before threading. Otherwise, over-pickling of threads will result during scale removal.
• Sharp manufacturing tools are mandatory. Ragged and torn threads open up in the pickling and galvanizing processes. Worn tools also increase bolt diameters. Frequent checking is necessary on long runs.
• Standard sized threads are cut on the bolt, while standard sized nuts are retapped oversize after galvanizing.

The table below shows the recommended overtapping for nuts and interior threads as detailed in ASTM A563, Specification for Carbon and Alloy Steel Nuts. On threads over 1.5 inches (38 mm) it is often more practical, if design strength allows, to have the male thread cut 0.031&#; (0.8 mm) undersize before galvanizing so a standard tap can be used on the nut.

Overtapping Guidelines for Nuts and Interior Threads

Design considerations courtesy of American Galvanizers Association. The AGA also has the publication, The Design of Products to be Hot-Dip Galvanized After Fabrication, available for download. 

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