Page 170 - CW E-Magazine (2-4-2024)

Special Report

result in accelerated corrosion of the For wear resistance, ceramics are com- Radiation;
titanium. In fact, in some cases the elec- posed of carbides or nitrides. Thermal degradation, involving
trode potential of the titanium can be depolymerisation and probably
increased, reducing the corrosion rate Polymers repolymerisation;
instead of accelerating it. This pheno- Polymeric materials don’t expe- Dehydration (rather uncommon); and
menon holds true except in environments rience specific corrosion rates; they’re Any combination of the above.

where titanium does not passivate (e.g., typically either fully corrosion-proof
in anhydrous conditions). against a selected corrodent (within The result of such attacks can
specific temperature ranges), or they include softening, charring, crazing,
Titanium’s corrosion-resistant pro- deteriorate quickly. They’re attacked delamination, embrittlement, discolora-
perties can be increased even further by either by chemical processes or by solva- tion, dissolving or swelling. The corro-
alloying it with other metals. Palladium tion. Solvation causes the swelling, sion of polymer matrix composites is
has been one of the most successful. softening and supreme failure of poly- additionally plagued by two factors: the
Generally, small amounts of palladium meric materials. Therefore, it is vital to nature of the laminate; and, within the
can significantly increase titanium’s grasp the complexities of corrosion in case of thermosetting resins, the cure.
resistance to reducing chemicals such as polymeric material. Improper or poor cure can adversely
sulphuric, hydrochloric and phosphoric impact corrosion resistance, whereas
acids. Additionally, this alloy combina- Polymers are used as thermoplas- correct cure time and procedures can
tion is known to raise the critical tem- tics, thermosets and elastomers. Most usually improve corrosion resistance.
perature at which crevice corrosion can polymers are very corrosion resistant,
occur in seawater. even more so than many corrosion Polymeric materials in outside
resistant alloys. Polymers, however, have applications are exposed to weather
Over time, with decreasing costs low strength, low melting points and extremes that may be extraordinarily
and increasing availability, the use of limited ultra-violet (UV) resistance. In injurious to the material. The foremost
titanium alloys has continued to spread corrosive, high-pressure applications, harmful weather element – exposure
and is now widely used in numerous polymers can be used for corrosion- to UV radiation – will cause embrittle-
industrial applications. Some of the resistant liners on metal surfaces. High ment, fading, surface cracking and
many uses of titanium and its alloys density polyethylene (HDPE), poly- chalking. When a polymeric material is
include paints and coatings, steel alloy- vinylchloride (PVC) and polytetra- exposed to direct daylight for a period
ing, jewellery, industrial storage tanks, fluoroethylene (PTFE) are examples of of years, it usually exhibits poor impact
electronic components, construction thermoplastics, but despite being in the resistance, reduced overall mechanical
materials and automotive components. same group, they have different proper- performance, and changes in appea-
Although titanium has superior corro- ties and applications. Thermosets are rance. The ability to resist weathering
sion resistance in comparison to many not usually used as solid materials, varies with the type of polymer and
other naturally occurring metals, it is but rather as coatings or in a matrix of among grades of a specific resin.
essential to understand the source of a composite. Elastomers have unique Several resin grades are obtainable with
this resistance and its benefits and limi- mechanical properties and are used UV-absorbing additives to boost weather-
tations, in order to make the best use of for sealing components in many pres- ability.
this metal in any given environment. surised applications.
Plastic materials tend to be an order
Ceramics According to their attack mechanism, of magnitude larger in porosity than
Because of their brittle nature – and the corrosion of polymeric materials is metals. Gases, liquids or vapours can
despite their high corrosion resistance – often classified in the following ways: permeate polymers (molecular migra-
ceramics are not used in pressure-con- Disintegration or degradation of a tion through micro-voids). Permeation
taining equipment. Ceramics are used physical nature owing to absorp- is harmful once a polymer is applied to
in high-temperature equipment and tion, permeation, solvent action or line piping or instrumentation. In lined
parts, such as refractory materials, and various factors; equipment, permeation may end in:
for protection against wear. Ceramics Oxidation, wherever chemical bonds Failure of the substrate as a result of

for refractory materials are generally are attacked; corrosion;
composed of a combination of oxides Hydrolysis, wherever ester linkages Bond failure and blistering ensuing

of aluminium, magnesium and silicon. are attacked; from the build-up of fluids at the
170 Chemical Weekly April 2, 2024

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