Design for Galvanizing

When designing a structure which is to be hot dip galvanized, it must be borne in mind that articles are immersed into and withdrawn from a bath of molten zinc at a temperature of about 450 degrees C. Design and fabrication is required that conforms to acceptable standards, but in the case for galvanizing a few extra provisions apply. These relate to allowing zinc to flow freely over and into every part of the fabrication and allowing the trapped air to escape, including from inside hollow items. This ensures the same level of corrosion protection inside hollow items as the outside. Without these provisions it can be impossible to process some items.

Some items that are not hollow require smilar provisions. Where air could be trapped under a portion of steel, or where liquid zinc could be trapped inside, allowance must be made to "vent" and "drain" adequately.

 

Venting and Draining

Galvanizing is the only corrosion protection system offering the same level of protection inside hollow items as outside. But to do that, it is necessary to allow the various liquids, including molten zinc, to reach and alloy with every internal part.

The term "hollow item" isn‘t always obvious. Tanks are readily recognisable as possibly a problem, but less obvious are tubular handrails, box section beams, square section posts, and some plate fabrications that are able to contain air or liquids.

Immersing such fabrications into zinc requires that all air can be expelled and the molten zinc can flow inside. Molten zinc is viscous, so it‘s like trying to fill a hollow section with liquid honey. The best surface quality is obtained where the immersion can be quick and clean. If the item floats on the surface while the zinc enters slowly through a tiny hole, a rough surface with poor quality results.

Incorrect Venting Correct Venting
 
 

Distortion

Distortion in Galvanized fabrications is an issue that can cause concern, but understanding the issue can often be a way to avoid it, and in most cases it is possible to design structures to avoid distortion.

There are two main classes of distortion in galvanizing: ‘Thermal expansion distortion‘ and ‘stress relief distortion‘.

Thermal Expansion Disortion

Galvanizing is conducted at 450 degrees C. Zinc melts at about 419C and the coating process is best done about 30 degrees above melting point. Steel heated to 450C expands about 4mm per metre at the galvanizing temperature. When the steel cools, it then contracts by this same amount.

If two pieces of steel are joined along their length, and they heat and cool at different rates, then that can change their respective length during heating and cooling.

Bolted connections between items, avoiding large variations in thickness in joined components, and use of symetrical designs are all ways to avoid distortion. One easy technique often overlooked is to involve the galvanizer at design stage. Having seen many examples of designs that work, and many that don‘t we can offer experienced advice on this issue. There are so many variations of designs that only generic examples can be given here.

Stress Relief Distortion.

This is sometimes a case of distortion "built in". Stresses introduced into a design during fabrication sometimes relieve themselves at the galvanizing temperature. Full stress relief doesn‘t occur until somewhere over 700 degrees, but at 450 some unpredictable, partial stress relief can occur.

Stresses can be introduced inadvertently during fabrication. A simple example is the use of a G cramp or similar to pull the pieces together during welding. This stress isn‘t obvious on the cold steel. But at 450 it might partially relieve itself.

 

Venting Gussets

Gussets are often used to add strength to beams, truss elements, rafters etc. During galvanizing these can trap air or zinc, preventing a good surface finish. Introducing crops or "mouse holes" can help eliminate potential problems.

The techniques below are some of the many ways of eliminating galvanizing problems while not compromising the structural integrity of the piece. If added at design stage they are easily fabricated in. If left until after welding, they are added at far greater cost. If not added at all, then quality suffers, and finish is compromised.

Design for Venting Gussets
 

Moving Parts

Moving parts and galvanizing don‘t always go together well, but there are ways to manage the issues. Such moving parts as gate drop bolts, trailer pins, gate hinges and so on, are examples that have been very succesfully galvanized for many years. The secret is in allowing enough clearance for the thickness of the coating, remembering that there are four layers of coating to consider across the diameter of the hole and the shaft.

A good guide for the tolerance is to allow 2mm radial clearance for shafts up to 30mm diameter, and 2.5mm for larger shafts.

Design for Moving Parts
 

Handrails

Handrails usually require a good smooth finish, and in galvanizing that‘s only possible with good design and fabrication. The Galvanizer cannot make up for inadequate venting and draining design.

