Surface Conditions

Very thick coatings may form on items that are large and consist of heavy steel sections. Longer immersion and handling times required to process these sections may result in the metallurgical properties of the steel having high reactivity with the zinc whilst in the galvanizing bath.

There are a number of factors to take in consideration when galvanizing large or thick steel sections that can have an effect on the surface condition of the end product.

• Steel Thickness
  When immersing ferrous articles into the zinc bath the material reacts with the molten zinc and metallurgical starts the bonding process. However thick steel articles require longer immersion times which in most cases will result in a thicker dull coating of zinc present over the steel surface.
• Larger Items
  Pending shape, size and thickness of any large articles and what other materials are fabricated with it, this will also have a similar effect to thick steel articles often producing dull thicker coatings and possible distortion issues due to immersion times.
• Metallurgical Properties
  Ferrous articles become very reactive in molten zinc that fall outside the recommend levels of phosphorous and silicone. This often attracts more zinc to the steels surface which can lead to a number of different surface conditions from delamination, striations, to very rough coatings.

Paint pigmentation from marking pens may be resistant to chemical cleaning. During pre-treatment processing, paint layers are removed, however, non-visible pigments may remain. The zinc coating will form around the residue pigmentation and remnants of workshop markings may remain evident after galvanizing. Oil-based paint markings should be removed by suitable means from steel surfaces before delivery for galvanizing.

In some applications, small mating or threaded areas may be required to be uncoated. This can be achieved by applying a small amount of a suitable adhesive or sealing product on the area, creating a barrier to pre-treatment acids and zinc. The following products will have varying success in preventing zinc coatings from forming and some clean-up of the surrounding surfaces will be required by the fabricator after galvanizing.

Weld-checks dyes can create issues if unable to be removed by pre-treatment chemicals. Dyes should be removed from steel surfaces with a suitable paint-removing solution or by light sanding before despatch for galvanizing.

Most pipe and tube sections manufactured in Australia are painted with water-based coatings, which can generally be removed within our process. Some local manufacturers of pipe and tube products and most offshore producers coat their product range with clear varnish or black bituminous paint. These coatings are resistant to chemical removal within our galvanizing plant and are required to be abrasive blasted before delivery. To avoid additional costs and extended processing times, hollow sections should be stipulated that they be suitable for hot-dip galvanizing when ordering from your steel supplier. All steel sections with powder-coated, brush or spray-paint coatings must be abrasive blasted before despatch to Hunter Galvanizing.

Before hot-dip galvanizing, all pre-existing zinc coatings must be stripped from all items including in-line produced lengths of hollow section, purlins, wire and mesh. Additional charges are necessary for this procedure and lead times may be increased. Some pipe and tube products have an external light coating of zinc and a painted internal surface. These items will require internal abrasive blasting in addition to acid stripping before delivery to our facility.

Flame, laser and plasma cutting will change the structure of the steel composition in the immediate area of the heat source. These areas may present thinner coating thicknesses and a lack of adhesion, reducing the ability of the zinc alloy layers to bond with the base metal.

The high temperatures required to cut material depletes the alloying elements in the surface of the steel. The formation of the coating structure relies on the alloying of iron and zinc in the galvanizing bath. Should insufficient alloying elements be present, the minimum coating thickness may not be able to be achieved; and cohesion of the galvanized coating will also be limited. To eliminate such issues, grind the heat-affected surface and bevel the cut edge.

Hunter Galvanizing uses proprietary printed tags to assist with traceability. Tags are produced and wired to individual items upon receipt. They withstand pre-treatment acids and galvanizing conditions, and remain on the item for identification at the customer’s location.

Hot-dip galvanizing is a long-lasting and cost-effective means of protecting steel from corrosion. When organic coatings such as paint or powder coatings are applied over hot-dip galvanized steel, the resulting combination is known as a duplex coating. To achieve a successful adhesion for the paint coating to bond to the galvanizing, the following procedures should be followed:

• A blast pressure of no more than 40 p.s.i or 275kpa
• Using an abrasive grade of 0.2 – 0.5mm clean ilmenite or garnet
• A sweeping blast to the surface of a 45degree angle
• Approximately 300-400mm away from the surface
• Using a Venturi type Nozzle

Not following the above procedure could result in damaging the galvanizing coating which is outside the control of the galvanizer

There are a number of factors that should be followed if you plan on powder coating you ferrous articles after galvanizing, failing to follow these could have impacts with the finished coating. These issues can range from a number of different defects such as pin holes, blistering, rough surfaces and not adhering to the surface.

• It’s good practice prior to dropping work off at Hunter Galvanizing to advise a staff member that your work will be powder coated after Galvanizing.
• Making a note of this on your Delivery Docket or Purchase Order will help identify the process changes.
• Any blemishes that may occur during the galvanizing process will not be repaired with a zinc enriched epoxy coating.
• Extra dressing may be required by the customer to smooth out any lumps, pimples or imperfections prior to taking it to the powder coaters, otherwise these will be easily seen through the powder coating finish.
• Please ensure the galvanized product is free from dust, oil, grease and is fully dry prior to taking to the powder coaters.
• Not passivating work prior to powder coating is a fallacy that has been discussed a number of times in the galvanizing industry. A good powder coater will understand this process and should preheat to degas the work prior to powder coating.