Environmental Engineering Reference
In-Depth Information
quenching from these temperatures produces martensitic microstructure. These
steels are used where high strength or hardness along with moderate corrosion
resistance is required. Typical applications are ball bearings, surgical instruments,
cutlery, cutters, valve parts, turbine blades, nuts and bolts, and the like.
The
austenitic stainless steels
retain austenite at room temperature because of
the addition of nickel and/or manganese and nitrogen. These are readily deformed
and can be hardened by cold working. Austenitic stainless steels are in general
more corrosion-resistant than the ferritic and martensitic grades. This coupled
with their good weldability has made austenitic stainless steels the principal
choice for structural units in process industries. Other typical applications include
kitchenware, automobile parts, pump shafts, fasteners, architectural trims in in-
dustrial and marine atmospheres, and cryogenic applications. The varieties con-
taining molybdenum are better suited in marine applications for their resistance
to pitting.
The
age-hardening
or
precipitation-hardening stainless steels
are alloys of
higher mechanical strength because of the precipitation of intermetallic phases
on aging after quenching from high temperature. Copper and aluminum are added
for this purpose. These alloys usually have poorer corrosion resistance than the
other types and are used in relatively mild corrosive environments. Typical appli-
cations are aircraft and missile components.
The
duplex stainless steels
contain both ferrite formers (Cr, Mo) and austenite
stabilizers (Ni, Mn) in such amounts as to have a favorable combination of both
phases. These alloys are particularly resistant to stress corrosion cracking (SCC).
Although the stainless steels have been developed for corrosion resistance,
they nevertheless are not the answer to all corrosion problems. Rather, they are
more susceptible to localized attack like pitting, crevice corrosion, and SCC than
carbon steels, particularly in chloride-containing media. Moreover, there is a mis-
nomer that nonmagnetic stainless steels have better performance as corrosion-
resistant materials. Some alloys belonging to the nonmagnetic austenitic group
become magnetic on cold working because of the transformation of the metasta-
ble austenite to ferrite, but this does not hamper their corrosion resistance.
Stainless steels are, in general, resistant to nitric acid over a wide range of
concentrations and temperatures, aerated dilute sulfuric acid at room temperature,
sulfuric acid of somewhat higher concentration (e.g., 10%) and at boiling temper-
ature if Fe
3
,Cu
2
, or HNO
3
is added as inhibitor, or at lower temperature if
small amounts of Cu, Pt, or Pd are alloyed. They can handle sulfuric acid of all
concentrations up to the boiling point, if anodically protected. They are resistant
to many organic acids. Alkalis do not attack stainless steels, but some varieties
are susceptible to SCC in hot concentrated caustic solutions. Stainless steels do
not rust in atmospheric exposures.
Stainless steels are not resistant to reducing acids HCl, Br, or HF because the
passive film is damaged in these media. Stainless steels are not suitable for sea-
