Environmental Engineering Reference
In-Depth Information
All that is needed to save them from corrosion is to provide suitable corrosion
protection measures. These measures have been discussed in the sections to fol-
low, mostly with reference to steels.
Low-alloy steels have principally been developed for improved mechanical
properties. They normally contain alloying elements like Ni, Cr, Mo, V, Nb, and
Ti with a total content of not more than 2%. The corrosion behavior of such
steels in aqueous media is not very different from that of carbon steels. However,
the addition of Cu, Si, Cr, Ni, and P improves the resistance to atmospheric
corrosion because the rust produced becomes nonporous and adherent. Copper
plays a definite role in altering the morphology of the rust and is used to the
extent of 0.1-0.4% in all weathering steels.
4.1.3 Stainless Steels
Stainless steels are basically iron-chromium alloys containing a minimum of 11%
Cr, the amount necessary to keep the alloy in the passive state in normal oxidizing
exposures. Chromium apart, nickel (or manganese) is another principal alloying
element in several grades of stainless steels. Molybdenum, titanium, niobium,
silicon, and copper are also added to several grades for specific effects.
Stainless steels are available both as wrought and cast products and are of
four principal types: ferritic, martensitic, austenitic, and precipitation hardening.
Duplex stainless steels containing both ferrite and austenite constitute the fifth
group. The wrought varieties are known generally by their numerical AISI
(American Iron and Steel Institute) designations (Table 4.1). The cast varieties
contain 1-2.5% silicon additionally for better castability.
The
ferritic stainless steels
are characterized by their ferrite microstructure.
For low-carbon iron-chromium alloys the high-temperature austenite phase exists
only up to 12% Cr; beyond this composition the alloys are ferritic at all tempera-
tures up to the melting point. They can be hardened moderately by cold working
but not by heat treatment. Ferritic stainless steels with straight chromium were
the first stainless steels to be developed. The low solubility of carbon and nitrogen
in the ferrite phase tends to render them easily sensitized, hence embrittled. Weld-
ing has also been a problem with the conventional ferritic steels. These difficulties
have been overcome in the recently developed high-purity ferritic stainless steels
containing the total interstitials below 250 ppm. These grades contain high chro-
mium (26-29%) with 1-4% molybdenum and in some varieties up to 2% nickel.
Typical applications of ferritic stainless steels are architectural and automobile
trims, tableware, ammonia oxidation plants for making nitric acid, tank cars and
tanks for storage of nitric acid, thin-wall tubings for heat exchangers, and other
high-temperature applications.
The
martensitic stainless steels
contain a higher percentage of carbon than
ferritic steels, so that the austenite phase is available at higher temperatures and
