HOT DIP GALVANIZING
©Copyright1999 Mario S Pennisi
Email: mariopen@coatfab.com and
Derek Jones

For those of you seeking galvanizing services in Australia, go to Yellow Pages Online (http://www.yellowpages.com.au/) and type in "galvanising & tinning" and your State for a list of providers.

INTODUCTION
The first use of hot dip galvanizing on steel was some work reported by the Frenchman P J Malouin in 1742. A French patent was issued to Sorel in 1837 and an English patent to H V Craufurd in the same year. Very little has changed in the process since that time. (See “The Origins of Galvanizing” in Corrosion Management, August 1995 page 3).

Zinc is very successful as a protective coating for steel because in most environments to which steel will be subjected, zinc will act as the anode; ie it will dissolve in preference to the steel. In simplistic terms, while there is zinc on the surface the steel will be protected from corrosion.

Hot dip galvanizing is one of a number of methods available to the surface finisher for applying a zinc coating to an item. Other techniques include electroplating, mechanical plating, and sherardising, painting with zinc-rich coatings and zinc spraying or metallising.

In the hot dip galvanizing process, a uniform coating of metallurgically bonded zinc-iron alloy layers and pure zinc is produced.

The life expectancy of zinc coatings is independent on the coating process – an equivalent coating of zinc will provide the same life expectancy regardless of the coating process. Hot dip galvanizing will provide over 80 microns of zinc coating, while zinc electroplates are normally less than 25 micron.

The hot dip galvanizing process is widely used in a number of applications, particularly constructional. Galvanizing is normally carried out to AS/NZS4680: Hot-dip galvanized (zinc) coatings on fabricated ferrous articles, AS/NZS4791: Hot-dip galvanized (zinc) coatings on ferrous open sections, applied by an in-line process, or AS/NZS4792: Hot-dip galvanized (zinc) coatings on ferrous hollow sections, applied by a continuous or a specialised process.

THE GALVANIZING PROCESS
Chemically clean items are galvanized by full immersion in molten zinc. The total coating thickness is automatically determined by the mass of the steel being galvanized and the composition of the steel, particularly silicon and phosphorus. In all other zinc coating processes there is no relationship between mass, composition and thickness of coating. In hot dip galvanizing the zinc covers corners; seals edges, seams and rivets; and penetrates some recesses to give a full zinc coating and thus protection to areas, which might be potential corrosion spots with other coating systems. The galvanized coating is slightly thicker at corners and narrow edges, giving greatly increased protection compared to organic coatings, which thin out in these critical areas. Complex shapes and open vessels may be galvanized inside and out in one operation. However, the process is not foolproof – bad welds may not be covered completely and burrs, etc should be removed before coating.

Virtually any article may be coated. Articles ranging in size from small fasteners to structures hundreds of metres high may be protected. Large galvanizing vats, together with modular design techniques of construction and double-end dipping allow almost any sized structure to be galvanized. Visual inspection of aged galvanized products shows that items are completely protected.

PREPARTION OF WORK FOR GALVANIZING
The first requirement is to remove oils and greases and old paint coatings. This is normally done in hot strong alkali solutions. If scale, rust and other surface contaminants are present abrasive blasting may be necessary, otherwise these contaminants are removed by acid cleaning or pickling in sulphuric or hydrochloric acids, followed by rinsing. Hydrochloric acid is preferred because it is more easily reclaimed. Iron and steel castings are usually abrasive blast cleaned followed by a brief acid dip. In some circumstances items may be cleaned electrolytically to remove foundry sand and surface carbon.

Hot rolled steel surfaces covered by heavy mill scale may require abrasive blast cleaning prior to acid cleaning.

FLUXING
Before a component can be offered to the hot zinc bath, all moisture must be dried from the surface and other cavities. The surface of acid cleaned steel is very active and will oxidise rapidly, more quickly than the surface can be dried. The metallurgical reactions between the molten zinc and the steel surface will not occur if oxides are present on the steel surface. To prevent this oxidation, the chemically clean, highly active steel surface is immersed in a flux solution, usually 30% zinc ammonium chloride with wetting agents, maintained at about 65°C. The flux solution removes any oxide film that might have formed on the highly reactive steel surface after acid cleaning, and prevents further oxidation before galvanizing. The work is then dried ready for galvanizing.

GALVANIZING
The galvanizing reaction takes place at between 445 and 465°C.

