What is galvanized?

Hot dip galvanizing, or “galvanization”as it is commonly known, is a method of applying a coating of zinc on iron parts (carbon steel) by immersing in a bath of molten zinc. For good results the design of the parts must be appropriate and it is necessary that certain conditions are verified, such as:

Surface of the parts

Control surface condition of the parts to be galvanized.

Dimensions

The dimensions of the parts must settle in the crucible galvanization. In our case, we have the largest in the Southeast of Spain with 12.50 m. long x 1.70 m. wide x 2.70 m. deep with 400,000 kg of molten zinc.

The weight

The weight of the pieces is conditioned by the existence of lifting and transport devices in the galvanizing plant; we have the largest lifting capacity of the Southeast with possibility to galvanize parts up to 10.000 kg.

Suitable steels

Using suitable steels for galvanizing.

The fulfillment of these conditions depends largely on designers and manufacturers of the parts or metal constructions that will be galvanized.

The main stages of the process are illustrated in the following diagram:

 

First of all,inspection of material is carried out in order to classify it and get work scheduling.

DEGREASEDDegreased

Parts with any grease or oil are subjected to be degreased in an alkaline solution hot (70ºC); after degreasing parts, these are rinsed in a water bath to prevent the drag of this solution to the next stage.

PICKLINGPickling

Pickling is used to remove rust and calamine, which are the most common contaminants of ferrous products and besides a chemically clean steel surface can be obtained with this method. It is done with dilute hydrochloric acid at room temperature, for that we have five tanks of 70 m3 volume and capacity for parts up to 15 m. length. The pickling time depends on the degree of surface oxidation of the pieces and the concentration of the acid solution.

RINSINGRinsing

After degreased and pickled, parts are rinsed in a water bath to prevent skidding acid residues to the following process steps.

FLUXINGFluxing

The fluxing or treatment with salts (mixture of ammonium chloride and zinc chloride), aims to remove any remaining impurities and produce an intense cleaning of the metal surface. These salts act as “flux” welding, that is, they favour wettability of the Steel surfaces by molten zinc. These salts are applied by dipping the pieces in an aqueous solution thereof.

DRYINGDrying

The pieces treated in the above step are dried before their introduction into the zinc bath to avoid traces of aqueous solutions and preheating the parts to be treated, thus facilitating the alloying zinc-iron.

GALVANIZATIONGalvanization

Galvanization operation itself is performed by dipping the pieces in the crucible with molten zinc at a temperature betweem 440ºC and 460ºC. the quality of zinc used is 99.995% purity.

During immersion of the parts in the molten zinc, diffusion of zinc in the surface of the steel occurs, leading to the formation of different layers of zinc-iron alloys with different composition. When the pieces are removed from the galvanizing bath, theses alloys are coated with an outer layer of zinc similar to zinc bath composition.

The time which the parts should be immersed in the zinc bath, to obtain a correct galvanized coating depends, among other factors, on the composition of the steel, temperature of the zinc bath and thickness of steel parts. In any case, the pieces must be soaked in zinc until it reaches the bath temperature. Before removing the pieces from the galvanizing it is necessary to remove the thin layer of zinc oxide and traces of salt from the surface thereof so that these do not remain adhered to the surface and they can not produce superficial imperfections in the coating.

Cooling and inspection

Once out of galvanizing bath parts are allowed to cool air. Then, they are reviewed to remove burrs, sharp drops and superficial adhesion of ashes or residues of salt and finally they undergo inspection. Galvanized coatings on various items must meet a number of requirements on surface appearance, adherence and thickness, all of them are specified in national and international standards. National or international standard currently applicables is the UNE-EN ISO 1461. Finally, the pieces are weighed because this is the criterion normally used for billing.

Passivated (optional, after the process)

The parts can be passivated after galvanization if the costumer requires it. This additional treatment prevents stains due to wet storage (rest of white zinc oxide can affect aesthetics of the product but not its service life) in those parts that are transported in high humidity and poor ventilation conditions, that is, in maritime transport long distance or in the case of prolonged stacks outdoors.

Shotblasting-abrasive blasting (optional, pre-process)

Optionally, materials with paint, lacquers, varnishes, strong corrosion, cements, resins… which subsequently are going to be galvanized, or only cleaned for other uses, can be shotblasted to get a surface free of contaminants.

This operation consists in projecting abrasive steel particles on the material surface that needs be cleaned. They are propelled with compressed air at high pressure; for this operation we have a shotblasting booth 15 m long x 4.50 m. wide x 5 m. height with bridge crane of 5,000 kg.

Result

The end result is a versatile and durable coating that provides very effective protection againts corrosion in a wide variety of items which can be painted, welding, machining … whose cost- protection relation is unbeatable. The thickness of galvanized coatings in one of the essential criterion for the quality thereof. It is usually expressed in micrometers (microns), although it can also be done in g/m2 (coating mass per square meter of surface). In the UNE-EN ISO 1461 allowable minimum values on thickness galvanized coatings are specified based on the thickness of the base material.