1. What is Hot-dip Galvanization?

Hot-dip galvanization is a method whereby freshly cleaned iron and steel is coated with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around 450°C. The first of these two steel alloys is harder than the base metal. This gives the hot-dip galvanizing its excellent abrasion resistance property. The outer layer, smooth and pure zinc, absorbs shocks; the underlying alloy layers offer effective protection for the base metal.

2. What is ‘cold galvanizing’?

Cold galvanizing is a term (not process) used to refer to zinc rich paint, suitable for repairing galvanized surfaces. Galvanizing is defined as a metallurgical reaction between zinc and iron that produces a zinc-steel bond of approximately 3,600 psi. There is no chemical reaction when applying zinc-rich paint and the bond’s strength is only a few hundred psi.

3. What is the approximate service life of galvanized parts?

The zinc corrosion rate and the duration of protection is determined by the quality of the coating and the number of corrosive elements present in the immediate environment. For example, in a rural environment where vehicle traffic is reduced, the virtual absence of corrosive emissions allows galvanizing to easily last for 100-150 years without maintenance. Conversely, in an industrial/marine environment where the presence of corrosive elements (chlorides, sulphides) is significant, galvanized steel can last 50-100 years.

4. What is ‘double-dipping’?

Double-dipping is a progressive immersion procedure for parts that are too large for a single dip in the kettle. This procedure cannot be used to obtain additional coating.

5. What are the steps in the galvanizing process?

1. Pre-inspection: Fabricated structural steel is viewed to ensure it has, if necessary, the proper venting and draining holes, bracing, and overall design characteristics necessary to yield a quality galvanized coating.
2. Cleaning: Steel is immersed in a caustic solution to remove organic material such as grease and dirt, followed by dipping in an acid bath (HCl or H₂SO₄) to remove mill scale and rust, and finally lowered into a bath of flux that promotes zinc & steel reaction and retards further oxidation of the steel.
3. Galvanizing: Clean steel is lowered into a kettle containing 450°C molten zinc where the steel and zinc metallurgically react to form three zinc-iron intermetallic layers and one pure zinc layer.
4. Final Inspection: The newly galvanized steel is visually inspected, followed up by measurement of coating thickness with a magnetic thickness gauge.

6. Will overloading the jig have any effect on galvanizing?

Yes, overloading the jigs could risk thermally “shocking” a small zinc bath. This means, the local temperature upon jig entry causes a high heat loss resulting in generation of fine dross. This issue can be overcome by reducing jig loading and increasing dips per hour.

7. What is the function of Nickel, Bismuth and Tin in galvanizing?

Nickel at 400-500ppm is essential to control the zinc coatings as it gives more even coatings, better astatic and reduces runs and spikes. Bismuth gives better fluidity and is a replacement to lead. Tin gives a brighter spangle.

8. What factors affect HDG coatings?

The factors that affect HDG coatings appearance and characteristics are:

• Size and shape of the item

Zn freezes at 420°C so there is very little superheat in the molten Zn as the item is withdrawn from the zinc bath. Thicker sections will retain heat longer and promote better drainage of the molten Zn from their surfaces than thinner sections. The angle with which the work can be withdrawn from the bath will influence the drainage characteristics. One-dimensional items that are short enough to dip vertically will produce smoother and more uniform coatings than 2- or 3-dimensional items where some surfaces will be at shallow withdrawal angles from the molten Zn.

• Steel chemistry

The rate at which molten zinc reacts with steel to form the galvanized coating is dependent on the steel chemistry. Pure iron has a very low reaction rate and for this reason, galvanizing kettles are manufactured from steel of this type. Structural grades of steel always contain alloying elements, the most common of which are carbon, manganese and silicon. Sulphur and phosphorous are residual elements arising from the raw material used to make the steel. Of these elements, silicon and phosphorous have the most significant effect on the galvanized coating’s characteristics, High silicon levels (>0.20%) may result in the galvanized coating have a duller or matt grey appearance, or a blotchy variable appearance. High phosphorous levels (not normally found in structural steels) can give rise to dark grey or browning coatings that are prone to delamination from the steel’s surface

• Steel surface condition

Rougher surfaces have a higher surface area per unit of surface and thus generate thicker galvanized coatings.

