Hot-dip galvanisation is a form of galvanisation. It is the process of coating iron, steel or aluminium with a thin zinc layer, by passing the metal through a molten bath of zinc at a temperature of around 860 °F (460 °C).
When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material
that stops further corrosion in many circumstances, protecting the steel below from the elements. Galvanized steel is widely used in applications where rust resistance is needed, and can be identified by the crystallization
patterning on the surface (often called a “spangle”).
Galvanized steel can be welded; however, one must exercise caution around the resulting zinc fumes. Galvanized steel is suitable for high-temperature applications of up to 392°F (200°C). The use of galvanized steel at temperatures
above this will result in peeling of the zinc at the intermetallic layer. Electro galvanized sheet steel is often used in automotive manufacturing to enhance the corrosion performance of exterior body panels; this is, however,
a completely different process.
ISO 10684 : Fasteners-- Hot-dip Galvanized coatings
ISO 1461 : Zink used for process
A mixture of zink oxide and varying quantities of metallic zink. The former is formed as a result of oxidation of clear zink on the bath surface and when the oxide is skimmed off, certain amount of Metallic zink gets entrapped
and removed along with it.
An intermetallic compound, which is a complex mixture of zink and iron, forms in the galvanizing bath as a result of the reaction of molten zink with iron or iron salts and settles down in at the bottom of the bath.
A chemical compound applied in the form of an aqueous solution and dried on to the work in the dry process or Spread as a molten blanket over the zinc bath in the wet process. The primary purpose of the flux is to help in Keeping
the surface of both work and molten zinc free from oxide at the time of reaction. In both the galvanizing Processes fluxing helps maintaining the surface of work free from oxides.
The undue attack of the underlying ferrous surface by the pickling solution after the removal of scale.
A white corrosion product, mainly containing zinc oxide and basic zinc carbonate, that accumulates on the Galvanized surface exposed to water film or moist atmosphere.
A substance added to pickling and prefluxing solution to facilitate wetting of the work surface.
HOT DIP GALVANIZING & PROCEDURE
1. Surface Preparation
3. Baking (eliminating hydrogen embrittlement)
Surface Preparation is the critical step in the application of any coating. In most instances where a coating fails before the end of its expected service life, It is because of incorrect or inadequate surface preparation. The
galvanizing process has its own built-in means of quality control because zink will not react with an unclean steel surface. Any failures or inadequacies in surface preparation will be Immediately apparent when the steel is
withdrawn from the zink bath because of the unclean areas remain Uncoated, and immediate corrective action can be taken. We take extra precautions while doing surface Preparations, so that the right amount of eco-friendly acids
or alkaly chemicals are used during surface Preparation. Also we use shot blasting for making the surface clean and ready for the next step.
1.1 Degreasing/Caustic Cleaning
A hot alkali solution, mild acidic bath, or biological cleaning bath removes organic contaminants such as dirt, paint marking, grease and oil from the metal surface. Epoxies, vinyl’s, asphalt or Welding slag, which cannot be removed
by degreasing, must be removed before galvanizing by grit-blasting, sand blasting, or other mechanical means.
1.1 Note :
Caustic Soda ( NaOH )
Concentrate 8 - 10 %
Temp :50 - 70 'C
A dilute solution of heated sulphuric acid or ambient hydrochloric acid removes mill scale and iron Oxides(rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using
abrasive cleaning or air blasting sand , metallic shot, or grit into the steel.
1.1 Note :
Hydrochloric Acid ( HCL )
Concentrate 8 - 15 %
Temp : Room
The final surface preparation step in the galvanizing process, a zink ammonium chloride solution, serves two purposes. It removes any remaining oxide and deposits a protective layer on the steel to prevent any further oxides from
forming on the surface prior to immersion in the molten zink.
During the true galvanizing step of the process, the material is completely immersed in a bath of molten zink. The Bath chemistry is specified by ASTM-B6, and requirements at least 98% pure zink maintained at 815-850 F. While immersed
in the kettle, the zink reacts with the iron in the steel to form a series ofmetallurgical bonded zink-iron intermetallic alloy layers, commonly topped bya layer of impact-resistant pure zink Once the fabricated items coating
growth is complete, its is with drawn slowly from the galvanizing bath, and the excess zink is removed by draining, vibrating and or centrifuging. The metallurgical reaction will continue after the materials are with drawn
from the bath, as long asit remains near bath temperature. Galvanized articles are cooled either by immersion in a passivation solution or water or by being left in open air.
Galvanising time : 5-10 min
Temprature : 445 degrees C to 465 degrees C
3. SURFACE & PROTECTION OF HYDROGEN EMBRITTLEMENT(Baking)
The potentially deleterious effects of hydrogen absorbed during surface preparation or HDG can usually be eliminated by " BAKING " the fasteners after processing. Baking is a low temp. heat treatment that has been shown
to either extract hydrogen by effusion or cause it to migrate to trap sites, thus making it immobile
The key factors that influence baking effectiveness are : (i) temperature, (ii) time. In addition of increasing the rate o f heating has also been shown to improve the effectiveness of baking.
Because of the high procedural risks, extreme care has to be taken when preparing, carrying out and monitoring the hot-dip galvanising process of high-strength fasteners. Hot-dip galvanising is not admissible for property classes
12 and 12.9 .
Only baths where the concentration of free hydrochloric acid is constantly monitored may be used for pickling. The HCL concentration in these baths should be between 15% (freshly prepared) and 8%. When a bath reaches the lower
concentration limit it has to be discarded. Regeneration by adding acid is not admissible. Only pickle liquor with an added inhibitor may be used. The inhibitor must be dosed according to the manufacturer's instructions for
the freshly prepared bath. Subsequent addition of inhibitor to the pickle liquor during its period of use is not admissible, testing of the effectiveness of the inhibit- ed pickle liquor can be carried out e.g. by means of
"Circlip tensioning tests" in the context of the manufacturer's in-house production quality control. This eliminates the restrictions on regenerating the pickle liquor by adding acid and corrosion inhibitor additives, The maximum
admissible pickling time without subsequent tempering is 15 minutes. If the maxi-mum pickling time (
< 30 minutes) is exceeded, the fasteners have to be tempered at220°C before galvanising. The holding time is 2 hours for fasteners that have been completely heated.
All tempered fasteners must be pickled again for
< 1 minute immediately before galvanising in order to activate the surface. High-temperature galvanising (530–560°C) is admissible for dimensions up to M24. From M27 on, only normal temperature galvanising (445–470°C) may be used.
Temperature : 200 - 210'C
Surface protection – Inorganic
Time : 4 hrs
ISO 10683 applies with regard to inorganic surface protection. Fasteners in property classes 10 and 10.9 and may not be pickled in acid during pre-treatment.
In the inspection of Hot -dip Galvanized steel is simple and quick. The two properties of the Hot-dip Galvanised coating most closely scrutinized are coating thickness and appearance/surface condition A variety
of sample physical tests can be performed to determine thickness, uniformity, adherence and appearance.
Products are galvanized according to long established, accepted and approved standards of the international Standards Organisation (ISO). These standards cover everything from the minimum coating thickness required
for various categories of galvanized items to the composition of the zink metal used in the process.
Coating Thickness : as per ISO 1461:2009
Appearance : According to 10684
Visual : According to 10684
Photographs of the sample parts, before inspection must be included. The photograph or image Caption must clearly include the part reference, measured values and exposure conditions.
The technical staff member and the laboratory manager must sign and certify the results presented in the report at the end of the document