Where handrails, (and possibly their posts), are made from hollow sections, adequate provision for allowing air to escape, and zinc to flow both into and out of the interior of the tube, RHS etc, is essential.

The best orientation to dip a handrail panel is at about 30-40 degrees from horizontal. Ideally every component member would be vertical, but where sections are at 90 degrees to each other, some compromise is called for. 30 degrees usually suffices. But the vents and drains need to be positioned to suit this processing angle.

Design for Handrails

The diagram shows a typical tubular handrail section with the holes for vent (topside) and drain (bottomside), in appropriate positions. Note that the holes need to be big enough (see "Hole Sizes"), and positioned as close to the butt weld as possible. Leaving holes well back from the weld means leaving air trapped inside, preventing speedy immersion, and afterward leaving extra zinc inside, (adding to the weight, and so to the cost).

Every section of tube in such a panel needs a vent and a drain. They are far easier to cut or drill a hole before welding, and are more properly positioned. The Galvanizer can drill holes after fabrication, but the position is not ideal, access is difficult, and the cost reflects that.

Further Designs for Handrails

Where holes would be considered unsightly or undesirable, internal venting is possible. There still needs to be a way for air to escape the whole fabrication, but intersections between tube sections can be vented and drained this way, provided the holes are big enough. Internal vents require twice the diameter of external vent holes.

 

Baseplates

Baseplates on stanchions, columns, handrail posts and similar uprights require some thought in design before Galvanizing.

In the same way that some other sections require venting and draining, baseplates can have similar issues. Even where the upright is not a hollow section (could be UB, UC, PFC or RSJ), a baseplate can still cause air to be trapped underneath, or zinc to be trapped in a pocket above. This is largely due to the need to dip the length of the upright at an angle, probably about 30 degrees from horizontal. At this angle the baseplate becomes a stopper on the end of a channel, or between the flanges of a beam section.

Designs for Baseplates

Where the upright is a hollow section, a drain hole to drain the zinc from the inside is required. Common practice is to put this in the centre of the baseplate, but this is a bad place to position it. As shown above, the hole is best placed right to one side of the intersection between baspelate and tubular. In the case of non-hollow sections, place the holes as shown. For hole size recommendations, see "Hole sizes".

Baseplate Types

Types of Baseplates
 

Mesh Infills

Mesh infill in panels can give rise to problems with galvanizing, but they are largely possible to design out.

Being so much thinner than the surrounding material, the wire of the mesh heats up and cools off much faster than the frame holding it. The result is that, on heating up, the mesh expands before the frame and, on cooling, it contracts before the frame. This means the mesh becomes loose on heating, and overtight on cooling. Where both ends of any individual wire in the mesh is secured to the frame, the the tension created is easily enough to snap the wire or break the weld. If nothing else, the mesh will be left distorted, being bowed out in places.

 

Tanks

Galvanizing tanks can be impossible unless careful design and fabrication precedes the galvanizing.

A tank is designed to contain liquids without leakage, but the galvanizing process requires items not to retain liquids. It is necessary to sink the tank in molten zinc, venting all the air out, then to withdraw the tank draining all the zinc out, and all at a rate that allows for a reasonable surface finish.

Tanks for liquids usually require an inspection hatch, an inlet and an outlet, and sometimes a vent, or instrument inlets. These orifices can all double up as vent and drain holes if placed in appropriate positions. Make these orifices as big as possible (fitting reducers afterward if required) to faciltate draining during galvanizing. Wherever the holes, it must be possible to hang the tank from a crane, lower it into the zinc and it must sink, with all the air coming out, and quickly. When the tank is lifted out, the zinc must drain completely.

If its possible, leave the tank completely open on one side, with perhaps a flanged lid to close afterward. If the tank walls are sheeted, and not reinforced, consider adding stiffeners (as shown above) to minimise ripple distortion in the sides.

Tank Design
 

Threaded Components

Galvanizing threaded components can be problematic. The issues fall into two categories:

  • Galvanizing a thread for protecting it, where it is intended that it be galvanized.
  • Not galvanizing a thread, where the coating would be a problem, but the item that the thread is attached to is to be galvanized. (in other words masking a thread).

