11. METALLIC & NON METALLIC
79
THERMOWELL MATERIALS
Passivating
: Involves immersing 300 series stainless steel in 10% nitric acid for
10-30 minutes. The acid removes any particles of iron which may have
become embedded in the surface during processing but doesn’t attack
the stainless steel. Actually, being a strong oxidizing acid, the chromium
oxide flm is improved thereby increasing the steels ability to withstand
corrosion.
Reducing atmospheres
: Contain hydrogen of carbon compounds and will not form protective oxi-
des on an alloy. If hydrogen is present, this may diffuse into thermowells
and thermocouples. It produces ‘green rot’ attack, so called from the dark
green surface colour produced, although this may not be very obvious.
In the case of chromel-alumel thermocouples the ‘green rot attack cau-
ses the chromed wire to become magnetic, which results in an erroneous
lower output. This effect is easy to confrm with a magnet; if both wires
are magnetic ‘green rot’ has occurred. (Actually this is not strictly a ‘redu-
cing’ phenomenon. It occurs only when a very small amount of oxygen is
present in an essentially reducing atmosphere. Under these conditions,
preferential oxidation of the chromium in the alloy will occur.)
Stress-corrosion cracking : When a metal is subjected to both stress and corrosion at the same time,
there is the possibility it may crack. Often stress-corrosion cracking oc-
curs in the presence of chlorides.
Stress relief
: A heat treating process, used to reduce internal stresses in a part to avoid
stress corrosion cracking from occurring.
Sulphidizing atmospheres : Contain sulphur compounds, which often arise when coal or fuel oil is
burned. The sulphur may be present as sulphur dioxide, which is the case
under oxidizing conditions, or as hydrogen sulphide (H2S) under redu-
cing conditions. The latter is worse as the atmosphere does not help the
formation of protective oxide flms. Alloys containing nickel (as almost all
the commonly used high temperature alloys do) are subject to attack by
sulphur because sulphur forms a low melting point compound with the
nickel in the alloy. Alloys high in chromium (over about 18%) containing
aluminium form an oxide flm which offers resistance to sulphur under
oxidizing conditions. To resist sulphur under reducing conditions, the best
protection is an aluminized flm.
Weld decay
: Localized corrosion on each side of a weld caused by carbide precipita-
tion.