Page 45 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

LOCALIZED CORROSION 23

(vi) It is useful to examine the strained portions of metal such as weld zones that
tend to be anodic with respect to unstrained areas (cathodic areas) and the bot-
tom of tanks in aggressive environments.
Some selected procedures/standards: measurement of weight loss along with
visual comparison of the pitted surface may suffice to rank the alloys with respect
to corrosion resistance. Both visual and microscopic examination of pits can
determine the size, shape, and density of pits. Metallographic examination should
show the correlation with the microstructure and to distinguish pits between pitting,
intergranular corrosion (IGC), or dealloying (30). The test ASTM G 46 gives
the standard rating chart for pits and the various methods of examination of pits
and describes nondestructive examination of pitting that includes radiographic,
electromagnetic, ultrasonic, and dye penetration methods. A brief outline of the
statistical approach may be found in ASTM G 46 and the detailed approach (30) in
ASTM G 16. It is customary to compare the resistance to pitting as a function of the
chloride concentration, which causes pit initiation in different alloys. It is customary
to determine the depths of 10 deepest pits and the deepest one along with pitting
potentials. The pit depths may be determined with the aid of micrometer, a depth
gage, or a microscope. The average of 30 deep pits and the deepest pit depth are
usually determined. From these data, the pitting factor is the ratio of the deepest
metal penetration to the average depth of penetration. Standard test procedures
for pitting and crevice corrosion resistance are described in ASTM G48. Standard
procedure for crevice corrosion is given in ASTM G78 (30).

1.4.10 Corrosion Tests

Five corrosion tests for iron- and nickel-based alloys have been identified of which
two tests pertain to crevice corrosion. The method MTI-2 originating from ASTM
G 48 involves the use of 6% ferric chloride for determining the relative resistance of
alloys to crevice corrosion. The method MTI-4 uses high levels of chloride concen-
tration in the range 0.1–1% NaCl to establish the minimum concentration of chloride
that results in crevice corrosion (33, 34).
In one of the tests, the corrosion potential of the sample in 9 g/l sodium chloride is
noted. The pH, initial, and final potential are noted. The current at +0.8 V versus SCE
is noted. If localized corrosion is not simulated in the first 20 s, the polarizing cur-
rents will be small and decrease with time. If localized corrosion is not stimulated in
15 min, the test is terminated, and the sample is resistant to localized corrosion. Local-
2
ized corrosion is indicated by increasing polarizing current with time (>500 μA/cm ).
The potential is then returned to corrosion potential to determine if the sample will
repassivate or the localized corrosion will continue to occur. Evidence of pitting and
crevice corrosion should be noted in ASTM F 746 (30).

1.4.11 Changes in Mechanical Properties
The change in mechanical property may be used to identify as well as quantify the
extent of pitting corrosion in the case of intense pitting corrosion. It is also useful