Page 625 - Corrosion Engineering Principles and Practice

588 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 589

most weathers. They are common on appliances and plumbing
fixtures. Glass-lined metals are used in process industries where there
is concern over corrosion or contamination of the product. Conversion
coatings are produced by intentionally corroding the metal surface in
a controlled manner. This is done to produce an adherent corrosion
product that protects the metal from further corrosion. Anodization
of aluminum, one of the more commonly used conversion coating
techniques, produces a protective aluminum oxide film on the
aluminum metal [2].
Metallic coatings also create a barrier between the metal substrate
and the environment. In addition, metallic coatings can sometimes
provide cathodic protection when the coating is compromised.
Metallic coatings and other inorganic coatings are produced using a
variety of techniques, including hot dipping, electroplating, cladding,
thermal spraying, chemical vapor deposition, or surface modification
using directed energy (laser or ion) beams.
The most widely used metallic coating method for corrosion
protection is galvanizing, which involves the application of metallic
zinc to carbon steel for corrosion control purposes. Hot-dip
galvanizing is the most common process, and as the name implies, it
consists of dipping the steel member into a bath of molten zinc.
According to the U.S. Department of Commerce approximately
8.6 × 10 t of hot-dip galvanized steel and 2.8 × 10 t of electrolytic
6
6
galvanized steel were produced in 1997. The total market for
metallizing and galvanizing in the United States is estimated at
$1.4 billion.

14.2 Why Coatings Fail
Coating systems for the protection of materials need and probably
demand more organized study than they have been given in many
instances, and the benefits attainable from proper preliminary
planning, job execution, on-site inspection, and postcompletion
surveillance are well worth the effort and expense. Figure 14.1
describes how coating failure responsibilities were broken down in a
throughout investigations made on 84 coating failures [3].
A coating may fail as a result of a large number of potentially
adverse conditions. Some of these can be defined as mechanical, as
when abrasion or impact removes the coating or more subtly and
locally damage the coating and create a corrosion site that will show
in a few months. A common reason protective coatings do not perform
well is that they have not been considered as systems. Successful
coating engineers approach a coatings project in much the same way
that they approach any other engineering problem, beginning with
the design of surfaces to be protected and ending with schedules for
monitoring the completed work.