When the dried steel part is immersed in the galvanizing bath the steel surface is wetted by the molten zinc and reacts to form a series of zinc-iron alloy layers. The work remains in the bath until its temperature reaches that of the molten zinc, so that all the galvanizing reactions can go to completion. After the surface of the molten zinc is skimmed to remove the dross from the surface, the job is withdrawn from the bath at a controlled rate. When the surplus surface metal has drained off, the item is either air quenched or quenched in water containing potassium dichromate. The item carries with it an outer layer of molten zinc that solidifies to form the relatively pure outer zinc coating. This shiny pure zinc layer does not always form – in the presence of high levels of silicon in the steel the pure zinc layer does not form and the surface will be a dull grey colour.

The period of immersion in the galvanizing bath varies from several minutes for relatively light articles, up to half an hour or longer for massive structural members. This is one of the reasons that hot dip galvanizing is charged by the weight of the item being coated.

The resulting galvanized coating is tough and durable, normally comprising a surface of relatively pure zinc covering zinc-iron alloy layers bonded metallurgically to the underlying steel. This coating completely covers the article and provides excellent resistance to abrasion.

One of the advantages of the hot dip galvanizing process is that a visual inspection can show that the coating is continuous. Defects such as uncoated areas due to incorrect preparation, carry over of dross (dull grey rough deposits) can be found easily.

QUENCHING
The galvanized zinc surface is highly reactive. It will oxidise rapidly in moist air (a relative humidity above 65% is sufficient to cause the onset of corrosion) and white corrosion products form rapidly when two zinc surfaces are stacked on each other (storage corrosion). To delay this corrosion, galvanizers normally water quench items in a solution of potassium dichromate that applies a chromate film to the zinc surface.

The chromate film formed from this dichromate solution is not suitable for powder coating. If the item is to be powder coated the galvanizer should air quench the item after galvanizing. To prevent the generation of corrosion products before powder coating, it is essential that the powder coating be applied within a few hours of galvanizing. If this is not possible, then a light acid etch, followed with a zinc phosphate or chromate conversion coating is mandatory.

GALVANIZING DEFECTS
As the item is withdrawn from the galvanizing kettle, zinc drips off the item causing spikes, dags and a rough surface. Tilting the parts as they emerge from the bath reduces the incidence of these defects. Thus, any drips are found at the drip point. These spikes are removed prior to returning the parts to the customer. The lighter the section the quicker the zinc will freeze so that drips and dags are possible on thin sections.

When designing and fabricating parts, which are to be galvanized, it is important to follow a number of design strategies that will result in good draining and venting of closed sections. This action will allow the galvanizer to do a good job safely.

STORAGE OF GALVANIZED PARTS
Zinc coated parts will sweat in humid environments when stacked closely. The sweating will lead to corrosion and ugly white corrosion products will be produced. Beneath each of the locations of white powder, a pit will have been produced and the coating thickness at these locations will have been reduced so that corrosion of the steel will occur prematurely. Separate galvanized parts during storage so that air can circulate freely between the items. Use spacers to separate flat sheets. Preparing galvanized coatings for powder coating Galvanized coatings are a bright silvery grey colour. In these days of brilliant colours a bright silver finish no longer is sufficient in domestic applications. The coating of choice is a powder coating that eventually will be able to provide almost any colour that the mind can conceive. The powder coating is cathodic to the zinc and if there is an adhesion defect or there is a holiday in the coating, the underlying zinc will corrode under the powder coating film (under film corrosion). The corrosion products will ooze out through and over the powder coating giving an ugly salt like appearance to the surface. To reduce the possibility of this occurring it is essential to prepare the corrosion free zinc surface with a conversion coating, either zinc phosphate or chromate. A new Australian Standard AS/NZS4506: Metal finishing-Thermoset powder coatings has been issued for these types of coatings.

GALVANIZING FASTENERS AND SMALL COMPONENTS
Fasteners and small components are loaded into perforated cylindrical steel baskets. They are degreased, acid pickled, pre-fluxed and dried. The dry baskets containing the components are lowered into the galvanizing bath. At the end of the galvanizing treatment, the baskets of galvanized components are raised from the molten zinc and immediately placed into a centrifuge or spinner and rotated at high speed for 15 to 20 seconds. Excess zinc is thrown off. The resultant coating is smooth and uniform.

