98 Steels: Metallurgy and Applicationscharacteristics of the material. The product is then passivated by immersion in a
dichromate solution which deposits a very thin film of chromium on the surface.
After passivation, a thin film of oil is applied in order to preserve the surface
from attack and also to enhance the lubrication properties in subsequent handling
operations. The oil applied, dioctyl sebacate, is a synthetic organic oil and is
acceptable for use in food packaging.
Electrolytic tinplate can be produced with equal or differential coatings on
each surface, the former carrying the prefix E in the designation and the latter
having the prefix D. The mass on each surface is expressed in g/m 2 and thus
E 2.8/2.8 has 2.8 grams of tin per square metre on each surface, giving a total
of 5.6 g/m 2. A tin coating of 5.6/5.6 g/m 2 is equivalent to a coating thickness
of 0.75 ttm per surface. The normal range of equally coated products is from
E 1.4/1.4 to E 11.2/11.2. However, low-fin coatings have been developed down to
1.0/1.0 g/m 2. In differentially coated products, the normal range is from D 2.0/1.0
to D 11.2/5.6.
Tinplate manufacturers also produce other products such as uncoated and
oiled sheet (blackplate) and material coated with metallic chromium. According
to Morgan is~ blackplate has not achieved significant usage in canmaking
operations due to problems in providing adequate resistance to corrosion by
lacquering techniques. However, electrolytic chromium/chromium oxide-type
coatings have enjoyed greater success and were developed primarily because of
the extremely variable price of tin. The coating is duplex and consists of about
80% metallic chromium adjacent to the steel substrate and 20% hydrated chromic
oxide/hydroxide in a layer above. It is recommended that this type of coating is
lacquered to provide added surface protection and to enhance fabrication.
Canmaking processes
Three basic procedures are employed:
- Three-piece can manufacture.
- Drawing and wall-ironing.
- Draw and redraw.
Three-piece cans consist of a welded cylindrical body and two ends, one of
which (the base) will be attached to the body of the can by the canmaker and
the other will be applied after filling. The cylinder is rolled into shape from
fiat, pre-lacquered, rectangular blanks and the two edges are joined by electrical
welding. A further coat of lacquer is then applied to the weld seam. The can ends
are pressed from circular blanks in an operation that requires a high degree of
precision. The ends are contoured with a series of expansion tings so that they
can support the internal pressure through tensile rather than bending stresses. The
rims of the ends are also carefully stamped and curled so as to accept a sealing
compound which forms an airtight seal with the body of the can. Cans of this
type are used for most human foods and also for paints, oils and chemicals.
The drawing and wall-ironing (DWI) process is more modem than three-piece
canmaking and is illustrated schematically in Figure 1.100. Thus DWI eliminates