Steels_ Metallurgy and Applications, Third Edition

Stainless steels 329

is to accelerate the dissolution process. In the hydrogen embrittlement theory,
hydrogen is generated due to the cathodic reaction (H + 4- e- --, H) and migrates
to the crack tip where it is absorbed within the metal lattice, causing mechanical
rupture. This extends the crack and further anodic dissolution then occurs on the
newly exposed surfaces.
Although laboratory tests are carried out in a variety of salt solutions, that
involving boiling 42% magnesium chloride solution is probably the most popular.
The test specimens may be U-bend specimens which are clamped so as to provide
a large tensile stress on the outside of the bend or else tensile specimens are
employed, loaded to perhaps 0.8 times the 0.2% PS of the material.
Major research effort has been devoted to the effects of composition on
the stress corrosion behaviour of stainless steels. One of the most important
elements is nickel and the celebrated Copson curve l~ is shown in Figure 4.19.
This relates to stainless steels containing 18-20% Cr in a magnesium chloride
solution boiling at 154~ This figure indicates a minimum resistance to SCC at
nickel contents in the range 8-10%, i.e. typical of those in standard austenitic
steels such as Types 302 and 304. A reduction in nickel from these levels leads
to an improvement in the SCC behaviour but of course this is associated with
the progressive replacement of austenite by delta ferrite in the microstructure.
Alternatively, the resistance to SCC can be improved by increasing the nickel
content to levels of 40% and above. The beneficial effect of nickel at levels
above 8-10% has been attributed to an increase in the stacking fault energy of
the materials but nickel must also exert an ennobling effect which inhibits anodic
dissolution.
Silicon has a beneficial effect on the SCC behaviour in magnesium chloride
solutions but has virtually no effect in high-temperature solutions of sodium
chloride.

1000

~" 100 Cracking

10 - --

l[ /" I Testsln n

It / I bo~,ng 42~

1

0 20 40 60

% Nickel

Figure 4.19 Effect of nickel on the stress corrosion behaviour of austenitic stainless steels
(After Copson l~ )