Corrosion Science Chemistry Research Article | Page 8

8
Corrosion: Understanding the Basics
Inorganic coatings include porcelain enamels, chemical-setting sili-
cate cement linings, glass coatings and linings, and other corrosion-
resistant ceramics. Like organic coatings, inorganic coatings for corro-
sion applications serve as barrier coatings. Some ceramic coatings,
such as carbides and silicides, are used for wear-resistant and heat-
resistant applications, respectively.
Inhibitors
Just as some chemical species (e.g., salt) promote corrosion, other
chemical species inhibit corrosion. Chromates, silicates, and organic
amines are common inhibitors. The mechanisms of inhibition can be
quite complex. In the case of the organic amines, the inhibitor is adsorbed
on anodic and cathodic sites and stifles the corrosion current. Other in-
hibitors specifically affect either the anodic or cathodic process. Still oth-
ers promote the formation of protective films on the metal surface.
The use of inhibitors is favored in closed systems where the necessary
concentration of inhibitor is more readily maintained. The increased
use of cooling towers stimulated the development of new inhibitor/
water-treatment packages to control corrosion and biofouling.
Inhibitors can be incorporated in a protective coating or in a primer for
the coating. At a defect in the coating, the inhibitor leaches from the
coating and controls the corrosion.
Cathodic Protection
Cathodic protection suppresses the corrosion current that causes dam-
age in a corrosion cell and forces the current to flow to the metal struc-
ture to be protected. Thus, the corrosion or metal dissolution is pre-
vented. In practice, cathodic protection can be achieved by two
application methods, which differ based on the source of the protective
current. An impressed-current system uses a power source to force cur-
rent from inert anodes to the structure to be protected. A sacrificial-an-
ode system uses active metal anodes, for example, zinc or magnesium,
which are connected to the structure to provide the cathodic-protection
current.
Design
The application of rational design principles can eliminate many cor-
rosion problems and greatly reduce the time and cost associated with
corrosion maintenance and repair. Corrosion often occurs in dead spaces
or crevices where the corrosive medium becomes more corro- sive.
These areas can be eliminated or minimized in the design process.
Where stress-corrosion cracking is possible, the components can be de-
signed to operate at stress levels below the threshold stress for cracking.