• Design of the item with respect to

The item needs to be suspended during galvanizing. The provision of lifting lugs to eliminate touch marks from wire or chains is an example of design improving quality. The item needs to have adequate venting and draining provisions to ensure the smooth flow of zinc into and out of the item. The dimensions of the item need to comfortably fi t within the bath dimensions to allow adequate withdrawal angles. Double dipping of the item will frequently produce a better quality than single dipping an item that can be barely submerged in the bath.

• Metallurgy of the galvanizing process

When the steel is immersed in the molten zinc bath, a number of reactions take place. These are:

1. The preflux is activated by the molten zinc and performs a final cleaning action on the steel surface. Oxidation products are produced by this reaction that contain a combination of zinc chloride and zinc oxide. These oxidation products are lighter than zinc and float to the surface. This is called zinc ash.
2. When the steel reaches galvanizing bath temperature, the zinc reacts with the steel to form a series of zinc-iron alloys. Some of these zinc-iron alloy crystals float off the surface of the steel and enter the zinc bath.
3. The zinc bath becomes saturated with iron in solution, either from work entering the bath or from the steel galvanizing kettle. This saturation level is typically about 250 parts of iron per million at galvanizing temperature.
4. Over time, these zinc-iron crystals coalesce and sink to the bottom of the zinc bath. This forms a mushy layer called ‘dross’. The dross is periodically removed by scooping it from the bath with special equipment. Zinc dross is crystalline and has a much higher melting point than zinc (650°C versus 420°C).
5. The molten zinc surface is constantly oxidising, and this oxide film is always present on the zinc surface to a greater or lesser degree.

9. What is passivation?

Passivation is the procedure of special treatment of zinc coating to prevent the occurrence of white rust in the primary phase.

10. How thick is the zinc coating at hot-dip galvanizing?

The thickness of zinc coating is in average between 50-100mm, which depends on the chemical composition of steel.

11. What are the advantages of hot-dip galvanizing?

• Economic efficiency
• Quality
• Long-term protection corrosion resistance
• Less maintenance
• Better resistance to mechanical stress
• Enables additional decorative coating with paint
• Very good corrosion resistance in different environments
• Is hot-dip galvanizing an environmentally sustainable surface treatment method?

Hot-dip galvanizing is probably the most environmentally friendly process available to counter corrosion. There are major economic benefits in the corrosion protection of buildings and structures, while effective corrosion protection is also a powerful way to reduce CO₂ load.

12. Can hot-dip galvanized steel be reused?

Yes, hot-dip galvanized products can be re-galvanized when the original zinc layer has corroded away.

13. Can hot-dip galvanized steel be recycled?

Yes, hot-dip galvanized steel can be recycled with other scraps when melted in an arc furnace. The zinc evaporates early in the process and is collected in the arc furnace filter, from which it is extracted and reused in zinc production.

14. What is the reason for incorporating venting and drainage holes into a project’s design?

The primary reason for vent holes is to allow otherwise trapped air and gases to escape; the primary reason for drain holes is to allow cleaning solutions and molten zinc metal to flow entirely into, over and throughout the part, and then back into the tank or kettle.

1. What are the causes of white stain/wet storage stain?

White/wet storage stains are generally white powder like substances that can be in the form of black/grey spots during initial development. It is a mixture of Zinc Oxide (ZnO) and Zinc Hydroxide (ZnOH) which are natural corrosion after galvanizing. It takes some time to form the zinc carbonate layer. Once this layer is formed then it is protected and the usual oxidation takes place according to the environment. It can happen due to exposure of moisture (e.g., rain), condensation and air flow over the surface therefore stacking and bundling should be done properly. If white stains appear, wipe it off immediate. If needed use non-metallic but a bristle brush or appropriate chemicals.

