Environmental Engineering Reference
In-Depth Information
towards the reduction of susceptibility, but in order to achieve complete immunity
this is to be lowered to very low value. For example, for 18Cr-2Mo alloys to be
immune to intergranular corrosion, the maximum level of carbon plus nitrogen
is 60-80 ppm. Ferritic stainless steels stabilized with titanium or niobium have
also been developed.
3.6.3 Tests for Sensitization
Special tests have been designed to detect the susceptibility of stainless steels
to intergranular corrosion. These serve as acceptance tests for the material for
construction or repair and also indicate the actual metallurgical condition of the
material in the evaluation of a plant process. Both chemical and electrochemical
tests are available.
The sulfuric acid-copper sulfate test, or Strauss test (ASTM A708-86), in-
volves exposure of the specimen in a boiling 16% H
2
SO
4
6% CuSO
4
solution
for 72 h and evaluation by bending of the specimen over a mandrel and examining
macroscropically for fissures or peeling caused by intergranular corrosion. Since
the rate of attack in this test is low, an accelerated test has been devised by
coupling the stainless steel specimen to a copper sheet (ASTM A262-86, Prac-
tice E).
The Huey test (ASTM A262-86, Practice C) consists of exposure of specimens
to boiling 65% nitric acid for five 48-h periods and corrosion rates are calculated.
Accepted rates for different materials have been specified. The test is time con-
suming.
The Streicher test (ASTM 262-86, Practice B), or oxalic acid etch test, is a
rapid test. It consists of etching the 3-zero emery paper finished specimens in
10% oxalic acid for 1.5 min under an applied current density of 1 amp/cm
2
and
then examining the surface at a magnification of 250-500
. The specimen acts
as an anode and a stainless steel beaker as cathode. A ''step'' structure indicates
a nonsensitized condition, whereas a ''ditch'' structure indicates susceptibility
to intergranular corrosion. Figure 3.20 illustrates these features.
Electrochemical tests involve anodic polarization of the specimen. Potentio-
static polarization shifts the passive region of the polarization curve progressively
toward higher current density value with increasing sensitization. A difference
of one to two orders of magnitude in the current density value with respect to
nonsensitized specimen depending on the degree of sensitization is indicated.
The electrochemical potentiokinetic repassivation (EPR) technique is based
on a reverse or reactivation potential scan from a potential back to the corrosion
potential. Either hot concentrated sulfuric acid or a solution of 0.5 M H
2
SO
4
0.01 M KSCN at 30
C is employed. A polished stainless steel sample of con-
trolled surface area is passivated at
°
2 V (SCE) for 2 min. Potential is then
scanned in the active direction at a rate of 6 V/h, down to the corrosion potential,
where no current is indicated. Sensitized material shows a current peak that in-
