Environmental Engineering Reference
In-Depth Information
environment so that they do not come in contact with each other. This is achieved
fully with some types of coatings and partially with others. Apart from this, the
coatings may release substances that inhibit the corrosive attack of the environ-
ment on the substrate or the coatings may act as a sacrificial anode (Section 3.3.3)
to protect the substrate cathodically. However, it is to be remembered that all
coatings have their own life, i.e., they will not resist the attack permanently.
Nonmetallic coatings are divided into two groups:
inorganic
and
organic
. In-
organic coatings comprise vitreous enamel, cement, and chemical conversion
coatings. Organic coatings include paints, varnishes, and lacquers. Thicker coat-
ings of enamel, cement, tar, or rubber are sometimes applied to negotiate aggres-
sive chemical conditions; such coatings are referred to as
linings
. Most of the
coatings have been developed to protect steel in varied aqueous environments
and the discussion that follows will be with reference to steel unless otherwise
stated.
4.3.1 Metallic Coatings
Classification
From a corrosion point of view, metal coatings can be divided into two classes:
noble and sacrificial. Silver, copper, nickel, chromium, tin, and lead coatings on
steel constitute the first group, whereas the coatings of zinc, aluminum, and cad-
mium belong to the second group. As discussed in Section 3.3.3, any damage
or discontinuity in the noble metal coating creates a small anode-large cathode
condition leading to rapid localized attack on the substrate at the damaged areas.
On the other hand, such damages in the sacrificial coating will not pose a problem
as the exposed substrate will be cathodic with respect to the coating metal and
will be protected at the cost of the corrosion of the coating metal. Naturally, the
noble metal coatings should be free from pores and this is usually achieved
through an increase in the coating thickness.
Reversal of polarity between zinc and steel occurs in many aerated waters
above 60
C, which means that the zinc coating behaves as a noble metal coating
on steel. Under the circumstances the base steel becomes vulnerable to attack at
coating discontinuities. Tin is cathodic to iron, but tin coating inside food cans
becomes anodic to steel because stannous (Sn
2
) ions are complexed with the
food product, thereby greatly reducing the stannous ion activity (see Nernst equa-
tion). However, the galvanic protection of steel by tin is lost in the presence of
dissolved oxygen and food should not be retained inside the tin cans after opening
to avoid contamination by corrosion products.
Methods of Application
Metallic coatings are applied by several methods, the principal among these being
hot dipping, electroplating, flame spraying, cementation or diffusion-coating
cladding, and vapor deposition.
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