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American Institute of Marine Underwriters |
Commodity Search Form
GALVANIZED IRON SHEETS
- Description: The galvanizing process varies from factory to factory and country to country, and in some cases, if the galvanizing is poor and the metal thinly coated, flaking takes place, leaving the metal bare beneath; this may give rise to rust. If the galvanizing process has been properly carried out, any corrosion is usually limited to flaking and powdering and causes little, if any, damage. Generally speaking, white rust, so frequently found on galvanized iron sheets, wire, etc., is a basic zinc carbonate and zinc oxide which is common to damage caused by seawater, fresh water, atmospheric oxidation, etc. Care must be exercised in attributing a cause of white rust and it is preferable simply to report the analytical findings. In some countries, as a protection against rusting the sheets are given a coating of oil. Galvanized steel coils are typically packaged with an inner wrapping of water-resistant paper and an outer galvanized steel sacrificial wrapper. The ends and cores of the coils are often provided with heavier gauge galvanized sheets, to resist handling damage from forklift blades. The coils are typically strapped together with a series of equally spaced metal bands, running longitudinally and transversely about the coils. Galvanized steel may be hot dipped or cold dipped. The hot dipped galvanized coils are more susceptible to rust damage than cold dipped coils. On exposure to moist air, the surface becomes tarnished owing to it becoming covered with a film of oxide of zinc. This film of oxide passes gradually into a basic carbonate of zinc by absorption from the air of carbonic acid, which protects the zinc from further action. It is evident that contact with fresh water will result in the production of oxide of zinc. Whereas the film of oxide of zinc thus produced is converted into basic carbonate when exposed to the air, affording protection to the zinc coating, experience has shown that when the sheets are in close contact with one another, with water between them, the oxide formed does not so readily become converted into basic carbonate, with the result that further quantities of oxide of zinc are formed on the surfaces in contact. forming in many cases a fairly thick crust of oxide of zinc. The acid used in the galvanizing process is sometimes not altogether eliminated. This defect may, if the sheets are packed closely together, cause a soft powder to form which is liable to cause crusting in the form of corrosion and when this occurs it may be thought that the damage has been brought about by salt water. With the layer of zinc covering the metal completely or partly gone, the iron has a value at the current price of iron not galvanized. Damage by seawater can only be satisfactorily determined by means of a laboratory analysis on account of the fact that in addition to the ordinary chlorides which may be found in the rust deposit seawater contains salts of magnesium, calcium, sodium, potassium, sulphates, bromides, and these are present in certain well defined proportions. The analyst should, therefore, find these salts present if damage is due to contact with seawater. If, after damage has been set up by contact with sea salts, the goods are subsequently exposed to rain, the salts present in seawater might well be washed away, thus destroying the original evidence of salt water contact.
- Index: 336
- Commodity Name: GALVANIZED IRON SHEETS