Galvanizing threads for corrosion protection requires the thread to have the surplus zinc removed, othewise the thread gets filled with zinc, and becomes useless. Its possible to use a die nut, or a tap to rethread the component afterwards, but thats not always practical.

Spin galvanizing is often the solution. This specialist technique, available from us at our Elgin plant, is suitable for external threads from about 20 diameter up to 120mm dia, and lengths from about 50mm to 500mm. Studbolts, hex bolts and the like are coated, but allowance is required for the new dimensions. This is usually by way of overcutting the nuts to fit.

Masking threads to prevent galvanizing.

This is perhaps one of the most asked questions, but the one with the most disappointing answer!

In our experience, there is no effective masking material for preventing galvanizing. It requires a material that is impervious to alkalis, acids, heat (450C) and no one material has proved reliable. So what can you do?

  • Leave room to re-cut the external threads with a die-nut after galvanizing.
  • Arrange that male threads are fitted after galvanizing.
  • Put some nuts on the thread before galv and remove them afterwards. (only suitable for threads of about 15mm dia and over - smaller ones often break the bolt/stud when breaking the zinc that "welds" the two in galvanizing.)
  • For internal (female) threads, place a hex head bolt in the hole, to be removed after galvanizing. (again only for >15mm)
 

Acid Weep

This name is given to a phenomenon that occurs when welds are not closed properly, and some other circumstances.

Where a cavit exists between two welded pieces of steel, with a hole to atmosphere, the cleaning chemicals involved in galvanizing manage to get inside. But when the steel is immersed in molten zinc (450C), the moisture is driven off as steam, leaving the dry residue behind. This has reacted with steel such that the residue is iron hydroxide, a brown powdery material that looks like rust. During the next shower of rain (and galv is supposed to be good for out doors!), the iron hydroxide forms a slurry which oozes out looking like rust weeping from the hole. The hole might be a pinhole in a weld, or an open seam between "hit and miss" welding.

Fortunately the stain is on the surface of the zinc coating, and only looks unsightly. It can be removed with a stiff brush and soapy water. It only occurs in the first shower or two, and doesn‘t keep happening for ever!

 

Castings

Fabrications that include castings are becoming increasingly common, particularly in decorative work. Their use occasionally raises issues, normally to do with different rates of expansion and contraction, so careful design needs to be considered. In additon, it is essential that the casting is steel, not cast iron. Welding cast iron to steel is difficult, but possible. Galvanizing will almost certainly crack such a weld, with the casting coming loose, and falling off into the zinc tank, never to be recovered!

 

Weld Slag

While most fabricators use mig welding systems, it is sometimes easier, or appropriate to use stick welding. Stick welding, with its flux coated welding rods, leaves a residue of slag behind on the weld. (It was this slag that prevented air oxidising the molten metal, so ensuring a good weld.)

Normal welding practice is to chip off this slag with a chipping hammer. This not only leaves a tidier looking weld, but allows visual examination of the weld to ensure it is a good one.

Sometimes some slag is not removed. In galvanizing, none of the process removes weld slag, so it stays there, looking untidy, and in most cases leaving a patch of bare, ungalvanized steel underneath. It eventually falls off, with the bare patch the only sign that someone forgot to chip the slag.

 

Old Steel

Steel manufactured before about 1945 is sometimes unsuitable for galvanizing. The older the steel, the less likely it is to be suitable. Steelmaking technology has come a long way in the last 60 years, and the type of steel fault seen in steel of that day is not found now.

As galvanizing is seen as the best corrosion protection, people often think to refurbish old gates, fences and other structures and have them galvanized. But this can be a problem. Laminations in the steel, dowel joints, mortice and tenon joints, riveted joints all can loosen and even break in galvanizing. Laminated steel can allow zinc to enter the laminations, expanding the gaps and leaving a layered effect like lasagne!

Here‘s examples of very old steel that should not be galvanized.