METALLURGY OF GALVANIZING

 

 

Galvanized layers and expected hardness

 

 

The molten zinc in the galvanizing kettle removes the flux so that the zinc wets the steel surface. Immediately, a metallurgical alloy is formed between the zinc and the steel – a thin molecular layer of brittle, hard high-iron zinc alloy (the gamma layer – 21-28% iron). Next a much thicker, hard, brittle alloy (the delta layer 7-12% iron) forms. On top of this the zeta layer forms containing about 6% iron and finally the surface layer is virtually pure zinc (the eta layer).

The silicon and phosphorous content of the steel affect the formation of these layers. If the silicon content is very high, the eta layer may not form and a dull galvanized surface will result. Each of these layers is metallurgically bonded to each other so that they cannot be separated from each other or from the steel as can a paint coating.

ABRASION RESISTANCE OF GALVANIZED COATINGS
Although the outer eta layer is soft and lacks abrasion resistance, the zeta and delta layers are harder and more abrasion resistant than the steel. The outer eta layer may be removed in service. The exposed harder, more abrasion resistant zeta and delta layers gives the galvanized coating outstanding abrasion resistance so that mechanical damage to galvanized coatings is minimised.

COATING THICKNESS
During the first minute of immersion in the galvanizing bath zinc-iron alloy layers grow rapidly on the surface of the steels, which are most commonly galvanized. The rate of alloy layer growth then diminishes and is finally very slow. When the work is withdrawn from the bath an outer layer of relatively pure zinc is also carried out. The total zinc coating mass applied depends mainly on the mass and thickness of the steel being galvanized.

As illustrated below, galvanized coatings are slightly thicker at corners and edges, an important advantage over most organic coatings, which thin out in these critical areas.

The structure of the galvanized coating and the relative thickness of its zinc-iron alloy layers have little or no effect on the protective life of the coating. Protective life depends basically on total coating mass.

ADVANTAGES OF GALVANIZING

  1. For most classes of steelwork galvanizing may provide the lowest long-term cost. In many cases galvanizing may also provide lowest initial cost.
  2. The galvanized coating becomes part of the steel surface it protects.
  3. The metallurgical structure of the galvanized coating provides good toughness and resistance to mechanical damage in transport, erection and service.
  4. The galvanized coating is subject to corrosion at a predicably slow rate, between one-seventeenth and one-eightieth that of steel, depending on the environment to which it is exposed.
  5. Zinc’s anodic protection for steel ensures that small areas of the base steel exposed through severe impacts or abrasion are protected from corrosion by the surrounding galvanized coating.
  6. An inherent advantage of the process is that a standard minimum coating thickness is applied to a given section and a particular steel composition.
  7. During galvanizing the work is completely immersed in molten zinc and the entire surface is coated, even recesses and returns, which often cannot be coated using other processes. If required, internal surfaces of vessels and containers can be coated simultaneously.
  8. Galvanized coatings are virtually self-inspecting because the reaction between the steel and molten zinc in the galvanizing bath will not occur unless the steel surface is chemically clean. Generally, a galvanized coating, which appears sound and continuous, is sound and continuous although there is evidence that defects can be hidden beneath an apparently sound coating.
  9. Galvanizing is a highly versatile process. Items ranging from small fasteners and threaded components, up to massive structural members can be coated.
  10. The mechanical properties of commonly galvanized steels are not significantly affected by galvanizing.
  11. Galvanizing provides outstanding corrosion performance in a wide range of environments.
  12. Duplex coatings of galvanizing-plus-paint are often the most economic solution to the problem of protecting steel in highly corrosive environments. Such systems provide a synergistic effect in which the life of the combined coatings exceeds the total life of the two coatings if they were used alone.

DISADVANTAGES OF GALVANIZING

  1. The location of galvanizing plants is limited. In Queensland, plants are currently located in Brisbane and Cairns.
  2. The limited number of plants requires that steelwork fabricated in other centres must be transported to the plant and then to the site adding to time and cost.
  3. The necessity to transport steelwork exposes it to damage in transit not only to the galvanizing, but also to the steelwork itself.
  4. The limited size of galvanizing tanks requires that steelwork must be designed in suitable sizes or in modules.
  5. Steelwork must be designed to permit safety to the work and to workers in the galvanizing plant.
  6. In-situ steelwork (eg existing structures) cannot be hot dip galvanized.

OTHER GALVANIZING PROCESSES
Ferrous open sections and ferrous hollow sections are being fabricated from pre-galvanized strip either hot dipped or electroplated