2. How to avoid white stains?

There are many ways to avoid white stains such as:

• Give space between articles
• Avoid leaving it in open especially during rain
• Avoid leaving on site on the soil especially if it is wet
• Avoid where chemicals are stored especially acid based

3. What is the cause of ungalvanized weld areas?

The weld areas may have some coating misses which could be due to welding shag being present on the welds. The fabricator is required to remove the welding slag before it is delivered to the galvanizer.

4. What is the cause of Mottled or dull grey coatings?

Thicker galvanized coatings will be generated by reactive steels which are dull when compared to standard coatings. Such coatings tend to have longer life due to their greater thickness. Moreover, their appearance is steel metallurgy’s function, which is beyond the galvanizer’s control.

5. What is the cause of dark stains present adjacent to welds?

Unsealed overlaps may witness preparation chemicals entering them or poor-quality welds simply boil from connection during the galvanizing process. This will only cause coating misses and surface contamination during galvanizing. Moreover, anhydrous fluxing salts remaining within the connection are likely to absorb atmospheric moisture, while leaching onto adjacent galvanized surface. Such salt leaching will each eventually the equilibrium. It is necessary to clean wash the affected areas for removing those slight corrosive leachate.

6. What is the cause of inclusions/dross pimples?

During galvanizing process, dross is generated as zinc iron crystals or in the form in higher melting point when compared to metal in zinc bath. Galvanized coating experiences trapped dross which gives out that gritty or rough appearance to the coating. Dross inclusion presence in coatings will not be found to be detrimental to performance of the coating, since zinc dross resistance is identical, similar to that of galvanized coating.

7. What is the cause of ash staining?

During the galvanizing process, there is formation of zinc ash, since it is in zinc that the work gets immersed. The ash that is generated gets skimmed off the molten zinc surface before withdrawal of the work from galvanizing bath. At times, ash can be found to be trapped within the inaccessible areas, while sticking to the external part of the coating, when the bath is excited by the work. A light brown or dull surface stain is left by ash after removal. However, the galvanized coating performance is not affected in any manner

8. What is the cause of white storage staining?

Once galvanizing is completed, the items stacked or stored in poorly ventilated, wet conditions will become white bulky zinc hydroxide deposits upon the galvanized coating surface due to atmospheric moisture reaction.

9. What is the cause of general surface irregularities and striations?

Lines thicker than adjacent galvanized coating and ridges may appear due to varying zinc reaction with steel surface. This is more due to the stress areas upon the steel surface or probably the welds presence or weld metal that is modified metallurgy into parent metal. It is regarded to be a common phenomenon faced with tube and pipe products. However, coating performance is not affected in any manner.

10. What is the cause of bare patches?

The uncoated areas of the galvanized work surface are because of inadequate pre-treatment in pickling, degreasing, pre-fluxing and poor surface preparation. It is necessary to get the areas repaired by making use of recommend repair procedure or re-galvanise the item, if defect is found of sufficient size.

11. What is the cause of puddling, drainage spikes and runs?

They are termed to be unavoidable defects in general item hot dip galvanizing process. They are acceptable unless the function assembly of the item is not affected or safety hazard present in service or handling.

12. What is the cause of rust staining?

Uncoated steel exposed to galvanized coatings will quicken corrosion of the coating and stain the coating brown where contact occurs. Wire brushing will need to be done to remove it.

13. What is the cause of delamination?

Very heavy galvanized coatings (more than 250 µm in thickness) may be fragile and delaminate from the surface upon impact and need more cautious handling in transport and erection. Thin, cold-rolled items having a very smooth surface finish and made from reactive steel may also cause coating delamination.

14. What is the cause of black spots?

Scattered black spotting is caused by residual galvanizing flux crystallizing on the surface of the work and is mostly because of flux contaminated rinse water or poor rinsing after galvanizing. This flaw is typically seen in galvanizing baths using the “wet” galvanizing process where the flux is on top of the molten zinc. Surplus aluminum in the galvanizing bath can also be the reason for this defect

15. What is the cause of spangled coatings?

Certain hot-dip galvanized coatings display a high level of “spangling” brought about by zinc crystal patterns on the surface. This is the case with galvanizing alloys created in specific smelting processes and these alloys are usually used for hot-dip galvanizing. The coating performance remains unaffected.