Page 173 - CW E-Magazine (2-4-2024)

Special Report

conditions, titanium and its alloys Area of anodic metal should be larger Intergranular corrosion

have shown superior resistance. than that of the cathodic metal; Intergranular corrosion of austenitic
Dissimilar materials should be insu- SS can be prevented by:
The corrosion resistance of a pure lated wherever possible; Employing high temperature
metal is usually better than that of one Threaded joints of materials far solution heat treatment, commonly

containing impurities or small amounts apart in the galvanic series should termed as quench-annealing or
of other elements. However, pure metals be avoided; solution-quenching;
are usually expensive and are rela- Anodic part of the material should Adding elements that are strong

tively soft and weak. Therefore, these be easily replaceable or should be carbide formers (called stabilisers);
materials are used in special cases only; made thicker; and and
e.g., commercially pure aluminium A third metal, which is anodic to Lowering the carbon content to

(99.5%), which is not very expensive, both metals in the galvanic series, below 0.03%.
is used for handling hydrogen peroxide. may be installed.
Another example is arc-melted zirco- Dezincification
nium, which is more corrosion resis- Crevice corrosion For severely corrosive environ-
tant than induction-melted zirconium It can be prevented by taking fol- ments where dezincification occurs, or
because of more impurities in the latter. lowing measures: for critical parts, cupronickels (70-90%
This is a special case in an atomic Welded butt joints, instead of rive- Cu, 30-10% Ni) should be utilised.

energy application where even a little ted or bolted joints, should be
corrosion is too much. ensured in new equipment. Sound Graphitisation
welds and complete penetration Nodular or malleable cast irons
Table-1 indicates the common metals are necessary to avoid porosity and should be used instead of grey cast
and their corrosion resistance to crevices on the inside (if welded iron. Also, white cast iron can be used
aggressive fluids like acids, bases and only from one side); as graphitisation does not take place
more. The degree of corrosion resis- Crevices in the existing lap joints in white cast iron as no free carbon is
tance is expressed as 1, 2, and 3, signi- should be closed by continuous present in it.
fying ‘good’, ‘be careful’, and ‘not welding, caulking, or soldering;
suitable’ respectively. However; the Solid non-absorbent gaskets, such Erosion corrosion

information given is just a guide and as Teflon, should be used wherever The addition of a third element to
should be used with care as corrosion possible; an alloy often increases its resistance
depends on combination of materials Welding should be done, instead of to erosion corrosion. The addition of

and fluids, fluid temperature, surround- rolling in tubes in tube sheets; and iron to cupronickel produces a marked
ing environment and galvanic currents Vessels should be designed for increase in resistance to erosion corro-

in construction, etc. complete drainage. sion by seawater. The addition of molyb-
denum to 18-8 to make type 316 makes
Selection of materials based on Filiform corrosion it more resistant to erosion corrosion.
mechanism of corrosion(1, 4) It can be retarded or prevented by Aluminium brasses show better erosion
applying films of very low water per- corrosion resistance than straight brass.
Uniform attack meability on the metal to be stored for Durimet 20 (30% Ni, 20% Cr, 3.5%
It can be reduced or prevented by a long time. Cu, 2% Mo) exhibits better perfor-
selecting proper material of construc- mance than 18-8 steals in fuming nitric
tion resistant to the prevailing environ- Pitting corrosion acid, seawater, and many other envi-
ment/atmosphere and by application of Some materials are more resistant ronments. Applied coatings of various
appropriate coating. to pitting corrosion than others. For types that produce a resilient barrier
example, the addition of 2% molybdenum between the metal and its environment
Galvanic corrosion to type 304 SS producing type 316 SS are sometimes utilised. Hard facings, or
It can be prevented by selecting results in very large increase in resis- welded overlays, are sometimes help-
materials as given below: tance to pitting. The increase in pitting ful. Repair of attacked areas by weld-
Combination of metals as close resistance is in the following order: SS- ing is often practical. Ceramic metal
together as possible in the galvanic 304, SS-316, Hastelloy F, Hastelloy C, coating is successfully applied to pipe
series; Titanium. and pipe bends, pump bodies and im-

Chemical Weekly April 2, 2024 173

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