|
Galvanizing - the act of coating a carbon steel tank with a continuous zinc coating inside and out in order to provide barrier and cathodic protection from corrosion.
The vessel is immersed in a bath of molten zinc at between 815-850 ºF (435-455 ºC).
The zinc metallurgically bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, commonly topped by a layer of impact-resistant pure zinc.
In addition to forming a physical barrier against corrosion, zinc, applied as a hot-dip galvanized coating, cathodically protects exposed steel. The zinc coating sacrifices itself slowly by galvanic action to protect the base steel. This sacrificial action continues as long as any zinc remains in the immediate area.
Zinc melts at 787 ºF (420 ºC), and boils at 1,665 ºF (907 ºC).
Before the tank is galvanized, a hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings. Dilute solutions of either hydrochloric or sulfuric acid remove surface rust and mill scale to provide a chemically clean metallic surface.
The tank is then immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the molten zinc bath. In the dry galvanizing process, the item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats on top of the molten zinc and the item passes through the flux immediately prior to galvanizing.
After the tank is withdrawn from the galvanizing bath, excess zinc is removed by draining and then it is air-cooled. Hanson Tank usually includes a handhole or manway in the vessel design for cleanout purposes. Extra tappings are also added if necessary to enable quick and complete draining. The threads in these and all the other threaded connections are heated and wire brushed clean before shipment.
Coating-thickness and surface-condition inspections complete the process.
Galvanizing adds about 10-15% to the overall weight of the basic vessel, on which the cost is based.
|