 

Hole Sizes

The size of vent and drain holes depends on the volume of the section being drained. The larger the volume the larger the hole area is required. This can be by a single large diameter hole, or by several smaller diameter holes. Remember that it‘s not water that is being drained out, it‘s molten zinc, the consistency of liquid honey. The larger the hole, generally the better the surface finish will be. If it‘s not necessary to close the end of a hollow section, consider leaving it totally open for the best result of all. Where a hollow section is long, the hole size needs to be increased. The table below is for 2m sections. Double these holes sizes (or number) for double the length, etc.

Diameter of section Minimum Hole size mm
<25mm 10
25-50mm 12
50-100mm 18
100-150mm 20
>150 25
 

Fabricating from Galvanized Steel

It‘s always tempting to use up offcuts of steel in the yard when fabricating a new piece. After all that‘s economical isn‘t it?

No! It‘s actually more expensive.

Using painted (see "painted") or already galvanized steel presents an additional problem to the galvanizer.

These cause delays and incur extra costs.

A fabrication with some galvanized and some bare steel in the same piece requires the old galv to be stripped off before the process can begin. It‘s an extra process that can take as long as a day, and can double the costs.

 

Oily Steelwork

Steelwork can get oily very easily in a fabrication shop, and that can create problems for the galvanizer. As the steel is to be cleaned in hydrochloric acid (which is a water based acid), oily steel will not "wet", until the oil is removed.

The degreaser that most galvanizers have as the first chemical stage of processing will remove light oil, but heavy oil and grease requires costly manual removal.

Oil sources include:

  • Drilling lubricant, such as in the image above.
  • Mig welding anti-spatter spray
  • Bandsaw lubricant
  • General workshop litter

For Mig welding anti-spatter spray, the hardest to remove is any silicone based spray. Some sprays are designed for use on steel which is to be galvanized. Using them saves money.

 

Painted Steel

Painted items cause problems in galvanizing, but the reason and the solution isn‘t always clear to fabricators.

Most of the time involved in galvanizing is taken in cleaning the steel fabrications to be coated. Of the 3-8 hours that can be involved in processing any one item, all but about 30 minutes are cleaning.

Cleaning mainly involves removing rust and millscale. These are oxides of iron, and are removed chemically by immersion in acid, usually hydrochloric acid. This acid is a water based solution, and it will not penetrate grease, oil or paint. As its difficult to fabricate without some oily desposit ending up on the steel, a degreaser is used prior to the acid. But this degreaser doesn‘t remove paints.

If the steel does not have all the oxides removed, then that uncleaned part will not galvanize, and leave a bare patch in the coating.

The coating isn‘t just a layer of zinc on the surface, galvanizing is an alloying reaction with the steel, and if its not chemically clean, no reaction takes place. A dirty piece of steel immersed in zinc gets hot, (450C) but not coated!

If the paint is not removed, the oxide underneath doesn‘t get cleaned, and while the paint will burn off in the hot zinc, the steel underneath gets no coating. So afterward someone will ask "Why is that bit not galvanized?". The culprit paint has by then eliminated its evidence, in being burned off.

What constitutes "painted" isn‘t always obvious! Pipe and pipe elbows are good examples of material that often is supplied by stockists with a coating. Elbows often come with black paint, and pipe often with clear shellac like coating. It must all come off before galvanizing, and the most common method is shotblasting. The cost of this extra work often exceeds the cost of the galvanizing, but is mostly avoidable.

 

Seam Welds

Hollow sections (CHS, RHS, SHS etc) are typically made in a pipe rolling mill from flat and seam welded to form the section. Depending on the quality of the mill, the seam can be raised or scored.

After galvanizing, this raised seam can be very noticeable, and even could give rise to complaint, depending on the circumstances. If the seam is on the top of a handrail, the roughness would be unacceptable. But the fault is fabricated in! The galvanizer cannot do anything about this, can‘t rotate the seam to the underside, can‘t choose another batch of steel, can‘t prevent it.

Whats the solution?

  • Put seams to the underside of handrails or other critical faces.
  • Examine the pipe on receipt from your stockist and reject raised seams.
  • Grind the seam off before sending to the galvanizer