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American Institute of Marine Underwriters |
Commodity Search Form
GALVANIZED IRON SHEETS
- Description: Damage by fresh water and atmospheric oxidation - rain water is commonly given as the cause of fresh water damage, but the composition of white rust on galvanized iron, whether caused by rain water or by atmospheric oxidation, is the same, and distinction between them by chemical means is not generally possible. Where, however, fresh water finds its way between galvanized sheets a definite line of incrustation is usually formed, possibly through out the length of the sheets, and the crust is thicker than normally may be expected from atmospheric action alone. Damage by improper processing, drying, etc - If hydrochloric acid or ammonium chloride are not properly removed after processing, corrosion is likely to be accelerated. The stacking of the sheets while still warm may result in moisture condensation, particularly in a cool climate, and any damage from this cause would not be distinguishable upon analysis from that caused by fresh water and atmospheric oxidation, etc. If, while still warm, sheets are packed and strapped ready for shipment under conditions where moisture is present in the atmosphere or on the sheets, such moisture is trapped between the sheets. This would result in condensation on the sheets when they cool, causing an electrolytic action to commence almost immediately, resulting finally in the formation of white rust by the time the sheets arrive at their destination. It may be accepted that no analysis can distinguish between damage due to improper processing or drying and fresh water or atmospheric oxidation. Similarly, no analysis will disclose whether the presence of seawater salts has been brought about by contact with actual seawater or whether the salts have become deposited in the rust as a result of their being present in the atmosphere. It is possible for these salts to become present in the atmosphere as a result of spray, either on the ocean voyage or at certain ports at certain times of the year, especially in the Tropics and the Far East. The process for testing for the cause of damage to galvanized iron sheets should be undertaken with considerable care. Stained portions of the sheets should be treated with boiling distilled water and a solution obtained, or a solution similarly obtained from the scrapings of the white rust found present. This solution will normally contain chlorides and sulphates from the galvanizing process, but if, in addition, there are found salts of magnesium, calcium, potassium, etc., then the rust may be due to contact with seawater or from a salt-laden atmosphere. The object of the analysis is to determine whether these salts are present, but the analyst is not normally in a position to certify the actual cause of the damage, namely, damage due to contact with seawater, fresh water, etc., and the analyst should issue a report confined to a statement of what the analysis actually discloses. When salts present in seawater are found, the report should mention the proportions in which they are present. It has been frequently noted that the staining of a sheet in the center does not extend to the edges, and although laboratory tests usually show the presence of salt in such cases, this is probably due to the presence of salt from the air during transit. The effects of water and moisture on galvanized sheets are largely due to the excess of moisture and a limited supply of air. This could be avoided by spacing the sheets or storing or stowing them in a dry, well-ventilated place. Any sheets which have become wet in transit should be wiped dry before storage. See also remarks under GALVANIZED WIRE.
- Index: 337
- Commodity Name: GALVANIZED IRON SHEETS