Cold drawn wire can be cleaned and then coated by dipping into a bath of molten zinc spelter, or by electro galvanizing methods, or by a fused in alloy method. An inadequate percentage of alloys in manufacture can be a cause of discoloration and oxidation.
Oxidation (rust) may also be due to abrasion in handling or transportation, causing the wire to rub and wear the protective coating, which, when exposed to the atmosphere or to periods of precipitation due to changes in temperature with high humidity, will cause oxidation. Electrolytic action occurs wherever two dissimilar metals are exposed to an acid i.e. salty, atmosphere. etc., and in the case under consideration the slightest break in the continuity of the zinc coating serves to bring together all the elements needed for electrolysis to take place. Such electrolytic action causes oxidation of the iron and also accounts for the presence of sodium chloride if found on analysis.
Good outturn depends a great deal on how wire is prepared for export. Open roll packing is often employed, and shipment from the mill to seaboard is often made in open vehicles; storage at shipping point and at the arrival point of the wire may be in open areas, or open trucks may be used for haulage at the port of destination. In such cases the wire is subjected to the elements and to changes of climate, such conditions being conducive to rust.
Discoloration and oxidation are often the result of abrasion and an exposure to the elements or to a moisture laden atmosphere, and the process is accelerated owing to temperature changes because of repeated cycles of heating and cooling of the mass, which causes water to condense on cooling and to evaporate on heating. There is also the electrolytic action as described above, which accounts for the presence of sodium chloride on chemical analysis.
Seawater salts may be present, although there may be no evidence that wire has been in contact with salt water or subjected to salt water spray. The observed condition is the natural effect which is produced when ferrous metals that may be imperfectly coated with zinc spelter are exposed to salt air and normal climatic changes, which combine with abrasion to cause the coating to wear and oxidize. See also remarks under GALVANIZED IRON SHEETS.
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