
274 Fundamentals of Corrosion
is very small in case the tin should dissolve. Good coverage requires good
and uniform nucleation of FeSn
2
. Many nuclei form when electrodeposition
of tin is carried out from the alkaline stannate bath.
Compared to either iron or tin, FeSn
2
is chemically inert in all but the stron-
gest oxidizing environments.
Most of the tinplate (tin coating on steel) produced is used for manufactur-
ing food containers (tin cans). The nontoxic nature of tin salts makes tin an
ideal material for the handling of foods and beverages.
An inspection of the galvanic series will indicate that tin is more noble
than steel and, consequently, the steel will corrode at the base of the pores.
On the outside of the tinned container, this is what happens — the tin is
cathodic to the steel. However, on the inside of the container, there is a rever-
sal of polarity because of the complexing of the stannous ions by many food
products. This greatly reduces the activity of the stannous ions, resulting in
a change in the potential of tin in the active direction.
This change in polarity is absolutely necessary because most tin coatings
are thin and therefore porous. To avoid perforation of the can, the tin must act
as a sacricial coating. Figure 8.10 illustrates this reversal of activity between
the outside and inside of the can.
The environment inside a hermetically sealed can varies depending upon
the contents, which include general foods, beverages, oils, aerosol products,
liquid gases, etc. For example, pH values vary for different contents:
Contents pH Range
Acidic beverage 2.4–4.5
Beer and wine 3.5–4.5
Meat, sh, marine products, and
vegetables
4.1–7.4
Fruit juices, fruit products 3.1–4.3
Nonfood products 1.2–1.5
The interior of cans is subject to general corrosion, localized corrosion,
and discoloring. The coating system for tinplate consists of tin oxide, metal-
lic tin, and alloy. The dissolution of the tin layer in acid fruit products is
caused by acids such as citric acid. In acid fruit products, the potential
reversal occurs between the tin layer and the steel substrate, as shown in
Figure 8.11. The potential reversal of a tin layer for steel substrate occurs at a
pH <3.8 in a citric acid solution. This phenomenon results from the potential
shift of the tin layer to a more negative direction. Namely, the activity of the
stannous ion, Sn
2+
, is reduced by the formation of soluble tin complexes, and
thereby the corrosion potential of the tin layer becomes more negative than
that of steel. Thus, the tin layer acts as a sacricial anode for steel so that
the thickness and density of the pores in the tin layer are important factors